Lev S. Sochava
		Papers photorecharging of paramagnetic centers in perovskites (1995 - 2000),  high Tc superconductors and fullerites (1989 - 1996),  off-center impurities (1975 - 1997), other.

Physics of the Solid State 43 (6), (2001).

An EPR study has revealed photoinduced recharging and optical alignment of the Fe⁴⁺_Ta--V_O tetragonal complexes in KTaO₃. The data on the optical creation and destruction of this center by light of different polarizations and wavelengths are discussed together with similar results obtained for the Fe³⁺_K--O_i center. These two centers were established to undergo mutual charge transfer, in which the electron released in the photoionization of the Fe²⁺_K--O_i center is trapped by the Fe⁵⁺_Ta--V_O center. Irradiation by light with photon energy below 2.05 eV, which is the ionization threshold of Fe²⁺_K--O_i, reverses this process. In both cases, the absorption cross section depends on the orientation of the center axis relative to the light polarization vector. As a result, the Fe⁴⁺_Ta--V_O and Fe³⁺_K--O_i tetragonal centers acted upon by polarized light undergo orientation-sensitive photorecharging, and the defects with the given charge state are no longer characterized by an equally probable axial distribution over the three <100> directions. This mechanism, which does not involve real reorientations of the Fe_Ta--V_O and Fe_K--O_i complexes, gives rise, nevertheless, to the alignment of the centers along (at right angles to) the light polarization vector.

Physics of the Solid State 42 (9), 1640-1642 (2000).

Fe³⁺_K--O_i^2- impurity centers in a KTaO₃ sample to which a dc electric field E = 75 kV/cm is applied are shown to be oriented at temperatures T greater than or equal to 120 K. In these conditions, the effective local field acting on the electric dipole moment of a center exceeds the applied field by a factor 7.6.

Physics of the Solid State 41 (8), 1303-1306 (1999).

The EPR spectrum of the non-Kramers iron ion Fe⁴⁺ (S = 2) in a KTaO₃:Fe crystal appearing after illumination of the sample in the visible has been detected and studied. Because of the large initial splitting (\D\ = 4.15 cm(-1)), only transitions within the \+/- 1 > and \+/- 2 > doublets are seen experimentally. Superhyperfine structure in the spectrum of a non-Kramers ion in perovskites has been detected for the first time. A structure is proposed for the center responsible for the new EPR spectrum, which represents a complex of a Fe⁴⁺ ion substituting for Ta with an oxygen vacancy at the nearest anion site.

Physics of the Solid State 39 (4), 547-549 (1997).

Superhyperfine structure is observed in the lines of the ESR spectra of two tetragonal iron centers in the KTaO₃ crystal, namely, ''Fe 4/2'' and Fe³⁺_Ta--V_O. Analysis of the superhyperfine structure of the former of these centers shows that the iron ion replaces Ta5+ and is found in the charge state 5+. Justification is given for assuming that the tetragonal symmetry of the center is due to the displacement of Fe⁵⁺ from the Ta⁵⁺ site along a [100] direction to an off-center position.

Proc. of the 13th Internat. Conf. on Defects in Insulating Materials (ICDIM 96), Materials Science Forum 239-241, 345-348 (1997).

Orientation sensitive recharging of tetragonal Fe_K--O_i centers in KTaO₃ under polarized optical irradiation was observed resulting in the ''alignment'' of the centers in particular charge state. The sign of the alignment altered in the course of irradiation. A phenomenological model involving photo charge transport allowed for a semiquantitative description of the experiments.

Izvestija Akad. Nauk, ser.fiz., 60 (10), 20-29 (1996). In Russian.

SPIE Proc. 2706, 73-82 (1996).

Ferroelectrics 183, 255-264 (1996).

Journal of Luminescence 66&67 (1-6), 526-530 (1995).

Spectral and kinetic measurements of ²E --> ⁴A₂ photoluminescence (PL) in SrTiO₃ crystals containing small amounts of Cr were performed and a strong effect of IR illumination on the PL was observed. The dominant photoionization mechanism of Cr³⁺ PL excitation was established. The locations in the SrTiO₃ band gap of Cr³⁺ donor levels (1.85 eV below the bottom of the conduction band) and of Cr⁴⁺ acceptor levels (1.4 eV above the top of the valence band) were found. A new mechanism of optical erasing of burnt spectral holes of the R lines in SrTiO₃:Cr was determined.

Radiation Effects and Defects in Solids 134 (1-4), 325-328 (1995).

The temperature-frequency behaviour of the permittivity epsilon'(T, f) and tg delta(T, f) (20-300 K, 120 Hz-9 MHz), EPR, optical absorption and luminescence of KTaO₃ single crystals, pure and with Li 0.1; Cr 0.02; Cu 3; Fe 1; Li 0.1 + Cr 0.1, and Li 0.1 + Cr 0.1 + Cu 0.02 wt.%, were studied. Below 70 K, in all the doped samples, dielectric dispersion occurs, with tg delta(T) maxima obeying Arrhenius law. This is associated with a local ordering in dipole-dipole clusters (which contain reorienting defects of various kind) with correlations due to interactions via the TO-soft mode.

Journal of Physics: Condensed Matter 7 (19), 3765-3777 (1995).

The temperature-frequency behaviour of the permittivity epsilon'(T,f) and tan[delta(T,f)] (20-300 K; 120 Hz-9 MHz), EPR, optical absorption and luminescence of KTaO₃, nominally pure and weakly doped (0.1 wt% Li; 0.01 wt% Cu; 1 wt% Fe; 0.1 wt% Li + 0.1 wt% Cr and 0.1 wt% Li+0.1 wt% Cr+0.02 wt% Cu) were studied. Below 70 K, in all doped samples, dielectric dispersion occurs, with tan[delta(T)] maxima positions obeying the universal Arrhenius law with activation energy DELTA = 1000 +/- 50 K. This is associated with local ordering in the dipole-dipole clusters containing reorienting defect pairs of various kinds, with correlations due to interaction via the TO soft mode.

		Papers photorecharging of paramagnetic centers in perovskites (1995 - 2000),  high Tc superconductors and fullerites (1989 - 1996),  off-center impurities (1975 - 1997), other.

Molecular Crystals and Liquid Crystals Science and Technology Section C - Molecular Materials 7 (1-4), 235-236 (1996).

Spin dynamics of the hole paramagnetic center in undoped C₇₀ fullerite was investigated on the basis of spin-echo method. The possible role of quasi-local C₆₀⁺ molecular vibrations in spin-lattice relaxation was considered.

Radiation Effects and Defects in Solids 137 (1-4), 1341-1343 (1995).

The hole paramagnetic center (S = 1/2, g = 2.0028) was observed in undoped C₇₀ fullerite for the first time. It was shown that the possible model of this center is C₆₀⁺ hole center with charge compensation at the surface.

Soviet Journal of Low Temperature Physics 17 (9), 557-558 (1991).

JETP Lett. 50 (3), 135-139 (1989).

A threshold in the magnetic field has been observed for several characteristic effects in the low-field absorption by high-T_c single crystals. A signal of a new type corresponds to the threshold field in the microwave absorption spectrum. The appearance of the new signal in the low-field absorption and of effects which involve a threshold in the magnetic field is linked with a transition from an individual pinning of vortices to a collective pinning.

Sov.Phys.Solid State 31 (7), 1219-1225 (1989).

Experimental and theoretical investigations were made of a low-field signal representing microwave absorption in Bi₂Sr₂CaCu₂O_y high-temperature superconductor single crystals. A complex temperature-dependent structure of the signal was observed. There was a threshold value of the magnetic field above which there were no memory effects. A narrow (<~0.2 Oe) signal of negative polarity exhibited a strong temperature dependence of the intensity with a maximum. The mechanisms responsible for the signal were investigated, including relaxation due to modulation of the magnetic moment by hopping transitions between superconducting regions in a Josephson medium and a resonance associated with tunnel transitions induced by a microwave field in the Josephson medium.

		Papers photorecharging of paramagnetic centers in perovskites (1995 - 2000),  high Tc superconductors and fullerites (1989 - 1996),  off-center impurities (1975 - 1997), other.

Proc. of the 13th Internat. Conf. on Defects in Insulating Materials (ICDIM'96), Materials Science Forum 239-241, 349-352 (1997).

The temperature dependence of the dynamics of two different defects was studied in single-crystalline SrO by pulsed spin excitation techniques: Rotational motion of paraelectric OH ^-, electronic spin flips of paramagnetic Co²⁺ and rotational motion of off-center Co²⁺. The findings are interpreted in the framework of microscopic models.

Sov.Phys.Solid State 32 (5), 844-846 (1990).

The temperature dependence of the rate 1/T₁ of spin–lattice relaxation of Co²⁺ ions occupying noncentral positions in the crystal lattice of SrO was investigated. This dependence had a linear part, as well as two exponential regions characterized by energies delta₁=79 ± 5 and delta₂=200 ± 20 cm^–1. Possible mechanisms of spin–lattice relaxation of Co²⁺ ions in SrO were analyzed. The experimentally determined energies could represent the gaps between the ground (J=1/2) and excited (J=3/2, J=5/2) spin–orbit multiplets. An excited vibrational state (delta=149 cm^–1) could also contribute to the exponential process characterized by the energy delta₁ because of the anharmonic Raman scattering by a quasilocal vibration.

Izvestija Akad. Nauk, ser.fiz. 52 (3), 477-481 (1988). In Russian.

Sov.Phys.Solid State 29 (3), 481-484 (1987).

A time dependence was discovered of the electric field splitting of ESR lines for the off-center Co²⁺ impurity ion in the strontium oxide crystal. It was found that such an effect should exist in the spectrum of a reorientable dipole paramagnetic center under both external and internal (defect) fields. A very simple model is used to calculate the ESR line position and profile for the off-center ion in the electric field; the results of the calculation give a qualitative explanation of the main features of the effect.

Izvestija Akad. Nauk, ser.fiz. 50 (2), 256-259 (1986).

Fizika Tverdogo Tela 27 (2): 532-534 1985
Soviet Physics - Solid State, Volume 27, Issue 2, pp.327-328 (1985).

JETP Letters, Volume 40, Issue 10, pp. 1248-1251 (1984)

A spin polarization of paramagnetic ions which results from a coherent tunneling between noncentral positions has been observed for the first time. This tunneling can occur either with or without a change in the spin state of the ions.

Fizika Tverdogo Tela 26 (9): 2661-2668 1984
Soviet Physics - Solid State, Volume 26, Issue 9, pp.1613-1617 (1984)

The SrO:Co²⁺ system was used for an experimental and theoretical study of paramagnetic ion tunneling between noncentral minima. The long ion–lattice relaxation time of the Co²⁺ ions (~ 80sec at 4.2K) made possible a detailed ESR study of both the kinetics of establishment of an equilibrium population at noncentral minima, and the dependence of the rate of this process on the electric field and temperature. The tunneling rate was found to be much greater in an external electric field. This effect cannot be explained by the usual model of phonon-induced tunneling: it was interpreted by taking into account the contribution of excited single-well states of the center, for which the tunneling and the interaction with deformations differ considerably from their values for ground states. A comparison of experiment and theory gave estimated values of new tunnel and deformation parameters taking account of excited states, and a mean value of the internal defect field in the crystal.

OPTIKA I SPEKTROSKOPIYA 50 (2): 326-333 1981
Optics and Spectroscopy, Volume 50, Issue 2, pp. 175-179

Fizika Tverdogo Tela 22 (8): 2388-2394 1980
Soviet Physics - Solid State, Volume 22, Issue 8, pp.1389-1393 (1980)

An investigation was made of the visible and infrared absorption spectra of SrO:Co²⁺ single crystals and of the corresponding electromodulation spectra. Vacuum annealing of SrO:Co²⁺ crystals produced one type of Co²⁺ centers which were trigonal and showed no inversion. Reorientation of these centers in electric fields was observed at 78 and 4.2K: it was manifested by changes in the zeroth and first moments of the absorption lines. The electric dipole moment of the centers was p=7.4±0.7D. The results obtained indicated that the Co²⁺ ions were located mnoncentrally in the crystal lattice of SrO with a displacement from the cation sites in the 111 direction. An analysis was made of classical oscillator models which could be used to explain the three main absorption bands of the Co²⁺ ions in SrO.

physica status solidi (a) - applied research 56 (2), 615-621 (1979).

Fizika Tverdogo Tela 21 (10): 3090-3094 1979
Soviet Physics - Solid State, Volume 21, Issue 10, pp.1778-1780 (1979)

An investigation of the ESR spectra of SrO:Co crystals showed that annealing in oxygen produced complexes formed from an interstitial oxygen ion and a noncentral CO²⁺ ion. Annealing in vacuum caused the excess oxygen to diffuse out of the crystal, leaving behind isolated noncentral Co²⁺ ions in the form of trigonal centers. Investigation of the trigonal centers in an external electric field indicated that an electric dipole moment pcorr=0.31 eÅ was associated with these centers: this moment was due to the displacement of Co²⁺ from a cation site along the 11 direction. Two relaxation processes occurring at very different rates were observed on application and removal of the electric field.

Fizika Tverdogo Tela 21 (9): 2759-2764 1979
Soviet Physics - Solid State, Volume 21, Issue 9, pp.1586-1589 (1979)

A study was made of the Stark effect exhibited by the zero-phonon absorption line 7404.5 Å in the optical spectrum of Ni²⁺ in an SrO crystal. This effect is linear in the field, and corresponds to a center with symmetry C₃ or C_3v. At 78 °K, there is a distortion of the symmetrical pattern in the linear Stark splitting of the line, caused by a reorientation of the dipole moment associated with the center, in the electric field. The dipole moment is found to be p=2.7 eÅ. The experimental results indicate a displacement of the Ni²⁺ ion equilibrium position in the SrO lattice along a 111 direction, so that it has a noncentral position.

Fizika Tverdogo Tela 21 (7): 2083-2090 1979
Soviet Physics - Solid State, Volume 21, Issue 7, pp. 1193-1197 (1979)

Models of noncentral Jahn–Teller (JT) ions Ni⁺ in SrO and Cu²⁺ in SrO considered previously ^{1 – 6} are discussed. Models proposed for such an ion include an interaction of the dipole-active strains causing noncentrality with the JT tetragonal strains. Occurrences of a new type of multiwell potential XY₂₄ of the noncentral ion are examined. The JT effect is investigated for a doublet in a cubic environment, including fourth-order anharmonicity. In such an E×e JT-effect model, there is a change in the position and in the number of minima of the adiabatic potential in comparison with the model that takes account of third-order anharmonicity or quadratic JT interaction. The models of the noncentral JT ion that take account of the influence of two tetragonal JT strains occurring in the E×e problem with fourth-order anharmonicity on the dipole-active strains account for the experimental results for SrO:Ni⁺.

Fizika Tverdogo Tela 21 (6): 1789-1797 1979
Soviet Physics - Solid State, Volume 21, Issue 6, pp.1023-1028 (1979)

Fizika Tverdogo Tela 20 (8), 2412-2419 (1978).
Soviet Physics - Solid State, Volume 20, Issue 8, pp.1392-1397 (1978)

The ESR method was used in an investigation of the nature of transitions of a Cu²⁺ ion between noncentral energy minima in an SrO crystal. A detailed study of the averaging of the ESR spectrum by the hopping motion of the copper ion between the minima made it possible to determine the temperature dependence of the hopping rate (1/tau). The experimental results were interpreted on the basis of a model of phonon-induced tunneling of a copper ion in its ground vibrational state and of tunnel-controlled transitions of a noncentral ion between energy minima of its excited state.
Broadening of the ESR lines of Cu²⁺ in an electric field was observed. This effect was also explained on the basis of the adopted tunnel hopping model.

Fizika Tverdogo Tela 19 (6): 1901-1903 1977
Soviet Physics - Solid State, Volume 19, Issue 6, pp.1115-1116 (1977)

Fizika Tverdogo Tela 18 (6): 1622-1626 1976
Soviet Physics - Solid State, Volume 18, Issue 6, pp. 943-945 (1976)

Crystals of copper-activated SrO exhibited—in addition to spectrum I investigated earlier [Tolparov et al., Fiz. Tverd. Tela (Leningrad) 16, 895 (1974) and Sochava et al., ibid. 17, 217 (1975)]—also a weaker ESR spectrum II. The latter spectrum, which had an orthorhombic symmetry with g_z=2.361, g_x=2.082, and g_y=2.050 at 77°K, was averaged in the temperature range 120–200°K and this increased its symmetry to the tetragonal form. Possible mechanisms of this averaging process were analyzed.

Fizika Tverdogo Tela  17 (9): 2590-2593 1975
Soviet Physics - Solid State, Volume 17, Issue 9, pp.1725-1726 (1975)

The ESR and optical spectra of cubic SrO crystals doped with divalent cobalt are investigated. The ESR spectra (taken at T=4.2 °K) indicate that the crystals contain two types of Co²⁺ (3d⁷) centers with S=1/2 and I=7/2. Centers type I have C_2v symmetry and g factors: g_z=2.134, g_x=4.171, g_y=4.994. Centers type II have C_s symmetry and g factors: g_z=7.331, g_x=2.386, g_y=1.694. The optical spectrum at T=21 and 300°K contains Co²⁺ bands corresponding to the transitions 4T1(F)    4A2 and 4T1(P). The 4T (P) band is split into three components (total splitting between outermost components 1200 cm – 1) and the oscillator strengths of the two bands are independent of temperature. The parameters of the cubic part of the crystal field are determined: Dq=380 cm^–1, B=760 cm^–1. Thus, both the ESR and the optical spectra indicate that the SrO:Co crystals contain low-symmetry Co²⁺ centers and no cubic centers. Models of the two types of centers are discussed in which the Co²⁺ ion is assumed to occupy an off-center position in the SrO crystal lattice.

Fizika Tverdogo Tela  17 (1): 217-226 1975
Soviet Physics - Solid State, Volume 17, Issue 1, pp. 127-132 (1975)

An investigation was made of the ESR spectrum of the Cu2 + ions in a cubic crystal of SrO subjected to a static electric field at T=4.2 °K. It was found that an electric dipole moment was associated with each paramagnetic Cu2 + center because of the off-center position of the copper ion. Off-center displacements of the copper ions were oriented by an external electric field and when this field exceeded 1.6 · 105 V/cm, a complete orientation was achieved. The dipole moment, calculated allowing for the internal crystal field, was (1.3 ± 0.2)D, which corresponded to a displacement of the copper ion from a cation site by 0.14 Å along the 111 direction. A displacement of the ESR lines of the noncentral Cu2 + ions in an electric field (''electrofield effect'') was observed. The results of this ESR study of the SrO:Cu2 + system in an electric field were used in discussing a model of paramagnetic centers.

		Papers photorecharging of paramagnetic centers in perovskites (1995 - 2000),  high Tc superconductors and fullerites (1989 - 1996),  off-center impurities (1975 - 1997), other.

Fizika Tverdogo Tela 35 (8), 2285-2287 (1993).

		Papers photorecharging of paramagnetic centers in perovskites (1995 - 2000),  high Tc superconductors and fullerites (1989 - 1996),  off-center impurities (1975 - 1997), other.