Optical evidence for mass enhancement of quasiparticles in pyrochloreCd2Re2O

Abstract

We report on the results of optical studies of the newly discovered superconductor Cd2Re2O7 in the normal state. We show that the compound has an exotic metallic state at low temperature. The optical conductivity spectrum exhibits two distinct features: a sharp renormalized resonance mode at zero frequency and a broad mid-infrared excitation band. Detailed analysis reveals a moderate enhancement of the effective mass at low temperature and low frequency.PACS numbers: 71.27.+a, 74.25.Gz, 74.70.Tx The magnetically frustrated pyrochlore oxides, which have a general formula A 2 B 2 O 7 where B stands for a transition metal, have attracted considerable interest recently. The B cations are six-fold coordinated and located within distorted octahedra. Those octahedra are corner-sharing and form a three-dimensional network.[1] Many of the compounds undergo a metal to nonmetal transition without an associated structural change. Cd 2 Re 2 O 7 is one of a few exceptions, which displays solely metallic behavior below room temperature. This compound undergoes a second-order phase transition at around 200 K and enters a better metallic state in low temperature.[2] Very recently it was found that Cd 2 Re 2 O 7 becomes superconducting below 2 K. This makes this compound the first superconductor in the pyrochlore family. [3][4][5] It is of great interest to investigate the peculiar electronic state lying behind the superconductivity. For this purpose we have investigated the optical properties of the compound at different temperatures in the normal state. An exotic metallic phase with strongly correlated electrons was revealed in the study.The single crystals of Cd 2 Re 2 O 7 were grown using a vapor-transport method described in detail in ref [6]. They were well characterized by x-ray diffraction, electron diffraction, resistivity, specific heat and magnetic susceptibility measurements, showing superconductivity below T c ∼ 1.5 K. [2,5,6] The crystal structure is facecentered cubic. Near normal incidence reflectivity spectra, R(ω), were measured from 30 to 40000 cm −1 on a natural growth surface normal to the a-axis. Standard Kramers-Kronig transformations were employed to derive the frequency-dependent conductivity and dielectric function.Cd 2 Re 2 O 7 exhibits an unusual temperature-dependent dc resistivity in the normal-state: it is almost Tindependent at high temperature but decreases rapidly below 200 K. The behavior, displayed in the inset of Fig. 1, is inconsistent with the traditional electron-phonon scattering mechanism, which should yield a linear Tdependent behavior at high temperature. Below 50 K, the dc resistivity follows an approximately quadratic Tdependence, implying electron-electron scattering and a Fermi-liquid-like state at low temperature. The reflectivity data at 300 K, 150 K and 24 K are shown in Fig. 1. We note immediately that the reflectivity in the farinfrared spectral range increases with decreasing temperature, characteristic of metallic response. However, the reflectivity in the mid-infrared r...

Order-disorder-order phase transitions in the pyrochlore superconductorCd2Re2O7

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