Current–voltage characteristics of Nb–carbon–Nb junctions

We report on properties of Nb(/Ti)–carbon–(Ti/)Nb junctions fabricated on graphite flakes using e-beam lithography. The devices were characterized at temperatures above 1.8 K where a Josephson current was not observed, but the differential conductivity revealed features below the critical temperature of Nb, and overall metallic conductivity, in spite of a high-junctions resistance. Since the conductivity of graphite along the planes is essentially two-dimensional (2D), we use a theoretical model developed for metal/graphene junctions for interpretation of the results. The model involves two very different graphene “access” lengths. The shorter length characterizes ordinary tunneling between the three-dimensional Nb(/Ti) electrode and 2D graphene, while the second, much longer length, is associated with the Andreev reflections (AR) inside the junction and involves also “reflectionless” AR processes. The relevant transmission factors are small in the first case and much larger in the second, which explains the apparent contradiction of the observed behavior

De Haas-Van Alphen measurements of one-electron and many-body effects in transition metals and intermetallic compounds

Examples are given which demonstrate the power and versatility of the dHvA effect in studying electronic behavior in metals. In transition metals the parametrization schemes give a very complete and consistent picture of the k-dependent and surface averaged electronic properties. Because the one-electron behavior is fairly well known, the many body contribution to the Fermi velocity can be isolated and its detailed anisotropy can be displayed. This kind of information is directly relevant to the calculation of electron-phonon interaction effects and cannot be derived by any other means

Stability of π\pi junction configurations in ferromagnet-superconductor heterostructures

We investigate the stability of possible order parameter configurations in clean layered heterostructures of the $SFS...FS$ type, where $S$ is a superconductor and $F$ a ferromagnet. We find that for most reasonable values of the geometric parameters (layer thicknesses and number) and of the material parameters (such as magnetic polarization, wavevector mismatch, and oxide barrier strength) several solutions of the {\it self consistent} microscopic equations can coexist, which differ in the arrangement of the sequence of ``0'' and ``$\pi$'' junction types (that is, with either same or opposite sign of the pair potential in adjacent $S$ layers). The number of such coexisting self consistent solutions increases with the number of layers. Studying the relative stability of these configurations requires an accurate computation of the small difference in the condensation free energies of these inhomogeneous systems. We perform these calculations, starting with numerical self consistent solutions of the Bogoliubov-de Gennes equations. We present extensive results for the condensation free energies of the different possible configurations, obtained by using efficient and accurate numerical methods, and discuss their relative stabilities. Results for the experimentally measurable density of states are also given for different configurations and clear differences in the spectra are revealed. Comprehensive and systematic results as a function of the relevant parameters for systems consisting of three and seven layers (one or three junctions) are given, and the generalization to larger number of layers is discussed.Comment: 17 pages, including 14 Figures. Higher resolution figures available from the author

Micromagnetic simulations of absoption spectra

Further development of a previously introduced method for numerically simulating magnetic spin waves is presented. Together with significant improvements in speed, the method now allows one to calculate the energy absorbed by the various modes excited by a position- and time-dependent H1 field in a ferromagnetic body of arbitrary shape in the presence of a (uniform or non uniform) static H0 field as well as the internal exchange and anisotropy fields. The method is applied to the case of the single vortex state in a thin disc, a ring, and various square slabs, for which the absorption spectra are calculated and the most strongly excited resonance modes are identified

Critical and non-critical coherence lengths in a two-band superconductor

We study the peculiarities of coherency in a two-gap superconductor. The both intraband couplings, inducing superconductivity in the independent bands, and interband pair-transfer interaction have been taken into account. On the basis of the Ginzburg-Landau equations derived from the Bogoliubov-de Gennes equations and the relevant self-consistency conditions for a two-gap system, we find critical and non-critical coherence lengths in the spatial behaviour of the fluctuations of order parameters. The character of the temperature dependencies of these length scales is determined by the relative contributions from intra- and interband interaction channels.Comment: Accepted for publication in Journal of Superconductivity and Novel Magnetis

Structure and shear response of lipid monolayers

We have been studying lipid monolayers both on the surface of water and after transfer to solid substrates. On the surface of water, we have determined in detail the structure of heneicosanoic acid monolayers along a high-pressure isobar, and correlated this information with viscosity data. We have also determined the effect of ions in the subphase on acid monolayer structure (as a function of pH). We have studied capillary waves on monolayer-covered water surfaces, and we find that these monolayers can be quite anisotropic in some phases, so that the monolayer pressure' is to always a meaningful quantity. Using atomic force microscopy, we have seen islands' in Langmuir-Blodgett monolayers without the need for a dye, plus features that are to visible using fluorescence microscopy

Structures and shear response of lipid monolayers. Progress report, August 1, 1993--January 31, 1996

Of the many systems now classified as {open_quotes}soft condensed matter{close_quotes}, lipids are some of the best known and most studied. Lipids occur most commonly in membranes, but the artificially created lipid systems known as Langmuir films (on water) and Langmuir-Blodgett films (on solid substrates) are in some ways better-defined and more easily controlled systems with which to address many of the same questions. Studies of these systems have a long and distinguished history, but in the past decade there has been an explosion of activity in this area, driven by the availability of a or more powerful experimental probes but also in part by the hope of producing new structured molecular materials and devices. Today the focus of device-oriented research is shifting to self-assembled (chemisorbed) films, because it is recognized that these films are somewhat more stable under application conditions. This trend has resulted in a generally more appropriate view of Langmuir and Langmuir Blodgett films as model systems with which to study the properties of organized molecular assemblies. These films are part of a larger class that includes membranes, lamellar paraffins and liquid crystals as well as self-assembled films, but with certain experimental and conceptual advantages (such as the ease with which the density may be varied, and the tethering to a flat plane). This report describes the continued studies of the phase diagrams of Langmuir monolayers, and efforts to understand the variables that affect the structures formed. It also describes studies of the structure of a transferred monolayer, and how this evolves as further layers are added. Finally, the authors describe their studies of the mechanical response of Langmuir-Blodgett films using a small-strain torsion balance at the center of a circular trough

Detection of ultrasound using a tunneling microscope

We have developed a scanning tunneling microscope capable of detecting high frequency (1.8\u201312.5 MHz) acoustic waves. Acoustic pulses arriving at a conducting surface are detected as a high frequency perturbation of the tunneling current. Near and below the rolloff frequency of the current to voltage amplifier, the instantaneous displacement of the surface can be reconstructed. For higher frequencies, the nonlinear (rectifying) properties of the tunneling behavior allow the detection of the envelope (and hence the time of arrival and total amplitude) of the acoustic burst. The technique\u2019s sensitivity is comparable to that of optical detection schemes but offers much better lateral resolution.Peer reviewed: YesNRC publication: N

Sound propagation in normal and superfluid /sup 3/He

Liquid /sup 3/He, despite the rather simple nature of its interatomic force, displays striking properties at ultralow temperatures. Below about 2 x 10/sup -3/ K it behaves as a superfluid whose properties are related to, but are much more varied and complex than, those of superconducting electrons in metals. Ultrasonic waves prove to be a powerful probe of this system; here the acoustic properties of both the normal and superfluid phases of this liquid are briefly reviewed

Technique to produce coherent x-ray radiation via laser pumping of a relativistic ion beam

The level population of a beam of relativistic positive ions with Z greater than or equal to 2 having a single bound electron may be inverted by the application of a ''..pi.. pulse'' of laser radiation tuned to the Doppler shifted 1s-2p transition. When the laser beam and ion beam move in opposite directions the required laser frequency is reduced by a factor 2..gamma... Subsequently applied short wavelength resonant radiation moving in the same direction as the ion beam (with an inverted population) will be amplified via stimulated emission, the wavelength in the lab frame now being shorter than the original laser wavelength by a factor (2..gamma..)/sup 2/. 7 refs