National Institute Of Materials Physics - Romania

Publications

144 articles found

101. Kondo peaks and dips in the differential conductance of a multi-lead quantum dot: Dependence on bias conditions

Authors: Tolea, M; Dinu, IV; Aldea, A

Published: JAN 2009, PHYSICAL REVIEW B, 79, DOI: 10.1103/PhysRevB.79.033306

We study the differential conductance in the Kondo regime of a quantum dot coupled to multiple leads. When the bias is applied symmetrically on two of the leads (V and -V, as usual in experiments) while the others are grounded, the conductance through the biased leads always shows the expected enhancement at zero bias. However, under asymmetrically applied bias (V and lambda V, with lambda>0), a suppression-dip-appears in the differential conductance if the asymmetry coefficient lambda is beyond a given threshold lambda(0)=3 root 1+r determined by the ratio r of the dot-lead couplings. This is a recipe to determine experimentally this ratio which is important for the quantum-dot devices. This finding is a direct result of the Keldysh transport formalism. For the illustration we use a many-lead Anderson Hamiltonian, the Green's functions being calculated in the Lacroix approximation, which is generalized to the case of nonequilibrium.

102. Fano effect in a double T-shaped interferometer

Authors: Moldoveanu, V; Dinu, IV; Tanatar, B

Published: JAN 2009, EUROPEAN PHYSICAL JOURNAL B, 67, 238, DOI: 10.1140/epjb/e2009-00012-0

We 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.

103. Peak-dip crossover of the differential conductance in mesoscopic systems with quantum impurities

Authors: Aldea, A; Tolea, M; Dinu, IV

Published: 2009, 25TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT25), PART 2, 150, DOI: 10.1088/1742-6596/150/2/022001

We investigate the differential conductance dI/dV for the interacting T-shape model, using an approach based on the Keldysh formalism and the Lacroix solution for the equation of motion. A peak-dip crossover has been noticed by changing the hybridization between the two dots. For the same model, the combined interaction and interference processes give rise to the Fano-Kondo effect with an interesting crossing point of the isoterms below the Kondo temperature A tentative explanation of these effects is given in terms of the many-body spectral properties of the system.

104. Coulomb drag effect in parallel quantum dots

Authors: Tanatar, B; Moldoveanu, V

Published: 2009, TRANSPORT AND OPTICAL PROPERTIES OF NANOMATERIALS, 1147, +, DOI:

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.

105. Measurement-induced decoherence in electronic interferometry at nanoscale

Authors: Moldoveanu, V; Tanatar, B; Tolea, M

Published: SEP 29 2008, PHYSICS LETTERS A, 372, 6196, DOI: 10.1016/j.physleta.2008.07.079

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. (C) 2008 Elsevier B.V. All rights reserved.

106. Optica and structural properties of polythiophene-like films deposited by plasma polymerization

Authors: Galca, AC; Satulu, V; Ionita, MD; Bercu, M; Barna, E; Dumitru, M; Mitu, B; Dinescu, G

Published: AUG 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 2037, DOI:

Polymer-like organic thin films obtained by plasma techniques are of wide interest for various applications. Polythiophene-like thin films were deposited by plasma polymerization in a parallel plate radiofrequency reactor starting from thiophene vapors. The optical properties of the films were obtained from spectroscopic ellipsometry, in the 250-1250 nm range. The calculated band gap is in agreement with a polythiophene-like material consisting of cross-linking of oligomers. The presence of functional groups and film stability against oxidation is analyzed by FTIR spectrometry. The monitoring of functional groups incorporation during the film deposition was done by analyzing the integral intensities of specific absorption bands and led to the conclusion that homogeneous films are obtained whatever the process duration.

107. Transport through quantum dots with magnetic impurities

Authors: Tolea, M; Aldea, A; Bulka, BR

Published: JUL 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 1758, DOI:

We analyze the electronic transport through a quantum dot that contains a magnetic impurity. The coherent transport of electrons is governed by the quantum confinement inside the dot, but is also influenced by the exchange interaction with the impurity. The interplay between the two gives raise to the singlet-triplet splitting of the energy levels available for the tunneling electron. In this paper, we focus on the charge fluctuations and, more precisely, the height of the conductance peaks. We show that the conductance peaks corresponding to the triplet levels are three times higher than those corresponding to singlet levels, if electronic correlations are neglected (for non-interacting dots, when an exact solution can be obtained). Next, we consider the Coulomb repulsion and the many-body correlations. In this case, the singlet/triplet peak height ratio has a complex behavior. Usually the highest peak corresponds to the state that is lowest in energy (ground state), regardless if it is singlet or triplet. In the end, we get an insight on the Kondo regime for such a system, and show the formation of three Kondo peaks. We use the equation of motion method with appropriate decoupling.

108. Charge fluctuations in quantum point contact

Authors: Bulka, BR; Dinu, IV; Tolea, M

Published: MAY 2008, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 40, 2617, DOI: 10.1016/j.physe.2007.08.001

We present studies of the role of charge fluctuations in transport through a quantum point contact (QPC). Our model of QPC is based on assumption of a specific electronic energy structure with a resonant level below the electronic sub-band. We show that charge fluctuations lead to a dynamical Coulomb blockade effect, which is responsible for reduction of the conductance to the value 2/3 x (2e(2)/h). The evolution of conductance with a magnetic field and as a function of source-drain voltage is presented as well. The conductance plateau at 2/3 x (2e(2)/h) evolves continuously with an increase of the voltage to a 0.8 or 0.4 plateau, depending on the relative position of the resonant state with respect to the Fermi energy at zero bias. Our simple model shows many similarities with experimental characteristics. (C) 2007 Elsevier B.V. All rights reserved.

109. Coulomb effects in open quantum dots within the random-phase approximation

Authors: Moldoveanu, V; Tanatar, B

Published: MAY 2008, PHYSICAL REVIEW B, 77, DOI: 10.1103/PhysRevB.77.195302

The 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 GreenKeldysh 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.

110. Coherent and incoherent transport through T-shaped double quantum dots

Authors: Moldoveanu, V; Tolea, M; Tanatar, B

Published: MAR 2008, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 40, 1104, DOI: 10.1016/j.physe.2007.08.012

We 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. (C) 2007 Elsevier B.V. All rights reserved.