Optical spectroscopy shows that the normal state of URu
2
Si
2
is an anomalous Fermi liquid
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abstract
Fermi showed that, as a result of their quantum nature, electrons form a gas of particles whose temperature and density follow the so-called Fermi distribution. As shown by Landau, in a metal the electrons continue to act like free quantum mechanical particles with enhanced masses, despite their strong Coulomb interaction with each other and the positive background ions. 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
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 ± 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
≠ 4 have been observed in a number of disparate systems, but the significance of this result has not been recognized.
