Insulator-metal transition in biased finite polyyne systems

A method for the study of the electronic transport in strongly coupled electron-phonon systems is formalized and applied to a model of polyyne chains biased through metallic Au leads. We derive a stationary non equilibrium polaronic theory in the general framework of a variational formulation. The numerical procedure we propose can be readily applied if the electron-phonon interaction in the device hamiltonian can be approximated as an effective single particle electron hamiltonian. Using this approach, we predict that finite polyyne chains should manifest an insulator-metal transition driven by the non-equilibrium charging which inhibits the Peierls instability characterizing the equilibrium state.Comment: to appear at EPJ

Optical spectroscopy of negative-U centers

The identification of strongly correlated double electron local sites in semiconductors usually requires a nonequilibrium situation in order to observe a single occupied state of a local site. We present a way of a direct observation of negative-U centers by means of a single photon optical transition from band states into local states provided controlled occupancy of local levels via doping the semiconductor or via change of the gate voltage. The photoabsorption spectrum shows single and double electron transitions as the consequence of the hybridization of local states and band electron states. The value of the correlation energy in localized states determine positions and intensities of peaks while the occupancy of local levels reflects itself in the line shape. A qualitative comparison with available data on installed atoms as possible double electron centers in semiconductors is made

Bipolaron localization for increasing electron-phonon coupling in a small cluster

By exact diagonalization of a small cluster, we show that an interplay of electron–phonon and onsite electron–electron interactions results in “intersite” or “onsite” two-electron (bipolaronic) solutions of the Holstein–Hubbard model, depending on the strengths of the interactions. On this basis, we argue that the decrease in the superconducting transition temperature of Bi-2212 compounds, following the enhancement of the electron–phonon interaction recently reported by Devereaux et al. [10] might be a consequence of the above mechanism, which leads to a transition from itinerant (intersite) to bound immobile (onsite) bipolarons, thus effectively reducing the number of superconducting carriers

Critical roles of metal-molecule contacts in electron transport through molecular-wire junctions

We study the variation of electron transmission through Au-S-benzene-S-Au junctions and related systems as a function of the structure of the Au:S contacts. For junctions with semi-infinite flat Au(111) electrodes, the highly coordinated in-hollow and bridge positions are connected with broad transmission peaks around the Fermi level, due to a broad range of transmission angles from transverse motion, resulting in high conductivity and weak dependence on geometrical variations. In contrast, for (unstable) S adsorption on-top of an Au atom, or in the hollow of a 3-Au-atom island, the transmission peaks narrow up due to suppression of large transmission angles. Such more one-dimensional situations may describe more common types of contacts and junctions, resulting in large variations in conductivity and sensitivity to bonding sites, tilting, and gating. In particular, if S is adsorbed in an Au vacancy, sharp spectral features appear near the Fermi level due to essential changes of the level structure and hybridization in the contacts, admitting order-of-magnitude variations of the conductivity. Possibly such a system, can it be fabricated, will show extremely strong nonlinear effects and might work as uni- or bi-directional voltage-controlled two-terminal switches and nonlinear mixing elements. Finally, density-functional theory based transport calculations seem relevant, being capable of describing a wide range of transmission peak structures and conductivities. Prediction and interpretation of experimental results probably require more precise modeling of realistic experimental situations

Quantum Spin Transport in Carbon Chains

First-principles and non-equilibrium Green’s function approaches are used to predict spin-polarized electronic transport in monatomic carbon chains covalently connected to graphene nanoribbons, as recently synthetized experimentally (Jin, C.; et al. Phys. Rev. Lett. 2009, 102, 205501−205504). Quantum electron conductances exhibit narrow resonant states resulting from the simultaneous presence of open conductance channels in the contact region and on the chain atoms. Odd-numbered chains, which acquire metallic or semiconducting character depending on the nature of the edge at the graphene contact, always display a net spin polarization. The combination of electrical and magnetic properties of chains and contacts results in nanodevices with intriguing spintronic properties such as the coexistence of magnetic and semiconducting behaviors