Far-IR excitations in Cd2Re2O7 in the normal and superconducting states.

Cd(2)Re(2)O(7) is a pyrochlore superconductor with a transition temperature T(c) near 1 K. We report on the far-infrared optical properties of Cd(2)Re(2)O(7) at temperatures above and below T(c) with a particular emphasis on changes in the spectrum below T(c). Seventeen phonon modes are observed in the normal state optical conductivity spectrum of Cd(2)Re(2)O(7) at low temperatures in good agreement with a factor group analysis. In the superconducting state, a softening (~1 cm(-1)) of the phonon modes at 35 and 61 cm(-1) occurs and thermal reflectance spectra show the development of two additional strong absorption features, near 9.6 and 19.3 cm(-1). The dominant presence of lattice vibrational modes in the optical spectrum suggests that electron-phonon interaction plays an important role in the normal and superconducting state properties of Cd(2)Re(2)O(7).

Kramers-Kronig-constrained variational dielectric fitting and the reflectance of a thin film on a substrate.

A test was made of the ability of Kramers-Kronig-constrained variational dielectric fitting to extract the optical conductivity of a thin film from reflectance data containing structure due to both thin film and substrate. The reflectance of a series of well-characterized thin films of SrRu(x)Mg(1-x)O(3) and SrRu(x)O(3) with a variety of thicknesses (approximately 56-300 nm) and dc resistivities (approximately 250-2200 micro Omega cm) was measured. The low frequency values of the extracted optical conductivities agree with the dc measurements, however, removal of features due to the substrate improves with increasing film thickness.

Superconductivity in heavy alkaline-Earth intercalated graphites.

We report the discovery of superconductivity below 1.65(6) K in Sr-intercalated graphite SrC(6), by susceptibility and specific heat (C(p)) measurements. In comparison with CaC(6), we found that the anisotropy of the upper critical fields for SrC(6) is much reduced. The C(p) anomaly at T(c) is smaller than the BCS prediction, indicating an anisotropic superconducting gap for SrC6 similar to CaC6. The significantly lower T(c) of SrC(6) as compared to CaC(6) can be understood in terms of "negative" pressure effects, which decreases the electron-phonon coupling for both in-plane intercalant and the out-of-plane C phonon modes. We observed no superconductivity for BaC(6) down to 0.3 K.

Formation and thickness evolution of periodic twin domains in manganite films grown on SrTiO3(001) substrates.

We present an extended synchrotron x-ray scattering study of the structure of thin manganite films grown on SrTiO3(001) substrates and reveal a new kind of misfit strain relaxation process which exploits twinning to adjust lattice mismatch. We show that this relaxation mechanism emerges in thin films as one-dimensional twinning waves which freeze out into a twin domain pattern as the manganite film continues to grow. A quantitative microscopic model which uses a matrix formalism is able to reproduce all x-ray features and provides a detailed insight into this novel relaxation mechanism. We further demonstrate how this twin angle pattern affects the transport properties in these functional films.

Specific heat of the Ca-intercalated graphite superconductor CaC6.

The superconducting state of Ca-intercalated graphite CaC6 has been investigated by specific heat measurements. The characteristic anomaly at the superconducting transition (Tc = 11.4 K) indicates clearly the bulk nature of the superconductivity. The temperature and magnetic field dependence of the electronic specific heat are consistent with a fully gapped superconducting order parameter. The estimated electron-phonon coupling constant is lambda = 0.70 +/- 0.04, suggesting that the relatively high Tc of CaC6 can be explained within the intermediate coupling BCS approach.