Tunable BCS-BEC crossover, reentrant, and hidden quantum phase transitions in two-band superconductors with tunable valence and conduction bands

TL;DR

This paper explores the complex physics of two-band superconductors with tunable gaps, revealing phenomena like BCS-BEC crossover, reentrant transitions, and hidden quantum phase transitions through a mean-field analysis.

Contribution

It introduces a detailed mean-field study of two-band superconductors with tunable parameters, uncovering new quantum phases and reentrant behaviors not seen in single-band systems.

Findings

Density-induced and band-selective BCS-BEC crossover observed.

Reentrant superconducting-normal phase transition identified.

Complex redistribution of electrons among bands with tunable parameters.

Abstract

Two-band electronic structures with a valence and a conduction band separated by a tunable energy gap and with pairing of electrons in different channels can be relevant to investigate the properties of two-dimensional multiband superconductors and electron-hole superfluids, as monolayer FeSe, recently discovered superconducting bilayer graphene, and double-bilayer graphene electron-hole systems. This electronic configuration allows also to study the coexistence of superconductivity and charge density waves in connection with underdoped cuprates and transition metal dichalcogenides. By using a mean-field approach to study the system above mentioned, we have obtained numerical results for superconducting gaps, chemical potential, condensate fractions, coherence lengths, and superconducting mean-field critical temperature, considering a tunable band gap and different filling of the conduction band, for parametric choice of the pairing interactions. By tuning these quantities, the electrons redistribute among valence and conduction band in a complex way, leading to a new physics with respect to single-band superconductors, such as density induced and band-selective BCS-BEC crossover, quantum phase transitions, and hidden criticalities. At finite temperature, this phenomenon is also responsible for the non-monotonic behavior of the superconducting gaps resulting in a superconducting-normal state reentrant transition, without the need of disorder or magnetic effects.