Electrical properties of the high-pressure phases of gallium and indium tellurides

A study has been made of the resistance ρ, the thermopower S, and magnetoresistance MR of Ga₂Te₃ and α-In₂Te₃ single crystals at pressures P up to 25 GPa. It is found that the resistance ρ and |S| sharply decrease at ∼0–5 and 1.5–3 GPa, respectively. The semiconductor-metal phase transitions in the temperature range from 77 to 300 K are established from the sign reversal of the temperature coefficient of ρ to occur at P>4.4 and >1.9 GPa. The values S ≈+(10–20)μ V/K for the metallic phases with a Bi₂Te₃-type structure agree with those for liquid In₂Te₃ and Ga₂Te₃. Negative MR is revealed in In₂Te₃ at P≈1.9 GPa. No MR is observed in Ga₂Te₃ up to 25 GPa. The variation of the electronic structure of In₂Te₃ and Ga₂Te₃ under pressure is discussed. __________ Translated from Fizika Tverdogo Tela, Vol. 42, No. 6, 2000, pp. 1004–1008. Original Russian Text Copyright ? 2000 by Shchennikov, Savchenko, Popova.     

Galvanomagnetic properties of La0.7Sr0.3Mn0.9Cu0.1O3

La_0.7Sr_0.3Mn_0.9Cu_0.1O₃ ceramic samples have been obtained by the conventional method of the solid-phase reaction, and their resistivity ρ has been investigated in a temperature range from 50 to 300 K in magnetic fields B = 0–20 T. Dependences are typical of perovskite manganites with a maximum at T _max = 140–150 K and an increase in ρ near T _max with increasing external magnetic field B. It has been established that the behavior of resistivity is caused by the variable range hopping conduction mechanism ρ(T) = ρ₀(T)exp[(T ₀/T)^1/4], where ρ₀(T) ～ T ^25/4. The Mott variable range hopping conduction has been observed below the Curie temperature for La_0.7Sr_0.3Mn_0.9Cu_0.1O₃ samples (T _C ～ 300 K) in a temperature range from 300 to 200 K. The influence of Cu doping on the properties of La_0.7Sr_0.3MnO₃ samples is apparently caused by an additional distortion introduced into the crystal lattice of the material and by a weakening of the double-exchange mechanism.     

Observation of a transition from BCS to HTSC-like superconductivity in Ba1 − x KxBiO3 single crystals

The temperature dependences of the upper critical field B _c2(T) and surface impedance Z(T) = R(T) + iX(T) have been measured in Ba_{1 ? x} K_xBiO₃ single crystals with transition temperatures 6 ≤ T _c ≤ 32 K (0.6 > x > 0.4). A transition from the BCS to an unusual type of superconductivity has been revealed: B _c2(T) curves of the crystals with T _c > 20 K have positive curvature (as in some HTSCs), and those of the crystals with T _c < 15 K described by the usual Werthamer-Helfand-Hohenberg (WHH) formula. The R(T) and X(T) dependences of the crystals with T _c ≈ 32 K and T _c ≈ 11 K in the temperature range T ? T _c are linear (as in HTSCs) and exponential (BCS), respectively. The experimental results are discussed using the extended saddle point model by Abrikosov.     

Effect of lateral tensile stresses on the low-temperature electrical resistivity and magnetoresistance of La0.67Ca0.33MnO3 nanofilms

The thickness of the buffer layer of strontium titanate introduced between an La_0.67Ca_0.33MnO₃ manganite film and a (001)La_0.29Sr_0.71Al_0.65Ta_0.35O₃ substrate is varied (d ₁ = 7–70 nm) to influence effective misfit m in their lattice parameters. As m increases, electrical resistivity ρ of the film increases sharply and the maximum in the ρ(T) dependence shifts toward low temperatures. At T < 150 K, the temperature dependence of ρ of the manganite film obeys the relationship ρ = ρ₁ + ρ₂ T ^4.5, where parameter ρ₁ is independent of the temperature and magnetic field. Coefficient ρ₂ decreases with increasing magnetic field and increases with the misfit between the lattice parameters of the film and substrate, i.e., when the effective hole concentration in the manganite layer decreases.     

Low-Temperature Transport Properties of Lanthanum Hexaboride La<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Ce<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>B<Subscript>6</Subscript> Single Crystals

Resistivity (ρ), thermal conductivity (k) and Seebeck coefficient (S) of La_1–xCe_xB₆ single crystals with various concentrations of cerium Ce ions was measured in a wide temperature range 3?300 K. The obtained data were analyzed in the framework of the Coqblin–Shrieffer model. The contributions of scattering of carriers on magnetic ions Ce for all transport parameters ρ(T), k(T), S(T) are revealed. Strong dependence of the magnetic scattering on concentration of the cerium ions are identified. The anomalous behavior of the transport parameters ρ(T), k(T), S(T) in the region near 30 K is attributed to the Δ ~ 30 K splitting of Г₈ level.     

High-pressure synthesis of superconducting Nb1−xB2 (x=0–0.48) with the maximum Tc=9.2 K

Superconductivity with T_c above 9 K was found in metal-deficient NbB₂ prepared under 5 GPa, while no clear superconductivity was observed down to 3 K in stoichiometric NbB₂. The superconductivity was observed above x=0.04 in Nb_1−xB₂, and the lattice parameters also changed abruptly at x=0.04. As x increased, the transition temperature T_c slightly rose and fell with the maximum value of 9.2 K at x=0.24 for the samples sintered at 5 GPa and 1200 °C. The T_c-value changed in the range from 7 to 9 K, depending on the sintering pressure. A series of Ta_1−xB₂ (0⩽x⩽0.24) was also synthesized under high pressure to examine a special effect of high-pressure synthesis.     

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).     

Magnetic ordering in Mg−Zn ferrites

The magnetic ordering of a series of magnesium-zinc ferrite, Zn_0.3Mg _x Fe_2.7?x O_4±δ (0.5≤x≤1.1; 0≤δ≤0.2) has been investigated using Mössbauer measurements in the temperature range 295–620 K. The samples were found to be magnetic at room temperature with a hyperfine field at each site which increases with iron content. The Curie temperature was also observed to increase in a similar manner. The slope of this increase forB _hf andT _c is steeper forx≤0.6 thanx≥0.7. It has also been observed that Mg²⁺ substitution by Zn²⁺ in MgFe₂O₄ affects the magnetic ordering and the internal hyperfine field. The Curie temperature decreases by ～200 K andB _hf by ～20%.     

Electrical resistivity and magnetoresistance of nonuniformly mechanically strained films (30 nm) of the La0.67Ca0.33MnO3 manganite

The structure, electrical resistivity, and magnetoresistance of predominantly oriented La_0.67Ca_0.33MnO₃(30 nm)/LaAlO₃ films are investigated after partial relaxation of biaxial mechanical stresses. The negative magnetoresistance MR of the films reaches a maximum at T = 235–240 K. The full width at half-maximum of the peak in the curve MR(T) for a film is five to six times greater than that for a manganite layer grown on a substrate with a small lattice mismatch. For T < 150 K, the temperature dependence of the electrical resistivity ρ of the films is fitted well by the relationship ρ = ρ₀ + ρ₁ (H)T ^4.5, where ρ₀ ≡ ρ(T = 4.2 K) and ρ₁(H) is a parameter that is independent of temperature but dependent on the magnetic field H. The parameter ρ₁(H = 0) for the La_0.67Ca_0.33MnO₃(30 nm)/LaAlO₃ films is several times larger than that for thin manganite layers only weakly strained by the substrate. The electrical resistivity ρ₁ decreases almost linear as the quantity μ₀ H increases in the field range 1–5 T.     

Ferromagnetism in the high-pressure phases of (GaSb)1−x Mnx

The electrophysical and magnetic properties of recently discovered high-pressure phases in the GaSb-Mn system with simple cubic and tetragonal structures have been examined. It has been shown that samples with the primitive cubic structure for low temperatures are in the ferromagnetic state and the Curic temperature depends on the initial manganese content and reaches T _c = 280 K for x = 0.6. It has been shown that these samples for a manganese content x ≤ 0.5 are in the semiconducting state with large impurity conduction and pass to the metallic state as x increases. The GaSbMn phase with the tetragonal structure has ferromagnetic properties up to temperatures of T ～450 K (at which the phase begins to decay) and exhibits metallic properties. The magnetization at T = 77.3 K is equal to M = 0.58 μ_B and 0.28 μ_B per manganese ion for the simple cubic and tetragonal phases, respectively.     

Electrical properties of (PbS)0.59TiS2 crystals at high pressure up to 20 GPa

The electrical resistance ρ and thermopower S of the (PbS)_0.59TiS₂ single-crystal compound with mismatched layers and the TiS₂ crystals have been investigated at room temperature in high-pressure chambers with synthetic diamond dies. The decrease in ρ and |S| observed in (PbS)_0.59TiS₂ under the pressure P≈2 GPa is associated with the structural transformation of PbS from the cubic phase into the orthorhombic phase. The jumps of ρ and |S| are presumably caused by the increase in the electron concentration in the TiS₂ layers. For P≥4 GPa, at which the gap is absent in the electronic spectrum of TiS₂, a decrease in ρ(P) is observed for the (PbS)_0.59TiS₂ samples.     

Correlation between the magnetic and electrical properties of the (VS)x(Fe2O3)2−x oxysulfide system

The correlation between the magnetic and electrical properties of the (VS)_x(Fe₂O₃)_2?x (0.9<x<1.25) oxysulfide solid solutions has been studied. The crossover of conductivity from the semimetallic to semiconducting type is accompanied by changes in the magnetic susceptibility, which are characteristic of the transition from delocalized to localized electrons. For x=1.25, a region of the ferromagnetic ordering has been established in the temperature range 90–120 K.     

非晶态FeZrB合金的电阻率与温度的关系

本文研究了非晶态(Fe_1-xZr_x)_84.5B_15.5(x=0,0.02,0.04,0.06,0.08,0.1,0.15)和Fe_90-xB_xZr₁₀(x=0,4,10,16,20)合金的电阻率ρ与温度T的关系。实验结果表明,当Zr含量在0.02≤x≤0.08时,ρ-T曲线出现两个线性斜率,在略高于居里温度T_c处出现转折,在T关键词：     

Thermal conductivity of a moderate heavy-fermion compound YbZnCu4

The thermal conductivity κ and electrical resistivity ρ of a cast polycrystalline sample of YbZnCu₄, which belongs to the class of moderately heavy-fermion compounds, are measured and studied in the temperature range 5–300 K. It is shown that the phonon thermal conductivity of the sample follows an amorphous-like pattern throughout the temperature range under investigation, which should be assigned to the presence of Yb ions with a homogeneous mixed valence in this compound. The temperature dependence ρ(T) has two specific portions: a high-temperature portion (T > 220 K) characteristic of conventional metals and a moderate-temperature portion (14–35 K) typical of Kondo compounds.     

Hall and transverse even effects in the vicinity of a quantum critical point in Tm1 ? x Yb x B12

The angular, temperature, and magnetic field dependences of the resistance recorded in the Hall effect geometry are studied for the rare-earth dodecaboride Tm_{1 ? x} Yb _x B₁₂ solid solutions where the metal-insulator and antiferromagnetic-paramagnetic phase transitions are observed in the vicinity of the quantum critical point x _c ?? 0.3. The measurements performed on high-quality single crystals in the temperature range 1.9?C300 K for the first time have revealed the appearance of the second harmonic contribution, a transverse even effect in these fcc compounds near the quantum critical point. This contribution a is found to increase drastically both under the Tm-to-ytterbium substitution in the range x > x _c and with an increase in the external magnetic field. Moreover, as the Yb concentration x increases, a negative peak of a significant amplitude appears on the temperature dependences of the Hall coefficient R _H(T) for the Tm^{1 ? x} Yb _x B₁₂ compounds, in contrast to the invariable behavior R _H(T) ?? const found for TmB₁₂. The complicated activation-type behavior of the Hall coefficient is observed at intermediate temperatures for x ?? 0.5 with activation energies E _g /k _B ?? 200 K and E _a/k _B = 55?C75 K, and the sign inversion of R _H(T) is detected at liquid-helium temperatures in the coherent regime. Renormalization effects in the electron density of states induced by variation of the Yb concentration are analyzed. The anomalies of the charge transport in Tm_{1 ? x} Yb _x B₁₂ solid solutions in various regimes (charge gap formation, intra-gap many-body resonance, and coherent regime) are discussed in detail and the results are interpreted in terms of the electron phase separation effects in combination with the formation of nanosize clusters of rare earth ions in the cage-glass state of the studied dodecaborides. The data obtained allow concluding that the emergence of Yb-Yb dimers in the Tm_{1 ? x} Yb _x B₁₂ cage-glass matrix is the origin of the metal-insulator transition observed in the achetypal strongly correlated electron system of YbB₁₂.     

Interface phenomena and microwave magnetoresistance in polycrystalline La1−x CaxMnO3 lanthanum manganites

A study is reported of the dc and 9.2-GHz electrical and magnetoresistance (ρ₀, MR ₀, ρ_mw, and MR _mw ) in La_1?x Ca_xMnO₃ polycrystals (x=0–0.3) in the 77–300 K temperature interval. The microwave magnetoresistance exhibits a sharp peak within a narrow interval near the Curie temperature T _C , while MR _mw ≈0 is far from T _C . The microwave absorption in low magnetic fields is shown to be due to a variation of the microwave magnetic, rather than electrical losses. The peaks of ρ₀, ρ_mw, MR ₀, and MR _mw do not coincide in temperature. The specific features in the behavior of ρ₀, MR ₀, ρ_mw, and MR _mw are explained as being due to an inhomogeneity of the grains, which generates in the grain close to the interface a magnetic-field-dependent contact potential difference. The origin of the inhomogeneities can either be traced to the formation on a grain of a surface layer with properties differing from those in the grain bulk, or understood in terms of the model postulating grain separation into a conducting and a nonconducting phase.     

Bl结构MoNx薄膜的制备及物理性能研究

我们对以反应性溅射法制备的MoN_x薄膜测量了超导转变温度T_c,电阻率ρ(T)(从T_c起始到300K)。用X射线衍射技术、卢瑟福背散射(RBS)、俄歇谱仪和X射线光电子能谱(XPS)技术对这些样品进行了分析。实验结果表明T_c和ρ(T)随N含量改变而变化。当样品是B1结构时,T_c小于4.2K,而且样品内还有过量的N存在。俄歇分析表明,样品内有O,C杂质存在。这些因素都可能导致T_c很低,ρ(T)呈负的温度系数。 关键词：     

Peculiarities in the electron spectrum of superconducting laves phase ZrxHf1−xV2

Structurally unstable superconducting Laves phases Zr_xHf_1?xV₂ were investigated in the temperature range 4.2 – 300 K for temperature dependence of magnetic susceptibilities χ, resistivity ρ, and X-ray fluorescent emission 2K-spectra of vanadium. Anomalous dependences χ and ρ as compared to common metals was discovered below 150 K. This may be explained in terms of a phase transition of Peierl's type when the lattice instability results from the instability of the electron spectrum due to the dielectric slit that appears on flat parts of the Fermi surface. The presence of regions with localised electron density in the Laves phases on the low-energy side of the vanadium emission spectrum at low temperatures (T=10K) indicates an essential rebuilding in the electron spectrum of valence electrons.     

Structural and electrical transport properties of FeH x under high pressures and low temperatures

We present the first successful in situ simultaneous measurement of the electrical resistance and X-ray diffraction of FeH _x (x～ 1) under high-pressure H₂ up to 25.5 GPa and low temperatures down to 9 K. The electrical resistivity ρ showed a sharp increase with the formation of iron-hydride FeH _x (x～ 1) at 3.5 GPa. The ?′-phase of FeH _x was found to be metallic up to 25.5 GPa. The ρ vs. T curves up to 16.5 GPa approximately follow Fermi-liquid law below 25 K. However, T ⁵ was found to be better fitting at 25.5 GPa. This change can be considered to be related to the previously reported ferromagnetism collapse at corresponding pressure.     

Electrical properties of zinc oxide under uniform pressure up to 25 GPa

The dependences of the Hall coefficient R _H (P) and resistivity ρ(P) of bulk n-ZnO crystals on hydrostatic pressure up to 8 GPa and quasi-hydrostatic pressure up to 25 GPa at T = 300 K have been measured. With an increase in pressure up to the polymorphic transition P _pt ? 9 GPa, an exponential increase in R(P) and ρ(P) is observed, which is caused by the increase in the ionization energy of the shallow-energy donor center. At P > P _pt, the resistivity decreases by several orders of magnitude.