Multiband Superconductors Close to a 3D–2D Electronic Topological Transition

Within the two-band model of superconductivity, we study the dependence of the critical temperature T _c and of the isotope exponent α in the proximity to an electronic topological transition (ETT). The ETT is associated with a 3D–2D crossover of the Fermi surface of one of the two bands: the σ subband of the diborides. Our results agree with the observed dependence of T _c on Mg content in A $_{1-x}{\rm Mg}_x{\rm B}_2$ (A?=?Al or Sc), where an enhancement of T _c can be interpreted as due to the proximity to a ‘shape resonance.’ Moreover we have calculated a possible variation of the isotope effect on the superconducting critical temperature by tuning the chemical potential.     

The Material-Dependent Parameter Controlling the Universal Phase Diagram of Cuprates

The critical temperature T _c in the universal phase diagram of cuprate superconductors is a function of two variables: the hole-doping δ and a material dependent parameter. Here we focus on the behavior of T _c,max as a function of the material dependent parameter (MDP) at the optimum hole doping. We have discussed the correlation between (1) the average Cu—O (planar) distance, or the strain of the Cu—O bond, (2) the nearest-neighbor hopping t′ and (3) the Lifshitz parameter z. These Lifshitz parameter z = μ_{δ = 0.16}−E _vHs which are all material dependent parameters, where μ_{δ = 0.16} is the chemical potential at optimum doping and E_vHs is the energy of the Van Hove singularity, defines the proximity to the Fermi surface topological transition from electron-like to hole-like. The results show that the striped phases occur for z < 0, the highest T _c,max for and the drop of T _c,max for z > 75 meV.     

Turbidity Data of Weightless SF<Subscript>6</Subscript> Near its Liquid–Gas Critical Point

Light transmission measurements performed in SF₆ close to its liquid–gas critical point are used to obtain turbidity data in the reduced temperature range (T is temperature, T _c is the critical temperature). Automatic experiments (ALICE 2 facility) were made at a near critical density, i.e., , in the one-phase homogeneous region, under the microgravity environment of the Mir Space Station ( is the average density, ρ _c is the critical density). The turbidity data analysis verifies the theoretical crossover formulations for the isothermal compressibility and the correlation length ξ. These latter formulations are also used to analyze very near T _c thermal diffusivity data obtained under microgravity conditions by Wilkinson et al. (Phys. Rev. E 57 436, 1998).     

Theory of Plane Copper and Oxygen Nuclear Spin–Lattice Relaxation Rates in Lightly Doped La2− x Sr x CuO4

Using a Mori-Zwanzig projection operator procedure the relaxation function theory of doped two-dimensional Heisenberg antiferromagnetic (AF) system in the paramagnetic state is presented taking into account the hole subsystem as well as both the electron and AF correlations. At low temperatures the main contribution to the nuclear spin–lattice relaxation rate, ⁶³(1/T ₁), of plane ⁶³Cu, arises from the AF fluctuations, and ¹⁷(1/T ₁), of plane ¹⁷O, has the contributions from the wave vectors in the vicinity of (π,π) and small q ∼ 0. The effects of thermal spin-wave damping Γ _q on ¹⁷(1/T ₁) in lightly doped regime are investigated, suggesting either a polynomial of up to third order (not simply (T/J)³) or exponential temperature dependence of Γ _q at low temperatures. It is shown that the theory is able to explain the main features of experimental data on temperature and doping dependence of ^17,63(1/T ₁) in the paramagnetic state of La_{2− x} Sr _x CuO₄ compounds.     

Investigation on the Structure and Superconducting Properties of Mg1 ? xMxB2 (M = deficiency or Ca)

We report the preparation of Mg_{1 – x} M _x B₂ (M = deficiency or Ca) compounds and their structure and superconducting properties. For Mg_{1 – x} B₂, although nearly single-phase samples can be obtained for x = 0, MgB₄ coexists with the MgB₂ phase and some minor impurity phases, and the amount of MgB₄ increases with x for 0 < x 0.5. The lattice parameters a and c of MgB₂ decreases and increases, respectively, with the increase of x, and T _c also decreases. While for Mg_{1 – x} Ca _x B₂, the superconducting transition temperature remains unchanged for x 0.3 and loss of superconductivity occurs for x > 0.3. X-ray diffraction patterns for x 0.3 samples show that MgB₂ phase coexists with CaB₆, Mg, and MgO. With increasing x, the amount of CaB₆, Mg, and MgO increases, while the amount of MgB₂ decreases. The lattice parameters of MgB₂ phase do not show any obvious change in contrast to Mg_{1 – x} B₂. The results were discussed by considering some possible contributions.     

Lattice Anomalies in (La,Sr)2CuO4 Under Epitaxial Strain Probed by Polarized X-Ray Absorption Spectroscopy

Local lattice anomalies in optimally doped T-(La,Sr)₂CuO₄ single crystal like thin films (T _c = 43.4 K) grown by molecular-beam epitaxy have been studied by the in-plane polarized Cu K-edge extended X-ray absorption fine structure (EXAFS). The results indicate temperature-dependent local atomic displacements which are anomalous at the T _c and below a higher temperature T _s as demonstrated by a change in the mean square relative displacement of the Cu–O bond , i.e., a sharp drop at the T _c and a gradual deviation from a noncorrelated Debye-like behavior below T _s where the spatial inhomogeneity appears. We find that the magnitude of the Cu–O displacement changes at the T _c, is enhanced by compressive strain while the tendency of charge segregation is suppressed. The results suggest that the uniaxial pressure effects stabilize the system by decreasing the onset temperature and magnitude of spatial heterogeneity.     

High-Pressure Studies on Superconductivity in LaFeAsO1−x F x and SmFeAsO1−x F x

The pressure dependence on the superconducting transition temperature (T _c ) was investigated for the iron-based superconductors LaFeAsO_1−x F _x and SmFeAsO_1−x F _x . The T _c ’s increase largely for LaFeAsO_1−x F _x with a small increase of pressure, while a sharp decrease of T _c was observed for SmFeAsO_1−x F _x . The electrical resistivity measurements reveal pressure-induced superconductivity for undoped LaFeAsO and SmFeAsO. These pressure effects seem to be related to an anisotropic decrease of the lattice constants under high pressure from the x-ray diffraction measurements up to 10 GPa for the LaFeAsO_1−x F _x system.     

HRTEM and EELS Studies on Structural Defects in the Superconductor Sr2CuO2+δCl2−y Newly Synthesized Under High Pressure

Structural defects in the newly high-pressure synthesized superconductor Sr₂CuO_2+δCl_2−y have been studied by means of high-resolution transmission electron microscopy and parallel electron energy-loss spectroscopy. Two types of grain boundaries were found in the material. One is the amorphous boundaries usually lying in the ab-planes. The other is the structurally intact grain boundaries, and most of them are large-angle boundaries. One of the large angles frequently observed is the 90°〈110〉 asymmetrical tilt boundary. In some domains, periodicity lengths were changed due to the lattice distortion usually associated with atom deficiency. These domains can be regarded as “second-phase particles” embedded in the perfect grains. O-K electron energy-loss spectroscopy results suggest that those defect domains are in the hole-poor states. Quantitative composition analysis from energy dispersive of X-ray suggests the hole-poorness is caused by oxygen deficiency. Dislocations, such as the a[110] and c[001] edge dislocations, were also observed in the material. In most cases, those dislocations are very complex and exhibit stacking faults. PACS: 68.37.Lp, 67.80.Mg, 74.72.Jt     

How Grain-Boundaries Influence the Intergranular Critical Current Density of Cu1−x Tl x Ba2Ca3Cu4O12−δ Superconductor Thin Films?

The insulating and metallic behavior of the grain-boundary weak links has been studied in thallium rich and the samples with small amount of thallium in the charge reservoir layer of Cu_1−x Tl _x Ba₂Ca₃Cu₄O_12−δ superconductor thin films. The influence of the nature of grain boundaries on the inter-granular critical current density (J _c) has also been investigated. From the power law dependence of H _ac∼(1−T _p/T _c) ⁿ , it was observed that n=1 gives a best fit for the J _c of thallium rich samples and n=2 provides a best fit for the J _c of the samples with small amount of thallium. The polycrystalline thin film samples showing the power law dependence of J _c as n=1 make superconductor-insulator-superconductor (SIS) type while the samples with n=2 follow superconductor-normal metal-superconductor (SNS) types of Josephson junctions. The insulating grain boundaries decrease the inter-granular Josephson coupling and hence the transport properties are suppressed.      

Partial Molar Volumes and Viscosity B-Coefficients of Nicotinamide in Aqueous Resorcinol Solutions at <Emphasis Type="Italic">T</Emphasis> = (298.15, 308.15, and 318.15) K

Partial molar volumes and viscosity B-coefficients for nicotinamide in (0.00, 0.05, 0.10, 0.15, and 0.20) mol·dm⁻³ aqueous resorcinol solutions have been determined from solution density and viscosity measurements at (298.15, 308.15, and 318.15) K as a function of the concentration of nicotinamide (NA). Here the relation , has been used to describe the temperature dependence of the partial molar volume . These results and the results obtained in pure water were used to calculate the standard volumes of transfer and viscosity B-coefficients of transfer of nicotinamide from water to aqueous resorcinol solutions to study various interactions in the ternary solutions. The partial molar volume and experimental slopes obtained from the Masson equation have been interpreted in terms of solute–solvent and solute–solute interactions, respectively. The viscosity data have been analyzed using the Jones–Dole equation, and the derived parameters B and A have also been interpreted in terms of solute–solvent and solute–solute interactions, respectively, in the ternary solutions.The structure making or breaking ability of nicotinamide has been discussed in terms of the sign of . The activation parameters of viscous flow for the ternary solutions studied were also calculated and explained by the application of transition state theory.