Crossover from multiple- to single-gap superconductivity in Nb5Ir3−xPtxO alloys

Abstract

By using mostly the muon-spin rotation/relaxation (µSR) technique, we investigate the superconductivity (SC) of Nb 5 Ir 3−x Pt x O (x = 0 and 1.6) alloys, with T c = 10.5 K and 9.1 K, respectively. At a macroscopic level, their superconductivity was studied by electrical resistivity, magnetization, and specific-heat measurements. In both compounds, the electronic specific heat and the low-temperature superfluid density data suggest a nodeless SC. The superconducting gap value and the specific heat discontinuity at T c are larger than that expected from the Bardeen-Cooper-Schrieffer theory in the weak-coupling regime, indicating strongcoupling superconductivity in the Nb 5 Ir 3−x Pt x O family. In Nb 5 Ir 3 O, multigap SC is evidenced by the field dependence of the electronic specific heat coefficient and the superconducting Gaussian relaxation rate, as well as by the temperature dependence of the upper critical field. Pt substitution suppresses one of the gaps, and Nb 5 Ir 1.4 Pt 1.6 O becomes a single-gap superconductor. By combining our extensive experimental results, we provide evidence for a multiple-to single-gap SC crossover in the Nb 5 Ir 3−x Pt x O family.

Multigap superconductivity in centrosymmetric and noncentrosymmetric rhenium-boron superconductors

“…For Re 3 B, T c /T F = 4.8 × 10 −4 is almost identical to the analogous value for multigap ReBe 22 and LaNiC 2 , the latter representing a typical example of NC-SCs. However, for Re 7 B 3 , T c /T F = 1.16 × 10 −3 is very close to the multigap Nb 5 Ir 3 O and elementary rhenium superconductors [27,30,63], the latter showing a breaking of TRS in the superconducting state and exhibiting a centrosymmetric crystal structure. In general, most of the weakly correlated NCSCs, e.g., ReT , Mo 3 Al 2 C, Li 2 (Pd, Pt) 3 B, and LaNiC 2 , exhibit T c /T F values between the unconventional and conventional bands [64], and this is also the case for Re 7 B 3 .…”

“…Conversely, many conventional superconductors, as e.g., Al, Sn, and Zn, are located at T c /T F 10 −4 . Between these two categories lie several multigap superconductors, e.g., LaNiC 2 , Nb 5 Ir 3 O, ReBe 22 , NbSe 2 , and MgB 2 [60][61][62][63][64]. Although there is no conclusive evidence for them to be classified as unconventional superconductors, the rhenium-boron superconductors lie clearly far off the conventional band.…”

Materials Research on High-Entropy Alloy Superconductors

“…Nb 5 Ir 3 O, crystallizing into the Ti 5 Ga 4 -type structure, which is the ordered derivative of Mn 5 Si 3 -type structure, is well known as a two-band superconductor [41]. By substituting Pt into Ir, the crossover to single-band superconductivity is observed [44], which is a rare phenomenon. This result promotes us to investigate multisite HEA superconductors for further search of the crossover phenomenon, and the high-entropy state may be a new route of controlling the superconducting band.…”

“…The second purpose of this chapter is the materials research on the hexagonal Mn 5 Si 3 -type HEAs, possessing multiple Wyckoff positions. Recently, several superconductors with the Mn 5 Si 3type-or its ordered derivative Ti 5 Ga 4 -type-structure have been found and attract much attention [38][39][40][41][42][43][44]. Besides, many intermetallic compounds are crystallizing into these crystal structures [45,46].…”

Cutting Edge of High-Entropy Alloy Superconductors from the Perspective of Materials Research

“…Even in this case, the δ-value in each layer would not be as large, e.g., a δ of 5. 56 Table 4). In other words, the multisite HEAs No.…”