Non-Fermi liquid over extended range at zero temperature without quantum criticality

TL;DR

This paper discovers an extended non-Fermi liquid phase in a Holstein model away from half-filling, characterized by a discontinuous transition and a mixture of NFL and Fermi liquid states, with implications for understanding strange metals.

Contribution

It reveals a novel NFL phase with extended stability due to a discontinuous transition in a non-engineered Hamiltonian, advancing the understanding of strange metallicity.

Findings

Extended NFL phase observed away from half-filling.

Discontinuous phase transition between NFL and Fermi liquid.

Emergence of a spin gap without a charge gap.

Abstract

A strange metal state appears in many strongly correlated materials from the cuprates, pnictides, and twisted bilayer graphenes to even bosonic systems. Enormous efforts are being made to unravel the nature of the non-Fermi liquid (NFL) which underlies the strange metallicity, yet progress is rather slow. Understanding the NFL behavior is crucial, in addition to its importance in replacing the Fermi liquid paradigm, in that NFL is the normal state from which new states of matter emerge like high temperature superconductivity. Here, we report the appearance of an NFL phase in a discontinuous metal-NFL-insulator transition in a ``non-engineered'' Hamiltonian of the Holstein model away from half-filling. The discontinuity of the phase transition between NFL and Fermi liquid metal renders the ground state at the phase boundary a mixture between them over an extended range like the extended criticality observed in cuprates and other quantum materials. The dynamical mean-field theory was employed in combination with Wilson's numerical renormalization group technique. We identify the origin of the NFL metal phase as the opening of a spin gap in the absence of a charge gap. Also, strong lattice fluctuations emerge at the boundary which should enhance superconductivity.