Heat capacity peak at the quantum critical point of the transverse Ising magnet CoNb2O6

Tian Liang, S. M. Koohpayeh, J. W. Krizan, T. M. McQueen, R. J. Cava and N. P. Ong ()
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Tian Liang: Princeton University
S. M. Koohpayeh: Institute for Quantum Matter, Johns Hopkins University
J. W. Krizan: Princeton University
T. M. McQueen: Institute for Quantum Matter, Johns Hopkins University
R. J. Cava: Princeton University
N. P. Ong: Princeton University

Nature Communications, 2015, vol. 6, issue 1, 1-7

Abstract: 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 CoNb2O6 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. Here, 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.

Date: 2015
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DOI: 10.1038/ncomms8611

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