Browsing by Author "Moldoveanu, V."
Now showing 1 - 20 of 25
Results Per Page
Sort Options
Item Open Access Analysis of the phase lapse problem in closed interferometers(Elsevier B.V., 2012) Tolea, M.; Moldoveanu, V.; Dinu, I. V.; Tanatar, BilalWe investigate the connection between the asymmetry of the Fano resonances in a mesoscopic interferometer with an embedded quantum dot and the π lapses in the phase of the bare dot transmittance. Consecutive Fano resonances with the same (opposite) sign of the Fano parameter imply the presence (absence) of a phase lapse with π between the corresponding resonances of the dot. Our results suggest that the famous phase lapse problem, first reported by Schuster et al. [R. Schuster, E. Buks, M. Heiblum, D. Mahalu, V. Umansky, H. Shtrikman, Nature 385 (1997) 417], can therefore be experimentally addressed in closed interferometers. It is also proposed that the Fano effect can be used to extract the phase distributions of the eigenfunctions for a mesoscopic 2D shape, via the parity of the resonances. In the presence of electron-electron interaction, one can calculate the phases of the T-matrix elements. The numerical results lead to the same conclusions as for the non-interacting case.Item Open Access Coherent and incoherent transport through T-shaped double quantum dots(Elsevier B.V., 2008) Moldoveanu, V.; Ţolea, M.; Tanatar, BilalWe investigate the measurement induced dephasing of the Fano effect in the electronic transport through a double quantum dot mesoscopic interferometer coupled to a charge detector. The current and the differential conductance are computed within the Keldysh formalism, taking into account of the inelastic processes due to the dot-detector interaction. We show that the visibility of the Fano lineshape is reduced by applying a finite bias on the charge detector.Item Open Access Controlled dephasing in single-dot Aharonov-Bohm interferometers(2007) Moldoveanu, V.; Ţolea, M.; Tanatar, BilalWe study the Fano effect and the visibility of the Aharonov-Bohm oscillations for a mesoscopic interferometer with an embedded quantum dot in the presence of a nearby second dot. When the electron-electron interaction between the two dots is considered the nearby dot acts as a charge detector. We compute the currents through the interferometer and detector within the Keldysh formalism and the self-energy of the nonequilibrium Green's functions is found up to the second order in the interaction strength. The current formula contains a correction to the Landauer-Büttiker formula. Its contribution to transport and dephasing is discussed. As the bias applied on the detector is increased, the amplitude of both the Fano resonance and Aharonov-Bohm oscillations are considerably reduced due to controlled dephasing. This result is explained by analyzing the behavior of the imaginary part of the interaction self-energy as a function of energy and bias. We emphasize as well the role of the ring-dot coupling. Our theoretical results are consistent with the experimental observation of Buks [Nature 391, 871 (1998)].Item Open Access Coulomb drag effect in parallel quantum dots(American Institute of Physics, 2009) Tanatar, Bilal; Moldoveanu, V.We study theoretically the electronic transport in parallel few-level quantum dots in the presence of both intradot and interdot long-range Coulomb interaction. Each dot is connected to two leads and the steady-state currents are calculated within the Keldysh formalism using the random-phase approximation for the interacting Green functions. Due to the momentum transfer mechanism between the two systems it is possible to get a nonvanishing current through an unbiased Coulomb-blockaded dot if the other dot is set in the nonlinear transport regime. The transitions between the levels of the passive dot reduce the drag current and lead to negative differential conductance.Item Open Access Coulomb effects in open quantum dots within the random-phase approximation(The American Physical Society, 2008) Moldoveanu, V.; Tanatar, BilalThe effect of electron-electron interactions on coherent transport in quantum dot systems is theoretically investigated by adapting the well-known random-phase approximation (RPA) to the nonequilibrium Green-Keldysh formalism for open mesoscopic systems. The contour-ordered polarization operator is computed in terms of the Green functions of the noninteracting system. We apply the proposed RPA-Keldysh scheme for studying Coulomb-modified Fano lines and dephasing effects in interferometers with side-coupled many-level dots. Our method allows us to treat on equal footing the decoherence induced by the intradot interaction and that by the Coulomb coupling to a nearby system. In the case of a single interferometer, we show that the intradot Coulomb interaction leads to a reduction of the Fano line amplitude. From the analysis of the interaction self-energy, it follows that this effect originates in inelastic scattering processes in which electron-hole pairs are involved. The interplay between the interdot and the intradot interactions in decoherence is discussed for two nearby identical T-shaped interferometers. We also show that the intradot interaction does not prevent the observation of controlled dephasing due to a nearby charge detector, as long as the latter is subjected to a sufficiently large bias.Item Open Access Dynamic correlation effects on the plasmon dispersion in a two-dimensional electron gas(The American Physical Society, 2003) Yurtsever, A.; Moldoveanu, V.; Tanatar, BilalThe charge-density oscillations (plasmons) of a low-density two-dimensional uniform electron gas are studied within the framework of finite temperature and frequency dependent (dynamic) version of Singwi, Tosi, Land, and Sjölander theory and compared with the recent experimental results. The use of the Hartree-Fock approximation for the static structure factor leads to a finite temperature dynamical counterpart of the static Hubbard approximation. We observe important differences between dynamic and static local-field factors as well as between the corresponding plasmon dispersion laws. Our calculated plasmon energies that include dynamic correlations are in very-good agreement with the recent experimental results.Item Open Access Fano effect in a double T-shaped interferometer(Springer, 2009) Moldoveanu, V.; Dinu, I. V.; Tanatar, BilalWe study the coherent transport in a one-dimensional lead with two side-coupled quantum dots using the Keldysh's Green function formalism.The effect of the interdot Coulomb interaction is taken into account by computing the firstand second order contributions to the self-energy.We show that the Fano interference due to the resonance of one dotis strongly affected by the fixed parameters that characterize the second dot. If the second dot is tuned close to resonance an additionalpeak develops between the peak and dip of the Fano line shape of the current. In contrast, when the second dotis off-resonance and its occupation number is close to unity the interdot Coulomb interaction merely shifts the Fano line and no other maxima appear.The system we consider is more general than the single-dot interferometer studied experimentally by Kobayashi et al. [Phys. Rev. B 70, 035319 (2004)] and may be used for controlling quantum interference and studying decoherence effects in mesoscopic transport.Item Open Access Inelastic transitions and counterflow tunneling in double-dot quantum ratchets(The American Physical Society, 2010) Moldoveanu, V.; Tanatar, BilalThe ratchet regime of unbiased double quantum dots driven out of equilibrium by an independently biased nearby detector has been theoretically studied using the nonequilibrium Keldysh formalism and the random-phase approximation for the Coulomb effects. When the detector is suitably biased the energy exchange between the two systems removes the Coulomb blockade on the double dot via inelastic interdot tunneling. The energy detuning determines whether the current flows in the same direction as the driving current (positive flow) or in the opposite direction (electronic counterflow). In both cases the intradot transitions lead to negative-differential conductance. Besides the ratchet contribution to the current we also single out a Coulomb drag component.Item Open Access Interaction and size effects in open nano-electromechanical systems(WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2019) Tanatar, Bilal; Moldoveanu, V.; Dragomir, R.; Stanciu, S.The time‐dependent transport of a 2D quantum wire (QW) connected to source/drain leads and electrostatically coupled to a singly‐clamped InAs cantilever is investigated. The latter is placed above the nanowire and acts as a nanoresonator (NR) in the quantum regime. The vibron dynamics and the transport properties of this nano‐electromechanical system (NEMS) are described within a generalized master equation approach which is exact with respect to the electron‐vibron coupling. A detailed description of the electron‐vibron coupling by taking into account its dependence on the wavefunctions of the quantum nanowire is introduced. It is shown that the tunneling processes in the QW trigger periodic oscillations of the average vibron number even in the absence of a bias. The time‐dependent filling of the vibronic states changes as the nanoresonator is swept along the quantum wire.Item Unknown Light-hole exciton mixing and dynamics in Mn-doped quantum dots(American Physical Society, 2016) Moldoveanu, V.; Dinu, I. V.; Dragomir, R.; Tanatar, BilalWe investigate theoretically the spectral and dynamical effects of the short-range exchange interaction between a single manganese (Mn) atom hosted by cylindrical CdTe quantum dots and its light-hole excitons or biexcitons. Our approach is based on the Kohn-Luttinger k·p theory and configuration interaction method, the dynamics of the system in the presence of intraband relaxation being derived from the von Neumann-Lindblad equation. The complex structure of the light-hole exciton absorption spectrum reveals the exchange-induced exciton mixing and depends strongly on the Mn position. In particular, if the Mn atom is closer to the edges of the cylinder, the bright and dark light-hole excitons are mixed by the hole-Mn exchange alone. Consequently, their populations exhibit exchange-induced Rabi oscillations which can be viewed as optical signatures of light-hole spin reversal. Similar results are obtained for mixed biexcitons, in this case the exchange-induced Rabi oscillations being damped by the intraband hole relaxation processes. The effect of light-hole heavy-hole mixing is also discussed.Item Unknown Many-body effects in the Coulomb drag between low density electron layers(Elsevier Science, 2003) Yurtsever, A.; Moldoveanu, V.; Tanatar, BilalRecent Coulomb drag experiments in low-density double-layer electron systems have the power of distinguishing various many-body formulations of the effective interactions. In this work we theoretically study the correlation effects on the drag resistivity in these systems within various models. The effective inter-layer interactions are best described by the generalization to the double-layer case of the Kukkonen-Overhauser approach which differs significantly from the self-consistent field approach of Singwi et al. [Phys. Rev. 176 (1968) 589]. Following the formulation of Vignale and Singwi [Phys. Rev. B 32 (1985) 2156] we derive an expression for the effective inter-layer interaction which embodies the many-body correlations through the local-field corrections. The drag resistivity is calculated within this approach together with the Hubbard approximation for the intra-layer local-field factor and a simple model for the inter-layer correlations. Comparison with the recent measurements of Kellogg et al. [Solid State Commun. 123 (2002) 515] yields very good agreement. Our results are also contrasted with the corresponding drag resistivities given by the Singwi et al. theory, the dynamic random-phase approximation and the Hubbard approximation. The significant differences found between these theories emphasize the strong sensitivity of the drag resistivity to the effective inter-layer interactions.Item Unknown Measurement-induced decoherence in electronic interferometry at nanoscale(Elsevier B.V., 2008) Moldoveanu, V.; Tanatar, Bilal; Ţolea, M.We introduce a theoretical formalism describing a wide class of 'Which Path' experiments in mesoscopic/nanoscopic transport. The physical system involves a mesoscopic interferometer (e.g. an Aharonov-Bohm ring with embedded dots or a side-coupled quantum dot) which is electrostatically coupled to a nearby quantum point constriction. Due to the charge sensing effect the latter acts as a charge detector. Therefore the interference pattern can be monitored indirectly by looking at the current characteristics of the detector as shown in the experimental work of Buks et al. [E. Buks, R. Schuster, M. Heiblum, D. Mahalu, V. Umansky, Nature (London) 391 (1998) 871]. We use the non-equilibrium Green-Keldysh formalism and a second order perturbative treatment of the Coulomb interaction in order to compute the relevant transport properties. It is shown that in the presence of the Coulomb interaction the current through the detector exhibits oscillations as a function of the magnetic field applied on a single-dot AB interferometer. We also discuss the dependence of the visibility of the Aharonov-Bohm oscillations on the gate potential applied to the dot.Item Unknown Mesoscopic Fano effect in a spin splitter with a side-coupled quantum dot(Elsevier B.V., 2012) Moldoveanu, V.; Tolea, M.; Tanatar, BilalWe investigate the interplay between the spin interference and the Fano effect in a three-lead mesoscopic ring with a side-coupled quantum dot (QD). A uniform Rashba spin-orbit coupling and a perpendicular magnetic field are tuned such that the ring operates as a spin splitter in the absence of the QD: one lead is used to inject unpolarized electrons and the remaining (output) leads collect almost polarized spin currents. By applying a gate potential to the quantum dot a pair of spin-split levels sweeps the bias window and leads to Fano interference. The steady-state spin and charge currents in the leads are calculated for a finite bias applied across the ring via the non-equilibrium Green's function formalism. When the QD levels participate to transport we find that the spin currents exhibit peaks and dips whereas the charge currents present Fano lineshapes. The location of the side-coupled quantum dot and the spin splitting of its levels also affect the interference and the output currents. The opposite response of output currents to the variation of the gate potential allows one to use this system as a single parameter current switch. We also analyze the dependence of the splitter efficiency on the spin splitting on the QD.Item Unknown Mesoscopic fano effect in Aharonov-Bohm rings with an embedded double dot(American Institute of Physics, 2006) Tanatar, Bilal; Moldoveanu, V.; Tolea, M.; Aldea, A.We investigate theoretically in a tight-binding model the transport properties of the Aharonov-Bohm interferometer (ABI) with one dot embedded in each of its arms. For weak interdot coupling the model Hamiltonian describes the system considered in the experiments of Holleitner et al. [Phys. Rev. Lett. 87, 256802 (2001)]. The electronic transmittance of the interferometer is computed within the Landauer-Büttiker formalism while the coexistence of resonant and coherent transport is explicitly emphasized by using the Feschbach formula. The latter produces effective Hamiltonians whose spectral properties describe the tunneling processes through each dot. We reproduce numerically the stability charging diagrams reported in the experiments of Holleitner et al. When the magnetic flux is fixed and one dot is set to resonance the interferometer transmittance shows Fano lineshapes as a function of the gate voltage applied to the other dot. Our model includes the effect of the magnetic field on the dot levels and explains the change of the asymmetric tail as the magnetic flux is varied. The transmittance assigned to the Fano dips located in the almost crossing point of the charging diagrams shows Aharonov-Bohm oscillations.Item Unknown Mesoscopic Fano effect in an Aharonov-Bohm interferometer Coulomb-coupled to a nearby quantum dot(Wiley, 2007) Tolea, M.; Moldoveanu, V.; Tanatar, BilalMotivated by the pionieering experiments of Buks et al. [Nature 391, 871 (1998)] we investigate the visibility of the Fano effect in a single-dot Aharonov-Bohm interferometer which is Coulomb-coupled to a nearby quantum dot. The latter acts as a 'Which Path Detector' and is coupled to two leads on which a finite bias is applied. Using the non-equilibrium Keldysh-Green function formalism we compute the currents through the detector and the interferometer. We take into account the first two contributions to the interaction selfenergy and emphasize the correction to the Landauer formula which appears beyond the single-particle approximation. Particular attention is given to the coherence properties of the interferometer in the presence of the electron-electron interaction between the embedded dot and the detector. We show that when the detector is subjected to a finite bias the amplitude of Aharonov-Bohm oscillations of the current through the interferometer decreases. The Fano line is in turn rather stable under interaction. Our results generalize an earlier work of Silva and Levit [Phys. Rev. B 63, 201309 (2001)] and complement the existing description of the controlled dephasing.Item Unknown Non-equilibrium transport and spin dynamics insingle-molecule magnets.(Academic Press, 2015) Moldoveanu, V.; Dinu, I. V.; Tanatar, BilalThe time-dependent transport through single-molecule magnets (SMM) coupled to mag-netic or non-magnetic electrodes is studied in the framework of the generalized Masterequation (GME) method. We calculate the transient currents which develop when themolecule is smoothly coupled to the source and drain electrodes. The signature of theelectrically induced magnetic switching on these transient currents is investigated. Oursimulations show that the magnetic switching of the molecular spin can be read indirectlyfrom the transient currents if one lead is magnetic and it is much faster if the leads haveopposite spin polarizations. We identify effects of the transverse anisotropy on thedynamics of molecular states.Item Unknown Orbital magnetization of single and double quantum dots in a tight-binding model(American Physical Society, 2003) Aldea, A.; Moldoveanu, V.; Niţǎ, M.; Manolescu, A.; Gudmundsson, V.; Tanatar, BilalWe 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 anticrossing 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. [Phys, Rev. Lett. 80, 4951 (1998)].Item Open Access Quantum turnstile operation of single-molecule magnets(Institute of Physics Publishing, 2015) Moldoveanu, V.; Dinu, I. V.; Tanatar, Bilal; Moca, C. P.The time-dependent transport through single-molecule magnets coupled to magnetic or non-magnetic electrodes is studied in the framework of the generalized master equation method. We investigate the transient regime induced by the periodic switching of the source and drain contacts. If the electrodes have opposite magnetizations the quantum turnstile operation allows the stepwise writing of intermediate excited states. In turn, the transient currents provide a way to read these states. Within our approach we take into account both the uniaxial and transverse anisotropy. The latter may induce additional quantum tunneling processes which affect the efficiency of the proposed read-and-write scheme. An equally weighted mixture of molecular spin states can be prepared if one of the electrodes is ferromagnetic. © 2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.Item Open Access Quantum turnstile regime of nanoelectromechanical systems(American Physical Society, 2020) Dragomir, R.; Moldoveanu, V.; Stanciu, S.; Tanatar, BilalThe effects of a turnstile operation on the current-induced vibron dynamics in nanoelectromechanical systems (NEMS) are analyzed in the framework of the generalized master equation. In our simulations each turnstile cycle allows the pumping of up to two interacting electrons across a biased mesoscopic subsystem which is electrostatically coupled to the vibrational mode of a nanoresonator. The time-dependent mean vibron number is very sensitive to the turnstile driving, rapidly increasing/decreasing along the charging/discharging sequences. This sequence of heating and cooling cycles experienced by the nanoresonator is due to specific vibron-assisted sequential tunneling processes along a turnstile period. At the end of each charging/discharging cycle the nanoresonator is described by a linear combination of vibron-dressed states sν associated to an electronic configuration ν. If the turnstile operation leads to complete electronic depletion the nanoresonator returns to its equilibrium position, i.e., its displacement vanishes. It turns out that a suitable bias applied on the NEMS leads to a slow but complete cooling at the end of the turnstile cycle. Our calculations show that the quantum turnstile regime switches the dynamics of the NEMS between vibron-dressed subspaces with different electronic occupation numbers. We predict that the turnstile control of the electron-vibron interaction induces measurable changes on the input and output transient currents.Item Open Access Resonant and coherent transport through Aharonov-Bohm interferometers with coupled quantum dots(The American Physical Society, 2005) Moldoveanu, V.; Ţolea, M.; Aldea, A.; Tanatar, BilalA detailed description of the tunneling processes within Aharonov-Bohm (AB) rings containing two-dimensional quantum dots is presented. We show that the electronic propagation through the interferometer is controlled by the spectral properties of the embedded dots and by their coupling with the ring. The transmittance of the interferometer is computed by the Landauer-Büttiker formula. Numerical results are presented for an AB interferometer containing two coupled dots. The charging diagrams for a double-dot interferometer and the Aharonov-Bohm oscillations are obtained, in agreement with the recent experimental results of Holleitner et al. [Phys. Rev. Lett. 87, 256802 (2001)] We identify conditions in which the system shows Fano line shapes. The direction of the asymetric tail depends on the capacitive coupling and on the magnetic field. We discuss our results in connection with the experiments of Kobayashi et al. [Phys. Rev. Lett. 88, 256806 (2002)] in the case of a single dot. ©2005 The American Physical Society.