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Description: A theoretical discussion is presented of the P-N junction theory and the junction transistor. A P-N juncion is where the two regions of a Ge crystal meet, one side of which is P-type, the other N-type. An NPN junction transistor consists of a single crystal, one end of which is N-type, a middle section which is P-type, and the composite end which is N-type. Silicon and Ge are the semiconductors commonly used. (W.L.H.)
Date: June 1, 1958
Creator: Leivo, W J
Partner: UNT Libraries Government Documents Department

Single Molecule Junctions: A Laboratory for Chemistry, Mechanics and Bond Rupture

Description: Simultaneous measurement [1] of junction conductance and sustained force in single molecule junctions bridging metal electrodes provides a powerful tool in the quantitative study of the character of molecule-metal bonds. In this talk I will discuss three topics. First, I will describe chemical trends in link bond strength based on experiments and Density Functional Theory based calculations. Second, I will focus on the specific case of pyridine-linked junctions. Bond rupture from the high conductance junction structure shows a requires a force that exceeds the rupture force of gold point contacts and clearly indicates the role of additional forces, beyond the specific N-Au donor acceptor bond. DFT-D2 calculations with empirical addition of dispersion interactions illustrates the interplay between the donor-acceptor bonding and the non-specific van der Waals interactions between the pyridine rings and Au asperities. Third, I will describe recent efforts to characterize the diversity of junction structures realized in break-junction experiments with suitable models for the potential surfaces that are observed. [1] Venkataraman Group, Columbia University.
Date: July 8, 2013
Creator: S., Hybertsen M.
Partner: UNT Libraries Government Documents Department

Single Molecule Junctions: Probing Contact Chemistry and Fundamental Circuit Laws

Description: By exploiting selective link chemistry, formation of single molecule junctions with reproducible conductance has become established. Systematic studies reveal the structure-conductance relationships for diverse molecules. I will draw on experiments from my collaborators at Columbia University, atomic-scale calculations and theory to describe progress in two areas. First, I will describe a novel route to form single molecule junctions, based on SnMe3 terminated molecules, in which gold directly bonds to carbon in the molecule backbone resulting in near ideal contact resistance [1]. Second, comparison of the conductance of junctions formed with molecular species containing either one backbone or two backbones in parallel allows demonstration of the role of quantum interference in the conductance superposition law at the molecular scale [2].
Date: April 11, 2013
Creator: S., Hybertsen M.
Partner: UNT Libraries Government Documents Department

Dynamics of current driven disordered Josephson junction arrays

Description: We present dynamical simulations of disordered Josephson junction arrays with a bias current. We study the IV characteristics and vortex dynamics as a function of integer frustration f = n, weak frustration f = n + {delta} ({delta} {much_lt} 1) and full frustration pairs close to the critical current i{sub c}. We focus on the study of the plastic flow of vortices (f = n + {delta}), of vortex-antivortex pairs (f = n), and the dynamics of domain walls (f = n + 1/2) close to i{sub c}. We analyze the voltage noise and vortex fluctuations in the plastic flow regimes. We obtain the phase diagram for the different dynamical regimes as a function of disorder and applied current. We also study the dynamical critical behavior of depinning close to i{sub c} in the gauge glass limit of the model, f {yields} {infinity}, calculating critical exponents for the voltage onset and voltage fluctuations. We discuss our results within the context of present theories of the non-linear dynamics of disordered media.
Date: April 1, 1995
Creator: Dominguez, D.; Gronbech-Jensen, N. & Bishop, A.R.
Partner: UNT Libraries Government Documents Department


Description: We report on the processing optimization and fabrication of ramp-edge high-temperature superconducting junctions by using alternative materials for both superconductor electrodes and normal-metal barrier. By using Ag-doped YBa{sub 2}Cu{sub 3}O{sub 7{minus}x} (Ag:YBCO) as electrodes and a cation-modified compound of (Pr{sub y}Gd{sub 0.6{minus}y})Ca{sub 0.4}Ba{sub 1.6}La{sub 0.4}Cu{sub 3}O{sub 7} (y = 0.4, 0.5, and 0.6) as a normal-metal barrier, high-temperature superconducting Josephson junctions have been fabricated in a ramp-edge superconductor/normal-metal/superconductor (SNS) configuration. By using Ag:YBCO as electrodes, we have found that the processing controllability /reproducibility and the stability of the SNS junctions are improved substantially. The junctions fabricated with these alternative materials show well-defined RSJ-like current vs voltage characteristics at liquid nitrogen temperature.
Date: June 1999
Creator: Jia, Q. & Fan, Y.
Partner: UNT Libraries Government Documents Department

Mode-locking and the transition to chaos in dissipative systems

Description: Dissipative systems with two competing frequencies exhibit transitions to chaos. We have investigated the transition through a study of discrete maps of the circle onto itself, and by constructing and analyzing return maps of differential equations representing some physical systems. The transition is caused by interaction and overlap of mode-locked resonances and takes place at a critical line where the map losses invertibility. At this line the mode-locked intervals trace up a complete Devil's Staircase whose complementary set is a Cantor set with universal fractal dimension D approx. 0.87. Below criticality there is room for quasiperiodic orbits, whose measure is given by an exponent ..beta.. approx. 0.34 which can be related to D through a scaling relation, just as for second order phase transitions. The Lebesgue measure serves as an order parameter for the transition to chaos. The resistively shunted Josephson junction, and charge density waves (CDWs) in rf electric fields are usually described by the differential equation of the damped driven pendulum. The 2d return map for this equation collapses to ld circle map at and below the transition to chaos. The theoretical results on universal behavior, derived here and elsewhere, can thus readily be checked experimentally by studying real physical systems. Recent experiments on Josephson junctions and CDWs indicating the predicted fractal scaling of mode-locking at criticality are reviewed.
Date: January 1, 1984
Creator: Bak, P.; Bohr, T. & Jensen, M.H.
Partner: UNT Libraries Government Documents Department

Macroscopic quantum effects in the zero voltage state of the current biased Josephson junction

Description: When a weak microwave current is applied to a current-biased Josephson tunnel junction in the thermal limit the escape rate from the zero voltage state is enhanced when the microwave frequency is near the plasma frequency of the junction. The resonance curve is markedly asymmetric because of the anharmonic properties of the potential well: this behavior is well explained by a computer simulation using a resistively shunted junction model. This phenomenon of resonant activation enables one to make in situ measurements of the capacitance and resistance shunting the junction, including contributions from the complex impedance presented by the current leads. For the relatively large area junctions studied in these experiments, the external capacitive loading was relatively unimportant, but the damping was entirely dominated by the external resistance.
Date: May 1, 1985
Creator: Clarke, J.; Devoret, M.H.; Martinis, J. & Esteve, D.
Partner: UNT Libraries Government Documents Department

Macroscopic quantum tunnelling in a current biased Josephson junction

Description: We discuss in this work an attempt to answer experimentally the question: do macroscopic variables obey quantum mechanics. More precisely, this experiment deals with the question of quantum-mechanical tunnelling of a macroscopic variable, a subject related to the famous Schrodinger's cat problem in the theory of measurement.
Date: November 1, 1984
Creator: Martinis, J.M.; Devoret, M.H.; Clarke, J. & Urbina, C.
Partner: UNT Libraries Government Documents Department

Macroscopic quantum tunneling in Josephson tunnel junctions and Coulomb blockade in single small tunnel junctions

Description: Experiments investigating the process of macroscopic quantum tunneling in a moderately-damped, resistively shunted, Josephson junction are described, followed by a discussion of experiments performed on very small capacitance normal-metal tunnel junctions. The experiments on the resistively-shunted Josephson junction were designed to investigate a quantum process, that of the tunneling of the Josephson phase variable under a potential barrier, in a system in which dissipation plays a major role in the dynamics of motion. All the parameters of the junction were measured using the classical phenomena of thermal activation and resonant activation. Theoretical predictions are compared with the experimental results, showing good agreement with no adjustable parameters; the tunneling rate in the moderately damped (Q {approx} 1) junction is seen to be reduced by a factor of 300 from that predicted for an undamped junction. The phase is seen to be a good quantum-mechanical variable. The experiments on small capacitance tunnel junctions extend the measurements on the larger-area Josephson junctions from the region in which the phase variable has a fairly well-defined value, i.e. its wavefunction has a narrow width, to the region where its value is almost completely unknown. The charge on the junction becomes well-defined and is predicted to quantize the current through the junction, giving rise to the Coulomb blockade at low bias. I present the first clear observation of the Coulomb blockade in single junctions. The electrical environment of the tunnel junction, however, strongly affects the behavior of the junction: higher resistance leads are observed to greatly sharpen the Coulomb blockade over that seen with lower resistance leads. I present theoretical descriptions of how the environment influences the junctions; comparisons with the experimental results are in reasonable agreement.
Date: April 1, 1991
Creator: Cleland, A.N.
Partner: UNT Libraries Government Documents Department

Flux lattice relaxation, noise and symmetry-breaking in frustrated Josephson junction arrays

Description: The purpose of the present report is to study: (1) Relaxation from an initially random flux state. We wish to exhibit the multiple length and time scales involved in this relaxation, and the influence of thermal fluctuations. This noisy relaxation is controlled by the dynamics of various mesoscopic defect'' structures defined with respect to the underlying ground-state flux structure. The defects, their mesoscopic collective patterns, and their dynamics control a complex macroscopic response, but are themselves microscopically controlled by the competitions producing the ground-state flux completely. Such multiscaleresponses have been observed in other competing interaction systems e.g., spin glasses, random field magnets, and weakly pinned charge-density waves, as well as noisy responses in high-temperature superconductors. Various creep'' and stretched-exponential'' regimes have been proposed, as well as phenomenological scaling theories attempting to relate spatial domain sizes with temporal scales. (2) Noisy (i.e.,multitime-scale) voltages responses, and their relation to multilength scales, in current driven JJA's in the presence of a uniform extended magnetic field which frustrates the flux order.
Date: January 1, 1993
Creator: Gronbech-Jensen, N.; Bishop, A.R. & Lomdahl, P.S.
Partner: UNT Libraries Government Documents Department

Vortex lattices in theory and practice

Description: The formal simplicity of ideal point vortex systems in two dimensions has long attracted interest in both their exact solutions and in their capacity to simulate physical processes. Attention here is focused on infinite, two-fold periodic vortex arrays, including an expression for the energy density of an arbitrary vortex lattice (i.e., an arbitrary number of vortices with arbitrary strengths in a unit cell parallelogram of arbitrary shape). For the case of two vortices per unit cell, the morphology of stable lattices can be described completely. A non-trivial physical realization of such lattices is a rotating mixture of /sup 3/He and /sup 4/He at temperatures so low that both isotopic components are superfluid. The structure of the expected lattices is quite different from the usual triangular structure. Magnetic flux lines in high-temperature superconductors show a one-parameter family of degenerate ground state of the lattice due to the anisotropy of the vortex--vortex interaction. A final topic, closely related to Josephson-junction arrays, is the case of vortices confined to a grid. That is, the vortices interact pair-wise in the usual manner but are constrained to occupy only locations on an independent periodic grid. By using vortex relaxation methods in the continuum and then imposing the grid it is possible to find low-lying states extremely rapidly compared to previous Monte Carlo calculations. 11 refs., 8 figs.
Date: January 1, 1988
Creator: Capmbell, Laurence J.
Partner: UNT Libraries Government Documents Department

Flux flow pinning and resistive behavior in superconducting networks

Description: We have studied the behavior of superconducting networks in terms of XY and Coulomb gas models. The dynamics of frustrated Josephson junction arrays has been simulated, with a view toward understanding the effects of vortex correlations on flux flow resistance. Randomness has been introduced, and its effects on the superconducting transition, and vortex mobility, have been studied. A three dimensional network has been simulated to study the effects of vortex line entanglement in high temperature superconductors. Preliminary calculations are in progress. The two dimensional classical Coulomb gas where charges map onto vortices in the superconducting network, has been simulated. The melting transitions of ordered charge (vortex) lattices have been studied, and we find clear evidence that these transitions do not have the critical behavior expected from standard symmetry analysis.
Date: October 1, 1990
Creator: Teitel, S.
Partner: UNT Libraries Government Documents Department

Quantum noise in Josephson junctions and squids

Description: A theory for noise in the resistively shunted Josephson junction in the quantum limit is outlined and extended to the dc SQUID. Experimental results for both the single junction and the SQUID are reported.
Date: April 1, 1981
Creator: Koch, R.H.; Van Harlingen, D.J. & Clarke, J.
Partner: UNT Libraries Government Documents Department

Microstructure of artificial 45{sup degree} [001] tilt grain boundaries in YBCO films grown on (001) MgO

Description: High-angle grain boundaries in YBa{sub 2}Cu{sub 3}O{sub 7-x} (YBCO) show weak-link effects and behave as Josephson junctions. This kind of grain boundary junction (GBJs) has potential applications in magnetic field measurement and electronic devices. This work studies the microstructure of artificially made GBJs in YBCO films on (001) MgO and the mechanism of boundary formation, with the goal to improve GBJ quality and obtain a better understanding of the junctions` transport properties.
Date: March 1996
Creator: Huang, Y.; Vuchic, B. V.; Merkle, K. L.; Buchholz, D. B. & Chang, R. P. H.
Partner: UNT Libraries Government Documents Department

Determination of the Built-in Electric Field near Contacts to Polycrystalline CuInSe{sub 2} - Probing Local Charge Transport Properties by Photomixing

Description: The built-in electric field in polycrystalline CuInSe{sub 2} (CIS) near gold co-planar contacts was quantitatively revealed for the first time by the photomixing technique. A He-Ne laser beam was focused locally on the CIS sample near one of its contact. While both dc dark and photo-currents showed ohmic behavior, the high frequency ac current was non-zero for zero applied dc bias, which reveals a built-in electric field of {approx}1000V/cm. The capability of the photomixing technique to probe local charge transport properties is expected to be very useful for, e.g., the quantitative evaluation of the quality of ohmic contacts and the investigation of electric field induced p-n junction formation in CIS and related materials.
Date: November 19, 1998
Creator: Tang, Y.; Dong, S.; Sun, G. S.; Braunstein, R. & von Roedern, B.
Partner: UNT Libraries Government Documents Department

Nonlinear response of superconductors to alternating fields and currents

Description: This report discusses the following topics on superconductivity: nonlinearities in hard superconductors such as surface impedance of a type II superconductimg half space and harmonic generation and intermodulation due to alternating transport currents; and nonlinearities in superconducting weak links such as harmonic generation by a long Josephson Junction in a superconducting slab.
Date: October 8, 1997
Creator: McDonald, J.
Partner: UNT Libraries Government Documents Department