arXiv subjects

Kamil Walczak

Publications and source records attributed to Kamil Walczak.

At least 19 recordsLinked to original sources

arXiv2016

Nonlinearities and Noise-Signal Relations in Electronic Heat Transport via Molecules

We examine the electronic heat transport phenomena in nanoscale junctions composed of organic molecules coupled to two metallic reservoirs of different temperatures. The electronic heat flux and its dynamical noise properties are calculated within the scattering (Landauer) formalism with the transmission probability determined by using non-equilibrium Green's functions (NEGF technique). The method based on Taylor series expansion is used to determine nonlinear corrections to the electronic heat flux and its noise power spectral density with up to the second order terms with respect to the temperature difference. Our results show only limited applicability of ballistic Fourier's law and fluctuation-dissipation theorem to heat transport in molecular systems. We derived and tested numerically several signal-signal, noise-signal, and noise-noise relations applicable to nanoscale heat flow carried by electrons at strongly non-equilibrium conditions (similar formulas are expected for phonons and photons). Importantly, the special treatment proposed by us may be extended to higher order terms in order to address a variety of problems related to nonlinear thermal and electro-thermal effects which may occur at nanoscale.

cond-mat.mes-hall↗

arXiv2009

Coupling optical and electrical gating for electronic read-out of quantum dot dynamics

We explore the coherent transfer of electronic signatures from a strongly correlated, optically gated nanoscale quantum dot to a weakly interacting, electrically backgated microscale channel. In this unique side-coupled `T' geometry for transport, we predict a novel mechanism for detecting Rabi oscillations induced in the dot through quantum, rather than electrostatic means. This detection shows up as a field-tunable split in the Fano lineshape arising due to interference between the dipole coupled dot states and the channel continuum. The split is further modified by the Coulomb interactions within the dot that influence the detuning of the Rabi oscillations. Furthermore, time-resolving the signal we see clear beats when the Rabi frequencies approach the intrinsic Bohr frequencies in the dot. Capturing these coupled dynamics, including memory effects and quantum interference in the channel and the many-body effects in the dot requires coupling a Fock-space master equation for the dot dynamics with the phase-coherent, non-Markovian time-dependent non-equilibrium Green's function (TDNEGF) transport formalism in the channel through a properly evaluated self-energy and a Coulomb integral. The strength of the interactions can further be modulated using a backgate that controls the degree of hybridization and charge polarization at the transistor surface.

cond-mat.mes-hall↗

arXiv2007

Vibrational features in inelastic electron tunneling spectra

A theoretical analysis of inelastic electron tunneling spectroscopy (IETS) experiments conducted on molecular junctions are presented, where the second derivative of the current with respect to voltage is usually plotted as a function of applied bias. Within the nonperturbative computational scheme, adequate for arbitrary parameters of the model, we consider the virtual conduction process in the off-resonance region. Here we study the influence of few crucial factors on the IETS spectra: the strength of the vibronic coupling, the phonon energy, and the device working temperature. It was also shown that weak asymmetry in the IETS signal with respect to bias polarity is obtained as a result of strongly asymmetric connection with the electrodes.

cond-mat.mes-hall↗

arXiv2006

Thermoelectric properties of vibrating molecule asymmetrically connected to the electrodes

Here we present a theoretical analysis of inelastic effects on thermoelectric properties of molecular-scale junction in both linear and nonlinear response regimes. Considered device is composed of molecular quantum dot (with discrete energy levels) asymmetrically connected to metallic electrodes (treated within the wide-band approximation) via potential barriers, where molecular vibrations are modeled as dispersionless phonon excitations. Nonperturbative computational scheme, used in this work, is based on Green's function theory within the framework of polaron transformation (GFT-PT) which maps the many-body electron-phonon interaction problem into a one-body multi-channel single-electron scattering problem. It is shown that all the thermoelectric characteristics are dominated by quantum transport of virtual polarons due to a strong electron-phonon coupling.

cond-mat.mes-hall↗

arXiv2006

Transfer-matrix approach to the problem of electrical conduction through a series of absorbers

Here we study incoherent transport through molecular wire treated as a linear chain of absorbers, where the phase-breaking processes are modeled by the use of imaginary point-like potentials. The calculations are performed within a transfer-matrix method of the scattering theory. An analytic expression for the transmission of a finite chain in obtained, while the electrical current is then computed with the help of the Tsu-Esaki formula. In particular, it is shown that the maximal current dependence on the wire length is exponential.

cond-mat.mes-hall↗

arXiv2006

Rate-equation calculations of the current flow through two-site molecular device and DNA-based junction

Here we present the calculations of incoherent current flowing through the two-site molecular device as well as the DNA-based junction within the rate-equation approach. Few interesting phenomena are discussed in detail. Structural asymmetry of two-site molecule results in rectification effect, which can be neutralized by asymmetric voltage drop at the molecule-metal contacts due to coupling asymmetry. The results received for poly(dG)-poly(dC) DNA molecule reveal the coupling- and temperature-independent saturation effect of the current at high voltages, where for short chains we establish the inverse square distance dependence. Besides, we document the shift of the conductance peak in the direction to higher voltages due to the temperature decrease.

cond-mat.mes-hall↗

arXiv2006

Polaronic transport through molecular quantum dots: charging-induced NDR and rectifying behavior

Here we study the polaronic transport through molecules weakly connected to metallic electrodes in the nonlinear response regime. Molecule itself is treated as a quantum dot with discrete energy levels, its connection to the electrodes is described within the wide-band approximation, while the charging is incorporated by means of the self-consistent potential. Nonperturbative computational scheme, used in this work, is based on the Green's function theory within the framework of mapping technique (GFT-MT). This method transforms the many-body electron-phonon interaction problem into a one-body multi-channel single-electron scattering problem with occupation of polaron levels calculated in the self-consistent way. In particular, three different phenomena as a result of charging in polaronic transport via discrete quantum states are discussed in detail: the suppression of the current at higher voltages, negative differential resistance (NDR effect), and rectification.

cond-mat.mes-hall↗

arXiv2006

Coulomb blockade in molecular quantum dots

The rate-equation approach is used to describe sequential tunneling through a molecular junction in the Coulomb blockade regime. Such device is composed of molecular quantum dot (with discrete energy levels) coupled with two metallic electrodes via potential barriers. Based on this model, we calculate nonlinear transport characteristics (conductance-voltage and current-voltage dependences) and compare them with the results obtained within a self-consistent field approach. It is shown that the shape of transport characteristics is determined by the combined effect of the electronic structure of molecular quantum dots and by the Coulomb blockade. In particular, the following phenomena are discussed in detail: the suppression of the current at higher voltages, the charging-induced rectification effect, the charging-generated changes of conductance gap, and the temperature-induced as well as broadening-generated smoothing of current steps.

cond-mat.mes-hall↗

arXiv2006

Decoherence in elastic and polaronic transport via discrete quantum states

Here we study the effect of decoherence on elastic and polaronic transport via discrete quantum states. The calculations are performed with the help of nonperturbative computational scheme, based on the Green's function theory within the framework of polaron transformation (GFT-PT), where the many-body electron-phonon interaction problem is mapped exactly into a single-electron multi-channel scattering problem. In particular, the influence of dephasing and relaxation processes on the shape of the electrical current and shot noise curves is discussed in detail under the linear and nonlinear transport conditions.

cond-mat.mes-hall↗

arXiv2005

Spin-dependent shot noise of inelastic transport through molecular quantum dots

Here we present a theoretical analysis of the effect of inelastic electron scattering on spin-dependent transport characteristics (conductance, current-voltage dependence, magnetoresistance, shot noise spectrum, Fano factor) for magnetic nanojunction. Such device is composed of molecular quantum dot (with discrete energy levels)connected to ferromagnetic electrodes (treated within the wide-band approximation), where molecular vibrations are modeled as dispersionless phonons. Non-perturbative computational scheme, used in this work, is based on the Green's function theory within the framework of mapping technique (GFT-MT) which transforms the many-body electron-phonon interaction problem into a single-electron multi-channel scattering problem. The consequence of the localized electron-phonon coupling is polaron formation. It is shown that polaron shift and additional peaks in the transmission function completely change the shape of considered transport characteristics.

cond-mat.mes-hall↗

arXiv2005

Spin-dependent transport through magnetic nanojunctions

Coherent electronic transport through a molecular device is studied using non-equilibrium Green's function (NEGF) formalism. Such device is made of a carbon nanowire which is connected to ferromagnetic electrodes. The molecule itself is described with the help of Hubbard model (Coulomb interactions are treated by means of the Hartree-Fock approximation), while the coupling to the electrodes is modeled through the use of a broad-band theory. It was shown that magnetoresistance varies periodically with increasing the length of atomic wire (in the linear response regime) and oscillates with increasing of bias voltage (in the nonlinear response regime). Since the TMR effect for analyzed structures is predicted to be large (tens of percentage), considered junctions seem to be suitable for application as a magnetoresistive elements in the future electronic circuits.

cond-mat.mes-hall↗

arXiv2005

The influence of Coulomb interactions on electrical conduction through short molecular wires

Electrical conduction through a two-terminal molecular device is studied using non-equilibrium Green's functions (NEGF) formalism. Such junction is made of a short linear wire which is connected to the metallic electrodes. Molecule itself is described with the help of Huckel (tight-binding) model with the electron interactions treated within extended Hubbard model (EHM), while the coupling to the electrodes is described with the help of a broad-band theory. Coulomb interactions within molecular wire are treated by means of the restricted Hartree-Fock (RHF) approximation. In particular, the influence of short-range and long-range Coulomb interactions on electrical transport characteristics is discussed in detail.

cond-mat.mes-hall↗

arXiv2005

The influence of vibronic coupling on the shape of transport characteristics in inelastic tunneling through molecules

Here we present theoretical studies of the effect of vibronic coupling on nonlinear transport characteristics (current-voltage and conductance-voltage) in molecular electronic devices. Considered device is composed of molecular quantum dot (with discrete energy levels) weakly connected to metallic electrodes (treated within the wide-band approximation), where molecular vibrations are modeled as dispersionless phonon excitations. Nonperturbative computational scheme, used in this work, is based on the Green's function theory within the framework of mapping technique (GFT-MT) which transforms the many-body electron-phonon interaction problem into a one-body multi-channel single-electron scattering problem. In particular, it is shown that quantum coherent transport of virtual polarons through the molecule can be a dominant factor justifying some well-known discrepancies between theoretical calculations and experimental results.

cond-mat.mes-hall↗

arXiv2005

Modeling transport through single-molecule junctions

Non-equilibrium Green's functions (NEGF) formalism combined with extended Huckel (EHT) and charging model are used to study electrical conduction through single-molecule junctions. Analyzed molecular complex is composed of asymmetric 1,4-Bis((2'-para-mercaptophenyl)-ethinyl)-2-acetyl-amino-5-nitro-benzene molecule symmetrically coupled to two gold electrodes [Reichert et al., Phys. Rev. Lett. Vol.88 (2002), pp. 176804]. Owing to this model, the accurate values of the current flowing through such junction can be obtained by utilizing basic fundamentals and coherently deriving model parameters. Furthermore, the influence of the charging effect on the transport characteristics is emphasized. In particular, charging-induced reduction of conductance gap, charging-induced rectification effect and charging-generated negative value of the second derivative of the current with respect to voltage are observed and examined for molecular complex.

cond-mat.mes-hall↗

arXiv2004

Tunnel magnetoresistance of polymeric chains

Coherent spin-dependent electronic transport is investigated in a molecular junction made of polymeric chain attached to ferromagnetic electrodes (Ni and Co, respectively). Molecular system is described by a simple Huckel model, while the coupling to the electrodes is treated through the use of a broad-band theory. The current flowing through the device is calculated within non-equilibrium Green's function approach. It is shown that tunnel magnetoresistance of molecular junction can be quite large (over 100 %)and strongly depends on: (i) the lenght of the polymeric chain and (ii) the strength of the molecule-to-electrodes coupling.

cond-mat.soft↗

arXiv2004

Nonlinear transport through a finite Hubbard chain connected to the electrodes

Coherent electronic transport through a molecular device is studied using non-equilibrium Green's function (NEGF) formalism. Such device is made of a short linear wire which is connected to para- and ferromagnetic electrodes. Molecule itself is described with the help of Huckel (tight-binding) model with the electron interactions treated within Hubbard approach, while the coupling to the electrodes is modeled through the use of a broad-band theory. Coulomb interactions within molecular wire are treated by means of the Hartree-Fock (HF) approximation. For the case of asymmetric coupling to paramagnetic electrodes, charging-induced rectification effect in biased molecular devices is discussed as a consequence of Coulomb repulsion. For the system with ferromagnetic electrodes, a significant magnetoresistance (MR) is predicted and its oscillations generated by Coulomb interactions are considered.

cond-mat.mes-hall↗

arXiv2004

Charging effects in biased molecular devices

The influence of the charging effects on the transport characteristics of a molecular wire bridging two metallic electrodes in the limit of weak contacts is studied by generalized Breit-Wigner formula. Molecule is modeled as a quantum dot with discrete energy levels, while the coupling to the electrodes is treated within a broad band theory. Owing to this model we find self-consistent occupation of particular energy levels and orbital energies of the wire in the presence of transport. The nonlinear conductance and current-voltage characteristics are investigated as a function of bias voltage in the case of symmetric and asymmetric coupling to the electrodes. It is shown that the shape of that curves are determined by the combined effect of the electronic structure of the molecule and by electron-electron repulsion.

physics.chem-ph↗

arXiv2004

Theoretical analysis of STM measurements

Modeling of electron transport through organic molecules is presented in order to interpret the experimental data of Rosink et al. [PRB 62, 10459 (2000)]. Such results are understand as coherent off-resonance tunneling through the junction composed of molecular wires weakly coupled to the electrodes (Au-substrate and STM-tip, respectively). The influence of physical conditions on the characteristic parameters of the model is discussed in detail.

cond-mat.soft↗