Proximity effect in a superconductor–topological insulator heterostructure based on first principles

Abstract

Superconductor-topological insulator (SC-TI) heterostructures were proposed to be a possible platform to realize and control Majorana zero modes. Despite experimental signatures indicating their existence, univocal interpretation of the observed features demands theories including realistic electronic structures. To achieve this, we solve the Kohn-Sham-Dirac-Bogoliubov-de Gennes equations for ultrathin Bi 2 Se 3 films on superconductor palladium telluride within the fully relativistic Korringa-Kohn-Rostoker method and investigate quasiparticle spectra as a function of chemical potential and film thickness. We find multiple proximity-induced gaps where the gap sizes highly depend on characteristics of the TI states. The TI Dirac interface state is relevant to the induced gap only when the chemical potential is close to the Dirac-point energy. Otherwise, at a given chemical potential, the largest induced gap arises from the highest-energy quantum-well states, whereas the smallest gap arises from the TI topological surface state with its gap size depending on the TI pairing potential.

Scrutinizing a superconductor-topological insulator interface as a platform for topological superconductivity

“…It was demonstrated that the proximity effect decays exponentially in the topological surface state (TSS) with its distance to the SC/TI interface (i.e. with the TI film thickness) [11,13,20]. Further analysis even revealed possible signatures of Majorana zero modes in form of zero-bias voltage peaks inside supercurrent vortices of the SC/TI heterostructures [10,11].…”

“…For example, recently a TI-based Josephson junction was fabricated that demonstrated the usability of TIs in the field of superconducting transmon qubits [9]. The fundamental property of the superconducting proximity effect in SC/TI heterostructures has been a major research focus in the past years, which was pursued both experimentally [9][10][11][12][13][14][15][16] and theoretically [1,[17][18][19][20][21]. For instance, using scanning tunneling microscopy [10][11][12], angle-resolved photoemission [13,14], or transport experiments [15], it was demonstrated that TIs of the (Bi,Sb) 2 (Te,Se) 3 family can indeed be proximitized by an interface to a superconductor.…”

“…This is reflected (i) in the different extent to which the quantum-well states and the TSS of thin TI films are gapped out upon contact with a SC and (ii) in how quickly the proximity-induced gap in the different states decay from the SC/TI interface [13,14,20]. These differences in the proximity effect were attributed to the degree of hybridization of the respective TI wave functions with the superconductor and the resulting possibility for Cooper pair tunneling into the TI [14,20]. Only in the recent past, it was realized that at the SC/TI contact charge transfer and band bending effects play an important role for the proximity effect and the opening of a topological gap [12,21].…”

“…For instance, using scanning tunneling microscopy [10][11][12], angle-resolved photoemission [13,14], or transport experiments [15], it was demonstrated that TIs of the (Bi,Sb) 2 (Te,Se) 3 family can indeed be proximitized by an interface to a superconductor. Elemental s-wave superconductors like Nb [9,13] or Pb [12] but also two-dimensional van der Waals superconductors like NbSe 2 [10,11,14] or PdTe 2 [15,20] have been used successfully to open a superconducting gap in the electronic structure of a TI. It was demonstrated that the proximity effect decays exponentially in the topological surface state (TSS) with its distance to the SC/TI interface (i.e.…”

“…Comparing different SC/TI interfaces, it is clear that the details of the interface chemistry strongly influence the proximity effect. This is reflected (i) in the different extent to which the quantum-well states and the TSS of thin TI films are gapped out upon contact with a SC and (ii) in how quickly the proximity-induced gap in the different states decay from the SC/TI interface [13,14,20]. These differences in the proximity effect were attributed to the degree of hybridization of the respective TI wave functions with the superconductor and the resulting possibility for Cooper pair tunneling into the TI [14,20].…”