Modulation of superconductivity across a Lifshitz transition in alternating-angle twisted quadrilayer graphene

TL;DR

This study demonstrates how electric field tuning of the Fermi surface topology in alternating-angle twisted quadrilayer graphene influences its superconducting properties, revealing a Lifshitz transition that enhances $T_c$ within a symmetry-broken state.

Contribution

It provides experimental evidence linking Fermi surface topology changes to superconductivity modulation in TQG, highlighting the role of Lifshitz transitions.

Findings

Superconductivity vanishes at high displacement fields due to increased bandwidth.

A Lifshitz transition occurs at lower displacement fields, merging flat and dispersive Fermi surfaces.

Superconducting transition temperature $T_c$ is enhanced near the Lifshitz transition.

Abstract

We report electric field-controlled modulation of the Fermi surface topology and explore its effects on the superconducting state in alternating-angle twisted quadrilayer graphene (TQG). The unique combination of flat and dispersive bands in TQG allows us to simultaneously tune the band structure through carrier density, $n$, and displacement field, $D$. From density-dependent Shubnikov-de Haas quantum oscillations and Hall measurements, we quantify the $D$-dependent bandwidth of the flat and dispersive bands and their hybridization. In the high $|D|$ regime, the increased bandwidth favors the single particle bands, which coincides exactly with the vanishing of the superconducting transition temperature $T_c$, showing that superconductivity in TQG is strongly bound to the symmetry-broken state. For a range of lower $|D|$ values, a Lifshitz transition occurs when the flat and dispersive band Fermi surfaces merge within the $\nu=+2$ symmetry-broken state. The superconducting state correspondingly shows an enhanced $T_c$, suggesting that the superconducting condensate is strongly dependent on the Fermi surface topology and density of states within this symmetry-broken state.