Observation of Dirac surface states in the noncentrosymmetric superconductor BiPd

Abstract

Materials with strong spin-orbit coupling (SOC) have in recent years become a subject of intense research due to their potential applications in spintronics and quantum information technology. In particular, in systems which break inversion symmetry, SOC facilitates the Rashba-Dresselhaus effect, leading to a lifting of spin degeneracy in the bulk and intricate spin textures of the Bloch wave functions. Here, by combining angular resolved photoemission (ARPES) and low temperature scanning tunneling microscopy (STM) measurements with relativistic first-principles band structure calculations, we examine the role of SOC in single crystals of noncentrosymmetric BiPd. We report the detection of several Dirac surface states, one of which exhibits an extremely large spin splitting. Unlike the surface states in inversion-symmetric systems, the Dirac surface states of BiPd have completely different properties at opposite faces of the crystal and are not trivially linked by symmetry. The spin-splitting of the surface states exhibits a strong anisotropy by itself, which can be linked to the low in-plane symmetry of the surface termination. PACS numbers: 79.60.Bm,73.20.-r,71.70.EjThe interplay of strong spin-orbit coupling (SOC) with superconductivity has become a major focus of research in recent years, as both are essential ingredients to stabilize Majorana bound states. The spin-orbit interaction affects the electronic states in a material in various ways and in particular can lead to non-trivial topologies of the band structure. In topological insulators SOC separates the conduction and valence bands, leading to an insulating state with an inverted band gap [1][2][3]. The latter leads directly to the presence of Dirac surface states protected by time-reversal symmetry [4][5][6]. Another consequence of SOC is the Rashba effect [7-10], which in the absence of inversion symmetry lifts the spin degeneracy of the electronic bands, generating intricate spin textures in the electronic wave functions [11][12][13]. Commonly observed at surfaces or interfaces, in noncentrosymmetric materials the Rashba-Dresselhaus effect leads to a lifting of spin-degeneracy of the bulk bands. Combined with superconductivity this can lead to mixing of spin-singlet and spin-triplet pairing components [14,15] and, more interestingly, to a topologically nontrivial superconducting phase [16][17][18][19].Noncentrosymmetric BiPd [20-24] becomes superconducting below 3.8 K [25-30] and offers a unique opportunity to study the interplay between SOC and superconductivity. The large spin-orbit interaction of the heavy element Bi results in a sizeable spin splitting of the bulk bands of BiPd [29]. This in turn can lead to nontrivial wavefunction topologies and unconventional superconducting states [31,32]. Along with the half-Heusler compounds [33][34][35] and PbTaSe 2 [36][37][38], BiPd constitutes a rare example of a noncentrosymmetric superconductor which cleaves easily, enabling high-resolution surfacesensitive spectroscopy of its electronic states [29,39...

Correct Brillouin zone and electronic structure of BiPd

“…[20][21][22][23] While the data broadly agree, the interpretation of the band structure does not. Neupane et al 20 and Setti et al 22 place all surface states at the Γ point, consistent with their band structure calculations, whereas Benia et al 21 locate those below the Fermi level at the S point and those above the Fermi level at the Γ point, consistent with their own band structure calculations 19,21 . Here, we set out to address these differing interpretations, and clarify how they are related -tracing them back to the unit cell used for electronic structure calculations.…”

Depth-resolved core level spectroscopy of noncentrosymmetric solid BiPd

“…Signature of multiple superconducting gap has been observed in point-contact spectroscopy results [4]. Angle resolved photoemission (ARPES) studies have shown signature of Dirac like surface states in BiPd [5][6][7]. The spin-resolved photoemission (SPPES) studies have also been carried out to confirm the spin polarization of the surface states [5].…”

Quantum oscillations and a nontrivial Berry phase in the noncentrosymmetric topological superconductor candidate BiPd

“…The NCS compound BiPd is a type-II superconductor with a transition temperature of T c ∼3.8 K [24,25], which has attracted much interest recently due to the possible unconventional nature of its superconductivity. Here, the unconventional superconducting state is thought to be due to the large SOC caused by the presence of the heavy element Bi [26].…”