Dirac states in the noncentrosymmetric superconductor BiPd

BiPd is a noncentrosymmetric superconductor and possesses Dirac states at the surface. Extensive angle-resolved photoemission (ARPES) studies [1-3] of this material reported conflicting results about the dimensionality of the Dirac states and the momentum of the Dirac point, which are essential for studying the properties of the Dirac fermions. While some studies [1,2] reported the Dirac node at the center of the surface Brillouin zone (SBZ), $\overline{\Gamma}$, the other reports [3,4] show it at the edge of SBZ, $\overline{S}$. The Dirac states show dispersion along with an opening up of an energy gap at the Dirac node with the change in photon energies indicating a possible three-dimensional behavior. Moreover, the Dirac states have been found to be anisotropic [2], which is attributed to anisotropic Rashba effect [3]. Clearly, BiPd provides a novel platform to study the exotic properties of the Dirac fermions where the material shows superconductivity at low temperatures. In the present work [5], we have employed high-resolution ARPES using multiple photon energies to investigate the properties of the Dirac surface states. We have carried out the Fermi surface mapping at multiple photon energies and also optimized the sample position carefully at different photon energies. We discover that the Dirac node can be recovered via sample optimization and the Fermi surface mapping shows absence of gap at the Dirac node at multiple photon energies non-equivalent with respect to $k_z$. These results establish $\overline{S}$ to be the location of the Dirac node. Evidently, the deviation from the two-dimensional behavior of the Dirac fermions is not a material property and arises due to the finite momentum of the Dirac node; the corresponding emission angle changes with the change in photon energy. Furthermore, the anisotropy of the Dirac states in this system is found to be unique exhibiting isotropy close to the Dirac node and anisotropy away from the node. We have constructed a model Hamiltonian up to third order in momentum and demonstrate that it explains the observed anisotropy well.

[1] M. Neupane et. al., Nat. Commun., 7, 13315 (2016).
[2] S. Thirupathaiah et. al., Phys. Rev. Lett., 117, 177001 (2016).
[3] H. M. Benia et. al., Phys. Rev. B, 94, 121407(R) (2016).
[4] A. Yaresko et. al., Phys Rev. B. 97, 075108 (2018).
[5] A. Pramanik et. al., Phys. Rev. B, 103, 155401 (2021).

E-mail for corresponding author: [email protected]