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Dmitry A. Ryndyk

Publications and source records attributed to Dmitry A. Ryndyk.

17 recordsLinked to original sources
arXiv2025

Light-induced deformation of side-chain azo-polymer: Insights from atomistic modeling

In this study, we apply, for the first time, the fully atomistic force field approach to modeling light-induced deformations of azo-polymers, thereby establishing a relationship between macroscopic parameters and the microscopic molecular architecture of the used azo-polymers. We apply an orientation potential to mimic the illumination of the sample, in which the action of light is represented through controlled redistribution of azo-chromophores relative to the polarization direction. This strategy allows us to capture both the microscopic details of chromophore behaviour and the collective, anisotropic response of the polymer matrix. By combining these complementary perspectives, the simulations not only resolve the local mechanism of light-induced motion but also provide a pathway to bridge molecular-scale dynamics with mesoscopic deformation phenomena in azo-polymer films.

cond-mat.soft↗
arXiv2024

Thermal with Electronic Excitation for the Unidirectional Rotation of a Molecule on Surface

Exploring the limits of the microscopic reversibility principle, we investigated the interplay between thermal and electron tunneling excitations for the unidirectional rotation of a molecule-rotor on the Au(111) surface. We identified a range of moderate voltages and temperatures where heating the surface enhances the unidirectional rotational rate of a chemisorbed DMNI-P rotor. At higher voltage, inelastic tunneling effects dominate while at higher temperature the process becomes stochastic. At each electron transfer event during tunneling, the quantum mixing of ground and excited electronic states brings part of the surface thermal energy in the excited electronic states of the molecule-rotor. Thermal energy contributes therefore to the semi-classical unidirectional rotation without contradicting the microscopic reversibility principle.

cond-mat.mes-hall↗
arXiv2024

Tuning the Planarity of an Aromatic Thianthrene-Based Molecule on Au(111)

Non-planar aromatic molecules are interesting systems for organic electronics and optoelectronics applications due to their high stability and electronic properties. By using scanning tunneling microscopy and spectroscopy, we investigated thianthrene-based molecules adsorbed on Au(111), which are non-planar in the gas phase and the bulk solid state. Varying the molecular coverage leads to the formation of two different kinds of self-assembled structures: close-packed islands and quasi one-dimensional chains. We found that the molecules are non-planar within the close-packed islands, while the configuration is planar in the molecular chain and for single adsorbed molecules. Using vertical tip manipulation to isolate a molecule from the island, we demonstrate the conversion of a non-planar molecule to its planar configuration. We discuss the two different geometries and their electronic properties with the support of density functional theory calculations.

cond-mat.mtrl-sci↗
arXiv2020

On-surface synthesis of nitrogen-doped nanographenes with 5-7 membered rings

We report on the formation of nitrogen-doped nanographenes containing five- and seven-membered rings by thermally induced cyclodehydrogenation on the Au(111) surface. Using scanning tunneling microscopy and supported by calculations, we investigated the structure of precursor and targets, as well as of intermediates. Scanning tunneling spectroscopy shows that the electronic properties of the target nanographenes are strongly influenced by the additional formation of non-hexagonal rings.

cond-mat.mtrl-sci↗
arXiv2018

Vibronic Dephasing Model for Coherent-to-Incoherent Crossover in DNA

In this work we investigate the interplay between coherent and incoherent charge transport in cytosine-guanine (GC) rich DNA molecules. Our objective is to introduce physically grounded approach to dephasing in large molecules and to understand the length dependent charge transport characteristics and especially the crossover from coherent tunneling to incoherent hopping regime at different temperatures. Therefore, we apply the vibronic dephasing model and compare the results to the Büttiker probe model which is commonly used to describe decoherence effects in charge transport. Using the full ladder model and simplified 1D model of DNA, we consider molecular junctions with alternating and stacked GC sequences and compare our results to recent experimental measurements.

cond-mat.mes-hall↗
arXiv2015

Enhanced thermoelectric figure of merit in polycrystalline carbon nanostructures

Grain boundaries are commonly observed in carbon nanostructures, but their influence on thermal and electric properties are still not completely understood. Using a combined approach of density functional tight-binding theory and non-equilibrium Green functions we investigate electron and phonon transport in carbon based systems. In this work, quantum transport and thermoelectric properties are summarized for graphene sheets, graphene nanoribbons and carbon nanotubes with a variety of grain boundary types in a wide temperature range. Motivated by previous findings that disorder scatters phonons more effectively than electrons, a significant improvement in the thermoelectric performance for polycrystalline systems is expected. As the effect is marginally sensitive to the grain boundary type, we demonstrate that grain boundaries are a viable tool to greatly enhance the figure of merit, paving the way for the design of new thermoelectric materials.

cond-mat.mtrl-sci↗
arXiv2013

Combined effect of strain and defects on the conductance of graphene nanoribbons

We investigate the combined influence of structural defects and uniaxial longitudinal strain on the electronic transport properties of armchair graphene nanoribbons using the numerical approach based on the semiempirical tight-binding model, the Landauer formalism, and the recursion method for Green functions. We calculate the conductance of graphene nanoribbons in the quantum coherent regime with different types and concentrations of defects. Further, we apply uniform planar tension to the nonideal graphene ribbons with randomly distributed and oriented single and double vacancies and Stone-Wales defects. Since transport characteristics of graphene nanoribbons are found to be very sensitive to edge termination and aspect ratio, and it has been shown that energy gaps can emerge under critical strain; the interplay of both effects needs to be studied. We show that band gap engineering using strain is still possible for nonideal armchair ribbons with a small defect concentration, as the oscillatory behavior of the gap is preserved.

cond-mat.mes-hall↗
arXiv2013

Influencing the conductance in biphenyl-like molecular junctions with THz radiation

We investigate the torsional vibrations in biphenyl-like molecular junctions and transport properties in the presence of an external THz field. Ab-initio calculations including external electric fields show that the torsional angle ϕ of a thiolated biphenyl junction exhibits virtually no response. However, if functional groups are added to the molecule, creating a dipole moment in each of the rings, an external field becomes more effective for changing ϕ. A model based on the cos2ϕ dependence of the current is proposed for the biphenyl-like molecular junctions in presence of an external THz field including 2,2'-bipyridine, 3,3'-bipyridine and 2,2',4,4'- tetramethyl-3,3'-bipyridine. The current through these molecules is shown to change if the THz frequency gets in resonance to the torsional vibration mode.

cond-mat.mes-hall↗
arXiv2012

Edge state effects in junctions with graphene electrodes

We consider plane junctions with graphene electrodes, which are formed by a single-level system ("molecule") placed between the edges of two single-layer graphene half planes. We calculate the edge Green functions of the electrodes and the corresponding lead self-energies for the molecular levels in the cases of semi-infinite single-layer electrodes with armchair and zigzag edges. We show two main effects: first, a peculiar energy-dependent level broadening, reflecting at low energies the linear energy dependence of the bulk density of states in graphene, and, second, the shift and splitting of the molecular level energy, especially pronounced in the case of the zigzag edges due to the influence of the edge states. These effects give rise to peculiar conductance features at finite bias and gate voltages.

cond-mat.mes-hall↗
arXiv2012

Many-body localized molecular orbital approach to molecular transport

An ab initio based theoretical approach to describe nonequilibrium many-body effects in molecular transport is developed. We introduce a basis of localized molecular orbitals and formulate the many-body model in this basis. In particular, the Hubbard-Anderson Hamiltonian is derived for single-molecule junctions with intermediate coupling to the leads. As an example we consider a benzenedithiol junction with gold electrodes. An effective few-level model is obtained, from which spectral and transport properties are computed and analyzed. Electron-electron interaction crucially affects transport and induces multiscale Coulomb blockade at low biases. At large bias, transport through asymmetrically coupled molecular edge states results in the occurrence of "anomalous" conductance features, i.e., of peaks with unexpectedly large/small height or even not located at the expected resonance energies.

cond-mat.mes-hall↗
arXiv2009

Single-spin polaron memory effect

The single-spin memory effect is considered within a minimal polaron model describing a single-level quantum dot interacting with a vibron and weakly coupled to ferromagnetic leads. We show that in the case of strong electron-vibron and Coulomb interactions the rate of spontaneous quantum switching between two spin states is suppressed at zero bias voltage, but can be tuned through a wide range of finite switching timescales upon changing the bias. We further find that such junctions exhibit hysteretic behavior enabling controlled switching of a spin state. Spin lifetime, current and spin polarization are calculated as a function of the bias voltage by the master equation method. We also propose to use a third tunneling contact to control and readout the spin state.

cond-mat.mes-hall↗
arXiv2008

Charge-memory effect in a polaron model: equation-of-motion method for Green functions

We analyze a single-level quantum system placed between metallic leads and strongly coupled to a localized vibrational mode, which models a singlemolecule junction or an STM setup. We consider a polaron model describing the interaction between electronic and vibronic degrees of freedom and develop and examine different truncation schemes in the equation-of-motion method within the framework of non-equilibrium Green functions. We show that upon applying gate or bias voltage, it is possible to observe charge-bistability and hysteretic behavior which can be the basis of a charge-memory element. We further perform a systematic analysis of the bistability behaviour of the system for different internal parameters such as the electron-vibron and the lead-molecule coupling strength.

cond-mat.mes-hall↗
arXiv2008

Charge-memory polaron effect in molecular junctions

The charge-memory effect, bistability and switching between charged and neutral states of a molecular junction, as observed in recent STM experiments, is considered within a minimal polaron model. We show that in the case of strong electron-vibron interaction the rate of spontaneous quantum switching between charged and neutral states is exponentially suppressed at zero bias voltage but can be tuned through a wide range of finite switching timescales upon changing the bias. We further find that, while junctions with symmetric voltage drop give rise to random switching at finite bias, asymmetric junctions exhibit hysteretic behavior enabling controlled switching. Lifetimes and charge-voltage curves are calculated by the master equation method for weak coupling to the leads and at stronger coupling by the equation-of-motion method for nonequilibrium Green functions.

cond-mat.mes-hall↗
arXiv2007

Molecular junctions in the Coulomb blockade regime: rectification and nesting

Quantum transport through single molecules is very sensitive to the strength of the molecule-electrode contact. Here, we investigate the behavior of a model molecular junction weakly coupled to external electrodes in the case where charging effects do play an important role (Coulomb blockade regime). As a minimal model we consider a molecular junction with two spatially separated donor and acceptor sites. Depending on their mutual coupling to the electrodes, the resulting transport observables show well defined features such as rectification effects in the I-V characteristics and nesting of the stability diagrams. To be able to accomplish these results, we have developed a theory which allows to explore the charging regime via the nonequilibrium Green function formalism parallel to the widely used master equation technique. Our results, beyond their experimental relevance, offer a transparent framework for the systematic and modular inclusion of a richer physical phenomenology.

cond-mat.mes-hall↗
arXiv2007

Nonequilibrium resonant spectroscopy of molecular vibrons

Quantum transport through single molecules is essentially affected by molecular vibrations. We investigate the behavior of the prototype single-level model with intermediate electron-vibron coupling and arbitrary coupling to the leads. We have developed a theory which allows to explore this regime via the nonequilibrium Green function formalism. We show that the nonequilibrium resonant spectroscopy is able to determine the energies of molecular orbitals and the spectrum of molecular vibrations. Our results are relevant to scanning tunneling spectroscopy experiments, and demonstrate the importance of the systematic and self-consistent investigation of the effects of the vibronic dynamics onto the transport through single molecules.

cond-mat.mes-hall↗
arXiv1995

Synchronization in one-dimensional array of Josephson coupled thin layers

We obtain self-consistent macroscopic equations describing interlayer Josephson effect and intralayer disequilibrium in one-dimensional array of Josephson coupled layers. We show that ``nonequilibrium coupling'' can lead to effective spatial and time synchronization and formation of coherent dynamic resistive state (collective Josephson effect) in Nb-AlO-Nb stacked junctions and HTSC (intrinsic Josephson effect). We propose it to be the origin of collective switching phenomena observed in PbBiSrCaCuO.

supr-con↗