Topologically protected surface states in a centrosymmetric superconductor β-PdBi2

Abstract

The topological aspects of electrons in solids can emerge in real materials, as represented by topological insulators. In theory, they show a variety of new magneto-electric phenomena, and especially the ones hosting superconductivity are strongly desired as candidates for topological superconductors. While efforts have been made to develop possible topological superconductors by introducing carriers into topological insulators, those exhibiting indisputable superconductivity free from inhomogeneity are very few. Here we report on the observation of topologically protected surface states in a centrosymmetric layered superconductor, β-PdBi2, by utilizing spin- and angle-resolved photoemission spectroscopy. Besides the bulk bands, several surface bands are clearly observed with symmetrically allowed in-plane spin polarizations, some of which crossing the Fermi level. These surface states are precisely evaluated to be topological, based on the Z2 invariant analysis in analogy to three-dimensional strong topological insulators. β-PdBi2 may offer a solid stage to investigate the topological aspect in the superconducting condensate.

Magnetic field dependence of the density of states in the multiband superconductorβ−Bi2Pd

“…In β-Bi 2 Pd, T c is practically not modified by scattering and the gap is already nearly isotropic, but H c2 (T ) behavior shows the influence of multiband Fermi surface properties and interband scattering. β-Bi 2 Pd has a Fermi surface with two nearly cylindrical sheets and two 3D sheets, derived from Pd 4d and Bi 6p states [31,32]. States from both Pd and Bi contribute to the density of states at the Fermi level, and in particular anisotropic 4d xy+yz orbitals.…”

“…We measure β-Bi 2 Pd (T c = 5 K [17,30]) with a very low-temperature STM. Our sample is moderately in the dirty limit (mean free path smaller than superconducting coherence length ξ , < ξ) and the Fermi surface shows multiple sheets of mixed orbital character [17,31,32]. The situation is opposed to MgB 2 , with σ and π sheets that remain well separated even in presence of defects [10].…”

Single-gap superconductivity inβ−Bi2Pd

“…The subkelvin scanning tunneling spectroscopy (STS) proved only a single superconducting gap, too. Thus, a conventional superconductivity has been proved for both bulk and surface of β-Bi 2 Pd which is noteworthy in the system where topologically protected surface states had been reported [14]. Yet some characteristics as the upper critical magnetic field inferred from the magnetoresistive transitions and the superconducting coupling ratio from STS deviate from those obtained from the bulk probes indicating that some surface properties may differ from the bulk.…”

“…In their case it was taken as evidence for the concept of multigap superconductivity in β-Bi 2 The surface superconductivity in β-Bi 2 Pd is particularly interesting because the topologically protected surface states have recently been detected by ARPES measurements [14]. It could have implications on superconductivity and feature unconventional superconducting order parameter with gapless states on the surface [30].…”

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

“…For 5 TLs, the two bands are almost touching each other at an energy level of -2.19 eV and the gap is around 0.01 eV. Once the thickness of the slab reaches 9 TLs, the gap completely disappears and the two bands with nearly linear dispersion cross each other forming a surface Dirac cone (S-DC) [46]. We find that the energy of the DP is E D =2.20 eV, very close to the E D =2.41 eV value extracted from the ARPES data [46].…”