Optical study of hybridization and hidden order in URu₂Si₂

Abstract

We summarize existing optical data of URu₂Si₂ to clarify the nature of the hidden order transition in this heavy fermion metal. Hybridization develops between 50 K and 17.5 K, and a coherent Drude peak emerges which mirrors the changes in the dc resistivity. The Drude weight indicates that there is little change in the effective mass of these carriers in this temperature range. In addition, there is a flat background conductivity that develops a partial hybridization gap at 10 meV as the temperature is lowered, shifting spectral weight to higher frequencies above 300 meV. Below 30 K the carriers become increasingly coherent and Fermi-liquid-like as the hidden order transition is approached. The hidden order state in URu₂Si₂ is characterized by multiple anisotropic gaps. The gap parameter  meV in the ab-plane. In the c-direction, there are two distinct gaps with magnitudes of  meV and  meV. These observations are in good agreement with other spectroscopic measurements. Overall, the spectrum can be fit by a Dynes-type density of states model to extract values of the hidden order gap. The transfer of spectral weight strongly resembles what one sees in density wave transitions.

Keywords:

• heavy-fermion metals
• optical properties
• URu₂Si₂
• hybridization
• hidden order

Acknowledgments

We thank the following colleagues for helpful discussions: Bill Buyers, Jules Carbotte, Andre Chubukov, Piers Coleman, Seamus Davis, Gabriel Kotliar, Brian Maple, Dmitrii Maslov and Peter Oppeneer. In particular we owe thanks to Nick Butch for allowing us to use his high resolution resistivity data shown in Figures 1 and 6. Much of the work shown here was done in collaboration with Urmas Nagel and Toomas Rõõm in Tallinn, Estonia and Ricardo Lobo in Paris. This work was supported by the Natural Sciences and Engineering Research Council of Canada and the Canadian Institute for Advanced Research.