Spin-Triplet Pairing State Evidenced by Half-Quantum Flux in a Noncentrosymmetric Superconductor

Abstract

A prime category of superconducting materials in which to look for spin-triplet pairing and topological superconductivity are superconductors without inversion symmetry. It is predicted that the breaking of parity symmetry gives rise to an admixture of spin-singlet / spin-triplet pairing states; a triplet pairing component, being substantial, seems all but guaranteed. However, the experimental confirmation of pair mixing in any particular material remains elusive. In this work, we perform phase-sensitive experiment to examine the pairing state of noncentrosymmetric superconductor α−BiPd. The Little-Parks effect observed in mesoscopic polycrystalline α−BiPd ring devices reveals the presence of half-integer magnetic flux quantization, which provides a decisive evidence for the spin-triplet pairing state. We find mixed half-quantum fluxes and integer-quantum fluxes, consistent with the scenario of singlet-triplet pair mixing.

Reveal the nematic topological superconductivity from the anisotropic unconventional Josephson effect: Theory and application to doped Bi2Se3

“…The Little-Parks effect reflects the oscillation of the free energy of the superconductor as a function of the applied magnetic flux, resulting in a corresponding periodicity of the resistance near the superconducting transition temperature T c [79]. The minimum of the free energy [80,81], as well as the minimum of resistance, is achieved when the applied magnetic flux becomes (n + 1/2)Φ 0 for a superconducting loop containing an odd number of π-junctions [82][83][84][85], instead of nΦ 0 for a conventional s-wave superconducting loop. Here, n is an integer and Φ 0 = h/2e is the magnetic flux quantum with h Planck constant and e the elemental charge [86,87].…”

“…We propose to fabricate a square-shaped loop of Cu x Bi 2 Se 3 with outer dimensions of 800 nm by 800 nm and line width of 100 nm [83,85], which contains a U-shaped Josephson junction with a metal bridge of Au as shown in Figure 9(b). The superconducting gap of Cu x Bi 2 Se 3 is about 0.45 meV [88,89] and the Fermi velocity in the normal region (yellow bridge in Figure 9(b)) is about 1.4 × 10 6 m/s, which results in the junction coherence length ξ ≈ 652 nm.…”

Signatures of triplet correlations in density of states of Ising superconductors

“…Uranium-based superconductors are perhaps the only uncontroversial exception [33]. Some success in measuring the singlet-triplet ratio has been achieved in noncentrosymmetric superconductors by Little-Parks effect experiments [50]. Identifying triplets in DOS in noncentrosymmetric superconductors is harder and indirect, because the potentially dominant singlet component gaps out the low energy states.…”

“…0 } and {f 49) and (50) this will allow us to complete the Landau expansion of the thermodynamic potential.…”

Full superconducting gap and type-I to type-II superconductivity transition in single crystalline NbGe2

“…For example, topological bands in the RhSi family have been experimentally observed with characters of chiral crystals, and their helicoid Fermi arcs on surface are claimed to have a Chern number of ± 2 7 . On the other hand, noncentrosymmetric superconductors can in principle allow an admixture of spinsinglet and -triplet parings 1,2,8 , and serve as a promising platform to realize intrinsic topological superconductors, such as PbTaSe 2 [9][10][11] and BiPd [12][13][14][15] . Among them, noncentrosymmetric NbGe 2 has been reported to have a superconducting transition temperature T c ∼ 2.09 ± 0.02 K 16 decades ago.…”