Paramagnetic reentrant effect in high purity mesoscopic AgNb proximity structures

We discuss the magnetic response of clean Ag coated Nb proximity cylinders in the temperature range 150 \mu K < T < 9 K. In the mesoscopic temperature regime, the normal metal-superconductor system shows the yet unexplained paramagnetic reentrant effect, discovered some years ago [P. Visani, A. C. Mota, and A. Pollini, Phys. Rev. Lett. 65, 1514 (1990)], superimposing on full Meissner screening. The logarithmic slope of the reentrant paramagnetic susceptibility chi_para(T) \propto \exp(-L/\xi_N) is limited by the condition \xi_N=n L, with \xi_N=\hbar v_F/2 \pi k_B T, the thermal coherence length and n=1,2,4. In wires with perimeters L=72 \mu m and L=130 \mu m, we observe integer multiples n=1,2,4. At the lowest temperatures, \chi_para compensates the diamagnetic susceptibility of the \textit{whole} AgNb structure.Comment: 4 pages, 4 figures (color

Rough Surface Effect on Meissner Diamagnetism in Normal-layer of N-S Proximity-Contact System

Rough surface effect on the Meissner diamagnetic current in the normal layer of proximity contact N-S bi-layer is investigated in the clean limit. The diamagnetic current and the screening length are calculated by use of quasi-classical Green's function. We show that the surface roughness has a sizable effect, even when a normal layer width is large compared with the coherence length $\xi =v_{\rm F}/\pi T_{\rm c}$. The effect is as large as that of the impurity scattering and also as that of the finite reflection at the N-S interface.Comment: 12 pages, 3 figures. To be published in J. Phys. Soc. Jpn. Vol.71-

Atomic Scale Modelling of Two-Dimensional Molecular Self-Assembly on a Passivated Si Surface

International audienceThe self-assembly of two-dimensional (2D) molecular structures on a solid surface relies on the subtle balance between non covalent intermolecular and molecule-surface forces. The energetics of 2D molecular lattices forming different patterns on a passivated semiconductor surface are here investigated by a combination of atomistic simulation methods. Density-functional theory provides structure and charges of the molecules, while metadynamics with empirical forces provides a best guess for the lowest-energy adsorption sites of single molecules and dimers. Subsequently, molecular dynamics simulations of extended molecular assemblies with empirical forces yield the most favorable lattice structures at finite temperature and pressure.The theoretical results are in good agreement with scanning tunneling microscopy observations of self-assembled molecular monolayers on a B-doped Si(111) surface, thus allowing to rationalize the competition of long-range dispersion forces between the molecules and the surface. Such a result demonstrates the interest of this predictive approach for further progress in supramolecular chemistry on semiconductor surface

Studies in molecular structure, symmetry and conformation I

Crystals of 1-aminocyclooctanecarboxylic acid hydrobromide are orthorhombic, with a = 26·026, b =7·087, c = 6·149, Z = 4 and space group P 2 1 2 1 2 1 .The structure was solved in projections by direct methods and later refined with three-dimensional data using a full-matrix least-squares treatment. All hydrogen atoms were located from a difference Fourier and the final R factor for the 1128 observed reflections was 8·62 %. The molecules are held together by a series of hydrogen bonds in a three-dimensional network. A detailed discussion of the intramolecular and the intermolecular features of the structure is presented. The cyclooctane ring is found to exist in the boat-chair conformation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44829/1/10870_2005_Article_BF01198532.pd

On the nature of the reentrant effect in susceptibility of mesoscopic cylindrical samples

Theory of the reentrant effect in susceptibility of mesoscopic cylindrical NS samples is proposed, which is essentially based on the properties of the Andreev levels. The specific feature of the quantum levels of the structure is that in a varying magnetic field (or temperature) each level periodically comes into coincidence with the chemical potential of the metal. As a result, the state of the system becomes strongly degenerate and the amplitude of the paramagnetic contribution to the susceptibility increases sharply.Comment: 6 pages. "Low Temperature Physics" to be publishe

Coherent quantum phenomena in a normal cylindrical conductor with a superconducting coating

The thermodynamic properties of a mesoscopic-size, simply connected cylindrical normal metal in good metallic contact with superconducting banks are studied theoretically. It is commonly accepted that if the superconductor thickness is quite small (of the order of the coherence length), as is assumed to be the case here, a vector potential field, whose value can be varied, exists inside the normal layer. It is further assumed that the quasiparticles with energy E<Δ (2Δ is the superconducting gap) move ballistically through the normal metal and undergo Andreev scattering caused by the off-diagonal potential of the superconductor. An equation is obtained within the multidimensional quasiclassical method which permits us to determine the spectrum of the Andreev levels and to calculate the density of states of the system in question. It is shown that the Andreev levels shift as the trapped flux Φ changes inside the normal conductor. At a certain flux value they coincide with the Fermi level. A resonance spike in the density of states ν(E) appears in this case, since near E=0 there is strong degeneracy of the quasiparticle states in respect to the quantum number q characterizing their motion along the cylinder axis. As a result, a macroscopic number of q states contribute to the amplitude of the effect. As the flux is increased, the density of states v(E) behaves as a stepwise function of Φ. The distance between the steps is equal to the superconducting flux quantum hc/2e