Modern Physics Letters B

We report on tunneling and Andreev-reflection conductance spectra of 39 K superconducting magnesium diboride, obtained with Pb and Au counter-electrodes. Two distinct steps at close to 2.7 and 7.1 meV appear in a low-resistance metallic-type Au–MgB₂ junction characteristic, whereas a tunneling-like spectrum measured for the same junction, annealed by the application of DC current, exhibits only a rounded contribution of the larger gap. Junctions with a superconducting lead counter-electrode pressed into a bulk MgB₂ sample reveal two conductance peaks that are interpreted as the result of the formation of a highly-transmitting break junctions inside the magnesium diboride ceramic. Our results strongly support the two-band model with two different gap values on quasi-two-dimensional σ (7.1 meV) and three-dimensional π (2.7 meV) Fermi surface sheets of MgB₂.

The two-dimensional ferromagnetic t–J model is studied in the framework of slave-boson mean field theory. After solving the mean field self-consistent equations, we find that there exists a hole-doping region δ₁<δ<δ₂ in which p-wave superconductivity and ferromagnetism coexist. There also exists a T* line, under which pre-paired p-wave cooper pairs appear. The similarity and differences between our theoretical results and the experimental ones made for UGe₂ are compared.

A variational method is used to study the ground-state binding energies of interface light-hole excitons in ZnTe/CdSe type-II heterojunctions under the influence of hydrostatic pressure. The finite triangle potential well approximation is introduced considering the band bending near the interface. The asymptotic transfer method is adopted to obtain the sub-band energies and wave functions of the electrons and light holes. The pressure influence on the band offsets, the effective masses and the dielectric constant are considered in the calculation. The obvious pressure-induced increase of the exciton binding energy is demonstrated and the influences of the pressure-depended parameters on the binding energy are compared.

Spin-dependent ballistic transport is studied for a two-dimensional electron gas (2DEG) heterostructure, sandwiched between two ferromagnetic metal (FM) layers. The device exhibits a strong spin-filtering behavior when the magnetization directions of the FM layers are anti-parallel to one another. Additionally, a pronounced magnetoconductance (MC) effect is predicted when the relative magnetization direction of the FM layers is changed, e.g. by applying an external magnetic field sweep. Both the spin-filter and spin-valve (SV) functions can be tuned externally by adjusting the applied voltage and the fringe field strength from the FM layers. Compared to conventional all-metal SVs, the proposed 2DEG-based SV potentially has the advantages of a higher MC ratio (of as high as 90%), external tenability, and less susceptibility to interfacial imperfections.

The surface superconductivity of a mesoscopic superconducting cylinder in an external parallel magnetic field is discussed within the framework of the linearized Ginzburg–Landau equation under different boundary conditions. For the superconductor/insulator interface, our calculations give the corrugation-like phase boundary H_c3(T), with oscillating ripples as a result of the switching of the "giant vortex state" wave function between different angular quantum number L. The tangent for the linear background of the phase boundary H_c3(T) is about 1.7 times H_c2(T), consistent with the well-known result in bulk surface superconductivity. For the superconductor/normal-metal interface, surface superconductivity disappears, accompanied by a decrease in the superconducting transition temperature T_c(H=0).

The magnetic properties of U_1-xDy_xAl_yNi_5-y (y=0,1) systems were investigated in the 2(5)–600 K temperature range and for fields up to 80 kOe. The systems having x≥0.2 are magnetically ordered with low transition temperatures and magnetization mainly due to the Dy contribution. The nickel exhibits magnetic moments, very weak in the low temperature range and well-defined effective moments over transition temperatures. The nickel behavior is discussed in terms of the spin fluctuation model.

We obtain analytically a universal expression of the resonant energies for any one-dimensional (1D) models with the defects having symmetric internal structures. In a 1D periodic system with the on-site energy ε₀=0 and a nearest-neighbor matrix element t₀=1.0, two classes of the most interesting and simplest wavefunction behaviors are numerically obtained for the resonant energies around (a) 0, ±1, (b) , respectively. We show that similar wavefunction behaviors can be found widely in many quasiperiodic and random systems where the delocalization phenomena are predicted. We suggest that the envelope of these wavefunctions can be generally used as a criterion of delocalization of electronic states in 1D random and quasiperiodic lattices.