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

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.

14 pages total, 8 figures

References in corpus (4)

• Evolution of Quantum Fluctuations Near the Quantum Critical Point of the Transverse Field Ising Chain System CoNb${}_2$O${}_6$
• A hierarchy of bound states in the 1D ferromagnetic Ising chain CoNb${}_2$O${}_6$ investigated by high resolution time-domain terahertz spectroscopy
• Energy spectrum of bound-spinons in the quantum Ising spin-chain ferromagnet
• Excitations in the quantum paramagnetic phase of the quasi-one-dimensional Ising magnet CoNb${}_2$O${}_6$ in a transverse field: Geometric frustration and quantum renormalization effects

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