Unveiling the quantum critical point of an Ising chain

TL;DR

This paper experimentally identifies the quantum critical point in a quasi-one-dimensional Ising ferromagnet by measuring thermal conductivity at ultra-low temperatures, confirming theoretical predictions about the gap closing at the transition.

Contribution

It provides the first experimental verification of the quantum critical point in CoNb2O6 through thermal conductivity measurements, aligning with theoretical models of the Ising chain.

Findings

Quantum critical point identified at the transverse field where the gap closes.

Phonon scattering by magnetic excitations observed above a certain temperature.

The gap decreases linearly with the transverse field, confirming theoretical predictions.

Abstract

Quantum phase transitions occur at zero temperature upon variation of some nonthermal control parameters. The Ising chain in a transverse field is probably the most-studied model undergoing such a transition, from ferromagnetic to paramagnetic state1,2. This model can be exactly solved by using a Jordan-Wigner transformation, which transforms the spins into noninteracting spinless fermions1. At the quantum critical point, the magnetic excitations can carry arbitrarily low energy and dominate the low temperature properties. Here we report the unveiling of such quantum critical point in quasi-one-dimensional Ising ferromagnet CoNb2O6 by ultra-low-temperature thermal conductivity measurements. We find that in the paramagnetic state, phonons are scattered by the magnetic excitations above certain temperature Ts, which corresponds to a gap. As predicted by the theoretical model1, this gap linearly goes to zero with decreasing the transverse field, thus determining the quantum critical point of the Ising chain.