Anisotropic low-temperature in-plane magnetoresistance in electron doped Nd<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Ce<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CuO<Subscript>4+δ</Subscript>

Nominally electron doped antiferromagnetic tetragonal nonsuperconducting Nd_2?xCe _x CuO_4+δ(x=0.12) has been shown to manifest strong angular dependence of the in-plane magnetoresistance on the orientation of the external magnetic field within the ab plane in many aspects similar to that observed in hole doped YBa₂Cu₃O_7?δ and La_2?xSr_xCuO₄. Specific fourfold angular magnetoresistance anisotropy amounting to several percents was observed in oxygen annealed films at low temperatures and in an external magnetic field up to 5.5 T. The strong temperature dependence and fourfold symmetry observed in our sample points to a specific role of rare-earth (Nd) ions in magnetoresistance anisotropy. At low temperature T = 1.4 K, we observed the unusual transformation of magnetoresistance response with increasing the external magnetic field, which seems to be a manifestation of a combined effect of a crossover between first and second order spin-flop transitions and a field-dependent rare-earth contribution to quasiparticle magnetotransport.     

Pairing type change upon an increase in the cerium doping level in the Nd<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ce<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CuO<Subscript>4 + δ</Subscript> electronic superconductor

We report on the results of analyzing the temperature dependences of the resistivity of the Nd_{2 − x} Ce _x CuO_{4 + δ} electronic superconductor with x = 0.14 (underdoped range), x = 0.15 (optimal doping), and x = 0.18 (overdoped range) and with various degrees of annealing in an oxygen-free atmosphere in magnetic fields up to H = 90 kOe (H ‖ c, J ‖ ab) in the temperature range T = (0.4–300) K. It is shown that the observed differences in the dependences of the slope of the upper critical field (dH_c2 /dT)|_Tc (dH_{c2} /dT)|_{T_c } on the degree of disorder in the Nd_{2 − x} Ce _x CuO_{4 + δ} system upon a change in the cerium doping level indicate a change in the symmetry of d-type pairing to anisotropic s pairing.     

Magnetic structure of Ce<Subscript>2</Subscript>Fe<Subscript>17 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> intermetallic compounds

The magnetic structure of intermetallic compounds Ce₂Fe_{17 − x} Mn _x (0 ≤ x ≤ 3) was studied using neutron diffraction. The neutron diffraction patterns measured at 4.2 K contain satellites indicating a modulated structure with the wave vector k = [0, 0, τ]. As the concentration x increases, the value of τ increases, while the average magnetic moment of Fe/Mn atoms decreases. A change in the magnitudes of the average magnetic moment and wave vector k is explained by competition between exchange interactions at distances of nearest neighbor transition element atoms.     

High-temperature study of SrFe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mo<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3−<Emphasis Type="Italic">z</Emphasis></Subscript> perovskites

The structure of high-temperature SrFe_{1 − x} Mo _x O_{3 − z} (0 ≤ x ≤ 0.5) phases was studied. Such studies are necessary to understand the mechanism of oxygen transport in membrane materials used for high-temperature oxygen separation.     

Metallic-like conductivity of asymmetric <Emphasis Type="Italic">p</Emphasis>-In<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>As quantum wells

The temperature dependence of the conductivity of the two-dimensional hole gas in an asymmetric GaAs/In _x Ga_{1 ? x} As/GaAs quantum well has been investigated. It is shown that fast spin relaxation leads to metallic-like behavior of the temperature dependence of the conductivity.     

The magnetic field dependence of the low temperature (0.1K to 3K) residual specific heat of Pr<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Ce<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CuO<Subscript>4−δ</Subscript>

Measurements are presented of the low temperature specific heat of three polycrystalline samples of Pr_2?x Ce _x CuO₄ in magnetic fields of 0, 2 and 4 Tesla. The residual specific heat contribution in zero field seems to have a form αT ⁿ (n<1) for all of the samples, whether superconducting or non-superconducting. The behaviour in magnetic fields is similar to that reported for other high Tc superconductors where a change in exponent of the residual term occurs at some crossover temperature in magnetic fields.     

High-temperature heat capacity of orthovanadates Ce<Subscript>1–<Emphasis Type="Italic">x</Emphasis> </Subscript>Bi<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>VO<Subscript>4</Subscript>

Orthovanadates Ce_1–xBi_xVO₄ (1 ≥ x ≥ 0) have been produced by solid-phase synthesis from initial oxides CeO₂, Bi₂O₃, and V₂O₅ upon step-by-step burning. The high-temperature heat capacity of Ce_1–xBi_xVO₄ has been measured by differential scanning calorimetry. The experimental data on C_p = f(T) were used to calculate the thermodynamic properties (the enthalpy changes, the entropy changes, and the Gibbs energy).     

Magnetostriction of Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>S (<Emphasis Type="Italic">x</Emphasis> = 0.27) crystals

The magnetostriction of Fe _x Mn_{1 − x} S (x = 0.27) single crystals in strong magnetic fields up to 120 kOe has been investigated. It has been found that the magnetostriction reaches colossal values (±3 × 10⁻⁴) atypical of compounds of 3d elements. It has been found that the magnetostriction changes sign when varying temperature and magnetic field; this behavior indicates an important role of the spin-phonon interactions in the formation of the magnetic order in solid solutions of iron-manganese sulfides.     

Electrical conductivity and thermopower in Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Mn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript> S sulfides

This paper reports on the results of investigations into the structural, electrical, and thermoelectrical properties of sulfides Co _x Mn_{1 ? x} S (0 ≤ x ≤ 0.4) in the temperature range 80–950 K. It is established that the thermopower coefficient α decreases significantly with an increase in the cobalt concentration in the lattice of the α-MnS compound. The Co _x Mn_{1 ? x} S compounds with cobalt concentrations in the range 0 ≤ x ≤ 0.3 are semiconductors with hole conduction (α > 0), whereas the compound with x = 0.4 exhibits metallic conduction (α < 0). It is found that the band gap E _g of the compounds under investigation varies in the range from 1.46 eV for α-MnS (x = 0) to 0.26 eV for Co _x Mn_{1 ? x} S (x = 0.4).     

Unusual behavior of the lattice thermal conductivity and of the Lorenz number in the YbIn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>4+<Emphasis Type="Italic">x</Emphasis></Subscript> system

Samples of various compositions were obtained in the homogeneity range of the Yb-In-Cu system (YbIn_1?xCu_4+x), from stoichiometric (YbInCu₄) to YbIn_0.905Cu_4.095. Their lattice constant (at 300 K and in the range 20–100 K), total thermal conductivity, and electrical resistivity (from 4 to 300 K) were measured. All the compositions studied exhibited an isostructural phase transition at T _v ?40–80 K driven by a change in the Yb ion valence state. It was shown that within the YbIn_1?xCu_4+x homogeneity range, the lattice thermal conductivity κ_ph decreases with increasing x; at T>T _v , κ_ph grows with temperature and the Lorenz number (which enters the Wiedemann-Franz law for the electronic component of thermal conductivity) of the light heavy-fermion system, to which YbIn_1?xCu_4+x belongs for T<T _v , behaves as it does in classical heavy-fermion systems. Thermal cycling performed through T _v generates stresses in the YbIn_1?xCu_4+x lattice, which entails an increase in the electrical resistivity and a decrease in the thermal conductivity. “Soft anneal” (prolonged room-temperature aging of samples) makes the effect disappear. A conclusion is drawn as to the nature of the effects observed.     

Rubidium-cation conductivity of Rb<Subscript>3–2<Emphasis Type="Italic">x</Emphasis></Subscript>Pb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>PO<Subscript>4</Subscript> solid solutions

New Rb₃PO₄-based ceramic materials with high rubidium-cation conductivity in the Rb_3–2x Pb _x PO₄ system have been synthesized and studied. Introduction of Pb²⁺ cations leads to a sharp increase in the conductivity of rubidium orthophosphate due to formation of cation vacancies and, at temperatures 350–550°C, also due to the stabilization of high-temperature cubic modification Rb₃PO₄. At high temperatures, the electrolytes prepared have very high ion conductivity higher than 10^–1 S cm^–1 at 700°C, which is higher than the values previously obtained in similar systems with additions of tin and cadmium ions. The factors influencing the transport properties of the materials under study are discussed.     

Magnetic phase transitions in Nd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Emphasis Type="Italic">x</Emphasis>MnO<Subscript>3</Subscript> manganites

The magnetic properties of manganites of the Nd_1?xCa_xMnO₃ system with x≤0.15 have been studied. It is shown that, in the 0.06≤x≤0.1 interval, the results can be interpreted using a model according to which the concentrational transition from a weakly ferromagnetic (WFM) state (x=0) to a ferromagnetic (FM) state (x>0.15) proceeds via a mixture of the exchange-coupled FM and WFM phases. In the vicinity of T=9 K, samples with 0.06≤x≤0.1 exhibit a spontaneous magnetic phase transition involving reorientation of the magnetization vectors of the WFM and the exchange-coupled FM phases. In the temperature interval between 5 and 20 K, a sample with the composition Nd^0.92Ca^0.08MnO^2.98 exhibits metamagnetic behavior. Magnetic phase diagrams in the H?T and T?x coordinates are presented. The appearance of the spin-reorientation transitions is explained in terms of the magnetic analog of the Jahn-Teller effect with allowance for the fact that, according to the neutron diffraction data, the magnetic moments of neodymium ions in the FM phase are parallel to the magnetic moments of manganese ions.     

Physical properties of Fe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Dy<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Si crystals

The results of experimental investigation of magnetic and electric properties of Fe_1?x Dy _x Si crystals are reported. It is shown that the magnitude and position of the anomaly observed in the temperature dependences of magnetization are controlled to a considerable extent by the external magnetic field. It is found that the introduction of Dy ions leads to a weak magnetoresistive effect.     

Electrical conductivity of Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O ferromagnetic films at low temperatures

The results of experiments on electrical conductivity and magnetic properties of thin cobalt-doped zinc oxide films are reported. The results indicate the predominance of the hoping conduction mechanism at low temperatures and the band mechanism at high temperatures. An increase in the cobalt concentration from 1.5 to 6.3 at % leads to the reduction of the electrical conductivity of the films. The contribution of hopping conduction to the conductivity increases due to a decrease in the crystallinity of the films and localization of a part of electron states upon an increase in the cobalt concentration. For cobalt-containing films, a hysteresis of the magnetic moment as a function of the magnetic field is observed. The dependence of the shape of the magnetization curves on the cobalt concentration is irregular. The paramagnetic contribution to the magnetic susceptibility increases with the cobalt concentration.     

Magnetic state of a Zn<Subscript>1 −<Emphasis Type="Italic">x</Emphasis></Subscript>Cr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se bulk crystal

The spin system of a Zn_{1 ?x} Cr _x Se bulk crystal (x = 0.045) was studied using thermal-neutron diffraction and magnetic measurements. Previously, it was reported in the literature that thin films (～200 nm thick) of this type of semiconductors exhibit a ferromagnetic order. In this study, the ferromagnetic order is found to be absent in the bulk crystal.     

Charge density of Ga<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Al<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sb

Charge density calculations and electronic band structures for Ga _x Al_{1 − x} Sb with x = 1.0, 0.5 and 0.0 are presented in this work. The calculations are performed using the empirical pseudopotential method. The charge density is computed for a number of planes, i.e. z = 0.0, 0.125 and 0.25 A ₀ by generating the potential through a number of potential parameters available in the literature. The virtual crystal approximation was applied for the semiconducting alloy. The characteristics of the band structure and charge density are observed to be affected by the potential parameters. Calculated band gaps and the nature of gaps are in good agreement with the experimental data reported. The ionicity is also reasonably in good agreement with other scales proposed in the literature; however the formulation needs to be improved. The present work also demands indirect experimental band gap for the alloy.      

Theory of the giant magnetoresistance in La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>A<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript>

To clarify the origin of the giant magnetoresistance (GMR) observed in La_1?x A _x MnO₃ (A²⁺=Sr²⁺, Pb²⁺, Ba²⁺ and Ca²⁺), we have investigated theoretically the electrical resistivity ρ of carriers in the background of Mn spins which interact with each other through the double exchange interaction. It has been found that extraordinarily large pin fluctuations caused by the instability of the ferromagnetic state are responsible for the transport anomalies including the GMR.     

Electronic structure and ground state parameters of Ru<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript><Emphasis Type="Italic">Me</Emphasis><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Al refractory alloys

The electronic structure and ground state parameters of B2 RuAl-based refractory alloys have been investigated in the framework of the density functional theory using the exact muffin-tin orbital method in combination with the coherent potential approximation. It has been demonstrated that the number of states at the Fermi level for the Ru_{1 − x} Me _x Al alloys as a function of the alloying metal content has a minimum, which indicates a change in the Fermi surface topology and the presence of specific features in the behavior of elastic constants. It has been concluded that the electronic structure of the alloys can be described in terms of the rigid band model. The nonlinear variations of the lattice parameters of the alloys has been explained.     

Effect of surface roughness on thermal conductivity of VLS-grown rough Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> nanowires

The recent demonstration of thermal conductivity of rough electrolessly etched Si nanowire (Hochbaum et al., Nature, 451:163, 2008) attracted a lot of interest, because it could not be explained by the existing theory; thermal conductivity of rough Si nanowires falls below the boundary scattering of the thermal conductivity. However, nanoscale pores presented in the nanowires (Hochbaum et al., Nano Letters, 9:3550–3554, 2009) hinder one to be fully convinced that the surface roughness solely made a contribution to the significant reduction in thermal conductivity. In this study, we synthesized vapor–liquid–solid (VLS) grown rough Si_1−x Ge _x nanowire and measured and theoretically simulated thermal conductivity of the nanowire. The thermal conductivity of rough Si_0.96Ge_0.04 nanowire is an order of magnitude lower than that of bulk Si_0.96Ge_0.04 and around a factor of four times lower than that of smooth Si_0.96Ge_0.04 nanowire. This significant reduction could be explained by the fact that the surface roughness scatters medium-wavelength phonons, whereas the long-wavelength phonons are scattered by phonon boundary scattering, and the short-wavelength phonons are scattered by alloy scattering.     

Low temperature features of the local structure of Sm<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Y<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>S

The particular features of the local electronic and local crystal structures of the mixed-valence compound Sm_{1 ? x} Y _x S are studied by the XAFS spectroscopy methods in the temperature range 20–300 K for the yttrium concentration x = 0.17, 0.25, 0.33, and 0.45. The temperature behavior of the valence of Sm, as well as of the lengths and the Debye-Waller factors of the bonds Sm-S, Sm-Sm(Y), Y-S, and Y-Sm(Y), has been determined. The violation of the Vegard law has been observed. A model for the estimation of the energy width of the 4f level and of its position with respect to the Fermi level is proposed.