Hidden Orders: The Chirality Density Wave and Orbital Antiferromagnetism in URu₂Si₂

In the uranium-based compound URu₂Si₂, two long range orders compete below a second order phase transition at about 17.5 kelvin: the mysterious non-magnetic “hidden order”, and an unconventional antiferromagnetic phase. The two orders are strikingly similar thermodynamically, yet symmetry-wise different, and the relation between them pose a thirty years old enigma. In this talk, I will introduce, 1) direct evidence of symmetry breaking in the "hidden order" probed by Raman spectroscopy, and 2) a new type of collective mode associated with the two competing orders.
In our study, we tune the "hidden order" into antiferromagnetism by substituting iron for ruthenium. In the “hidden order” phase, we discovered evidence of local vertical and horizontal reflection symmetries breaking at the uranium sites, leading to staggering of chiral 5f electron states, forming a commensurate Chirality Density Wave as the ground state. Above critical Fe concentration, staggering of orbital magnetic moment from the 5f electrons leads to an antiferromagnetic ground state. We drive and detect dynamical oscillations between the "hidden order" and the antiferromagnetic ground states using polarized light, and as such provide direct experimental evidence for an unified order parameter describing the two competing phases. The dynamical oscillations between the two phases present a new type of collective mode, which softens at the first order phase boundary.
Main References:
[1] H.-H. Kung et al., Science 347, 1339 (2015)
[2] H.-H. Kung et al., PRL 117, 227601 (2016)