arXiv subjects

S. Ihnatsenka

Publications and source records attributed to S. Ihnatsenka.

At least 19 recordsLinked to original sources

arXiv2023

Edge channels in a graphene Fabry-Perot interferometer

Quantum-mechanical calculations of electron magnetotransport in graphene Fabry-Pérot interferometers are presented with a focus on the role of spatial structure of edge channels. For an interferometer that is made by removing carbon atoms, which is typically realized in nanolithography experiments, the constrictions are shown to cause strong inter-channel scattering that establishes local equilibrium and makes the electron transport non-adiabatic. Nevertheless, two-terminal conductance reveals a common Aharonov-Bohm oscillation pattern, independent of crystallographic orientation, which is accompanied by single-particle states that sweep through the Fermi energy for the edge channels circulating along the physical boundary of the device. The interferometer constrictions host the localized states that might shorten the device or disrupt the oscillation pattern. For an interferometer that is created by electrostatic confinement, which is typically done in the split-gate experiments, electron transport is shown to be adiabatic if the staggered potential is introduced additionally into the model. Interference visibility decays exponentially with temperature with a weaker dependence at low temperature.

cond-mat.mes-hall↗

arXiv2022

Fabry-Perot and Aharonov-Bohm interference in ideal graphene nanoribbons

Quantum-mechanical calculations of electron magneto-transport in ideal graphene nanoribbons are presented. In noninteracting theory, it is predicted that an ideal ribbon that is attached to wide leads should reveal Fabry-Perot conductance oscillations in magnetic field. In the theory with Coulomb interaction taken into account, the oscillation pattern should rather be determined by the Aharonov-Bohm interference effect. Both of these theories predict the formation of quasi-bound states, albeit of different structures, inside the ribbon because of strong electron scattering on the interfaces between the connecting ribbon and the leads. Conductance oscillations are a result of resonant backscattering via these quasi-bound states.

cond-mat.mes-hall↗

arXiv2021

Model of the thermoelectric properties of anisotropic organic semiconductors

A model of charge hopping transport that accounts for anisotropy of localized states and Coulomb interaction between charges is proposed. For the anisotropic localized states the degree of orientation relates exponentially to the ratio of conductivities in parallel and perpendicular directions, while the ratio of Seebeck coefficients stays nearly unaffected. However, the ratio of Seebeck coefficients increases if Coulomb interaction is screened stronger in a direction parallel to the predominant orientation of the localized states. This implies two different physical mechanisms responsible for the anisotropy of thermoelectric properties in the hopping regime: electronic state localization for conductivities, and screening for Seebeck coefficients. This provides explanation for recent experimental findings on tensile drawn and ribbed polymer films.

cond-mat.mes-hall↗

arXiv2016

Activated hopping transport in anisotropic systems at low temperatures

Numerical calculations of anisotropic hopping transport based on the resistor network model are presented. Conductivity is shown to follow the stretched exponential dependence on temperature with exponents changing from 1/4 to 1 as the wave functions become anisotropic and their localization length in the direction of charge transport decreases. For sufficiently strong anisotropy, this results in nearest-neighbor hopping at low temperatures due to the formation of a single conduction path, which adjusts in the planes where the wave functions overlap strongly. In the perpendicular direction, charge transport follows variable-range hopping, a behavior that agrees with experimental data on organic semiconductors.

cond-mat.mes-hall↗

arXiv2013

Electron interaction, charging and screening in grain boundaries in graphene

Electronic, transport, and spin properties of grain boundaries (GBs) are investigated in electrostatically doped graphene at finite electron densities within the Hartree and Hubbard approximations. We demonstrate that depending on the character of the GBs, the states residing on them can have a metallic character with a zero group velocity or can be fully populated losing the ability to carry a current. These states show qualitatively different features in charge accumulation and spin polarization. We also demonstrate that the semiclassical Thomas-Fermi approach provides a satisfactory approximation to the calculated self-consistent potential. The conductance of GBs is reduced due to enhanced backscattering from this potential.

cond-mat.mes-hall↗

arXiv2013

Effect of edge reconstruction and electron-electron interactions on quantum transport in graphene nanoribbons

We present numerical studies of conduction in graphene nanoribbons with reconstructed edges based on the standard tight-binding model of the graphene and the extended Huckel model of the reconstructed defects. We performed atomic geometry relaxation of individual defects using density functional theory and then explicitly calculated the tight-binding parameters used to model electron transport in graphene with reconstructed edges. The calculated conductances reveal strong backscattering and electron-hole asymmetry depending on the edge and defect type. This is related to an additional defect-induced band whose wave function is poorly matched to the propagating states of the pristine ribbon. We find a transport gap to open near the Dirac point and to scale inversely with the ribbon width, similarly to what has been observed in experiments. We predict the largest transport gap to occur for armchair edges with Stone-Wales defects, while heptagon and pentagon defects cause about equal backscattering for electrons and holes, respectively. Choosing the heptagon defect as an example, we show that although electron interactions in the Hartree approximation cause accumulation of charge carriers on the defects, surprisingly, their effect on transport is to reduce carrier backscattering by the defects and thus to enhance the conductance.

cond-mat.mes-hall↗

arXiv2012

Spin polarization and g-factor enhancement in graphene nanoribbons in magnetic field

We provide a systematic quantitative description of spin polarization in armchair and zigzag graphene nanoribbons in a perpendicular magnetic field. We first address spinless electrons within the Hartree approximation studying the evolution of the magnetoband structure and formation of the compressible strips. We discuss the potential profile and the density distribution near the edges and the difference and similarities between armchair and zigzag edges. Accounting for the Zeeman interaction and describing the spin effects via the Hubbard term we study the spin-resolved subband structure and relate the spin polarization of the system at hand to the formation of the compressible strips for the case of spinless electrons. At high magnetic field the calculated effective g-factor varies around a value of ~2.25 for armchair nanoribbons and ~3 for zigzag nanoribbons. An important finding is that in zigzag nanoribbons the zero-energy mode remains pinned to the Fermi-energy and becomes fully spin-polarized for all magnetic fields, which, in turn, leads to a strong spin polarization of the electron density near the zigzag edge.

cond-mat.mes-hall↗

arXiv2012

Computation of electron quantum transport in graphene nanoribbons using GPU

The performance potential for simulating quantum electron transport on graphical processing units (GPUs) is studied. Using graphene ribbons of realistic sizes as an example it is shown that GPUs provide significant speed-ups in comparison to central processing units as the transverse dimension of the ribbon grows. The recursive Green's function algorithm is employed and implementation details on GPUs are discussed. Calculated conductances were found to accumulate significant numerical error due to single-precision floating-point arithmetic at energies close to the charge neutrality point of the graphene.

physics.comp-ph↗

arXiv2012

Self-consistent Hartree theory of the role of electron-electron interactions in the electronic structure and conductance quantization of graphene nanoconstrictions

We present self-consistent calculations of electron transport in graphene nanoconstrictions within the Hartree approximation. We consider suspended armchair ribbons with V-shaped constrictions having perfect armchair or zigzag edges as well as mesoscopically smooth but atomically stepped constrictions with cosine profiles. Our calculations are based on a tight-binding model of the graphene and account for electron-electron interactions in both the constriction and the semi-infinite leads explicitly. We find that electron interactions result in (i) Electrons accumulating along edges of the uniform ribbon and along zigzag and cosine constriction edges but not along armchair constriction edges. (ii) The first subband showing almost perfect transmittance due to localization at the uniform graphene boundary except at low energies for the cases of zigzag and cosine constrictions where Bloch stop-bands form in related periodic structures. (iii) The second subband being almost perfectly blocked by the constriction. (iv) Electron interactions favor intra-subband scattering while the non-interacting electron theory predicts predominance of inter-subband scattering. (v) Conductance quantization for the first few conductance steps being more pronounced for armchair constrictions but less so for zigzag constrictions. (vi) A much more prominent 2e^2/h conductance plateau for the cosine constriction than is found in the absence of electron interactions. Possible implications for recent experiments are briefly discussed.

cond-mat.mes-hall↗

arXiv2012

Conductance quantization in graphene nanoconstrictions with mesoscopically smooth but atomically stepped boundaries

We present the results of million atom electronic quantum transport calculations for graphene nanoconstrictions with edges that are smooth apart from atomic scale steps. We find conductances quantized in integer multiples of 2e2/h and a plateau at ~0.5*2e2/h as in recent experiments [Tombros et al., Nature Physics 7, 697 (2011)]. We demonstrate that, surprisingly, conductances quantized in integer multiples of 2e2/h occur even for strongly non-adiabatic electron backscattering at the stepped edges that lowers the conductance by one or more conductance quanta below the adiabatic value. We also show that conductance plateaus near 0.5*2e2/h can occur as a result of electron backscattering at stepped edges even in the absence of electron-electron interactions.

cond-mat.mes-hall↗

arXiv2011

First-principles study of electron transport in few-electron open quantum dots by the Hartree-Fock approach

Electron transport properties of few-electron open quantum dots within the spin-restricted Hartree-Fock approximation are studied. The self-consistent numerical calculations were performed for a whole device, including the semi-infinite leads, without employing any phenomenological or adjustable parameters. Inclusion of the non-local Fock potential brings qualitatively new physics in comparison to the Hartree approach: electron screening decreases, resonant energy levels become less pinned to the Fermi energy and clearly correlate with conductance peaks. When coupling between the dot and leads decreases the number of electrons inside the dot becomes quantized and the model predicts the Coulomb blockade of electron transport. This is confirmed by comparison with the master equation approach for an equivalent quantum dot.

cond-mat.mes-hall↗

arXiv2011

Dirac Point Resonances, Transport Gaps and Conductance Quantization in Graphene Nanoribbons with Adsorbed Atoms and Molecules

We present calculations of electronic quantum transport in graphene nanoribbons with adsorbed H, F, OH and O, based on a tight binding model derived from extended Huckel theory. The relaxed atomic geometries of the adsorbates and graphene are calculated using density functional theory. Our model includes the effects of the local rehybridization of the graphene from the sp2 to sp3 electronic structure that occurs when H, F, OH or O bonds covalently to the graphene. It captures the physics of the scattering resonances that are induced in the graphene near the Dirac point by the presence of these adsorbates. We find these Dirac point resonances to play a dominant role in quantum transport in ribbons with these adsorbates: Even at low adsorbate concentrations the conductance of the ribbon is strongly suppressed and a transport gap develops for electron Fermi energies near the resonance. The transport gap is centered very near the Dirac point energy of for H, below it for F and OH and above it for O. We predict ribbons with these adsorbed species under appropriate conditions to exhibit quantized conductance steps of equal height similar to those that have been observed by Lin et al. [Phys. Rev. B 78, 161409 (2008)] at moderately low temperatures, even for ribbons with conductances a few orders of magnitude smaller than 2e2/h.

cond-mat.mes-hall↗

arXiv2011

Dirac point resonances due to atoms and molecules adsorbed on graphene and transport gaps and conductance quantization in graphene nanoribbons with covalently bonded adsorbates

We present a tight binding theory of the Dirac point resonances due to adsorbed atoms and molecules on an infinite 2D graphene sheet based on the standard tight binding model of the graphene p-band electronic structure and the extended Huckel model of the adsorbate and nearby graphene carbon atoms. The relaxed atomic geometries of the adsorbates and graphene are calculated using density functional theory. Our model includes the effects of the local rehybridization of the graphene from the sp^2 to sp^3 electronic structure that occurs when adsorbed atoms or molecules bond covalently to the graphene. Unlike in previous tight-binding models of Dirac point resonances, adsorbed species with multiple extended molecular orbitals and bonding to more than one graphene carbon atom are treated. More accurate and more general analytic expressions for the Green's function matrix elements that enter the T-matrix theory of Dirac point resonances than have been available previously are obtained. We study H, F, OH and O adsorbates on graphene and for each we find a strong scattering resonance (two resonances for O) near the Dirac point of graphene, by far the strongest and closest to the Dirac point being the resonance for H. We extract a minimal set of tight binding parameters that can be used to model resonant electron scattering and electron transport in graphene and graphene nanostructures with adsorbed H, F, OH and O accurately and efficiently. We also compare our results for the properties of Dirac point resonances due to adsorbates on graphene with those obtained by others using density functional theory-based electronic structure calculations, and discuss their relative merits. We then present calculations of electronic quantum transport in graphene nanoribbons with these adsorbed species...

cond-mat.mes-hall↗

arXiv2011

Nonlinear conductance quantization in graphene ribbons

We present numerical studies of non-linear conduction in graphene nanoribbons when a bias potential is applied between the source and drain electrodes. We find that the conductance quantization plateaus show asymmetry between the electron and hole branches if the potential in the ribbon equals the source or drain electrode potential and strong electron (hole) scattering occurs. The scattering may be at the ends of a uniform ballistic ribbon connecting wider regions of graphene or may be due to defects in the ribbon. We argue that, in ribbons with strong defect scattering, the ribbon potential is pinned to that of the drain (source) for electron (hole) transport. In this case symmetry between electron and hole transport is restored and our calculations explain the upward shift of the conductance plateaus with increasing bias that was observed experimentally by Lin et al. [Phys. Rev. B 78, 161409 (2008)].

cond-mat.mes-hall↗

arXiv2009

Origin of conductance quantization in disordered graphene ribbons

We present numerical studies of conduction in graphene nanoribbons with different types of disorder. We find that even when defect scattering depresses the conductance to values two orders of magnitude lower than 2e^2/h, equally spaced conductance plateaus occur at moderately low temperatures due to enhanced electron backscattering near subband edge energies if bulk vacancies are present in the ribbon. This work accounts quantitatively for the surprising conductance quantization observed by Lin et al. [Phys. Rev. B 78, 161409 (2008)] in ribbons with such low conductances.

cond-mat.mes-hall↗

arXiv2009

Band-gap engineering and ballistic transport in corrugated graphene nanoribbons

We calculate the band structure and the conductance of periodic corrugated graphene nanoribbons within the framework of the tight-binding $p$-orbital model. We consider corrugated structures based on host ribbons with armchair and zigzag edges and three different types of corrugations (armchair edges, zigzag edges as well as a rectangular corrugation). We demonstrate that for armchair host ribbons, depending on the type of corrugation, a band gap or low-velocity minibands appear near the charge neutrality point. For higher energies the allowed Bloch state bands become separated by mini-stopbands. By contrast, for corrugated ribbons with the zigzag host, the corrugations introduce neither band gaps nor stopbands (except for the case of the rectangular corrugations). The conductances of finite corrugated ribbons are analyzed on the basis of the corresponding band structures. For a sufficiently large number of corrugations the conductance follows the number of the corresponding propagating Bloch states and shows pronounced oscillations due to the Fabry-Perot interference within the corrugated segments. Finally we demonstrate that edge disorder strongly affects the conductances of corrugated ribbons. Our results indicate that observation of miniband formation in corrugated ribbons would require clean, edge-disorder free samples, especially for the case of the armchair host lattice.

cond-mat.mes-hall↗

arXiv2009

Electron-electron interactions in antidot-based Aharonov-Bohm interferometers

We present a microscopic picture of quantum transport in quantum antidots in the quantum Hall regime taking electron interactions into account. We discuss the edge state structure, energy level evolution, charge quantization and linear-response conductance as the magnetic field or gate voltage is varied. Particular attention is given to the conductance oscillations due to Aharonov-Bohm interference and their unexpected periodicity. To explain the latter we propose the mechanisms of scattering by point defects and Coulomb blockade tunneling. They are supported by self-consistent calculations in the Hartree approximation, which indicate pinning and correlation of the single-particle states at the Fermi energy as well as charge oscillation when antidot-bound states depopulate. We have also found interesting phenomena of anti-resonance reflection of the Fano type.

cond-mat.mes-hall↗

arXiv2009

Origin of the 0.25-anomaly in the nonlinear conductance of a quantum point contact

We calculate the non-linear conductance of a quantum point contact using the non-equilibrium Greens function technique within the Hartree approximation of spinless electrons. We quantitative reproduce the 0.25-anomaly in the differential conductance (i.e. the lowest plateau at 0.25-0.3*2e^2/h) as well as an upward bending of higher conductance half-integer plateaus seen in the experiments, and relate these features to the non-linear screening and pinning effects.

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