Unusual Dirac Fermions on the Surface of a Noncentrosymmetric α -BiPd Superconductor

Abstract

Combining multiple emergent correlated properties such as superconductivity and magnetism within the topological matrix can have exceptional consequences in garnering new and exotic physics. Here, we study the topological surface states from a noncentrosymmetric α-BiPd superconductor by employing angle-resolved photoemission spectroscopy (ARPES) and first principle calculations. We observe that the Dirac surface states of this system have several interesting and unusual properties, compared to other topological surface states. The surface state is strongly anisotropic and the inplane Fermi velocity varies rigorously on rotating the crystal about the y-axis. Moreover, it acquires an unusual band gap as a function of ky, possibly due to hybridization with bulk bands, detected upon varying the excitation energy. Coexistence of all the functional properties, in addition to the unusual surface state characteristics make this an interesting material. α-BiPd is recently synthesized with all the aforementioned properties obtained intrinsically, and thus provides the long-sought material for new experiments and applications. So far, there have been few works on this material, mainly studying the magnetic and transport phenomena [9][10][11]. Again, for the spectroscopic investigations, there is only one ARPES [12] and one scanning tunnelling spectroscopy (STS) study [13]. The ARPES work reported the electronic structure of this compound with the detection of topological Dirac cone at -0.7 eV below the Fermi level (E F ), without detailing the properties of Dirac cone, while the STS work reported the states above the Fermi level.In this paper we report several interesting and unusual properties of the topological Dirac states present on the surface of this noncentrosymmetric α-BiPd superconductor by employing ARPES and first principle calculations. We detect the surface states that are having a Dirac node at a binding energy of 0.7 eV below Fermi level (E F ) at the Γ point dispersing along the Γ − X high symmetry line. Upon varying the photon energy, we notice surface states that are gapped as a function of k y at the node due to a possible hybridization with bulk bands, a unique feature of the surface state that is not disclosed in this compound so far. Upon varying the photon polarization we identify the orbital character of the detected bands.We further show that the Dirac fermions in α-BiPd are highly anisotropic on rotating the crystal about the y-axis such that, in going from Γ−X to Γ−Z the massless linear dispersive Dirac states become flat dispersive massive fermions. Therefore, the Dirac states found in this compound are of one dimensional (1D) character which could provide a natural route for the quantum wires. In general, the Dirac states found on the surface of 2D compounds such as graphene [14] [24]. Unusual band structure of this material, surface state is gapped and asymmetric, make this a unique compound which will attract tremendous future research interests.Single crystals of stoichiometric α-BiPd were g...

Possible origin of linear magnetoresistance: Observation of Dirac surface states in layered PtBi2

“…2(j)]. A similar observation was made earlier in the case of noncentrosymmetric BiPd [37]. There, it was suggested that the gap opening is due to the hybridization between the surface and the bulk states located in the same binding energy range.…”

“…Interestingly, in addition to this, we have uncovered for the first time an another surface Dirac cone at the zone center with a node at ≈ 900 meV below E F . Much like in BiPd [37], the Dirac states at the zone center in PtBi 2 show a gap opening as a function of photon energy, possibly due to a surface-bulk hybridization. Based on these studies, we suggest that the linear MR observed in the hexagonal PtBi 2 could have originated from the linear dispersive surface states in the quantum limit, like in the case of topological insulators [2,3] where the large linear MR is observed only from the surface band structure.…”

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.…”

Fermi surface investigation of the noncentrosymmetric superconductor α -PdBi

“…4) [13] and exhibits bulk type-II weak-coupling clean-limit superconductivity below T c = 3.7K [14][15][16][17][18][19][20]. Strong SOC has been clearly evidenced experimentally by means of angle-and spin-resolved photoemission spectroscopy (ARPES) [21][22][23][24] as well as scanning tunneling spectroscopy (STS) [24,25]. Furthermore, both experimental techniques revealed the presence of a surface Dirac state located at about 700 meV below the Fermi level and this has been supported by first-principle band-structure calculations [21][22][23][24][25].…”