Multiple-gap response of type-I noncentrosymmetric BeAu superconductor

Abstract

Precise measurements of the thermodynamic critical field (Bc) in type-I noncentrosymmetric superconductor BeAu were performed by means of the muon-spin rotation/relaxation technique. The temperature evolution of Bc can not be described within the single gap scenario and it requires the presence of at least two different types of the superconducting order parameters. The selfconsistent two-gap approach, adapted for analysis of Bc(T ) behavior, suggests the presence of two superconducing energy gaps with the gap to Tc ratios 2∆/kBTc 4.52 and 2.37 for the big and the small gap, respectively. This implies that the superconductivity in BeAu is unconventional and that the supercarrier pairing occurs at various energy bands.BeAu is an old known superconductor with the transition temperature T c 3.2 K. Superconductivity in BeAu was originally discovered by Matthias in 1959, 1 i.e. just in two years after the formulation of the BCS theory. 2 In this short report, Matthias was noted the absence of a superconductivity in a pure Be and Au (Be was later found to have T c 26 mK, Ref.3) and performed a search within the gold-rich site of the Be-Au phase diagram. The superconductivity was found to appear in a stoichiometric (i.e. 1:1 Be to Au ratio) BeAu sample. 1Recently, the interest to BeAu was renewed. [4][5][6][7][8] This mostly relates to the realisation of their noncentrosymmetric crystal structure, which was expected to give rise to unconventional superconductivity due to spin-orbit coupling and/or mixed singlet/triplet pairing state (see e.g. Refs. 9-16 and references therein). In addition, the B20 FeSi-type of the crystal structure of BeAu becomes particualry interesting since such materials were predicted to host chiral fermions in topological semimetals. [17][18][19] Moreover, B20 structure is the only known crystal structure for bulk magnetic skyrmions in materials such as MnSi, Fe 1−x Co x Si, FeGe, MnGe, Cu 2 OSeO 3 etc. [20][21][22][23][24] All these make BeAu an intriguing candidate material to search for unconventional superconductivity, associated with its noncentrosymmetric crystal structure in combination with the possible existence of exotic quasiparticles.

Y4Be33Pt16 – a non-centrosymmetric cage superconductor with multi-centre bonding in the framework

“…27,28 However, a more comprehensive analysis of the superconducting state in Y 4 Be 33 Pt 16 is currently underway. In particular, because several in-depth muon spin-rotation, relaxation, and resonance experiments performed on another Be-containing non-centrosymmetric superconductor BeAu 24 revealed that not only type-I character, 29 but also a multi-gap nature of superconductivity 30 may exist in such unconventional systems.…”

Isotropic single-gap superconductivity of elemental Pb

“…1), the measured B c (T ) dependences should be analyzed within the multigap scenario. Such an approach was recently used by us to describe the two-gap superconductivity in noncentrosymmetric binary alloy AuBe [18,19]. However, as will be shown later, in elemental Pb, the single-gap approach was found to describe the experimental data reasonably precisely, so any type of admixture of the second superconducting en-ergy gap was not expected [see also the single-gap theory curves in panels (a) and (b) of Fig.…”

“…[B c (0) = B c (T = 0), N F (0) is the density of states at the Fermi level, and is the zero-temperature value of the superconducting energy gap], becomes a direct probe of multiple superconducting energy gaps [16][17][18][19], as well as allows one to probe the superconducting gap anisotropy [20][21][22]. As the temperature dependence of B c normalized to its zerotemperature value follows very closely a nearly quadratic * rustem.khasanov@psi.ch behavior, the deviation function D(T ) is normally considered:…”

“…The transverse-field muon-spin rotation/relaxation (TF-μSR) experiments were conducted using the Dolly spectrometer (π E1 beam line) at the Paul Scherrer Institute, Switzerland. Measurements were performed in the intermediate state of superconducting Pb, i.e., when the sample volume is separated into the normal-state and the superconductingstate (Meissner) domains [15,18,22,[26][27][28][29][30][31][32]. Two sets of experiments were performed.…”

“…The magnetic-field distribution in a type-I superconductor in the intermediate state, which is probed directly by means of TF-μSR, consists of two sharp peaks corresponding to the response of the domains remaining in the Meissner state (B = 0) and in the normal-state (B ≡ B c > B ex ). Consequently, the value of B c could be directly and very precisely determined by measuring the position of the B > B ex peak [18,22,[26][27][28][29][30][31][32][35][36][37][38][39][40]. The details of the TF-μSR experiments on elemental Pb and the data analysis procedure are discussed in the Supplemental Material [41]. )…”