Gapless spin excitations in a quantum spin liquid state of S=1/2 perfect kagome antiferromagnet

TL;DR

This study investigates the quantum spin liquid state in a perfect kagome antiferromagnet, revealing gapless spin excitations and field-induced spin gap formation, challenging existing fermionic quasiparticle theories.

Contribution

It provides experimental evidence for gapless bosonic excitations and field-induced quantum phase transition in a kagome antiferromagnet, offering new insights into its ground state.

Findings

Susceptibility remains nearly temperature-independent at low temperatures.

Absence of a linear specific heat contribution despite gapless excitations.

Magnetic field induces a spin gap and a quantum phase transition.

Abstract

Quantum spin liquids (QSLs) represent an exotic quantum many-body state characterized by the suppression of long-range magnetic order due to strong quantum fluctuations. The kagome spin-1/2 antiferromagnet (AFM) is a prime candidate for realizing QSLs, but its ground state remains an unresolved conundrum. Here we investigate the recently discovered perfect kagome AFM YCu$_3$(OH)$_{6.5}$Br$_{2.5}$ to elucidate two central enigmas surrounding the kagome AFM. Ultra-sensitive torque magnetometry experiments reveal that the intrinsic magnetic susceptibility arising from the kagome layer remains nearly temperature-independent down to exceedingly low temperatures. This observation seemingly implies the emergence of gapless fermionic spin excitations akin to Pauli paramagnetism in metals. However, most strikingly, these results stand in stark contrast to the conspicuous absence of a temperature-linear contribution to the specific heat. These findings appear irreconcilable with the widely-discussed theoretical frameworks assuming fermionic quasiparticles (QPs), instead suggesting a transition of bosonic QPs into a superfluid state with a gapless Goldstone mode. Furthermore, magnetocaloric measurements evince an entropy anomaly, constituting thermodynamic evidence that magnetic fields instigate the opening of a spin gap, driving a quantum phase transition into a 1/9 magnetization plateau state. These results shed light on the nature of the low-energy excitations in zero and strong magnetic fields, providing crucial insights into the long-standing unresolved issues of the ground state of the kagome AFM.