No abstract

We report the observation of half-integer magnetic flux quantization in mesoscopic rings of superconducting β-Bi 2 Pd thin films. The half-quantum fluxoid manifests itself as a π phase shift in the quantum oscillation of the critical temperature. This result verifies unconventional superconductivity of β-Bi 2 Pd, in accord with the expectation of a topological superconductor. We also discuss the strong indication that β-Bi 2 Pd is a spin-triplet superconductor. The condensation of Cooper pairs gives rise to superconductivity [1]. A key signature of the electron pairing is the quantization of the magnetic flux through a multiply-connected superconducting body, in discrete units of Φ 0 = hc 2e. Indeed, the observations of the fluxoid quantization served as the first experimental verifications of the BCS theory [2, 3, 4]. Shortly after the initial magnetometry measurements, Little and Parks further demonstrated the oscillatory feature of the superconducting transition temperature T c , as a result of the periodic free energy of the superconducting state as a function of the applied magnetic flux [5]. The minimumof the free energy, or the maximum of the T c , is always achieved when the applied magnetic flux takes Φ = nΦ 0 , where n is an integer number. In the following decades, the Little-Parks effect, as a stringent test for the electron pairing, has been observed in numerous superconducting materials [6,7,8,9]. On the other hand, Geshkenbein, Larkin and Barone (GLB) predicted that

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.