Distributions of Time- and Distance-Headways in the Nagel-Schreckenberg Model of Vehicular Traffic: Effects of Hindrances

In the Nagel-Schreckenberg model of vehicular traffic on single-lane highways vehicles are modelled as particles which hop forward from one site to another on a one dimensional lattice and the inter-particle interactions mimic the manner in which the real vehicles influence each other's motion. In this model the number of empty lattice sites in front of a particle is taken to be a measure of the corresponding distance-headway(DH). The time-headway(TH) is defined as the time interval between the departures (or arrivals) of two successive particles recorded by a detector placed at a fixed position on the model highway. We investigate the effects of spatial inhomogeneities of the highway (static hindrances) on the DH and TH distributions in the steady-state of this model.Comment: 21 pages LATEX, 5 postscript figures; European Physical Journal B, vol.5, 781 (1998

From ballistic transport to tunneling in electromigrated ferromagnetic breakjunctions

We fabricate ferromagnetic nanowires with constrictions whose cross section can be reduced gradually from 100 nm to the atomic scale and eventually to the tunneling regime by means of electromigration. These devices are mechanically stable against magnetostriction and magnetostatic effects. We measure magnetoresistances ~ 0.3% for 100*30 nm^2 constrictions, increasing to a maximum of 80% for atomic-scale widths. These results are consistent with a geometrically-constrained domain wall trapped at the constriction. For the devices in the tunneling regime we observe large fluctuations in MR, between -10 and 85%.Comment: 4 pages, 5 figure

Electron Transport in Molecular Transistors

In this thesis I will describe the conductance properties of certain organic molecules. I will first show that two metal electrodes can be fabricated with a nm-scale gap between them by causing electomigration-induced failure in a nanoscale wire. These two electrodes are separated by a few nanometers from a metallic gate electrode. Organic molecules can be incorporated into the gap between the electrodes creating a transistor geometry. Transport measurements on metal-organic complexes at low temperatures show Coulomb blockade and Kondo-assisted tunneling. Using the specially designed molecule C$_{140}$, I have studied the coupling between the vibrational modes of the molecule and electron flow through transistors made from it. I have also been able to make magnetic electrodes to pass spin-polarized current through molecules. Using this, I have studied the coexistence of the Kondo effect and ferromagnetism in the electrodes. I have also modified the technique to have a mechanically adjustable distance between the two electrodes, which is useful for studying the influence of the contact on the conductance of a device