Electrical properties of TlGaTe2 single crystals under hydrostatic pressure

The effect of hydrostatic pressure (up to 0.82 GPa) on the electric properties of chain TlGaTe₂ single crystals has been investigated in the temperature range 77-296 K. It has been shown that pressure leads to a considerable increase of conductivity (σ_⊥) across the chains of TlGaTe₂ single crystals. Parameters of localized states in the band gap of TlGaTe₂ single crystal according to the low-temperature electrical measurements were obtained at various pressures.     

Phase transitions in CuBiP2Se6 crystals

Investigation results of dielectric (20?Hz–1?MHz) properties of layered CuBiP₂Se₆ crystals are presented. The temperature dependence of the static dielectric permittivity reveals the first-order “displacive” antiferroelectric phase transition at T _c?=?136?K. In the paraelectric phase, at low frequencies, dielectric spectra are highly influenced by the high ionic conductivity with the activation energy of 2473?K (0.21?eV). In the antiferroelectric phase the electrical conductivity and its activation energy (531.1?K (0.045?eV)) are considerably smaller. At low temperatures, the temperature behaviour of the distribution of relaxation times reveals complex freezing phenomena. A part of long relaxation time distribution is strongly affected by external direct current (DC) electric field and it is obviously caused by antiferroelectric domain dynamics.     

Anisotropy of the hopping conductivity in TlGaSe2 single crystals

The temperature dependences of the conductivities parallel and perpendicular to the layers in layered TlGaSe₂ single crystals are investigated in the temperature range from 10 K to 293 K. It is shown that hopping conduction with a variable hopping length among localized states near the Fermi level takes place in TlGaSe₂ single crystals in the low-temperature range, both along and across the layers. Hopping conduction along the layers begins to prevail over conduction in an allowed band only at very low temperatures (10–30 K), whereas hopping conduction across the layers is observed at fairly high temperatures (T?210 K) and spans a broader temperature range. The density of states near the Fermi level is determined, N _F=1.3×10¹⁹eV·cm³)^?1, along with the energy scatter of these states J=0.011 eV and the hopping lengths at various temperatures. The hopping length R along the layers of TlGaSe₂ single crystals increases from 130 Å to 170 Å as the temperature is lowered from 30 K to 10 K. The temperature dependence of the degree of anisotropy of the conductivity of TlGaSe₂ single crystals is investigated.     

Field emission theory of dislocation-sensitized photo-stimulated exo-electron emission from coloured alkali halide crystals

A new field emission theory of dislocation-sensitized photo-stimulated exo-electron emission (DSPEE) is proposed, which shows that the increase in the intensity of photo emission fromF-centres during plastic deformation is caused by the appearance of an electric field which draws excited electrons out of the deeper layer and, therefore, increases the number of electrons which reach the surface. The theory of DSPEE shows that the variation of DSPEE flux intensity should obey the following relation

Pecularity of lithium electrodiffusion into fulleride single crystals

Diffusion of lithium cations in C₆₀ single crystals driven by an electric field has been studied. It was found that the stoichiometry of C₆₀ single crystals can be changed with respect to lithium by injection of Li⁺ ions through the heterojunction Li₇SiPO₈|C₆₀ and electrons through the heterojunction C₆₀| Graphite. The double charge injection changes the stoichiometry of lithium in a C₆₀ single crystal and increases both the lithium ionic and electronic conductivity. The electronic conductivity in Li_xC₆₀ crystals is non-metallic in nature. The temperature dependence of the electronic conductivity lithium doped C₆₀ single crystals was investigated. It was found that electrons occupied the lowest singlet exited states (LUMO) and that this leads to the appearance of an intensive EPR signal. The temperature dependence of the concentration of paramagnetic centers was investigated. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Photocurrent oscillations in luminescent epitaxial thin films of ZnSe (Cu,Ga)

Photoluminescent epitaxial thin films of ZnSe (Cu, Ga), n-type, vacuum-deposited onto CaF₂ crystals, or air cleaved mica substrates, have been studied. At temperatures between 80 K and 220 K, slow photocurrent oscillations can be observed if ZnSe (Cu, Ga) thin films are illuminated with light (200 nm ? λ ? 530 nm) and an electric field is simultaneously applied, whose value is above a definite threshold. These oscillations, with amplitude and frequency varying as a function of temperature, applied electric field and light intensity, are attributed to moving high-field domains similar to those found by Boer and co-workers on CdS single crystals. Field quenching of luminescence is observed, which visualizes moving high-field domains. Anode-adjacent high-field domains occur at applied voltages above the range at which moving domains are observed and a simultaneous electroluminescence is initiated near the anode.     

Anisotropy of the electrical conductivity of lithium heptagermanate crystals

The electrical conductivity σ of single crystals of lithium heptagermanate Li₂Ge₇O₁₅ is studied in an electric field in the frequency range 0.5–100 kHz at temperatures ranging from 300 to 700 K. Heating the crystal above 500 K gives rise to a pronounced anisotropy in the electrical conductivity, which differs in magnitude by one to two orders of magnitude for different directions of the measurement field along the crystallographic axes. It is shown that an increase in the electrical conductivity σ with increasing temperature originates from charge transfer with an activation energy U = 1.04 eV. It is assumed that the thermally activated contribution to the electrical conductivity is governed by transport of lithium interstitial ions along channels in the structure of the Li₂Ge₇O₁₅ compound.     

Influence of composition nonstoichiometry on the electrical conductivity of LiNaGe4O9 crystals

The electrical conductivity σ of Li_{2 ? x} Na _x Ge₄O₉ (x = 1, 0.5, 0.2) crystals in an alternating-current electric field has been investigated at a frequency of 1 kHz in the temperature range of 300–800 K. A considerable anisotropy of the electrical conductivity has been revealed for crystals with a sodium concentration x = 1 at T > 500 K. It has been shown that the electrical conductivity σ along certain crystallographic directions increases by more than three orders of magnitude with a change in the sodium concentration from x = 1 to x = 0.2. The results have been discussed taking into account the specific features of the structure of the crystals under investigation. Presumably, the major charge carriers are interstitial Li ions migrating along channels of the framework structure of the Li_{2 ? x} Na _x Ge₄O₉ crystals.     

Magnetic and electric properties of antimony selenide (Sb2Se3) crystals

The magnetic susceptibility in the three principal crystallographic directions, and the electrical conductivity and thermoelectric power parallel and perpendicular to the basal plane of Sb₂Se₃ single crystals, have been measured over the temperature range 100–550 K. The different results obtained from the magnetic and electric studies have been accounted for on the basis of non-stoichiometry of the compound and localized lone electrons. A hopping mechanism in the higher temperature region simultaneously with impurity excitation is suggested.     

Low-temperature thermal conductivity of terbium-gallium garnet

Thermal conductivity of paramagnetic Tb₃Ga₅O₁₂ (TbGG) terbium-gallium garnet single crystals is investigated at temperatures from 0.4 to 300 K in magnetic fields up to 3.25 T. A minimum is observed in the temperature dependence κ(T) of thermal conductivity at T _min = 0.52 K. This and other singularities on the κ(T) dependence are associated with scattering of phonons from terbium ions. The thermal conductivity at T = 5.1 K strongly depends on the magnetic field direction relative to the crystallographic axes of the crystal. Experimental data are considered using the Debye theory of thermal conductivity taking into account resonance scattering of phonons from Tb³⁺ ions. Analysis of the temperature and field dependences of the thermal conductivity indicates the existence of a strong spin-phonon interaction in TbGG. The low-temperature behavior of the thermal conductivity (field and angular dependences) is mainly determined by resonance scattering of phonons at the first quasi-doublet of the electron spectrum of Tb³⁺ ion.     

Conductivity anisotropy of a TlGaTe<Subscript>2</Subscript> chain single crystal under hydrostatic pressure

Pressure and temperature dependences of the conductivity anisotropy of TlGaTe₂ chain single crystals have been investigated. It has been found that the conductivity anisotropy of TlGaTe₂ can be controlled by choosing the values of temperature and pressure. The temperatures (216, 193, and 77 K) and corresponding pressures (0, 0.31, and 0.71 GPa) have been determined, at which the conductivity of the TlGaTe₂ single crystal becomes isotropic.     

Response of the capacitance and dielectric loss of the SrRuO<Subscript>3</Subscript>/SrTiO<Subscript>3</Subscript>/SrRuO<Subscript>3</Subscript> film heterostructures to variations in temperature and electric field

Three-layer epitaxial heterostructures with a 750-nm-thick intermediate strontium titanate layer between two strontium ruthenate conductive thin-film electrodes have been grown by laser deposition. Photolithography and ion etching have been used to form film parallel-plate capacitors based on the grown heterostructures. The capacitance (C) and dielectric loss tangent (tanδ) of the parallel-plate capacitors have been measured in the temperature range T = 4.2–300 K at an applied bias voltage of up to ±2.5 V and without it. At T > 100 K, the temperature dependence of the dielectric permittivity (ε) of the SrTiO₃ intermediate layer is well approximated by the Curie–Weiss law taking into account the capacitance induced by the penetration of an electric field into the oxide electrodes. At T ≈ 20 K, the dielectric permittivity ε of the SrTiO₃ intermediate layer decreases by approximately 20% in an electric field of 25 kV/cm. The dielectric loss tangent of the film capacitor heterostructures decreases monotonically with a decrease in the temperature in the range from 300 to 80 K and almost does not depend on the electric field strength. However, in the range from 80 to 4.2 K, the dielectric loss tangent increases nonmonotonically (abruptly) with a decrease in the temperature and decreases significantly in an applied electric field.     

准一维多链无序体系跳跃电导特性

在单电子紧束缚近似下,建立了准一维多链无序体系直流、交流电子跳跃输运模型,通过计算探讨了无序模式、维度效应、温度及外场对其直流、交流电导率的影响.计算结果表明:准一维多链无序体系的直流、交流电导率随着格点能量无序度的增大而减小,非对角无序具有增强体系电子输运能力的作用.随着链数的增加,体系的直流、交流电导率增大,但格点能量无序度较小时,维度效应的影响不明显.在对角无序情况下准一维多链无序体系的交流电导率随温度的升高而增大,而在非对角无序模式下却随温度的升高而减小,但对于直流情况,体系的直流电导率随温度的升     

Investigation of dc hopping conduction in TlGaS2 and TlInS2 single crystals

It is established that variable-range hopping conduction takes place between states localized near the Fermi level in layered TlGaS₂ and TlInS₂ single crystals both along and across their natural layers in a constant electric field at T⩽200 K. The densities of states near the Fermi level and the hopping distances at different temperature are estimated. The occurrence of activationless hopping conduction is established in TlGaS₂ and TlInS₂ single crystals in the temperature range 110–150 K. Fiz. Tverd. Tela (St. Petersburg) 40, 612–615 (April 1998)     

Ferroelectric phase transition in Ga2Te3 single crystals

Measurements of the electrical conductivity and Hall effect were carried out in a wide temperature range (200-500 K) for Ga₂Te₃ crystals. The crystals were grown in single crystalline form by making a modification of the travelling heater method technique. The measurements revealed unusual observations in the electric conductivity and Hall mobility indicating the presence of some type of phase transitions at about 430 K. So, ferroelectric behavior was examined for confirming the presence of second-order (ferroelectric) phase transition. An energy gap of 1.21 eV and depth of the impurity center of 0.11 eV were found.     

蓝青铜K0.3MoO3单晶Peierls相变研究

测量了蓝青铜K_0.3MoO₃单晶Ｒ-Ｔ曲线，发现曲线在２８０Ｋ左右有异常变化，计算得到１８０Ｋ以下的半导体能隙为１３２０Ｋ（０.１１ｅＶ）．液氮温度下测量了晶体的非线性导电性，得到电场阈值为０.1２９Ｖ／ｃｍ．样品ＤＳＣ研究表明，样品在２４０Ｋ处经历一新的Ｐｅｉｅｒｌｓ相变，且为一级相变，据此对相变的微观性质进行了定量计算．１８０Ｋ附近Δｃ_p-Ｔ曲线表明，１８０Ｋ处的相变为一个二级相变加一个一级相变 关键词：     

Electronic properties of C60 single crystals doped with lithium by electrodiffusion

Diffusion of lithium cations in C₆₀ single crystals driven by electric field has been detected and studied. A novel technique for fullerene crystal doping based on injection of ions through a “superionic crystal/C₆₀ single crystal” heterojunction has been suggested. It has been found that lithium doping of C₆₀ single crystals brings about an ESR signal, and this signal as a function of time has been investigated. The electronic conductivity in Li_xC₆₀ crystals has a nonmetallic nature. Reflection spectra measured in the IR band have shown that the reflectivity due to free electrons gradually decreases with time, which correlates with the evolution of signals due to ESR and microwave conductivity. Lithium doping of crystals increases the oscillator strength of the T _1u (4) vibrational mode and shifts it to lower frequencies (from 1429 cm⁻¹ to 1413 cm⁻¹), which indicates that one electron is present at the C₆₀ molecule, and this fact may be treated as evidence that the LiC₆₀ phase is generated in a C₆₀ crystal. Zh. éksp. Teor. Fiz. 116, 1706–1722 (November 1999)     

Anomalous increase in the unipolarity of doped lithium niobate crystals in the temperature range 300–400 K

The lithium niobate single crystals doped with B, Zn, and Gd at a content of 0.002–0.44 wt % have been grown. Their domain structure, static and dynamic piezoelectric properties, dielectric properties, and conductivity are investigated over a wide range of frequencies. The dielectric dispersion associated with the Debye-type relaxation process and considerable anomalies in ?′₂₂(T) and conductivity are revealed in the temperature range ～300–400 K. At these temperatures, the piezoelectric modulus d ₃₃ of the initial polydomain crystals LiNbO₃: Gd jumpwise increases up to the values close to those for the undoped single-domain crystal. This increase is accompanied by a substantial change in the etch patterns due to the domain structure of the crystal. The nature of the anomalies observed in LiNbO₃ in the above temperature range is discussed.     

The contribution of anion disorder to ionic conductivity on single crystals of β-PbF2

The effect of the different cooling processes on the disorder of flourine ions and ionic conductivity in β-PbF₂ has been studied by X-ray method and ionic conductivity measurements on single crystals below the transition temperature T_c. The spike-like diffuse scattering was observed along the <111>¹ directions around the Bragg reflections. The activation energies for the conduction process are 0.40 eV for the sample quenched from 970 K and 0.54 eV for the one from 720 K. The higher the quenching temperature is, the higher the conductivity and the lower the activation energy become. The dependence of conductivity on the different cooling processes is more evident in single crystals than in polycrystalline samples. The contribution of the different cooling processes to ionic conductivity can be quantitatively explained by the extent of ordering of mobile fluorine ions. Time dependence of ionic conductivity has not been observed.     

Isotope effect on the Peierls transition temperature of TTF-TCNQ

Conductivity measurements on isotopically substituted single crystals of TTF-TCNQ show a relatively large shift in the phase transition temperatures at 52.5 K (T₁) and 37.7 K (T₃). Deuteration of both TTF and TCNQ chains increases T₁ by 0.75 ± 0.15 K, N¹⁵ substitution increases T₁ by 0.6 ± 0.1 K while deuteration of the TTF chains apparently has no significant effect. Different possible interpretations of these results are briefly discussed.