Quasiparticle Relaxation Dynamics in URu$_{2-x}$Fe$_{x}$Si$_{2}$ Single Crystals

We investigate quasiparticle relaxation dynamics in URu$_{2-x}$Fe$_{x}$Si$_{2}$ single crystals using ultrafast optical-pump optical-probe (OPOP) spectroscopy as a function of temperature ($T$) 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 $T$, 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 $T$ paramagnetic (PM) phase. In contrast, for $x$ = 0.1, two transitions occur over a narrow $T$ range (from ~15.5 - 17.5 K). A PM to HO transition occurs at an intermediate $T$ followed by a transition to the LMAFM phase at lower $T$. While the data at low $T$ are consistent with the expected coexistence of LMAFM and HO, the data in the intermediate $T$ 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 $T$ resolution, providing a complementary approach to study low energy electronic structure in quantum materials.