The ab-plane optical spectra of one underdoped and one nearly optimally doped single crystal of HgBa2Ca2Cu3O 8+δ were investigated in the frequency range from 40 to 40,000 cm −1 . The frequency dependent scattering rate was obtained by Kramers Kronig analysis of the reflectance. Both crystals have a scattering rate gap of about 1000 cm −1 which is much larger than the 700 cm −1 gap seen in optical studies of several cuprates with maximum Tc around 93 K. There appears to be a universal scaling between scattering rate gap and maximum Tc for the cuprate superconductors.PACS numbers: 74.25. Gz, 74.72.Gr, 78.20.Ci Although high temperature superconductivity in the copper oxides was discovered over a decade ago, an understanding of the mechanism that gives rise to the high transition temperature, T c , is still elusive. Most of the research has been directed towards the one-and twolayer systems which have a maximum T c in the 93 K range. Recently, however, high quality single crystals of the three-layer Hg based materials with a maximum T c ≈ 135 K have become available [1] making it possible to examine, spectroscopically, oxide superconductors that have significantly higher T c .Of particular interest is the magnitude of the normal state pseudogap. This is a partial suppression of the density of low energy excitations seen well above T c in all underdoped high temperature superconductors. A pseudogap has been seen with a variety of techniques such as angle-resolved photoemission (ARPES), tunneling spectroscopy, specific heat, dc resistivity, nuclear magnetic resonance and optical spectroscopy [2]. Measurements of dc resistivity [3] and NMR 63 Cu T 1 [4] on the threelayer underdoped HgBa 2 Ca 2 Cu 3 O 8+δ (Hg-1223) show evidence of a pseudogap with onset temperatures, T * , of 320 K and 230 K respectively.The size of the pseudogap in the one-and two-layer materials is of the order of 9.5k B T c at optimal doping and essentially independent of temperature. As T c is reduced below optimal doping the pseudogap increases in size [5][6][7].In ab-plane infrared response, the pseudogap is most clearly seen in the frequency-dependent scattering rate which can be calculated from the reflectance after Kramers-Kronig analysis. Here the pseudogap is a suppression of scattering below a characteristic energy which is taken to be a measure of the "size" of the pseudogap.

The interplane optical spectrum of the organic superconductor -(BEDT-TTF) 2 Cu͓N(CN) 2 ͔Br was investigated in the frequency range from 40 to 40 000 cm Ϫ1 . The optical conductivity was obtained by Kramers-Kronig analysis of the reflectance. The absence of a Drude peak at low frequency is consistent with incoherent conductivity but in apparent contradiction to the metallic temperature dependence of the dc resistivity. We set an upper limit to the interplane transfer integral of t b 2 /t ac Ϸ10 Ϫ7 eV. A model of defect-assisted interplane transport can account for this discrepancy. We also assign the phonon lines in the conductivity to the asymmetric modes of the BEDT-TTF molecule.

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...

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The nonlinear susceptibilities of the random-exchange Pd5 Co50 Al45 and Pd8 Co50 Al42 alloys are derived from dc magnetic measurements. They are investigated in the critical region and spin-glass exponents characterizing these materials are obtained. A progressive development of a canonical spin-glass phase is traced from Pd5 Co50 Al45 to Pd8 Co50 Al42 . The conventional spin-glass phase in Pd8 Co50 Al42 is characterized with the exponents = 3.25 and = 6.4 and a phase transition temperature Tg = 25.3 K.

We present optical conductivity data for Lu(Ni1−xCox)2B2C over a wide range of frequencies and temperatures for x=0 and x=0.09. Both materials show evidence of being good Drude metals with the infrared data in reasonable agreement with dc resistivity measurements at low frequencies.An absorption threshold is seen at approximately 700 cm −1 . In the cobalt-doped material we see a superconducting gap in the conductivity spectrum with an absorption onset at 24 ± 2 cm −1 = 3.9±0.4kB Tc suggestive of weak to moderately strong coupling. The pure material is in the clean limit and no gap can be seen. We discuss the data in terms of the electron-phonon interaction and find that it can be fit below 600 cm −1 with a plasma frequency of 3.3 eV and an electron-phonon coupling constant λtr = 0.33 using an α 2 F (ω) spectrum fit to the resistivity.

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