Steplike electric conduction in a classical two-dimensional electron system through a narrow constriction in a microchannel

Using molecular dynamics simulation, we investigate transport properties of a classical two-dimensional electron system confined in a microchannel with a narrow constriction. As a function of the confinement strength of the constriction, the calculated conductance in the simulations exhibits steplike increases as reported in a recent experiment [D. G. Rees et al., Phys. Rev. Lett. 106, 026803 (2011)]. It is confirmed that the number of the steps corresponds to the number of stream lines of electrons through the constriction. We verify that density fluctuation plays a major role in smoothing the steps in the conductance.Comment: 11 pages, 9 figure

Superconductivity in Fullerides

Experimental studies of superconductivity properties of fullerides are briefly reviewed. Theoretical calculations of the electron-phonon coupling, in particular for the intramolecular phonons, are discussed extensively. The calculations are compared with coupling constants deduced from a number of different experimental techniques. It is discussed why the A_3 C_60 are not Mott-Hubbard insulators, in spite of the large Coulomb interaction. Estimates of the Coulomb pseudopotential $\mu^*$, describing the effect of the Coulomb repulsion on the superconductivity, as well as possible electronic mechanisms for the superconductivity are reviewed. The calculation of various properties within the Migdal-Eliashberg theory and attempts to go beyond this theory are described.Comment: 33 pages, latex2e, revtex using rmp style, 15 figures, submitted to Review of Modern Physics, more information at http://radix2.mpi-stuttgart.mpg.de/fullerene/fullerene.htm

Muon spin rotation and SQUID investigation of superconductivity in (NH₃)_xNaK₂C₆₀ (x~0.7)

The family of superconducting fullerides (NH_3)_xNaK_2C_60 shows an anomalous correlation between T_c and lattice parameter. To better understand the origin of this anomaly we have studied a representative x=0.75 compound using SQUID magnetometry and MuSR spectroscopy. The lower critical field H_c1, measured by the trapped magnetization method, is less than 1 G, a very small value as compared with that of other fullerides. Muon spin depolarization in the superconducting phase shows also quite small local field inhomogeneities, of the order of those arising from nuclear dipolar fields. On the other hand, the 40 T value for H_c2, as extracted from magnetometry data, is comparable to that of other fullerides. We show that these observations cannot be rationalized within the framework of the Ginzburg-Landau theory of superconductivity. Instead, the anomalous magnetic properties could be interpreted taking into account the role played by polaronic instabilities in this material.Comment: 7 pages, 6 figures, RevTex4, submitted to Phys. Rev.