光折变晶体钛酸钡锶电荷传输参数的测量

报道用两波耦合技术测量光折变晶体钛酸钡锶(Ba_(1-x)Sr_xTiO-3,BST)的电荷传输参数φμτ在激光波长λ=515nm和光功率密度I—1W/cm~2下测得光折变响应时间为0.5sec.考虑到BST晶体吸收系数的光强相关性,修改了光栅形成率的函数变量,得到BST晶体的暗电导σ_a和电荷传输参数φμτ的拟合值分别为1.0×10~(-11)(Ωcm)~(-1)和2.8×10~(-10)cm~2/V.     

Excess space charge in strontium titanate

An analysis is made of mechanisms of the formation and redistribution of space charge which influence the dielectric hysteresis in SrTiO₃. The excess space charge density in SrTiO₃ at 4.2 K is estimated by comparing calculated and experimental dependences of the initial capacitance and the dielectric hysteresis parameter ΔC/C ₀ as a function of the crystal thickness. Fiz. Tverd. Tela (St. Petersburg) 40, 245–247 (February 1998)     

Ab initio calculations of neutral and charged impurity centers of manganese and chromium in strontium titanate

This paper presents the results of ab initio calculations of the equilibrium geometry, the electronic structure, and the spin and charge densities for neutral and negatively charged defects produced by the Mn and Cr impurities in the B position of the SrTiO₃ structure. It has been shown that, in both cases, the neutral defect is an acceptor center, while the singly charged defect is a donor center. It has been found that doubly charged defects are polar, have the symmetry C _4v, and reside in the ionic configurations ⁵Mn³⁺ + ³Ti³⁺ and ⁴Cr³⁺ + ³Ti³⁺, respectively. In each case, there is a pair of almost energy-degenerate electronic states (⁴ B ₁ and ⁶ B ₁ for Mn and ³ A ₁ and ⁵ A ₁ for Cr), which differ only in the direction of the spin of the electron polaron localized at one of the neighboring titanium atoms. For the manganese impurity, the energy of the polar state ⁶ B ₁ is only 0.174 eV lower than that of the state ⁶ A _1g (O _h ) with the Mn²⁺ ion in the high-spin state. A new mechanism of dielectric relaxation in STO: Mn has been proposed.     

Spectra and color centers of strontium titanate crystals

The results of studying the spectral properties of strontium titanate single crystals (grown under different conditions), both pure and activated by iron-group or lanthanide ions, are presented. The data on the main color centers and the formation of systems of ordered crystallites from 10⁻⁷ to 10⁻⁹ m in size on the sample surface after treatment in plasma are obtained by optical and X-ray spectroscopy, cathodoluminescence, electron microscopy, and atomic-force microscopy.     

Central peak in the strontium titanate crystal near the tetragonal-to-cubic phase transition

Raman spectra of the SrTiO3 crystal have been measured in wide temperature (22?C316 K) and frequency (2?C1020 cm^?1) ranges. It has been shown that a central peak appears in low-frequency Raman spectra at temperatures above 70 K. In the spectral geometry with polarization rotation near the temperature T _c = 106 K of the cubic-to-tetragonal phase transition, the central peak exhibits properties of the order-disorder phase transition. Such a behavior of the central peak has been explained by the interaction of the low-frequency soft mode E _g with the relaxation mode near T _c .     

Redistribution of excess space charge in structures based on single-crystal strontium titanate

A band mechanism for transport in single-crystal strontium titanate (STO) is used as a basis for estimating the parameters characterizing the localization and redistribution of space charge in STO for T<100 K. A comparison of the experimental and calculated temperature dependences of the charge carrier concentration yield an estimate for the location of the Fermi level in STO at T=4.2 K with E _F-E _v∼10 meV. The phenomena at metal/STO and YBCO/STO contacts are discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 349–352 (February 1997)     

ESR study of impurities in strontium titanate films

The paper reports on an ESR study of Cr-and Ca-codoped SrTiO₃ films, 1700 and 350 nm thick, before and after UV irradiation (λ=365 nm). The spectrum of the thick film (1700 nm) exhibits two ESR lines with g factors of 1.977 and 1.974, which belong to the Cr³⁺ centers. In the spectrum of the thin film (350 nm), one observes only one line, which is due to the chromium center with a g factor of 1.974. Calculations showed that the line with the smaller g factor belongs to the Cr³⁺ center located close to the film surface. The weak line observed in the spectrum after UV irradiation (g factor = 2.012) is most likely due to the O^? center. The regions of thermal stability of the observed centers were studied. A comparative analysis of the characteristics of impurities in bulk samples and films was carried out.     

EPR of trivalent chromium centers with monoclinic symmetry in SrF2 crystals

Cr³⁺ centers with monoclinic symmetry in SrF₂:Cr crystals are studied by EPR. Based on an analysis of the EPR data and a study of the conditions under which the observed samples were formed, a model is proposed which explains the low symmetry of the magnetic properties in terms of the simultaneous effect on the impurity ion state of interactions with the interstitial F⁻ ion and with normal vibrations of the crystal lattice in a trigonal symmetry. Fiz. Tverd. Tela (St. Petersburg) 39, 302–305 (February 1997)     

Electroreflectance of strontium titanate

The room temperature electroreflectance (ER) measurements for cubic SiTiO₃ are reported with particular emphasis on experimental conditions important for consistent reproducible lineshapes. This study includes for the first time a determination of the position of zero band perturbation (flatband) by making use of the even field nature of the ER lineshapes. The ER spectra are dependent upon dc bias, indicating the importance of a knowledge of the flatband position. A Kramers-Kronig analysis is performed and the lineshapes are reported in terms of ΔR/R, Δε₁ and Δε₂. The results in terms of Δε₂ show that the strongest direct transition occurs near 3.8 eV. The weak structure in the 3.4 eV region is due to either indirect transitions or weak direct transitions. Our unpolarized flatband ER spectra give rise to structures which for (110) samples are resolved into contributions from [001] and [12&#x0304;10] polarized spectra. In addition to the large oscillations observed above the band gap, weaker oscillatory structure is observed in the range 1.5 to 2.8 eV in the polarized spectra of (110) faced samples. The oscillations for [11&#x0304;0] polarization are 180° out of phase with those for [001] polarization.     

Surface of strontium titanate

We report the first complete determination, using surface x-ray diffraction, of the surface structure of TiO2-terminated SrTiO3(001), both at room temperature in vacuum, and also hot, under typical conditions used for thin film growth. The cold structure consists of a mixture of a (1x1) relaxation and (2x1) and (2x2) reconstructions. The latter disappear over several minutes upon heating. The structures are best modeled by a TiO2-rich surface similar to that proposed by Erdman et al. [Nature (London) 419, 55 (2002).10.1038/nature01010]. Both reconstructions have been shown by density functional theory to be energetically favorable. The calculated (1x1) surface energy is higher, indicating that it may be a disordered mixture of the reconstructions. Atomic displacements are significant down to three unit cells, which may have important implications on possible surface ferroelectric phenomena in SrTiO3.     

Elastic anomalies in strontium titanate

The anomalies of the second and third-order elastic constants have been considered for the phase transition of strontium titanate within the framework of Landau’s theory. All the anomalies of the second-order elastic constants have been obtained in a single formula using Kronecker delta functions and relations among them have been established. The real parts ofC*₁₁ andC*₄₄ decrease steeply across the transition temperature and thereafter flatly tend to their asymptotic values in the low temperature phase agreeing qualitatively with experimental observations. We have also derived expressions for the third-order elastic anomalies and discussed the temperature variation of the real part ofC*₁₁₁. We have derived expressions for the attenuation of the longitudinal and transverse waves along certain simple symmetry directions and have shown that there is nearly good agreement with experimental observations.     

Time resolved EPR of triplet excitons in Phenazine-TCNQ charge transfer crystal

Transient nutation EPR spectroscopy has been applied to study the dynamical properties of the excited triplets in the Phenazine-Tetracyanoquinodimenthane 1: 1 CT crystal. Measurements have been carried out with the magnetic field set along the principal axes of the ZFS tensor. Spin-spin and spin-lattice relaxation times have been determined at different temperatures together with the decay rate constants from the triplet sublevels which are found to be highly spin selective. The temperature dependence of the initial optical electron polarization carried by the triplet has been also analyzed. It is shown that the single fission and the intersystem crossing caused by spin-orbit coupling are both responsible for the generation of the triplet in this crystal, the former prevailing at room temperature. Our results are in agreement with previous investigations on the same crystal.     

A study of the structural phase transition in strontium titanate single crystal by coherent and incoherent second optical harmonic generation

The surface phase transition in a SrTiO₃ crystal was studied by second optical harmonic generation. Nonlinear optical response singularities were observed at temperature T*=145 K, which was 40 K higher than the T_c structural phase transition temperature in the crystal volume. Nonlinear critical opalescence in the crystal volume caused by the presence of point defects was studied. The second harmonic field and the intensity of critical opalescence were calculated based on the phenomenological model of nonlinear optical processes with the use of the Landau theory of phase transitions.     

EPR of trivalent iron centers in a BaF2: Fe crystal

An orthorhombic paramagnetic Fe³⁺ ion center (concentration of iron ions is 0.1 at. %) was found using EPR in BaF₂: Fe crystals irradiated by x rays. The EPR spectra recorded in the Q range at a temperature T = 77 K exhibit both the fine structure typical of a center with effective spin S = 5/2 and a superhyperfine structure (SHFS) indicating the SHFS interaction of the electronic moment of the center with the nuclear magnetic moments of its six ligands (F^? ions). An analysis of the SHFS reveals that this center forms through the replacement of a Ba²⁺ cation by a Fe²⁺ cation, which transforms into a Fe³⁺ (⁶ A _1g ) cation under x-ray irradiation and shifts into a neutral position along the C₂ axis of the cubic coordination shell of the replaced host cation.     

EPR of N2− centers in strontium chloride

We report the EPR observation of two different color centers due to N₂^? molecular ions. The signals observed between 4 and 120K are attributed to an N₂^? ion substituting for two chlorines. The model is very similar to the O₂^? ions in the same matrix. Above 120K, thermally activated processes induce a spin-lattice relaxation and smear out the spectra. A simple technique is proposed to measure the activation energy of the reorientation.     

Raman spectrum of strontium titanate

The Raman spectrum of strontium titanate has been recorded using λ 4358 of mercury as exciter. The observed spectrum consists of 7 Raman lines, one of which is of low frequency, as expected from the recent theory of Cochran. 6 of these Raman lines have been interpreted as the first order spectrum arising from a small deviation of the cubic strontium titanate from its idealized symmetry. It has been shown that one normal mode of SrTiO₃ neglected by J.T.Last, will be really active in infrared absorption in the region of 440 cm^?1 and that it has to be taken into account in the interpretation of the infrared spectra of titanates. The four vibrational modes of the unit cell of SrTiO₃ correspond to frequencies of 90, 335, 441 and 620 cm^?1 observed in Raman effect. The large width of the Raman lines and the additional lines at 256 cm^?1 and 726 cm^?1 have been attributed to a splitting of the longitudinal and transverse optical modes. With the observed frequencies it has been found possible to account for in a satisfactory manner the specific heat of SrTiO₃ in the range 54·84° K to 1800° K.     

Electronic structure of strontium titanate

The spectra of three sets of optical functions for SrTiO₃ crystals are determined in a broad energy range of fundamental absorption. The calculations are carried out using the experimental reflectivity spectrum in the range 1–35 eV and two theoretical permittivity spectra in the ranges 0–30 eV and 0–14 eV. The special features of these spectra have been determined. The theoretical spectra of the optical functions are compared with the spectra determined using the experimental reflectivity spectrum.     

EPR study of the effect of partial pressure of oxygen in the growth atmosphere on concentration of chromium centers in synthetic forsterite

Samples of synthetic forsterite doped with chromium and grown in an argon atmosphere with different oxygen concentrations (at a partial pressure of oxygen in the range from 0.03 to 0.78 kPa) have been studied using multifrequency electron paramagnetic resonance (EPR) spectroscopy. It has been demonstrated how the oxidizing properties of the atmosphere during the growth of crystals affect the relative concentration of di-, tri-, and tetravalent chromium ions in the samples. The structure of trivalent chromium impurity centers has been discussed.     

Kinetics of oxygen exchange in strontium titanate

The kinetics of oxygen exchange are of primary importance for the application of titanates as fast resistive oxygen sensors. The sensor’s conductivity is correlated with the oxygen partial pressure pO₂ of the surrounding atmosphere: Due to oxygen surface transfer and subsequent diffusion of oxygen vacancies V _O ^·· , a pO₂ change gives rise to a conductivity change of the sample. While bulk diffusion usually occurs very fast, the surface transfer reaction becomes the rate determining step for thin samples and for low temperatures. We have shown that in the case of acceptor doped SrTiO₃ the kinetics of the surface transfer reaction can be strongly influenced through stoichiometric changes brought about by thin coatings of alkaline earth metal oxides (e.g. SrO). In contrast to the commonly used jump method (conductivity response to a sudden pO₂ change in the time domain), a model is presented which is based on the frequency-domain analysis of amplitude and phase shift of the response signal obtained from a pO₂ modulation in a fast kinetic measurement set-up. This method allows not only for measuring response times in the sub-millisecond range but also for distinguishing between behaviour either controlled by volume diffusion or by surface transfer reaction. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.