Scaling of the critical temperature with the Fermi temperature in diborides

Abstract

The experimental determination of the scaling of the superconducting critical temperature (T-c) vs the Fermi temperature (T-f) of the holes in the boron sigma subband is presented. The Fermi level has been tuned near the "shape resonance," i.e., the two- to three-dimensional crossover of the Fermi surface of the boron sigma subband by changing the Al/Mg content in Al1-xMgxB2. The product k(f)xi(0) of the Fermi wave vector (k(f)) times the superconducting Pippard coherence length (xi(0)), that is a measure of the pairing strength, remains constant, k(f)xi(0)=90 for x>0.66. This high-T-c phase occurs in the boron superlattice under a tensile microstrain in the range 3%<epsilon<6%

Feshbach Shape Resonance in Multiband Superconductivity in Heterostructures

“…6, where the superconducting gaps in the π and in the σ band and critical temperature T c obtained by aluminium [143], carbon [183,184] and scandium [177] substitutions are reported on the same scale. These results show that the two gap behaviour is present over all the range of the Lifshitz parameter of ±400 meV around z=0.…”

Thermodynamics of two-band superconductors: The case ofMgB2

“…16,17,18,19 Interband scattering is weak in MgB 2 , 20 but this is not necessarily the case in samples in which Mg has been replaced by Al or B by C (Ref. 21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37) or which have been exposed to neutron irradiation. 38 Such samples exhibit considerably reduced T c , while the two gaps persist even at very low critical temperatures.…”

Anomalous Thermal Expansion in Superconducting Mg1− x Al x B2 System

“…In the case of the Mg 1-x Al x B 2 system the chemical potential is tuned at the 2D/3D ETT point for x=0.33 [27]. It has been shown [21] that the interband pairing term shows a broad resonance as expected for a superconducting Feshbach shape resonance.…”

“…In fact it is actually a composite intermetallic formed by a multilayer of first planes of a bulk low T c superconductor, boron that shows T c =11K under pressure [14]: graphene B monolayers, intercalated by second metallic planes of a non superconducting element, magnesium [14]: hcp Mg monolayers. Therefore following the discovery of Nagamatsu et al [13] it was first proposed by the Rome group [15][16][17] that MgB 2 shows mutigap superconductivity. The multigap superconductivity in MgB 2 was quickly confirmed by experiments on the upper critical field H c2 (T) [18], tunneling [19] and angular resolved photoemission (ARPES) [20] and many other measurements [2].…”