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Search Results: 1 - 10 of 425052 matches for " James M. Valles Jr "
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Early stage morphology of quench condensed Ag, Pb and Pb/Ag hybrid films
Zhenyi Long,James M. Valles Jr
Physics , 2004, DOI: 10.1007/s10909-005-4732-3
Abstract: Scanning Tunneling Microscopy (STM) has been used to study the morphology of Ag, Pb and Pb/Ag bilayer films fabricated by quench condensation of the elements onto cold (T=77K), inert and atomically flat Highly Oriented Pyrolytic Graphite (HOPG) substrates. All films are thinner than 10 nm and show a granular structure that is consistent with earlier studies of QC films. The average lateral diameter, $\bar {2r}$, of the Ag grains, however, depends on whether the Ag is deposited directly on HOPG ($\bar {2r}$ = 13 nm) or on a Pb film consisting of a single layer of Pb grains ($\bar {2r}$ = 26.8 nm). In addition, the critical thickness for electrical conduction ($d_{G}$) of Pb/Ag films on inert glass substrates is substantially larger than for pure Ag films. These results are evidence that the structure of the underlying substrate exerts an influence on the size of the grains in QC films. We propose a qualitative explanation for this previously unencountered phenomenon.
"Super weakly" coupled superconductivity in ultrathin superconductor-normal-metal bilayers
Zhenyi Long,M. D. Stewart Jr,James M. Valles Jr
Physics , 2005, DOI: 10.1103/PhysRevB.73.140507
Abstract: Tunneling measurements of the temperature dependence of the electronic density of states (DOS) of ultrathin bilayers of Pb and Ag reveal that their superconducting energy gap, $\Delta (T)$, evolves similarly to BCS predictions despite the presence of a large anomalous DOS at the Fermi energy that persists as $T\to 0$. The gap ratio, ${2\Delta (0)}\over {k_BT_c}$, systematically decreases more than 20% below the BCS universal value of 3.52 in bilayers with the lowest superconducting transition temperatures, $T_c$; a behavior we deem super weakly coupled. A semi-quantitative model suggests that the reduced gap ratios result from a systematic depletion of the DOS available for pairing that occurs with the growth of the anomalous DOS at the Fermi energy.
Ordered Nano-Crystal Arrays Spontaneously Form in Films Evaporated onto Nanopore Array Substrates
Niravun Pavenayotin,M. D. Stewart Jr.,James M. Valles Jr.,Aijun Yin,J. M. Xu
Physics , 2005,
Abstract: We present SEM images of films thermally evaporated onto Anodic Aluminum Oxide substrates that are patterned with a hexagonal array of 34 and 80 nm diameter holes spaced by 100 nm. Over a range of film thicknesses, Pb and Sn films spontaneously self assemble into an array of nano-crystals in registry with the underlying hole lattice. The development of the arrays with thickness indicates that surface energies drive coalescing grains to move over the holes. Materials that wet the substrate or whose grains do not coalesce at these substrate temperatures do not form arrays. We discuss some potential applications.
Magnetic Flux Periodic Response of Nano-perforated Ultrathin Superconducting Films
M. D. Stewart Jr.,Zhenyi Long,James M. Valles Jr.,Aijun Yin,J. M. Xu
Physics , 2005, DOI: 10.1103/PhysRevB.73.092509
Abstract: We have patterned a hexagonal array of nano-scale holes into a series of ultrathin, superconducting Bi/Sb films with transition temperatures 2.65 K $
Observation of a subgap density of states in superconductor-normal metal bilayers in the Cooper limit
Zhenyi Long,M. D. Stewart, Jr.,Taejoon Kouh,James M. Valles Jr
Physics , 2004, DOI: 10.1103/PhysRevLett.93.257001
Abstract: We present transport and tunneling measurements of Pb-Ag bilayers with thicknesses, $d_{Pb}$ and $d_{Ag}$, that are much less than the superconducting coherence length. The transition temperature, $T_c$, and energy gap, $\Delta$, in the tunneling Density of States (DOS) decrease exponentially with $d_{Ag}$ at fixed $d_{Pb}$. Simultaneously, a DOS that increases linearly from the Fermi energy grows and fills nearly 40% of the gap as $T_c$ is 1/10 of $T_c$ of bulk Pb. This behavior suggests that a growing fraction of quasiparticles decouple from the superconductor as $T_c$ goes to 0. The linear dependence is consistent with the quasiparticles becoming trapped on integrable trajectories in the metal layer.
Polymerization Force Driven Buckling of Microtubule Bundles Determines the Wavelength of Patterns Formed in Tubulin Solutions
Yongxing Guo,Yifeng Liu,Jay X. Tang,James M. Valles Jr
Physics , 2007, DOI: 10.1103/PhysRevLett.98.198103
Abstract: We present a model for the spontaneous formation of a striated pattern in polymerizing microtubule solutions. It describes the buckling of a single microtubule (MT) bundle within an elastic network formed by other similarly aligned and buckling bundles and unaligned MTs. Phase contrast and polarization microscopy studies of the temporal evolution of the pattern imply that the polymerization of MTs within the bundles creates the driving compressional force. Using the measured rate of buckling, the established MT force-velocity curve and the pattern wavelength, we obtain reasonable estimates for the MT bundle bending rigidity and the elastic constant of the network. The analysis implies that the bundles buckle as solid rods.
Cooper pair islanding model of insulating nanohoneycomb films
S. M. Hollen,J. M. Valles Jr
Physics , 2011, DOI: 10.1088/1742-6596/376/1/012002
Abstract: We first review evidence for the Cooper pair insulator (CPI) phase in amorphous nanohoneycomb (NHC) films. We then extend our analysis of superconducting islands induced by film thickness variations in NHC films to examine the evolution of island sizes through the magnetic field-driven SIT. Finally, using the islanding picture, we present a plausible model for the appearance and behavior of the CPI phase in amorphous NHC films.
Observation of Critical Amplitude Fluctuations near the Two Dimensional Superconductor-Insulator Transition
J. A. Chervenak,J. M. Valles, Jr
Physics , 1999, DOI: 10.1103/PhysRevB.59.11209
Abstract: We report results of transport measurements in the quantum critical regime of the disorder tuned, 2D superconductor-insulator transition (SIT) in homogeneously disordered films. We show that, as the superconducting transition temperature decreases, the transition width grows, appearing to diverge at the SIT. In addition, structure develops in the DC current-voltage characteristics of films closest to the SIT indicating that the 2D superconductivity is driven into a regime of extreme inhomogeneity. The data suggest a picture of the phase transition in which large amplitude fluctuations occur as the amplitude is suppressed to near zero by disorder.
Absence of a Zero Temperature Vortex Solid Phase in Strongly Disordered Superconducting Bi Films
J. A. Chervenak,J. M. Valles, Jr
Physics , 1999, DOI: 10.1103/PhysRevB.61.R9245
Abstract: We present low temperature measurements of the resistance in magnetic field of superconducting ultrathin amorphous Bi films with normal state sheet resistances, $R_N$, near the resistance quantum, $R_Q={\hbar\over {e^2}}$. For $R_NR_Q$ there is no vortex solid and, hence, no zero resistance state in magnetic field. We describe how disorder induced quantum and/or mesoscopic fluctuations can eliminate the vortex solid and also discuss implications for the magnetic-field-tuned superconductor-insulator transition.
Deviations from mean-field behavior in disordered nanoscale superconductor-normal-metal-superconductor arrays
Taejoon Kouh,J. M. Valles, Jr.
Physics , 2002, DOI: 10.1103/PhysRevB.67.140506
Abstract: We have fabricated quasi-two-dimensional arrays of nano-scale Pb grains coupled by an overlayer of Ag grains. Their temperature dependent resistive transitions follow predictions for an array of mesoscopic superconductor-normal-superconductor junctions. The decrease of their transition temperatures with Ag overlayer thickness systematically deviates from the Cooper limit theory of the proximity effect as the Pb grain size decreases. The deviations occur when the estimated number of Cooper pairs per grain is less than or equal to 1 and suggest the approach to a superconductor to metal transition.
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