The effect of oxygen stoichiometry on electrical transport and magnetic properties of La0.9Te0.1MnOy

The effect of the variation of oxygen content on structural, magnetic and transport properties in the electron-doped manganites La_0.9Te_0.1MnO_y has been investigated. All samples show a rhombohedral structure with the space group The Curie temperature T_C decreases and the paramagnetic-ferromagnetic (PM-FM) transition becomes broader with the reduction of oxygen content. The resistivity of the annealed samples increases slightly with a small reduction of oxygen content. Further reduction in the oxygen content, the resistivity maximum increases by six orders of magnitude compared with that of the as-prepared sample, and the ρ(T) curves of samples with y=2.86 and y=2.83 display the semiconducting behavior (dρ/dT<0) in both high-temperature PM phase and low-temperature FM phase, which is considered to be related to the appearance of superexchange ferromagnetism and the localization of carriers. The results are discussed in terms of the combined effects of the increase in the Mn²⁺/(Mn²⁺+Mn³⁺) ratio, the partial destruction of double exchange interaction, and the localization of carriers due to the introduction of oxygen vacancies in the Mn-O-Mn network.     

Effects of doping Na on the structure and physical properties of La2/3Ca1/3MnO3

Effects of doping Na on the structure, electrical and magnetic properties of La_2/3Ca_1/3MnO₃ are investigated. A structural phase transition from orthorhombic to rhombohedral structure takes place at y=0.375. All samples show metal-insulator (M-I) transition at the transition temperature and undergo the transition from paramagnetism to ferromagnetism at the Curie temperature T_C. and T_C increase monotonically with increasing Na content. However the Na-doped samples have a shoulder in their electrical transport curves found below and shows a widened magnetic transition process. On the other hand, intrinsic colossal magnetoresistance (CMR) peaks are observed in all the samples, but samples with y around 0.25 show two MR peaks which can be attributed to magnetic inhomogeneity induced by the doped Na⁺ ions. Here we propose a method to broaden the CMR peak of perovskite manganite, which is beneficial for practical applications.     

Pressure cycle of superconducting Cs0.8Fe2Se2 : A transport study

We report measurements of the temperature and pressure dependence of the electrical resistivity (ρ) of single-crystalline iron-based chalcogenide Cs_0.8Fe₂Se₂. In this material, superconductivity with a transition temperature develops from a normal state with extremely large resistivity. At ambient pressure, a large “hump” in the resistivity is observed around 200 K. Under pressure, the resistivity decreases by two orders of magnitude, concomitant with a sudden T_c suppression around . Even at 9 GPa a metallic resistivity state is not recovered, and the ρ(T) “hump” is still detected. A comparison of the data measured upon increasing and decreasing the external pressure leads us to suggest that the superconductivity is not related to this hump.     

Thermal and electronic transport in CeRu2Al10: Evidence for a metal-insulator transition

A first report of physical properties of the ternary intermetallic compound CeRu₂Al₁₀ is given. The electrical resistivity below room temperature shows activated behaviour with a narrow gap of before the onset of a sharp peak in ρ(T) below . The Hall coefficient as well as the thermoelectric power are overall positive, and both increase in a similarly sharp manner below T^*. The lattice part of the thermal conductivity indicates phonon coupling of the heat transport at T^*, possibly underlying a lattice transformation that accompanies the putative metal-to-insulator and magnetic phase transitions.     

Non-Fermi liquid behavior in polycrystalline Ce2PdIn8

We report on the formation of a novel ternary compound Ce₂PdIn₈ that is isostructural with the heavy-fermion superconductors Ce₂CoIn₈ and Ce₂RhIn₈. Its magnetic, electrical transport and thermodynamic properties were studied on polycrystalline samples in wide ranges of temperature and magnetic field strength. The results revealed Ce₂PdIn₈ to be a paramagnetic Kondo lattice with a coherence temperature of about 12 K. The C/T ratio of the specific heat reaches at 350 mK a strongly enhanced magnitude of about per Ce-atom, thus clearly indicating a heavy-fermion nature of this material. Moreover, a logarithmic divergence of C/T vs. T, observed below 3 K, which is accompanied by a linear temperature dependence of the electrical resistivity below 6 K, hint at a non-Fermi liquid character of the electronic ground state in the new compound reported.     

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.     

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.     

Magnetic properties of PrRh2Si2: A neutron diffraction study

The magnetic properties of polycrystalline PrRh₂Si₂ sample have been investigated by neutron diffraction measurements. Antiferromagnetic transition with an anomalously high ordering temperature (T_N∼68 K) is clearly observed in magnetic susceptibility, specific heat, electrical resistivity and neutron diffraction measurements. Neutron diffraction study shows that Pr³⁺ ions carry an ordered moment of 2.99(7)μ_B/Pr³⁺ and align along the crystallographic±c-directions for the ions located at the (0,0,0) and positions. The magnetoresistance at 2 K and 10 T is rather large (∼35%).     

Effect of Mn-doping on the electrical properties of Cu2GeSe3

In this work we report the temperature dependence of the resistivity ρ of p-Cu₂GeSe₃ and manganese-doped p-Cu₂GeSe₃ at low temperature. It was found that for a intrinsic sample ρ obeys the Shklovskii-Efros-type variable-range hopping resistivity law in the temperature range from 4 to 63 K. This behaviour is governed by generation of a Coulomb gap Δ=78 meV in the density of localized states. We find a low activation term T₀=0.24 K, which is an indication of a large localization length ξ. For Mn-doped sample a metal-insulator transition (MIT) is observed at T=65 K. On the basis of the Mott criterion for metal-insulator transition, the critical carrier density n_c is determined. From the analysis of resistivity data it is concluded that Mn acts as acceptor impurity.     

Low temperature magnetism of Cd-doped Ce2RhIn8 heavy fermion antiferromagnet

We report the low temperature magnetic properties of Cd-doped single crystals of Ce₂RhIn₈ grown from In-flux. Measurements of temperature-dependent magnetic susceptibility, heat capacity, electrical resistivity and X-ray magnetic scattering revealed that Cd-doping enhances the antiferromagnetic ordering temperature from for crystals with nominal Cd-concentration of ~20%. Similarly to the pure compound, Cd-doped Ce₂RhIn₈ presents just below T_N a commensurate antiferromagnetic structure with a propagation vector . Comparisons between our results and the general effects of Cd-doping on the single layer related family CeMIn₅ (M=Co, Rh and Ir) will also be given.     

Synthesis, crystal structure and thermoelectric properties of IrSn1.5Te1.5 -based skutterudites

The mixed anion skutterudite IrSn_1.5Te_1.5 has been synthesized and characterized by X-ray powder diffraction, thermopower and electrical resistivity measurements. Structural analysis reveals that Sn and Te order in layers perpendicular to the [111] direction of the skutterudite unit cell, and a distortion of the anion sublattice is evident. The thermopower (S) is 160 μV/K at room temperature, while the electrical resistivity (ρ) is . The effects of chemical substitutions on the Ir site (Ru, Pd) and Sn site (In, Sb) have been investigated. The power factor (S²/ρ) was found to improve with In substitution but, at 0.9 μW/K² cm, is too small for this material to be useful for thermoelectric applications.     

Structure, magnetic, and transport properties of the Co-doped manganites La0.9Te0.1Mn1−xCoxO3 (0≤x≤0.25)

The effect of Co doping at Mn-site on the structural, magnetic and electrical transport properties in electron-doped manganties La_0.9Te_0.1Mn_1−xCoxO₃ (0≤x≤0.25) has been investigated. The room temperature structural transition from rhombohedra to orthorhombic (Pbnm) symmetry is found in these samples with x≥0.20 by the Rietveld refinement of X-ray powder diffraction patterns. All samples undergo the paramagnetic-ferromagnetic (PM-FM) phase transition. The Curie temperature T_C of these samples decreases and the transition becomes broader with increasing Co-doping level. The magnetization magnitude of Co-doping samples increases at low temperatures with increasing Co-doping level for x≤0.15 and decreases with increasing Co-doping content further. The metal-insulator (M-I) transitions observed in the sample with x=0 are completely suppressed with Co doping, and the resistivity displays semiconducting behavior within the measured temperature region for these samples with x>0. All results are discussed according to the changes of the structure parameters and magnetic exchange interaction caused by Co-doping. In addition, the different effects between the Co doping and Cu doping in the Mn site for the electron-doped manganites are also discussed.     

Electrical transport properties of Ag3Sn compound

The temperature behaviors of the electrical resistivity in polycrystalline Ag₃Sn bulk samples were investigated experimentally. We found that the temperature dependence of resistivity shows concave function characteristics from 305 down to 26 K, and can be described by a parallel resistor model [H. Wiesmann et al., Phys. Rev. Lett. 38 (1977) 782]. The resistivities of all samples reveal T² behavior from 26 down to . We compared our data to the existing theories in which the T² dependence of resistivity has been proposed. However, we found none of them can describe the experimental data, and the physical mechanism of the T² behavior of resistivity at low temperatures is still enigmatical.     

Oxygen vacancy doping effect on the electrical and magnetic behavior of Ba5−xLaxNb4−xTixO15

We report electric and magnetic properties of oxygen deficient Ba_5−xLa_xNb_4−xTi_xO_15−δ phases, which have been prepared by solid-state reaction method followed by a controlled reduction process under hydrogen atmosphere. The extra electrons added by the formation of the oxygen vacancies (δ) introduce localized spins and the magnetic susceptibility can be described by a temperature-independent contribution and a Curie-Weiss term associated to the Ti³⁺ ion formation. Besides, the experimental resistivity (ρ) data of these four reduced compounds are well described in a wide temperature range with the equation , which suggests the presence of small polarons in the system. Although, all samples present electrical insulating behavior, the electrical resistivity decreases four orders of magnitude for intermediate x values. We interpreted this fact as a consequence of the mix between the localized bands of the Nb and Ti ions, which favors the promotion of carriers due to reduction of the band gap.     

Quadratic temperature dependence of electron-phonon scattering in disordered V1−xPdx alloys

We report the results of a comprehensive study of weak localization and electron-electron interaction effects in disordered V_1−xPd_x alloys whose compositions are close to the (low T_c) A15 V₃Pd compound. Magnetoresistivity and zero field resistivity have been measured within the temperature range 1.5≤T≤300 K. The low-temperature resistivity obeys a law, which is explained by electron-electron interaction. We have determined the electron-phonon scattering time (τ_e-ph) for V_1−xPd_x alloys. Our results indicate an anomalous electron-phonon scattering rate obeying quadratic temperature dependence. This observation is interpreted by the existing theories of electron-phonon interactions.