Direct observation of the hybridization gap in both the hidden order and large moment antiferromagnetic phases in URu2Si2

Despite extensive research on the heavy fermion superconductor $\mathrm{U}{\mathrm{Ru}}_{2}{\mathrm{Si}}_{2}$ in the past three decades, the nature of the hidden order (HO) phase transition occurring at 17.5 K remains ambiguous. Here we report a comparative scanning tunneling microscopy/spectroscopy (STM/STS) study on different terminations of the parent $\mathrm{U}{\mathrm{Ru}}_{2}{\mathrm{Si}}_{2}$ and Fe-doped samples. A small gap, which was ascribed to the HO parameter by previous STM/STS studies, emerges in both the HO and large moment antiferromagnetic phases on the U terminations, indicating it is not the unique hallmark of the HO parameter. Moreover, a peak-gap-peak structure is observed on the Si terminations. Variations of the two spectral features with Fe concentration and temperature show that they stem from the alteration of $f\text{\ensuremath{-}}c$ hybridization. The higher vanishing temperatures and larger sizes of the gap in the Fe-substituted samples indicate stronger $f\text{\ensuremath{-}}c$ hybridization strength compared to $\mathrm{U}{\mathrm{Ru}}_{2}{\mathrm{Si}}_{2}$. Our studies demonstrate hybridization is not the driving force for the HO phase transition.