# Fermionic spin liquid analysis of the paramagnetic state in Volborthite

## TL;DR

This paper investigates fermionic spin liquid phases in Volborthite's paramagnetic state, proposing that spinon Fermi surfaces could explain the observed thermal Hall effect and identifying multiple candidate states through symmetry and mean field analysis.

## Contribution

It introduces a non-symmorphic Kagome lattice model, classifies twelve $ ext{Z}_2$ spin liquids, and links fermionic states with spinon Fermi surfaces to experimental thermal Hall observations.

## Key findings

- Identification of twelve $ ext{Z}_2$ spin liquid states.
- Four states with spinon Fermi surfaces consistent with experimental data.
- Potential explanation for finite thermal Hall conductivity in Volborthite.

## Abstract

Recently, thermal Hall effect has been observed in the paramagnetic state of Volborthite, which consists of distorted Kagome layers with $S=1/2$ local moments. Despite the appearance of a magnetic order below $1 \, \mathrm{K}$, the response to external magnetic field and unusual properties of the paramagnetic state above $1 \, \mathrm{K}$ suggest possible realization of exotic quantum phases. Motivated by these discoveries, we investigate possible spin liquid phases with fermionic spinon excitations in a non-symmorphic version of the Kagome lattice, which belongs to the two-dimensional crystallographic group $p2gg$. This non-symmorphic structure is consistent with the spin model obtained in the density functional theory (DFT) calculation. Using projective symmetry group (PSG) analysis and fermionic parton mean field theory, we identify twelve distinct $\mathbb{Z}_2$ spin liquid states, four of which are found to have correspondence in the eight Schwinger boson spin liquid states we classified earlier. We focus on the four fermionic states with bosonic counterpart and find that the spectrum of their corresponding root $U(1)$ states feature spinon Fermi surface. The existence of spinon Fermi surface in candidate spin liquid states may offer a possible explanation of the finite thermal Hall conductivity observed in Volborthite.