Discovery of segmented Fermi surface induced by Cooper pair momentum

TL;DR

This study directly visualizes how a supercurrent induces a segmented Fermi surface in a topological insulator-superconductor heterostructure, revealing the impact of finite Cooper pair momentum on quasiparticle spectra.

Contribution

It provides the first direct imaging of a segmented Fermi surface caused by Cooper pair momentum using quasiparticle interference in a topological insulator-superconductor system.

Findings

Finite supercurrent induces a segmented Fermi surface.

Finite Cooper pair momentum significantly alters quasiparticle spectra.

Observation supports theories of pair density wave and FFLO states.

Abstract

Since the early days of Bardeen-Cooper-Schrieffer theory, it has been predicted that a sufficiently large supercurrent can close the energy gap in a superconductor and creates gapless Bogoliubov quasiparticles through the Doppler shift of quasiparticle energy due to the Cooper pair momentum. In this gapless superconducting state, zero-energy quasiparticles reside on a segment of the normal state Fermi surface, while its remaining part is still gapped. The finite density of states of field-induced quasiparticles, known as the Volovik effect, has been observed in tunneling and specific heat measurements on d- and s-wave superconductors. However, the segmented Fermi surface of a finite-momentum state carrying a supercurrent has never been detected directly. Here we use quasiparticle interference (QPI) technique to image field-controlled Fermi surface of Bi$_2$Te$_3$ thin films proximitized by the superconductor NbSe$_2$. By applying a small in-plane magnetic field, a screening supercurrent is induced which leads to finite-momentum pairing on topological surface states of Bi$_2$Te$_3$. Our measurements and analysis reveal the strong impact of finite Cooper pair momentum on the quasiparticle spectrum, and thus pave the way for STM study of pair density wave and FFLO states in unconventional superconductors.