No abstract

No abstract

The London penetration depth λ(T ) was measured in a single crystal of V 3 Si. The superfluid density obtained from this measurement shows a distinct signature of two almost decoupled superconducting gaps. This alone is insufficient to distinguish between s ± and s ++ pairing states, but it can be achieved by studying the effect of controlled nonmagnetic disorder on the superconducting transition temperature T c . For this purpose, the same V 3 Si crystal was sequentially irradiated by 2.5-MeV electrons three times, repeating the measurement between the irradiation runs. A total dose of 10 C/cm 2 (6.24 × 10 19 electrons/cm 2 ) was accumulated, for which T c changed from 16.4 K in a pristine state to 14.7 K (9.3%). Not only is this substantial suppression impossible for a single isotropic gap, but also it is not large enough for a sign-changing s ± pairing state. Our electronic band structure calculations show how five bands crossing the Fermi energy can be naturally grouped to support two effective gaps, not dissimilar from the physics of iron pnictides. We analyze the results using two-gap models for both λ(T ) and T c which describe the data very well. Thus the experimental results and theoretical analysis provide strong support for an s ++ superconductivity with two unequal gaps, 1 (0) ≈ 2.53 meV and 2 (0) ≈ 1.42 meV, and a very weak interband coupling in the V 3 Si superconductor.

The orthorhombic compound AuSn 4 is compositionally similar to the Dirac node arc semimetal PtSn 4 . AuSn 4 is, contrary to PtSn 4 , superconducting with a critical temperature of T c = 2.35 K. Recent measurements present indications for quasi-two-dimensional superconducting behavior in AuSn 4 . Here we present measurements of the superconducting density of states and the band structure of AuSn 4 through scanning tunneling microscopy and angular resolved photoemission spectroscopy (ARPES). The superconducting gap values in different portions of the Fermi surface are spread around 0 = 0.4 meV, which is close to but somewhat larger than = 1.76k B T c expected from BCS theory. We observe superconducting features in the tunneling conductance at the surface up to temperatures about 20% larger than bulk T c . The band structure calculated with density functional theory follows well the results of ARPES. The crystal structure presents two possible stackings of Sn layers, giving two nearly degenerate polytypes. This makes AuSn 4 a rather unique case with a three-dimensional electronic band structure but properties ressembling those of low-dimensional layered compounds.