Heat capacity anomaly at the quantum critical point of the Transverse Ising Magnet CoNb_2O_6

TL;DR

This study reveals a significant band of gapless spin excitations in CoNb2O6 at its quantum critical point, with a peak in spin entropy and fermion-like behavior, highlighting novel low-energy modes involved in the quantum phase transition.

Contribution

It provides the first heat capacity evidence of gapless spin excitations and their fermion-like behavior at the quantum critical point of CoNb2O6, a realization of the transverse Ising model.

Findings

Gapless spin excitations exist at the QCP.

Spin entropy peaks at the QCP, involving 30% of spin degrees.

Fermion-like, linear heat capacity observed below 1 K.

Abstract

The transverse Ising magnet Hamiltonian describing the Ising chain in a transverse magnetic field is the archetypal example of a system that undergoes a transition at a quantum critical point (QCP). The columbite CoNb$_2$O$_6$ is the closest realization of the transverse Ising magnet found to date. At low temperatures, neutron diffraction has observed a set of discrete collective spin modes near the QCP. We ask if there are low-lying spin excitations distinct from these relatively high energy modes. Using the heat capacity, we show that a significant band of gapless spin excitations exists. At the QCP, their spin entropy rises to a prominent peak that accounts for 30$\%$ of the total spin degrees of freedom. In a narrow field interval below the QCP, the gapless excitations display a fermion-like, temperature-linear heat capacity below 1 K. These novel gapless modes are the main spin excitations participating in, and affected, by the quantum transition.