Carrier scattering mechanisms in P-type indium selenide

Carrier scattering mechanisms in p-indium selenide are investigated by means of the temperature dependence of the Hall mobility (160–500 K) and thermopower (200–300 K). An anomalous behaviour of the Hall voltage, which changes sign below 215 K, is interpreted through the existence of planar aggregates of ionized shallow donor impurities that create potential wells behaving as deep donors and in which a low concentration of two-dimensional free electrons can exist. This fact is taken into account through equations for two-carrier Hall effect. Holes are found to be predominantly scattered by anA ₁ homopolar phonon with an energy of 27.8 meV. From thermopower results, the density of states effective mass in the valence band is determined,m _dv ^* =(1.5±0.2)m ₀.     

Magnetic anomalies in Gd2PdSi3

The results of ac and dc magnetic susceptibility, thermopower and Hall effect measurements of a compound, Gd₂PdSi₃, establish that this compound orders magnetically below T_n = 20 K. Though the ordering appears to be of an antiferromagnetic-type, the paramagnetic Curie temperature is positive with the magnitude being nearly the same as that of T_N, suggestive of the existence of ferromagnetic correlations. The thermopower at 300 K is large, apparently due to Pd 4d electrons, decreasing monotonically with temperature. There is a change in the sign of Hall constant well below T_N Also considering the observation of Kondo-like characteristics above 21K earlier by us, the overall thermal, transport and magnetic behaviour of this compound is interesting.     

Transport properties in single phase superconductor Ba2YCu3O9-δ

The electrical resistivity and the thermopower are measured on the single phase superconductor Ba₂YCu₃O₉-δ (δ=2.1). The results indicate that the temperature dependences of the resistance and thermopower exhibit typical metallic behaviour, and the sample conducts via electrons at high temperatures. The behaviour of the thermopower can be described with Mott's semi-classical model. The specific heat of electrons in normal state has been estimated 780mJ/K·mole at 200K, i.e. γ=3.9mJ/K²·mole. Unusual phonon-drag effect is observed above the superconducting transition temperature T_c. Below T_c, the electrical resistivity and the thermopower all drop to zero corresponding to a superconducting ground state.     

Thermoelectric power of quasi-one-dimensional Nb3X4 with X=S, Se andTe

The thermoelectric power of linear chain synthetic metals Nb₃X₄ (with X=S, Se and Te) was measured from 5 to 300 K. The thermopower is negative indicating a dominant transport by electrons. Common to three compounds, in lower temperature regions the thermopower rises linearly with temperature but soon saturates. With respect to Nb₃S₄ and Nb₃Se₄ we have found no special anomaly of the thermopower except for a little higher magnitude.With respect to Nb₃Te₄ anomalies in the thermopower vs temperature appear at about 80 and 20 K which are explained in terms of the charge- density-wave phase transition from the simultaneous measurement of the resistivity and the observation of the electron diffraction patterns.     

Temperature dependence of thermoelectric power in charge density wave compound (TaSe4)2I

We have measured the thermoelectric power of quasi 1-D chain compound, (TaSe₄)₂I, in the temperature range 120–335 K. Below the CDW transition temperature (260 K) the thermopower is observed to be a linear function of reciprocal temperature consistent with semiconducting behaviour. The electron mobility is obtained to be 1.24 times the hole mobility suggesting that the majority carriers of heat and current transport in this compound are electrons. The results are qualitatively very similar to those of TaS₃ system.     

Thermoelectric power of mixed electronic-ionic conductors III. Case of calcium titanate

The primary purpose of the present work is the determination of the thermopower component related to electronic charge carriers for undoped calcium titanate. The second purpose of this work is establishment of the relationship between this thermopower component and the electronic component of electrical conductivity. An essential part of the present study includes the determination of the thermopower components corresponding to different charge carriers (electrons, electron holes and ions). The determination procedures are based on the following three models:

Symmetrical model. This model assumes consistency between thermopower and electrical conuctivity in terms of the n-p transition (this model assumes that minimum of electrical conductivity corresponds to the electronic component of the thermopower equal zero). It was shown that this model does not apply for CaTiO₃.

The Heikes model. This model is based on Heikes formula and also hopping mechanism of the transport of electrons. It was shown that thermopower of CaTiO₃ cannot be described by this model and, consequently, thermopower vs. electrical conductivity cannot be considered within the Jonker formalism.

General model. This model is based on a general thermopower equation for mixed conductors without any simplifying assumptions. Application of this model indicates that the electronic component of thermopower is not consistent with the minimum of electrical conductivity.

     

新型超导体MgB2的热电势和电阻率研究

测量了MgB₂的热电势和电阻率与温度的依赖关系.在100K—300K区间,热电势呈近似线性温度依赖关系,其斜率为正,表明载流子为空穴型且与能带贡献的图像相一致.与此对应,在此温区电阻率呈T²依赖关系.在100K以下,热电势和电阻率各自转变了其高温区的温度依赖关系.热电势在超导转变温度T_c(零电阻366K)到100K间有一宽峰,具有声子曳引峰的特征,表明电子-声子相互作用很强.估算了一些重要的参数,如带米能E_F、能带宽度 关键词： 新型超导体 热电势 电阻率     

Suppression of charge ordering and thermal hysteresis of electronic transport and magnetisation in La0.5Ca0.5Mn1−xNixO3

A series of polycrystalline La_0.5Ca_0.5Mn_1?xNi_xO₃ (x = 0.00, 0.025, 0.050, 0.075, 0.100 and 0.125) was synthesised using solid state reaction. Measurements in a cooling and warming cycle between 300 and 80 K were carried out to study the Ni-doping effects on the electrical resistivity, thermopower and magnetisation of single-phase La_0.5Ca_0.5Mn_1?xNi_xO₃. Partial substitution of Ni for Mn leads to the suppression of charge ordering state, the evidence of which is shown by the dramatic decrease in electrical resistivity and thermal hysteresis width in electrical resistivity, thermopower and magnetisation. However, the magnitude of both electrical resistivity and thermopower increases with increasing Ni content. This can be attributed to an increase in the Mn⁴⁺ concentration, which favours the antiferromagnetic state and leads to a gradual disappearance of ferromagnetic double exchange interaction. Besides, the metal–nonmetal transition temperature decreases with increasing Ni content until x = 0.075, which might arise from increased electron–phonon coupling due to less ordered spins at temperatures above ferromagnetic transition. For samples with x greater than 0.075, no metal–nonmetal transition is observed due to the suppression of double exchange mechanism.     

Spin-polarized transport and thermopower of organometallic nanocontacts

We present a theoretical study of the length dependence of both conductance and thermopower of organometallic vanadium-benzene molecules (V _nBz_n+1) sandwiched between magnetic Co(100) electrodes. We show that the molecules with n≥3 are efficient spin filters. Namely, we find that the zero bias conductance of the majority electrons is small and decays exponentially with increasing length of the molecule and is in the tunneling regime while the minority electrons show metallic conductance. We show furthermore that the thermopower strongly depends on the length of the molecules and can even change sign as a function of length and temperature.     

Thermoelectric power of TmS and TmSe

The thermopower has been measured from 1.6 to 300 K for TmS and from 0.065 to 300 K for TmSe. The experimental results suggest a dominant scattering of the conduction electrons by heavy 4? electrons with a strongly temperature-dependent density of states at the Fermi level.     

Thermokräfte abschreckend kondensierter Metallschichten

For the first time it is reported on the annealing behaviour of the thermopower of quenched condensed metal films, which are evaporated in vacuum at liquid helium temperatures. The measurements concern Pb- and Sn-films, Au- and Cu-films as well as Au-films with small amounts of Fe, and Bi-films between 2 and 300 °K. It is shown that in all quenched films the phonon drag part of thermopower S_g is completely suppressed. In Pb and Sn the part of thermopowerS _e caused by electron diffusion is not changed remarkably by quenching, whereas in Au- and Cu-films it is much greater than in bulk material up to 250 °K. Annealing at higher temperatures leads to an irreversible decrease ofS _e. The well known giant thermopower due to Fe-impurities in gold is very much reduced by quenching. The characteristic minima appear again after annealing of these films. Amorphous Bi-films have thermopowers which are comparable with those of Pb and Sn. After crystallisation the negative thermopower is much greater and increases with increasing temperature of annealing.     

Sr2RuO4的热电势

Sr2RuO4是第一个无CuO面的层状强关联氧化物超导体.测量了9至260K温度范围内Sr2RuO4的热电势,观测到在此温度范围内其热电势为正值.用两种载流子模型对实验数据进行了拟合,并且与Hall系数的实验结果进行了比较,发现低温下两种载流子对热电势和Hall系数的贡献比较类似,但在高温区空穴对热电势的贡献很大而相应地对Hall系数的贡献不占主要地位. 关键词： Sr2RuO4 热电势     

Thermoelectric power of mixed electronic-ionic conductors II. Case of titanium dioxide

The purpose of the present work is the determination of the thermopower components corresponding to different charge carriers (electrons, electron holes and ions) for TiO₂ and the use of these data for evaluation of the effect of symmetry between these two properties. The procedure of the determination of these components was based on the following two approximations:

The first approximation is based on a symmetrical model assuming a consistency between thermopower and electrical conductivity within the n-p transition (minimum of electronic component of the electrical conductivity corresponds to zero value of the electronic component of thermopower).

The second approximation is based on the apparent asymmetry between thermopower and electrical conductivity within the n-p transition as determined from the first approximation.

The analysis, based on the data of the electronic components of thermopower and electrical conductivity for TiO₂ single crystal, results in the band gap (using the Jonker formalism). The determined band gap is equal to 2.77 eV and 2.57 eV at the first and the second approximations, respectively, while the band gap determined from the experimentally measured data is equal 3.35 eV. These values are consistent with the band gap determined from the data of electrical conductivity corresponding to the n-p transition point (E_g=3.16 eV) and for the data measured experimentally and those free of the ionic conductivity component (E_g=2.79 eV). The obtained results indicate that thermopower and electrical conductivity most likely exhibit the effect of symmetry.     

Thermopower of a moderate heavy-fermion compound YbZnCu4

The thermopower coefficients S of samples of a moderate heavy-fermion compound YbZnCu₄ and metallic LuZnCu₄ are measured in the temperature range 5–300 K. Data on the temperature dependence of the thermopower coefficient S of YbZnCu₄ suggest that this material is a heavy-fermion compound with a Kondo temperature of ～50 K.     

Transport effects in single-crystal La0.82Ca0.18MnO3

This paper reports experimental data on the temperature dependences of the electrical resistivity, magnetoresistance, thermopower, magnetothermopower, and normal and spontaneous Hall coefficients of the La_0.82Ca_0.18MnO₃ single crystal with a Curie temperature of 180 K. It is shown that, at low temperatures, electrons are the majority carriers. For T < 110 K, the electrical resistivity depends substantially on the position of the magnetization vector with respect to the crystallographic axes, which implies a significant role played by the spin-orbit interaction. For T > 137 K, holes are dominant. In the vicinity of the Curie temperature, electrical conduction is effected primarily by holes activated to the mobility edge. The local activation energy of the resistivity exhibits a critical behavior. The temperature dependence of the local activation energy is determined by spin correlation functions. For T > 240 K, the activation energy does not depend on temperature.     

Role of phonon dynamics in the thermopower of niobium hydrides

The thermopower of NbH_x alloys for x varying from 0.607 to 0.883 has been examined at temperatures ranging from 100 to 430 K. It was found that the transition from orthorhombic β to cubic α' phase is accompanied by a strong change of thermopower. It is concluded that differences in phonon spectra of these two phases are responsible for the observed differences in the transport properties. The sensitivity of the thermopower to any other phase transitions in this system has been observed as well.     

The thermopower of amorphous CuTi-alloys as a function of concentration and temperature

We report on thermopower measurements of several Cu_100–x Ti _x -alloys (x=30, 35, 40, 45, 50, 55, 60 and 65) in the temperature range from 4.4 to 320 K. In all cases the thermopower is positive. It shows a strong variation with concentration. An analysis of the temperature dependence is presented at low and high temperatures. The correlation of the thermopower with the pressure dependence of the electrical resistivity and the Hall-coefficient is discussed and compared with theoretical predictions.We wish to dedicate this article to the memory of A.B. Bhatia who died on September 27, 1984     

Peak in the magnetic-field dependence of diffusion-induced thermopower for n-Bi-Sb semiconducting alloys

A peak is detected on the dependence of the diffusion-induced thermopower on transverse magnetic field in degenerate semiconducting alloys n-Bi_1?x Sb_x (0.07≤x≤0.15) doped with tellurium donor impurity. The temperature gradient is directed along the bisector axis C ₁ of the monocrystalline sample and the magnetic field is along the triad axis C ₃. The electron spectrum of the Bi-Sb alloys under investigation consists of three equivalent ellipsoids with distinctly different effective masses along the axes of the ellipsoid (m _∥/m _⊥). A simple kinetic theory shows that the presence of the peak on the diffusion thermopower is a manifestation of this strong anisotropy in the electron spectrum and of the additive contribution of all three ellipsoids to electron transport. The nonmonotonic dependence of thermopower on the transverse magnetic field makes it possible to determine the electron relaxation time, while the temperature dependence of this relaxation time can be used to separate the relaxation time for electrons scattered from ionized impurities and from acoustic phonons.     

The transport properties in Nd0.75Sr1.25CoO4 film: Evidence for polaronic transport

The experiments of electrical resistivity and thermopower on Nd_0.75Sr_1.25CoO₄ film in the temperature range 90 K<T<310 K were carried out. The great difference in the activation energies estimated from thermopower and resistivity, a characteristic of small polarons, is observed, providing strong evidence for polaron-dominated transport mechanism in this material. Furthermore, the activation energy at intermediate-temperature region is larger than that at low-temperature region in resistivity, but it is not observed in thermopower, indicating that the energy for the creation of the carriers is slightly lower at low-temperature region than that at intermediate-temperature region. At the same time, the abrupt drop in the thermopower and the abnormal peak in the differential curve of resistivity indicate that a phase transition between a paramagnetic state and a ferromagnetic state occurs at temperature about 218 K. The positive thermopower in the whole temperature range measured suggests that the carriers are holes in this system.     

Phonon drag thermopower in doped bismuth

The low-temperature (2<T<80 K) thermopower in bismuth doped by tellurium, a donor impurity (0<c≤0.07 at. % Te), is dominated by the phonon component, which shifts to higher temperatures with increasing dopant concentration. The temperature and concentration dependences of the phonon thermopower of doped bismuth are satisfactorily described by the theory of phonon drag of electrons. The theory is developed for a strongly anisotropic electron spectrum and includes both direct and two-step phonon drag.