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In this paper a comparative study of the electronic and magnetic properties of quasi-two-dimensional electrons in an artificial graphene-like superlattice composed of circular and elliptical quantum dots is presented. A complete orthonormal set of basis wave functions, which has previously been constructed in the frame of the Coulomb gauge for the vector potential has been implemented for calculation of the energy dispersions, the Hofstadter spectra, the density of states and the orbital magnetization of the considered systems, taking into account both the translational symmetry of the superlattice and the wave function phase-shifts due to the presence of a transverse external magnetic field. Our calculations indicate a topological change in the miniband structure due to the ellipticity of the quantum dots, and non-trivial modifications of the electron energy dispersion surfaces in reciprocal space with the change of the number of magnetic flux quanta through the unit cell of the superlattice. The ellipticity of the QDs leads to an opening of a gap and considerable modifications of the Hofstadter spectrum. The orbital magnetization is shown to reveal significant oscillations with the change of the magnetic flux. The deviation from the circular geometry of quantum dots has a qualitative impact on the dependencies of the magnetization on both the magnetic flux and the temperature.

We apply quantum electrodynamical density functional theory to obtain the electronic density, the spin polarization, as well as the orbital and the spin magnetization of square periodic arrays of quantum dots or antidots subjected to the influence of a far-infrared cavity photon field. A gradient-based exchange-correlation functional adapted to a two-dimensional electron gas in a transverse homogeneous magnetic field is used in the theoretical framework and calculations. The obtained results predict a non-trivial effect of the cavity field on the electron distribution in the unit cell of the superlattice, as well as on the orbital and the spin magnetization. The number of electrons per unit cell of the superlattice is shown to play a crucial role in the modification of the magnetization via the electron-photon coupling. The calculations show that cavity photons strengthen the diamagnetic effect in the quantum dots structure, while they weaken the paramagnetic effect in an antidot structure. As the number of electrons per unit cell of the lattice increases the electron-photon interaction reduces the exchange forces that would otherwise promote strong spin splitting for both the dot and the antidot array.

We use a recently proposed quantum electrodynamical density functional theory (QEDFT) functional in a real-time excitation calculation for a two-dimensional electron gas in a square array of quantum dots in an external constant perpendicular magnetic field to model the influence of cavity photons on the excitation spectra of the system. The excitation is generated by a short elecrical pulse. The quantum dot array is defined in an AlGaAs-GaAs heterostructure, which is in turn embedded in a parallel plate far-infrared photon-microcavity. The required exchange and correlation energy functionals describing the electron-electron and electron-photon interactions have therefore been adapted for a two-dimensional electron gas in a homogeneous external magnetic field. We predict that the energies of the excitation modes activated by the pulse are generally red-shifted to lower values in the presence of a cavity. The red-shift can be understood in terms of the polarization of the electron charge by the cavity photons and depends on the magnetic flux, the number of electrons in a unit cell of the lattice, and the electron-photon interaction strength. We find an interesting interplay of the exchange forces in a spin polarized two-dimensional electron gas and the square lattice structure leading to a small but clear blue-shift of the excitation mode spectra when one electron resides in each dot., RevTeX - pdfLaTeX, 11 pages with 8 included pdf figures

The electronic, thermal, magnetic and optical properties of BN-codoped strontium oxide (SrO) monolayers are studied taking into account the interaction effects between the B and the N dopant atoms. The indirect band gap of a pure two dimensional SrO is modified to a narrow direct band gap by tuning the B-N attractive interaction. The B or N separately doped SrO leads to a metallic behavior, while a BN-codoped SrO has a semiconductor character. The strong B-N attractive interaction changes a non-magnetic SrO to a magnetic system and reduces its heat capacity. An ab initio molecular dynamics, AIMD, calculations are also utilized to check the thermodynamic stability of the pure and BN-codoped SrO monolayers. The band gap reduction of SrO increases the optical conductivity shifting the most intense peak from the Deep-UV to the visible light region. The red shifted optical conductivity emerges due to the B-N attractive interaction. In addition, both iso- and anisotropic characters are seen in the optical properties depending on the strength of the B-N attractive interaction. It can thus be confirmed that the interaction effects of the BN-codopants can be used to control the properties of SrO monolayers for thermo- and opto-electronic devices., Comment: RevTeX - pdfLaTeX, 9 pages with 10 included pdf figures

A conversion of AA- to AB-stacking bilayer graphene (BLG) due to interlayer interaction is demonstrated. Two types of interlayer interactions, an attractive and a repulsive, between the Boron and Nitrogen dopant atoms in BLG are found. In the presence of the attractive interaction, an AA-stacking of BN-codoped BLG is formed with a less stable structure leading to weak mechanical properties of the system. Low values of the Young modulus, the ultimate strength and stress, and the fracture strength are observed comparing to a pure BLG. In addition, the attractive interaction induces a small bandgap that deteriorates the thermal and optical properties of the system. In contrast, in the presence of a repulsive interaction between the B and N atoms, the AA-stacking is converted to a AB-stacking with a more stable structure. Improved mechanical properties such as higher Young modulus, the ultimate strength and stress, fracture strength are obtained comparing to the AA-stacked BN-codoped BLG. Furthermore, a larger bandgap of the AB-stacked bilayer enhances the thermal and the optical characteristics of the system., RevTeX, 10 pages with 8 included figures

We theoretically study the electronic and the thermal characteristics of Tantalum, Ta, doped Molybdenum disulfide, MoS$_2$, using density functional theory. It has been shown that the MoS$_2$ monolayer is not a good material for thermoelectric devices due to its relatively large band gap. We find that a Ta doped MoS$_2$ forming a TaMoS$_2$ monolayer can be useful for thermoelectric devices. The particular attention of this work is paid to the interaction effect between the Ta atoms in the MoS$_2$ structure. We find that the interaction type is repulsive. It introduces an asymmetry in the density of states, DOS, reducing the band gap. In the presence of a strong repulsive interaction of Ta-Ta atoms, new states in the DOS around the Fermi energy are found leading to a reduction of the band gap. Consequently, a high Seebeck coefficient and figure of merit are seen over a wide range of energy around the Fermi energy. In contrast, a small reduction of the band gap and a vanishing degeneracy of the valence and the conduction bands are observed for the case of a weak Ta-Ta repulsive interaction leading to less promising thermoelectric properties., Comment: 7 pages, 7 figure

Physical quilities such as electronic, mechanical, thermal, and optical properties of Al and P codoped silicene forming AlSi$_6$P nanosheets are investigated by first-principle calculations within density functional theory. A particular attention of this study is paid to the interaction effect between the Al and P atoms in the buckled silicene structure. We infer that the interaction type is attractive and it leads to an asymmetry in the density of states opening up a band gap. In contrast to BN-codoped silicene, the buckling in the Al-P codoped silicene is not much influenced by the dopants as the bond lengths of Si-P and Al-P are very similar to the Si-Si bond lengths. On the other hand, the longer bond length of Si-Al decreases the stiffness and thus induces fractures at smaller values of applied strain in AlSi$_6$P. The elastic and nonelastic regions of the stress-strain curve of AlSi$_6$P depend on the placement of the Al and P atoms. The finite band gap caused by the Al-P dopant leads to enhancement of the Seebeck coefficient and the figure of merit, and induces a redshift of peaks in the dielectric response, and the optical conductivity. Finally, properties of the real and imaginary parts of the dielectric function, the excitation spectra, the refractive index, and the optical response of AlSi$_6$P are reported., Comment: 9 pages, 8 figures

We present theoretical calculations for the cyclotron resonance and various magnetoplasmon modes of a Coulomb interacting two-dimensional GaAs electron gas (2DEG) modulated as a lateral superlattice of quantum dots subjected to an external perpendicular constant magnetic field. We use a real-time excitation approach based on the Liouville-von Neumann equation for the density operator, that can go beyond linear response delivering information of all longitudinal and transverse collective modes of interest to the same order. We perform an extensive analysis of the coexisting collective modes due to the lateral confinement and the magnetic field for a different number of electrons in each dot. In the limit of vanishing dot modulation of the 2DEG we find signs of the structure of the Hofstadter butterfly in the excitation spectra., Comment: RevTeX - pdfLaTeX, 10 pages with 9 pdf-figures included

The properties of graphene-like BC$_6$N semiconductor are studied using density functional theory taking into account the attractive interaction between B and N atoms. In the presence of a strong attractive interaction between B and N dopant atoms, the electron charge distribution is highly localized along the B-N bonds, while for a weaker attractive interaction the electrons are delocalized along the entire hexagonal ring of BC$_6$N. Furthermore, when both B and N atoms are doped at the same site of the hexagon, the breaking of the sub-lattice symmetry is low producing a small bandgap. In contrast, if the dopant atoms are at different sites, a high sub-lattice symmetry breaking is found leading to a large bandgap. The influences of electron localization/delocalization and the tunable bandgap on thermal behaviors such as the electronic thermal conductivity, the Seebeck coefficient, and the figure of merit, and optical properties such as the dielectric function, the excitation spectra, the refractive index, the electron energy loss spectra, the reflectivity, and the optical conductivity are presented. An enhancement with a red shift of the optical conductivity at low energy range is seen while a reduction at the high energy range is found indicating that the BC$_6$N structure may be useful for optoelectronic devices in the low energy, visible range., Comment: 9 pages with including 11 figures

In this work we investigate magneto-optical properties of two-dimensional semiconductor quantum-ring excitons with Rashba and Dresselhaus spin-orbit interactions threaded by a magnetic flux perpendicular to the plane of the ring. By calculating the excitonic Aharonov-Bohm spectrum, we study the Coulomb and spin-orbit effects on the Aharonov-Bohm features. From the light-matter interactions of the excitons, we find that for scalar excitons, there are open channels for spontaneous recombination resulting in a bright photoluminescence spectrum, whereas the forbidden recombination of dipolar excitons results in a dark photoluminescence spectrum. We investigate the generation of persistent charge and spin currents. The exploration of spin orientations manifests that by adjusting the strength of the spin-orbit interactions, the exciton can be constructed as a squeezed complex with specific spin polarization. Moreover, a coherently moving dipolar exciton acquires a nontrivial dual Aharonov-Casher phase, creating the possibility to generate persistent dipole currents and spin dipole currents. Our study reveals that a manipulation of the spin-orbit interactions provides a potential application for quantum-ring spinor excitons to be utilized in nano-scaled magneto-optical switches., Comment: RevTeX, 13 pages with included 8 eps figures

We analyze how a multilevel many-electron system in a photon cavity approaches the steady state when coupled to external leads. When a plunger gate is used to lower cavity photon dressed one- and two-electron states below the bias window defined by the external leads, we can identify one regime with nonradiative transitions dominating the electron transport, and another regime with radiative transitions. Both transitions trap the electrons in the states below the bias bringing the system into a steady state. The order of the two regimes and their relative strength depends on the location of the bias window in the energy spectrum of the system and the initial conditions., RevTeX, 6 pages with 7 included eps-figures, references and graphs added in resubmission

A propagation matrix method is proposed to investigate spin-resolved electronic transport in single finger-gate (SFG) and double finger-gate (DFG) controlled split-gate quantum devices. We show how the interplay of the Rashba and Dresselhaus spin-orbit (SO) interactions as well as a Zeeman (Z) field influences the quantum transport characteristics. Without the Dresselhaus effect, the conductance reveals a mirror symmetry between the hole-like and the electron-like quasi-bound states in the SO-Z gap energy regime in the SFG system, but not for the DFG system. For the Dresselhaus interaction, we are able to analytically identify the binding energy of the SFG and DFG bound states. Furthermore, the DFG resonant states can be determined by tuning the distance between the finger gates., Comment: 7 pages, 6 figures. arXiv admin note: text overlap with arXiv:1407.2369

A single cavity photon mode is expected to modify the Coulomb interaction of an electron system in the cavity. Here we investigate this phenomena in a parallel double quantum dot system. We explore properties of the closed system and the system after it has been opened up for electron transport. We show how results for both cases support the idea that the effective electron-electron interaction becomes more repulsive in the presence of a cavity photon field. This can be understood in terms of the cavity photons dressing the polarization terms in the effective mutual electron interaction leading to nontrivial delocalization or polarization of the charge in the double parallel dot potential. In addition, we find that the effective repulsion of the electrons can be reduced by quadrupolar collective oscillations excited by an external classical dipole electric field., PDF-LaTeX, RevTeX, 8 pages with 8 embedded PNG-figures

We compare the response of five different models of two interacting electrons in a quantum dot to an external short lived radial excitation that is strong enough to excite the system well beyond the linear response regime. The models considered describe the Coulomb interaction between the electrons in different ways ranging from mean-field approaches to configuration interaction (CI) models, where the two-electron Hamiltonian is diagonalized in a large truncated Fock space. The radially symmetric excitation is selected in order to severely put to test the different approaches to describe the interaction and correlations of an electron system in a nonequilibrium state. As can be expected for the case of only two electrons none of the mean-field models can in full details reproduce the results obtained by the CI model. Nonetheless, some linear and nonlinear characteristics are reproduced reasonably well. All the models show activation of an increasing number of collective modes as the strength of the excitation is increased. By varying slightly the confinement potential of the dot we observe how sensitive the properties of the excitation spectrum are to the Coulomb interaction and its correlation effects. In order to approach closer the question of nonlinearity we solve one of the mean-field models directly in a nonlinear fashion without resorting to iterations., PDFLaTeX, RevTeX, 12 pages with 16 included pdf figures. Scales in Fig. 10(b) and 12(b) repaired, minor spelling mistakes corrected, and the conclusion section extended

We calculate the persistent spin current inside a quantum ring as a function of the strength of the Rashba or Dresselhaus spin-orbit interaction. We provide analytical results for the spin current of a one-dimensional (1D) ring of non-interacting electrons for comparison. Furthermore, we calculate the time evolution in the transient regime of a two-dimensional (2D) quantum ring connected to electrically biased semi-infinite leads using a time-convolutionless non-Markovian generalized master equation. In the latter case, the electrons are correlated via the Coulomb interaction and the ring can be embedded in a photon cavity with a single mode of linearly polarized photon field. The electron-electron and electron-photon interactions are described by exact numerical diagonalization. The photon field can be polarized perpendicular or parallel to the charge transport. We find a pronounced charge current dip associated with many-electron level crossings at the Aharonov-Casher phase $\Delta\Phi=\pi$, which can be disguised by linearly polarized light. Qualitative agreement is found for the spin currents of the 1D and 2D ring. Quantatively, however, the spin currents are weaker in the more realistic 2D ring, especially for weak spin-orbit interaction, but can be considerably enhanced with the aid of a linearly polarized electromagnetic field. Specific spin current symmetries relating the Dresselhaus spin-orbit interaction case to the Rashba one are found to hold for the 2D ring in the photon cavity.

We demonstrate that with a stepwise introduction of complexity to a model of an electron system embedded in a photonic cavity and a carefully controlled stepwise truncation of the ensuing many-body space it is possible to describe the time-dependent transport of electrons through the system with a non-Markovian generalized quantum master equation. We show how this approach retains effects of an external magnetic field and the geometry of an anisotropic electronic system. The Coulomb interaction between the electrons and the full electromagnetic coupling between the electrons and the photons are treated in a non-perturbative way using "exact numerical diagonalization"., RevTeX, 14 pages with included eps figures, replaced to mend scaling in figure axes for time "t" and current "J"

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., Comment: 10 pages 9 figures

We use a generalized master equation (GME) to describe the nonequilibrium magnetotransport of interacting electrons through a broad finite quantum wire with an embedded ring structure. The finite quantum wire is weakly coupled to two broad leads acting as reservoirs of electrons. The mutual Coulomb interaction of the electrons is described using a configuration interaction method for the many-electron states of the central system. We report some non-trivial interaction effects both at the level of time-dependent filling of states and on the time-dependent transport. We find that the Coulomb interaction in this non-trivial geometry can enhance the correlation of electronic states in the system and facilitate it's charging in certain circumstances in the weak coupling limit appropriate for the GME. In addition, we find oscillations in the current in the leads due to the correlations oscillations caused by the switched-on lead- system coupling. The oscillations are influenced and can be enhanced by the external magnetic field and the Coulomb interaction., RevTeX (pdf-LaTeX), 10 pages with 15 included jpg figures

Time-dependent transport through two capacitively coupled quantum dots is studied in the framework of the generalized master equation. The Coulomb interaction is included within the exact diagonalization method. Each dot is connected to two leads at different times, such that a steady state is established in one dot before the coupling of the other dot to its leads. By appropriately tuning the bias windows on each dot we find that in the final steady state the transport may be suppressed or enhanced. These two cases are explained by the redistribution of charge on the many-body states built on both dots. We also predict and analyze the transient mutual charge sensing of the dots., Comment: 9 pages, 5 figures

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 $\phi$. 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 $\phi$ at which the charge current disappears, while the spin current reaches a maximum or a minimum value., Comment: 8 pages, 5 figures

We investigate theoretically the transport properties of a mesoscopic system driven by a sequence of rectangular pulses applied at the contact to the input (left) lead. The characteristics of the current which would be measured in the output (right) lead are discussed in relation with the spectral properties of the sample. The time-dependent currents are calculated via a generalized non-Markovian master equation scheme. We study the transient response of a quantum dot and of a narrow quantum wire. We show that the output response depends not only on the lead-sample coupling and on the length of the pulse but also on the states that propagate the input signal. We find that by increasing the bias window the new states available for transport induce additional structure in the relaxation current due to different dynamical tunneling processes. The delay of the output signal with respect to the input current in the case of the narrow quantum wire is associated to the transient time through the wire., 10 pages, 9 figs

In a two-dimensional quantum wire in a perpendicular magnetic field with a smooth embedded repulsive scattering potential we find in the multimode conductance resonances caused by bound states with negative binding energy. These resonances are the counterexamples to well known dips in the conductance and evanescent states caused by quasi-bound states of attractive scattering centers in the wire., Comment: RevTeX, 4 pages with included 4 postscript figures, high resolution version is available at http://hartree.raunvis.hi.is/~vidar/Rann/VGCSTAM_05.pdf

In order to understand the orbital and spin magnetization of a confined electronic system we analyze these ground state properties in the transition from a quantum dot to a quantum ring of finite thickness. The Coulomb interaction between the electrons is treated in the Hartree and Hartree-Fock approximations and special care is taken to include also the contributions of the nonlocal current to the summation of the magnetic moments of the occupied states. We identify changes in the many-body structure of the ground state and in the magnetization curves versus the magnetic field and other parameters characterizing the system. We compare the results of two models for quantum dots (or rings), one with the electrons moving continuously in the system, and one with the electrons moving on a lattice., Comment: LaTeX, iopart.cls, 7 pages with 5 integrated color figures

We explore the structural changes in charge the density and the electron configuration of quantum dots caused by the presence of other dots in an array, and the interaction of neighboring dots. We discuss what recent measurements and calculation of the far-infrared absorption reveal about almost isolated quantum dots and investigate some aspects of the complex transition from isolated dots to dots with strongly overlapping electron density. We also address the the effects on the magnetization of such dot array., Comment: 29 papes, 17 included figures. This article is slated to appear as a chapter in "Nano-Physics and Bio-Electronics", edited by T. Chakraborty, F. Peeters, and U. Sivan (to be published by Elsevier Co.)