Applying the extended molecule approach to correlated electron transport: important insight from model calculations

Theoretical approaches of electronic transport in correlated molecules usually consider an extended molecule, which includes, in addition to the molecule itself, parts of electrodes. In the case where electron correlations remain confined within the molecule, and the extended molecule is sufficiently large, the current can be expressed by means of Laudauer-type formulae. Electron correlations are embodied into the retarded Green function of a sufficiently large but isolated extended molecule, which represents the key quantity that can be accurately determined by means of ab initio quantum chemical calculations. To exemplify these ideas, we present and analyze numerical results obtained within full CI calculations for an extended molecule described by the interacting resonant level model. Based on them, we argue that for organic electrodes the transport properties can be reliably computed, because the extended molecule can be chosen sufficiently small to be tackled within accurate ab initio methods. For metallic electrodes, larger extended molecules have to be considered in general, but a (semi-)quantitative description of the transport should still be possible particularly in the typical cases where electron transport proceeds by off-resonant tunneling. Our numerical results also demonstrate that, contrary to the usual claim, the ratio between the characteristic Coulomb strength and the level width due to molecule-electrode coupling is not the only quantity needed to assess whether electron correlation effects are strong or weak

Metal-insulator transition in the one-dimensional Holstein model at half filling

We study the one-dimensional Holstein model with spin-1/2 electrons at half-filling. Ground state properties are calculated for long chains with great accuracy using the density matrix renormalization group method and extrapolated to the thermodynamic limit. We show that for small electron-phonon coupling or large phonon frequency, the insulating Peierls ground state predicted by mean-field theory is destroyed by quantum lattice fluctuations and that the system remains in a metallic phase with a non-degenerate ground state and power-law electronic and phononic correlations. When the electron-phonon coupling becomes large or the phonon frequency small, the system undergoes a transition to an insulating Peierls phase with a two-fold degenerate ground state, long-range charge-density-wave order, a dimerized lattice structure, and a gap in the electronic excitation spectrum.Comment: 6 pages (LaTex), 10 eps figure

Molecule-Electrode Interface Energetics in Molecular Junction: a Transition Voltage Spectroscopy Study

We assess the performances of the transition voltage spectroscopy (TVS) method to determine the energies of the molecular orbitals involved in the electronic transport though molecular junctions. A large number of various molecular junctions made with alkyl chains but with different chemical structure of the electrode-molecule interfaces are studied. In the case of molecular junctions with clean, unoxidized electrode-molecule interfaces, i.e. alkylthiols and alkenes directly grafted on Au and hydrogenated Si, respectively, we measure transition voltages in the range 0.9 - 1.4 V. We conclude that the TVS method allows estimating the onset of the tail of the LUMO density of states, at energy located 1.0 - 1.2 eV above the electrode Fermi energy. For oxidized interfaces (e.g. the same monolayer measured with Hg or eGaIn drops, or monolayers formed on a slightly oxidized silicon substrate), lower transition voltages (0.1 - 0.6 V) are systematically measured. These values are explained by the presence of oxide-related density of states at energies lower than the HOMO-LUMO of the molecules. As such, the TVS method is a useful technique to assess the quality of the molecule-electrode interfaces in molecular junctions.Comment: Accepted for publication in J. Phys. Chem C. One pdf file including manuscript, figures and supporting informatio

Interplay between disorder and quasi-regularity in CDW systems

We develop here a microscopical model of quasi-regular impurity distribution driven by the CDW appropriate for highly mobile impurities or materials with fictitious CDW transition temperature. Thermal hysteresis effects at the CDW transition reported in the quasi-1D materials NbSe3 and Ta2NiSe7, could be interpreted in the framework of the present model. Possible connections with other (alloy, heavy-fermion) systems are also suggested

Three-dimensional vibronic analysis of the B' system of Na

The vibronic structure of the B$^{\prime}$ system in the two-photon ionization spectrum of Na3 is investigated theoretically, based on an earlier coupling scheme with three interacting potential energy surfaces (pseudo Jahn-Teller coupling). This is extended in the present work to allow for additional ab initio data in the modeling, to include the totally symmetric vibrational mode and to treat a higher excited state in the electronic manifold. Important features of the experimental recording can thus be reproduced, although some unsatisfactory aspects remain. The implications for the nonadiabatic nature of the underlying nuclear motion, as well as directions for future work, are discussed

Life-time and pinning effects in CDW systems

We develop here a microscopical formalism enabling the simultaneous treatment of the two main effects due to electron-impurity interaction: finite electronic life-time and pinning of the CDW phase. It is found that these two effects affect each other in a way which is particularly non-trivial in the strong pinning limit

Extending the Newns-Anderson model to allow nanotransport studies through molecules with floppy degrees of freedom

The Newns-Anderson model is ubiquitous in studies of the molecular transport in the presence of solvent (outer) reorganization. The present work demonstrates that intramolecular reorganization can also be significant for the transport through molecules with floppy degrees of freedom, for which the Newns-Anderson model can be extended. The expressions of the model parameters deduced from electronic structure calculations for (4, 4')-bipyridine (44BPY) quantitatively differ from those characteristic for outer reorganization due to strong intramolecular anharmonicities. These expressions can be utilized as input in transport calculations for 44BPY-based molecular junctions of experimental interest [Science 301 (2003) 203, J. Am. Chem. Soc. 130 (2008) 16045, Nano Lett. 12 (2012) 354].Comment: accepted for publication in EPL (Europhys. Lett.