Field Driven Quantum Criticality in the Spinel Magnet ZnCr$_2$Se$_4$

TL;DR

This study investigates the magnetic phase transitions and quantum critical behavior of the frustrated magnet ZnCr$_2$Se$_4$, revealing a field-driven quantum critical point with unconventional low-energy excitations and strong quantum fluctuations.

Contribution

It provides the first detailed phase diagram and evidence of a direct quantum phase transition in ZnCr$_2$Se$_4$, highlighting its status as a rare 3D magnet with unconventional quantum criticality.

Findings

Identification of a quantum phase transition from spiral to polarized state

Observation of unconventional $T^2$ specific heat behavior

Detection of temperature-independent thermal mean free path

Abstract

We report detailed dc and ac magnetic susceptibilities, specific heat, and thermal conductivity measurements on the frustrated magnet ZnCr$_2$Se$_4$. At low temperatures, with increasing magnetic field, this spinel material goes through a series of spin state transitions from the helix spin state to the spiral spin state and then to the fully polarized state. Our results indicate a direct quantum phase transition from the spiral spin state to the fully polarized state. As the system approaches the quantum criticality, we find strong quantum fluctuations of the spins with the behaviors such as an unconventional $T^2$-dependent specific heat and temperature independent mean free path for the thermal transport. We complete the full phase diagram of ZnCr$_2$Se$_4$ under the external magnetic field and propose the possibility of frustrated quantum criticality with extended densities of critical modes to account for the unusual low-energy excitations in the vicinity of the criticality. Our results reveal that ZnCr$_2$Se$_4$ is a rare example of 3D magnet exhibiting a field-driven quantum criticality with unconventional properties.