A comparison of the hidden order transition in URu$_2$Si$_2$ to the $\lambda$-transition in $^4$He

TL;DR

This paper compares the hidden order transition in URu$_2$Si$_2$ to the lambda transition in superfluid helium, proposing that both involve a change from ungapped to gapped elementary excitations, explaining various experimental observations.

Contribution

It introduces a unified framework linking the hidden order in URu$_2$Si$_2$ to superfluid helium's transition through elementary excitations, providing new insights into the phase transition mechanisms.

Findings

The hidden order transition involves a change to gapped elementary excitations.

The behavior of the HO phase resembles a gas of weakly interacting excitations.

This model explains entropy release, resistivity jumps, and Fermi surface reduction.

Abstract

The low-temperature states of ambient URu$_2$Si$_2$ and superfluid $^4$He are both characterized by momentum-dependent energy gaps between the ground and excited states. This behavior weakly persists even above the transition temperatures but becomes over-damped (ungapped) because of the number of excitations present at elevated temperature. We show that akin to the normal fluid to superfluid transition in $^4$He, the hidden-order (HO) transition in URu$_2$Si$_2$ can be understood by a change of the ungapped excitations to the gapped, elementary excitations (EE) of the unknown ordered state. These under-damped EEs reflect the basic character and order parameters of the different phase transitions. This view accounts for the full amount of entropy released in these transitions, the jumps in the resistivity and thermal conductivity directly below the transition, as well as the reduction of the Fermi surface. We argue that the behavior in the HO phase is that of a gas of weakly interacting excitations from charge density wave or crystal field states in a similar manner to that of the phonon-roton excitations of the superfluid $^4$He phase. We discuss the influence of applying pressure and magnetic fields within this scenario and the role of the small moment antiferromagnetic clustering in the hidden order phase.