# Signatures for spinons in the quantum spin liquid candidate Ca$_{10}$Cr$_7$O$_{28}$

## TL;DR

This study provides experimental evidence and theoretical modeling supporting the existence of spinon Fermi surfaces and potential weak pairing in the quantum spin liquid candidate Ca$_{10}$Cr$_7$O$_{28}$, indicating a $bZ_2$ quantum spin liquid state.

## Contribution

It introduces a spinon hopping model that explains experimental heat capacity and spin structure factor data, proposing a $bZ_2$ quantum spin liquid with weak pairing in Ca$_{10}$Cr$_7$O$_{28}$.

## Key findings

- Heat capacity shows linear temperature dependence suggesting fermionic spinons.
- Spin structure factor indicates a spinon Fermi surface with diffuse scattering.
- Evidence of weak $f$-wave spinon pairing inducing a partial gap.

## Abstract

We present new experimental low-temperature heat capacity and detailed dynamical spin-structure factor data for the quantum spin liquid candidate material Ca$_{10}$Cr$_7$O$_{28}$. The measured heat capacity shows an almost perfect linear temperature dependence in the range $0.1$ K $\lesssim T\lesssim0.5$ K, reminiscent of fermionic spinon degrees of freedom. The spin structure factor exhibits two energy regimes of strong signal which display rather different but solely diffuse scattering features. We theoretically describe these findings by an effective spinon hopping model which crucially relies on the existence of strong ferromagnetically coupled triangles in the system. Our spinon theory is shown to naturally reproduce the overall weight distribution of the measured spin structure factor. Particularly, we argue that various different observed characteristic properties of the spin structure factor and the heat capacity consistently indicate the existence of a spinon Fermi surface. A closer analysis of the heat capacity at the lowest accessible temperatures hints towards the presence of weak $f$-wave spinon pairing terms inducing a small partial gap along the Fermi surface (except for discrete nodal Dirac points) and suggesting an overall $\mathbb{Z}_2$ quantum spin liquid scenario for Ca$_{10}$Cr$_7$O$_{28}$.