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 LandauFermi 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.hidden order | resistance | infrared conductivity | resonant scattering A mong the heavy Fermion metals, URu 2 Si 2 is one of the most interesting: it displays, in succession, no fewer than four different behaviors. As is shown in Fig. 1, where the electrical resistivity is plotted as a function of temperature, at 300 K the material is a very bad metal in which the conduction electrons are incoherently scattered by localized uranium f electrons. Below T K ∼ 75 K, the resistivity drops and the material resembles a typical heavy Fermion metal (1-3). At T 0 = 17.5 K the "hiddenorder" phase transition gaps a substantial portion of the Fermi surface but the nature of the order parameter is not known. A number of exotic models for the ordered state have been proposed (4-7), but there is no definitive experimental evidence to support them. Finally, at 1.5 K URu 2 Si 2 becomes an unconventional superconductor. The electronic structure, as shown by both angle-resolved photoemission experiments (8) and bandstructure calculations (9) is complicated, with several bands crossing the Fermi surface. To investigate the nature of the hidden-order state we focus on the normal state just above the transition. This approach has been used in the high-temperature superconductors where the normal state shows evidence of discrete frequency magnetic excitations that appear to play the role that phonons play in normal superconductors (10). The early optical experiments of Bonn et al. (11) showed that URu 2 Si 2 at 20 K, above the hidden order transition, has an infrared spectrum consisting ...

We have measured the far infrared reflectance of the heavy fermion compound URu2Si2 through the phase transition at THO=17.5 K dubbed 'hidden order' with light polarized along both the aand c-axes of the tetragonal structure. The optical conductivity allows the formation of the hidden order gap to be investigated in detail. We find that both the conductivity and the gap structure are anisotropic, and that the c-axis conductivity shows evidence for a double gap structure, with ∆1,c = 2.7 meV and ∆2,c = 1.8 meV respectively at 4 K, while the gap seen in the a-axis conductivity has a value of ∆a = 3.2 meV at 4 K. The opening of the gaps does not follow the behaviour expected from mean field theory in the vicinity of the transition.I.

We present data on the optical conductivity of URu2−x(Fe,Os)xSi2. While the parent material URu2Si2 enters the enigmatic hidden order phase below 17.5 K, an antiferromagnetic phase is induced by the substitution of Fe or Os onto the Ru sites. We find that both the HO and the AFM phases exhibit an identical gap structure that is characterized by a loss of conductivity below the gap energy with spectral weight transferred to a narrow frequency region just above the gap, the typical optical signature of a density wave. The AFM phase is marked by strong increases in both transition temperature and the energy of the gap associated with the transition. In the normal phase just above the transition the optical scattering rate varies as ω 2 . We find that in both the HO and the AFM phases, our data are consistent with elastic resonant scattering of a Fermi liquid. This indicates that the appearance of a coherent state is a necessary condition for either ordered phase to emerge. Our measurements favor models in which the HO and the AFM phases are driven by the common physics of a nesting-induced density-wave-gap.

We present an analytic Bogoliubov description of a BEC of polar molecules trapped in a quasi-2D geometry and interacting via internal state-dependent dipole-dipole interactions. We derive the mean-field ground-state energy functional, and we derive analytic expressions for the dispersion relations, Bogoliubov amplitudes, and dynamic structure factors. This method can be applied to any homogeneous, two-component system with linear coupling, and direct, momentum-dependent interactions. The properties of the mean-field ground state, including polarization and stability, are investigated, and we identify three distinct instabilities: a density-wave rotonization that occurs when the gas is fully polarized, a spin-wave rotonization that occurs near zero polarization, and a mixed instability at intermediate fields. These instabilities are clarified by means of the real-space density-density correlation functions, which characterize the spontaneous fluctuations of the ground state, and the momentum-space structure factors, which characterize the response of the system to external perturbations. We find that the gas is susceptible to both density-wave and spin-wave response in the polarized limit but only a spin-wave response in the zero-polarization limit. These results are relevant for experiments with rigid rotor molecules such as RbCs, $\Lambda$-doublet molecules such as ThO that have an anomalously small zero-field splitting, and doublet-$\Sigma$ molecules such as SrF where two low-lying opposite-parity states can be tuned to zero splitting by an external magnetic field.Comment: 21 pages, 18 figures, comments welcom

We summarize existing optical data of URu 2 Si 2 to clarify the nature of the hidden order transition in this heavy fermion metal. Hybridization develops between 50 K and 17.5 K, and a coherent Drude peak emerges which mirrors the changes in the dc resistivity. The Drude weight indicates that there is little change in the effective mass of these carriers in this temperature range. In addition, there is a flat background conductivity that develops a partial hybridization gap at 10 meV as the temperature is lowered, shifting spectral weight to higher frequencies above 300 meV. Below 30 K the carriers become increasingly coherent and Fermi-liquid-like as the hidden order transition is approached. The hidden order state in URu 2 Si 2 is characterized by multiple anisotropic gaps. The gap parameter ∆a = 3.2 meV in the ab-plane. In the c-direction, there are two distinct gaps with magnitudes of ∆ c1 = 2.7 meV and ∆ c2 = 1.8 meV. These observations are in good agreement with other spectroscopic measurements. Overall, the spectrum can be fit by a Dynes-type density of states model to extract values of the hidden order gap. The transfer of spectral weight strongly resembles what one sees in density wave transitions.