Observation of multiple-gap structure in hidden order state of URu2Si2from optical conductivity

Abstract

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.

Optical study of hybridization and hidden order in URu2Si2

“…As the temperature is lowered the single flat background that dominates at 300 K develops a Drude peak at low frequency and a broad gap-like depression centred at 10 meV. The c-axis conductivity is rather higher than that of the a-axis, in keeping with transport measurements [11], but is similar in overall features to what is shown in figure 2. In the heavy Fermion literature this gap-like depression has been identified as the hybridization gap that is expected to develop as the localized f electrons hybridize with the mobile s and p electrons [19].…”

“…7 from Hall et al [11]is strongly reminiscent of a spin density wave state for light polarized parallel to both the a-axis and the c-axis. It was initially noted that the characteristic structure of a suppression of conductivity below the gap energy with spectral weight recovery in a symmetric peak at higher frequency bore a striking resemblance to the spin density wave state in Cr [4,36].…”

“…A transition temperature of 17.5 K implies a gap energy of 2∆ = 5.3 meV. Recent measurements of the optical conductivity of URu 2 Si 2 [11] reveal the anisotropy of the hidden order parameter in both the different magnitudes of the optical gaps when measured with light polarized along either the a-or the c-axis, but also in the appearance of a second gap, presumably on a different part of the Fermi surface. The strong suppression of the conductivity indicates that a substantial portion of the Fermi surface is being gapped.…”

“…It is possible to fit the measured optical conductivity [11] with a simple Dynes [38] model for an s-wave gap in the density of states:…”

“…The spectral weight lost in the gap was transferred to frequencies just above the gap, typical of a density wave transition. Subsequent optical work has centred around investigations of the effects of doping on the hidden order [5,6], of the effect of hybridization on the electrodynamics [7][8][9][10] and the anisotropy of the hidden order parameter [11]. The superconducting state occurs below the temperatures available in typical optical experiments; likewise, the large moment antiferromagnet state (LM-AFM) induced by hydrostatic pressure has not yet been investigated optically.…”