arXiv subjects

D. A. Ryndyk

Publications and source records attributed to D. A. Ryndyk.

13 recordsLinked to original sources

arXiv2023

A Nanocar and Rotor in One Molecule

Depending on its adsorption conformation on the Au(111) surface, a zwitterionic single-molecule machine works in two different ways under bias voltage pulses. It is a unidirectional rotor while anchored on the surface. It is a fast-drivable molecule-vehicle (nanocar) while physisorbed. By tuning the surface coverage, the conformation of the molecule can be selected to be either rotor or nanocar. The inelastic tunneling excitation producing the movement is investigated in the same experimental conditions for both the unidirectional rotation of the rotor and the directed movement of the nanocar.

physics.chem-ph↗

arXiv2013

Comparison of electron and phonon transport in disordered semiconductor carbon nanotubes

Charge and thermal conductivities are the most important parameters of carbon nanomaterials as candidates for future electronics. In this paper we address the effects of Anderson type disorder in long semiconductor carbon nanotubes (CNTs) to electron charge conductivity and lattice thermal conductivity using the atomistic Green function approach. The electron and phonon transmissions are analyzed as a function of the length of the disordered nanostructures. The thermal conductance as a function of temperature is calculated for different lengths. Analysis of the transmission probabilities as a function of length of the disordered device shows that both electrons and phonons with different energies display different transport regimes, i.e. quasi-ballistic, diffusive and localization regimes coexist. In the light of the results we discuss heating of the semiconductor device in electronic applications.

cond-mat.mes-hall↗

arXiv2009

Effect of Microwaves on the Current-Phase-Relation in SNS Josephson Junctions

We investigate the current-phase-relation (CPR) of long diffusive superconductor-normal metal-superconductor (SNS) Josephson junctions in thermodynamic equilibrium and under microwave irradiation. While in equilibrium good agreement with the predictions of quasi-classical theory is found, we observe that the shape of the CPR can be strongly affected by microwave irradiation. Close to a Josephson-phase difference of pi, the supercurrent can be strongly suppressed when increasing the rf-power. Our results can be understood in terms of microwave excitation of low-lying Andreev bound states across the mini-gap in the junction. In the frequency interval studied, this mechanism becomes important when the mini-gap closes at a phase difference of pi.

cond-mat.mes-hall↗

arXiv2008

Green function techniques in the treatment of quantum transport at the molecular scale

The theoretical investigation of charge (and spin) transport at nanometer length scales requires the use of advanced and powerful techniques able to deal with the dynamical properties of the relevant physical systems, to explicitly include out-of-equilibrium situations typical for electrical/heat transport as well as to take into account interaction effects in a systematic way. Equilibrium Green function techniques and their extension to non-equilibrium situations via the Keldysh formalism build one of the pillars of current state-of-the-art approaches to quantum transport which have been implemented in both model Hamiltonian formulations and first-principle methodologies. We offer a tutorial overview of the applications of Green functions to deal with some fundamental aspects of charge transport at the nanoscale, mainly focusing on applications to model Hamiltonian formulations.

cond-mat.mes-hall↗

arXiv2005

Nonequilibrium excitations of molecular vibrons

We consider the nonequilibrium quantum vibrations of a molecule clamped between two macroscopic leads in a current-carrying state at finite voltages. Our approach is based on the nonequilibrium Green function technique and the self-consistent Born approximation. Kinetic equations for the average populations of electrons and vibrons are formulated in the weak electron-vibron coupling case and self-consistent solutions are obtained. The effects of vibron emission and vibronic instability are demonstrated using few-orbital models. The importance of the electron-vibron resonance is shown.

cond-mat.mes-hall↗

arXiv2004

Static charge-imbalance effects in intrinsic Josephson systems

Nonequilibrium effects created by stationary current injection in layered d-wave superconductors forming a stack of intrinsic Josephson junctions are studied. Starting from a nonequilibrium Green function theory we derive microscopic expressions for the charge-imbalance (difference between electron- and hole-like quasi-particles) on the superconducting layers and investigate its influence on the quasi-particle current between the layers. This nonequilibrium effect leads to shifts in the current-voltage curves of the stack. The theory is applied to the interpretation of recent current injection experiments in double-mesa structures.

cond-mat.supr-con↗

arXiv2004

Inelastic resonant tunneling through single molecules and quantum dots: spectrum modification due to nonequilibrium effects

Resonant electron transport through a mesoscopic region (quantum dot or single molecule) with electron-phonon interaction is considered at finite voltage. In this case the standard Landauer-Büttiker approach cannot be applied. Using the nonequilibrium Green function method we show that due to a nonequilibrium distribution function of electrons in the mesoscopic region, the inelastic scattering rate and spectral function of the dot become functions of the voltage and have to be calculated self-consistently.

cond-mat.mes-hall↗

arXiv2003

Thermal fluctuations in ultrasmall intrinsic Josephson junctions

Current-voltage curves of small area hysteretic intrinsic Josephson junctions, for which the Josephson energy $E_J=\hbar J_c/2e$ is of order of thermal energy $kT$, are investigated. A non-monotonic temperature dependence of the switching current is observed and explained by thermal phase fluctuations. At low temperatures premature switching from the superconducting into the resistive state is the most important effect of fluctuations. At high temperatures only a single resistive branch is observed. At the cross-over temperature a hysteretic phase-diffusion branch exists. It shows the importance of a frequency-dependent impedance of an external circuit formed by the leads.

cond-mat.supr-con↗

arXiv2003

Electronic compressibility and charge imbalance relaxation in cuprate superconductors

In the material SmLa$_{1-x}$Sr$_x$CuO$_{4-δ}$ with alternating intrinsic Josephson junctions we explain theoretically the relative amplitude of the two plasma peaks in transmission by taking into account the spatial dispersion of the Josephson Plasma Resonance in $c$ direction due to charge coupling. From this and the magnetic field dependence of the plasma peaks in the vortex solid and liquid states it is shown that the electronic compressibility of the CuO$_2$ layers is consistent with a free electron value. Also the London penetration depth $λ_{ab} \approx 1100 {\rm Å}$ near $T_c$ can be determined. The voltage response in the $IV$-curve of a Bi$_2$Sr$_2$CaCu$_2$O$_8$ mesa due to microwave irradiation or current injection in a second mesa is related to the nonequilibrium charge imbalance of quasiparticles and Cooper pairs and from our experimental data the relaxation time $\sim 100 {\rm ps}$ is obtained.

cond-mat.supr-con↗

arXiv2002

Superlattice with hot electron injection: an approach to a Bloch oscillator

A semiconductor superlattice with hot electron injection into the miniband is considered. The injection changes the stationary distribution function and results in a qualitative change of the frequency behaviour of the differential conductivity. In the regime with Bloch oscillating electrons and injection into the upper part of the miniband the region of negative differential conductivity is shifted from low frequencies to higher frequencies. We find that the dc differential conductivity can be made positive and thus the domain instability can be suppressed. At the same time the high-frequency differential conductivity is negative above the Bloch frequency. This opens a new way to make a Bloch oscillator operating at THz frequencies.

cond-mat.mes-hall↗

arXiv2002

Charge-imbalance effects in intrinsic Josephson systems

We report on two types of experiments with intrinsic Josephson systems made from layered superconductors which show clear evidence of nonequilibrium effects: 1. In 2-point measurements of IV-curves in the presence of high- frequency radiation a shift of the voltage of Shapiro steps from the canonical value hf/(2e) has been observed. 2. In the IV-curves of double-mesa structures an influence of the current through one mesa on the voltage measured on the other mesa is detected. Both effects can be explained by charge-imbalance on the superconducting layers produced by the quasi-particle current, and can be described successfully by a recently developed theory of nonequilibrium effects in intrinsic Josephson systems.

cond-mat.supr-con↗

arXiv2001

Nonequilibrium effects due to charge fluctuations in intrinsic Josephson systems

Nonequilibrium effects in layered superconductors forming a stack of intrinsic Josephson junctions are investigated. We discuss two basic nonequilibrium effects caused by charge fluctuations on the superconducting layers: a) the shift of the chemical potential of the condensate and b) charge imbalance of quasi-particles, and study their influence on IV-curves and the position of Shapiro steps.

cond-mat.supr-con↗

arXiv2001

Dynamics and transformations of Josephson vortex lattice in layered superconductors

We consider dynamics of Josephson vortex lattice in layered superconductors with magnetic, charge (electrostatic) and charge-imbalance (quasiparticle) interactions between interlayer Josephson junctions taken into account. The macroscopic dynamical equations for interlayer Josephson phase differences, intralayer charge and electron-hole imbalance are obtained and used for numerical simulations. Different transformations of the vortex lattice structure are observed. It is shown that the additional dissipation due to the charge imbalance relaxation leads to the stability of triangular lattice.

cond-mat.supr-con↗