### QuMat seminar

## Bogoliubov Fermi surface in a proximitized topological insulator Speaker: Jens Brede – University of Cologne |

Abstract:

The Fermi surface of an ideal two-dimensional metal is just a circle in momentum space that separates the occupied (p^{2}/2m<E_{F}) and empty (p^{2}/2m>E_{F}) states at zero temperature and can be obtained by solving for the zero-excitation energy E(p)=0. When the metal undergoes a superconducting transition, the electrons on the circle pair up and the Fermi surface “vanishes” as only quasi-particle excitations above the superconducting gap are possible. However, as pointed out theoretically by Volovik [1] there are circumstances where a Fermi surface, i.e. a surface of zero energy excitations for Bogoliubov quasiparticles, appears also in a superconductor.

Zu et al. [2] demonstrated this experimentally by creating sufficiently strong supercurrents that Doppler shift the quasiparticle energy at the surface of a topological insulator proximitized by a superconductor. Such a 2D gapless superconducting state in spin-helical systems under the in-plane magnetic field is predicted by Papaj et al. [3] to host Majorana bound states when the topological 1D channels are formed by quantum confinement of quasiparticles via Andreev reflection.

I will show by STM/STS at 400 mK and in-plane magnetic fields of up to 2 T that a Bogoliubov Fermi surface is also realized in the topological surface state (TSS) of bismuth telluride [Bi_{2}Te_{3}(0001)] thin films grown on thin superconducting niobium [Nb(110)] layers.

In zero magnetic field the STS on the top surface of the few nanometers thick Bi_{2}Te_{3} films shows an induced hard superconducting gap of about 0.5 meV. Increasing the field, we observe in-gap states due to Doppler shifted Bogoliubov quasiparticles from the TSS which led to a zero bias conductance peak at a magnetic field of about 750 mT. At this field the induced screening current match the depairing current and quasiparticle interference at the Fermi energy shows distinct scattering vectors, capturing the Bogoliubov Fermi surface.

Thereafter, I will show that we can exploit spatial inhomogeneity in electronic (E_{F}) and superconducting (Delta) to spatially confine the gapless superconducting state and find spectroscopic signatures in-line with the predicted Majorana bound states.

Publications/References:

[1] Volovik, Lect. Notes Phys.718:31-73 (2007)

[2] Zhu et al., Science 374, 1381–1385 (2021)

[3] Papaj et al., Nat. Commun. 12, 577 (2021)