Zhe Wang, HZDR and S. Zherlitsyn, HLD Dresden

Understanding quantum phase transitions, i.e., phase transitions at zero temperature driven by non-thermal parameters, has become one of the most significant topics in condensed-matter physics. It is generally believed that universal scaling occurs near a quantum critical point, which can be thermodynamically characterized by a divergent Grüneisen parameter. However, for the quantum critical point of the one-dimensional transverse-field Ising chain, a recent theory work shows that the Grüneisen parameter should be finite, when the quantum critical point is approached. The transverse-field Ising spin chain plays an important role in quantum statistical and condensed matter physics, because quantitative understanding of the relevant physics can be achieved exactly, based on its rigorous solvability by analytical as well as numerical methods. However, it remains very
challenging to experimentally realize such a paradigmatic model in a real material for the study of quantum critical behavior.
By performing magnetization, sound velocity, and magnetocaloriceffect measurements in pulsed high magnetic fields, researchers from Augsburg, Cologne, Dresden, and Tokyo have identified a quantum critical regime induced by a transverse field in the Ising-like spin-1/2 antiferromagnetic chain material BaCo2V2O8.
In this quantum critical regime at about 40 T, the system is disordered and the thermal energy is much smaller than the intrachain exchange interaction. The high-field experimental results show that the magnetic Grüneisen parameter follows a universal divergence on approaching the quantum critical point as a function of magnetic field at the lowest temperature, but it converges with decreasing temperature at the critical field (Figure). These features jointly point to the one-dimensional transverse-field Ising universality class of the underlying quantum critical point.

B-St-2

Figure: (a) Magnetocaloric effect of BaCo2V2O8 measured for different starting temperatures in pulsed magnetic fields. Experimental values of the Grüneisen parameter, (1/T)(dT/dB), as a function of (b) magnetic field and (c) temperature.

Quantum Criticality of an Ising-like Spin1/2 Antiferromagnetic Chain in Transvers Magnetic Field, Zhe Wang, T. Lorenz, D. I. Gorbunov, P. T. Cong, Y. Kohama, S. Niesen, O. Breunig, J. Engelmayer, A. Herman, Jianda Wu, K. Kindo, J. Wosnitza, S. Zherlitsyn, and A. Loidl, Phys. Rev. Lett. 120, 207205 (2018). https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.120.207205