Gap symmetry of the noncentrosymmetric superconductor W3Al2C

Abstract

A detailed zero-field and transverse-field muon spin relaxation/rotation (μSR) experiments have been carried out on the recently discovered noncentrosymmetric superconductor W 3 Al 2 C to speculate about its superconducting ground state. Bulk nature of superconductivity below 7.6 K is confirmed through magnetization and specific heat measurements. No change in the zero field μSR spectra collected above and below T c is visible, ruling out the possibility of spontaneous magnetic field below T c . This confirms that time-reversal symmetry is preserved for W 3 Al 2 C upon entering in the superconducting ground state. Temperature-dependent superfluid density [ρ s (T )], which directly reflects the superconducting gap symmetry is obtained by the analysis of spectra obtained from the transverse-field μSR experiments. Despite a noncentrosymmetric structure, W 3 Al 2 C adopts a fully gapped spin-singlet superconducting ground state as confirmed through the analysis of ρ s (T ) and C e (T ). The zero-temperature value of gap is 0 = 1.139(5) meV with gap-to-T c ratio 2 0 /k B T c ≈ 3.48, classifying this material as a weakly coupled superconductors. We have also revealed that the ratio T c /λ −2 eff is comparable to the class of unconventional superconductors, which points towards an unconventional pairing mechanism in W 3 Al 2 C.

$s$-wave superconductivity in the noncentrosymmetric W$_3$Al$_2$C superconductor: An NMR study

“…In the superconducting phase, the electronic properties of the system can be described by the standard BCS Here, the reduced shift reflects an electron pairing forming spincompensated (typically s-wave) Cooper pairs. This result is consistent with a recent muon-spin rotation (µSR) study of W 3 Al 2 C [12], where s-wave superconductivity with the same gap ratio was reported. The superconducting gap value we find in W 3 Al 2 C is similar to the average SC gap reported in Mo 3 Al 2 C, where an NMR study claimed a 2∆ 0 /k B T c ratio of 2.8(2) [14], while specific-heat measurements implied a ratio of 4.03 [13].…”

“…27 Al NMR in the normal-and superconducting phases NMR is a powerful local technique for investigating the electronic properties of materials, in particular, their electronic correlations, magnetic order, and superconductivity [24]. Although the magnetic field reduces the T c value, a high upper critical field (above 50 T [12]), still allows us to explore the superconducting behaviour of W 3 Al 2 C. In addition, we also investigated the NMR response in the normal state. The 27 Al NMR measurements were performed in different external magnetic fields, from 4 to 7 T, corresponding to T c values between 5.9 K and 4.6 K. In all cases, the 27 Al NMR reference frequency ν 0 was calculated with respect to a standard Al(NO 3 ) 3 reference.…”

“…Indeed, from the existing literature, it is known that, while Li 2 Pd 3 B is a conventional superconductor [8], its high-Z Pt counterpart, Li 2 Pt 3 B, exhibits unconventional superconductivity [9], here identified by the presence of gap nodes [10]. The superconducting properties of the isostructural Mo 3 Al 2 C vs W 3 Al 2 C compounds are, however, still under study, with various groups reporting clearly contradictory types of superconductivity, for either of them, both conventional [11,12,13,14,15,16,17] and unconventional [18,19]. Thus, in Mo 3 Al 2 C, a power-law behavior of the 27 Al NMR spin-lattice relaxation rates possibly suggests superconducting gap nodes [18], while the exponential temperature dependence of the magnetic penetration depth and the absence of time-reversal symmetry breaking are more consistent with conventional nodeless SC [16,20].…”

Superconductivity with a Violation of Pauli Limit and Evidences for Multigap in η-Carbide Type Ti4Ir2O

“…One possible cause of the enhancement of 𝐻 c2 is the spin-triplet component in the order parameter, which can be induced by non-centrosymmetric crystal structures. [7,8] However, this should not be the case since Ti 4 Ir 2 O hosts a centrosymmetric crystal structure. Another possibility of enhancement due to anisotropy, as observed in many low-dimensional superconductors, such as Li 0.9 Mo 6 O 17 , [52] LaO 0.5 F 0.5 BiS 2 , [15] La 2 IRu 2 , [53] and K 2 Mo 3 As 3 , [54] is also ruled out.…”

“…𝐻 c2 of most type-II superconductors falls below 𝐻 P , regardless of the value of 𝑇 c . [4] However, a violation of the Pauli limit is often regarded as a side evidence for unconventional superconductivity, as in heavy fermions, [5] iron-based superconductors, [6] noncentrosymmetric superconductors, [7,8] and more re-cently, in 𝐴 2 Cr 3 As 3 (𝐴 = Na, K, Rb, Cs) [9−11] or magic-angle twisted trilayer graphene. [12] In addition to unconventional superconductivity, large spin-orbit scattering, [13] strong electron-phonon coupling, [14] or a highly anisotropic structure [15] all tend to increase 𝐻 c2 , pushing it beyond 𝐻 P .…”