Quasiparticle relaxation dynamics in URu2-xFexSi2 single crystals

We investigate quasiparticle relaxation dynamics in URu2-xFexSi2 single crystals using ultrafast optical-pump optical-probe (OPOP) spectroscopy as a function of temperature and Fe substitution (x), crossing from the hidden-order (HO) phase (x=0) to the large-moment antiferromagnet (LMAFM) phase (x=0.12). At low temperature, the dynamics for x=0 and x=0.12 are consistent with the low-energy electronic structure of the HO and LMAFM phases that emerge from the high-temperature paramagnetic (PM) phase. In contrast, near the bicritical point separating HO and LMAFM (x=0.1), two transitions occur over a narrow temperature range (from 15.5-17.5 K). A PM to HO transition occurs at an intermediate temperature followed by a transition to the LMAFM phase at lower temperature. While the data at low temperatures are consistent with the expected coexistence of LMAFM and HO, the data in the intermediate temperature phase are not, and instead suggest the possibility of an unexpected coexistence of HO and PM. Additionally, the dynamics in the PM phase reflect the presence of a hybridization gap as well as strongly interacting spin and charge degrees of freedom. OPOP yields insights into meV-scale electrodynamics with sub-Kelvin temperature resolution, providing a complementary approach to study low-energy electronic structure in quantum materials.