$Z_2$ fractionalized phases of a solvable, disordered, $t$-$J$ model

TL;DR

This paper introduces a solvable, disordered $t$-$J$ model exhibiting $Z_2$ fractionalized phases, with one phase modeling underdoped cuprates and another resembling overdoped cuprates, highlighting emergent gauge charges and fractionalization effects.

Contribution

It presents a novel solvable model with $Z_2$ fractionalization, capturing phases analogous to underdoped and overdoped cuprates, and analyzes their critical states.

Findings

Identification of a gapped boson, gapless fermion phase with gapped electron spectral function.

Discovery of a critical 'quasi-Higgs' phase with Fermi liquid-like propagator but fractionalized excitations.

Description of the phase transition between the gapped and gapless fractionalized phases.

Abstract

We describe the phases of a solvable $t$-$J$ model of electrons with infinite-range, and random, hopping and exchange interactions, similar to those in the Sachdev-Ye-Kitaev models. The electron fractionalizes, as in an `orthogonal metal', into a fermion $f$ which carries both the electron spin and charge, and a boson $\phi$. Both $f$ and $\phi$ carry emergent $\mathbb{Z}_2$ gauge charges. The model has a phase in which the $\phi$ bosons are gapped, and the $f$ fermions are gapless and critical, and so the electron spectral function is gapped. This phase can be considered as a toy model for the underdoped cuprates. The model also has an extended, critical, `quasi-Higgs' phase where both $\phi$ and $f$ are gapless, and the electron operator $\sim f \phi$ has a Fermi liquid-like $1/\tau$ propagator in imaginary time, $\tau$. So while the electron spectral function has a Fermi liquid form, other properties are controlled by $\mathbb{Z}_2$ fractionalization and the anomalous exponents of the $f$ and $\phi$ excitations. This `quasi-Higgs' phase is proposed as a toy model of the overdoped cuprates. We also describe the critical state separating these two phases.