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

Abstract

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 Mg x 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.

Two-band superconductivity in (AlMg)B2: Critical temperature and isotope exponent as a function of carrier density

“…Moreover, we have found that the change of dimensionality in the electronic band dispersion from 2D to 3D dimensions, is signaled by the isotope exponent that exhibits a minimum. The small value of the minimum of the isotope coefficient at the ETT is in agreement with previous results of Angilella et al [39]. Therefore, experimental investigation of the variation of the isotope coefficient by tuning the Fermi level, e.g., by Al doping, will provide relevant information for the pairing process in diborides.…”

Is MgB2a BCS-like charged superfluid?

“…This supports the view of Pickett and co-workers that MgB 2 is a BCS-like superfluid. We note here the interesting proposal of Bianconi and co-workers, where T c is correlated crucially with features of the band structure of the layered material MgB 2 [10,11]. This is proposed as a complementary mechanism to that of Pickett and co-workers.…”

“…Both references [10] and [11] invoke the multiband nature of the electronic structure of MgB 2 . Though the Feshbach resonance proposal cannot be discounted as a possible alternative to the electron-phonon mechanism advocated by An and Pickett [4,5] and other authors, the proximity of MgB 2 to the curve containing the five bcc transition metals shown in figure 2, which materials are manifestly superconducting due to an electron-phonon mechanism from figure 1, would have to be accounted for by any such alternative mechanism as put forward by Bianconi et al [10].…”

Superconductivity of KFe2As2 Under Pressure: Ab Initio Study of Tetragonal and Collapsed Tetragonal Phases

“…Theoretical calculations for the multi-band superconductors show that the change of the character of the electrons from 3D to 2D behavior (which can be observed as a modification of a shape of the Fermi surface, e.g., from cilidricallike to spherical-like) leads to a stabilization of the superconductivity [95][96][97] due to the emergence of the narrow bands [98]. In such a case, the van Hove singularity plays an important role in tuning of the superconductivity [99,100].…”