Marian Nita - Academia.edu (original) (raw)

Papers by Marian Nita

Physical Review B, 2003

We calculate the orbital magnetization of single and double quantum dots coupled both by Coulomb ... more We calculate the orbital magnetization of single and double quantum dots coupled both by Coulomb interaction and by electron tunneling. The electronic states of the quantum dots are calculated in a tight-binding model and the magnetization is discussed in relation to the energy spectrum and to the edge and bulk states. We identify effects of chirality of the electronic orbits and of the anti-crossing of the energy levels when the magnetic field is varied. We also consider the effects of detuning the energy spectra of the quantum dots by an external gate potential. We compare our results with the recent experiments of Oosterkamp et al.

Physical Review E, 2012

The phase of the electronic wave function is not directly measurable but, quite remarkably, it be... more The phase of the electronic wave function is not directly measurable but, quite remarkably, it becomes accessible in pairs of isospectral shapes, as recently proposed in the experiment of Christopher R. Moon et al., Science 319, 782 (2008). The method is based on a special property, called transplantation, which relates the eigenfunctions of the isospectral pairs, and allows to extract the phase distributions, if the amplitude distributions are known. We numerically simulate such a phase extraction procedure in the presence of disorder, which is introduced both as Anderson disorder and as roughness at edges. With disorder, the transplantation can no longer lead to a perfect fit of the wave functions, however we show that a phase can still be extracted-defined as the phase that minimizes the misfit. Interestingly, this extracted phase coincides with (or differs negligibly from) the phase of the disorder-free system, up to a certain disorder amplitude, and a misfit of the wave functions as high as ∼ 5%, proving a robustness of the phase extraction method against disorder. However, if the disorder is increased further, the extracted phase shows a puzzle structure, no longer correlated with the phase of the disorder-free system. A discrete model is used, which is the natural approach for disorder analysis. We provide a proof that discretization preserves isospectrality and the transplantation can be adapted to the discrete systems.

Physical Review B, 2007

We consider the superfluid-insulator transition for cold bosons under an effective magnetic field... more We consider the superfluid-insulator transition for cold bosons under an effective magnetic field. We investigate how the applied magnetic field affects the Mott transition within mean-field theory and find that the critical hopping strength ͑t / U͒ c increases with the applied field. The increase in the critical hopping follows the bandwidth of the Hofstadter butterfly at the given value of the magnetic field. We also calculate the magnetization and superfluid density within mean-field theory.

Physica B: Condensed Matter, 1998

We calculate the quantum states corresponding to the drifting and channeled classical orbits in a... more We calculate the quantum states corresponding to the drifting and channeled classical orbits in a two-dimensional electron gas (2DEG) with strong magnetic and electric modulations along one spatial direction, x. The channeled states carry high, concentrated currents along the y axis, and are confined in an effective potential well. The quantum and the classical states are compared.

Journal of Physics: Conference Series, 2012

When subjected to a linearly polarized terahertz pulse, a mesoscopic ring endowed with spin-orbit... more When subjected to a linearly polarized terahertz pulse, a mesoscopic ring endowed with spin-orbit interaction (SOI) of the Rashba-Dresselhaus type exhibits non-uniform azimuthal charge and spin distributions. Both types of SOI couplings are considered linear in the electron momentum. Our results are obtained within a formalism based on the equation of motion satisfied by the density operator which is solved numerically for different values of the angle φ, the angle determining the polarization direction of the laser pulse. Solutions thus obtained are later employed in determining the time-dependent charge and spin currents, whose values are calculated in the stationary limit. Both these currents exhibit an oscillatory behavior complicated in the case of the spin current by a beating pattern. We explain this occurrence on account of the two spin-orbit interactions which force the electron spin to oscillate between the two spin quantization axes corresponding to Rashba and Dresselhaus interactions. The oscillation frequencies are explained using the single particle spectrum.

Spin magnetization of a strongly correlated electron gas confined in a two-dimensional finite lattice

Physical Review B, 2004

Physica E: Low-dimensional Systems and Nanostructures, 2005

Scientific Reports, Jan 10, 2017

We study Coulomb interacting electrons confined in polygonal quantum rings. We focus on the inter... more We study Coulomb interacting electrons confined in polygonal quantum rings. We focus on the interplay of localization at the polygon corners and Coulomb repulsion. Remarkably, the Coulomb repulsion allows the formation of in-gap states, i.e., corner-localized states of electron pairs or clusters shifted to energies that were forbidden for non-interacting electrons, but below the energies of corner-sidelocalized states. We specify conditions allowing optical excitation to those states. Core-shell quantum wires are vertically grown nanoscale structures consisting of a core which is covered by at least one layer of a different material (shell). Recently these structures attracted considerable attention as building blocks of quantum nanodevices 1-10. A characteristic feature of core-shell systems is a non-uniform carrier distribution in different parts of the wire 11-17. It is a consequence of the polygonal cross section which most commonly is hexagonal 18-20 , but may also be triangular 21-26 , square 27 , or dodecagonal 28. Some of the properties of those wires, such as the band alignment 29,30 , may be controlled to a high extent. An appropriate combination of sample size and shell thickness allows to induce electron concentration on the shell area 18. Moreover, the present technology allows for etching out the core part and producing nanotubes 19,20. Both nanowires and nanotubes may be viewed as polygonal quantum rings if they are sufficiently short, i.e., shorter than the electron wavelength in the growth direction. In this geometry the single-particle states with the lowest energy are localized in the corners of the polygon and are separated by a gap from the states localized on the sides. The gap can be of tens of meV or larger, depending on the shape of the polygon 31,32. The single-particle energy levels of a polygonal quantum ring are two-and fourfold degenerate and their arrangement is determined only by the number of polygon vertices. Similarly to the case of bent parts of quantum wires 33-40 , in the corner areas of polygonal quantum rings effective quantum wells are formed and thus low energy levels localize between internal and external boundaries. The number of such corner states is the number of vertices times two spin orientations. An energy gap may separate single-particle corner states from higher-energy states, the latter being distributed over the polygon sides 31,41,42. In this paper we extend the single-particle model of refs 31 and 32 to systems of few Coulomb interacting electrons. We show how this coupling allows the formation of states corresponding to electron pairs, or larger clusters, that localize on the corners and whose energies lie in the gap between corner and corner-side states of the uncoupled system. We focus on the formation and excitation of those many-body in-gap states, with particular emphasis on their fingerprints in optical absorption. As general motivations to study in-gap states in polygonal rings we mention their potential application in quantum information devices, exploiting the corner occupation as information unit, or their use as quantum simulators of discrete lattice models 43 .

We demonstrate the theoretical possibility of obtaining a pure spin current in a 1D ring with spi... more We demonstrate the theoretical possibility of obtaining a pure spin current in a 1D ring with spin-orbit interaction by irradiation with a non-adiabatic, two-component terahertz laser pulse, whose spatial asymmetry is reflected by an internal dephasing angle phi\phiphi. The stationary solutions of the equation of motion for the density operator are obtained for a spin-orbit coupling linear in the electron momentum (Rashba) and used to calculate the time-dependent charge and spin currents. We find that there are critical values of phi\phiphi at which the charge current disappears, while the spin current reaches a maximum or a minimum value.

Phys Rev B, 2001

We investigate the transport properties of quantum dots placed in strong magnetic field using a q... more We investigate the transport properties of quantum dots placed in strong magnetic field using a quantum-mechanical approach based on the 2D tightbinding Hamiltonian with direct Coulomb interaction and the Landauer-Büttiker (LB) formalism. The electronic transmittance and the Hall resistance show Coulomb oscillations and also prove multiple addition processes. We identify this feature as the 'bunching' of electrons observed in recent experiments and give an elementary explanation in terms of spectral characteristics of the dot. The spatial distribution of the added electrons may distinguish between edge and bulk states and it has specific features for bunched electrons. The dependence of the charging energy on the number of electrons is discussed both for strong and vanishing magnetic field. The crossover from the tunneling to quantum Hall regime is analyzed in terms of dot-lead coupling.

Persistent, oscillatory charge and spin currents are shown to be driven by a two-component terahe... more Persistent, oscillatory charge and spin currents are shown to be driven by a two-component terahertz laser pulse in a one-dimensional mesoscopic ring with Rashba-Dresselhaus spin orbit interactions (SOI) linear in the electron momentum. The characteristic interference effects result from the opposite precession directions imposed on the electron spin by the two SOI couplings. The time dependence of the currents is obtained by solving numerically the equation of motion for the density operator, which is later employed in calculating statistical averages of quantum operators on few electron eigenstates. The parameterization of the problem is done in terms of the SOI coupling constants and of the phase difference between the two laser components. Our results indicate that the amplitude of the oscillations is controlled by the relative strength of the two SOI's, while their frequency is determined by the difference between the excitation energies of the electron states. Furthermore, the oscillations of the spin current acquire a beating pattern of higher frequency that we associate with the nutation of the electron spin between the quantization axes of the two SOI couplings. This phenomenon disappears at equal SOI strengths, whereby the opposite precessions occur with the same probability.

Journal of Physics Condensed Matter, May 8, 2006

We show that due to the Landau band mixing the eigenstate localization within the disordered band... more We show that due to the Landau band mixing the eigenstate localization within the disordered bands get an asymmetric structure: the degree of localization increases in the lower part of the band and decreases in the upper one. The calculation is performed for a 2D lattice with the Anderson disorder potential and we prove that this effect is related to the upper shift of the extended states within the band and is enhanced by the disorder strength. The asymmetric localization and the energy shift dissapear when the interband coupling is switched off.

Physica Status Solidi B-basic Solid State Physics, 1998

The transport properties of a rectangular mesoscopic plaquette in the presence of a perpendicular... more The transport properties of a rectangular mesoscopic plaquette in the presence of a perpendicular magnetic field are studied in a tight-binding model with randomly distributed traps. The longitudinal and Hall resistances are calculted in the four-probe Landauer-B\"{u}ttiker formalism which accounts automatically both for the quantum coherence and the trapping-induced localization. The localized character of eigenvectors and the specific aspect of

Physica E-low-dimensional Systems & Nanostructures, 2010

We address the quantum dot phase measurement problem in an open Aharonov–Bohm interferometer, ass... more We address the quantum dot phase measurement problem in an open Aharonov–Bohm interferometer, assuming multiple transport channels. In such a case, the quantum dot is characterized by more than one intrinsic phase for the electrons transmission. It is shown that the phase which would be extracted by the usual experimental method (i.e. by monitoring the shift of the Aharonov–Bohm oscillations,

The spectrum and the eigenstates of a finite 2D tight-binding electronic system, with Dirichlet b... more The spectrum and the eigenstates of a finite 2D tight-binding electronic system, with Dirichlet boundary conditions, in magnetic field and external linear potential are studied. The eigenstates show an equipotential character and may cross the plaquette in the direction perpendicular to the electric field. When leads are added to the plaquette, the channels carrying the current may be shortcut by equipotentials, resulting in additional plateaus situated inbetween the usual IQHE plateaus. This idea is confirmed by a numerical calculation within the four-terminal Landauer-Büttiker approach.