Über die Supraleitung von Hf-Rh-Legierungen

The transition temperaturesT _c of quenched Hf- Rh alloys are strongly increased in the hexagonal phase by Rh, similar to the homologous systems Ti-Rh and Zr-Rh. On the other hand a stabilization of the cubic high-temperature phase by Rh has not been possible. At higher amounts of Rh(>6 at%) the intermetallic compound Hf₂Rh with aT _c of 1.98 °K is formed. Moreover a transition temperature of 1.73 °K is found which is probably due to the intermetallic compound HfRh. The results above are interpreted with the help of X-ray diffraction and metallurgic techniques.     

Structural transformations of K<Subscript>3</Subscript>H(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals with variations in temperature

Single crystals of the K₃H(SO₄)₂ compound are investigated using X-ray diffraction on Xcalibur S and Bruker diffractometers. The structure of the low-temperature monoclinic phase is refined (space group C2/c, z = 4, a = 14.698(1) Å, b = 5.683(1) Å, c = 9.783(1) Å, β = 103.01(1)°, T = 293 K, Bruker diffractometer), the structural phase transition is revealed, and the structure of the high-temperature trigonal phase is determined (space group R \(\bar 3\) m, z = 3, a = 5.73(1) Å,c = 21.51(1) Å,T = 458 K, Xcalibur diffractometer).     

Superconducting transition temperature in hafnium under pressures up to 64 GPa

The superconducting transition temperature T _c of hafnium is measured as a function of pressure up to 64 GPa. The character of the pressure dependence of T _c observed at α–ω–β transitions in Hf is found to be similar to that observed for Zr. In the regions of α and β phases, T _c increases with pressure with the slopes dT _c /dP=0.05 and 0.16 K/GPa, respectively. At the α–ω transition, T _c (P) exhibits a tendency to a decrease, while at the ω–β transition, T _c increases stepwise from 5.8 to 8.0 K. The α–ω transition occurs at pressures between 31.2 and 35.9 GPa, and the ω–β transition, at a pressure of 62±2 GPa.     

X-ray study of [N(CH<Subscript>3</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> at low temperatures

The unit cell parameters a, b, and c of [N(CH₃)₄]₂ZnCl₄ have been measured by x-ray diffraction in the temperature range 80–293 K. Temperature dependences of the thermal expansion coefficients α_a, α_b, and α_c along the principal crystallographic axes and of the unit cell thermal expansion coefficient α_V were determined. It is shown that the a=f(T), b=f(T), and c=f(T) curves exhibit anomalies in the form of jumps at phase transition temperatures T₁=161 K and T₂=181 K and that the phase transition occurring at T₃=276 K manifests itself in the a=f(T) and b=f(T) curves as a break. A slight anisotropy in the coefficient of thermal expansion of the crystal was revealed. The phase transitions occurring at T₁=161 K and T₂=181 K in [N(CH₃)₄]₂ZnCl₄ were established to be first-order.     

Low-temperature anomalies in the specific heat and thermal conductivity of MgB<Subscript>2</Subscript>

The temperature dependences of the specific heat C(T) and thermal conductivity K(T) of MgB₂ were measured at low temperatures and in the neighborhood of T _c . In addition to the well-known superconducting transition at T _c ≈40 K, this compound was found to exhibit anomalous behavior of both the specific heat and thermal conductivity at lower temperatures, T≈10–12 K. Note that the anomalous behavior of C(T) and K(T) is observed in the same temperature region where MgB₂ was found to undergo negative thermal expansion. All the observed low-temperature anomalies are assigned to the existence in MgB₂ of a second group of carriers and its transition to the superconducting state at T_c2≈10?12 K.     

Einfluß von Gitterstörungen auf die Supraleiteigenschaften von Blei

The transition temperatureT _c and the critical fieldH _c of lead were measured as a function of the concentration of lattice defects. The defects were generated by plastic deformation at liquid Helium temperatures and reduced by annealing. T _c is rather insensitive to defects. With increasing residual resistance ratio ρ the transition temperature increases and finally reaches a constant value with onlyΔT _c ≈4.5 · 10^?3 °K. On the other hand a deformation of the same amount increasesH _c more than twice as much as the starting value. Annealing to room-temperature reducesρ, T _c andH _c to their initial values. During the annealing process,T _c shows a distinct maximum and ρ a marked step. Contrary to this behaviourH _c decreases linearly during the whole region of annealing. Taking into account the strong influence of ρ uponH _c a picture is given about the mechanism of deformation, which allows to understand the results qualitatively. The changes ofT _c produced by elastic strain were also measured. The results are in quantitative agreement with those of pressure experiments.     

Heat capacity and thermodynamic properties of HoMnO<Subscript>3</Subscript> in the range of 364–1046 K

The temperature dependence of the molar heat capacity of HoMnO₃ has been measured by differential scanning calorimetry. The experimental data have been used to calculate the thermodynamic properties of the oxide compound (changes in the enthalpy H°(T)–H°(364 K), entropy S°(T)–S°(364 K), and reduced Gibbs energy Φ°(T)). The data on the heat capacity of HoMnO₃ have been generalized in the range of 40–1000 K.     

Novel superconducting niobium beryllide Nb<Subscript>3</Subscript>Be with A15 structure

New niobium beryllide Nb₃Be with A15 structure and lattice parameter a=0.5187±0.0007 nm, coexisting (3–5%) with a tetragonal phase, presumably, of an ordered solid solution with parameters a=0.5414±0.0008 nm and c=0.6378±0.0009 nm, was synthesized by thermal treatment (875–110°C) of amorphous film coatings containing 26.8–32.4 at. % Be and formed from short-period Nb and Be layers by magnetron sputtering. The domain of existence of the Nb₃Be phase and the critical superconducting transition temperature (10.0 K and a transition width of 2.5 K) were determined and the X-ray structural data for structure identification were obtained.     

Anisotropic<Emphasis Type="Italic">s</Emphasis>-wave or<Emphasis Type="Italic">d</Emphasis>-wave pairing? Analysis of experiments on ion irradiation of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7-x</Subscript> films

The effect of ion irradiation on the superconducting transition temperatureT _c and resistivityρ _ab (T) of YBa₂Cu₃O_7-x films with different oxygen content (initial temperatureT _c0≈90 K and 60 K) is studied experimentally. The dependenciesT _c /T _c0 on residual resistivityρ _o are obtained in very wide range 0.2<T _c /T _c0 <1 andρ _o μΩ·cm. The critical values ofρ _o , corresponding to the vanishing of superconductivity, are found to be an order of magnitude larger then those predicted by theory ford-wave pairing. At 0.5÷0.6<T _c /T _c0<1 the experimental data are in close agreement with theoretical dependencies, obtained for the anisotropics-wave superconductor within the BCS-framework.     

Supraleitung von Niob unter uniaxialem Zug

The transition temperature of Nb-wires is increased by uniaxial stress. The average slope of the observedT _c (σ)-dependence equals 3.2 · 10^?5 °K cm²/kp.     

Crystallographic and dynamic characteristics of K<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Rb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiOPO<Subscript>4</Subscript> solid solutions in the low-temperature ranges

The unit cell parameters a and c of K_{1 ? x} Rb _x TiOPO₄ (x= 0, 0.3, and 0.5) crystals are investigated using x-ray diffraction in the temperature range 90–320 K. The thermal expansion coefficients along the principal crystallographic axes of the crystals are determined from the obtained temperature dependences of the unit cell parameters. It is found that the parameter a increases with increasing temperature, while the parameter c decreases. For the crystals under investigation, the elastic moduli c ₁₁ and c ₃₃ along the [100] and [001] directions are determined by the ultrasonic pulse-echo technique in the temperature range 100–350 K. It is shown that c ₃₃ > c ₁₁. The anomalies revealed in the temperature dependences of the crystallographic and dynamic characteristics of the samples at a temperature T ≈ 280 K indicate the occurrence of a phase transition. The temperature of the phase transition is found to increase with increasing rubidium content x.     

Anomalous temperature dependence of magnetic relaxation in YBa<Subscript>2</Subscript>Cu<Subscript>2</Subscript>O<Subscript>7 − δ</Subscript> oxygen-deficient single crystals

The dependence of the magnetization relaxation rate S = ?d lnM/dlnt on temperature T is measured in YBa₂Cu₃O_{7 ? δ} samples with various oxygen concentrations. It is found that the S(T) curve changes qualitatively when oxygen deficit δ exceeds the threshold value δ_th = 0.37. For δ < δ_th (T _c > 60 K, where T _c is the superconducting transition temperature), function S(T) has the well-known peak at T/T _c = 0.4. For δ > δ_th (at T _c < 51 K), this peak transforms into a plateau and a new sharp peak appears at T/T _c = 0.1. The threshold value δ_th of the oxygen deficit corresponds to the transition of the sample from the disordered state into the ordered state of oxygen vacancies. We consider the change in the shape of the S(T) curve as a macroscopic manifestation of this transition.     

Dielectric properties of betaine phosphite-betaine phosphate solid-solution crystals in the improper ferroelastic phase

Temperature dependences of dielectric permittivity in the improper ferroelastic phase, including the region of the improper ferroelastic phase transition occurring at T=T_c1, were studied in the betaine phosphite-betaine phosphate solid-solution crystals. At a betaine phosphate (BP) concentration of 10%, the phase transition temperature T_c1 was found to shift toward higher temperatures by about 5 K compared to betaine phosphite (BPI) crystals, where T_c1=355 K. The phase transition remains in the vicinity of the tricritical point. As the BP concentration in BPI is increased, the dielectric anomaly at T=T_c1 weakens substantially compared to pure BPI. The nonlinear temperature dependence of reciprocal dielectric permittivity in the improper ferroelastic phase of BPI_xBP_1?x crystals is described in the concentration region 0.9≤x≤1 in terms of a thermodynamic model taking into account the biquadratic relation of the nonpolar order parameter of the improper ferroelastic phase transition to polarization. The decrease in the ferroelectric phase transition temperature T_c1 (or in the temperature of loss of improper ferroelastic phase stability) with increasing BP concentration in the above limits is due to the decreasing effect of the nonpolar mode on the polar instability, which is accompanied by a weakening of the dielectric anomaly at T=T_c1     

Reflexivity and Correlation of Magnetic States of Nanostructures in the Nb(70 nm)/Ni<Subscript>0.65</Subscript>Cu<Subscript>0.35</Subscript>(6.5 nm)/Si Ferromagnet–Superconductor Heterostructure

We have studied the Nb(70 nm)/Ni_0.65Cu_0.35(6.5 nm)/Si layered structure in the temperature range T = 1.5–10 K using polarized neutron reflectometry. The correlation of the states of magnetic structures is observed at temperature T = 9 K, which is slightly higher than the superconducting transition temperature T_c = 8.5 K of the structure. At temperature T = 4 K, which is lower than T_c, the effect of reflexivity of magnetic states existing at T = 9 K was observed.     

Electrical and Magnetic Properties of Pb and In Nanofilaments in Asbestos near the Superconducting Transition

Bulk nanocomposites based on superconducting metals Pb and In embedded into matrices of natural chrysotile asbestos with the nanotube internal diameter d ~ 6 nm have been fabricated and studied. The low-temperature electrical and magnetic properties of the nanocomposites demonstrate the superconducting transition with the transition critical temperature T_c ≈ (7.18 ± 0.02) K for the Pb–asbestos nanocomposite (this temperature is close to T_{c bulk} = 7.196 K for bulk Pb). The electrical measurements show that In nanofilaments in asbestos have T_c ~ 3.5–3.6 K that is higher than T_{c bulk} = 3.41 K for bulk In. It is shown that the temperature smearing of the superconducting transition in the temperature dependences of the resistance R(T) ΔT ≈ 0.06 K for the Pb–asbestos and ΔT ≈ 1.8 K for the In–asbestos are adequately described by the fluctuation Aslamazov–Larkin and Langer–Ambegaokar theories. The resistive measurements show that the critical magnetic fields of the nanofilaments extrapolated to T = 0 K are H_c(0) ~ 47 kOe for Pb in asbestos and H_c(0) ~ 1.5 kOe for In in asbestos; these values are significantly higher than the values for the bulk materials (H\(H_{\rm{c}}^{\rm{bulk}}\) = 803 Oe for Pb and \(H_{\rm{c}}^{\rm{bulk}}\) = 285 Oe for In). The results of the electrical measurements for Pb?asbestos and In–asbestos agree with the data for the magnetic-field dependences of the magnetic moment in these nanocomposites.     

Supraleitung des Galliums bei hohem Druck

A high-pressure modification of Gallium was formed at liquid helium temperature by the application of a pressure in excess of 35000 atm. It was superconducting with a transition temperature of 6·38°K. The high-pressure form remained after the pressure was removed at helium temperature. This resulted, however in a shift in transition temperature to 7·5°K. The average derivativedT _c/dp was ?3·10^?5°K/atm.     

Magnetic properties of the Gd<Subscript>2</Subscript>V<Subscript>0.67</Subscript>Mo<Subscript>1.33</Subscript>O<Subscript>7</Subscript> and Y<Subscript>2</Subscript>VMoO<Subscript>7</Subscript> pyrochlore compounds

It is demonstrated that 50% substitution of vanadium for molybdenum in the pyrochlore lattice of the complex oxide Y₂(V _x Mo_{1 ? x} )₂O₇ results in a transition from the spin-glass ground state (at x = 0) to the ferromagnetic state in Y₂VMoO₇ (a = 10.1645(2) Å, T _C = 55 K). The Gd₂V_0.67Mo_1.33O₇ compound (a = 10.2862(3) Å) is a ferromagnet with T _C (84 K) exceeding that of undoped Gd₂MnO₂O₇.     

Temperature Dependence of the Upper Critical Field in Disordered Hubbard Model with Attraction

We study disorder effects upon the temperature behavior of the upper critical magnetic field in an attractive Hubbard model within the generalized DMFT+Σ approach. We consider the wide range of attraction potentials U—from the weak coupling limit, where superconductivity is described by BCS model, up to the strong coupling limit, where superconducting transition is related to Bose–Einstein condensation (BEC) of compact Cooper pairs, formed at temperatures significantly higher than superconducting transition temperature, as well as the wide range of disorder—from weak to strong, when the system is in the vicinity of Anderson transition. The growth of coupling strength leads to the rapid growth of H_c2(T), especially at low temperatures. In BEC limit and in the region of BCS–BEC crossover H_c2(T), dependence becomes practically linear. Disordering also leads to the general growth of H_c2(T). In BCS limit of weak coupling increasing disorder lead both to the growth of the slope of the upper critical field in the vicinity of the transition point and to the increase of H_c2(T) in the low temperature region. In the limit of strong disorder in the vicinity of the Anderson transition localization corrections lead to the additional growth of H_c2(T) at low temperatures, so that the H_c2(T) dependence becomes concave. In BCS–BEC crossover region and in BEC limit disorder only slightly influences the slope of the upper critical field close to T _c . However, in the low temperature region H_c2 (T may significantly grow with disorder in the vicinity of the Anderson transition, where localization corrections notably increase H_c2 (T = 0) also making H_c2(T) dependence concave.