Josephson Effect in Pb/I/NbSe2 Scanning Tunneling Microscope Junctions

We have developed a method for the reproducible fabrication of superconducting scanning tunneling microscope (STM) tips. We use these tips to form superconductor/insulator/superconductor tunnel junctions with the STM tip as one of the electrodes. We show that such junctions exhibit fluctuation dominated Josephson effects, and describe how the Josephson product IcRn can be inferred from the junctions' tunneling characteristics in this regime. This is first demonstrated for tunneling into Pb films, and then applied in studies of single crystals of NbSe2. We find that in NbSe2, IcRn is lower than expected, which could be attributed to the interplay between superconductivity and the coexisting charge density wave in this material.Comment: 3 pages, 2 figures. Presented at the New3SC-4 meeting, San Diego, Jan. 16-21 200

Reactions of Oxygen Atoms with Van der Waals Complexes: The Effect of Complex Formation on the Internal Energy Distribution in the Products

Reactions of atomic oxygen with complexes containing HCl are investigated and the OH product state distributions are compared to those observed for the corresponding reactions of HCl monomers. In previous studies of reactions of O(3P) with HCl and hydrocarbon complexes, rotationally colder OH product state distributions were observed, when compared to the corresponding reactions of monomers. In contrast, we find that reactions of O(1D) with HCl clusters yield OH rotational distributions that are unaffected by the incorporation of HCl into a van der Waals complex. Quasiclassical trajectories are run on collisions of oxygen with HCl and Ar⋯HCl at 1 eV collision energies to investigate the differences in the dynamics of the O(1D) and O(3P) reactions. It is found that when the van der Waals complex is longer lived than the collision complex, rotational and vibrational cooling are observed. In contrast, when the dissociation of the van der Waals complex is prompt, compared to the collision complex lifetime, the effects of complex formation on the internal energy of the OH product become negligible

Universal transport in 2D granular superconductors

The transport properties of quench condensed granular superconductors are presented and analyzed. These systems exhibit transitions from insulating to superconducting behavior as a function of inter-grain spacing. Superconductivity is characterized by broad transitions in which the resistance drops exponentially with reducing temperature. The slope of the log R versus T curves turns out to be universaly dependent on the normal state film resistance for all measured granular systems. It does not depend on the material, critical temperature, geometry, or experimental set-up. We discuss possible physical scenarios to explain these findings.Comment: 4 pages, 3 figure

The association of serotype and pulsed-field gel electrophoresis genotype in isolates of Streptococcus pneumoniae isolated in Israel

SummaryThe relationship between Streptococcus pneumoniae isolates causing invasive infections in children admitted to a single center in central Israel was examined by pulsed-field gel electrophoresis (PFGE) and serotyping. Although there was a close correlation between serotype and PFGE clone, the genetic diversity varied by serotype, with some genotypes comprising multiple serotypes. Additionally, clones C and D were associated with higher penicillin minimum inhibitory concentrations. Serotyping alone may be insufficient for epidemiological mapping of pneumococcal isolates in the era of pneumococcal conjugate polysaccharide vaccines

Quantum Fluctuations in the Chirped Pendulum

An anharmonic oscillator when driven with a fast, frequency chirped voltage pulse can oscillate with either small or large amplitude depending on whether the drive voltage is below or above a critical value-a well studied classical phenomenon known as autoresonance. Using a 6 GHz superconducting resonator embedded with a Josephson tunnel junction, we have studied for the first time the role of noise in this non-equilibrium system and find that the width of the threshold for capture into autoresonance decreases as the square root of T, and saturates below 150 mK due to zero point motion of the oscillator. This unique scaling results from the non-equilibrium excitation where fluctuations, both quantum and classical, only determine the initial oscillator motion and not its subsequent dynamics. We have investigated this paradigm in an electrical circuit but our findings are applicable to all out of equilibrium nonlinear oscillators.Comment: 5 pages, 4 figure

Fluctuation Dominated Josephson Tunneling with a Scanning Tunneling Microscope

We demonstrate Josephson tunneling in vacuum tunnel junctions formed between a superconducting scanning tunneling microscope tip and a Pb film, for junction resistances in the range 50-300 k$\Omega$. We show that the superconducting phase dynamics is dominated by thermal fluctuations, and that the Josephson current appears as a peak centered at small finite voltages. In the presence of microwave fields (f=15.0 GHz) the peak decreases in magnitude and shifts to higher voltages with increasing rf power, in agreement with theory.Comment: 4 pages, REVTeX, submitted to PR

Proximity effect and strong coupling superconductivity in nanostructures built with an STM

We present high resolution tunneling spectroscopy data at very low temperatures on superconducting nanostructures of lead built with an STM. By applying magnetic fields, superconductivity is restricted to length scales of the order of the coherence length. We measure the tunneling conductance and analyze the phonon structure and the low energy DOS. We demonstrate the influence of the geometry of the system on the magnetic field dependence of the tunneling density of states, which is gapless in a large range of fields. The behavior of the features in the tunneling conductance associated to phonon modes are explained within current models.Comment: 4 figures, 4 page