Raman scattering study on the hidden order and antiferromagnetic phases in URu$_{2-x}$Fe$_x$Si$_2$*

The heavy fermion compound URu$_2$Si$_2$ possesses an unusual ground state known as the ``hidden order'' (HO) phase below $T=17.5$\,K, which evolves into an large moment antiferromagnetic (LMAFM) phase under pressure. A recent Raman scattering study shows that an $A_{2g}$ symmetry ($D_{4h}$) in-gap mode emerges in the HO phase, characterizing the excitation from a chirality density wave\footnote{H. Kung, R.E. Baumbach, E.D. Bauer, V.K. Thorsmolle, W. Zhang, K. Haule, J.A. Mydosh and G. Blumberg \textit{Science} \textbf{347}, 1339 (2015)}. Here, we report Raman scattering results for single crystal URu$_{2-x}$Fe$_x$Si$_2$ with $x\leq 0.2$, where the Fe substitution acts as chemical pressure, shifting the system's ground state from HO to LMAFM. We found that the $A_{2g}$ mode softens with doping, vanishes at the HO and LMAFM phase boundary, then re-emerges and hardens with doping in the LMAFM phase. The relations between the $A_{2g}$ mode energy and the strength of the HO/LMAFM order parameters will be discussed in this talk.