Effect of annealing on the magnetic and superconducting properties of single-crystalline UCoGe

Single-crystals of the new ferromagnetic superconductor UCoGe have been grown. The quality of as-grown samples can be significantly improved by a heat-treatment procedure, which increases the residual resistance ratio (RRR) from ∼5 to ∼30. Magnetization M(T) and resistivity ρ(T) measurements show the annealed samples have a sharp ferromagnetic transition with a Curie temperature T_C is 2.8 K. The ordered moment of 0.06 μ_B is directed along the orthorhombic c-axis. Superconductivity is found below a resistive transition temperature T_s=0.65 K.     

Analysis of temperature-dependent electrical resistivity of ZnO nano-structures

The temperature–dependent electrical resistivity ρ(T) in metallic and semiconducting phase of ZnO nanostructures is theoretically analysed. ρ(T) shows semiconducting phase in low temperature regime (140 K<T<180 K), shows an absolute minimum near 180 K and increases linearly with T at high temperatures (200 K<T<300 K). The resistivity in metallic phase is estimated within the framework of electron–phonon and electron–electron scattering mechanism. The contributions to the resistivity by inherent acoustic phonons (ρ_ac) as well as high frequency optical phonons (ρ_op) were estimated using Bloch–Gruneisen (BG) model of resistivity. The electron–electron contributions ρ_e?e=BT² in addition with electron–phonon scattering is also estimated for complete understanding of resistivity in metallic phase. Estimated contribution to resistivity by considering both phonons, i.e., ω_ac and ω_op and the zero limited resistivity are added with electron–electron interaction ρ_e–e to obtain the total resistivity. Resistivity in Semiconducting phase is discussed with small polaron conduction (SPC) model. The SPC model consistently retraces the low temperature resistivity behaviour (140 K<T<180 K). Finally the theoretically calculated resistivity is compared with experimental data which appears favourable with the present analysis in wide temperature range.     

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.     

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.     

Hole transport anomaly in high-TC ferromagnet (Zn,Mn)GeP2

Anomalous Hall effect and a large negative magnetoresistance (up to −8.5%) have been found in the high-T_C ferromagnetic chalcopyrite (Zn,Mn)GeP₂. The elevated manganese concentration in the top diffusion layer grown on ZnGeP₂ isolated substrate gives rise to the increased hole conductivity and the temperature dependence of electric resistance ρ(T) typical of a metallic trace which indicate the charge carrier degeneration in combination with ferromagnetism. Additionally, we found a hysteresis of magnetoresistance Δρ/ρ₀ vs. H is associated with change in a magnetic order at low temperatures, T<50 K. The effect accompanies the reversal sign of Δρ/ρ₀ and is consistent with a singularity of magnetization vs. temperature. These anomalies observed for the first time in the high-T_C ferromagnetic chalcopyrites II-IV-V₂:Mn are explained by the phenomenological cluster model for ferromagnets.     

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.     

Thermal transport properties in the normal state of CaAlxSi2−x superconductors

Normal state electrical and thermal properties, including electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) of the CaAl_xSi_2−x (x=0.9-1.2) system were investigated. It is found that the electrical resistivity and Seebeck coefficient exhibit a typical metallic character throughout the temperature range investigated, and the metallicity of this series is enhanced with increase in Al/Si ratio. On the other hand, the thermal conductivity shows a weak temperature variation at low temperatures, whereas κ follows a T²-dependence for T>150 K. Analysis of the electronic thermal resistivity based on Klemen’s model reveals that the scattering of electrons from the defects and static imperfections becomes dominant as the temperature approaches T_c. These results are discussed in the light of simultaneous existence of various crystal structures and development of ultra-soft phonon mode recently observed in the CaAlSi system.     

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).     

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.     

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.     

Composition-dependent metal-insulator transition in perovskite CaXPb(1−X) TiO3 (CPT) ceramic

Ceramic compositions of a complex perovskite Ca_XPb(_1?X)TiO₃ (CPT) systems with x=0.6, 0.7 and 0.8 were prepared by mechanical mixing of their oxides (CaTiO₃ and PbTiO₃). The structure of the (CPT) ceramics was characterized by X-ray diffraction (XRD) The ceramics transform gradually from orthorhombic phase (pseudo cubic phase) to cubic phase by increasing pb content percent. The dc resistivity ρ(t) versus temperature (range 300–525 K) for x=0.6, 0.7 and 0.8. The ρ.T/curves reveal that samples exhibit a metallic behaviour at low temperature and undergo a metal-semiconductor transition with increasing temperature at T_p=373 K, 343 K and 333 K, for x=0.6, 0.7 and 0.8, respectively. The nature of conduction mechanism is studied in semiconductor region by studying the current–voltage temperature characteristics. The current–voltage characteristics were interpreted in terms of Poole–Frenkel type of conduction mechanism.     

Anisotropy of electric resistivity of Sapele-based biomorphic SiC/Si composites

The electrical resistivity of Sapele-based biomorphic SiC/Si materials was measured in a wide temperature range from 10 K to room temperature. The samples were fabricated by the reactive infiltration of molten silicon into a carbonized Sapele (African Entandrophragma Cylindricum) wood preform. All the samples studied contained residual Si (10–35 wt %). It was found that the resistivity-temperature (ρ(T)) dependences have semimetallic behavior which becomes very close to linear metallic behavior at 100 < T < 300 K. The obtained values of resistivity were quite low (ρ ≈ 0.002–0.02 Ω cm) and showed strong anisotropy: the resistivity along the wood growth axis was several times lower than that in the perpendicular direction. The extent of this anisotropy was in correlation with the amount of residual Si (and, hence, with the amount of residual porosity) in a sample. The resistivity perpendicular to the wood growth axis drastically increased with the Si content, whereas the resistivity parallel to it was practically independent of the Si content. It is suggested that the presence of residual carbon in the samples and carrier scattering at SiC/Si interphases could determine the observed character of ρ(T) dependences.     

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.     

Crystal growth, structure and electrical properties of thorium phosphorosulfide

Single crystals of thorium phosphorosulfide have been grown by the chemical vapour transport method. The X-ray diffraction examination showed that the unit cell of the crystals belongs to a tetragonal system of the PbFCl-type structure. The basal plane resistivity ρ(300 K)=64 μΩ cm and thermoelectric power S(300 K)=−7.7 μV/K, examined between 0.4 and 315 K, show metallic behaviour.     

Giant magnetoresistance in the Ge-rich magnetocaloric compound,Gd5(Si0.1Ge0.9)4

We have measured the zero-field electrical resistivity in the temperature range 5–295 K and magnetoresistance in magnetic fields of up to 12 T of Gd₅(Si_0.1Ge_0.9)₄. The resistivity changes drastically at the magnetostructural first-order transition (T_C≅80 K on heating). This transition can be induced reversibly by the application of an external magnetic field above T_C, producing a concomitant giant magnetoresistance (GMR) effect, Δρ/ρ≅−50%. This study demonstrates that (in addition to giant magnetocaloric and magnetoelastic effects) GMR can be tuned between ∼20 and ∼290 K in Gd₅(Si_xGe_1−x)₄ with x⩽0.5 by simply adjusting the Si : Ge ratio.     

Comparison of annealing effects on Zn-doped GaMnAs and undoped GaMnAs epilayers

To compare the annealing effects on GaMnAs-doped with Zn (GaMnAs:Zn) and undoped GaMnAs (u-GaMnAs) epilayers, we grew GaMnAs thin films at 200 °C by molecular beam epitaxy (MBE) on GaAs substrates, and they were annealed at temperatures ranging from 220 °C to 380 °C for 100 min in air. These epilayers were characterized by high-resolution X-ray diffraction (XRD), electrical, and magnetic measurements. A maximum resistivity at temperatures T_m close to the Curie temperatures T_c was observed from the measurement of the temperature-dependent resistivity ρ(T) for both the GaMnAs:Zn and the u-GaMnAs samples. We found, however, that the maximum temperature T_m observed for GaMnAs:Zn epilayers increased with increasing annealing temperature, which was different from the result with the u-GaMnAs epilayers. The formation of GaAs:Zn and MnAs or Mn-Zn-As complexes with increasing annealing temperature is most likely responsible for the differences in appearance.     

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.     

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.     

Electrical resistivity of alkali metal doped manganites LaxAyMnwO3 (A = Na,K, Rb): Role of electron–phonon,electron–electron and electron–magnon interactions

The electrical resistivity behaviour of alkali metal (Na, K, Rb) substitutions at La site in La_xA_yMn_wO₃ (A = Na, K, Rb) manganites is studied caused by electron–phonon, electron–electron and electron–magnon scattering. Substitutions affect average mass and ionic radii of A-site and hence resulting lattice and optical phonon softening. Estimated resistivity compared with reported metallic resistivity, accordingly ρ_diff. = [ρ_exp ? {ρ₀ + ρ_e–ph (=ρ_ac + ρ_op)}], infers electron–electron and electron–magnon dependence over most of the temperature range. Electron–phonon contribution indicates that alkali metal K doping provoked larger lattice distortion, while electron–electron interaction is more dominating process for Na and Rb doped compound favouring motion of excess charge carrier. Semiconducting nature is discussed with variable range hopping and small polaron conduction model. The change in activation energies and the density of states at the Fermi-level is consistently explained by cationic disorder and Mn valence.     

Structural and electrical transport properties of Nb-doped TiO2 films deposited on LaAlO3 by rf sputtering

We have investigated the structural and electrical transport properties of Nb-doped TiO₂ films deposited on (1 0 0) LaAlO₃ substrates by rf magnetron sputtering at temperatures ranging from 873 K to 1073 K. Films deposited below 998 K are anatase, and mixed phases between anatase and rutile exist in the film grown at higher temperatures. We find that films deposited at low temperatures exhibit semiconductor behavior, while metallic conductivity is observed in the most conducting film deposited at 998 K. For this sample, compared to electron-phonon scattering mechanism, electron-phonon-impurity interference effect plays an important role in its electron transport process. Moreover, the temperature coefficient of the resistivity for the film deposited at 1073 K is negative from 2 K to 300 K. The temperature dependence of resistivity for the film is described by ∼exp(b/T)^1/2 at temperatures from 80 K down to 30 K, and by the fluctuation induced tunneling model from 80 K to 300 K.