Fully gapped superconductivity in the topological superconductorβ−PdBi2

Abstract

The recent discovery of the topologically protected surface states in the β-phase of PdBi2 has reignited the research interest in this class of superconductors. Here, we show results of our muon spin relaxation and rotation (µSR) measurements carried out to investigate the superconducting and magnetic properties and the topological effect in the superconducting ground state of β-PdBi2. Zero-field µSR data reveal that no sizeable spontaneous magnetization arises with the onset of superconductivity implying that the time reversal symmetry is preserved in the superconducting state of β-PdBi2. Further, a strong diamagnetic shift of the applied field has been observed in the transverse-field (TF) µSR experiments, indicating that any triplet-pairing channel, if present, does not dominate the superconducting condensate. Using TF-µSR, we estimate that the magnetic penetration depth λ = 263(10) nm at zero temperature. The nature of λ(T ) provides evidence for the existence of a nodeless single s-wave type isotropic energy gap of 0.78(1) meV at zero temperature. Our results further suggest that the topologically protected surface states have no effect on the bulk of the superconductor.

Electron-phonon coupling and pairing mechanism in β−Bi2Pd centrosymmetric superconductor

“…The average value of the gap at zero temperature is estimated to be ∆(0) = 0.65 meV (without SOC) and ∆(0) = 0.70 meV (with SOC). These values are in good agreement with scanning tunneling microscopy and muon spin relaxation measurements yielding 0.76 meV [10] and 0.78 meV [12], respectively. Slightly larger values of 0.88 meV and 0.92 meV were reported based on calorimetric and Hallprobe magnetometry measurements [11,48], in the first case, and point-contact spectroscopy measurements [13], in the second case.…”

“…Since the strong spin-orbit coupling (SOC) inherent to the heavy element Bi induces significant changes in the electronic band structure [15,16], β-Bi 2 Pd compound offers the opportunity to further study the interplay of the spin-orbit coupling effect with superconductivity. We find that β-Bi 2 Pd displays BCS-like superconductivity with a single anisotropic superconducting gap in agreement with recent experimental findings [10][11][12][13].…”

Unconventional superconductivity

“…On one side of this continuing discussion, -phase PdBi2 -identified as a topological superconductor by Biswas et al [688] -as characterized by SR exhibits preservation of time reveral symmetry in a nodeless single band s-wave isotropic gap superconductor. The topologically protected surface states do not affect the bulk properties of the superconductor.…”

Evolution of the topologically protected surface states in superconductorβ-Bi2Pd from the three-dimensional to the two-dimensional limit

“…More recently, two members of the Bi-Pd family of superconductors have been found to exhibit topologically protected surface states with Rashba-like spin splitting. While the superconducting state of noncentrosymmetric BiPd appears to be topologically trivial [43][44][45], the pairing symmetry of centrosymmetric β-Bi 2 Pd remains to be elucidated [46][47][48][49][50][51][52]. Despite the fact that spin-and angleresolved photoemission spectroscopy (ARPES) has revealed the presence of notrivial surfaces states at the Fermi level [46], no Andreev bound states associated with Majorana fermions have been observed in pointcontact spectroscopy [48].…”