Complete Wetting of Pits and Grooves

For one-component volatile fluids governed by dispersion forces an effective interface Hamiltonian, derived from a microscopic density functional theory, is used to study complete wetting of geometrically structured substrates. Also the long range of substrate potentials is explicitly taken into account. Four types of geometrical patterns are considered: (i) one-dimensional periodic arrays of rectangular or parabolic grooves and (ii) two-dimensional lattices of cylindrical or parabolic pits. We present numerical evidence that at the centers of the cavity regions the thicknesses of the adsorbed films obey precisely the same geometrical covariance relation, which has been recently reported for complete cone and wedge filling. However, this covariance does not hold for the laterally averaged wetting film thicknesses. For sufficiently deep cavities with vertical walls and close to liquid-gas phase coexistence in the bulk, the film thicknesses exhibit an effective planar scaling regime, which as function of undersaturation is characterized by a power law with the common critical exponent -1/3 as for a flat substrate, but with the amplitude depending on the geometrical features.Comment: 12 page

Recent developments in the characterization of superconducting films by microwaves

We describe and analyze selected surface impedance data recently obtained by different groups on cuprate, ruthenate and diboride superconducting films on metallic and dielectric substrates for fundamental studies and microwave applications. The discussion includes a first review of microwave data on MgB2, the weak-link behaviour of RABiTS-type YBa2Cu3O7-d tapes, and the observation of a strong anomalous power-dependence of the microwave losses in MgO at low temperatures. We demonstrate how microwave measurements can be used to investigate electronic, magnetic, and dielectric dissipation and relaxation in the films and substrates. The impact of such studies reaches from the extraction of microscopic information to the engineering of materials and further on to applications in power systems and communication technology.Comment: Invited contribution to EUCAS2001, accepted for publication in Physica C in its present for

Shapes, contact angles, and line tensions of droplets on cylinders

Using an interface displacement model we calculate the shapes of nanometer-size liquid droplets on homogeneous cylindrical surfaces. We determine effective contact angles and line tensions, the latter defined as excess free energies per unit length associated with the two contact lines at the ends of the droplet. The dependences of these quantities on the cylinder radius and on the volume of the droplets are analyzed.Comment: 26 pages, RevTeX, 10 Figure

Monte Carlo Methods for Estimating Interfacial Free Energies and Line Tensions

Excess contributions to the free energy due to interfaces occur for many problems encountered in the statistical physics of condensed matter when coexistence between different phases is possible (e.g. wetting phenomena, nucleation, crystal growth, etc.). This article reviews two methods to estimate both interfacial free energies and line tensions by Monte Carlo simulations of simple models, (e.g. the Ising model, a symmetrical binary Lennard-Jones fluid exhibiting a miscibility gap, and a simple Lennard-Jones fluid). One method is based on thermodynamic integration. This method is useful to study flat and inclined interfaces for Ising lattices, allowing also the estimation of line tensions of three-phase contact lines, when the interfaces meet walls (where "surface fields" may act). A generalization to off-lattice systems is described as well. The second method is based on the sampling of the order parameter distribution of the system throughout the two-phase coexistence region of the model. Both the interface free energies of flat interfaces and of (spherical or cylindrical) droplets (or bubbles) can be estimated, including also systems with walls, where sphere-cap shaped wall-attached droplets occur. The curvature-dependence of the interfacial free energy is discussed, and estimates for the line tensions are compared to results from the thermodynamic integration method. Basic limitations of all these methods are critically discussed, and an outlook on other approaches is given

Analyse eines DC-Sputterprozesses zur Deposition epitaktischer YBa_2Cu_3O_7_-_#delta# Filme auf dielektrischen und metallischen Substraten

The planar DC-sputtering as a preparation process for epitaxial YBCO films on monocrystalline and textured substrates is investigated. For a reproducible deposition of YBCO films with a thickness d up to 1 #mu#m the substrate temperature has to be kept constant. Films deposited in this way on MgO and LaAlO_3 show an intrinsic decrease of J_c(0.85 T_c) with 1/#sq root#(d). In measurements on diameter 2 inch films T_c values of 89.5 K and J_c(77 K) values of 4.4 MA/cm"2 were obtained in average on an area of diameter = 35 mm. Maps of the J_c distribution indicate a decrease in J_c at the edge of the substrate caused by the shape of the plasma. The values of the surface resistance R_s(4.2 K) and R_s(77 K) measured at 19 GHz are in the range of 0.2 and 1.2 m#OMEGA# and remain constant up to surface magnetic fields of 30 and 5 mT respectively. XRD-and TEM-analysis indicate also a good crystalline structure. A double-sided deposition with the high-pressure DC-sputtering is only possible step-by-step. The superconducting properties of the a- and b-side are nearly the same as for one-sided films. However, the side deposited first is often slightly degraded after the rear side deposition showing a worse crystalline quality and contaminations. The use of a radiation heater also allows a successful HTS deposition on metallic substrates. For this the control of the growth temperature is essential. YBCO films grow epitaxially on cube-textured, CeO_2-buffered Ni tapes, but their superconducting properties are still limited because of grain boundaries and cracks within the buffer and the HTS layer. Values of the critical current densities in the range of 0.3 MA/cm"2 at 63 K and 0.1 MA/cm"2 at 77 K were achieved up to now. (orig.)Available from TIB Hannover: RO 9740(2001,1) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

High temperature superconducting elliptic filters for UMTS Cryogenic Receiver

High Temperature Superconducting (HTS) planar filters exhibit sharp skirts and very low insertion loss. Base station receivers with cooled low noise amplifiers and HTS filters can provide improved reduction of interference, better utilization of the spectrum and a higher signal to noise ratio. In this paper HTS filters for a UMTS cryogenic receiver are presented. The filters are based on an elliptic design and fabricated using YBCO coated sapphire substrates. The measured noise figure of a nine pole UMTS filter with a low noise amplifier doe not exceed 0.7dB