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

TL;DR

This study investigates the evolution of collective modes in URu$_{2-x}$Fe$_x$Si$_2$, revealing a unified order parameter underlying both hidden order and antiferromagnetic phases through Raman spectroscopy.

Contribution

It demonstrates the continuous evolution of a collective mode across phases, suggesting a common orbital origin for hidden order and antiferromagnetism in URu$_{2-x}$Fe$_x$Si$_2$.

Findings

The collective mode energy decreases and vanishes at the phase boundary.

The mode reappears with increasing energy in the antiferromagnetic phase.

Evidence for a unified orbital-based order parameter in both phases.

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.