G. Deutscher

Surface melting enhanced by curvature effects

Abstract Recent unambiguous experiments (melting of ultrafine particles, premelting at the surface of a bulk crystal, superheating, etc.) offer clear evidence of the key role of the surface in determining the melting of a material. In this work we concentrate our attention on spherical and non-spherical nanometric lead inclusions. We report experimental results on Pb/SiO and Pb/Al 2 O 3 systems obtained at different temperatures by two techniques: high-sensitivity optical reflectance and dark-field electron microscopy. The main result is the existence, below the melting temperature and at the surface of the inclusion, of a liquid layer whose thickness is much larger than that observed on the bulk (zero curvature). This thickness, which depends on local curvature, increases continuously with temperature until a uniform curvature of the solid core is attained; then the core melts suddenly. A phenomenological model, based on the minimization of the free energy, is proposed and reported in detail. It represents a significant improvement compared to previous theoretical approaches related to well-known thermodynamic size-effect models, particularly insofar as the agreement with the experimental results is concerned.

Transmittance and reflectance in situ measurements of semicontinuous gold films during deposition

We have continuously measured in situ reflectance and transmittance of percolating gold films over the entire range of surface coverage P, in the IR regime (1.7 and 2.2 μm). The samples present similar optical and electrical behavior, when compared on a normalized thickness scale. In the fractal region, close to the percolation threshold, the optical properties show a linear dependence on the surface coverage parameter, in agreement with a renormalization argument previously suggested. A strong absorption (about 40%), whose origin is not well understood, is found in the fractal region. The interpretations previously proposed by other authors are not applicable. We conclude that geometrical effects must dominate the optical properties over a large range of surface coverage.

Andreev-Saint-James reflections : A probe of cuprate superconductors

Electrical transport through a normal-metal/superconductor contact at biases smaller than the energy gap can occur via the reflection of an electron as a hole of opposite wave vector. The same mechanism of electron-hole reflection gives rise to low-energy states at the surface of unconventional superconductors having nodes in their order parameter. The occurrence of electron-hole reflections at normal-metal/superconductor interfaces was predicted independently by Saint-James and de Gennes and by Andreev, and their spectroscopic features were discussed in detail by Saint-James in the early sixties. They are generally called Andreev reflections but, in view of the literature, will here be referred to as Andreev\char21{}Saint-James (ASJ) reflections. This review presents a historical review of ASJ reflections and spectroscopy in conventional superconductors, and reviews their application to the high-

Polar Kerr-effect measurements of the high-temperature YBa2Cu3O6+x superconductor: evidence for broken symmetry near the pseudogap temperature.

{T}_{c}

Coupling superconducting-ferromagnetic point contacts by Andreev reflections

cuprates. The occurrence of ASJ reflections in all studied cuprates is well documented for a broad range of doping levels, implying that there is no large asymmetry between electrons and holes near the Fermi level in the superconducting state. In the underdoped regime, where the pseudogap phenomenon has been observed by other methods such as nuclear magnetic resonance (NMR), angular-resolved photoemission spectroscopy (ARPES), and Giaever tunneling, gap values obtained from ASJ spectroscopy are smaller than pseudogap values, indicating a lack of coherence in the pseudogap energy range. Low-energy surface bound states have been observed in all studied hole-doped cuprates, in agreement with a dominant

Solid-liquid transition in ultra-fine lead particles

d

Transition to zero dimensionality in granular aluminum superconducting films

-wave symmetry order parameter. Results are mixed for electron-doped cuprates. In overdoped

Self similarity and correlations in percolation

\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}\phantom{\rule{0.3em}{0ex}}(\ensuremath{\delta}l0.08)

Superconducting glass and related properties

and

Transmittance of thin metal films near the percolation threshold

{\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}