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

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 $\varphi$. Both $f$ and $\varphi$ carry emergent ${\mathbb{Z}}_2$ gauge charges. The model has a phase in which the $\varphi$ 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 $\varphi$ and $f$ are gapless, and the electron operator $\sim f\varphi$ 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 $\varphi$ 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.

30 pages, 9 figures