R. W. Henn

Far-infrared ellipsometry using synchrotron radiation

We discuss ellipsometric measurements using synchrotron radiation and estimate the errors that are due to focusing light onto the sample. Our results lead to the conclusion that ellipsometry is a brightness experiment and can benefit considerably from the use of synchrotron radiation in the mid and far infrared. We report implementation of a synchrotron far-infrared ellipsometer at the U4IR beam line at the National Synchrotron Light Source, Brookhaven National Laboratory, covering the 50–700-cm-1 wave-number range. To illustrate the capability of our setup we have determined the complex dielectric function of the ionic insulator LiF and of the heavily doped semiconductor GaAs (n≈8×1018 cm-3). We also report direct measurements of the optical interplane conductivity of single crystalline La2CuO4+δ and a twinned YBa2Cu3O7 crystal. From these data, dynamic atomic effective charges have been determined for La2CuO4+δ.

Cu-Isotope effect on the spin-peierls transition temperature of CuGeO3

The Cu-isotope effect on the spin-Peierls transition temperature of single crystals of CuGeO3 has been determined from magnetic susceptibility measurements. We observe a downshift of 0.08(2) K and 0.05(2)K of the spin-Peierls transition temperature of65CuGeO3 as compared to the transition temperatures ofnatCuGeO3 and63CuGeO3, respectively. These results are discussed with respect to the Cu isotope mass dependence of the coupling constant λ entering theoretical considerations.

Specific heat, magnetization and C-isotope effect of Y2C2(Br,I)2 superconductors

The specific heat cp(T) and the magnetization M(B,T) of the layered superconductors Y2C2I2, Y213C2I2 (Tc≈10 K), Y2C2Br2 (Tc≈5 K), and of Y2C2Br0.5I1.5 (Tc≈11.6 K) were measured from 2–80 K in magnetic fields up to 10 T. All samples exhibit sharp phase transitions in zero field. The jump Δcp at Tc is drastically reduced in amplitude and broadens significantly with increasing field. The temperature dependence of Δcp(T) and of Bc(T) is analyzed and is consistent with strong electron–phonon coupling. The 13C/12C isotope effect of Y2C2X2 is found to be nearly zero. In contrast to this finding the 3-dimensional parent compound YC2 shows weak-coupling BCS behavior. The anisotropic thermodynamic parameters of the materials are evaluated and discussed.

Thermodynamic properties of the superconducting state in YC2

We characterise the superconducting state of the binary carbide YC2 by means of magnetisation and specific heat measurements. The thermodynamic properties reveal weak coupling superconductivity close to the type I limit (κGL ≈ 0.8). The specific heat data in the superconducting state, as well as the12C/13C-isotope shift are in excellent agreement with weak coupling BCS predictions.

Superconductivity at 10 K in the layered compounds Y2C2X2 (X=Br,I)

Resistivity, magnetisation, and specific heat measurements on the new layered superconductors Y2C2Br2 and Y2C2I2 have been performed to characterise their superconducting state. An analysis of the specific heat anomalies atTc in terms of the two-fluid (2fl) model and McMillans coupling parameter reveal electron phonon coupling in the moderately strong regime. Indication for strong coupling is also rendered from an analysis of the magnetisation data by means of the α-model.