# Strong Landau-quantization effects in high-magnetic-field superconductivity of a two-dimensional multiple-band metal near the Lifshitz transition

## TL;DR

This paper explores how Landau quantization causes giant oscillations and reentrant superconductivity in a two-dimensional multiband superconductor near a Lifshitz transition under high magnetic fields.

## Contribution

It demonstrates the significant impact of Landau quantization on superconductivity near a Lifshitz transition, revealing reentrant behavior and oscillations in the transition temperature.

## Key findings

- Giant oscillations in transition temperature T_C2(H) due to Landau levels.
- Reentrant superconducting regions emerge at specific magnetic fields.
- Reentrance is suppressed by Zeeman spin splitting but can be restored under resonance conditions.

## Abstract

We investigate the onset of superconductivity in magnetic field for a clean two-dimensional multiple-band superconductor in the vicinity of the Lifshitz transition when one of the bands is very shallow. Due to small number of carriers in this band, the quasiclassical Werthamer-Helfand approximation breaks down and Landau quantization has to be taken into account. We found that the transition temperature TC2(H) has giant oscillations and is resonantly enhanced at the magnetic fields corresponding to full occupancy of the Landau levels in the shallow band. This enhancement is especially pronounced for the lowest Landau level. As a consequence, the reentrant superconducting regions in the temperature-field phase diagram emerge at low temperatures near the magnetic fields at which the chemical potential matches the Landau levels. The specific behavior depends on the relative strength of the intraband and interband pairing interactions and the reentrance is most pronounced in the purely interband coupling scenario. The reentrant behavior is suppressed by the Zeeman spin splitting in the shallow band, the separated regions disappear already for very small spin-splitting factors. On the other hand, the reentrance is restored in the resonance cases when the spin-splitting energy exactly matches the separation between the Landau levels. The predicted behavior may realize in the gate-tuned FeSe monolayer.