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Register a new userIdentifying contact effects in electronic conduction through buckyballs on silicon
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We present a theory of current conduction through buckyball(C60) molecules on silicon by coupling a density functional treatment of the molecular levels embedded in silicon with a non-equilibrium Greens function (NEGF) treatment of quantum transport. Several experimental variations in conductance-voltage(G-V) characteristics are quantitatively accounted for by varying the detailed molecule-silicon bonding geometries. We identify how variations in contact surface microstructure influence the number, positions and shapes of the conductance peaks, while varying separations of the scanning probe from the molecules influence their peak amplitudes.
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The transmission through a magnetic layer of correlated electrons sandwiched between non-interacting normal-metal leads is studied within model calculations. We consider the linear regime in the framework of the Meir-Wingreen formalism, according to which the transmission can be interpreted as the overlap of the spectral function of the surface layer of the leads with that of the central region. By analyzing these spectral functions, we show that a change of the coupling parameter between the leads and the central region significantly and non-trivially affects the conductance. The role of band structure effects for the transmission is clarified. For a strong coupling between the leads and the central layer, high-intensity localized states are formed outside the overlapping bands, while for weaker coupling this high-intensity spectral weight is formed within the leads continuum band around the Fermi energy. A local Coulomb interaction in the central region modifies the high-intensity states, and hence the transmission. For the present setup, the major effect of the local interaction consists in shifts of the band structure, since any sharp features are weakened due to the macroscopic extension of the configuration in the directions perpendicular to the transport direction.
We show experimentally how quantum interference can be produced using an integrated quantum system comprising an arch-shaped short quantum wire (or quantum point contact, QPC) of 1D electrons and a reflector forming an electronic cavity. On tuning the coupling between the QPC and the electronic cavity, fine oscillations are observed when the arch QPC is operated in the quasi-1D regime. These oscillations correspond to interference between the 1D states and a state which is similar to the Fabry-Perot state and suppressed by a small transverse magnetic field of 60mT. Tuning the reflector, we find a peak in resistance which follows the behavior expected for a Fano resonance. We suggest that this is an interesting example of a Fano resonance in an open system which corresponds to interference at or near the Ohmic contacts due to a directly propagating, reflected discrete path and the continuum states of the cavity corresponding to multiple scattering. Remarkably, the Fano factor shows an oscillatory behavior taking peaks for each fine oscillation, thus, confirming coupling between the discrete and continuum states. The results indicate that such a simple quantum device can be used as building blocks to create more complex integrated quantum circuits for possible applications ranging from quantum-information processing to realizing the fundamentals of complex quantum systems.
The electronic conduction of a novel, three-terminal molecular architecture, analogous to a heterojunction bipolar transistor is studied. In this architecture, two diode arms consisting of donor-acceptor molecular wires fuse through a ring, while a gate modulating wire is a pi-conjugated wire. The calculated results show the enhancement or depletion mode of a transistor by applying a gate field along the positive or negative direction. A small gate field is required to switch on the current in the proposed architecture. The changes in the electronic conduction can be attributed to the intrinsic dipolar molecular architecture in terms of the evolution of molecular wavefunctions, specifically the one associated with the terphenyl group of the modulating wire in the presence of the gate field.
The giant piezoresistance (PZR) previously reported in silicon nanowires is experimentally investigated in a large number of surface depleted silicon nano- and micro-structures. The resistance is shown to vary strongly with time due to electron and hole trapping at the sample surfaces. Importantly, this time varying resistance manifests itself as an apparent giant PZR identical to that reported elsewhere. By modulating the applied stress in time, the true PZR of the structures is found to be comparable with that of bulk silicon.
The thermal conductance of straight and corrugated monocrystalline silicon nanowires has been measured between 0.3 K and 5 K. The difference in the thermal transport between corrugated nanowires and straight ones demonstrates a strong reduction in the mean free path of the phonons. This averaged mean free path is remarkably smaller than the smaller diameter of the nanowire, evidencing a phonon thermal transport reduced below the Casimir limit. Monte Carlo simulations highlight that this effect can be attributed to significant multiple scattering of ballistic phonons occuring on the corrugated surfaces. This result suggests an original approach to transforming a monocrystalline material into a phonon glass.
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