Abstract
High-resolution thermal expansion and magnetostriction measurements were performed on single crystals of α−RuCl3 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 μ0Hc1=7.8(2)T where the antiferromagnetic zigzag order is suppressed. At higher fields, a kink in the magnetostriction coefficient signals an additional transition or crossover around μ0Hc2≈11T. The extracted Grüneisen ratio shows typical hallmarks for quantum criticality near Hc1 but also displays anomalous behavior above Hc1. We compare our experimental data with 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 region above Hc1 cannot be accounted for in our semiclassical modeling and hence suggest a genuine quantum nature. We construct a phase diagram for α−RuCl3 in a magnetic field along the Ru-Ru bonds, displaying a zigzag ordered state below Hc1, a quantum paramagnetic regime between Hc1 and Hc2, and a semiclassical partially polarized state above Hc2.
