Rashbons: Properties and their significance

TL;DR

This paper systematically studies rashbons, bound states of fermions induced by non-Abelian gauge fields, revealing their properties, dispersion, and potential for enhancing transition temperatures in fermionic condensates.

Contribution

It provides a detailed analysis of rashbon properties, including their dispersion and existence limits, and proposes methods to increase the transition temperature of rashbon BECs.

Findings

Rashbons cease to exist beyond a critical momentum related to gauge coupling.

Transition temperature can be enhanced by tuning the gauge field strength.

Normal state may be a mixture of rashbons and unpaired fermions, exhibiting pseudogap physics.

Abstract

In presence of a synthetic non-Abelian gauge field that induces a Rashba like spin-orbit interaction, a collection of weakly interacting fermions undergoes a crossover from a BCS ground state to a BEC ground state when the strength of the gauge field is increased [Phys. Rev. B {\bf 84}, 014512 (2011)]. The BEC that is obtained at large gauge coupling strengths is a condensate of tightly bound bosonic fermion-pairs whose properties are solely determined by the Rashba gauge field -- hence called rashbons. In this paper, we conduct a systematic study of the properties of rashbons and their dispersion. This study reveals a new qualitative aspect of the problem of interacting fermions in non-Abelian gauge fields, i.e., that the rashbon state induced by the gauge field for small centre of mass momenta of the fermions ceases to exist when this momentum exceeds a critical value which is of the order of the gauge coupling strength. The study allows us to estimate the transition temperature of the rashbon BEC, and suggests a route to enhance the exponentially small transition temperature of the system with a fixed weak attraction to the order of the Fermi temperature by tuning the strength of the non-Abelian gauge field. The nature of the rashbon dispersion, and in particular the absence of the rashbon states at large momenta, suggests a regime of parameter space where the normal state of the system will be a dynamical mixture of uncondensed rashbons and unpaired helical fermions. Such a state should show many novel features including pseudogap physics.