Quantum critical excitations in Ising-like spin chains: A heat transport perspective

Abstract

Regarded as the closest realization of the quantum Ising model in a magnetic field, which features stable quasiparticles with an emergent E₈ symmetry, CoNb₂O₆ keeps being a source of inspiration by manifesting intricacies beyond this model. A notable example is the recent finding of an unexpected band of localized gapless fermionic excitations around the field-induced quantum critical point (QCP), whose origin remains unclear. Despite differences in many aspects compared to CoNb₂O₆, BaCo₂V₂O₈ in a magnetic field also exhibits the E₈ physics, while allowing for better scrutiny of the critical excitations associated with its well-separated one- and three-dimensional QCPs. Here we study the low-lying magnetic excitations within the antiferromagnetic order of BaCo₂V₂O₈ by performing ultralow-temperature heat transport measurements. The field-independent thermal conductivity at the lowest temperatures and the suppressed thermal conductivity with an increasing field at higher temperatures point to heat-carrying phonons scattered by magnetic excitations. For the magnetic excitations, importantly, we find no evidence for the emergence, around the one-dimensional QCP, of gapless fermionic excitations similar to those observed in CoNb₂O₆. Based on the contrasting cases of BaCo₂V₂O₈ and CoNb₂O₆, we propose that the frustrated alignment of chains and the proximity of two QCPs in CoNb₂O₆ may put this system in a unique place to host the gapless fermionic excitations.

Keywords

quantum criticality / magnetic excitations / ultralow-temperature thermal conductivity / Ising chain

RIGHTS & PERMISSIONS

Higher Education Press 2026

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