Novel Phases of Planar Fermionic Systems

TL;DR

This paper explores a family of two-dimensional fermionic systems exhibiting a sequence of phase transitions from a Fermi liquid to an inhomogeneous state, and then to a state with a standard order parameter, with potential experimental realizations.

Contribution

It introduces a unified analysis of phase structures in planar fermionic systems, highlighting the existence of intermediate inhomogeneous phases with nonstandard ordering, supported by RG analysis and specific examples.

Findings

Identification of a second order transition to inhomogeneous states in superconductors and fermionic systems.

Existence of a sizeable parameter window for intermediate phases with nonstandard orderings.

Discussion of experimental conditions and RG methods for observing these phases.

Abstract

We discuss a {\em family} of planar (two-dimensional) systems with the following phase strucure: a Fermi liquid, which goes by a second order transition (with non classical exponent even in mean-field) to an intermediate, inhomogeneous state (with nonstandard ordering momentum) , which in turn goes by a first order transition to a state with canonical order parameter. We analyze two examples: (i) a superconductor in a parallel magnetic field (which was discussed independently by Bulaevskii)for which the inhomogeneous state is obtained for $1.86 T_c \stackrel{\sim}{<} B \stackrel{\sim}{<} 1.86 \sqrt{2} T_c$ where $T_c$ is the critical temperature (in Kelvin) of the superconductor without a field and $B$ is measured in Tesla, and (ii) spinless (or, as is explained, spin polarized) fermions near half-filling where a similar, sizeable window (which grows in size with anisotropy) exists for the intermediate CDW phase at an ordering momentum different from $(\pi , \pi )$. We discuss the experimental conditions for realizing and observing these phases and the Renormalization Group approach to the transitions.