Low temperature thermal expansion behavior in the Kondo system La1−x Ce x Al2

The thermal expansion coefficient for La_1–x Ce _x Al₂ has been measured from 4 to 50 K and compared to recent calculations based on a valence fluctuation model. The experimental results were found to be inconsistent with the predictions assuming valence fluctuations, even though the predictions of this model for the specific heat and magnetic susceptibility are in striking agreement with the measured quantities. The thermal expansion coefficient was not determined with sufficient precision to allow a quantitative comparison with prediction assuming resonant scattering-Kondo behavior.Work Supported by National Science Foundation Grant No. DMR-76-16433.     

d-Hole localization and the suppression of superconductivity in YBa2(Cu1-xZnx)3O7?y

The temperature and Zn concentration dependence of the electrical resistivity, specific heat, magnetic susceptibility, and electron paramagnetic resonance (EPR) spectra of YBa₂(Cu_1–x Zn _x )₃O_7–y withy0.1 has been measured forx0.16. In addition, the temperature and field dependence of the magnetization has been measured for 2<T<300K and 0<H<9.0T, along with the temperature and quasihydrostatic pressure dependence of the electrical resistivity for selected samples for 0<P<13 GPa. The substitution of Zn for Cu in YBa₂Cu₃O_7–y causes a rapid and nearly linear depression of the superconducting transition temperature,T _c , withT _c going to 0 K forx 0.10. YBa₂(Cu_1–x Zn _x )₃O_7–y retains the YBa₂Cu₃O_7-y orthorhombic structure forx0.16 for both the superconducting and nonsuperconducting samples. Initially, the unit cell volume increases nearly linearly with Zn content; however, an abrupt change occurs in the vicinityx=0.8–0.10. Forx<0.10, the temperature dependence of the electrical resistivity,(T), is metallic-like (d/dT>0) and increases gradually with increasing Zn content. However, forx 0.10,(T) becomes semiconductor-like, with a very rapid increase of the resistivity with increasingx. The electrical resistivity, magnetic susceptibility, EPR spectra, and specific heat all indicate that thed-holes associated with the Cu ions become localized in the nonsuperconducting phase,x>-0.10.