Thermodynamic evidence of fermionic behavior in the vicinity of one-ninth plateau in a kagome antiferromagnet

TL;DR

This study provides thermodynamic evidence of fermionic excitations and Dirac-like spectra near the 1/9 magnetization plateau in a kagome antiferromagnet, supporting the presence of quantum spin liquid states.

Contribution

It reports the first thermodynamic measurements confirming fermionic behavior and Dirac spectra near the 1/9 plateau in a kagome antiferromagnet, with anisotropic constant terms and double-peak features.

Findings

Confirmation of the 1/9 magnetization plateau in YCOB.

Observation of a linear T dependence in specific heat indicating Dirac spectrum.

Detection of a double-peak feature near 30 T as evidence of fermionic excitations.

Abstract

The spin-1/2 kagome Heisenberg antiferromagnets are believed to host exotic quantum entangled states. Recently, the report of 1/9 magnetization plateau and magnetic oscillations in a kagome antiferromagnet YCu$_3$(OH)$_6$Br$_2$[Br$_x$(OH)$_{1-x}$] (YCOB) have made this material a promising candidate for experimentally realizing quantum spin liquid states. Here we present measurements of the specific heat $C_p$ in YCOB in high magnetic fields (up to 41.5 Tesla) down to 0.46 Kelvin, and the 1/9 plateau feature has been confirmed. Moreover, the temperature dependence of $C_p/T$ in the vicinity of 1/9 plateau region can be fitted by a linear in $T$ term which indicates the presence of a Dirac spectrum, together with a constant term, which indicates a finite density of states (DOS) contributed by other Fermi surfaces. Surprisingly the constant term is highly anisotropic in the direction of the magnetic field. Additionally, we observe a double-peak feature near $30$~T above the 1/9 plateau which is another hallmark of fermionic excitations in the specific heat.