SU(2)-invariant spin liquids on the triangular lattice with spinful Majorana excitations

TL;DR

This paper introduces a novel class of SU(2)-invariant spin liquids on the triangular lattice featuring spinful Majorana fermions, gapless excitations, and unique thermal and magnetic properties, with potential experimental relevance.

Contribution

It presents a new SU(2)-invariant spin liquid state with Majorana excitations, stable mean-field solutions, and detailed physical property predictions, advancing understanding of quantum spin liquids.

Findings

Presence of gapless excitations along 3 Fermi lines

Absence of thermal Hall effect due to symmetry

Zeeman field gaps excitations and suppresses low-temperature responses

Abstract

We describe a new class of spin liquids with global SU(2) spin rotation symmetry in spin 1/2 systems on the triangular lattice, which have real Majorana fermion excitations carrying spin S = 1. The simplest translationally-invariant mean-field state on the triangular lattice breaks time-reversal symmetry and is stable to fluctuations. It generically possesses gapless excitations along 3 Fermi lines in the Brillouin zone. These intersect at a single point where the excitations scale with a dynamic exponent z = 3. An external magnetic field has no orbital coupling to the SU(2) spin rotation-invariant fermion bilinears that can give rise to a transverse thermal conductivity, thus leading to the absence of a thermal Hall effect. The Zeeman coupling is found to gap out two-thirds of the z = 3 excitations near the intersection point and this leads to a suppression of the low temperature specific heat, the spin susceptibility and the Wilson ratio. We also compute physical properties in the presence of weak disorder and discuss possible connections to recent experiments on organic insulators.