Gapped itinerant spin excitations account for missing entropy in the hidden order state of URu$_2$Si$_2$

TL;DR

This study reveals that gapped itinerant spin excitations in URu$_2$Si$_2$ account for the missing entropy in its hidden order phase, providing new insights into its complex electronic ground state.

Contribution

The paper presents neutron scattering evidence of itinerant-like spin excitations above the hidden order transition, linking these excitations to entropy changes in URu$_2$Si$_2$.

Findings

Itinerant spin excitations extend up to 10 meV from incommensurate wavevectors.

Gapping of excitations correlates with entropy release at the transition.

Explains the reduced electronic specific heat in the hidden order state.

Abstract

One of the primary goals of modern condensed matter physics is to elucidate the nature of the ground state in various electronic systems. Many correlated electron materials, such as high temperature superconductors, geometrically frustrated oxides, and low-dimensional magnets are still the objects of fruitful study because of the unique properties which arise due to poorly understood many-body effects. Heavy fermion metals - materials which have high effective electron masses due to these effects - represent a class of materials with exotic properties, such as unusual magnetism, unconventional superconductivity, and "hidden order" parameters. The heavy fermion superconductor URu2Si2 has held the attention of physicists for the last two decades due to the presence of a "hidden order" phase below 17.5 K. Neutron scattering measurements indicate that the ordered moment is 0.03 $\mu_{B}$, much too small to account for the large heat capacity anomaly at 17.5 K. We present recent neutron scattering experiments which unveil a new piece of this puzzle - the spin excitation spectrum above 17.5 K exhibits well-correlated, itinerant-like spin excitations up to at least 10 meV emanating from incommensurate wavevectors. The gapping of these excitations corresponds to a large entropy release and explains the reduction in the electronic specific heat through the transition.