Analogy between the "Hidden Order" and the Orbital Antiferromagnetism in URu$_{2-x}$Fe$_x$Si$_2$

Abstract

We study URu$_{2-x}$Fe$_x$Si$_2$, in which two types of staggered phases compete at low temperature as the iron concentration $x$ is varied: the nonmagnetic "hidden order" (HO) phase below the critical concentration $x_c$, and unconventional antiferromagnetic (AF) phase above $x_c$. By using polarization resolved Raman spectroscopy, we detect a collective mode of pseudovector-like $A_{2g}$ symmetry whose energy continuously evolves with increasing $x$; it monotonically decreases in the HO phase until it vanishes at $x=x_c$, and then reappears with increasing energy in the AF phase. The mode's evolution provides direct evidence for unified order parameter for both nonmagnetic and magnetic phases arising from the orbital degrees-of-freedom of the uranium-5$f$ electrons.