Room-temperature Coulomb blockade of charge transport through composite nanostructures containing organic inter-links has recently been observed. A pronounced charging effect in combination with the softness of the molecular links implies that charge transfer gives rise to a significant deformation of these structures. For a simple model system containing one nanoscale metallic cluster connected by molecular links to two bulk metallic electrodes we show that self-excitation of periodic cluster oscillations in conjunction with sequential processes of cluster charging and decharging appears for a sufficiently large bias voltage. This new `electron shuttle' mechanism of discrete charge transfer gives rise to a current through the nanostructure, which is proportional to the cluster vibration frequency.Comment: 4 pages, 4 figure
We show that quantum spin fluctuations in inhomogeneous ferromagnets drastically affect the Andreev reflection of electrons and holes at a ferromagnet-superconductor interface. As a result a strong long-range proximity effect appears, associated with electron-hole spin triplet correlations and persisting on a lenght scale typical for non-magnetic materials, but anomalously large for ferromagnets.In recent years much attention has been paid to normal conductor-superconductor (N/S) structures in which transport properties of the normal conductor are much modified in the vicinity of the superconductor (for a review, see paper [1]). The origin of this "proximity effect" has to do with correlations of normal-metal electrons and holes caused by Andreev reflection at the inteface with the superconductor. An important feature of such Andreev scattering, which converts electrons into holes and vice versa, is the selection rule that requires the energies of the electron and hole (as measured from the Fermi energy) -as well as their spin projections -to be equal in magnitude but opposite in sign. While the spin selection rule is irrelevant for the proximity effect in nonmagnetic normal materials, the energy selection results in a destructive interference between the electron and hole states corresponding to a decay of the electron-hole correlations at distances of order L T = hD/kT from the N/S interface (D is the diffusion constant of the normal conductor, T is temperature).In ferromagnetic materials electrons and holes acquire an additional exchange energy, which is sensitive to the direction of the spin. Hence the spin selection rule for Andreev reflection becomes relevant. As a result the proximity effect decays on the much smaller length scale L I0 = hD/I 0 (I 0 is the exchange energy). Typically, I 0 exceeds k B T by several orders of magnitude, resulting in a drastic reduction of the proximity effect in magnetic materials as indeed observed in a number of measurements Such a shortening was indeed observed in a number of measurements (see Refs. [2-5] and references therein).Recently, new experiments [6-8] revealed a large excess conductance of the F/S boundary, which was interpreted in terms of a long-range proximity effect in the ferromagnet. It was pointed out [7] that spin triplet fluctuations in the electron-hole correlations caused by the spin-orbit interaction and electron-impurity scattering [9] can not (by two orders of magnitude) explain the large effect observed in Refs. [6][7][8].The main message of our paper is that in magnetically inhomogeneous materials (such as multi-domain ferromagnets, inhomogeneous "cryptoferromagnetic" states imposed by the superconductor [10], F/S interfaces inducing electronic spin-flip processes [11] etc.), strong quantum fluctuations of the electron and hole spins make the proximity effect less sensitive to the spin selection rule that applies to Andreev reflections. As a result, a strong long-range, spin-triplet proximity effect in F/S structures persists on a length scale typ...
We study the Coulomb blockade in a superconducting grain, connected to two normal electrodes by tunnel junctions. At small bias, the conductance of this system is due to electrons passing in pairs through the grain. The linear conductance is periodic in the gate voltage. The period and the conductance activation energy are determined by the charge 2e, rather than e. At resonance the current first grows linearly with the applied bias and then drops as the quasiparticle transport channel opens up.
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