arXiv subjects

L. Jastrabik

Publications and source records attributed to L. Jastrabik.

16 recordsLinked to original sources

arXiv2017

Electron spin dynamics of Ce$^{3+}$ ions in YAG crystals studied by pulse-EPR and pump-probe Faraday rotation

The spin relaxation dynamics of Ce$^{3+}$ ions in heavily cerium-doped YAG crystals is studied using pulse-electron paramagnetic resonance and time-resolved pump-probe Faraday rotation. Both techniques address the 4$f$ ground state, while pump-probe Faraday rotation provides also access to the excited 5$d$ state. We measure a millisecond spin-lattice relaxation time $T_1$, a microsecond spin coherence time $T_2$ and a $\sim 10$ ns inhomogeneous spin dephasing time $T_2^*$ for the Ce$^{3+}$ ground state at low temperatures. The spin-lattice relaxation of Ce$^{3+}$ ions is due to modified Raman processes involving the optical phonon mode at 125 cm$^{-1}$. The relaxation at higher temperature goes through a first excited level of the $^{2}$F$_{5/2}$ term at about $\hbar ω\approx 228$ cm$^{-1}$. Effects provided by the hyperfine interaction of the Ce$^{3+}$ with the $^{27}$Al nuclei are observed.

cond-mat.mtrl-sci↗

arXiv2016

Electronic Structure, Optical and Dielectric Spectroscopy Study of TbMnO3

Spectral ellipsometry studies of the optical functions, first observation of photoluminescence and low frequency dielectric permittivity investigations were performed in ferroelectric - multiferroic TbMnO3 single crystals. The main attention was paid to electronic band structure, nature of the main optical transitions and unusual behavior of the dielectric permittivity in the region of the sharp narrow temperature maximum, associated with ferroelectric phase transition. It was found that magnitude of the permittivity maximum decreases strongly with frequency, whereas position of maximum is frequency independent. Observed features of polarization response can be described satisfactorily in the frame of on the basis of the Landau-Khalatnikov theory for critical relaxation polarization

cond-mat.mtrl-sci↗

arXiv2011

EPR studies of manganese centers in SrTiO3: Non-Kramers Mn3+ ions and spin-spin coupled Mn4+ dimers

X- and Q-band electron paramagnetic resonance (EPR) study is reported on the SrTiO3 single crystals doped with 0.5-at.% MnO. EPR spectra originating from the S = 2 ground state of Mn3+ ions are shown to belong to the three distinct types of Jahn-Teller centres. The ordering of the oxygen vacancies due to the reduction treatment of the samples and consequent formation of oxygen vacancy associated Mn3+ centres are explained in terms of the localized charge compensation. The EPR spectra of SrTiO3: Mn crystals show the presence of next nearest neighbor exchange coupled Mn4+ pairs in the <110> directions.

cond-mat.mtrl-sci↗

arXiv2011

Experimental manifestations of the Nb^{4+}-O^{-} polaronic excitons in KTa_{0.988}Nb_{0.012}O_{3}

The formation of the photo-polaronic excitons in ABO_{3} perovskite type oxides has been detected experimentally by means of the photoinduced electron paramagnetic resonance studies of KTa_{0.998}Nb_{0.012}O_{3} crystals. The corresponding microwave X-band spectrum at T < 10 K consists of a narrow, nearly isotropic signal located at g ~ 2 and a strongly anisotropic component. The first signal, which has a rich structure due to hyperfine interactions with the lattice nuclei, is attributed to the single trapped charge carriers: the electrons and/or the holes. The anisotropic spectrum is caused by the axial centers oriented along the C_{4} pseudo-cubic principal crystalline axes. The spectrum angular dependence can be described well by an axial center with S = 1, g_{\parallel) = 0.82, g_{\perp} = 0.52 and D = 0.44 cm^{-1}. The anisotropic spectrum is attributed to the Nb^{4+}-O^{-} polaronic excitons. The temperature dependence of the anisotropic component is characterized by two activation energies: the internal dynamics activation E_{a1} = 3.7\pm0.5 meV, which makes the EPR spectrum unobservable above 10 K, and the destruction energy E_{a2} = 52\pm4 meV. By comparing the anisotropic photo-EPR spectrum and the photoinduced optical absorption temperature dependencies, we found that the Nb^{4+}-O^{-} polaronic excitons also manifested themselves via the ~0.7 eV wide absorption band arising under UV light excitation in the weakly concentrated KTaO_{3}:Nb crystals.

cond-mat.mtrl-sci↗

arXiv2009

Tensile strain induced changes in the optical spectra of SrTiO3 epitaxial thin films

Effect of biaxial tensile strains on optical functions and band edge transitions of ultra thin epitaxial films was studied using as an example a 13 nm thick SrTiO3 films deposited on KTaO3(100) single-crystal substrates. Optical functions were determined by spectroscopic ellipsometry technique. It was found that tensile strains result in a shift of the low energy band gap optical transitions to higher energies and decrease the refractive index in the visible region. Comparison of the optical spectra for strained SrTiO3 films and for homoepitaxial strain-free SrTiO3:Cr (0.01 %at.) films deposited on SrTiO3(100) single crystalline substrates showed that this shift could not be related to technological imperfections or to reduced thickness. The observed effect is connected with changes in the lowest conduction and in the top valence bands that are due to increase of the in-plane lattice constant and/or onset of polar phase in the tensile strain-induced ultra-thin epitaxial SrTiO3 films.

cond-mat.mtrl-sci↗

arXiv2007

Electron spin resonance investigation of Mn^{2+} ions and their dynamics in manganese doped SrTiO_3

Using electron spin resonance, lattice position and dynamic properties of Mn2+ ions were studied in 0.5 and 2 % manganese doped SrTiO3 ceramics prepared by conventional mixed oxide method. The measurements showed that Mn2+ ions substitute preferably up to 97 % for Sr if the ceramics is prepared with a deficit of Sr ions. Motional narrowing of the Mn2+ ESR spectrum was observed when temperature increases from 120 K to 240-250 K that was explained as a manifestation of off-center position of this ion at the Sr site. From the analysis of the ESR spectra the activation energy Ea = 86 mV and frequency factor 1/?0 ? (2-10)x10^(-14) 1/s for jumping of the impurity between symmetrical off-center positions were determined. Both values are in agreement with those derived previously from dielectric relaxation. This proves the origin of dielectric anomalies in SrTiO3:Mn as those produced by the reorientation dynamics of Mn2+ dipoles.

cond-mat.soft↗

arXiv2006

Li Doping Effect on Properties and Phase Transfomations of Knbo3

Dielectric permittivity and infrared reflectivity spectra of Li doped KNbO3 single crystals have been studied for the first time for K1-xLixNbO3 (KLN) with x = 0.015, 0.02, 0.065. It was found that like in KTaO3, Li admixture results in appearance of dielectric relaxation with the relaxation parameters very close to those in KTaO3 quantum paraelectric. It was attributed to 90-grad dipole reorientation of Li+ <100> off centers substituted K+, which appear to be presents as in paraelectric cubic phase as in ferroelectric phase down to low temperatures. Besides, Li doping is accompanied by increasing of the cubic-tetragonal phase transition point, decreasing of tetragonal-orthorhombic-rhombohedral phase transition points and TO soft mode stiffen at room temperature.

cond-mat.other↗

arXiv2005

Bias field effect on the temperature anomalies of dielectric permittivity in PbMg1/3Nb2/3O3- PbTiO3 single crystals

In contrast to ordinary ferroelectrics where the temperature, Tm, of the permittivity maximum monotonically increases with bias field, E, in (1-x)PbMg1/3Nb2/3O3-(x)PbTiO3 (0<x<0.35) single crystals, Tm was found to remain constant or decrease with E up to a certain threshold field, Et, above which Tm starts increasing. Et decreases with x and almost disappears at about x=0.4. We explain this field dependence by the quenched random fields and coupling between the orientable dipoles with average polarization. For crystals with 0.06<x<0.13, the T-E phase diagrams are constructed, which include the relaxor, ferroelectric, glass and mixed ferroelectric-glass phases. The boundary between the ferroelectric and relaxor phases corresponds to a first-order phase transition and is characterized by thermal hysteresis with a large curvature of the line obtained in the field cooling mode, and by the presence of the end-point above which there is no difference between the ferroelectric and relaxor phases. The glass phase has a nearly field-independent boundary in the vicinity of the Vogel-Fulcher temperature TVF. The coupling between the glass order parameter and polarization changes the ferroelectric boundary at temperatures lower than TV

cond-mat.mtrl-sci↗

arXiv2004

Nontrivial dependence of dielectric stiffness and SHG on dc bias in relaxors and dipole glasses

Dielectric permittivity and Second Harmonic Generation (SHG) studies in the field-cooled mode show a linear dependence of dielectric stiffness (inverse dielectric permittivity) on dc bias in PMN-PT crystals and SHG intensity in KTaO$_{3}$:Li at small Li concentrations. We explain this unusual result in the framework of a theory of transverse, hydrodynamic-type, instability of local polarization.

cond-mat.mtrl-sci↗

arXiv2003

Random-Site Cation Ordering and Dielectric Properties of PbMg1/3Nb2/3o3-PbSc1/2Nb1/2o3

(1-x)PbMg1/3Nb2/3O3-(x)PbSc1/2Nb1/2O3 (PMN-PSN) solid solution crystals have been grown by the flux method in the whole concentration range. X-ray supercell reflections due to B-cation ordering were observed for as-grown crystals from the 0.1 < x < 0.65 compositional range. Though the ordered domains are rather large (~50 nm) the relaxor-like dielectric behavior is observed for compositions with x < 0.6. The diffusion of the dielectric permittivity maximum in as-grown crystals is the lowest at x = 0.6 and increases towards the end members of solid solution. Such behavior is explained within a Bragg-Williams approach by employing the random layer model. At x ~ 0.6 the excitation energy determined from the Vogel-Fulcher relation exhibits a jump which we regard to changing the kind of the polar regions from PbMg1/3Nb2/3O3 to PbSc1/2Nb1/2O3 related type.

cond-mat.mtrl-sci↗

arXiv2000

Random field based model of mixed ferroelectrics phase diagram

The equations for phase transitions temperatures, order parameters and critical concentrations of components have been derived for mixed ferroelectrics. The electric dipoles randomly distributed over the system were considered as a random field sources. We derive a random field distribution function for different orientations of the electric dipoles with nonlinear and spatial correlation effects included.

cond-mat.mtrl-sci↗

arXiv1999

Phase transitions in the antiferroelectric-ferroelectric mixed systems

Phase diagram of solid solutions of antiferroelectric and ferroelectric materials like PbZr$_{1-x}$Ti$_x$O$_3$ (PZT), Rb$_x$(NH$_4$)$_{1-x}$H$_2$PO$% _4$ (RADP) is calculated at the region $x\leq 0.1$. Antiferroelectric and ferroelectric order parameters (which correspond respectively to the difference and sum of two sublattices ions shifts) were introduced for antiferroelectric host lattice. Small admixture of ferroelectric component was considered as a random electric field source. The influence of this field on both aforementioned order parameters was calculated. Self consistent averaging with calculated random field distribution function made it possible to obtain the system of equations for order parameters of mixed system. The antiferroelectric $T_a$ and ferroelectric $T_c$ phase transition temperature concentrational dependence was expressed through the transition temperatures of the composition and members. The obtained $T_a(x)$ dependence was shown to fit pretty good the observed values for PZT and RADP. The application of the proposed theoretical approach to other types of mixed systems is discussed.

cond-mat.mtrl-sci↗

arXiv1999

On the physical mechanisms of relaxation time distribution in disordered dielectrics

The distribution function of relaxation times in disordered dielectrics has been calculated in the random field theory framework. For this purpose, we first consider the dynamics of single two-orientable impurity electric dipole in a random electric field $E$ created by the rest of impurities in disordered ferroelectric. This dynamics is conveniently described by Langevin equation. Relaxation time $τ$ is then a reciprocal probability (calculated on the base of Fokker-Planck equation) of the dipole transition through barrier in a double-well potential (corresponding to two possible dipole orientations), distorted by a random fields. The obtained dependence $τ(E)$ made it possible to obtain the expression for relaxation times distribution function $F(τ)$(via random fields distribution function $f(E)$. Latter function has been calculated self-consistently in the random field theory framework. Nonlinear random field contribution and effects of spatial correlations between impurities have also been taken into account. It was shown that nonlinear contribution of random field gives asymmetric shape of $F(τ)$, while in linear case it is symmetric. Comparison of calculated $F(τ)$ curves with those extracted from empirical Cole-Cole (CC), Davidson-Cole (DC), Kohlrausch-William-Watts (KWW) and Havriliak-Negami (HN) functions had shown, that they correspond to mixed ferro-glass phase with coexistence of short and long-range order. Different forms of $F(τ)$ are determined by linear (CC) or nonlinear (DC, KWW, HN) contributions of random field.

cond-mat.mtrl-sci↗

arXiv1998

Hopping Conductivity of a Nearly-1d Fractal: a Model for Conducting Polymers

We suggest treating a conducting network of oriented polymer chains as an anisotropic fractal whose dimensionality D=1+εis close to one. Percolation on such a fractal is studied within the real space renormalization group of Migdal and Kadanoff. We find that the threshold value and all the critical exponents are strongly nonanalytic functions of εas εtends to zero, e.g., the critical exponent of conductivity is ε^{-2}\exp (-1-1/ε). The distribution function for conductivity of finite samples at the percolation threshold is established. It is shown that the central body of the distribution is given by a universal scaling function and only the low-conductivity tail of distribution remains $ε$-dependent. Variable range hopping conductivity in the polymer network is studied: both DC conductivity and AC conductivity in the multiple hopping regime are found to obey a quasi-1d Mott law. The present results are consistent with electrical properties of poorly conducting polymers.

cond-mat.dis-nn↗

arXiv1997

Conductivity fluctuations in polymer's networks

Polymer's network is treated as an anisotropic fractal with fractional dimensionality D = 1 + εclose to one. Percolation model on such a fractal is studied. Using the real space renormalization group approach of Migdal and Kadanoff we find threshold value and all the critical exponents to be strongly nonanalytic functions of ε, e.g. the critical exponent of the conductivity was obtained to be ε^{-2}\exp(-1-1/ε). The main part of the finite size conductivities distribution function at the threshold was found to be universal if expressed in terms of the fluctuating variable, which is proportional to the large power of the conductivity, but with dimensionally-dependent low-conductivity cut-off. Its reduced central momenta are of the order of \exp(-1/ε) up to the very high order.

cond-mat.dis-nn↗

arXiv1996

Critical Percolation and Transport in Nearly One Dimension

A random hopping on a fractal network with dimension slightly above one, $d = 1 + ε$, is considered as a model of transport for conducting polymers with nonmetallic conductivity. Within the real space renormalization group method of Migdal and Kadanoff, the critical behavior near the percolation threshold is studied. In contrast to a conventional regular expansion in $ε$, the critical indices of correlation length, $ν=ε^{-1}+O(e^{-1/ε})$, and of conductivity, $t\simeq ε^{-2} exp (-1-1/ε)$, are found to be nonanalytic functions of $ε$ as $ε\to 0$. Distribution for conductivity of the critical cluster is obtained to be gaussian with the relative width $\sim \exp (-1/ε)$. In case of variable range hopping an ``1-d Mott's law'' $exp [ -( T_t/T)^{1/2}]$ dependence was found for the DC conductivity. It is shown, that the same type of strong temperature dependence is valid for the dielectric constant and the frequency-dependent conductivity, in agreement with experimental data for poorly conducting polymers.

cond-mat.dis-nn↗