Table of contents

Extended X-ray absorption fine-structure (EXAFS) measurements of evaporated and ion-implanted Fe-Co thin films have been obtained by using the conversion-electron detection technique. Experiments were performed on films 700 AA thick in order to get some structural information which may explain their high resistivity value and unusual behaviour after ion-irradiation. Compared with bulk Fe-Co, the electron-beam-evaporated sample yields a damping of the EXAFS signals. The authors show that this effect is correlated to the nanocrystalline character of the sample so obtained. A quantitative analysis provides an estimation of the grain boundary fraction. Electrical resistivity and EXAFS results suggest that implantations of Fe-Co nanocrystals with heavy ions locally smooth out the insulating effect of the grain boundary but increase the damage produced in the crystalline structure.

Angle resolved measurements of the de Haas-van Alphen effect are reported in the heavy-fermion compound CeCu₆. The experiments confirm the presence at low temperature of a sharply defined Fermi surface resulting from the existence of heavy long-lived charged fermion quasiparticles. LMTO electronic structure calculations have been performed within the local density approximation, in which the f electrons are included as either band or core states, but in neither case are the results in satisfactory agreement with experiment. The measured effective masses, which are in the range 6-80 m_e, are found to be magnetic field dependent in a manner which closely resembles that of the linear specific heat coefficient. The authors deduce that, in zero applied field, the mass renormalization in CeCu₆ is greater than 200.

The authors present precision measurements of the electrical resistivity rho on a cadmium single crystal normal to the c-axis below 9 K. The sample thickness d was reduced by chemical polishing from 1.1 to 0.04 mm. rho depends systematically on the quality of the surface (etching a polished surface results in an increased residual resistivity rho ₀ and a decreased temperature dependence of rho ). A minimum in the temperature dependent part, rho - rho ₀, is observed when the thickness is close to the size of the mean free path. A T²-variation of the surface resistivity is observed over a limited temperature range. While these features are in qualitative agreement with the Soffer model for specular scattering, the minimum is deeper and the T²-range more extended than predicted by this model. The results are compared with existing size effect data on copper and potassium.

The author studies the transport properties of disconnected Pb films from 77 down to 0.05 K, at zero and high applied magnetic fields. The conductivity between separated metal clusters is found to be dominated by voltage injection and not by thermal excitation of electrons at low temperatures. This explains temperature-independent resistance tails in both the superconductivity and the normal state of the films, observed at low temperatures in this and in other experiments. I-V characteristics were found to follow a power law I varies as Vⁿ, n>2, at low temperatures, consistent with a space-charge-limited current model for the conductivity.

The reflectivity spectra of Hg_1-xFe_xSe (0<or=x<or=0.15), Cd_1-xFe_xSe (0<or=x<or=0.12) and Cd_1-xFe_xTe (0<or=x<or=0.06) crystals have been investigated in the 1-12 eV energy range. The observed changes in the shapes of the spectra, caused by the increase in the Fe content, have been compared with the calculated band structure of binary compounds available in the literature. The band structure in some regions seems to be sensitive to the presence of Fe ions. Most of the structures of the spectra are broadened owing to structural, chemical and magnetic disorder occurring in the lattice due to the Fe contribution. The results obtained contribute to the model of the electronic structure of these compound proposed in the literature.

The different roles played by atomic size and electronic factors in stabilizing the transition metal Laves phases against the two competing phases C11_b(MoSi₂) and C16(CuAl₂) are studied within a simple tight binding d bond model. Good qualitative agreement with the experimental AB₂ structure map is found if size and electronic factors are both included.

Binding energies of surface polarons outside polar crystal surfaces are calculated quantum mechanically by considering the interactions of outside electrons with both electronic and ionic polarizations of the crystals. Results show that quantum mechanical modifications of image potentials due to electronic polarization reduce polaron binding energies by 40-50% compared with those calculated with image potential due to electronic polarization approximated by their electrostatic limits. Substitution of infinitely high frequencies for the finite vibrational frequencies of electronic polarizations changes the calculated polaron binding energies by less than 5% for the materials considered.

The effect of magnetic scattering on weak localization is investigated. The different contributions of elastic and spin-flip magnetic scattering and their field dependence are used to obtain ana expression for three-dimensional, weak localization magnetoresistance.

The AC susceptibility and magnetization of a pure single crystal of Dy have been measured in the temperature region from 2 K to 200 K. The AC susceptibility data show an abrupt drop near 6.5 K for all three crystal axes. The magnetization has been measured using a SQUID magnetometer with an applied field of 0.002-0.005 T. The basal plane magnetization data show a sharp step as the crystal is warmed past 4.3 K. The c-axis magnetization data show a corresponding anomaly. From the data it appears that a component of the Dy moment lifts out of the basal plane at temperatures below 4.3 K. The authors also observe an anomaly in the susceptibility data near 167 K.