arXiv subjects

I. V. Zozoulenko

Publications and source records attributed to I. V. Zozoulenko.

At least 37 records · Page 2Linked to original sources

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

Edge disorder and localization regimes in bilayer graphene nanoribbons

A theoretical study of the magnetoelectronic properties of zigzag and armchair bilayer graphene nanoribbons (BGNs) is presented. Using the recursive Green's function method, we study the band structure of BGNs in uniform perpendicular magnetic fields and discuss the zero-temperature conductance for the corresponding clean systems. The conductance quantized as 2(n+1)G_ for the zigzag edges and nG_0 for the armchair edges with G_{0}=2e^2/h being the conductance unit and $n$ an integer. Special attention is paid to the effects of edge disorder. As in the case of monolayer graphene nanoribbons (GNR), a small degree of edge disorder is already sufficient to induce a transport gap around the neutrality point. We further perform comparative studies of the transport gap E_g and the localization length in bilayer and monolayer nanoribbons. While for the GNRs E_{g}^{GNR}is proportional to 1/W, the corresponding transport gap E_{g}^{BGN} for the bilayer ribbons shows a more rapid decrease as the ribbon width W is increased. We also demonstrate that the evolution of localization lengths with the Fermi energy shows two distinct regimes. Inside the transport gap, xi is essentially independent on energy and the states in the BGNs are significantly less localized than those in the corresponding GNRs. Outside the transport gap ξgrows rapidly as the Fermi energy increases and becomes very similar for BGNs and GNRs.

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.

cond-mat.mes-hall↗

arXiv2008

Magnetic barriers in graphene nanoribbons

A theoretical study of the transport properties of zigzag and armchair graphene nanoribbons with a magnetic barrier on top is presented. The magnetic barrier modifies the energy spectrum of the nanoribbons locally, which results in an energy shift of the conductance steps towards higher energies. The magnetic barrier also induces Fabry-Perot type oscillations, provided the edges of the barrier are sufficiently sharp. The lowest propagating state present in zigzag and metallic armchair nanoribbons prevent confinement of the charge carriers by the magnetic barrier. Disordered edges in nanoribbons tend to localize the lowest propagating state, which get delocalized in the magnetic barrier region. Thus, in sharp contrast to the case of two-dimensional graphene, the charge carriers in graphene nanoribbons cannot be confined by magnetic barriers. We also present a novel method based on the Green's function technique for the calculation of the magnetosubband structure, Bloch states and magnetoconductance of the graphene nanoribbons in a perpendicular magnetic field. Utilization of this method greatly facilitates the conductance calculations, because, in contrast to excising methods, the present method does not require self-consistent calculations for the surface Green's function.

cond-mat.mes-hall↗

arXiv2008

Magnetic barrier induced conductance fluctuations in quantum wires

Quasi-ballistic semiconductor quantum wires are exposed to localized perpendicular magnetic fields, also known as magnetic barriers. Pronounced, reproducible conductance fluctuations as a function of the magnetic barrier amplitude are observed. The fluctuations are strongly temperature dependent and remain visible up to temperatures of about 10 K. Simulations based on recursive Green functions suggest that the conductance fluctuations originate from parametric interferences of the electronic wave functions which experience scattering between the magnetic barrier and the electrostatic potential landscape.

cond-mat.mes-hall↗

arXiv2008

Edge disorder induced Anderson localization and conduction gap in graphene nanoribbons

We study the effect of the edge disorder on the conductance of the graphene nanoribbons (GNRs). We find that only very modest edge disorder is sufficient to induce the conduction energy gap in the otherwise metallic GNRs and to lift any difference in the conductance between nanoribbons of different edge geometry. We relate the formation of the conduction gap to the pronounced edge disorder induced Anderson-type localization which leads to the strongly enhanced density of states at the edges, formation of surface-like states and to blocking of conductive paths through the ribbons.

cond-mat.mes-hall↗

arXiv2008

Interacting electrons in the Aharonov-Bohm interferometer

We present a microscopic picture of quantum transport in the Aharonov-Bohm (AB) interferometer taking into account electron interaction within the Hartree and the spin density functional theory approximations. We discuss the structure of the edge states for different number of the Landau levels in the leads, their coupling to the states in the central island and the formation of compressible/incompressible strips in the interferometer. Based on our results we discuss the existing theories of the unexpected AB periodicity, which essentially rely on specific phenomenological models of the states and their coupling in the interferometer. Our work provides a basis for such the theories, giving a detailed microscopic description of the propagating states and the global electrostatics in the system at hand.

cond-mat.mes-hall↗

arXiv2008

Magnetoconductance of interacting electrons in quantum wires: Spin density functional theory study

We present systematic quantitative description of the magnetoconductance of the split-gate quantum wires. Accounting for the exchange and correlation interactions within the spin density function theory (DFT) leads to the lifting of the spin degeneracy and formation of the spin-resolved plateaus at odd values of $e^{2}/h$. We show that the width of the odd conductance steps in the spin DFT calculations is equal to the width of the transition intervals between the conductance steps for the spinless electrons in the Hartree approximation. A detailed analysis of the structure of compressible/incompressible strips and the evolution of the Hartree and the spin-DFT subband structure provides an explanation of this finding. Our spin-DFT calculations reproduce not only qualitatively, but rather quantitatively all the features in the magnetoconductance observed in the recent experiment (I. P. Radu, J. B. Miller, S. Amasha, E. Levenson-Falk, D. M. Zumbuhl, M. A. Kastner, C. M. Marcus, L. N. Pfeiffer, and K. W. West, to be published) including the unexpected effect of the collapse of the odd conductance plateaus at lower fields.

cond-mat.mes-hall↗

arXiv2007

Spin polarization in modulation-doped GaAs quantum wires

We study spin polarization in a split-gate quantum wire focussing on the effect of a realistic smooth potential due to remote donors. Electron interaction and spin effects are included within the density functional theory in the local spin density approximation. We find that depending on the electron density, the spin polarization exhibits qualitatively different features. For the case of relatively high electron density, when the Fermi energy $E_{F}$ exceeds a characteristic strength of a long-range impurity potential $V_{donors}$, the density spin polarization inside the wire is practically negligible and the wire conductance is spin-degenerate. When the density is decreased such that $E_{F}$ approaches $V_{donors}$, the electron density and conductance quickly become spin polarized. With further decrease of the density the electrons are trapped inside the lakes (droplets) formed by the impurity potential and the wire conductance approaches the pinch-off regime. We discuss the limitations of DFT-LSDA in this regime and compare the obtained results with available experimental data.

cond-mat.mes-hall↗

arXiv2007

Quantum wires in magnetic field: A comparative study of the Hartree-Fock and the spin density functional approaches

We present a detailed comparison of the self-consistent calculations based on the Hartree-Fock and the spin density functional theory for a spit-gate quantum wire in the IQH regime. We demonstrate that both approaches provide qualitatively (and in most cases quantitatively) similar results for the spin-resolved electron density, spin polarization, spatial spin separation at the edges and the effective $g$ factor. The both approach give the same values of the magnetic fields corresponding to the successive subband depopulation and qualitatively similar evolution of the magnetosubbands. Quantitatively, however, the HF and the DFT subbands are different (even though the corresponding total electron densities are practically the same). In contrast to the HF approach, the DFT calculations predict much larger spatial spin separation near the wire edge for the low magnetic fields (when the compressible strips for spinless electrons are not formed yet). In the opposite limit of the large fields, the Hatree-Fock and the DFT approaches give very similar values for the spatial spin separation.

cond-mat.mes-hall↗

arXiv2007

Effect of Electron Interaction on Statistics of Conductance Oscillations in Open Quantum Dots: Does the Dephasing Time Saturate?

We perform self-consistent quantum transport calculations in open quantum dots taking into account the effect of electron interaction. We demonstrative that in the regime of the ultralow temperatures $2πk_BT\lesssimΔ$ ($Δ$ being the mean level spacing), the electron interaction strongly affects the conductance oscillations and their statistics leading to a drastic deviation from the corresponding predictions for noninteracting electrons. In particular, it causes smearing of conductance oscillations, which is similar to the effect of temperature or inelastic scattering. For $2πk_BT\gtrsimΔ$ the influence of electron interaction on the conductance becomes strongly diminished. Our calculations (that are free from phenomenological parameters of the theory) are in good quantitative agreement with the observed ultralow temperature statistics (Huibers \textit{et al.}, Phys. Rev. Lett. \textbf{81}, 1917 (1998)). Our findings question a conventional interpretation of the ultralow temperature saturation of the coherence time in open dots which is based on the noninteracting theories where the electron interaction is neglected and the agrement with the experiment is achieved by introducing additional phenomenological channels of dephasing.

cond-mat.mes-hall↗

arXiv2007

Resonant reflection at magnetic barriers in quantum wires

The conductance of a quantum wire containing a single magnetic barrier is studied numerically by means of the recursive Greens function technique. For sufficiently strong and localized barriers, Fano - type reflection resonances are observed close to the pinch-off regime. They are attributed to a magnetoelectric vortex-type quasibound state inside the magnetic barrier that interferes with an extended mode outside. We furthermore show that disorder can substantially modify the residual conductance around the pinch-off regime.

cond-mat.mes-hall↗

arXiv2007

Conductance of a quantum point contact based on spin-density-functional theory

We present full quantum mechanical conductance calculations of a quantum point contact (QPC) performed in the framework of the density functional theory (DFT) in the local spin-density approximation (LDA). We show that a spin-degeneracy of the conductance channels is lifted and the total conductance exhibits a broad plateau-like feature at 0.5*2e^{2}/h. The lifting of the spin-degeneracy is a generic feature of all studied QPC structures (both very short and very long ones; with the lengths in the range 40<l<500 nm). The calculated conductance also shows a hysteresis for forward- and backward sweeps of the gate voltage. These features in the conductance can be traced to the formation of weakly coupled quasi-bound states (magnetic impurities) inside the QPC (also predicted in previous DFT-based studies). A comparison of obtained results with the experimental data shows however, that while the spin-DFT based "first-principle" calculations exhibits the spin polarization in the QPC, the calculated conductance clearly does not reproduce the 0.7 anomaly observed in almost all QPCs of various geometries. We critically examine major features of the standard DFT-based approach to the conductance calculations and argue that its inability to reproduce the 0.7 anomaly might be related to the infamous derivative discontinuity problem of the DFT leading to spurious self-interaction errors not corrected in the standard LDA. Our results indicate that the formation of the magnetic impurities in the QPC might be an artefact of the LDA when localization of charge is expected to occur. We thus argue that an accurate description of the QPC structure would require approaches that go beyond the standard DFT+LDA schemes.

cond-mat.mes-hall↗

arXiv2007

"0.7 anomaly" and magnetic impurity formation in quantum point contacts

The origin of the 0.7-conductance anomaly in quantum point contacts (QPCs) has been a subject of lively discussions since its discovery more than 10 years ago. In a recent letter, based on spin density functional theory (DFT), Rejec and Meir explained the origin of the 0.7-anomaly as being due to the formation of a magnetic impurity in the QPCs [T. Rejec and Y. Meir, Nature 442, 900 (2006)]. They did not, however, perform transport conductance calculations so the central question whether the 0.7-anomaly is indeed related to the formation of a magnetic impurity has remained unanswered. In this communication, we perform spin DFT transport calculations for the structures considered by Rejec and Meir. While we recover the findings reported by Rejec and Meir concerning the formation of localized spin-degenerate quasi-bound states, our transport calculations do not contain the 0.7 anomaly in the conductance. We suggest that the inability of the DFT to reproduce quantitatively the 0.7 anomaly may be due to the uncorrected self-interaction errors in the DFT calculation of electronic transport for the case when localization of charge is expected to occur, so that the magnetic impurity formation may be an artefact of DFT due to the spurious self-interaction.

cond-mat.mes-hall↗

arXiv2007

Many-body effects in transport through open systems: pinning of resonant levels

The role of electron-electron interaction in transport properties of open quantum dots is studied. The self-consistent full quantum mechanical magnetotransport calculations within the Hartree, Density Functional Theory and Thomas-Fermi approximations were performed where a whole device, including the semi-infinitive leads, is treated on the same footing (i.e. the electron-electron interaction is accounted for both in the leads as well as in the dot region). The main finding of the present paper is the effect of pinning of the resonant levels to the Fermi energy due to the enhanced screening. Our results represent a significant departure from a conventional picture where a variation of external parameters (such as a gate voltage, magnetic field, etc.) causes the successive dot states to sweep past the Fermi level in a linear fashion. We instead demonstrate highly nonlinear behavior of the resonant levels in the vicinity of the Fermi energy. The pinning of the resonant levels in open quantum dots leads to the broadening of the conduction oscillations in comparison to the one electron picture. The effect of pinning becomes much more pronounced in the presence of the perpendicular magnetic field. This can be attributed to the enhanced screening efficiency because of the increased localization of the wave function. The strong pinning of the resonant energy levels in the presence of magnetic field can have a profound effect on transport properties of various devices operating in the edge state transport regime. We also critically examine an approximation often used in transport calculations where an inherently open system is replaced by a corresponding closed one.

cond-mat.mes-hall↗

arXiv2006

Hysteresis and spin phase transitions in quantum wires in the integer quantum Hall regime

We demonstrate that a split-gate quantum wire in the integer quantum Hall regime can exhibit electronic transport hysteresis for up- and down-sweeps of a magnetic field. This behavior is shown to be due to phase spin transitions between two different ground states with and without spatial spin polarization in the vicinity of the wire boundary. The observed effect has a many-body origin arising from an interplay between a confining potential, Coulomb interactions and the exchange interaction. We also demonstrate and explain why the hysteretic behavior is absent for steep and smooth confining potentials and is present only for a limited range of intermediate confinement slopes.

cond-mat.mes-hall↗

arXiv2006

Light propagation in nanorod arrays

We study propagation of TM- and TE-polarized light in two-dimensional arrays of silver nanorods of various diameters in a gelatin background. We calculate the transmittance, reflectance and absorption of arranged and disordered nanorod arrays and compare the exact numerical results with the predictions of the Maxwell-Garnett effective-medium theory. We show that interactions between nanorods, multipole contributions and formations of photonic gaps affect strongly the transmittance spectra that cannot be accounted for in terms of the conventional effective-medium theory. We also demonstrate and explain the degradation of the transmittance in arrays with randomly located rods as well as weak influence of their fluctuating diameter. For TM modes we outline the importance of skin-effect, which causes the full reflection of the incoming light. We then illustrate the possibility of using periodic arrays of nanorods as high-quality polarizers.

physics.optics↗