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[en] An analytical study of the gap equation in the Bogoliubov formulation is presented. The normal-superconducting phase boundary is simulated by the expression Δ (R/sup =/) = Δ/sub infinity/ tanh / α Δ/sub infinity/z/v/sub f/) theta(z) where Δ/sub infinity/(t) is the equilibrium gap, theta (z) a unit step function and v/sub f/ the Fermi velocity. The Bogoliubov-de Gennes equations are solved in a nonperturbative WKBJ approximation. The gap equation is expanded near T/sub c/ in powers of Δ/sub infinity/ and the major term is of the same order as that given by the Ginzburg-Landau-Gor'kov approximation. Discrepancies in the two values are discussed in detail. It is concluded that the present technique reproduces the Ginzburg-Landau-Gor'kov results except within a BCS coherence length. 25 references
[en] Revealing a universal relation between geometrical structures and electronic properties of capped carbon nanotubes (CNTs) is one of the current objectives in nanocarbon community. Here, we investigate the local curvature of capped CNTs and define the cap region by a crossover behavior of the curvature energy versus the number of carbon atoms integrated from the tip to the tube region. Clear correlations among the energy gap of the cap localized states, the curvature energy, the number of carbon atoms in the cap region, and the number of specific carbon clusters are observed. The present analysis opens the way to understand the cap states.
[en] Topological insulators are characterized by a massless Dirac surface state and a bulk energy gap. An insulating massive Dirac fermion state is predicted to occur if the breaking of the time reversal symmetry opens an energy gap at the Dirac point, provided that the Fermi-energy resides inside both the surface and bulk gaps. By introducing magnetic dopants into the three dimensional topological insulator Bi2Se3 to break the time reversal symmetry, we observed the formation of a massive Dirac fermion on the surface; simultaneous magnetic and charge doping furthermore positioned the Fermi-energy inside the Dirac gap. The insulating massive Dirac Fermion state thus obtained may provide a tool for studying a range of topological phenomena relevant to both condensed matter and particle physics.
[en] Highlights: • Ab-initio study of magnetic Heusler compounds. • GW has an important effect on the band gap and transition energies of spin-filter materials. • GW has littile effect in the case of spin-gapless semiconductors. - Abstract: Among Heusler compounds, the ones being magnetic semiconductors (also known as spin-filter materials) are widely studied as they offer novel functionalities in spintronic and magnetoelectronic devices. The spin-gapless semiconductors are a special case. They possess a zero or almost-zero energy gap in one of the two spin channels. We employ the GW approximation to simulate the electronic band structure of these materials. Our results suggest that in most cases the use of GW self energy instead of the usual density functionals is important to accurately determine the electronic properties of magnetic semiconductors.
[en] The half-metallic and elastic properties of Mn2IrAl full-Heusler compound was investigated using WIEN2k code. The ferromagnetic (FM) states were compared non-magnetic (NM) states in Hg2CuTi and Cu2MnAl structures for determined which phase was the most stable. The FM phase in Hg2CuTi structure was observed more stable energetically. The computed results showed that the spin-up (majority) electrons of Mn2IrAl compound had metallic feature while spin-down (minority) electrons had semiconduction behavior with an energy gap of 0.43 eV. According to calculated Cij elastic constants, Mn2IrAl compound was elastically stable as it provides stability conditions and it was a ductile material. Finally, Mn2IrAl compound was found true half-metallic ferromagnet within 2 μ B/f.u. (paper)
[en] Angle-resolved photoemission experiments reveal evidence of an energy gap in the normal state excitation spectrum of the cuprate superconductor Bi2Sr2CaCu2O8+σ. This gap exists only in underdoped samples and closes around the doping level at which the superconducting transition temperature Tc is a maximum. The momentum dependence and magnitude of the gap closely resemble those of the d2x-γ2 gap observed in the superconducting state. This observation is consistent with results from several other experimental techniques, which also indicate the presence of a gap in the normal state. Some possible theoretical explanations for this effect are reviewed. 30 refs., 8 figs
[en] We derive a formula for calculating the free-energy difference between the superconducting and the normal states of a strong-coupling superconductor with localized states within the gap (induced by magnetic impurities). The present formula is a generalization of the one given by Bardeen and Stephen (for the electron-phonon systems) to include the effect of Shiba-Rusinov impurities