Frozen photoconductivity in PbTe films

We have studied PbTe films of thicknessd=200/10000 A made with telluride vapour deposition on glass substrate at room temperature. The estimate of the donor concentration ～10¹⁹ cm^?3 of the fresh-deposited film compared with the impurity content in the bulk raw material ～10¹⁷ cm^?3 shows that the donors were mainly film defects or nonstoichiometric Pb atoms. Electrical conductivity of the freshly deposited film increased with lowering of the temperature. After deposition the donors were compensated with an oxidation in the laboratory air. Transition to the thermally activated conductivity resulted from oxidation. At temperatures belowT≈100 K the resistance of the compensated films followed Mott’s ruleR=R ₀ exp(T ₀/T)^1/3. The square film value 1 Mohm andT ₀≈100 K ford=1000 A. At low temperatures an exposure to light resulted in sharp decrease of the film resistance. At liquid helium temperatures the resistance dropped 10³–10⁶ times and stayed at the low value for an indeterminate time. The heating of the film aboveT=100 K gave rise to an initial high resistive state. The critical temperatureT _c, when the frozen photoconductivity became negligible, varied with samples in the temperature region 90–120 K. Near the critical temperature we could measure the time dependence of the film resistance after the light exposure, which followed the equationR=A+B.lnt fort>1 sec with the empirical constantsA andB. After a time intervalτ the resistance gained the initial “dark” value and remained stationary. The value lnτ≈α.(T _c?T), where the factorα approximately wasα≈0.5 K^?1. Some results of these experiments were published earlier (Krylov and Nadgorny 1982; Krylov and Pojarkov 1984).     

New results in the study of Brillouin scattering in thiourea

Below T₂ = 202 K, in the incommensurate phase, a Debye relaxation appears for c₃₃; it can be attributed to a linear coupling between an acoustical mode and a phason. The relaxation time is τ = τ₀/(T₀?T) with τ₀ = 6.2 x 10^?12 sec deg and T₀ = 200.9 K. The same phenomenon appears more weakly for c₁₁. The c₆₆ elastic constant has a double discontinuity around T₁ = 169 K; this shows that in the vicinity of T₁ there are two transitions, separated by a temperature interval of 3°.     

Influence of nonmagnetic impurities on the kondo effect

The influence of scattering by nonmagnetic impurities is studied in perturbation theory. While the finite lifetime of the electrons in intermediate states due to scattering by nonmagnetic impurities does not lead to a change in the logT-behaviour of the third-order self-energy, certain vertex-corrections give rise to an additional term which varies like 1/√T at low temperatures. Similar correction terms are found to occur in the higher order self-energy contributions. Although these terms diverge more strongly atT=0 than the logarithmic contributions they are quite small at finite temperatures since they depend on the lifetime τ of the electrons through a factor of (? _F τ)^?5/2 (? _F Fermi energy). The possibility of observing these interference effects experimentally is discussed.     

Exciton-magnon interactions in NicMg1−c O single crystals

The effect of chemical composition and temperature on exciton-magnon interactions in Ni_cMg_1?c O single crystals was studied using optical absorption spectra in the region of the magnetic-dipole ³ A _2g (G)→³ T _2g (F) and electric-dipole ³ A _2g (F)→¹ E _g (D) transitions. The two zero-phonon lines at ～7800 and ～7840 cm^?1 on the low-energy side of the magnetic-dipole transition band were assigned to a pure exciton transition and an exciton-one-magnon transition at the center of the Brillouin zone. The intensity of the exciton-one-magnon peak decreases rapidly with increasing magnesium ion concentration and/or temperature, to vanish altogether at T=6 K for c<0.95 and at T=130 K for c≥0.99. Thus, the contribution of long-wavelength magnons in optical absorption spectra of Ni_cMg_1?c O becomes negligible at temperatures substantially lower than the Néel point T _N (the antiferromagnetic ordering temperature). This observation can be explained as being due to a substantial decrease in the characteristic spin-spin interaction length with increasing concentration of the diamagnetic magnesium impurity ions (static disorder) and/or with increasing amplitude of thermal atomic vibrations (dynamic disorder). At the same time, the peak at ～14500 cm^?1, which lies in the electric-dipole transition region and corresponds to excitation of an exciton and two magnons at the Brillouin zone edge, remains visible up to the Néel temperature. This is accounted for by the short-wavelength magnons being sensitive to short-range magnetic order, which persists up to T _N .     

Transport properties of n-type ferromagnetic semiconductor HgCr2−xInxSe4(x = 0.100)

Measurements of the electrical conductivity, magnetoresistance, and Hall effect were performed on a n-type ferromagnetic semiconductor HgCr_2?xIn_xSe₄(x = 0.100) single crystal from 6.3 to 296 K in magnetic fields up to 1.19×l0⁶A/m. The conductivity decreases rapidly near the Curie temperatureT_c (≈120 K) as the temperature is raised. A large peak in the magnetoresistance is observed near T_c. The Hall effect measurements indicate that the temperature dependence of the conductivity and the magnetoresistance are due mostly to a change in electron mobility. The electron mobility is 1.2 × 10^?2 m²/V · s at 6.3 K, and decreases rapidly near T_c with the rise in temperature. Then it increases slowly from 5.5 × 10^?4 m²/V · s at 160 K to 7.5 × 10^?4 m²/V · s at 241 K. This temperature dependence of the electron mobility can be explained in terms of the spin-disorder scattering which takes into account the exchange interaction between charge carriers and localized magnetic moments.     

Spin correlations in Ni-Mn-Ga

Results of measuring small-angle neutron scattering and neutron depolarization in a Ni_49.1Mn_29.4Ga_21.5 single crystal in the temperature range 15<T<400 K and in the range of magnetic fields 0<H<4.5 kOe are presented. The characteristic temperatures of the alloy were found to be as follows: T _C=373.7 K and the martensite transition temperature T _m=301–310 K. The magnetic critical scattering at T _C and the scattering at T<T _C were adequately described by the relationship I _m=A(q ²+κ²)^?2 (q is the transferred wave vector and R _c=1/κ is the correlation radius), and the temperature dependences of the A and R _c scattering parameters were determined. Left-right asymmetry was observed at 150<T< T _m in the scattering of neutrons polarized along or opposite to the applied field. This asymmetry was due to the inelastic magnetic interaction of neutrons in the sample. The magnetization of the alloy at T _m, critical scattering at T?T _C, anomalies in scattering, and the softening of magnetic excitations at 150 <T<T _m are discussed.     

Cyclotron resonance study of conduction electrons in n-type indium antimonide under a strong magnetic field—I: Thermal equilibrium case

The far IR cyclotron resonance of conduction electrons is investigated in n-type indium antimonide in the quantum regimes, ?ω_ck_BT and ?ω_c?k_BT. The resonance peak position, half width, and the degree asymmetry in the line shape are studied as a function of temperature. In analyzing the experimental data, the three band model has been employed together with modern theoretical results of electron scattering by ionized impurities in the presence of a strong magnetic field. It is found that, for example for an experiment at 84 μm, the Une width depends very little on temperature between 4.2 and 45 K where the ionized impurity scattering is dominant, and increases rapidly with temperature above 45 K where the onset of phonon scattering is expected. Further details of the ionized impurity scattering were investigated by using three different laser wavelengths 84, 119 and 172μm. The line width at the phonon-limited temperature region depends very little on magnetic field and sample. The temperature dependence of the band gap was also determined by analysis of the resonance peak shift.     

ESR-Linienbreite der Ionen der Eisengruppe in Lösungen

Electron spin resonance linewidths of ions belonging to the first transition group with quenched orbital angular momentum are calculated using a modified relaxationmatrix theory including third and fourth order perturbation terms. We base our calculation on a Hamiltonian, which depends on electron spin, nuclear spin, orbital angular momentum, rotation of the whole complex, and vibration of the ligands. Quadrupol effects, intermolecular electron-electron and electron-nucleus interactions are neglected. The results show that the well-known formula of the transverse relaxation time derived by using the spin-Hamiltonian is correct, if first the contribution of the rotational spin orbit process is taken in consideration and second the rotational correlation timeτ _c is replaced byτ _v =(1/τ _c +1/τ(0))^?1. 1/τ(0) describes the linewidth of the lowest energy value of the electrostatic energy of unpaired electrons in the ligand field. The linewidth arises from the normal modes of the complex; the calculation gives τ(0)=l0^?11...10^?12 sec.     

Analysis of temperature and concentration dependence of flow stress in KCl–KBr single crystals with special reference to solute distribution

Available data on the temperature and concentration dependence of critical resolved shear stress (CRSS) of KCl–KBr solid-solution crystals containing 9, 17, 27 and 45?mol% KBr in the temperature range 77–230?K have been analyzed within the framework of the kink-pair nucleation model of plastic flow in solid- solution crystals. It is found that CRSS τ decreases with increasing temperature T in accordance with the model relation lnτ?=?A???BT, where A and B are positive constants. The CRSS τ at a given temperature depends on solute concentration c as τ?∝?c^p , where exponent p has a value between 0.33 and 0.57 as temperature T rises from 0 to 230?K. The model parameter W _o, i.e. binding energy between the edge-dislocation segment L _o involved in the unit activation process and the solute atoms close to it (T?→?0?K), which is inversely proportional to B, increases with solute concentration c monotonically as W _o?∝?c ^0.33 up to a critical value c _m?=?35?mol% KBr, which is in reasonable agreement with the model prediction W _o?∝?c ^0.25. However, W _o decreases with an increase in c beyond c _m, which indicates somewhat ordered distribution of solute in the host lattice of concentrated KCl–KBr solid solutions with c?>?c _m.     

Zero-bias tunneling anomaly in a two-dimensional electron system with disorder

The temperature dependence of a zero-bias anomaly in the tunneling conductance of an Al/δ-GaAs tunneling structure with a two-dimensional electron density in the δ-layer of 3.5 × 10¹² cm^?2 has been investigated. It has been shown that the respective drop Δρ(?, T) in the tunneling density of states ρ near the Fermi level E _F of the two-dimensional electron system depends logarithmically on the energy ? within the range of 2.7kT < |?| < ?/τ, where ? is measured with respect to E _F and τ is the momentum relaxation time of two-dimensional electrons. It has been found that the drop depth Δρ(0, T)/ρ is also proportional to ln(kT/?₀) in the temperature range T = 0.1–20 K and saturates below 0.1 K.     

The pressure dependence of the superconducting transition temperature of LaT4P12(T = Fe,Ru, Os)

The superconducting transition temperature, T_c, of the LaT₄P₁₂ compounds with T = Fe, Ru, or Os has been measured under hydrostatic pressure P up to 1.8 GPa. The T = Fe compound exhibits a substantial increase of T_c from T_c (P = 0) = 4.1 K at a rate (dT_c/dP)_P=0= +7.2 x 10^-1 K/GPa. In contrast, the Ru and Os compounds exhibit only weak decreases of T_c from T_c (P = 0) = 7.2 K and 1.8 K with (dT_c/dP)_P=0= -1.6 x 10^-1 K/GPa and -9.5 x 10^-2 K/GPa, respectively. An analysis of this strikingly divergent behavior of T_c(P) in terms of the structural characteristics of the RT₄X₁₂ class of compounds where R = rare earth element, T = Fe, Ru, or Os, and X = P, As, or Sb suggests that T_c(P) for these materials consists of two competing contributions: a depression of T_c due to the compression of the lattice (i.e., decrease in volume), and an enhancement of T_c due to the effect of pressure on La itself.     

Phase transitions in classical vector models

The equation of state obtained earlier /6/ for 3-dimensional Ising ferromagnets in the first order in$\text{1}\text{z}$ (z is the number of the nearest neighbours) is generalized with regard to the classical D-component vector model. The results obtained are valid in a wide temperature range |τ|?[z²(D+2)]^?1 (τ=(T_c-T)/T_c) and become exact in the limit D → ∞.     

Hall effect in α-RuCl3

The Hall effect measurements performed in the layer compound α-RuCl₃ show that, in the sheets perpendicular to the c-axis, μ(300 K) ? 0.2 cm²V sec^?1; the temperature dependence of μ appears to be μ(T) = μ₀T^-n, with n = 2.3 ± 0.1, in the 180–320 K range. The transport appears to be due to electrons that move with a band type of mechanism, the main scattering process being due to the homopolar high energy phonons which modulate the thickness of the layers.     

Thermal expansion op NbSe2 and TaS2

The lattice parameters a₀ and c₀ of the hexagonal 2H polytype of NbSe₂; have been measured over the temperature range 156–478 K for a₀, and 138–482 K for c₀. The lattice parameter c₀ of the hexagonal 2H polytype of TaS₂ has been measured between 151 and 472 K. The lattice parameters a₀ and c₀ for the octahedral 1T polytype of TaS₂ been measured between 165 and 488 K. Over these temperature ranges, the following average coefficients of thermal expansion have been measured; 2H-NbSe₂, 6.6 × 10^?6K^?1 along the a-axis, 19.9 × 10^?6 K^?1 along the c-axis; 2H-TaS₂, 15.6 × 10^?6 K^?1 along the c-axis; IT-TaS₂, 12.7 × 10^?6 K^?1 along the c-axis. The parameter c₀ of 1T-TaS₂ undergoes two transitions which may be explained in terms of charge density waves.     

Levelcrossing Experimente im Tm I-Spektrum

A semi phenomenological dynamical theory for the rotations of the BO₆ octahedra in ABO₃ perovskites is proposed, which accounts for the soft mode and the central peak forT≧T _c observed by neutron scattering in SrTiO₃. The neutron scattering cross section is discussed. The EPR line width is predicted to diverge as ?^?ν(1–2η) and as ?^?ν(2–2η) for three- and two dimensional correlations with ?=(T-T _c )/T _c . The theory is generalized to other structural transitions.     

Transverse Nernst-Ettingshausen effect, resonant scattering, and superconductivity in SnTe: In

The temperature dependences of the coefficient of the transverse Nernst-Ettingshausen effect in SnTe: In samples with different indium contents (1–16 at %) in the temperature range 100–300 K and the electrical resistivity at temperatures of 1.2–4.2 K and in magnetic fields of up to 10 kOe are investigated. The data obtained indicate the presence of resonant hole scattering into the band of quasi-local In impurity states in Sn_1?x In _x Te samples with In content x ≥ 0.05 and a superconducting transition with a critical temperature T _c ～ 1.5–2.2 K. The SnTe: In samples with the degree of filling of impurity states by electrons, which is close to 1/2, and the Fermi level ?_F pinned in the vicinity of the minimum energy dependence of the relaxation time τ(?) are characterized by inhomogeneities of a new type, i.e., inhomogeneities of the scattering parameter r = ?lnτ/?ln?| $ \varepsilon _F The temperature dependences of the coefficient of the transverse Nernst-Ettingshausen effect in SnTe: In samples with different indium contents (1–16 at %) in the temperature range 100–300 K and the electrical resistivity at temperatures of 1.2–4.2 K and in magnetic fields of up to 10 kOe are investigated. The data obtained indicate the presence of resonant hole scattering into the band of quasi-local In impurity states in Sn_1−x In _x Te samples with In content x ≥ 0.05 and a superconducting transition with a critical temperature T _c ∼ 1.5–2.2 K. The SnTe: In samples with the degree of filling of impurity states by electrons, which is close to 1/2, and the Fermi level ɛ_F pinned in the vicinity of the minimum energy dependence of the relaxation time τ(ɛ) are characterized by inhomogeneities of a new type, i.e., inhomogeneities of the scattering parameter r = ϖlnτ/∂lnɛ|, which are induced by fluctuations of the degree of filling of quasi-local states by electrons. Original Russian Text ? S.A. Nemov, V.I. Proshin, G.L. Tarantasov, R.V. Parfen’ev, D.V. Shamshur, A.V. Chernyaev, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 3, pp. 461–464.     

Spin-lattice relaxation of the Vk-center in LiF

The angular and temperature dependences of the spin-lattice relaxation (SLR) rate of V_k-centers in LiF doped with Mg or Ag have been investigated. In the temperature interval 4.2–100 K the results can be fitted by the formula $\text{τ}{}^{\mathrm{?1}}\text{= A(θ)T + Be}{}^{\mathrm{<mtext>?\Delta </mtext><mtext>T</mtext>}}$ with A(0°) = 0.11 sec^?1K^?1, A(90°) = 1.3 sec^?1K^?1, B = 3 × 10⁵sec^?1 and Δ = (175 ± 15)K.A mechanism for the SLR is considered, assuming the modulation of the hyperfine interaction by phononinduced transitions between the ground and excited states of the resonant molecular vibrations of the V_k-center. This mechanism is found to explain the value, the temperature dependence and the isotropy of τ^?1 in the interval T = 20–100 K.The one-phonon SLR mechanisms of the V_k-center in the T < 10 K region are discussed.     

Superconductivity in α- and ω-Zirconium under high pressure

The pressure dependence of the superconducting transition temperatureT _c (p) of α-Zr has been investigated in both solid and liquid pressure transmitting media. Up to about 45 kbardT _c /dp was measured to be + 3.5 × 10^?6 K/bar. Cold working at 4.2 K produced a strong irreversible effect onT _c . The superconductivity of the high pressure phase, ω-Zr, has been studied in its region of stability, i.e. above 60 kbar. For ω-Zr,dT _c /dp=+7.7 × 10^?6K/bar, andT _c (0)=0.72 K (by extrapolation).     

Stress-induced crossover phenomena in nickel above its Curie point

The temperature dependence of the anisotropic critical scattering of neutrons from a nickel single crystal was observed under the influence of elastic uniaxial mechanical stress of 140 g/mm². The temperature was varied between 10^-4 < τ = [T ? T_c(Δ)]/T_c(0) < 10^-2, and the momentum transfer between 1.4 × 10^-3 < $\text{q = (2π/λ) sin θ < 6.6 × 10}{}^{-3}\text{A}\text{?}{}^{\mathrm{-I}}$. The expected crossover from isotropic (Heisenberg) to anisotropic (XY) behaviour should occur between 10^-5 < τ < 10^-3, in good agreement with the experimental result.     

Anomalous temperature dependence of magnetic relaxation in YBa<Subscript>2</Subscript>Cu<Subscript>2</Subscript>O<Subscript>7 − δ</Subscript> oxygen-deficient single crystals

The dependence of the magnetization relaxation rate S = ?d lnM/dlnt on temperature T is measured in YBa₂Cu₃O_{7 ? δ} samples with various oxygen concentrations. It is found that the S(T) curve changes qualitatively when oxygen deficit δ exceeds the threshold value δ_th = 0.37. For δ < δ_th (T _c > 60 K, where T _c is the superconducting transition temperature), function S(T) has the well-known peak at T/T _c = 0.4. For δ > δ_th (at T _c < 51 K), this peak transforms into a plateau and a new sharp peak appears at T/T _c = 0.1. The threshold value δ_th of the oxygen deficit corresponds to the transition of the sample from the disordered state into the ordered state of oxygen vacancies. We consider the change in the shape of the S(T) curve as a macroscopic manifestation of this transition.