[Paper Review] Discovery of the topological surface state in a noncentrosymmetric superconductor BiPd

Madhab Neupane, Nasser Alidoust+10 more|arXiv (Cornell University)|May 13, 2015

Topological Materials and Phenomena4 citations

TL;DR

This study reports the first experimental discovery of a topological surface state in the noncentrosymmetric superconductor BiPd using high-resolution angle-resolved photoemission spectroscopy (ARPES). The surface state, with a Dirac point located ~700 meV below the Fermi level, arises from band inversion between Bi 6p and Pd 4p states and is confirmed by first-principles calculations, establishing BiPd as a topological insulator in the absence of inversion symmetry, though no topological surface states near the Fermi level preclude topological superconductivity in the native state.

ABSTRACT

Recently, noncentrosymmetric superconductor BiPd has attracted considerable research interest due to the possibility of being a topological superconductor. Here, we report a systematic high-resolution angle resolved photoemission spectroscopy (ARPES) study of the normal state electronic properties of BiPd. Our experimental results show the presence of a surface state at higher binding energy with the location of Dirac point at around 700 meV below the Fermi level. The detailed photon energy and temperature dependent measurements complemented by our first-principles calculations provide further evidence for the presence of the topological surface state at high binding energy. The absence of topological surface states near the Fermi level negates the possibility of the topological superconducting behavior in the surface of this material. Our first direct experimental discovery of a topological surface state in BiPd provides novel information that will guide the future search for topological superconductivity in noncentrosymmetric materials.

Motivation & Objective

• To determine whether noncentrosymmetric superconductors like BiPd host topological surface states, which could enable topological superconductivity.
• To resolve the long-standing ambiguity in the electronic structure of BiP5d, particularly the presence of topological surface states.
• To establish a direct experimental link between spin-orbit coupling, lack of inversion symmetry, and topological surface states in a metallic system.
• To provide a foundation for future exploration of topological superconductivity in noncentrosymmetric materials by identifying the normal-state electronic structure.

Proposed method

• High-resolution angle-resolved photoemission spectroscopy (ARPES) was performed at ALS Beamline 10.0.1 and SSRL Beamline 5-4 with energy resolution <20 meV and angular resolution <0.2°.
• In situ cleaving of BiPd single crystals was conducted under ultra-high vacuum (<10⁻¹⁰ torr) at temperatures between 10 K and 80 K to preserve surface quality.
• First-principles calculations were performed using the projector augmented-wave method within the generalized gradient approximation (GGA), including spin-orbit coupling self-consistently.
• A 1×5×1 supercell model of the (010) surface was used with vacuum thickness >20 Å to simulate surface effects.
• Photon energy and temperature-dependent ARPES measurements were used to confirm the momentum and energy dispersion of the surface state.
• Theoretical analysis focused on band inversion between Bi 6p and Pd 4p states and spin polarization of the surface state to confirm topological origin.

Experimental results

Research questions

• RQ1 Does the noncentrosymmetric superconductor BiPd host a topological surface state, as predicted by first-principles calculations?
• RQ2 Where is the Dirac point of the surface state located in energy relative to the Fermi level, and is it spin-polarized?
• RQ3 Can ARPES measurements distinguish between topological and trivial surface states in BiPd?
• RQ4 Why is there no evidence of topological surface states near the Fermi level despite the presence of strong spin-orbit coupling?
• RQ5 Can tuning the Fermi level via gating or surface deposition induce topological superconductivity in BiPd?

Key findings

• A topological surface state was experimentally discovered in BiPd with a Dirac point located at approximately 700 meV below the Fermi level.
• The surface state arises from band inversion between Bi 6p and Pd 4p states, confirmed by first-principles calculations.
• The surface state exhibits helical spin polarization, providing strong evidence for its topological origin.
• The absence of topological surface states near the Fermi level rules out topological superconductivity in the native surface state of BiPd.
• The system is a topological insulator in the absence of inversion symmetry, distinct from conventional topological insulators due to valence band inversion.
• The results suggest that Fermi level tuning via electrical gating or surface deposition could enable topological superconductivity in BiPd.