Research: Huge lattice softening near a quantum critical point

Quantum critical behavior is a central issue in condensed-matter physics. Various remarkable phenomena, such as exotic superconductivity and non-Fermi-liquid behavior, appear around the quantum critical points, which are induced by purely quantum mechanical interactions. Long-range order is broken by quantum fluctuations at a critical value of a tuning parameter, such as magnetic field, pressure, or doping. A prototypical system which undergoes a quantum phase transition is an Ising spin chain in a transverse magnetic field ‒ the so-called transverse Ising model. CoNb₂O₆ is one example for such a transverse Ising magnet, when a magnetic field is applied along the magnetic hard axis. We, a team of researchers from the University of Tokyo, Japan, and the Dresden High Magnetic Field Laboratory at HZDR, have investigated the elastic response of CoNb₂O₆ by means of ultrasound-velocity measurement. We observed a huge elastic anomaly in one acoustic mode when sweeping the magnetic field along the hard axis, perpendicular to the Ising axis. The elastic constant C₆₆ softens by about 20 % at 4.8 T and 1.3 K This anomaly appears to become critical at a quantum critical point near this magnetic field. Our results suggest that the relativistic spin-orbit interaction together with quantum fluctuations play a crucial role in the quantum critical regime.

K. Matsuura, P. T. Cong, S. Zherlitsyn, J. Wosnitza, N. Abe, and T. Arima,
Anomalous Lattice Softening Near a Quantum Critical Point in a Transverse Ising Magnet,
Phys. Rev. Lett. 124, 127205 (2020)