Publikationen 2002
T. Granzow, Th. Woike, M. Wöhlecke, M. Imlau, and W. Kleemann
Polarization-Based Adjustable Memory Behavior in Relaxor Ferroelectrics
Phys. Rev. Lett. 89, 127601 (2002)
Abstract
The irreversible decay of the spontaneous polarization above the
phase-transition temperature is a limiting factor in any application
of ferroelectric crystals. Here we show that electric fields applied
at high temperatures induce a preferred direction in the crystal
which is stable even after repeated heating and cooling through the
phase transition. This preference in direction leads to a
reorientation of domains in the ferroelectric phase. We use
pyroelectric measurements to show that the directional preference
originates from internal charge carriers interacting with domain
walls.
V. Szalay, L. Kovács, M. Wöhlecke, E. Libowitzky
Stretching potential and equilibrium length of the OH bond in solids
Chem. Phys. Letters 354, 56 - 61 (2002)
Abstract
Correlation has been observed between the experimentally determined harmonic fre-
quencies, w, and anharmonicity coefficients, X, of OH bond stretching vibrations in alkali
halide, oxide, and hydrate crystals. It has been shown that simple second order
perturbation approximation formulas of w and X can explain the
w-X correlation, and they can
be used to obtain the shape of the stretching potential and an estimate of the equilibrium
length of an OH bond in crystalline environment.
T. Granzow, U. Dörfler, Th. Woike, M. Wöhlecke, R. Pankrath, M. Imlau ,W. Kleemann
Local electric- field- driven repoling reflected in the ferroelectric polarization of Ce-doped Sr0.61Ba0.39Nb2O6.
Appl. Phys. Letters 80, 470 - 472 (2002)
Abstract
We present pyroelectric measurements with the relaxor-ferroelectric SBN in the phase transition regime. It is
demonstrated that domains poled at high temperatures are more stable than those oriented at room temperature. This yields a
higher phase transition temperature when heating the sample and a strong repoling during the cooling process. We explain this
difference within the framework of the random field model for relaxor ferroelectrics.
T. Granzow, U. Dörfler, Th. Woike, M. Wöhlecke, R. Pamkrath, M. Imlau, W. Kleemann
Evidence of random electric fields in the relaxor-ferroelectric Sr0.61Ba0.39Nb2O6
Europhysics Letters 57, 597 - 603 (2002)
Abstract
We present strong evidence of the existence of quenched random electric fields in relaxor ferroelectrics by
investigating the aging in ferroelectric hysteresis loops under light illumination. The temporal behaviour
of ferroelectric domains under external electric fields and light illumination is measured in Cerium-doped
Strontium-Barium-Niobate crystals using quasistatic surface charge detection. The aging, caused by electric
pinning centres which hinder domain wall motion, leads to a decrease in domain wall mobility. Raising the
sample conductivity by illumination eliminates the pinning centres and fully restores the polarization dynamics.
A. Ruediger, O.F. Schirmer, A.K. Kadashchuk, Yu.A. Skryshevskii
Pyroelectric luminescence via internal Poole-Frenkel effect
Europhysics Letters 57, 592 (2002)
Abstract
Pyroelectric luminescence is a universal property of pyroelectric materials
under changing temperature. In all compounds studied, inorganic and organic, it
has two possible manifestations: Short emission bursts caused by discharge of
the pyroelectric field through the surrounding atmosphere and a smooth
luminescence slowly varying with temperature. This latter emission,
characteristic for the specific material, still waits for an explanation. With
Sn2P2S6 as a sample pyroelectric we show that
this luminescence arises from a release of trapped charge carriers, triggered by
the pyrolectric field via the Poole-Frenkel effect.
Publikationen 2001
O. F. Schirmer, C. Veber, M. Meyer
Parameters of light-induced charge transfer processes in photorefractive crystals
Physics of the Solid State (in print)
Abstract
A method is outlined by which the parameters governing the light-induced
charge transfer processes in photorefractive crystals can be determined. The
system BaTiO3:Rh is treated as an example. EPR is used to obtain
information on the EPR-active defect charge states. By simultaneous observation
of light-induced EPR- and optical absorption changes the corresponding optical
absorption bands are established as fingerprints of the defect charge states.
Consistency arguments allow to label also EPR-silent absorption bands in this
way, significantly extending the scope of EPR based defect studies. On this
basis the charge transfer paths taking place under illumination are identified.
Quantitatively, the defect concentrations are directly or indirectly derived
from the available EPR signals. In addition the kinetics of the light-induced
changes of the densities of the occurring defect charge states are studied. In
conjunction with the defect concentrations, this allows to deduce the
responsible transfer parameters.
G. Malovichko, V. Grachev, R. Pankrath, O. Schirmer
Photosensitive centers and charge transfer processes in barium calcium titanate
Ferroelectrics (in print)
Abstract
The results of a study of
charge transfer processes in barium calcium titanate crystals, Ba0.77Ca0.23TiO3,
in the temperature range between 4.2 K and 300 K are reported. The nominally
pure and Rh doped BCT crystals were investigated with the help of electron
paramagnetic resonance, optical absorption spectroscopy and light induced
absorption change measurements.
S. Bernhardt, L. Mize, P. Delaye, R. Pankrath, O. Schirmer, G. Roosen
Orientation dependence of photorefractive gain in BCT: determination of
electro-optic coefficients
Trends in Optics and Photonics Series 62, 504 (2001)
Abstract
Photorefractive two-beam coupling measurements as a function of the
orientation of the grating spacing at 514 nm have enabled us to precise the
values of the electrooptic coefficients of BCT. We find r13T
= 33 pm V-1 and r33T = 170 pmV-1.
These values are in good accordance with other published values. For the
interesting coefficient r42T, we find a higher value r42T
= 465 pmV-1 than the one previously published. The observation of
beam-fanning and the fact that we have been able to implement a DPCM in a
45°-cut BCT (and not in a 0° cut crystal) confirms that the electrooptic
coefficient r42T is higher than r33T.
A. Ruediger, O. F. Schirmer, A. Kadashchuk, A. Grabar
Light-induced charge transfer in nominally pure and iron doped Sn2P2S6:
a study by combined EPR-optical absorption spectroscopy
Trends in Optics and Photonics Series 62, 568 (2001)
Abstract
The light-induced charge
transfer in ferroelectric tin hypothiodiphosphate Sn2P2S6 is investigated by means
of optical absorption and electron paramagnetic resonance spectroscopy and their
combination. By creating electrons and holes via optical band gap excitation
defect states in the gap are recharged. This situation is highly metastable,
even at room temperature. EPR reveals that hole capture probably transforms Sn2+
at one of the two inequivalent Sn sites into Sn3+. By 1.4 eV
illumination such holes are transferred to the other Sn site. Optical absorption
measurements at
298 K give evidence that the incorporation of iron leads to an absorption at
about 700 nm. Dielectric loss measurements show relaxation phenomena in the
frequency range of 20 Hz to 1 MHz and the influence of band gap illumination on
the behavior of the relaxing carriers. The recombination of the metastable
electron and hole arrangement is detected by thermoluminescence. It occurs only
if the pyroelectric field of the crystal has the right polarity. The phenomenon
appears to be connected to a Poole-Frenkel effect.
V. V. Gladkii, V. A. Kirikov, E. V. Pronina, T. R. Volk, R. Pankrath, M. Wöhlecke
Anomalies in the Slow Polaritation Kinetics of a Ferroelectric relaxor in the Temperature Region of a Diffuse Phase Transition
Physics of the Solid State 43, 2140-2145 (2001)
Abstract
It is shown experimentally in the specific example of Cr-doped
barium-strontium niobate that the anomalies in the infralow-frequency dielectric
properties characteristic of a ferroelectric relaxor persist over the whole
temperature region of the diffuse phase transition and decrease gradually with
increasing temperature. Experimental data are presented on the anomalous
quasi-static dielctric hysteresis loops, the slow polarization kinetics and a
quantitative measure of the structural disorder typical of a relaxor.
T. Volk, L. Ivleva , P. Lykov, D. Isakov, V. Osiko, M. Wöhlecke
Modification of the optical and photorefractive properties of Ce-doped strontium-barium niobate by co-doping with a nonphotorefractive La impurity
Appl. Phys. Letters 79, 854 (2001)
Abstract
We demonstrate a possibility to modify photorefractive properties of a ferroelectric crystal by means of controlling the
ferroelectric phase transition. This is shown by an example of co-doping a photorefractive SBN-0.61:Ce crystal with a
nonphotorefractive La impurity, which strongly lowers the ferroelectric phase transition and in turn drastically enhances
the linear electrooptic coefficients. The observed low coercive fields enable us to build in SBN:(Ce+La) a reproducible fast
switching of the energy transfer direction. This is done by an electric field switching which changes the sign of the two-beam
coupling gain factor G.
O. F. Schirmer, M. Meyer, A. Ruediger, C. Veber
COMBINED EPR/OPTICAL INVESTIGATIONS OF LIGHT-INDUCED CHARGE TRANSFERS IN PHOTOREFRACTIVE MATERIALS
Optical Materials 18, 1 (2001)
Abstract
A way to elucidate the light-induced charge transfer processes, which lead to
refractive index changes in photorefractive materials, is outlined. Using this method, the
EPR information on the chemical identity and structure of defects, between which the
transfers occur, is correlated to the corresponding optical absorption bands. On the basis
of these bands alone charge transfer phenomena can then be analyzed. The determination of
special charge transfer features is illustrated with examples on defects in BaTiO3.
A. Ruediger, O. Schirmer, S. Odoulov, A. Shumelyuk, A. Grabar
STUDIES OF LIGHT-INDUCED CHARGE TRANSFER IN Sn2P2S6 BY COMBINED EPR/OPTICAL ABSORPTION SPECTROSCOPY
Optical Materials 18, 123 (2001)
Abstract
The light-induced charge transfer in ferroelectric tin hypothiodiphosphate Sn2P2S6
is investigated by means of optical absorption and EPR spectroscopy and their combination.
Light-induced metastability at 298 K, known to affect the holographic sensitivity, is
observed via optical absorption. EPR measurements support the recent identification of
holes as the dominating charge carriers. For excitation energies exceeding the band gap of
2.5 eV at 10 K, EPR reveals that the following processes are likely to occur: a hole is
captured at one of two different Sn2+ sites, creating Sn3+. At an
energy of 1.5 eV the hole is first transferred to the other Sn2+ and for
excitation of 2.0 eV to a further center. Since these defects are intrinsic and therefore
not limited in quantity, the light induced sensitisation is a very effective way to
improve the holographic performance.
T. Volk, L. Ivleva, P. Lykov, N. Polozkov, V. Salobutin, R. Pankrath, M. Wöhlecke
Effects of rare-earth impurity doping on the ferroelectric and photorefractive properties of strontium-barium niobate crystals
Optical Materials 18, 179 (2001)
Abstract
Doping strontium-barium niobate (SBN) crystals with rare earth (RE)
impurities strongly lowers the ferroelectric phase transition Tc
which results in significant enhancement of the dielectric permittivity and
electrooptic coefficients and in a simultaneous decrease of the coercive filed Ec.
In photorefractive SBN:Ce crystals co-doped with a non-photorefractive impurity
La we studied the hysterese dependence of the two-beam coupling gain G
on the external field.
Th. Woike, T. Granzow, U. Dörfler, Ch. Poetsch, M. Wöhlecke, R. Pankrath
Refractive Indices of congruently melting Sr0.61Ba0.39Nb2O6
phys. stat. sol. (a) 186, R13 (2001)
Abstract
Refractive indices of pure congruently melting Strontium-Barium-Niobate single crystals poled
and unpoled have been measured in the wavelength range 400 to 2330 nm. The difference in the refractive indices
of poled and unpoled crystals is interpreted in the framework of the phase transition.
OH- ions in Oxide Crystals
Critical Reviews in Solid State and Materials Sciences, 26(1):1 - 86 (2001)
Abstract
This article reviews the spectroscopic properties of hydrogen bound to a large variety of synthetic compounds like simple
oxides, perovskites, the LiNbO3 family, KTP, sillenites, eulytites, borates, garnets, spinels and some others. In almost
all compounds the OH-stretch mode is found in the range 3200-3700 cm-1, with isotope replica of deuterium
(2350-2750 cm-1) and a few examples of tritium (2050-2250 cm-1). The fundamental vibration, the isotope
replica and the corresponding overtones are described with an anharmonic oscillator model. A nonlinear relation between the
anharmonicity and the harmonic frequency is found and interpreted using recent second order perturbation
calculations. An often pronounced temperature dependence of the stretch mode and phonon
coupling is discussed. Data for the stretching vibration influenced by additional weak or strong doping are presented.
Atomistic defect models are described and tested by means of light polarization, external perturbations like pressure and electric
field, and Raman scattering analyzed with the behavior-type method. The last section briefly reviews applications profiting
from doping with hydrogen and its easy detection by means of the OH stretch mode.
H.J. Reyher, M. Pape, N. Hausfeld
Photoactive Pb
J. Phys.: Condens. Matter 13, 3767 (2001)
Abstract
In photorefractive Pb5Ge3O11 and (Pb
T. Granzow, U. Dörfler, Th. Woike, M. Wöhlecke, R. Pankrath, M. Imlau, W. Kleemann
INFLUENCE of PINNING EFFECTS on the FERROELECTRIC HYSTERESIS in CERIUM-DOPED SrxBa1-xNb2O6
Phys. Rev B 63, 174101 (2001)
Abstract
The temporal behavior of ferroelectric domains under external electric field is investigated in
cerium-doped strontium-barium-niobate crystals using quasistatic surface-charge detection. The polarization reversal depends
strongly on frequency, temperature, dark conductivity, and doping
concentration. This so-called electrical aging is attributed to electric
pinning centers which hinder the domain-wall motion.
We discuss the existence of pinning centers on the basis of the quenched random-field model.
T. Volk, B. Maximov, T. Chernaya, N. Rubinina, M. Wöhlecke, V. Simonov
PHOTOREFRACTIVE PROPERTIES of LiNbO3:Zn CRYSTALS RELATED to THE DEFECT STRUCTURE
Appl. Phys. B 72, 647 (2001)
Abstract
LiNbO3:Zn single crystals and powders were studied by precise X-ray diffraction methods. A structure refinement yielded new
models of the intrinsic defect structure valid for different Zn
concentration ranges. For concentrations up to 7 at.% in the crystal Zn ions incorporate onto Li-sites; at higher
concentrations Zn ions are found on both Li and Nb sites. Photorefractive properties of
LiNbO3:Zn are discussed in the context of the deduced defect models. A smooth increase of the
photoconductivity up to 7 at.%Zn is accounted for by a decrease of Nb antisites. A steeper growth of the photoconductivity at
higher Zn concentrations is related to vanishing Li-vacancies.
PHOTOREFRACTIVE PROPERTIES of Cr-doped Sr0.61Ba0.39Nb2O6
RELATED to CRYSTAL PURITY and DOPING CONCENTRATION
Appl. Phys. B 72, 661 (2001)
Abstract
The purity and the concentrations of the constituents Sr, Ba, Nb and Cr were
determined in single crystals of chromium-doped Sr0.61Ba0.39Nb2O6 (SBN) by
instrumental neutron activation analysis (INAA) and X-ray fluorescence analysis
(XRF). Experiments with different Cr concentrations
CCr reveal a constant Sr and Ba concentration as well as a decreasing Nb concentration
with an increasing CCr . Therefore, Cr is incorporated on Nb lattice sites. The distribution coefficient
of Cr is 1.2 between 100 and about 7000 ppm Cr and 1 above. From measurements of the holographic two beam
coupling gain G for different CCr, we deduced response
times for the buildup of holographic gratings. The saturation value of G
depends in a nonlinear manner on CCr and reaches its maximum at about
6000 ppm Cr. The inverse photorefractive response time of the grating erasure depends linearly on the erasure intensity for all doping
concentrations. Thus a one-center model can be assumed for the charge transport in
SBN:Cr.
Publikationen 2000
H.J. Reyher, N. Hausfeld, M. Pape
A magnetic circular dichroism and optically detected magnetic resonance investigation of Fe2+ and Fe3+ centres in KTaO3
J. Phys.: Condens. Matter 12, 10599 (2000)
Abstract
A cubic and an axial Fe2+ centre, FeTa2+ and FeTa2+--VO,
are found in reduced Fe-doped KTaO3 crystals in conjunction with
their trivalent partners. These four defects are characterized by optically
detected magnetic resonance (ODMR) via the magnetic circular dichroism (MCD) of
the absorption. The g-values are determined by and interpreted using
crystal-field terms. For all centres, the optical absorption bands are
identified by the tagged-MCD method. The main MCD bands of the divalent ions are
attributed to the wall known 5T2 --> 5E
crystal-field transition in the 10 000 cm-1 region. Bands at higher
energies are interpreted as intervalence transfer transitions to conduction
band-like states.
AB INITIO STUDY OF OXYGEN VACANCIES IN BARIUM TITANATE
J. Phys.: Condens. Matter 12, 8239 (2000)
Abstract
In this paper we present embedded-cluster calculations on singly charged and neutral
oxygen vacancies (or F centres) in the oxide perovskite BaTiO3. The simulations
include Hartree-Fock theory with MP2 corrections and density-functional-theory
calculations for a central quantum defect cluster and a pair-potential description of the
embedding lattice. All important defect-induced lattice distortions are taken into account
in this way. We discuss the possible electronic states of charged F centres and the
effects of nearby acceptor-type defects. It is shown that isolated oxygen vacancies induce
electronic deep-gap levels. Scenarios are discussed to account for shallow-gap levels
observed experimentally.
EPR, ENDOR and OPTICAL ABSORPTION STUDY of Cr3+ CENTERS SUBSTITUTING FOR NIOBIUM in Li-rich LITHIUM NIOBATE CRYSTALS
Phys. Rev. B 62, 7779 (2000)
Abstract
A triplet of EPR lines with a relatively small zero-field splitting and
a ratio of peak-to-peak intensities of 1:50:1 was found in lithium niobate
crystals grown from the melt, with the addition of potassium and of 1 wt % of
Cr. Detailed investigations of the angular dependencies of EPR and electron
nuclear double-resonance (ENDOR) spectra, the temperature dependence of EPR
spectra, and optical absorption allowed us to attribute this triplet to a family
of Cr3 + centers with g = 1.995±0.005, |b20|
~ 0.0215±0.001 cm–1, and an optical band at 530 nm.
Since strong hyperfine interactions of chromium electrons with the nearest Li
nuclei were found by ENDOR, it was concluded that Cr in this center substitutes
for Nb. Several hydrogen lines were detected in ENDOR spectra. This sheds
light on one of the mechanisms of local charge compensation of the centers. The
second possible mechanism—charge compensation by additional Li ions in
structural vacancies Liv+—is also considered. The members
of the family of CrNb centers differ from each other by the location
of one or both of these compensating defects. An estimation of crystal-field
parameters for the observed low-symmetry exchange pairs in nonstoichiometric
crystals shows that they consist of CrLi+3+-CrLi+3+
centers 'glued' by the intrinsic defects, but not of self-compensated CLi+3+r-CrNb5+3+
centers.
M. Meyer, M. Wöhlecke, O. F. Schirmer
ON THE TEMPERATURE DEPENDENCE OF THE BAND EDGE OF Sr0.61Ba0.39Nb2O6
phys. stat. sol. (b) 221, R1 (2000)
Link to paper
S. Lenjer, R. Scharfschwerdt, Th. W. Kool, O. F. Schirmer
AN OFF-CENTER ION NEAR A BA SITE IN BaTiO3 AS STUDIED BY EPR UNDER UNIAXIAL STRESS
Solid State Commun. 116, 133 (2000)
Abstract
In BaTiO3 a paramagnetic defect is identified by EPR which is attributed
to a paramagnetic S =1/2 ion off-center near a Ba site. Twelve magnetically
distinguishable sites contribute to the angular dependence pattern of the spectra, each of
the sites being characterized by an orthorhombic g-tensor. The relative occupations of the
sites change under uniaxial stress in a manner consistent with the model. The features of
the center are highly characteristic for the indicated off-center position, as revealed by
comparison with the previously identified Ti3+ near a Sr2+ site in
SrTiO3. On the basis of the principal values of the g-tensor the paramagnetic
ion can be identified with Ni. It is discussed that Ni1+ is the most likely
charge state.
J. Wingbermühle, M. Meyer, O. F. Schirmer, R. Pankrath, R. K. Kremer
ELECTRON PARAMAGNETIC RESONANCE OF Ce3+ IN STRONTIUM-BARIUM NIOBATE
J. Phys.: Condens. Matter 12, 4277 (2000)
Abstract
The EPR of Ce3+ in congruent SrxBa1-xNb2O6
(x = 0.61) is characterized by the orthorhombic g-tensor {0.89, 3.55, 0.54} with principal
axes lying in the c-plane, tilted with respect to the a- and b-axes by 21.1°, and atong
the c-axis. Ce3+ occupies a single crystallographic site in the crystal,
leading to a unique EPR spectrum. The point symmetry of the Ce3+ position, C1,
is lower than that of the replaced alkaline-earth ions, C4 or CS.
This is attributed to an off-centre movement of Ce3+ . The Ce3+
signal decreases under illumination. No further EPR signal is detected to arise from the
photoionization of Ce3+.
EPR INVESTIGATIONS OF SMALL ELECTRON AND HOLE POLARONS IN OXIDE PEROVSKITES
in ‘Defects and surface-induced effects in advanced perovskites’, G. Borstel et al. (eds.), Kluwer 2000, p. 75 – 88
Abstract
Mobile charge carriers moving among equivalent sites in polar crystals tend to form
polarons. Their properties are determined by the interplay between symmetry breaking
interactions; such as coupling to the lattice and potential fluctuations, and those which
are symmetry restoring, such as tunneling between equivalent neighbors. Polarons are
classified by the alternatives 'small - large', 'electron - hole', 'single polaron -
bipolaron', 'flree - bound'. Information on the structural properties of small electron
polarons; based on EPR studies; in BaTiO3, WO3 and LiNbO3
is presented. On this firm basis some of the optical features of the polarons are
interpreted. In all three cases there are indications for the presence of bipolarons also.
An important polaron subclass are O- hole polarons bound to acceptor defects in
perovskite crystals. Their properties, especially the optical ones, are discussed in
comparison to those of free polarons.
RAMAN STUDY OF THE ORDERING IN Sr(B'0.5Nb0.5)O3 COMPOUNDS
Abstract
Strontium based complex perovskites are potential candidates for microwave integrated
circuit applications. In the present article, we report on Raman scattering studies of
cubic and non-cubic structures of Sr(B'0.5Nb0.5)O3
[B' = Ga, In, Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Yb] based complex-perovskite
materials for an improved understanding of structure-property relations. The spectral
results are compared to some tantalum analogues of known crystal structure. The present
study reveals a higher degree of ordering for the tantalum compounds compared to those of
the niobium analogues. Vibrational studies show a correlation between the tolerance factor
and symmetry of these materials.
T. R. Volk, V. Yu. Salobutin,
L. I. Ivleva, N. M. Polozkov, R. Pankrath, and M. Wöhlecke
Ferroelectric Properties of Strontium Barium Niobate Crystals Doped with Rare-Earth Metals
Physics of the Solid State 42, 2129-2136 (2000)
Abstract
The
influence of doping of the SrxBa1-xNb2O6
(x = 0.61) crystals (SBN-0.61) by rare-earth element (RE) dopants has been
investigated. Taking into account the data available in the literature, it is
demonstrated that the introduction of all rare-earth metals is accompanied by a
substantial decrease in the temperature of the phase transition Tp
and an increase in its smearing. The shift in the Tp
temperature for certain rare-earth metals ranges up to 20 K per atomic
percent of dopant in the crystal. It is experimentally established that, in the
SBN-0.61:Yb, SBN-0.61:Ce, SBN-0.61:Tm, SBN-0.61:La, and SBN-0.61:(Ce + La)
crystals, a decrease in Tp brings
about a considerable increase in the permittivity and the piezoelectric and
electrooptical coefficients. The conclusion is drawn that the doping by
rare-earth metals provides a means of optimizing the properties of strontium
barium niobates. The pulse switching of the SBN-0.61 and SBN-0.61:RE crystals is
studied for the first time. It is found that the switching is characterized by a
number of features, the most important of which is a decrease in the switched
charge after the application of external fields. This effect associated with the
specific features of the switching in the relaxor ferroelectric is assumed to be
responsible for the instability of the parameters for strontium barium niobate
material.
Publikationen 1999
Y. Lin, J.E. Eldridge, J. Sichelschmidt, S.W. Cheong, T. Wahlbrink
FRÖHLICH-INTERACTION INDUCED ONE-PHONON RAMAN SCATTERING IN La2CuO4 USING AN INFRARED LASER
Solid State Commun. 112, 315 (1999)
Abstract
The room-temperature Raman scattering from La2CuO4 is measured
using a Fourier-transform spectrometer with an infrared laser. Due to the low energy of
the laser, neither the two-magnon, nor the two-phonon scattering is observed, but a strong
resonant one-phonon scattering is obtained. The seven peak frequencies correspond to
longitudinal optic (LO) phonons with both Eu and A2u Symmetry. The
peaks correspond to phonons approximately halfway across the Brillouin zone, indicating a
violation of wavevector conservation.
O.F. Schirmer, A. Mazur, C. Veber, A. Rüdiger
PHOTOINDUCED CHARGE TRANSPORT IN BaTiO3 AND CONGRUENT Ba1-xCaxTiO3 DOPED WITH TRANSITION METAL AND ALKALI IONS
Trends in Optics and Photonics (TOPS) 27, 149 (1999)
Abstract
By investigating light-induced charge changes of defects in BaTiO3 (BT)
and Ba0.77Ca0.23TiO3 (BCT), using a combination of
EPR and fast optical absorption spectroscopy at low and room temperature, we identify the
charge transfer paths and their properties for the following systems: BT:Fe, BCT:Fe, BT:Cr
and BT:Rh. In the latter case it is proved that codoping with Na increases the infrared
response and the photorefractive speed of the material.
T. Volk, T. Woike, U. Dörfler, H. Schmitz, R. Pankrath, M. Wöhlecke
FERROELECTRICITY DRIVEN TWO-BEAM COUPLING GAIN IN STRONTIUM-BARIUM NIOBATE CRYSTALS
Trends in Optics and Photonics (TOPS) 27, 86 (1999)
Abstract
We report on contribution from ferroelectric phenomena to the two-beam coupling gain
factor G in Sr0.6Ba0.4.Nb2O6
crystals doped with rare-earth elements. The values of G
are sensitive to the ferroelectric phase transition and may be controlled by a variation
in Tc which is achieved, in particular by doping SBN with rare-earth
impurities.
M. Pape, H.J. Reyher, N. Hausfeld, S. Mendricks
PHOTOACTIVE Pb3+ HOST LATTICE IONS IN PHOTOREFRACTIVE Pb5Ge3O11 CHARACTERIZED BY ODMR AND EPR
Trends in Optics and Photonics (TOPS) 27, 42 (1999)
Abstract
In photorefractive Pb5Ge3O11 and (Pb0:98Ba0:02)5Ge3O11
crystals, Pb2+ host lattice ions can be recharged by visible light to Pb3+.
The magnetic and structural properties of this hole trapping center, metastable at low
temperatures, is characterized by EPR. This information is correlated with optical
properties by optically detected magnetic resonance (ODMR) via the magnetic circular
dichroism (MCD) of absorption. A pronounced photochromic effect can also be associated
with this charge trapping center. Several observations give evidence that this Pb3+
center is involved in the holographic recharging effects in lead germanate.
OPTICAL AND PHOTOREFRACTIVE PROPERTIES OF OPTICAL-DAMAGE RESISTANT LiNbO3:Zn CRYSTALS RELATED TO STRUCTURE PARAMETERS
Trends in Optics and Photonics (TOPS) 27, 144 (1999)
Abstract
In optical-damage resistant LiNbO3:Zn crystals a correlation is found
between dependencies of optical properties and the unit cell parameters on Zn
concentration. Non-monotonic shapes of these curves may be accounted for by different Zn
incorporation into the lattice depending on the Zn amount. Peculiarities observed at about
2-3 mol% Zn may be related to a removal of NbLi defects which is due to Zn
incorporation on Li-sites. At concentrations exceeding 6.5 mol% Zn a partial incorporation
of Zn ions on Nb-sites is detected. The wellknown extrema and anomalies in concentration
dependencies of various optical properties, including a drastic fall of photorefraction
observed formerly in the range from 6 to 7 %Zn may be accounted for in terms of these
structural data. Concentration dependencies of the electrooptical coefficients presented
here find a qualitative interpretation within the same scheme.
M. Wierschem, C. Bäumer, A. Jovanovic, M. Wöhlecke, S. Kapphan, L. Kovács
FUNDAMENTAL AND OVERTONE SPECTRA OF THE OH AND OD STRETCH MODE IN CONGRUENT AND IN STOICHIOMETRIC LiTaO3
phys. stat. sol. (b) 215, R7 (1999)
H.-J. Reyher, N. Hausfeld, M. Pape, J. Baur, J. Schneider
ATTRIBUTION OF THE NEAR-UV ABSORPTION BANDS OF YAG: Ce to Ce3+-IONS BY MCD AND ODMR
Solid State Commun. 110, 345 (1999)
AbstractG. Malovichko, V. Grachev, O. Schirmer
INTERRELATION OF INTRINSIC AND EXTRINSIC DEFECTS. CONGRUENT, STOICHIOMETRIC AND REGULARLY ORDERED LITHIUM NIOBATE
Appl. Phys. B 68, 785 (1999)
Abstract
A new approach to the intrinsic and extrinsic defect subsystems of complex oxides,
considering them as one integrated functional system, is developed. The strong
interrelation of these subsystems becomes especially apparent when concentrations of both
defect classes are comparable. A new parameter, reflecting the total intrin-sic defect
concentration, is introduced to determine the degree of crystal imperfection. The
necessity of at least two calibrated samples - congruent and regularly ordered crystals -
is substantiated. The accuracy, by which the parameter can be determined, can serve as a
criterion in comparison of different methods of crystal characterisa-tion. A procedure for
crystal calibration by several physical characteristics is illustrated. Such a heuristic
gener-alisation of the concept of mutual correlation of the whole defect system and the
material became only possible on the basis of experimental and theoretical study of large
numbers of LiNbO3 crystals. The spectra of EPR, NMR, ENDOR and other properties
of crystals with different compositions, diverse modifiers (K, Mg,...) and various probe
impurities have been analysed. It is found that crystals with vanishingly small
concentration of intrinsic defects offer extraordinary informative opportunities. Their
micro- and macroscopic properties are discussed. The developed ideas have a general
character; therefore they should also be valid for other non-stoichiometric complex
oxides.
R. Scharfschwerdt, O. F. Schirmer, H. Hesse, D. Rytz
FERMI LEVEL ENGINEERING OF BaTiO3 BY ALKALI CODOPING: INCREASING THE NEAR ABSORPTION BY RHODIUM
Appl. Phys. B 68, 807 (1999)
Abstract
Using electron paramagnetic resonance, optical absorption and fast spectroscopy of
light-induced absorption changes, it is shown that codoping BaTiO3:Rh with NaBa
acceptors raises the charge state of Rh3+ to Rh4+. Subsequent
oxidation under high oxygen pressures can lower the Fermi level to Rh4+/5+,
leading to increased infrared absorption. The light-induced charge transfer in such
specimens is characterised by "one center" behaviour.
INFLUENCE OF Zn DOPING ON ELECTROOPTICAL PROPERTIES AND STRUCTURE PARAMETERS OF LITHIUM NIOBATE CRYSTALS
Appl. Phys. B 68, 795 (1999)
Abstract
We report the dependence of the unit cell parameters and the EO coefficients on Zn
doping in optical-damage-resistant LiNbO3:Zn crystals. Both properties depend
in a non-monotonic manner on the Zn content. This is accounted for by different types of
Zn ion incorporation into the lattice depending on the Zn concentration in the melt.
Extrema observed in the concentration dependence of the EO coefficients at about 2-3 and
6.4 mol.% Zn correlate with an unusual concentration dependence of the unit cell
parameters a and c. The low concentration anomalies may be accounted for
by a decrease of the Li vacancy concentration due to the Zn incorporation into Li sites.
Anomalies at high concentrations are obviously due to a partial incorporation of Zn ions
on Nb sites, which is reflected in the structure data. Anomalies in the concentration
dependence of other optical properties at about 6 -7 mol.% Zn reported recently are
obviously related to a change in the localization of the Zn ions. The combination of high
EO coefficients with a reduced optical damage for these concentrations make these crystals
attractive for applications as Q-switching or electrooptical modulation.
A HOLOGRAPHIC METHOD FOR THE DETERMINATION OF ALL ELECTROOPTIC COEFFICIENTS APPLIED TO Ce-DOPED STRONTIUM-BARIUM-NIOBATE
Appl. Phys. B 68, 843 (1999)
Abstract
Using c- and a-cut plates of Strontium-Barium-Niobate doped with 0.66 mol.% cerium
all nonzero components of the linear electrooptic tensor r13, r42, r33
including the sign are determined from angular dependence ofthe two-beam coupling
coefficient G using transmission and reflection geometry of
holographic recording. We found: r13=56 pm/V, r42=38
pm/V and r33=333 pm/V. In addition we calculated the effective trap density Neff
to be 2.2*1017 cm-3. The advantages of this method as well as the
values obtained are discussed.
LIGHT-INDUCED CHARGE TRANSFER BETWEEN DEFECTS IN PHOTOREFRACTIVE MATERIALS
Radiation effects and defects in solids 149, 1 (1999)
Abstract
The basic features of the photorefractive effect in oxide materials are outlined and
the required knowledge on defects in such compounds is defined. The central question is:
Between what charge states of what defects does an inhomogeneous light pattern
redistribute charges in a photorefractive crystal? The joint investigation of
light-induced EPR and optical absorption changes is introduced as a very useful method to
answer this question, also for EPR silent defect valencies and also at room temperature.
Examples for the application of the method are given and its scope is discussed.
A. Mazur, C. Veber, O.F. Schirmer, C. Kuper, H. Hesse
Light-induced charge transport in photorefractive BaTiO3: Fe and Ba0,77 Ca0,23TiO3: Fe
Radiation effects and defects in solids 150, 281 (1999)
Abstract
Photorefractive barium titanate and barium-calcium titanate crystals with the
congruently melting composition (Ba0.23Ca0.77TiO3) doped
with iron are annealed in a hydrogen atmosphere at various temperatures. The samples are
studied with electron paramagnetic resonance (EPR) and optical absorption both performed
separately as well as simultaneously before and after illumination. The samples are
electron conductive and the charge transport at room temperature is governed by only one
level identified by photo-EPR as Fe2+/3+. The optical absorption of Fe2+
in BaTiO3:Fe is found to be at 2.1 eV and in (Ba0.23Ca0.77TiO3):Fe
at 1.9 eV.
A. Mazur, C. Veber, O.F. Schirmer, C. Kuper, H. Hesse
Light-induced charge transport processes in photorefractive Ba0,77Ca0,23TiO3 doped with iron
J. Appl. Phys. 85, 6751 (1999)
Abstract
Photorefractive barium-calcium titanate crystals with the congruently melting
composition (Ba0.77Ca0.23TiO3) doped with iron are
annealed in a hydrogen atmosphere at various temperatures. The samples are studied with
electron paramagnetic resonance (EPR) and optical absorption, both performed separately as
well as simultaneously before and after illumination. The reduced samples are electron
conductive and the charge transport at room temperature is governed by only one level
identified by photo-EPR as Fe2+/3+. The optical absorption of Fe2+
is found to be peaked at 1.9 eV.
S. Köhne, O.F. Schirmer, H. Hesse, T.W. Kool, V. Vikhnin
Ti3+ Jahn-Teller polarons and bipolarons in BaTiO3
J. Superconductivity 12, 193 (1999)
Abstract
In high quality BaTiO3 crystals without trapping defects conduction
electrons form Ti3+ polarons, their stabilisation energy being caused by a
Jahn-Teller effect. Depending on locally fluctuating potentials, tunable by uniaxial
stress, small, intermediate and banding electrons are identified by EPR. There are
indications that also bipolarons are present.
G.I. Malovichko, V. Grachev, E. Kokanyan, O. Schirmer
Axial and low symmetry centers of trivalent impurities in lithum niobate. Chromium in congruent and stoichiometric crystals
Phys. Rev. B 59, 9113 (1999)
Abstract
A systematic analysis and classification of clusters, consisting of impurity
(extrinsic) and intrinsic defects in lithium niobate crystals, was made in order to
understand the main features of the observed EPR spectra. It is shown, that possible
configurations for lithium substitution with charge compensators such as a lithium vacancy
(XLi-YLi), niobium vacancy
(XLi-YNb) and oxygen interstitial ions (XLi-Yi)
belong to C3 or C1 point group symmetries; this is also valid for
the complexes with niobium substitution and the NbLi antisite as compensator (XNb-YLi).
Clusters of an oxygen vacancy and an impurity on a niobium site (XNb-YO
configurations) have C1 symmetry only, but never C3 symmetry.
A detailed study of EPR spectra for a wide set of crystals with different chromium
concentrations and [Li]/[Nb] ratios was carried out. Besides the main axial Cr3+
center, eight satellite chromium centers were experimentally resolved and parameters of
their spin-Hamiltonians were determined by fitting angular dependencies of EPR lines. It
was found that in stoichiometric material less chromium is incorporated into the crystal
and that the satellite centers disappeared. A correlation of EPR, optical absorption and
luminescence spectra was observed and analyzed. The existence of the family of chromium
centers was explained on the basis of one common hypothesis about charge compensation by
intrinsic defects. In a minimal model, sufficient to explain all experimental data, it is
assumed that the satellite centers include two defects - CrLi and niobium
vacancy vNb in the first or further neighboring shells. Two vNb
compensate five CrLi. Since in conventional congruent crystals the relative
concentration of additional satellite centers is comparable with the concentration of the
main center, the conclusion was made, that both kinds of centers are equally responsible
for many of the LN properties.
Atomic Simulation of Electrooptic and Magnetooptic Oxide Materials
Springer Tracts in Modern Physics, Vol. 151, Springer (1999)
Abstract
This book reviews all the
state-of-the-art simulation methods used to investigate the atomic-scale properties of
technologically important oxide materials. Previous and many recent results are carefully
discussed.
Contents: Introduction. - Scope of Theoretical Methods. - Barium Titanate. - Potassium
Niobate and Potassium Tantalate. - Lithium Niobate. - Strontium Barium Niobate. - Yttrium
Iron Garnet.
Publikationen 1998
V. Grachev, G. Malovichko, O. Schirmer
Models of axial and low-symmetry impurity centers in lithium niobate crystals derived from EPR and ENDOR data
Radiation Effects and Defects in Solids (in print)
Abstract
Possible configurations of defect complexes in conventional lithium
niobate crystals, consisting of an impurity associated with intrinsic defects, are
analysed. Lithium or niobium vacancies can compensate a charge excess of di- or trivalent
impurities, if they substitute for a lithium ion or are incorporated in a structural
vacancy, whereas the antisite defect, niobium on a lithium site, can serve as charge
compensation only for impurity ions replacing niobium. From a comparison with experimental
data it is concluded that intrinsic defects in the first to ninth shells around the
impurity are responsible for clearly distinguishable satellite EPR lines (local charge
compensation). Small perturbations of the axial crystal field, caused by distant defects,
are revealed only in the observed line asymmetries and broadening.
G. Malovichko, V. Grachev, E. Kokanyan, O. Schirmer
Point imperfections and clusters of intrinsic and extrinsic defects in non-stoichiometric and stoichiometric Lithium Niobate: The Regularly ordered crystal
Radiation Effects and Defects in Solids (in print)
Abstract
The results of our investigations of different kinds of defects in
lithium niobate crystals are summarised in this report. Randomly distributed intrinsic
point imperfections are dominating in conventional congruent crystals. This leads to a
perturbation of the surroundings of optically or acoustically active impurities and to a
broadening of their spectral lines. The great narrowing of resonance lines in nearly
stoichiometric samples increases the spectral resolution sufficiently to allow the study
of even non-controlled trace impurities and satellite centers, consisting of the impurity
and intrinsic defects. The disappearance of satellite centers and the appearance of new
centers due to the change of charge compensation mechanism was discovered in
stoichiometric crystals. The quantitative characterisation of the degree of crystal
perfection, a definition of regularly ordered crystal and the necessity of the
re-investigation of physical properties for perfect samples are discussed.
OPTICAL DAMAGE RESISTANCE IN LITHIUM NIOBATE CRYSTALS
Ferroeelectrics Review 1, 195-262 (1998)
Abstract
This article reviews the current knowledge of the most important properties and
applications of damage resistant LiNbO3 crystals. Damage resistance (reduction
of optical damage or photorefraction) occurs due to doping with a few mol.% of Mg, Zn, In,
and Sc. As a result the photorefraction is reduced by two orders of magnitude. The damage
resistant impurities do not affect the transparency in the spectral range of interest.
These features allow the use of these crystals in optical setups with high stability at
strong laser intensities. The influence of damage resistant impurities on photorefraction,
the refractive index and nonlinear optical effects is discussed and the similar behavior
of all impurities explained. Photorefraction is reduced because of an enhanced
photoconductivity, which is interpreted in terms of the current model of photo-induced
charge transport. The refractive index of damage resistant LiNbO3 crystals of
different compositions is described by a generalized Sellmeier equation suitable to
calculate phase-matching conditions. Many experimental data are presented, which suggest a
common structure origin of the damage resistance. The impurity-induced removal of the
intrinsic Nb antisite defect up to a certain threshold concentration of the damage
resistant impurities is responsible for most features. The last section reviews
applications profiting from the reduced photorefraction combined with unchanged
transparency, like phase-matching configurations including quasi-phase-matching structures
and laser action.
Influence of Zn/In-codoping on the optical properties of Lithium Niobate
J. Appl. Phys. 84, 5191 (1998)
Abstract
The influence of a combined doping with Zn and In on the optical properties of
lithium niobate is studied. While each of these dopants is known to improve the material's
nonlinear optical properties, it is an open question whether further improvements
especially of the nonlinear optical features can be achieved by a suitable combination of
dopants. Our measurements yet show that lithium niobate imposes certain limits which can
not be overcome. This can be described by a simple model which is based on the defect
structure of lithium niobate.
H. Kröse, R. Scharfschwerdt, A. Mazur, O. F.Schirmer
A multichannel spectrometer for correlated EPR-optical absorption analysis of photochromic processes in crystals;
Appl. Phys. B-Lasers & Optics. 67, 79-86 (1998)
Abstract
A fast optical fiber multichannel spectrometer that has been developed for
simultaneous measurements of EPR and optical absorption of defects in crystals is
described. The setup is designed for establishing correlations between both phenomena by
observing their changes under varying pump light energies. In this way optical bands are
assigned to the defects causing
them. After this calibration step, the pump-light-induced variations of the band
intensities alone serve to unravel the responsible charge transfer processes. The speed of
the spectrometer 14 ms for one spectrum in the range 400 nm to 1100 nm - is the basis for
such experiments. A special scheme for evaluation of the spectra allows a fast overview of
the phenomena occuring on the atomic level. Examples based on two types of photochromic
BaTiO3 crystals are given. The extension of this type of spectroscopy to other
fields, where correlation between EPR and optical phenomena is essential, is discussed.
E. Ruza, H. J. Reyher, J. Trokss, M. Wöhlecke
Optically detected magnetic resonance via the blue luminescence of Ti-doped Al2O3
J. Phys. Condens. Matter 10, 4297 (1998)
Abstract
The UV-excited blue and green luminescence bands of Ti-sapphire are characterized by
ODMR. These emission bands are attributed to two Ti3+-O--centers,
which show very similar properties and are created as a result of a charge-transfer
transition of an electron from O-- to Ti4+-ions. In both centers,
the d-electron of Ti3+ and the hole of O- are strongly coupled and
form triplet states. Doublet systems can be ruled out as source of the blue-green
luminescence. The angular dependence of the ODMR can be explained with an appropriate spin
Hamiltonian assuming orthorhombic local symmetry. The orientation of the principal axes of
the g-tensor and the crystal field tensor, found for both centers, suggest that the hole
is localized on a single oxygen ion and that the Ti3+-O- bond is
almost aligned with the Al-O bonds of the undisturbed lattice.
T. Woike, T. Volk, U. Dörfler, R. Pankrath, L. Ivleva, M. Wöhlecke
Ferroelectric and optical hysteresis in SBN doped with rare-earth elements
Ferroelectric Letters 23, 127 (1998)
Abstract
In Sr0.61Ba0.39Nb2O6 crystals doped
with Ce and Tm the dependence of the two-beam coupling gain factor G
on an external dc- field E applied after recording shows a hysteresis, which reflects the
ferroelectric P-E hysteresis. At coercive fields Ec the sign of G abruptly reverses, i.e. the energy transfer direction between the
coupled beams. The field-controlled state of G is
stable and stored after turning off the fields. The results are discussed in terms of the
concept of electrooptical effect and ferroelectricity in oxygen-octahedra ferroelectrics.
K. Kasemir, K. Betzler, B. Matzas, B. Tiegel, M. Wöhlecke, N. Rubinina, T. Volk
Influence of In Doping on the Refractive Indices of Lithium Niobate
physica status solidi (a) 166/1, R7 (1998)
Publikationen 1997
B.I. Kochelaev, J. Sichelschmidt, B. Elschner, W. Lemor, A. Loidl
Three-Spin Polaron in La2-xSrxCuO4: Experimental evidence by EPR measurements, Phys. Rev. Lett. 79, 4274 (1997)
Abstract
Electron-paramagnetic resonance (EPR) measurements on La2-xSrxCuO4
provide experimental evidence of a three-spin polaron, consisting of two Cu2+
ions and one p hole. The symmetry properties and the peculiar temperature dependence of
the g values of the EPR line indicate the presence of dynamical Jahn-Teller distortions
and formation of a collective mode of polarons and surrounding strongly correlated Cu ions
(bottlenecked regime).
Computer Modeling of Trapped Holes in Barium Titanate
Materials Science Forum 239-241, 341 (1997)
Abstract
Based on shell-model and embedded-cluster simulations a characterization of holes
trapped at acceptor defects in barium titanate is given. The ab initio level of
the calculations corrresponds to Hartree-Fock theory, Möller-Plesset perturbation theory
and density functional theory.
H.-J. Reyher, B. Faust, B. Sugg, R. Rupp, L. Ackermann
Optically detected magnetic resonance via the magnetic circular dichroism of absorption of cerium impurities in bulk paramagnetic terbium gallium garnet
J. Phys.: Condens. Matter 9, 9065 (1997)
Abstract
The paramagnetic resonance of dilute Ce3+ ions in the bulk paramagnetic
host Tb3Ga5O12 is separated from the host response by
means of optically detected magnetic resonance (ODMR) using the magnetic circular
dichroism (MCD) of the absorption. This result shows that it is possible in principle to
extend by means of the ODMR of the absorption the regime of paramagnetic
resonance detection of impurity ions to magnetic hosts, which strongly interact with the
microwave field and, hence, make the impurity signals unobservable by means of
conventional EPR. The observed ODMR signals are attributed to Tb3+ and Ce3+
ions by correlation with the optical bands of these species. By means of ODMR via the
Faraday effect it is shown that this effect is due to transitions between Tb levels. The
identification of the observed magnetic resonance structures is confirmed by calculations
based on known information on Tb3+ and Ce3+ in diamagnetic hosts.
The role of molecular fields is discussed.
Light-Induced Charge Transport Processes in Photorefractive Barium Titanate Crystals Doped with Iron
Appl. Phys. B 65, 481 (1997)
Abstract
Iron-doped photorefractive barium titanate crystals
(BaTiO3) are annealed in a hydrogen atmosphere at various temperatures. After
these reducing treatments, absorption, light-induced absorption changes, two-beam coupling
direction, photo electron paramagnetic resonance (photo EPR), dark and
photoconductivity as well as bulk photovoltaic current density are investigated. The
samples are electron conductive and the charge transport is governed by only one level
which is identified by photo EPR as Fe2+/Fe3+. The photoconductivity
exceeds the dark conductivity for intensities above
1 kWm-2. A relation between the absorption constant and Fe2+
concentration is derived. From the known charge transport parameters the advantageous
photorefractive properties of optimized reduced BaTiO3:Fe are deduced; possible
response times in the millisecond range at an intensity of 10 kWm-2 are
estimated.
T. Volk, T. Woike, U. Dörfler, R. Pankrath, L. Ivleva, M. Wöhlecke
FERROELECTRIC PHENOMENA IN HOLOGRAPHIC PROPERTIES OF STRONTIUM-BARIUM NIOBATE CRYSTALS DOPED WITH RARE-EARTH ELEMENTS
Ferroelectrics 203, 457 (1997)
Abstract
We report on a relation between the two-beam coupling gain factor G
in Sr0.61Ba0.39Nb2O6 (SBN:0.6) crystals
and their ferroelectric properties. In SBN:0.6 doped with rare-earth-metal impurities Ce
and Tm we found a hysteresis dependence of G versus the
external dc-field E applied after recording, which is due to the ferroelectric P-E
hysteresis. Particularly, G-E hysteresis manyfests itself in an
alteration of the sign of G, i.e., of the direction of the
energy transfer between the coupling beams, under applying of E reversing Ps.
The value of G at room temperature essentially increases
with impurity concentrations. It is caused by an increase of the dielectric permittivity,
which in its turn is due to a significant lowering of the phase-transition temperature Tc
produced by a rare-earth doping.
T. Woike, T. Volk, U. Dörfler, R. Pankrath, L. Ivleva, M. Wöhlecke
FERROELECTRIC AND OPTICAL HYSTERESIS IN SBN DOPED WITH RARE-EARTH ELEMENTS
Ferroelectrics Letters, 23 No 5/6 im Druck
Abstract
In Sr0.61Ba0.39Nb2O6 crystals doped with
Ce or Tm the dependence of the two-beam coupling gain factor G
on an external c-field E applied after recording shows a hysteresis, which reflects the
ferroelectric P-E hysteresis. At coercive fields Ec the sign of G abruptly reverses, i.e. the energy transfer direction between the
coupled beams. The field-controlled state of G is stable
and stored after turning off the fields. The results are discussed in terms of the
electrooptical effect and ferroelectricity in oxygen-octahedra ferroelectrics.
A. Mazur, O.F. Schirmer, S. Mendricks
OPTICAL ABSORPTION AND LIGHT-INDUCED CHARGE TRANSPORT OF Fe2+ IN BATIO3
Applied Physics Letters. 70(18):2395-2397, 1997 May 5.
Abstract
Iran doped barium titanate was grown and reduced at various temperatures. Using
optical absorption and electron paramagnetic resonance (photo-EPR) simultaneously, a wide
absorption band at similar to 2.1 eV due to Fe2+ is established. The light-induced charge
transport between Fe2+/3+ and shallow Ti4+3+-V-0 is identified, showing a two-center type
response at low and one-center type response at room temperature.
H. Donnerberg, S. Többen, A. Birkholz
FORMATION OF LOCALIZED HOLE STATES IN COMPLEX OXIDES .1. HOLE STATES IN BATIO3
Journal of Physics-Condensed Matter. 9(30):6359-6370, 1997 Jul 28.
Abstract
Defect electrons (holes) play an important role in most technologically important
complex oxides. In this contribution we present the first detailed
characterization of localized hole states in such materials. Our investigations
employ advanced embedded-cluster calculations which consistently include
electron correlations and defect-induced lattice relaxations. This is necessary
in order to account for the variety of possible hole-state manifestations. Even
in highly ionic oxides such as MgO, there exists a delicate interplay between
electron correlations and defect-induced lattice deformations.
L. Kovács, G. Ruschhaupt, K. Polgár, G. Corradi, M. Wöhlecke
COMPOSITION DEPENDENCE OF THE ULTRAVIOLET ABSORPTION EDGE IN LITHIUM NIOBATE
Applied Physics Letters.70(21):2801-2803, 1997 May 26.
Abstract
A convenient and fast method to determine the composition of LiNbO3 single
crystals consisting of a measurement of the position of the fundamental absorption edge in
the near UV region is refined and extended to the stoichiometric range. Using
unsophisticated apparatus and simple reflection correction, the proposed two-parameter
calibration equation provides an absolute accuracy of 0.1 mol % with the relative accuracy
reaching an unusual value of better than 0.01 mol % for near stoichiometric compositions.
R. Scharfschwerdt, A. Mazur, O.F. Schirmer, H. Hesse, S. Mendricks
OXYGEN VACANCIES IN BATIO3
Physical Review B-Condensed Matter. 54(21):15284-15290, 1996 Dec 1.
Abstract
Using electron paramagnetic resonance two types of Ti3+-related centers are
identified in BaTiO3, which are attributed to Ti3+-V-O and to Ti3+-V-O associated with Na
or K, respectively. This assignment is based on the chemical and illumination conditions
leading to the detection of the defects. It is furthermore supported by the Ti hyperfine
structure, resolved for Ti3+-V-O-Na(K). The symmetry of the centers is broken in the sense
that an electron is localized near one of the two equivalent Ti4+ ions next to V-O. Also
the observed Ti hyperfine structure points to
electron capture at one Ti site. The V-O-type centers are contrasted to isolated Ti3+,
representing a free small polaron in crystals containing
Nb5+ and no oxygen vacancies. The participation of the V-O centers in photochromic
processes in BaTiO3 is studied. In crystals doped with Na or K an optical absorption
precursor of the fundamental absorption is identified and attributed to the creation of
ail exciton bound to the V-O-Na(K) complex.
G. Wübbeler, O.F. Schirmer, S. Köhne
PHASE SEPARATION IN LA2CUO4+d AS PROBED BY A PARAMAGNETIC SURFACE DEFECT
Physical Review B-Condensed Matter. 54(13):9054-9057, 1996 Oct 1.
Abstract
The phase separation into metallic and insulating regions, occurring in La2CuO4+delta
at low doping concentrations, is investigated with electron-paramagnetic resonance. A
paramagnetic center probing the phase-separation process is found in ceramic or powdered
specimens. The intensity of this resonance is shown to be directly related to the surface
area of the specimen. This leads to the conclusion that a paramagnetic Cu2+ center sited
at surfaces or at grain boundaries is present. An additional phase separation is proposed
taking place in material regions near these surfaces.
W. Lemor, G. Wübbeler, O.F. Schirmer
PHASE SEPARATION IN LA2CUO4+d BY ELECTRICAL TRANSPORT MEASUREMENTS
Journal of Superconductivity. 9(4):365-367, 1996 Aug.
Abstract
The dc conductivity and the high-frequency capacitance of a La2CuO4+delta single
crystals are determined by impedance spectroscopy in the frequency range 20 Hz to 1 MHz at
liquid helium temperature. Using isochronal annealing, the effect of the phase separation
on these quantitites is studied for E parallel and perpendicular to the CuO2 planes as
well as for different oxygen doping levels, delta, of the sample. The results are
consistent with the model of a diffusion-controlled formation of a metallic-and below T-c
superconducting-percolation network in an insulating background.
V.G. Grachev, G.I. Malovichko, O.F. Schirmer
Single, dimer and trimer chromium centers in LiNbO3
Ferroelectrics 185, 639 (1996)
Abstract
Chromium doped lithium niobate crystals of different stoichiometric composition were
investigated by means of electron paramagnetic resonance. It was found that
characteristics of observed spectra have the strong dependence on chromium concentration
and crystal stoichiometry. The parameters of exchange interaction of dimer chromium center
were determined.
R. Scharfschwerdt, O.F. Schirmer, H. Kröse, T.W. Kool
Oxygen vacancy related defects in BaTiO3
Ferroelectrics 185, 643 (1996)
Abstract
Depending on the acceptor content of BaTiO3 crystals, two types of
Ti3+ ESR signals are observed. Acceptor-poor samples, which thus also
have low concentrations of compensating oxygen vacancies, VO, contain isolated
Ti3+ free small polarons. Acceptor-rich crystals show signals to be attributed
to Ti3+ next to VO-alkali-acceptor complexes. This model is
supported by a comparison of the optical absorptions and light induced charge transfer
transitions of both types of specimens.
M. Wöhlecke, G. Corradi, K. Betzler
OPTICAL METHODS TO CHARACTERISE THE COMPOSITION AND HOMOGENEITY OF LITHIUM NIOBATE SINGLE CRYSTALS
Applied Physics B-Lasers & Optics.63(4):323-330, 1996 Oct.
Abstract
A number of the physical properties of lithium niobate strongly depend on sample
composition. Although several procedures for the determination of the composition have
already been published, a critical survey and an introduction to standard methods
recommendable also for laboratories active in R&D of LiNbO3 devices is
still missing. Within a detailed description of a series of methods, we summarise their
capabilities and accuracy. The proposed optical characterisation methods, in particular
those based on the generation of second harmonic light and those involving the measurement
of birefringence and the UV absorption edge are found to be most convenient for an
accurate and fast standard characterisation of LiNbO3 single crystals. An
absolute accuracy of 0.1 mol% based on a comparison with the Curie temperature calibration
method and a relative accuracy of up to 0.01 mol% are available. Some of these methods are
also suited for the two or three dimensional homogeneity control of LiNbO3
single crystals.
H.J. Reyher, B. Faust, M. Maiwald, H. Hesse
ODMR AND EPR INVESTIGATIONS OF FE CENTERS IN KTAO3
Applied Physics B-Lasers & Optics.63(4):331-337, 1996 Oct.
Abstract
The analysis of EPR spectra obtained from iron doped KTaO3 crystals in the
as-grown state revealed three dominant iron centers: Fe3+-O-I,
axial Fe-centers with spin S = 3/2 and rhombic Fe3+. By comparison with data
from literature possible assignments for the center with S = 3/2 are discussed. For the
rhombic species the temperature dependence of the main parameters of the Spin- Hamiltonian
was measured. The result makes it most plausible that only one rhombic iron center exists
in KTaO3, in contrast with literature. The understanding of the EPR spectra
allows us to assign transitions, observed at very low magnetic fields by optically
detected magnetic resonance (ODMR), to this rhombic Fe center. On this basis, the magnetic
circular dichroism (MCD) of this defect could be identified using the method of
tagged-MCD. This spectrum is compared to the tagged-MCD of Fe3+-O-I
and of axial Fe4+ centers, which may be generated metastably by optical charge
transfer. Considerably different structures in the MCD spectra of both Fe3+
centers indicate different local surroundings and electronic states.
M. Wöhlecke, T. Volk, H. Donnerberg
ON THE ROLE OF INTRINSIC CLUSTERS IN DAMAGE-RESISTANT LINBO3
Ferroelectrics Letters Section. 22(3-4):53-58, 1997.
Abstract
The intrinsic stoichiometry-related defect structure of LiNbO3 consists of
Nb antisites and of charge compensating cation vacancies. The electrooptic properties
of the material are mainly determined by the concentration of Nb antisites
acting as important electron traps. Recent atomistic computer simulations
suggested the agglomeration of Nb antisites and cation vacancies probably
resulting in spacious intrinsic defect clusters. Since the incorporation of
divalent impurity cations consumes stoichiometry-related defects, possible
clustering effects should be observable as peculiarities in the antisite
concentration dependence of physical properties. It is the purpose of this
letter to relate corresponding non-linearities to the existing intrinsic defect
clusters. The non-linearities, which are observed well below the threshold
concentrations of divalent impurity cations, cannot be understood on the basis
of isolated stoichiometry-related defects.
Reyher HJ. Faust B. Kading M. Hesse H. Ruza E. Wöhlecke M.
OPTICAL ALIGNMENT OF AXIAL FE CENTERS IN KTAO3 (VOL 51, PG 6707, 1995)
Physical Review B-Condensed Matter. 54(5):3660, 1996 Aug 1.
COMMENTS ON THE DEFECT CHEMISTRY OF MAGNESIUM-DOPED LITHIUM NIOBATE (LINBO3)
Journal of Solid State Chemistry. 123(2):208-214, 1996 May.
Abstract
Congruently grown LiNbO3 is known to be highly defective due to its significant
Li2O deficiency. At present two stoichiometry-related defect-chemical
models are discussed which can be distinguished with respect to the occurrence
of Li or Nb vacancies, The Nb-vacancy model takes advantage of
ilmenite-structured LiNbO3 and predicts a niobium antisite
concentration being at least five times as large as in the Li-vacancy model, In
the present contribution it is argued that the magnesium-doping mechanisms are
essentially independent of these particular differences. It is argued that
extended stoichiometry-related defect clusters represent a key for a proper
understanding of the defect chemistry of impurity cations, The discussion is
mainly based on our recent shell-model simulations of perfect and defective
LiNbO3.
Schlarb U. Reichert A. Betzler K. Wöhlecke M. Gather B. Volk T. Rubinina N.
SHG PHASE MATCHING CONDITIONS FOR UNDOPED AND DOPED LITHIUM NIOBATE
Radiation Effects & Defects in Solids. 136(1-4):1029-1032, 1995.
Abstract
We measured the refractive indices of undoped and Zn- or Mg-doped lithium niobate
in the wavelength range 400 to 1200 nm. The results are described by a
generalized Sellmeier equation which takes into account the defect structure of
the material. From the generalized Sellmeier equation a simple relation between
the phase matching temperature for noncritical type I second harmonic
generation (SHG) and the internal critical phase matching angle is derived.
Good correspondence with experimental data over a wide composition and
fundamental wavelength range is observed.
H.-J. Reyher, J. Ruschke, F. Mersch
PHOTOINDUCED LINEAR DICHROISM IN SILLENITE CRYSTALS AND IN DIAMOND
Radiation Effects & Defects in Solids. 136(1-4):1039-1042, 1995.
Abstract
By illumination with linearly polarized light, a strong optical linear dichroism
(LD) can be created in undoped sillenites of the type Bi12TiO20(BTO).
This photoinduced LD is stable in the dark at T < 77 K and is reversible by
changing the direction of the polarization appropriately. The spectra of the LD
and of the magnetic circular dichroism (MCD) of the Bi-Ti(4+) antisite reveal
some similarities; however, there is no direct evidence for attributing the LD
to Bi-Ti(4+). The observation of the LD as a function of the direction of the
polarization is shown to give information on the symmetry of the center causing
the LD. Comparative studies of nitrogen containing diamond support this fact.
B. Faust, H.J. Reyher, O.F. Schirmer
OPTICALLY DETECTED MAGNETIC RESONANCE OF FE4+-O-I IN KTAO3
Solid State Communications. 98(5):445-447, 1996 May.
Abstract
The electron spin resonance of Fe4+-O-I in KTaO3
has been observed optically (ODMR) via the magnetic circular dichroism (MCD) of
absorption. The charge state Fe3+-O-I was
present simultaneously. Fe4+-O-I was created metastably
at low temperatures by light used to detect the MCD.
U. Schlarb, B. Matzas, A. Reichert, K. Betzler, M. Wöhlecke, B.
Gather, T. Volk, N. Rubinina
Refractive indices of Zn/In-Co-doped lithium niobate
Ferroelectrics 185, 269 (1996)
Abstract
We measured the refractive indices of Zn/In-co-doped lithium niobate in the
wavelength rage of 400 to 1200 nm, using samples with 5.5 mol% ZnO and various additional
concentrations of In2O3. The results can be well described by a
generalized Sellmeier equation which takes into account the defect structure of the
material. The proposed Sellmeier equation allows the treatment of nonliniear optical
effects; calculated phase-mathing conditions for second-harmonic generation are in good
agreement with experimental results.
T. Volk, M. Wöhlecke, N. Rubinina, A. Reichert, N. Razumovski
Optical-damage-resistant impurities (Mg, Zn, In, Sc) in lithium niobate
Ferroelectrics 183, 291 (1996)
Abstract
The paper reviews the results of recent studies of optical-damage-resistant LiNbO3:Me
crystals, with Me substituting for Mg, Zn, In, Sc. The effects of Me ions on LiNbO3
are very similar. This similarity manifests itself drastically in photorefractive, optical
and nonlinear optical properties. The resemblance of these effects is due to a common
influence of Me impurities on the intrinsic lattice defects NbLi.
EMBEDDED-CLUSTER CALCULATIONS FOR TRANSITION-METAL IMPURITIES IN BaTiO3
Journal of Physics-Condensed Matter. 8(11):1687-1704, 1996 Mar 11.
Abstract
The embedded-cluster technique is used to simulate the local electronic structure
of transition-metal impurities in BaTiO3. The description of the central
defect cluster employs an ab initio SCF-MO approach. The quantum cluster
consists of 21 ions. Outer crystal regions are modelled on the basis of a
shell-model representation. In all cases defect-induced lattice relaxations
have been consistently included. Our results, demonstrated for Mn-Ti(4+),
concern optical transitions, Jahn-Teller effects and questions related to the
stability of this defect. The computational level of our ab initio calculations
corresponds to Hartree-Fock theory (HF) and the configuration interaction (CI).
Additionally, Moller-Plesset perturbation theory and density functional theory
have been applied to investigate charge-transfer transitions.
T. Varnhorst, O.F. Schirmer, H. Kröse, R. Scharfschwerdt, T.W. Kool
O- HOLES ASSOCIATED WITH ALKALI ACCEPTORS IN BATIO3
Physical Review B-Condensed Matter. 53(1):116-125, 1996 Jan 1.
Abstract
In BaTiO3 the alkali ions Na and K, replacing Ba, lead to acceptor levels
about 50 meV above the valence band edge. The structure of these defects can
be studied by electron spin resonance, after light induced valence-band holes
have been captured at neighboring O2- ions. Hyperfine interaction
with Na, Ba, and Sr impurities, adjacent to the paramagnetic O-,
is-identified. The holes are trapped stably below similar to 50 K. Their level
position is determined by the temperature dependence of the relaxation rate of
the light-induced nonequilibrium excess hole population. Photostimulated small
polaron transfer between O2- sites equivalent with respect to Na or
K leads to a strong optical-absorption band with peak at 1.3 eV, corresponding
to a stabilization energy of the bound hole polaron of about 0.65 eV. The Na+-O-
dipoles can be reoriented under uniaxial stress at 4.2 K; a differential stress
coupling coefficient of 1.23*10-4 m3 is derived. Several
O- centers associated with partly unassigned accepters on Ti sites
are identified. Their properties are compared to those of the Ba site
accepters. Vibronic g shifts are accounted for by a dynamic Jahn-Teller effect
involving the twofold degenerate O- P
-orbitals.
Publikationen 1995
LiNbO3 with the damage-resistant impurity indium
Appl. Phys. A 60, 217 (1995)
Abstract
We report on a detailed investigation (EPR, SHG, optical absorption, luminescence
and Raman scattering) on the new damage-resistant impurity indium in LiNbO3,
where the increased photoconductivity strongly reduces the photorefractive
effect. EPR and optical absorption measurements point to a complete
disappearance of the Nb antisite in LiNbO3:In for all In concentrations.
We believe that the very effective driving out of Nb antisites by In is due to
the trivalent charge state of In and the possibility of charge
self-compensation. Similarities in the properties of Mg-, Zn- or In-doped
samples are discussed. Simultaneous doping with In and Zn leads to an addition
of both contributions, in particular for optical frequency doubling and
luminescence. Raman studies prove that In does not improve the stoichiometry of
the crystals. Indium doping provides the possibility to control simultaneously
phase-matching conditions and to reduce drastically photorefraction. Therefore,
In co-doped LiNbO3 compositions are promising materials for applications after
solving contemporary growth problems.
T. Volk, M. Wöhlecke, A. Reichert, F. Jermann, N. Rubinina
The peculiar impurity concentration ranges in damage resistant LiNbO3 crystals doped with Mg, Zn, In and Sc
Ferroelectrics Lett. 20, 97 (1995)
Abstract
We report on a comparative study of the optical damage-resistant impurities Mg and
Zn, as well as In and Sc in LiNbO3. In LiNbO3 doped with these ions
a certain behavior of physical properties (e.g., the shape of OH stretch mode bands,
phase-matching temperatures of optical frequency doubling, etc.) on the impurity
concentration is observed. The corresponding ('threshold') concentrations are about 5-7
mol.% for the divalent cations Mg and Zn and only 1 mol.% for the trivalent In and Sc. The
results are discussed assuming that these ions incorporate on Li-sites thus removing the
intrinsic defect NbLi from the lattice.
F. Jermann, M. Simon, R. Böwer, E. Krätzig, O. F. Schirmer
Light-induced absorption changes in reduced lithium niobate
Ferroelectrics 165, 319 (1995)
Abstract
Reduced LiNbO3, crystals change their optical absorption under
illumination with blue or green pump light. We measure the dependence of this effect on
pump intensity I at different probe wavelengths. Steady-state absorption changes and
build-up velocities are found to be proportional to the square root of I. Rates of
relaxation towards equilibrium in the dark increase exponentially with temperature. Our
experimental results can be well described by light-induced dissociation of bipolarons
into small polarons unstable at room temperature.
H. Kröse, E. Possenriede, R. Scharfschwerdt, T. Varnhorst, O. F. Schirmer, H. Hesse, C. Kuper
Optically induced charge transfer paths between defects in BaTiO3 containing rhodium
Optical Materials 4, 153 (1995)
Abstract
An optical method is presented which allows to identify at room temperature the
photoinduced charge transfers between different defects in photorefractive crystals. The
analysis is based on the labelling of the optical spectra with the corresponding
microscopic defect models, obtained from simultaneously recording light induced ESR and
optical absorption changes at low temperatures. The investigation of as-grown BaTiO3
crystals containing Rh, using this method, shows that illumination with light of energies
between 1.2 eV and 2.5 eV transfers holes from Rh4+ to Fe3+. At E
> 2.5 eV holes are photoioized from Fe4+ to Rh3+ and to an
unidentified shallower hole trap.
Shell Model and Embedded Cluster Calculations of Hole Bipolarons in BaTiO3
Radiation Effects and Defects in Solids, 134, 165 (1995)
Abstract
Classical shell-model and ab initio embedded-cluster calculations are
employed in order to provide theoretical support in favour of existing hole bipolarons in
BaTiO3. In particular the embedded-cluster modelling studies, which explicitly
include the local electronic defect structure, emphasize the important role of lattice
relaxations and electron-correlation contributions in stabilizing diamagnetic O2-2
peroxy molecules (bipolarons) in the material. Our simulations show that hole bipolarons
are predominantly bound at Ti-site acceptor defects.
Donnerberg H.
REFINED EMBEDDED-CLUSTER CALCULATIONS FOR TRAPPED HOLE BIPOLARONS IN BaTiO3
Journal of Physics-Condensed Matter. 7(49):L 689-L 694, 1995 Dec 4.
Abstract
Embedded-quantum-cluster calculations are used to investigate the formation
of hole-type bipolarons in BaTiO3. These simulations improve on our
recent results because of the exact performance of geometry optimizations of
the embedded quantum cluster. Moreover, ion relaxations and electron
correlations have been treated consistently. The modelling of correlations
includes Moller-Plesset perturbation theory to second order (MP2) and density
functional theory (DFT) beyond the local density approximation. Both
defect-induced crystal relaxations and electron correlation contributions are
necessary in order to stabilize trapped hole bipolarons in BaTiO3.
B. Briat, H.-J. Reyher, A. Hamri, N.G. Romanov, J.C. Launay, F. Ramaz
MAGNETIC CIRCULAR DICHROISM AND THE OPTICAL DETECTION OF MAGNETIC RESONANCE FOR THE Bi ANTISITE DEFECT IN Bi12GeO20
Journal of Physics-Condensed Matter. 7(34):6951-6959, 1995 Aug 21.
Abstract
We report on the optical detection of electron paramagnetic resonance at 36
GHz, 67 GHz and 70 GHz for all the absorption bands of Bi12GeO20
(BGO) in the visible spectral range. This completes our previous studies and
demonstrates dearly that the three major magnetic circular dichroism features
do belong to the same isotropic centre, i.e., a Bi-Ge3+ + h defect
with the hole being delocalized onto the four surrounding oxygen atoms. We
provide a simple explanation for the unusual sign of ODMR and the transient
behaviour of the magnetic circular dichroism at resonance.
M. Exner, H. Donnerberg, C.R.A. Catlow, O.F. Schirmer
COMPUTER SIMULATION OF DEFECTS IN KTaO3
Physical Review B-Condensed Matter. 52(6):3930-3940, 1995 Aug 1.
Abstract
Using atomistic computer simulation methods we propose a general overview of
the point defect structure of KTaO3. A set of short-range potential parameters
was derived using an empirical fitting procedure. These parameters were then
applied within the framework of the shell model. We employed a two-region
strategy in combination with the Mott-Littleton technique for defect
calculations. Intrinsic defects including Frenkel and Schottky-type disorder,
polarons, and reduction/oxidation mechanisms are treated in the study as well
as a great number of extrinsic defects. The way these ions are introduced into
the crystal, their agglomeration into defect complexes and possible off-center
displacements are investigated. It is found that in KTaO3, intrinsic
disorder is of minor significance and small electron polarons possibly are
stable. The calculations support the idea that reduction is significantly
influenced by preexisting oxygen vacancies, which result from charge
compensation of accidental acceptor impurities in the material. Extrinsic
monovalent ions substitute on the potassium site for which no charge
compensation is necessary. Ions of other valencies are incorporated on both
cation lattice sites which then gives rise to a charge self-compensation, in
some cases combined with the formation of oxygen vacancies.
O. F. Schirmer, H. J. Reyher, M. Wöhlecke
Characterisation of point defects in photorefractive oxide crystals by paramagnetic resonance methods
Abstract
not available
New developments in the simulation of electrooptic oxides
Abstract
not available
W. Lemor, G. Wübbeler, O.F. Schirmer
PHASE SEPARATION IN LA2CUO4+d AS PROBED BY IMPEDANCE SPECTROSCOPY
Solid State Communications. 95(12):823-827, 1995 Sep.
Abstract
The dc-conductivity and the high-frequency capacity of a La2CuO4+d single crystal are determined by impedance spectroscopy in
the frequency range 20 Hz - 1 MHz at liquid helium temperature. Using
isochronal annealing, the effect of the phase separation on these quantities is
studied for E parallel and perpendicular to the CuO2-planes as well as
for different oxygen doping levels, delta, of the sample. The results are consistent
with the model of a diffusion controlled formation of a metallic - and below
T-c superconducting percolation network in an insulating background.
H. Kröse, R. Scharfschwerdt, O.F. Schirmer, H. Hesse
LIGHT-INDUCED CHARGE TRANSPORT IN BaTiO3 VIA THREE CHARGE STATES OF RHODIUM
Applied Physics B-Lasers & Optics. 61(1):1-7, 1995 Jul.
Abstract
By combined studies of electron spin resonance and optical absorption at low
temperatures, the charge-transfer bands of Rh5+ and Rh4+ are identified
to be peaked near 1.6 and 1.9 eV, respectively. On this basis, the
light-induced charge-transfer processes in BaTiO3:Rh are unraveled at room
temperature. It is shown that three charge states of Rh are involved, leading
to two levels: the shallow Rh4+/5+ and the deeper Rh3+/4+ level.
The optical behaviour of these two levels corresponds to those expected from
a 'two-center' model. The present paper represents the first atomic-scale
identification of three charge states of one element leading to optical
'two-level' response.
B. Sugg, N. Nürge, B. Faust, E. Ruza, R. Niehüser, H.-J. Reyher, R. A. Rupp
The photorefractive effect in terbium gallium garnet
Optical Materials 4, 343 (1995)
Abstract
A photorefractive effect in terbium gallium garnet with refractive index changes up
to D n = 5 ´ 10-8 for
elementary volume phase holograms is reported. The light-induced refractive index changes
can directly be attributed via Kramers-Kronig relations to the observed photochromic and
light-induced, linear dichroitic behavior. This can also explain the possibility of
writing holograms with orthogonally polarized beams.
H.-J. Reyher, B. Faust, M. Käding, H. Hesse, E. Ruza, M. Wöhlecke
OPTICAL ALIGNMENT OF AXIAL FE CENTERS IN KTAO3
Physical Review B-Condensed Matter. 51(10):6707-6710, 1995 Mar 1.
Abstract
Axial Fe3+-OI centers in KTaO3 can be aligned using
linearly polarized Ar+-ion laser light with l = 456
nm. The alignment is observed directly via the change of the characteristic line pattern
of the optically detected magnetic resonance (ODMR) of these centers. The ODMR is
monitored by the magnetic circular dichroism (MCD) of absorption and is unambiguously
attributed to Fe3+-OI, because the angular dependence of the ODMR
lines agrees well with known EPR results. By observing the strength of the ODMR as a
function of the photon energy, the MCD spectrum and, hence, the absorption bands of Fe3+-OI
centers in KTaO3 are identified. These bands are also found in the spectrum of
the linear dichroism (LD), which is produced by photo-alignment of the axial centers. The
results lead to the interpretation, that the orientation of Fe3+-OI
has a local, molecular character and is similar to the case of FA-centers in
alkali halides.
ATOMISTIC SIMULATION STUDIES OF TRAPPED HOLE BIPOLARONS IN BATIO3
Journal of Physics-Condensed Matter. 7(2):327-337, 1995 Jan 9.
Abstract
Classical shell-model- and embedded-cluster-type calculations are employed in
order to supply theoretical arguments in favour of hole bipolarons in BaTiO3
which have recently been speculated to exist in this photorefractive material.
Our investigations concern the geometrical structure of hole bipolarons trapped
at acceptor defects, their spin state and hole ionization energies. In
particular the embedded-cluster modelling studies, which explicitly include the
local electronic defect structure, suggest the importance of lattice relaxation
and electron correlation terms in order to stabilize diamagnetic O-2(2-)
molecules (bipolarons) in BaTiO3. Our simulations show that hole
bipolarons are predominantly bound at Ti-site acceptor defects. A trapping of
bipolarons at Ba-site accepters is in most Cases unfavourable. Finally, by
extrapolating our present results to the high-Tc superconducting
oxides we qualitatively discuss the possible role of small hole (peroxy)
bipolarons in these materials.
SHG phase matching conditions for undoped and doped lithium niobate
Radiation Effects and Defects in Solids 136, 1029 (1995)
Abstract
Refined embedded-cluster calculations for trapped hole bipolarons in BaTiO3
J. Phys.: Condens. Matter 7, L689 (1995)
Abstract
Embedded-quantum-cluster calculations are used to investigate the formation of
hole-type bipolarons in BaTiO3. These simulations improve on our recent results
because of exact performance of geometry optimizations of the embedded quantum cluster.
Moreover, ion relaxations and electron correlations have been treated consistently. The
modelling of correlations includes Mfller-Plesset
perturbation theory to second order (MP2) and density functional theory (DFT) beyond the
local density approximation. Both defect-induced crystal relaxations and electron
correlation contributions are necessary in order to stabilize trapped hole bipolarons in
BaTiO3.
U. Schlarb, M. Wöhlecke, B. Gather, A. Reichert, K. Betzler, T. Volk, N. Rubinina
Refractive indices of Zn-doped lithium niobate
Optical Materials 4, 791 (1995)
Abstract
We measured the index of refraction n(l) of Zn-doped LiNbO3
grown from a congruent melt as a function of the Zn concentration in the crystal. The Zn
concentration cZn in a sample was determined by electron microprobe technique.
The refractive index n(l,cZn)
is described by a generalized Sellmeir equation for 400 < l
1200 nm and cZn up to 8 mol% ZnO. Calculated phase-matching conditions
of nonlinear effects like second-harmonic generation are in excellent agreement with
experimental results obtained with the same set of crystals.