Electron-phonon coupling and two-band superconductivity of Al- and C-dopedMgB2

Abstract

We have studied the electron-phonon and superconducting properties of the Mg 1−x Al x B 2 and MgB 2(1−y) C 2y alloys within the framework of density functional theory using the self-consistent virtual-crystal approximation. For both alloys, the Eliashberg spectral functions and the electronphonon coupling constants have been calculated in the two-band model for several concentrations up to x(Al)= 0.55 and y(C)= 0.175. We solved numerically the two-band Eliashberg gap equations without considering interband scattering. Using a single parameter for the Coulomb pseudopotential, which was determined for the undoped compound, we were able to reproduce the experimental doping dependence of ∆ σ , ∆ π , and T c for both alloys on a quantitative level. In particular, the observed differences in the doping range of superconductivity between Al and C doping indicate a pronounced influence of the doping site, which can be explained naturally in the present approach without the need to invoke interband scattering, suggesting that this factor plays only a minor role.

Magnetic anisotropy energy of disordered tetragonal Fe-Co systems fromab initioalloy theory

“…The VCA has been implemented in various ab initio methods and used for a number of alloys of neighboring elements in the periodic table. 3,11,12 Its application to the bcc Fe-Co and fcc Co-Ni systems 3,13 yields concentration trends of the spin magnetic moment in a very good agreement with experiment; 1 the orbital magnetic moments seem to be described reliably in the VCA as well. However, a recent first-principles study of the random bct Fe 1−x Co x alloys, 14 based on a supercell technique applied to a few special Co concentrations (x = 0.5, 0.625, and 0.75), has shown that a realistic treatment of the chemical disorder can reduce the MAE by a factor of 1.5−3 as compared to the simple VCA.…”

Electronic Structure of Single-Crystalline MgxAl1−xB2 Probed by X-ray Diffraction Multipole Refinements and Polarization-Dependent X-ray Absorption Spectroscopy

“…[11][12][13][14] As a result, the superconducting state of MgB 2 is presently understood within a multiband approach of the Eliashberg theory where the pairing is split into σ and π intraband and σ-π interband contributions. 11,[15][16][17][18] It has been demonstrated that the coupling between the σ-band electrons and the B-B bond-stretching phonon mode with E 2g symmetry at the Γ point plays a decisive role for superconductivity in MgB 2 . 9,15,[19][20][21] The electron-phonon coupling for the π bands and the interband coupling seem to be weaker although not negligible.…”

Multi-Band Superconductivity and the Steep Band/Flat Band Scenario

“…Superconductivity in magnesium diboride [9] was early on identified to stem from two bands, namely a π and a σ-band [36][37][38][39][40][41][42][43][44][45]. Upon doping MgB2 with Al or Sc superconductivity persists over a rather broad range.…”