Field-induced phase transitions of the Kitaev material $\alpha$-RuCl$_3$ probed by thermal expansion and magnetostriction

• Main Authors: Gass, S, Cônsoli, P. M, Kocsis, V, Corredor, L. T, Lampen-Kelley, P, Mandrus, D. G, Nagler, S. E, Janssen, L, Vojta, M, Büchner, B, Wolter, A. U. B
• Format: Journal Article
• Language: English
• Published: 16.03.2020
• Subjects: Physics - Strongly Correlated Electrons
• DOI: 10.48550/arxiv.2003.07081

Phys. Rev. B 101, 245158 (2020) High-resolution thermal expansion and magnetostriction measurements were performed on single crystals of $\alpha$-RuCl$_3$ in magnetic fields applied parallel to the Ru-Ru bonds. The length changes were measured in the direction perpendicular to the honeycomb planes. Our data show clear thermodynamic characteristics for the field-induced phase transition at the critical field $\mu_0H_{c1} = 7.8(2)$ T where the antiferromagnetic zigzag order is suppressed. At higher fields, a kink in the magnetostriction coefficient signals an additional phase transition around $\mu_0H_{c2} \approx 11$ T. The extracted Gr\"uneisen parameter shows typical hallmarks for quantum criticality near $H_{c1}$, but also displays anomalous behavior above $H_{c1}$. We compare our experimental data with linear spin-wave calculations employing a minimal Kitaev-Heisenberg model in the semiclassical limit. Most of the salient features are in agreement with each other, however, the peculiar features in the high-field region above $H_{c1}$ cannot be accounted for in our modelling and hence suggest a genuine quantum nature. We construct a phase diagram for $\alpha$-RuCl$_3$ showing two low-temperature transitions induced by an in-plane field along the Ru-Ru bonds.