Conventional superconductivity in SrPd2Ge2

Abstract

The electronic structure of SrPd 2 Ge 2 single crystals is studied by angle-resolved photoemission spectroscopy (ARPES), scanning tunneling spectroscopy (STS), and band structure calculations within the local-density approximation (LDA). The STS measurements show a single s-wave superconducting energy gap (0) = 0.5 meV. The photon-energy dependence of the observed Fermi surface reveals a strongly three-dimensional character of the corresponding electronic bands. By comparing the experimentally measured and calculated Fermi velocities a renormalization factor of 0.95 is obtained, which is much smaller than typical values reported in Fe-based superconductors. We ascribe such an unusually low band renormalization to the different orbital character of the conduction electrons and, using ARPES and STS data, argue that SrPd 2 Ge 2 is likely to be a conventional superconductor, which makes it clearly distinct from isostructural iron pnictide superconductors of the "122" family.

Type II superconductivity in SrPd2Ge2

“…Hence, we have shown strong type II superconductivity in SrPd 2 Ge 2 , confirming the preliminary indirect indications presented in our previous study [22] and the magnetization studies [20,24].…”

“…This suggests that the sample is in the dirty limit, implying a rather short electron mean free path l [34]. The electron mean free path was not considered in the estimation of  0 and  0 using only the Fermi velocity, superconducting energy gap and the averaged length of the Fermi contours [22] and concurs with the fact that the effective penetration depth  is much larger than  0 and the effective coherence length  is much smaller than  0 . The notion of the dirty limit is further validated by the applicability of the pair breaking model of de Gennes [35] and Maki [36] to the differential conductance spectra in various magnetic fields (Figure 2 (a)), as described by Szabó et al [37] In this model, the SDOS N(E) of a dirty limit type II superconductor in the gapless region for small  in magnetic fields near B c2 is given by , ) ( 2 (6) where N N (0) is the density of states at the Fermi surface of the superconductor in the normal state and is the Maki -de Gennes pair-breaking parameter.…”

“…Consequently, in the low temperature limit (k B T « ), the differential conductance measures directly the SDOS. [26] (0) and T c and the ones presented in the previous study [22] possibly originate from the variations occurring in the sample fabrication process, just as different values of T c were observed for a polycrystalline sample [20] and a single crystal [24].…”

“…From ARPES, Kim et al determined the Fermi velocity ћv F = 4.7 eVÅ and the penetration depth  0 = 40 nm for SrPd 2 Ge 2 [22]. In combination with the above given superconducting energy gap value (0), the coherence length  0 = 348 nm was estimated from…”

“…In our latter work [22], we have shown, by means of angle resolved photoemission spectroscopy (ARPES), scanning tunneling spectroscopy (STS) and band structure calculations, that SrPd 2 Ge 2 , unlike the "122" family of pnictides, has a strongly three-dimensional electronic structure. It is well described within local density approximation (LDA) and features a single isotropic superconducting energy gap with 2/k B Tc close to the Bardeen -Cooper -Schrieffer (BCS) theory universal value, ruling out exotic electronic states.…”