Transport properties of dilute magnetic alloys

The electrical resistivity, thermopower, and the electronic part of the thermal resistivity of dilute magnetic alloys are calculated in the framework of the Suhl-Nagoaka theory. Using Bloomfield's and Hamann's solution of the Nagaoka equations, we derive expressions for the transport quantities in the limitT? ¯ T_K andT?¯ T_K to order (In ¯T _K /T)^?4 where ¯T _K is the Kondo temperature which may depend on the spin independent scattering. We find that the thermopower and deviations from the Wiedemann-Franz law in this limit decrease as ¦In ¯T_K/T¦^?3 if one neglects a trivial temperature dependence of the thermopower due to the electron-phonon interaction.     

Coulomb blockade and the thermopower of a suspended quantum dot

On the basis of the 2D electron gas in an AlGaAs/GaAs membrane separated from a wafer, a one-electron transistor is created that operates on the Coulomb blockade effect—a two-barrier structure with a quantum dot. The separation of the sample from the wafer, which has a large dielectric constant, leads to a sharp decrease in the total capacity C of the quantum dot and, as a result, to high charge energy E _C = e ²/C and critical temperature T _C = E _C/k _B ≈ 40 K. The dependence of the conductance of the quantum dot on the driving and gate voltages includes a rhombic structure characteristic of the Coulomb blockade effect. The phonon-drag thermopower is found in this system. This thermopower exhibits an anomalous alternating dependence on the gate voltage and intensity of the phonon flux. Possible mechanisms are proposed for explaining the indicated anomalies in the thermopower.     

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.     

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     

Temperature dependence of conductance and thermopower anomalies of quantum point contacts

It has been shown within the Landauer approach that the presence of the 0.7 anomaly in the conductance of a ballistic microcontact and the respective plateau in the thermopower implies pinning of the potential barrier height at a depth of k _B T below the Fermi level. A simple way of taking into account the effect of electron-electron interaction on the profile and temperature dependence of a smooth one-dimensional potential barrier in the lower subband of the microcontact has been proposed. The calculated temperature dependences of the conductance and Seebeck coefficient agree with the experimental gate-voltage dependences, including the emergence of anomalous plateaus with an increase in temperature.     

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.     

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.     

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.     

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.     

C-axis thermoelectric transport in low stage SbCl5 graphite intercalation compounds

c-axis thermal conductivity, electrical resistivity and thermopower measurements performed on stages-2, 3 and 4 SbCl₅-graphite intercalation compounds in the temperature range 3 < T < 300 K are reported. Contrary to the electrical resistivity and thermopower data, the temperature variation of the thermal conductivity is qualitatively different from that previously observed on other intercalation compounds.     

Heavy fermion behaviour of the nonmagnetic CeCu4Al compound

We present measurements of the temperature dependence of the electrical resistivity, the thermopower and the specific heat of the hexagonal compound CeCu₄Al. At high temperatures, the electrical resistivity is characterized by a nearly temperature independent behaviour, followed by a continuous increase below 100K. No maximum has been found down to 1.7 K. The thermopower shows a positive maximum at about 30 K. As in CeCu₆ no negative values are observable in the range from 4.2 K up to a room temperature. The specific heat data between 7 and 15 K reveal a γ value around 280 mJ mol^-1 K^-2. Below this temperature range the specific heat c_p/T shows a rapid rise and crosses the value of 1 J mol^-1 K^-2 at about 1.45 K.     

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.     

Thermoelectric power of potassium doped lanthanum manganites at low temperatures

The temperature-dependent resistivity and thermoelectric power of monovalent (K) doped La_1−xK_xMnO₃ polycrystalline pellets (x=0.05, 0.10 and 0.15) between 50 and 300 K are reported. K substitution enhances the conductivity of this system. Curie temperature (T_C) also increases from 260 to 309 K with increasing K content. In the paramagnetic region (T>T_C), the electrical resistivity is well represented by adiabatic polaron hopping, while in the ferromagnetic region (T<T_C), the resistivity data show a nearly perfect fit for all the samples to an expression containing, the residual resistivity, spin-wave and two-magnon scattering and the term associated with small-polaron metallic conduction, which involves a relaxation time due to a soft optical phonon mode. Small polaron hopping mechanism is found to fit well to the thermoelectric power (S) data for T>T_C whereas at low temperatures (T<T_C) in ferromagnetic region (S_FM), S_FM is well explained with the spin-wave fluctuation and electron–magnon scattering. Both, resistivity and thermopower data over the entire temperature range (50–300 K) are also examined in light of a two-phase model based on an effective medium approximation.     

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.     

Liquid Madelung energy and Schottky defect energy related to liquid structure and melting temperature for alkali halides

Motivated by the work of Reiss et al in which the melting temperature T_m of alkali halides is correlated with Coulomb energy, we consider the cohesive energy W of ionic melts and Schottky defect energy E_s in the hot crystal, relative to the thermal energy k_BT_m It is shown here that W is accurately approximated by the liquid Madelung energy and hence that Wk_BT_m relates to the charge-charge direct correlation function c_QQ(r) at r = 0 The existence of a “Madelung constant” for the liquid at T_m is thereby demonstrated through the alkali halide series An estimate of the ratio E_s/k_BT_m then considered, the basic additional ingredient being argued to be the static dielectric constant of the solid The BarrDawson-Lidiard empirical correlation between E_s, and k_BT_m can be understood in this way.     

Tunneling thermopower in magnetic granular alloys

The nature of the field dependence of thermopower in the Co-Al-O and Fe-Al-O magnetic granular alloys with tunneling conduction is shown to be related to tunneling thermopower. The tunneling thermopower is small and depends approximately linearly on temperature and squared magnetization, and its field dependence is described by a relation of the type S(H)/T=a+bρ(0)/ρ(H), where ρ is the alloy electrical resistivity and the parameters a and b are field-independent.     

Temperature Dependence of the Thermal Conductivity of a Trapped Dipolar Bose-Condensed Gas

The thermal conductivity of a trapped dipolar Bose condensed gas is calculated as a function of temperature in the framework of linear response theory. The contributions of the interactions between condensed and noncondensed atoms and between noncondensed atoms in the presence of both contact and dipole-dipole interactions are taken into account to the thermal relaxation time, by evaluating the self-energies of the system in the Beliaev approximation. We will show that above the Bose-Einstein condensation temperature (T?>?T _BEC ) in the absence of dipole-dipole interaction, the temperature dependence of the thermal conductivity reduces to that of an ideal Bose gas. In a trapped Bose-condensed gas for temperature interval k _B T?<<?n ₀ g _B , E _p ?<<?k _B T (n ₀ is the condensed density and g _B is the strength of the contact interaction), the relaxation rates due to dipolar and contact interactions between condensed and noncondensed atoms change as \( {\tau}_{dd12}^{-1}\propto {e}^{-E/{k}_BT} \) and τ _c12?∝?T ^?5, respectively, and the contact interaction plays the dominant role in the temperature dependence of the thermal conductivity, which leads to the T ^?3 behavior of the thermal conductivity. In the low-temperature limit, k _B T?<<?n ₀ g _B , E _p ?>>?k _B T, since the relaxation rate \( {\tau}_{c12}^{-1} \) is independent of temperature and the relaxation rate due to dipolar interaction goes to zero exponentially, the T ² temperature behavior for the thermal conductivity comes from the thermal mean velocity of the particles. We will also show that in the high-temperature limit (k _B T?>?n ₀ g _B ) and low momenta, the relaxation rates \( {\tau}_{c12}^{-1} \) and \( {\tau}_{dd12}^{-1} \) change linearly with temperature for both dipolar and contact interactions and the thermal conductivity scales linearly with temperature.     

Negative-ion formation from surface scattering at finite temperatures

Temperature effects on negative-ion formation in positive-ion-surface scattering are studied within the framework of the time-dependent Anderson-Newns model. It is shown that the negative-ion formation is significantly enhanced at finite temperature T, provided k_BT is not less than the Anderson correlation energy U, where k_B is the Boltzmann constant. In the transient region (femtosecond timescale), temperature effects are, however, masked by large energy fluctuations.     

Strong coupling superconductivity and prominent superconducting fluctuations in the new superconductor Bi4O4S3

Electric transport and scanning tunneling spectrum(STS)have been investigated on polycrystalline samples of the new superconductor Bi4O4S3.A weak insulating behavior in the resistive curve has been induced in the normal state when the superconductivity is suppressed by applying a magnetic field.Interestingly,a kink appears on the temperature dependence of resistivity near 4 K at all high magnetic fields above 1 T when the bulk superconductivity is completely suppressed.This kink associated with the upper critical field as well as the wide range of excess conductance at low fields and high temperatures is explained as the possible evidence of strong superconducting fluctuation.From the tunneling spectra,a superconducting gap of about 3 meV is frequently observed yielding a ratio of 2Δ/kB TC~16.6.This value is much larger than the one predicted by the BCS theory in the weak coupling regime(2Δ/kB TC~3.53),which suggests the strong coupling superconductivity in the present system.Furthermore,the gapped feature persists on the spectra until 14 K in the STS measurement,which suggests a prominent fluctuation region of superconductivity.Such a superconducting fluctuation can survive at very high magnetic fields,which are far beyond the critical fields for bulk superconductivity as inferred both from electric transport and tunneling measurements.