We demonstrate large rectification ratios (> 100) in single-molecule junctions based on a metal-oxide cluster (polyoxometalate), using a scanning tunneling microscope (STM) both at ambient conditions and at low temperature. These rectification ratios are the largest ever observed in a single-molecule junction, and in addition these junctions sustain current densities larger than 10(5) A cm(-2). By following the variation of the I-V characteristics with tip-molecule separation we demonstrate unambiguously that rectification is due to asymmetric coupling to the electrodes of a molecule with an asymmetric level structure. This mechanism can be implemented in other type of molecular junctions using both organic and inorganic molecules and provides a simple strategy for the rational design of molecular diodes.

An analysis of resistive broadening in the presence of magnetic fields up to 14 T for sulfur doped FeTe superconductors is presented. FeTe shows an abrupt change in resistivity at 70 K due to a structural transition. Vanishing of the structural transition and the appearance of superconductivity at ∼10 K and 9.7 K are seen in FeTe 0.9 S 0.1 and FeTe 0.8 S 0.2 respectively. The upper critical field and coherence lengths are estimated using the Werthamer-Helfand-Hohenberg and Ginzburg-Landau theories for different criteria for the transition temperatures. The estimated activation energy for thermally activated flux flow (TAFF) is an order of magnitude smaller than for the rare earth (R) based RFeAsO 1−x F x system, which indicates weaker pinning than for the RFeAsO 1−x F x system. The flux flow activation energy shows power law behavior with the two different exponents for fields above and below H = 6 T for FeTe 0.9 S 0.1 and H = 8 T for FeTe 0.8 S 0.2 . The fluctuation conductivity is analyzed using Aslamazov-Larkin theory and lowest Landau level (LLL) theory, respectively, for zero and nonzero magnetic fields. Incidentally, the field above which 2D LLL scaling is observed in these systems coincides with the crossover field observed in TAFF resistivity.

We report on the magneto-resistivity of FeSe 0.9−x M x (M = Si, Sb, x = 0.05, 0.1) down to 2 K in the presence of magnetic fields up to 14 T. The superconducting transition shows marginal differences for Sb-and Si-doped samples. Normal state resistivity shows marked changes at intermediate temperatures around 100 K, signaling the presence of a structural transition. It also shows linear behavior with temperature, reminiscent of high T c ceramics. Superconducting parameters like critical fields and coherence lengths are quantified for all samples. The broadening of superconducting transitions is studied through thermally activated flux flow (TAFF) and fluctuation conductivity. The activation energy of these superconductors is found to be one order smaller than the FeAs-1111 system, which may be explained in terms of larger penetration depth. The activation energy of thermally activated flux flow decreases with the Si and Sb doping and is explained by Kramer's scaling for grain boundary pinning. The zero-field and magnetic-field-induced fluctuation conductivity are studied using Aslamazov-Larkin (AL) and lowest Landau level (LLL) theories, respectively. Zero-field fluctuation conductivity shows the 2D to 3D crossover just above the mean-field transition and 3D LLL scaling is obtained near mean-field transitions for magnetic-field-induced fluctuation conductivity. This is a clear indication of the three-dimensional nature of these superconductors. The 3D nature of these superconductors signifies its potential for future technological applications.