Recent years have shown steady progress towards molecular electronics, in which molecules form basic components such as switches, diodes and electronic mixers. Often, a scanning tunnelling microscope is used to address an individual molecule, although this arrangement does not provide long-term stability. Therefore, metal-molecule-metal links using break-junction devices have also been explored; however, it is difficult to establish unambiguously that a single molecule forms the contact. Here we show that a single hydrogen molecule can form a stable bridge between platinum electrodes. In contrast to results for organic molecules, the bridge has a nearly perfect conductance of one quantum unit, carried by a single channel. The hydrogen bridge represents a simple test system in which to understand fundamental transport properties of single-molecule devices.

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We have studied 2H-NbSe2 by scanning tunneling spectroscopy with two different orientations, along the c and the a/b axis. The results can be understood in the framework of a two-gap model: along the c-axis, the large gap is dominant in the tunneling spectra, while a smaller gap is measured along the a/b axis. Our measurement thus shows unambiguously the existence of two gaps, where the orientation preferentially selects one gap or the other. Similarly as for MgB2, the tunneling spectra are well described by the McMillan equations for a two-band superconductor, showing that interband coupling originates from quasiparticle scattering from one band to the other. The electronic structure of 2H-NbSe2 is further studied theoretically by means of DFT calculations. Examining the different contributions to the Fermi level DOS, we conclude that the large gap observed in tunneling originates from states associated with the Fermi surface cylinders around K, whereas the small gap originates from the cylinders around Γ. In addition, we show that the tunneling current at large distance from the surface is dominated by the selenium orbitals. This finding suggests that the third component of the Fermi surface, the Se-based pancake around Γ, is strongly coupled to the cylinders around K, possibly due to the charge density wave state.

We present scanning tunneling microscopy and spectroscopy of the newly discovered superconductor CaC6. The tunneling conductance spectra, measured between 3 and 15 K, show a clear superconducting gap in the quasiparticle density of states. The gap function extracted from the spectra is in good agreement with the conventional BCS theory with Delta0=1.6+/-0.2 meV. The possibility of gap anisotropy and two-gap superconductivity is also discussed. In a magnetic field, direct imaging of the vortices allows us to deduce a coherence length in the ab plane xiab approximately 33 nm.

The conductance of a single-atom contact is sensitive to the coupling of this contact atom to the atoms in the leads. Notably for the transition metals this gives rise to a considerable spread in the observed conductance values. The mean conductance value and spread can be obtained from the first peak in conductance histograms recorded from a large set of contact-breaking cycles. In contrast to the monovalent metals, this mean value for Pt depends strongly on the applied voltage bias and other experimental conditions and values ranging from about 1 G0 to 2.5 G0 (G0 = 2e 2 /h) have been reported. We find that at low bias the first peak in the conductance histogram is centered around 1.5 G0. However, as the bias increases past 300 mV the peak shifts to 1.8 G0. Here we show that this bias dependence is due to a geometric effect where monatomic chains are replaced by single-atom contacts, where the former are destabilized by the electron current at high bias.

-The mutual interaction between Cooper pairs is proposed as a mechanism for the superconducting state. Above Tc, pre-existing but fluctuating Cooper pairs give rise to the unconventional pseudogap (PG) state, well-characterized by experiment. At the critical temperature, the pair-pair interaction induces a Bose-like condensation of these preformed pairs leading to the superconducting (SC) state. Below Tc, both the condensation energy and the pair-pair interaction β are proportional to the condensate density Noc(T ), whereas the usual Fermi-level spectral gap ∆p is independent of temperature. The new order parameter β(T ), can be followed as a function of temperature, carrier concentration and disorder -i.e. the phase diagrams. The complexity of the cuprates, revealed by the large number of parameters, is a consequence of the coupling of quasiparticles to Cooper-pair excitations. The latter interpretation is strongly supported by the observed quasiparticle spectral function. I. Introduction. -As Occam's razor would suggest, second order phase transitions often depend on few parameters [1]. Such is the case for the familiar magnetic, spin glass, charge-density wave, structural transitions, etc., and the more exotic Kosterlitz-Thouless case for two-dimensional systems [2]. The superconducting phase of 'classical' materials follows this trend wherein a weak attractive electron-electron interaction is responsible for the transition. Essentially a single energy scale, the SC gap parameter ∆ 0 at zero temperature, is relevant [3]. In the conventional theory of Bardeen, Cooper, Schrieffer (BCS) [3] pair-breaking quasiparticle excitations restore the normal-metal state at the transition, such that ∆ 0 = 1.76 k B T c . Moreover, ∆ 0 fixes the scale of the critical currents, for example in a Josephson junction, the upper critical field and the coherence length, such as the vortex core radius.

We address the question of probing the supercurrents in superconducting (SC) samples on a local scale by performing scanning tunneling spectroscopy (STS) experiments with a SC tip. In this configuration, we show that the tunneling conductance is highly sensitive to the Doppler shift term in the SC quasiparticle (QP) spectrum of the sample, thus allowing the local study of the superfluid velocity. Intrinsic screening currents, such as those surrounding the vortex cores in a type II SC in a magnetic field, are directly probed. With Nb tips, the STS mapping of the vortices, in single crystal 2H-NbSe(2), reveals both the vortex cores, on the scale of the SC coherence length xi, and the supercurrents, on the scale of the London penetration length lambda. A subtle interplay between the SC pair potential and the supercurrents at the vortex edge is observed. Our results open interesting prospects for the study of screening currents in any superconductor.

The signature of phase coherence on the electric and magnetic response of 10 5 nonconnected Aharonov-Bohm rings is measured by a resonant method at 350 MHz between 20 mK and 500 mK. The rings are etched in a GaAs-Al x Ga 1Ϫx As heterojunction. Both quantities exhibit an oscillating behavior with a periodicity consistent with half a flux quantum ⌽ 0 /2ϭh/2e in a ring. We find that electric screening is enhanced when time-reversal symmetry is broken by magnetic field, leading to a positive magnetopolarizability, in agreement with theoretical predictions for isolated rings at finite frequency. Temperature and electronic-density dependences are investigated. The dissipative part of the electric response, the electric absorption, is also measured and leads to a negative magnetoconductance. The magnetic orbital response of the very same rings is also investigated. It is consistent with diamagnetic persistent currents of 0.25 nA. This magnetic response is an order of magnitude smaller than the electric one, in qualitative agreement with theoretical expectations.