Temperature dependence of electrical resistivity for Ca-doped perovskite-type Y1−xCaxCoO3 prepared by sol-gel process

The temperature dependences of DC electrical resistivity for perovskite-type oxides Y_1−xCa_xCoO₃ (0?x?0.1), prepared by sol-gel process, were investigated in the temperature range from 20 K up to 305 K. The results indicated that with increase of doping content of Ca the resistivity of Y_1−xCa_xCoO₃ decreased remarkably, which was found to be caused mainly by increase of carrier (hole) concentration. In the whole temperature range investigated the temperature dependence of resistivity ρ(T) for the un-doped (x=0) sample decreased exponentially with decreasing temperature (i.e. ln ρ∝1/T), with a conduction activation energy ; the resisitivity of lightly doped oxide (x=0.01) possessed a similar temperature behavior but has a reduced E_a (0.155 eV). Moreover, experiments showed that the relationship ln ρ∝1/T existed only in high-temperature regime for the heavily doped samples (T?82 and ∼89 K for x=0.05 and 0.1, respectively); at low temperatures Mott's ln ρ∝T^−1/4 law was observed, indicating that heavy doping produced strong random potential, which led to formation of considerable localized states. By fitting of the experimental data to Mott's T^−1/4 law, we estimated the density of localized states N(E_F) at the Fermi level, which was found to increase with increasing doping content.     

Electrical behavior of some silver-doped lanthanum-based CMR materials

With a view to understand the structural, magnetic and electrical properties of La_1−xAg_xMnO₃ (x=0.05-0.3), a series of samples were prepared by polyvinyl alcohol (PVA) gel route. It has been found that both the metal-insulator and ferro- to paramagnetic transition temperatures after increasing up to the composition x=0.20, are found to remain constant thereafter. The electrical resistivity vs. temperature plot of the sample x=0.10 is found to exhibit an insulating behavior below 36 K, while the sample, x=0.20 exhibits two peaks, and the observed behavior is explained on the basis of the phase separation model. The low-temperature (T<T_P), electrical resistivity data were analyzed by a theoretical model, ρ=ρ₀+ρ₂T²+ρ_4.5T^4.5, indicating the importance of grain/domain boundary effects, electron-electron and two-magnon scattering processes. The low-temperature resistivity data (T<50 K) were fitted to an equation, which is based on the combined effect of weak localization, electron-electron and electron-phonon scattering.     

Influence of Pr-doping on structural, electronic transport, magnetic properties of perovskite molybdates Sr1−xPrxMoO3 (0≤x≤0.15)

The effect of Pr-doping on structural, electronic transport, magnetic properties in perovskite molybdates Sr_1−xPr_xMoO₃ (0≤x≤0.15) has been investigated. The Pr-doping at Sr-site does not change the space group of the samples, but decreases the lattice parameter a. The magnitude of resistivity ρ increases initially (x≤0.08) and then decreases with further increasing Pr-doping level x and ρ(T) behaves as T² and T dependence in the low-temperature range blow T* and high-temperature range of 150 K<T<350 K, related to the electron-electron (e-e) and electron-phonon (e-ph) scattering, respectively. The magnetic susceptibility χ value of the sample increases with increasing x and the χ(T) curve for all samples can be well described by the model of exchange-enhanced paramagnetism. The specific heat magnitude in the low-temperature region increases with increasing Pr-doping level. The specific heat value agrees with the classical Dulong-Petit phonon specific heat, C_cl=3k_BrN_A=124.7 J/mol K in the high-temperature region and the temperature dependence of the specific heat can be well described by the formula C_p(T)/T=γ_e+β_pT² in the low-temperature range. These behaviors can be explained by the competition between the increase in the density of state (DOS) at Fermi energy level and the disorder effect due to Pr-doping.     

The effective activation energy Ueff(T,B,J) in Hg-1223 single phase superconductors

We review the methods of calculating the effective activation energy U_eff(T,B,J) for both transport measurements and magnetic decay, together with some theoretical models. Then, we apply these methods to our Hg-1223 single-phase superconductor to obtain the activation energy. Transport results give that the magnetic field and temperature dependence of the U_eff can be well described as U₀B^−α(1−T/T_c)^m. Magnetic relaxation shows that the current density dependence of U(J) can be scaled onto a single curve, which can be considered as the activation energy at some temperature T₀. The pinning mechanism in the measured temperature range does not change, and the activation energy depends separately on the three variables: T, B, and J, are responsible for the magnetic decay data scaling onto a single curve at various temperatures. As temperatures close to zero and near T_c, thermally assisted flux motion model is no longer valid since other processes dominate.     

Kondo behaviour of the solid solution (Ce1−xLax)PtGa

The solid solution (Ce_1−xLa_x)PtGa has been studied through X-ray diffraction, magnetization (σ(B)), magnetic susceptibility (χ(T)), electrical resistivity (ρ(T)), magnetoresistivity (MR) and heat capacity (C_P(T)) measurements. The Néel temperature (T_N=3.3 K) for CePtGa is lowered upon La substitution as observed from χ(T) and ρ(T) measurements. The Kondo temperature T_K as calculated from MR measurements is comparable to T_N and also decreases with La substitution. The volume dependence of T_K is in accordance with the compressible Kondo lattice model and a Doniach diagram of the results is presented. C_P(T) measurements are presented for CePtGa, Ce_0.2La_0.8PtGa and LaPtGa and the results are discussed in terms of the electronic and magnetic properties. Other features of interest are anomalies in ρ(T) and C_P(T) due to crystalline electric field effects and metamagnetism as observed in σ(B) studies for samples with 0≤x≤ 0.3.     

Effects of Cr doping in bilayered manganite LaSr2Mn2O7: Resistivity, thermoelectric power, and thermal conductivity

Systematic studies of resistivity, thermoelectric power, and thermal conductivity have been performed on polycrystalline bilayered manganites LaSr₂Mn_2−xCr_xO₇ (0≤x≤0.2). It is found that the temperature dependence of both Seebeck coefficient S(T) and resistivity ρ(T) in the high temperature region follows the small polaron transport mechanism for all the samples. But in the low temperature region, variable-range-hopping (VRH) model matches the experimental data better. In addition, the maximum of absolute S(T) at low temperatures is gradually suppressed for the sample with Cr-doping level of x>0.04, implying that a new FM order probably arises. With decreasing the temperatures further, S(T) has a sign change and becomes positive for the sample with Cr-doping level of x>0.04, indicating that there may occur a variation of the type of charge carrier. As to thermal conduction κ(T), the low-temperature peak is suppressed due to Cr-doping. The variation of κ(T) is analyzed based on the combined effect due to the suppression of local Mn³⁺O₆ Jahn-Teller (JT) lattice distortion because of the substitution of Cr³⁺ ions for Mn³⁺ ions, which results in the increase in thermal conduction, and the introduction of the disorder due to Cr-doping, which contributes to the decrease in thermal conduction.     

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.     

Transport properties and magnetoresistance in CoNb1−xMnxSb half-Heusler alloys with a low temperature localization

The transport properties and magnetoresistance of half-Heusler CoNb_1−xMn_xSb (x=0.0-1.0) alloys have been investigated between 2 and 300 K. In this temperature range, a metallic conductivity has been observed for the alloys with higher (x=1.0) and lower (x=0.0-0.2) Mn contents. However, the middle Mn content alloys (x=0.4-0.8) exhibit non-metallic conductive behavior. Their temperature dependence of resistivity undergoes a Mott localization law ρ=ρ₀exp(T₀/T)^p (p=1/4) rather than a thermal excitation regime ρ=ρ₀exp(E_a/kT) at low temperature (). The localization can be attributed to atomic and magnetic disorder. Resistivity peaks from 25 to 300 K were also observed for these alloys. Magnetotransport investigation reveals that these resistivity peaks result from localization effect as well as spin-disorder scattering.     

Influence of carbon concentration on structural, magnetic and electrical transport properties for antiperovskite compounds AlCxMn3

The structural, magnetic and transport properties of the antiperovskite AlC_xMn₃ (1.0≤x≤1.4) are investigated. It is found that the lattice parameter a increases monotonously with nominal carbon concentration x. The Curie temperature T_C increases with increasing x from 1.0 to 1.1 and then decreases with further increasing x. The highest T_C value is 364 K, about 70 K higher than that of stoichiometric AlCMn₃ reported previously. This may be attributed to a competition between the lattice expansion and the strong Mn 3d-C 2p hybridization. Below 100 K, the resistivity can be well described as ρ(T)=ρ₀+AT², corresponding to the electron-electron scattering. A increases with x, suggesting certain changes in the electronic structure, e.g. carrier density. Above 250 K, all ρ(T) curves depart from the linear dependence on temperature and seem to take on a tendency towards saturation.     

Spin-state transition, magnetic, electrical and thermal transport properties of the perovskite cobalt oxide Gd0.7Sr0.3CoO3

The magnetization, resistivity ρ, thermoelectric power (TEP) S, and thermal conductivity κ in perovskite cobalt oxide Gd_0.7Sr_0.3CoO₃ have been investigated systematically. Based on the temperature dependence of susceptibility χ_g(T) and Seebeck coefficient S(T), a combination of the intermediate-spin (IS) state for Co³⁺ and the low-spin (LS) state for Co⁴⁺ can be suggested. A metal-insulator transition (MIT) caused by the hopping of σ* electrons (localized or delocalized e_g electrons) from the IS Co³⁺ to the LS Co⁴⁺ is observed. Meanwhile, S(T) curve also displays an obvious phonon drag effect. In addition, based on the analysis of the temperature dependence of S(T) and ρ(T), the high-temperature small polaron conduction and the low-temperature variable-range-hopping conduction are suggested, respectively. As to thermal conduction κ(T), rather low κ values in the whole measured temperature range is attributed to unusually large local Jahn-Teller (JT) distortion of Co³⁺O₆ octahedra with IS state.     

Effects of Cr doping in electron-doped manganites La0.9Te0.1MnO3

The effects of Cr doping on Mn sites in the electron-doped manganites La_0.9Te_0.1MnO₃ have been studied by preparing the series La_0.9Te_0.1Mn_1−xCr_xO₃ (0.05≤x≤0.20). Upon Cr doping, both the Curie temperature T_C and magnetization M are suppressed. The resistivity measurements indicate that there exists a weak metal-insulator (M-I) transition for the sample with x=0.05, with an increase in the doping level, the M-I transition disappears and the resistivity increases. Thermopower S(T) exhibits a maximum near T_C for all samples. By fitting the S(T) and ρ(T) curves, it is found that the temperature dependences of both S(T) and ρ(T) in the high temperature paramagnetic (PM) region follow the small polaron conduction (SPC) mechanism for all samples. The fitting parameters obtained imply changes of both the average-hopping distance of the polarons and the polaron concentration with Cr doping in our studied samples. In the case of the thermal conductivity κ(T), the variation of κ(T) is analyzed based on the combined effects due to the suppression of the local Mn³⁺O₆ Jahn-Teller (JT) lattice distortion because of the substitution of Cr³⁺ for Mn³⁺ ions, which results in the increase in κ, and the introduction of the disorder due to Cr-doping, which contributes to the decrease in κ.     

Large negative magnetoresistance in thiospinel CuCrZrS4

We report on large negative magnetoresistance observed in ferromagnetic thiospinel compound CuCrZrS₄. The electrical resistivity increased with decreasing temperature according to the exp(T₀/T)^1/2, an expression derived from variable range hopping with strong electron-electron interaction. The resistivity under a magnetic field was expressed by the same form with the characteristic temperature T₀ decreasing with increasing magnetic field. Magnetoresistance ratio ρ(T,0)/ρ(T,H) is 1.5 for H=90 kOe at 100 K and increases divergently with decreasing temperature reaching 80 at 16 K. Results of magnetization measurements are also presented. A possible mechanism of the large magnetoresistance is discussed.     

The Non-Adiabatic Polaron Model Revisited

We revisit Holstein’s polaron model to derive an extension of the expression for the thermal dependence of the electrical resistivity in the non-adiabatic small-polaron regime. Our analysis relaxes Holstein’s assumption that the vibrational-mode energies \(\hbar \omega _{k}\) are much smaller than the thermal energy k _B T and substitutes a fifth-order expansion in powers of \(\hbar \omega _{k}/k_{B}T\) for the linear approximation in the expression for the quasiparticle hopping probability in the original treatment. The resulting expression for the electrical resistivity has the form ρ(T)=ρ ₀ T ^3/2 exp(E _a /k _B T?C/T ³+D/T ⁵), where C and D are constants related to the molecule–electron interaction energy, or alternatively to the polaron binding energy, and the dispersion relation of the vibrational normal modes. We show that experimental data for the La _1?x Ca _x MnO ₃ (x=0.30,0.34,0.40, and 0.45) manganite system, which are poorly fitted by the conventional non-adiabatic model, are remarkably well described by the more accurate expression. Our results suggest that, under conditions favoring high resistivity, the higher-order terms associated with the constants C and D in the above expression should taken into account in comparisons between theoretical and experimental results for the temperature-dependent transport properties of transition-metal oxides.     

NiS2-xSex在x=1.00附近的反铁磁量子相变

针对NiS_2-xSe_x系统在x=1.0０附近发生的反铁磁量子相变,制备了一系列NiS_2-xSe_x(x=0.96, 0.98, 1.00, 1.05, 1.10和1.20)多晶样品,对其结构、磁性质和电阻率进行了系统的观测.结果发现：样品磁化率-温度关系呈现典型的强关联电子系统特征;与铜氧化物超导体相类似,它们的电阻率-温度关系在很宽的温 关键词： 量子相变 反铁磁自旋涨落 2-xSe_x体系')" href="#">NiS_2-xSe_x体系     

Scaling of the temperature-dependent resistivity in SrFe2−xNixAs2

We show that the zero-field normal-state resistivity of temperature-dependent resistivity ρ(T) of SrFe_2?xNi_xAs₂ can be reproduced by the expression ρ(T) = ρ₀ + c T exp(?2Δ/T). ρ(T) can be scaled using both this expression where the energy scale Δ, c and the residual resistivity ρ₀ are scaling parameters and a recently proposed model-independent scaling method (H.G. Luo, Y.H. Su, T. Xiang, Phys. Rev. B 77 (2008) 014529). The scaling parameters have been calculated and the compositional variation of 2Δ(x) has been determined. This dependence show almost a linear decreasing in the underdoped regime similar to that reported for cuprates. The existence of a universal metallic ρ(T) curve in a wide temperature range which, however, is restricted for the underdoped compounds to temperatures above a structural and anitiferromagnetic transition is interpreted as an indication of a single mechanism which dominates the scattering of the charge carriers in SrFe_2?xNi_xAs₂ (x = 0–0.3).     

Hopping conductivity of Ni-doped p-CdSb in strong magnetic fields

Magnetoresistance (MR) of oriented single crystals of the anisotropic semiconductor p-CdSb doped with 2 at% of Ni is investigated between T=1.5 and 300 K in transversal pulsed magnetic fields up to B=30 T. In fields B∼4-15 T at T below 4.2 K, the resistivity obeys the law ln ρ∼η[B?(B)]^1/2 with ?(B)=a(0)/a(B), where a is the carrier localization radius and parameter η depends on a(0), on the acceptor concentration N_A and on the direction of the magnetic field with respect to the crystallographic axes, but does not depend on T. Such behavior gives evidence for MR realized by hopping charge transfer over the nearest-neighbor sites in strong magnetic field. The analysis of the experimental data yields the values of η, agreeing with calculated ones within an error of 10%, taking into account the effects of the anisotropy of the acceptor states and of the explicit dependence of a(B) due to the increase in the activation energy of shallow acceptors in magnetic field and the sensitivity of the metal-insulator transition to B.     

Normal state transport properties in single crystals of Ba1−xKxFe2As2 andNdFeAsO1−xFx

Hall effect and magnetoresistance Δρ/ρ(0) (MR) in the normal state have been measured on single crystals of Ba_1?xK_xFe₂As₂ and NdFeAsO_1?xF_x. Detailed analysis reveal the following conclusions: (1) For the parent phases of Ba_1?xK_x Fe₂As₂ and NdFeAsO_1?xF_x, large Hall effect and MR with strong temperature dependence were observed below a characteristic temperature corresponding to the antiferromagnetic/ structural transition. The field dependence of the Hall resistivity ρ_xy exhibits a non-linear behavior, which is accompanied by the violation of the B-square feature of the longitudinal magnetoresistivity Δρ_xx(B)/ρ_xx(0). A closer inspection further indicates that they are well related to each other and could be attributed to the multi-band effect or spin-related scattering. (2) The superconducting samples show much smaller Hall coefficient and MR in the normal state. The Hall coefficient shows a weaker temperature dependence compared to the parent phase, while the mean scattering rate 1/τ_H has a power-law like temperature dependence as 1/τ_H ∝ Tⁿ (n = 2–3). (3) For a Ba_1?xK_xFe₂ As₂ sample with T_c = 36 K, the field dependence of MR is complicated and the feature varies in different temperature regions. A drastic change of Δρ/ρ(0) was found between 80 K and 100 K, which corresponds very well to the maximum of the temperature derivative of the resistivity. This may be attributed to the spin-related scattering of electrons. (4) A comparison between the parent phase and the superconducting sample with T_c = 50 K in NdFeAsO_1?xF_x suggests that the electronic transport properties in the normal state cannot be easily understood with the simple multi-band model, while a picture concerning a suppression to the quasiparticle density of states at the Fermi energy is more reasonable.     

Preexponential factor in variable-range hopping conduction in CuInTe2

We study the temperature dependence of the electrical resistivity in a single crystal of p-type uncompensated CuInTe₂ on the insulating side of the metal-insulator transition down to 0.4 K. We observe a crossover from Mott to Efros-Shklovskii variable-range hopping conduction. In Efros-Shklovskii-type conduction, the resistivity is best described by explicitly including a preexponential temperature dependence according to the general expression ρ=ρ₀T^αexp(T_ES/T)^1/2, with α≠0. A theory based on the resistor network model was developed to derive an explicit relation between α and the decay of the wavefunction of the localized states. A consistent correspondence between the asymptotic extension of the wavefunction and the conduction regime is proposed. The results indicate a new mechanism for a local resistivity maximum in insulators, not involving magnetic effects.     

Influence of Ce doping on electrical and thermal properties of La0.7−xCexCa0.3MnO3 (0.0≤x≤0.7) manganites

The effect of Ce-doping on structural, magnetic, electrical and thermal transport properties in hole-doped manganites La_0.7−xCe_xCa_0.3MnO₃ (0.0≤x≤0.7) is investigated. The structure of the compounds was found to be crystallized into orthorhombically distorted perovskite structure. dc Susceptibility versus temperature curves reveal various magnetic transitions. For x≤0.3, ferromagnetic regions (FM) were identified and the magnetic transition temperature (T_C) was found to be decreasing systematically with increasing Ce concentration. The electrical resistivity ρ(T) separates the well-define metal-semiconducting transition (T_MS) for low Ce doping concentrations (0.0≤x≤0.3) consistent with magnetic transitions. For the samples with 0.4≤x≤0.7, ρ(T) curves display a semiconducting behavior in both the high temperature paramagnetic (PM) phase and low temperature FM or antiferromagnetic phase. The electron–phonon and electron–electron scattering processes govern the low temperature metallic behavior, whereas small polaron hopping model is found to be operative in PM phases for all samples. These results were broadly corroborated by thermal transport measurements for metallic samples (x≤0.3) in entire temperature range we investigated. The complicated temperature dependence of Seebeck coefficient (S) is an indication of electron–magnon scattering in the low temperature magnetically ordered regime. Specific heat measurements depict a broadened hump in the vicinity of T_C, indicating the existence of magnetic ordering and magnetic inhomogeneity in the samples. The observation of a significant difference between ρ(T) and S(T) activation energies and a positive slope in thermal conductivity κ(T) implying that the conduction of charge carriers were dominated by small polaron in PM state of these manganites.     

Thermal expansion and kinetic coefficients of crystals

Electrical (ρ) and thermal (W) resistivities and thermal expansion coefficient (β) of Cu, Zn, Al, Pb, Ni, β-brass, Al₂O₃, NaCl, Si, SiO₂(∥), and SiO₂(⊥) were simultaneously measured with standard four-probe, absolute steady-state, and quartz dilatometer techniques. Measurements of Ni and β-brass were performed at temperatures from 300 to 1100 K and measurements of all other samples were made between 90 and 500 K. This temperature range includes the range below and above the Debye temperature (T_D). The total uncertainties of the specific electrical and thermal resistivities and thermal expansion coefficient (TEC) measurements are 0.5%, 3.0%, and (1.5-4.0%), respectively. The universal linear relationship between the electrical and thermal resistivities and βΤ over the wide temperature range was found experimentally. Using the Landau criterion for convection development for ideal phonon and electron gases in the solids, the universal relations, ρ^ph/ρ^*≡βT and W^ph/W^*≡βT (where ρ^ph is the phonon electrical resistivity, is the characteristic electrical resistivity, W^ph is the phonon thermal resistivity, and W^*=k_BG/qc_p is the characteristic thermal resistivity) between relative phonon electrical and phonon thermal resistivities and βΤ were derived. The derived universal relations provide a new method for estimating the kinetic coefficients (electrical and thermal resistivities) from TEC measurements.