Topology of the Fermi Surface Beyond the Quantum Critical Point

TL;DR

This paper proposes a topological framework for understanding phase transitions in strongly correlated Fermi systems at the quantum critical point, challenging conventional collective scenarios and revealing new Fermi surface behaviors.

Contribution

It introduces a topological scenario based on Landau quasiparticles and Poincaré mapping, explaining Fermi surface rearrangements beyond the QCP within standard Fermi liquid theory.

Findings

Fermi surface can become multi-connected beyond the QCP.

Failure of convergence leads to a Fermi volume with excess entropy.

Thermodynamics beyond the QCP significantly deviates from standard Fermi liquid predictions.

Abstract

We examine the nature of phase transitions occurring in strongly correlated Fermi systems at the quantum critical point (QCP) associated with a divergent effective mass. Conventional scenarios for the QCP involving collective degrees of freedom are shown to have serious shortcomings. Working within the original Landau quasiparticle picture, we propose an alternative topological scenario for the QCP, in systems that obey standard Fermi liquid (FL) theory in advance of the QCP. Applying the technique of Poincar\'e mapping, we analyze the sequence of iterative maps generated by the Landau equation for the single-particle spectrum at zero temperature. It is demonstrated that the Fermi surface is subject to rearrangement beyond the QCP. If the sequence of maps converges, a multi-connected Fermi surface is formed. If it fails to converge, the Fermi surface swells into a volume that provides a measure of entropy associated with formation of an exceptional state of the system characterized by partial occupation of single-particle states and dispersion of their spectrum proportional to temperature. Based on this dual scenario, the thermodynamics of Fermi systems beyond the QCP exhibits striking departures from the predictions of standard FL theory. Mechanisms for the release of the entropy excess of the exceptional state are discussed.