Optical spectroscopy shows that the normal state of URu$_2$Si$_2$ is an anomalous Fermi liquid

TL;DR

This paper uses optical spectroscopy to reveal that the normal state of URu$_2$Si$_2$ deviates from conventional Fermi liquid behavior, indicating unique electron scattering processes and incoherent f-electron interactions.

Contribution

The study demonstrates that URu$_2$Si$_2$ exhibits an anomalous Fermi liquid with a non-universal scattering ratio, challenging the traditional understanding of electron interactions in heavy fermion systems.

Findings

The coefficient b in resistivity is approximately 1, not 4 as in conventional Fermi liquids.

Electrons experience a unique scattering process, suggesting incoherent f-electron behavior.

Fermi liquid-like states with b ≠ 4 are more common than previously recognized.

Abstract

Fermi showed that electrons, as a result of their quantum nature, form a gas of particles where the temperature and density follow the so called Fermi distribution. In a metal, as shown by Landau, that despite their strong Coulomb interaction with each other and the positive background ions, the electrons continue to act like free quantum mechanical particles but with enhanced masses. This state of matter, the Landau-Fermi liquid, is recognized experimentally by an electrical resistivity that is proportional to the square of the absolute temperature plus a term proportional to the square of the frequency of the applied field. Calculations show that, if electron-electron scattering dominates the resistivity in a Landau Fermi liquid, the ratio of the two terms, $b$ has the universal value of {\em b} = 4. We find that in the normal state of the heavy Fermion metal URu$_2$Si$_2$, instead of the Fermi liquid value of 4 the coefficient $b$ =1 $\pm$ 0.1. This unexpected result implies that the electrons in this material are experiencing a unique scattering process. This scattering is intrinsic and we suggest that, the uranium $f$ electrons do not hybridize to form a coherent Fermi liquid but instead act like a dense array of elastic impurities, interacting incoherently with the charge carriers. This behavior is not restricted to URu$_2$Si$_2$. Fermi liquid like states with $b \neq$ 4 have been observed in a number of disparate systems but the significance of this result has not been recognized.