Density of states effective mass of n-type CuInSe2 from the temperature dependence of Hall coefficient in the activation regime

The Hall coefficient R_H of n-type CuInSe₂ single crystals is measured between 10 and 300 K in pulsed magnetic field up to 35 T. The threshold field B_th, above which the magnetic freezeout starts to occur, varies linearly with temperature. From the analysis of the temperature dependence of electron concentration in the activation regime above 100 K at different field values, it is established that the density of states effective mass is independent of the magnetic field B and the activation energy E_D, above around 6 T, varies as B^1/3. Similar B^1/3 dependence of the magnetoresistance in the high magnetic field regime, reported earlier in the same material, suggests that theoretical work that could explain this coincidence is needed.     

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.     

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.     

First-principles and Monte Carlo combinational study on Zn1−xCoxO diluted magnetic semiconductor

The electronic structures and magnetic properties of Zn_1−xCo_xO (x=5.55%,8.33%,12.5%) are studied using first-principles calculations in combination with Monte Carlo (MC) simulation. The combinational method makes possible a complete simulation from the microscopic magnetic interaction to macroscopic magnetic behavior. The calculated results from first principles indicate that the ferromagnetic ground state is stabilized by a half-metallic electronic structure which originates from the strong hybridization between Co 3d electrons and O 2p electrons. With the magnetic coupling strengths obtained from first-principles calculations, the MC simulation predicts the ferromagnetism of Zn_1−xCo_xO (x=5.55%,8.33%,12.5%) with , which is consistent with the experimental facts.     

Quantum critical behaviour in doped Y bB12 studied by ab initio calculations

The Kondo insulator Y bB₁₂ is known to undergo a transition to the metallic state with doping or under an external magnetic field. Within the virtual crystal approximation (VCA), we calculated the occupation of the Yb 4f and 5d shells, and , as a function of doping of Y bB₁₂ with the rare earths Tm and Lu. We found that exhibits an anomalous change at the critical concentration of the dopant, in agreement with experiment ( for Y b_1−xLu_xB₁₂ and for Y b_1−xTm_xB₁₂). We suggest that the critical behaviour seems to be strictly connected with the change of and in consequence the change of the Yb valency.     

Specific heat of (Ca1−xSrx)3Ru2O7 single crystals

We have measured the specific heat of crystals of (Ca_1−xSr_x)₃Ru₂O₇ using ac- and relaxation-time calorimetry. Special emphasis was placed on the characterization of the Néel () and structural () phase transitions in the pure, x=0 material. While the latter is believed to be first order, detailed measurements under different experimental conditions suggest that all the latent heat (with L∼0.3R) is being captured in a broadened peak in the effective heat capacity. The specific heat has a mean-field-like step at T_N, but its magnitude () is too large to be associated with a conventional itinerant electron (e.g. spin-density-wave) antiferromagnetic transition, while its entropy is too small to be associated with the full ordering of localized spins. The T_N transition broadens with Sr substitution while its magnitude decreases slowly. On the other hand, the entropy change associated with the T_c transition decreases rapidly with Sr substitution, and is not observable for our x=0.58 sample.     

Ac magnetotransport in La0.7Sr0.3Mn0.95Fe0.05O3 at low dc magnetic fields

We report the ac electrical response of La_0.7Sr_0.3Mn_1−xFe_xO₃(x=0.05) as a function of temperature, magnetic field (H) and frequency of radio frequency (rf) current (). The ac impedance (Z) was measured while rf current directly passes through the sample as well as in a coil surrounding the sample. It is found that with increasing frequency of the rf current, Z(T) shows an abrupt increase accompanied by a peak at the ferromagnetic Curie temperature. The peak decreases in magnitude and shifts down with increasing value of H. We find a magnetoimpedance of for at around room temperature when the rf current flows directly through the sample and when the rf current flows through a coil surrounding the sample. It is suggested that the magnetoimpedance observed is a consequence of suppression of transverse permeability which enhances skin depth for current flow. Our results indicate that the magnetic field control of high frequency impedance of manganites is more useful than direct current magnetoresistance for low-field applications.     

Physical properties of BaMg1/3Nb2/3O3-BaCo1/3Nb2/3O3 solid solutions

Structural, electric and magnetic properties of Ba₃Mg_1−xCo_xNb₂O₉ based dielectric ceramic compounds have been studied. The samples, prepared by a solid state reaction method, were characterised by X-ray powder diffraction (XRPD), electron microscopy (SEM), dielectric (ε(T)) and magnetic measurements (χ⁻¹(T)). The XRPD analyses showed that the crystal structure of these compounds does change by the increase of substitution degree, passing from a superstructure hexagonal-type, (no. 164), space group (SG) to a simple structure cubic-type, (no. 221), SG. However, the evolution of the elementary unit cell lattice parameter can be followed and it exhibit a linear increasing tendency with increase in the substitution, indicating the existence of a solid solution through out the investigated range of substitution (0-1). The microstructure analysis shows a variation in the grain size and also the porosity of the samples with the degree of substitution. The results are in good agreement with that of dielectric measurements, which also showed that the dielectric constant (ε) increases with the increase of cobalt content. The magnetic characterization of cobalt substituted samples showed an antiferromagnetic type super-exchange interaction between these magnetic ions. At the same time, the values of effective magnetic momentum (μ_eff) are close to the value that corresponds to Co²⁺ free ions. The study highlights the possibility of modelling these materials by substitutions, in order to improve properties of negative-positive-zero (NPO) type dielectric 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.     

4-benzyl pyridinium dihydrogenmonophosphate C6H5CH2C5H4NHH2PO4 : Single-crystal structure and its conductivity in polycrystalline form

The salt 4-benzyl pyridinium dihydrogenmonophosphate is monoclinic P2₁/c with the following unit cell dimensions: ; ; ; and β=97.328(11). Also, , D_x=1.403, , F(000)=560; ; and R=0.0495 and R_w=0.0964 for 3733 independent reflections. The structure consists of infinite parallel two-dimensional planes built of H₂PO₄⁻ anions and C₆H₅CH₂C₅H₄NH⁺ cations mutually connected by strong O-H ?O and N-H ?O hydrogen bonding. There are no contacts other than the normal Van der Waals interactions between the layers. The conductivity relaxation parameters associated with some H⁺ conduction have been determined from an analysis of the spectrum measured in a wide temperature range.     

Low temperature electrical transport in Ag substituted LaMnO3 polycrystalline pellets prepared by a pyrophoric method

A detailed investigation of the electrical resistivity of a series of monovalent (Ag) doped polycrystalline La_1−xAg_xMnO₃ pellets with x ranging from 0.05 to 0.30 and measured over the temperature range between 10 and 350 K is reported. La_1−xAg_xMnO₃ compounds have been prepared by a novel pyrophoric technique. XRD analysis of our samples indicates single phasic nature for samples with Ag content ≤0.15, while samples with higher Ag content (x≥0.2) show presence of both magnetic perovskite and non-magnetic Ag phases. A sharp insulator-metal transition with T_MI close to room temperature arising out of the paramagnetic to ferromagnetic transition, and a large magneto-resistance (MR=∼16%) near T_MI has been observed for all the Ag doped samples. Between T_C(T_p1) and 350 K, all the samples show activated conduction following the Emin-Holstein theory of adiabatic small polaron hopping, while at lower temperatures, in the ferromagnetic regime resistivity appears to be governed by various electron scattering processes. Between 20 and 50 K, a distinct minimum for both H=0 and has been observed, which is explained in terms of inelastic scattering and electron-electron interactions.     

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.     

Non-Fermi liquid behavior in the magnetotransport of quasi two-dimensional heavy Fermion compounds CeMIn5

We observe several non-Fermi liquid behaviors in the normal-state transport properties of CeMIn₅ (M: Rh and Co) under pressure at low temperatures: (1) The dc-resistivity shows T-linear dependence, ρ_xx∝T. (2) The magnitude of Hall coefficient |R_H| increases rapidly with decreasing temperature, and reaches a value much larger than |1/ne| at low temperatures. (3) The magnetoresistance displays T- and H-dependence that strongly violate Kohler's rule, and is well scaled by the tangent of the Hall angle, . These non-Fermi liquid properties in the electron transport are remarkably pronounced when the AF fluctuations are enhanced in the vicinity of the quantum critical point. Since all of these salient features have been also reported for high-T_c cuprates, we infer that the non-Fermi liquid transport properties capture universal features of strongly correlated electron systems in the presence of strong antiferromagnetic fluctuations.     

Formation of room-temperature ferromagnetic Zn1−xCoxO nanocrystals

Optical and magnetic properties of Co²⁺-doped ZnO nanocrystals were studied. Optical measurements confirm the incorporation of Co²⁺ in ZnO lattice with tetrahedral geometry. Optical absorption spectra also reveal the partial bleaching of the excitonic feature attributable to an increase in electron concentration. Magnetization measurements indicate the ferromagnetic ordering in Co²⁺-doped ZnO nanocrystals with saturation magnetization . No structural changes were observed in lightly doped ZnO nanocrystals. The present investigations are important in obtaining the ferromagnetic Zn_1−xCo_xO nanocrystals.