Thermodynamic properties of thin films of superfluid 3He-A

The pairing correlations in superfluid He-3 are strongly modified by quasiparticle scattering off a surface or an interface. We present theoretical results and predictions for the order parameter, the quasiparticle excitation spectrum and the free energy for thin films of superfluid He-3. Both specular and diffuse scattering by a substrate are considered, while the free surface is assumed to be a perfectly reflecting specular boundary. The results are based on self-consistent calculations of the order parameter and quasiparticle excitation spectrum at zero pressure. We obtain new results for the phase diagram, free energy, entropy and specific heat of thin films of superfluid He-3.Comment: Replaced with an updated versio

Tunneling into Current-Carrying Surface States of High Tc_c Superconductors

Theoretical results for the ab-plane tunneling conductance in the d-wave model for high Tc superconductors are presented. The d-wave model predicts surface bound states below the maximum gap. A sub-dominant order parameter, stabilized by the surface, leads to a splitting of the zero-bias conductance peak (ZBCP) in zero external field and to spontaneous surface currents. In a magnetic field screening currents shift the quasiparticle bound state spectrum and lead to a voltage splitting of the ZBCP that is linear in H at low fields, and saturates at a pairbreaking critical field of order 3 Tesla. Comparisons with recent experimental results on Cu/YBCO junctions are presented.Comment: 4 pages in a RevTex (3.0) file plus 3 Figures in PostScript. To appear in Phys. Rev. Let

The Effect of Surfaces on the Tunneling Density of States of an Anisotropically Paired Superconductor

We present calculations of the tunneling density of states in an anisotropically paired superconductor for two different sample geometries: a semi-infinite system with a single specular wall, and a slab of finite thickness and infinite lateral extent. In both cases we are interested in the effects of surface pair breaking on the tunneling spectrum. We take the stable bulk phase to be of $d_{x^2-y^2}$ symmetry. Our calculations are performed within two different band structure environments: an isotropic cylindrical Fermi surface with a bulk order parameter of the form $\Delta\sim k_x^2-k_y^2$, and a nontrivial tight-binding Fermi surface with the order parameter structure coming from an anti-ferromagnetic spin-fluctuation model. In each case we find additional structures in the energy spectrum coming from the surface layer. These structures are sensitive to the orientation of the surface with respect to the crystal lattice, and have their origins in the detailed form of the momentum and spatial dependence of the order parameter. By means of tunneling spectroscopy, one can obtain information on both the anisotropy of the energy gap, |\Delta(\p)|, as well as on the phase of the order parameter, \Delta(\p) = |\Delta(\p)|e^{i\varphi(\p)}.Comment: 14 pages of revtex text with 11 compressed and encoded figures. To appear in J. Low Temp. Phys., December, 199

Subharmonic gap structure in d-wave superconductors

We present a self-consistent theory of current-voltage characteristics of d-wave/d-wave contacts at arbitrary transparency. In particular, we address the open problem of the observation of subharmonic gap structure (SGS) in cuprate junctions. Our analysis shows that: (i) the SGS is possible in d-wave superconductors, (ii) the existence of bound states within the gap results in an even-odd effect in the SGS, (iii) elastic scattering mechanisms, like impurities or surface roughness, may suppress the SGS, and (iv) in the presence of a magnetic field the Doppler shift of the Andreev bound states leads to a very peculiar splitting of the SGS, which is an unambiguous fingerprint of d-wave superconductivity.Comment: Revtex4, 4 pages, 5 figure

Characterization of an 8-cm Diameter Ion Source System

Results of tests characterizing an 8-cm diameter ion source are presented. The tests were conducted in three separate vacuum test facilities at the University of Alabama-Huntsville, Colorado State University, and L3 Communications' ETI division. Standard ion optics tests describing electron backstreaming and total-voltage-limited impingement current behavior as a function of beam current were used as guidelines for selecting operating conditions where more detailed ion beam measurements were performed. The ion beam was profiled using an in-vacuum actuating probe system to determine the total ion current density and the ion charge state distribution variation across the face of the ion source. Both current density and ExB probes were utilized. The ion current density data were used to obtain integrated beam current, beam flatness parameters, and general beam profile shapes. The ExB probe data were used to determine the ratio of doubly to singly charged ion current. The ion beam profile tests were performed at over six different operating points that spanned the expected operating range of the DAWN thrusters being developed at L3. The characterization tests described herein reveal that the 8-cm ion source is suitable for use in (a) validating plasma diagnostic equipment, (b) xenon ion sputtering and etching studies of spacecraft materials, (c) plasma physics research, and (d) the study of ion thruster optics at varying conditions

Thermodynamics of a d-wave Superconductor Near a Surface

We study the properties of an anisotropically paired superconductor in the presence of a specularly reflecting surface. The bulk stable phase of the superconducting order parameter is taken to have $d_{x^2-y^2}$ symmetry. Contributions by order parameter components of different symmetries vanish in the bulk, but may enter in the vicinity of a wall. We calculate the self-consistent order parameter and surface free energy within the quasiclassical formulation of superconductivity. We discuss, in particular, the dependence of these quantities on the degree of order parameter mixing and the surface to lattice orientation. Knowledge of the thermodynamically stable order parameter near a surface is a necessary precondition for calculating measurable surface properties which we present in a companion paper.Comment: 12 pages of revtex text with 12 compressed and encoded figures. To appear in J. Low Temp. Phys., December, 199

Unconventional Pairing in Heavy Fermion Metals

The Fermi-liquid theory of superconductivity is applicable to a broad range of systems that are candidates for unconventional pairing. Fundamental differences between unconventional and conventional anisotropic superconductors are illustrated by the unique effects that impurities have on the low-temperature transport properties of unconventional superconductors. For special classes of unconventional superconductors the low-temperature transport coefficients are {\it universal}, i.e. independent of the impurity concentration and scattering phase shift. The existence of a universal limit depends on the symmetry of the order parameter and is achieved at low temperatures $k_B T \ll \gamma \ll \Delta_0$, where $\gamma$ is the bandwidth of the impurity induced Andreev bound states. In the case of UPt$_3$ thermal conductivity measurements favor an $E_{1g}$ or $E_{2u}$ ground state. Measurements at ultra-low temperatures should distinguish different pairing states.Comment: 8 pages in a LaTex (3.0) file plus 5 Figures in PostScript. To appear in the Proceedings of the XXI International Conference on Low Temperature Physics held in Prague, 8-14 August 199

Quasiparticle Bound States and Low-Temperature Peaks of the Conductance of NIS Junctions in d-Wave Superconductors

Quasiparticle states bound to the boundary of anisotropically paired superconductors, their contributions to the density of states and to the conductance of NIS junctions are studied both analytically and numerically. For smooth surfaces and real order parameter we find some general results for the bound state energies. In particular, we show that under fairly general conditions quasiparticle states with nonzero energies exist for momentum directions within a narrow region around the surface normal. The energy dispersion of the bound states always has an extremum for the direction along the normal. Along with the zero-bias anomaly due to midgap states, we find, for quasi two-dimensional materials, additional low-temperature peaks in the conductance of NIS junctions for voltages determined by the extrema of the bound state energies. The influence of interface roughness on the conductance is investigated within the framework of Ovchinnikov's model. We show that nonzero-bias peaks at low temperatures may give information on the order parameter in the bulk, even though it is suppressed at the surface.Comment: 14 pages, PostScrip

D-Wave Superconductors near Surfaces and Interfaces: A Scattering Matrix Approach within the Quasiclassical Technique

A recently developed method [A. Shelankov and M. Ozana, Phys. Rev. B 61, 7077 (2000)] is applied to investigate d-wave superconductors in the vicinity of (rough) surfaces. While this method allows the incorporation of arbitrary interfaces into the quasiclassical technique, we discuss, as examples, diffusive surfaces and boundaries with small tilted mirrors (facets). The properties of the surface enter via the scattering matrix in the boundary condition for the quasiclassical Green's function. The diffusive surface is described by an ensemble of random scattering matrices. We find that the fluctuations of the density of states around the average are small; the zero bias conductance peak broadens with increasing disorder. The faceted surface is described in the model where the scattering matrix couples m in- and m out-trajectories (m>=2). No zero bias conductance peak is found for [100] surfaces; the relation to the model of Fogelstrom et al. [Phys. Rev. Lett. 79, 281 (1997)] is discussed.Comment: RevTeX, 19 pages, 18 figure

Induction of non-d-wave order-parameter components by currents in d-wave superconductors

It is shown, within the framework of the Ginzburg-Landau theory for a superconductor with d_{x^2-y^2} symmetry, that the passing of a supercurrent through the sample results, in general, in the induction of order-parameter components of distinct symmetry. The induction of s-wave and d_{xy(x^2-y^2)-wave components are considered in detail. It is shown that in both cases the order parameter remains gapless; however, the structure of the lines of nodes and the lobes of the order parameter are modified in distinct ways, and the magnitudes of these modifications differ in their dependence on the (a-b plane) current direction. The magnitude of the induced s-wave component is estimated using the results of the calculations of Ren et al. [Phys. Rev. Lett. 74, 3680 (1995)], which are based on a microscopic approach.Comment: 15 pages, includes 2 figures. To appear in Phys. Rev.