Fluctuations in a superconducting quantum critical point of multi-band metals

TL;DR

This paper investigates the quantum critical behavior of superconductivity suppression in multi-band metals due to hybridization, revealing a transition akin to Bose-Einstein condensation and novel features like breached pair states.

Contribution

It provides a theoretical analysis of superconducting quantum critical points in multi-band metals, highlighting the role of hybridization and the universality class of the phase transition.

Findings

Superconductivity suppression linked to hybridization in multi-band metals.

Quantum critical point exhibits features similar to Bose-Einstein condensation.

Existence of breached pair superconductivity with unpaired electrons at the Fermi surface.

Abstract

In multi-band metals quasi-particles arising from different atomic orbitals coexist at a common Fermi surface. Superconductivity in these materials may appear due to interactions within a band (intra-band) or among the distinct metallic bands (inter-band). Here we consider the suppression of superconductivity in the intra-band case due to hybridization. The fluctuations at the superconducting quantum critical point (SQCP) are obtained calculating the response of the system to a fictitious space and time dependent field, which couples to the superconducting order parameter. The appearance of superconductivity is related to the divergence of a generalized susceptibility. For a single band superconductor this coincides with the \textit{Thouless criterion}. For fixed chemical potential and large hybridization, the superconducting state has many features in common with breached pair superconductivity with unpaired electrons at the Fermi surface. The T=0 phase transition from the superconductor to the normal state is in the universality class of the density-driven Bose-Einstein condensation. For fixed number of particles and in the strong coupling limit, the system still has an instability to the normal sate with increasing hybridization.