Hybridization, hidden order, and antiferromagnetism in URu2Si2: electrodynamics studied by infrared spectroscopy

Abstract

Infrared spectroscopy has been used to study URu2Si2. This heavy fermion material exhibits novel behaviour including a phase transition with an unknown order parameter. Optical spectroscopy has been applied to study the scattering mechanism of the quasiparticles in the coherent state above the transition. It is found that the scattering is incoherent above the hybridization temperature and gradually develops a Drude peak as the temperature is lowered into the coherent regime. As the temperature approaches the transition from above, the scattering is almost entirely Fermi liquid in character. It is observed that the scaling between the frequency and temperature terms in the Fermi liquid region is anomalous and diverges from the predicted value of 4, with profound implications for the nature of Fermi liquid behaviour generally. Infrared spectra also clearly show the charge gap in the ordered state. It is found that the gap is anisotropic, with a different character in the c-axis than in the ab-plane of the tetragonal crystal, and that a second small gap appears at lower temperature in the c-axis. The gap does not have a mean-field temperature dependence. The gap is well-modelled by a Dynes density of states with case I coherence factors, typical of a nesting-induced incommensurate density wave. Doping induces antiferromagnetism in place of the hidden order, and at higher temperatures, which is also studied by spectroscopy. It is found that the Fermi liquid temperature region rises in temperature in tandem with the phase transition, so that the transition is always preceded by Fermi liquid behaviour but with anomalous scaling between frequency and temperature. The character of the charge gap does not change between the hidden order and antiferromagnetic states, indicating that the same mechanism is responsible for the charge gap in both phases.