We outline a procedure for obtaining the electron-phonon spectral density by inversion of optical conductivity data, a process very similar in spirit to the McMillan-Rowell inversion of tunelling data. We assume both electron-impurity (elastic) and electron-phonon (inelastic) scattering processes. This procedure has the advantage that it can be utilized in the normal state. Furthermore, a very good qualitative result can be obtained explicitly, without iteration. We illustrate this technique on recently acquired far-infrared data in K3C60. We show that the electron-phonon interaction is most likely responsible for superconductivity in these materials.

We present results of the first optical and angle-resolved photoemission study on a layered ruthenium oxide system with various Ca͞Sr substitution levels. Using two-plane ͑Sr 12x Ca x ͒ 3 Ru 2 O 7 and one-plane Ca 2 RuO 4 crystals we were able to study evolution of the electronic properties in a range from a band metal ͑Sr 3 Ru 2 O 7 ͒ to a "bad" metal ͑Ca 3 Ru 2 O 7 ͒ to a Mott-Hubbard insulator ͑Ca 2 RuO 4 ͒. Apart from a Mott-Hubbard metal-insulator transition (MIT), we have uncovered a qualitative change of the electronic properties at a critical x 0.33. We suggest that the latter is a result of a quantum phase transition into an antiferromagnetic phase that precedes the real Mott-Hubbard MIT. [S0031-9007(98)07196-8]

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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.

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The specific heat of the single crystals of UPd 2 Si 2 and UNi 2 Si 2 has been studied using the step heating method for the temperature range between 2 and 250 K. Successive phase transitions at high temperatures were observed for both samples, which are consistent with previously reported magnetization and neutron scattering results. No evidence of any phase transition was observed at temperatures below 70 K, which has been previously reported. The Debye temperature and the electronic specific heat coefficient ␥ of UPd 2 Si 2 are found to be 117 K and 19 mJ/mol K 2 , respectively. In low temperatures between 2 and 4 K, an upturn in specific heat with T Ϫ2 dependence was observed. Several possibilities for this upturn in the specific heat are discussed.