arXiv subjects

G. Fagas

Publications and source records attributed to G. Fagas.

17 recordsLinked to original sources

arXiv2010

Deformation Potentials and Electron-Phonon Coupling in Silicon Nanowires

The role of reduced dimensionality and of the surface on electron-phonon (e-ph) coupling in silicon nanowires is determined from first principles. Surface termination and chemistry is found to have a relatively small influence, whereas reduced dimensionality fundamentally alters the behavior of deformation potentials. As a consequence, electron coupling to "breathing modes" emerges that cannot be described by conventional treatments of e-ph coupling. The consequences for physical properties such as scattering lengths and mobilities are significant: the mobilities for [110] grown wires are 6 times larger than those for [100] wires, an effect that cannot be predicted without the form we find for Si nanowire deformation potentials.

cond-mat.mtrl-sci↗

arXiv2009

Comment on "Electron transport through correlated molecules computed using the time-independent Wigner function: Two critical tests"

The many electron correlated scattering (MECS) approach to quantum electronic transport was investigated in the linear response regime [I. Baldea and H. Koeppel, Phys. Rev. B. 78, 115315 (2008)]. The authors suggest, based on numerical calculations, that the manner in which the method imposes boundary conditions is unable to reproduce the well-known phenomena of conductance quantization. We introduce an analytical model and demonstrate that conductance quantization is correctly obtained using open system boundary conditions within the MECS approach.

cond-mat.mes-hall↗

arXiv2007

Electronegativity in quantum electronic transport

Electronegativity is shown to control charge transfer, energy level alignments, and electron currents in single molecule tunnel junctions, all of which are governed by correlations contained within the density matrix. This is demonstrated by the fact that currents calculated from the one-electron reduced density matrix to second order in electron correlation are identical to the currents obtained from the Green's function corrected to second order in electron self-energy.

cond-mat.mes-hall↗

arXiv2005

Magnetic-field dependence of transport in normal and Andreev billiards: a classical interpretation to the averaged quantum behavior

We perform a comparative study of the quantum and classical transport probabilities of low-energy quasiparticles ballistically traversing normal and Andreev two-dimensional open cavities with a Sinai-billiard shape. We focus on the dependence of the transport on the strength of an applied magnetic field $B$. With increasing field strength the classical dynamics changes from mixed to regular phase space. Averaging out the quantum fluctuations, we find an excellent agreement between the quantum and classical transport coefficients in the complete range of field strengths. This allows an overall description of the non-monotonic behavior of the average magnetoconductance in terms of the corresponding classical trajectories, thus, establishing a basic tool useful in the design and analysis of experiments.

cond-mat.mes-hall↗

arXiv2003

Conductance of a molecular junction mediated by unconventional metal-induced gap states

The conductance of a molecular junction is commonly determined by either charge-transfer-doping, where alignment of the Fermi energy to the molecular levels is achieved, or tunnelling through the tails of molecular resonances within the highest-occupied and lowest-unoccupied molecular-orbital gap. Here, we present an alternative mechanism where electron transport is dominated by electrode surface states. They give rise to metallization of the molecular bridge and additional, pronounced conductance resonances allowing for substantial tailoring of its electronic properties via, e.g. a gate voltage. This is demonstrated in a field-effect geometry of a fullerene-bridge between two metallic carbon nanotubes.

cond-mat.mes-hall↗

arXiv2002

Fingerprints of mesoscopic leads in the conductance of a molecular wire

The influence of contacts on linear transport through a molecular wire attached to mesoscopic tubule leads is studied. It is shown that low dimensional leads, such as carbon nanotubes, in contrast to bulky electrodes, strongly affect transport properties. By focusing on the specificity of the lead-wire contact, we show, in a fully analytical treatment, that the geometry of this hybrid system supports a mechanism of channel selection and a sum rule, which is a distinctive hallmark of the mesoscopic nature of the electrodes.

cond-mat.mes-hall↗

arXiv2002

A molecular wire sandwiched between nanotube leads: analytic results for the conductance

Analytic results for the conductance of a molecular wire attached to mesoscopic tubule leads are obtained. They permit to study linear transport in presence of low dimensional leads in the whole range of parameters. In particular contact effects can be addressed in detail. By focusing on the specificity of the lead-wire contact, we show that the geometry of this hybrid system supports a mechanism of channel selection, which is a distinctive hallmark of the mesoscopic nature of the electrodes.

cond-mat.mes-hall↗

arXiv2002

Molecular wire-nanotube interfacial effects on electron transport

We discuss the conductance of a molecular bridge between mesoscopic electrodes supporting low-dimensional transport and bearing an internal structure. As an example for such nanoelectrodes we assume semi-infinite (carbon) nanotubes. In the Landauer scattering matrix approach, we show that the conductance of this hybrid is very sensitive to the geometry of the contact unlike the usual behaviour in the presence of bulk electrodes.

cond-mat.mes-hall↗

arXiv2001

Fullerene based devices for molecular electronics

We have investigated the electronic properties of a C_60 molecule in between carbon nanotube leads. This problem has been tackled within a quantum chemical treatment utilizing a density functional theory-based LCAO approach combined with the Landauer formalism. Owing to low-dimensionality, electron transport is very sensitive to the strength and geometry of interfacial bonds. Molecular contact between interfacial atoms and electrodes gives rise to a complex conductance dependence on the electron energy exhibiting spectral features of both the molecule and electrodes. These are attributed to the electronic structure of the C_60 molecule and to the local density of states of the leads, respectively.

cond-mat.mes-hall↗

arXiv2001

An `all-carbon' molecular device

We have performed parameter-free calculations of electron transport across a carbon molecular junction consisting of a C$_{60}$ molecule sandwiched between two semi-infinite metallic carbon nanotubes. It is shown that the Landauer conductance of this carbon hybrid system can be tuned within orders of magnitude not only by varying the tube-C$_{60}$ distance, but more importantly at fixed distances by i) changing the orientation of the Buckminsterfullerene or ii) rotating one of the tubes around its cylinder axis. Furthermore, it is explicitely shown that structural relaxation determines qualitatively the transmission spectrum of such devices.

cond-mat.mes-hall↗

arXiv2001

Conductance of a molecular wire attached to mesoscopic leads: contact effects

We study linear electron transport through a molecular wire sandwiched between nanotube leads. We show that the presence of such electrodes strongly influences the calculated conductance. We find that depending on the quality and geometry of the contacts between the molecule and the tubular reservoirs, linear transport can be tuned between an effective Newns spectral behavior and a more structured one. The latter strongly depends on the topology of the leads. We also provide analytical evidence for an anomalous behavior of the conductance as a function of the contact strength.

cond-mat.mes-hall↗

arXiv2000

Electron transport in nanotube--molecular wire hybrids

We study contact effects on electron transport across a molecular wire sandwiched between two semi-infinite (carbon) nanotube leads as a model for nanoelectrodes. Employing the Landauer scattering matrix approach we find that the conductance is very sensitive to parameters such as the coupling strength and geometry of the contact. The conductance exhibits markedly different behavior in the two limiting scenarios of single contact and multiple contacts between the molecular wire and the nanotube interfacial atoms. In contrast to a single contact the multiple-contact configuration acts as a filter selecting single transport channels. It exhibits a scaling law for the conductance as a function of coupling strength and tube diameter. We also observe an unusual narrow-to-broad-to-narrow behavior of conductance resonances upon decreasing the coupling.

cond-mat.mes-hall↗

arXiv1999

Phonon-mediated thermal conductance of mesoscopic wires with rough edges

We present an analysis of acoustic phonon propagation through long, free-standing, insulating wires with rough surfaces. Due to a crossover from ballistic propagation of the lowest-frequency phonon mode at $ω<ω_{1}=πc/W$ to a diffusive (or even localized) behavior upon the increase of phonon frequency, followed by re-entrance into the quasi-ballistic regime, the heat conductance of a wire acquires an intermediate tendency to saturate within the temperature range $T\sim \hbar ω_{1}/k_{B}$.

cond-mat↗

arXiv1999

Correlations in optical phonon spectra of complex solids

Spectral correlations in the optical phonon spectrum of a solid with a complex unit cell are analysed using the Wigner-Dyson statistical approach. Despite the fact that all force constants are real, we find that the statistics are predominantly of the GUE type depending on the location within the Brillouin zone of a crystal and the unit cell symmetry. Analytic and numerical results for the crossover from GOE to GUE statistics are presented.

cond-mat.mes-hall↗

arXiv1999

Lattice-dynamics of a Disordered solid-solid Interface

Generic properties of elastic phonon transport at a disordered interface are studied. The results show that phonon transmittance is a strong function of frequency and the disorder correlation length. At frequencies lower than the van Hove singularity the transmittance at a given frequency increases as the correlation length decreases. At low frequencies, this is reflected by different power-laws for phonon conductance across correlated and uncorrelated disordered interfaces which are in approximate agreement with perturbation theory of an elastic continuum. These results can be understood in terms of simple mosaic and two-colour models of the interface.

cond-mat↗

arXiv1998

Universal correlations in vibrational spectra of complex crystals

We study spectral statistics of lattice modes in a disordered crystal and in a crystal with a complex unit cell. The correlations of the eigenmode frequencies of a block of a disordered solid is found to obey the GOE Wigner-Dyson statistics. In contrast, the set of eigenfrequencies of a crystal with a complex unit cell taken at the same point ${\bf Q}\not=0$ in the Brillouin zone exhibit correlations specific to the GUE universality class.

cond-mat↗

arXiv1998

Lattice-dynamical calculation of phonon scattering at a disordered interface

For an fcc crystal with central force interactions and separately for a scalar model on a square lattice, we compute exactly the phonon transmission coefficient $T(ω)$ through a disordered planar interface between two identical semi - infinite leads. At high frequencies $T(ω)$ exhibits a strong frequency dependence which is determined by the correlation length of the disorder.

cond-mat↗