Transport properties in the pseudobinary (Gd,Y) Al2 series

The temperature dependence of the electrical resistivity, the thermal conductivity and the thermopower of the cubic isostructural (Gd_xY_1–x)Al₂ series will be presented. The magnetic properties of this system are characterized by the existence of ferromagnetism for Gd concentrations x>0.3 while for low Gd contents cluster and spinglass behaviour is observed. The spin dependent scattering contribution to the transport phenomena has been obtained by comparing the experimental data of the magnetic compounds with those of the isostructural nonmagnetic YAl₂. For the ferromagnetic concentration range and forT>T _c (T _c =Curie temperature) we revealed a temperature independent contribution to the electrical resistivity, a contribution with a temperature variation of 1/T to the thermal resistivity and a linear temperature dependence of this part to the thermopower. These results are in good agreement with the temperature dependence calculated by solving the linearized Boltzmann equation for this type of scattering processes.     

Resistivity,thermopower and single-particle tunneling studies on some zinc-doped yttrium barium copper oxide superconductors

Abstract

Resistance and thermopower measurements have been made on a series of compounds, YBa₂Cu₃- _x Zn _x O_7-y, with x = 0.025, 0.05, 0.1, 0.15 and 0.2. The superconducting transition temperature decreases as the zinc concentration increases. In a range of temperatures below T_M , the mid point of the transition, the resistance shows an exponential temperature dependence fitting the phenomenological formula proposed by Ausloos et al. From the plot of logarithm of resistivity vs. (T_M ? T) ^1/2/T, one deduces a value of the average dimension of the Josephson junction to be a few tens of Å, suggesting the microtwin boundaries to be the location of the junctions. The thermopower shows a peak always just above T_c . This conclusively shows that phonon drag is not the cause of the peak. The temperature dependence of the thermopower appears to resemble closely the earlier observations of Srinivasan et al. on yttrium barium copper oxide. Single-particle tunneling measurements carried out for two concentrations, x = 0 and 0.05, appear to indicate that the energy gap parameter scales with T_c , and 2Δ/kT_c has an approximate value of 5.5.     

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.     

Non-fermi liquid transport in <Emphasis Type="Italic">R</Emphasis>Co<Subscript>2</Subscript>-based alloys

Results on the experimentally investigated dependence of the electrical resistivity of quasibinary alloys R _x Y_1−x Co₂, where R stands for magnetic rare earth elements, on temperature (2–300 K), composition (x = 0–1), and magnetic field (0–15 T) are presented. Non-Fermi liquid behavior of electrical resistivity in a magnetic field was observed in paramagnetic Gd _x Y_1−x Co₂ alloys. The relative magneto-thermopower of these alloys exhibits diverging behavior with decreasing temperature, indicating anomalous temperature variation of the thermopower in the non-Fermi liquid regime.     

Thermopower of Al1−x Six solid solutions in vicinity of lattice instability

The thermopower coefficient as a function of temperature, S(T), has been measured in nonequilibrium superconductors, such as Al_1−x Six substitutional solid solutions and Al-Si alloys on various decay stages. When aluminum is substituted with silicon, the contribution to the thermopower due to phonon-drag effects, which are dominant in pure aluminum at low temperatures, is suppressed, and low-temperature anomalies in S(T) detected in compositions near lattice instability limit are determined by the diffusion component of the thermopower. The low-temperature anomalies in the thermopower and the notable increase in the coefficient in front of the linear term in S(T) are attributed to effects of thermopower renormalization due to the electron-phonon interaction enhancement with “soft modes” in the face-centered cubic (FCC) lattice of Al_1−x Six solid solutions. The nature of these anomalies in S(T) is analyzed in terms of the Kaiser and Reizer-Sergeev models. Zh. éksp. Teor. Fiz. 113, 339–351 (January 1998)     

LixNayCoO2的制备和热电性质

用固相反应方法，成功地合成了具有交互生长结构的Li_xNa_yCoO₂单相多晶系列样品. 热电势测量表明，该系统的室温热电势比Na_xCoO₂体系约高出60μV/K. 基于空穴载流子具有O2p特征，提出Li_xNa_yCoO₂中额外的热电势来源于氧空穴的占位熵. 关键词： xNa_yCoO₂')" href="#">Li_xNa_yCoO₂ 热电势 热电材料     

Electrical properties of (SN)x films

The temperature dependence of the thermopower S and the conductivity σ of (SN)_x films has been investigated as a function of the substrate temperature during film deposition T_s. In contrast to the transport properties of SN_x films previously reported, films made with high T_s have metallic-like transport properties approaching those of (SN)_x crystals. For low substrate temperatures σ and S are found to increase strongly with rising temperature and thermopower values up to 0.9 mV/K are observed. This conduction behavior is explained by a heterogeneous model with areas having different electronic gaps. Annealing as well as decomposition effects suggest that the gap width is correlated to the (SN)_x chain length. No evidence was seen for superconductivity in these films.     

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.     

Kinetic effects in manganites La1 − x Ag y MnO3 (y ≤ x)

We have measured the resistivity, magnetoresistance, and thermopower of ceramic manganite samples La_{1 ? x} Ag _y MnO₃ (y ≤ x) doped with silver as functions of temperature (4.2–350 K) and magnetic field (up to 26 kOe). A metal-insulator phase transition is observed in all investigated samples at temperatures close to room temperature. The behavior of the resistivity and thermopower in the high-temperature paramagnetic region is interpreted using the concept of small radius polaron; the activation energy decreases with increasing doping level. The resistivity in the low-temperature ferromagnetic region is approximated by the expression ρ_FM(T) = ρ₀ + AT ² + BT ^4.5 presuming the existence of electron-electron and electron-magnon interactions. A resistivity minimum and a strong magnetoresistive effect are observed at low temperatures. The latter effect is associated with scattering of charge carriers at grain boundaries, which are antiferromagnetically ordered relative to one another. The temperature dependence of thermopower in the magnetically ordered phase is described in the framework of a model taking into account the drag of charge carriers by magnons.     

The electronic properties of [CH(FeCl4)0.061]x

The temperature dependence of d.c. conductivity, thermopower and electron paramagnetic resonance (EPR) results on [CH(FeCl₄)_0.061]_x are reported. The d.c. conductivity and thermopower measurements indicate metallic charge transport along the polyacetylene chain interrupted by the interfibril contact resistances and also ‘dragged’ primarily by the dopant ions. The initial measurements of EPR show Dysonian lineshape with very broad linewidth (ΔH ≈ 600 G at room temperature). The temperature dependence of EPR absorption intensity implies that there exist localized magnetic moments. The observed g value (g ≈ 2.03) suggests the dopant anion is in a form of (FeCl₄)^-.     

Effect of elastic stresses on the thermoelectric properties of Pb1−x GexTe epitaxial layers

The temperature dependence of the thermopower of Pb_1?x Ge_xTe (x = 0.01?0.05) epitaxial films grown on BaF₂(111) substrates was measured. The temperature of the ferroelectric phase transition was found to disagree with that obtained for bulk single crystals of the same composition. This disagreement was explained as resulting from the effect of elastic stresses arising in the crystal lattice of the Pb_1?x Ge_xTe compound due to the difference between the thermal expansion coefficients of the film and the substrate during cooling of the layers from the growth temperature to the temperature of measurement.     

Electronic transport properties of stuffed compositions of ferromagnetic copper chromium telluride: Cu1+xCr2Te4 (x=0-1)

We present here a detailed study of electronic transport properties of the metallic-ferromagnetic compounds Cu_1+xCr₂Te₄, having excess Cu atoms with x=0-1, from 2 to 400 K. The stuffing of the copper atoms in the parent structure reduces the ferromagnetic ordering temperature T_C from 325 to 156 K, while for the entire range the dependence of the electrical resistance and the thermopower with temperature and the anomalies in them on the magnetic ordering remain similar. All the compounds show a magnon-drag contribution in thermopower as a positive maximum around T_C/3, and a T² - dependence of resistivity at low temperatures. The increasing effects of the short range magnetic ordering in the paramagnetic resistivity are seen with the increase in the stuffing of atoms in these compounds. The transport properties are explained by the current carriers —the holes in a wide energy band dominated by the p-state of Te-atoms, which are scattered by the spindisorder in the paramagnetic phase and from the magnons in the ferromagnetic phase.     

The behavior of thermopower in the YBa2−x LaxCu3Oy system. Correlation between the band parameters in normal state and critical temperature

The thermopower in the YBa_2−x La_xCu₃O_y system (x=0–0.5) has been studied. An analysis made in terms of the narrow-band model revealed trends in the variation of the main band parameters with increasing lanthanum concentration. The dependence of the conduction band width on lanthanum concentration was found to pass through a weak minimum at x=0.05–0.1, which correlates in position with the maximum in the concentration dependences of the critical temperature and degree of sample orthorhombicity. The results obtained and their comparison with Fe, Co→Cu substitution data permit a conclusion that the main factor affecting the properties of the YBa₂Cu₃O_y system under nonisovalent substitution of various cations is the increasing disorder in the chain-oxygen subsystem caused by increasing impurity content. Fiz. Tverd. Tela (St. Petersburg) 40, 17–22 (January 1998)     

Kinetic Properties of the Mn<Subscript>1 – <Emphasis Type="Italic">x</Emphasis></Subscript>Gd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se Solid Solutions

The results of kinetic study of the Mn_{1 – x}Gd_xSe chalcogenide solid solutions with different substitute concentrations (0 ≤ x ≤ 0.15) in the temperature range of 80–400 K are reported. The difference between the Hall constant and thermopower signs has been found. The electron-type conductivity determined from the Hall constant and hysteresis of the I–V characteristics have been explained by the existence of nanoareas with local electric polarizations. The sharp extrema observed in the temperature dependence of thermopower are explained by splitting of a narrow 4f subband by the crystal field.     

Transport effects in La0.67−x R x Sr0.33MnO3 (R=Eu, Gd)

An experimental study is reported on the temperature dependences of the magnetic susceptibility, electrical resistivity, magnetoresistance, thermo-and magnetothermopower, and on the Hall effect of the polycrystalline lanthanum manganites La_0.67?x R _x Sr_0.33MnO₃ (x=0 and 0.07, R=Eu, Gd) within the 77-to 430-K temperature range. Replacement of a small amount of lanthanum by europium or gadolinium was found to result in a considerable decrease in the resistivity and in a change in the pattern of its temperature dependence. The temperature dependence of both the ordinary and anomalous Hall coefficients near the Curie temperature is shown to be determined by the change in the number of carriers in delocalized states. A method is proposed for separation of the hole and electronic contributions to thermopower in the ferromagnetic region. The conduction mechanisms are discussed in terms of the concept of mobility edge motion.     

超巨磁电阻材料Pr2/3Sr1/3MnO3薄膜输运性质的研究

对1/f噪声缺口的超巨磁电阻Pr_2/3Sr_1/3MnO₃薄膜样品从10K到室温的热电势值进行了测量，热电势为负值，低温下随温度线性变化，表现为金属扩散热电势，在150K开始急剧减小，过渡到与温度T成反比的变化关系，与小极化子模型相符.结合在电阻极大温度附近对电阻和1/f噪声行为的讨论，对于发生在电阻极大温度附近的相变过程，结果支持源于相分离的渝渗模型. 关键词： 稀土锰氧化物 热电势 渝渗模型     

Electrical and thermal conductivities and Seebeck coefficient of liquid copper–bismuth alloys

The electrical resistivity and absolute thermoelectric power of the liquid Cu _x –Bi_{(1? x )} system have been measured over the whole composition range from the liquidus to 1100°C. The thermal conductivity is deduced from measurements of these two properties. The experimental results are interpreted and discussed in term of the extended Faber–Ziman formula using the t-matrix formalism with hard-sphere structure factors. The concentration and energy dependence of the phase shifts have been taken into account for a complete conductivity and thermopower calculation.     

Mixed valence on dilute Eu in YCu2Si2

We report measurements of electrical resistivity, thermopower, thermal conductivity, magnetic susceptibility, Mößbauer andL _III x-ray absorption of intermediate valent Eu in Y_1–x Eu _x Cu₂Si₂ alloys withx=0.03,x=0.07 andx=0.10. A qualitative comparison of the physical properties of dilute Eu in YCu₂Si₂ with concentrated Eu in EuCu₂Si₂ shows only slight differences; the dominant feature is a very strong variation of the Eu valence with temperature, from about 2.8 at 4 K to about 2.45 at 800 K, in the concentrated compound as well as in the dilute alloys.     

The effect of Fe substitution on the electrical and thermal conductivity and thermopower of Ca3(FexCo1−x)4O9 synthesised by a sol–gel process

The effect of Fe substitution for Co on direct current (DC) electrical and thermal conductivity and thermopower of Ca₃(Co_1−xFe_x)₄O₉ (x = 0, 0.05, 0.08), prepared by a sol–gel process, was investigated in the temperature range from 380 down to 5K. The results indicate that the substitution of Fe for Co results in an increase in thermopower and DC electrical resistivity and substantial (14.9–20.4% at 300K) decrease in lattice thermal conductivity. Experiments also indicated that the temperature dependence of electrical resistivity ρ for heavily substituted compounds Ca₃(Co_1−xFe_x)₄O₉ (x = 0.08) obeyed the relation lnρ ∝ T^−1/3 at low temperatures, T < ~55K, in agreement with Mott’s two-dimensional (2D) variable range hopping model. The enhancement of thermopower and electrical resistivity was mainly ascribed to a decrease in hole carrier concentration caused by Fe substitution, while the decrease of thermal conductivity can be explained as phonon scattering caused by the impurity. The thermoelectric performance of Ca₃Co₄O₉ was not improved in the temperature range investigated by Fe substitution largely due to great increase in electrical resistivity after Fe substitution.     

Mechanism responsible for the modification of the band spectrum and superconducting properties in the Tl2Ba2Ca1 ? x Y x Cu2 ? y Co y O z system

The modification of the temperature dependences of the thermopower coefficient for the Tl₂Ba₂Ca_{1 − x} Y _x Cu_{2 − y} Co _y O _z system is investigated as a function of the dopant content. The model of an asymmetric narrow band is shown to be applicable to high-temperature superconductors in the thallium system. The parameters of the band spectrum and charge carrier system for samples of a close-to-optimum composition are determined by analyzing the temperature dependences of the thermopower coefficient within the narrow band model. The character and mechanism of their variation in the course of the transition to an underdoped regime are analyzed together with the dynamics of changes in the superconducting properties. The critical temperature T _c is found to correlate with the effective conduction-band width W _D . It was shown that the dependence T _c (W _D ) for Tl₂Ba₂Ca_{1 − x} Y _x Cu_{2 − y} Co _y O _z has a close-to-universal character, but the suppression of T _c upon the broadening of the band occurs faster than in the case of the yttrium system. Original Russian Text ? O.A. Martynova, V.é. Gasumyants, 2007, published in Fizika Tverdogo Tela, 2007, Vol. 49, No. 9, pp. 1537–1542.