National Institute Of Materials Physics - Romania

Publications

142 articles found

121. Criteria for the occurrence of negative resonant magnetoresistance in magnetic tunnel junctions

Authors: Tolea, M; Aldea, A; Tolea, F

Published: SEP 2006, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 243, R86, DOI: 10.1002/pssb.200642347

The dependence of the driven-current through a tunnel junction on the relative orientation of the magnetization in the electrodes deviates from the Julliere formula when the current is resonant through impurity levels. For asymmetric coupling of the impurities to the electrodes, the magnetoresistance can even be negative on the resonance. Such inversion in sign is due to the fact that not only the density of states in the electrodes is important (for both orientations of spins), but also the density of states at the impurity position, which has a specific behavior. We find the exact conditions under which the negative magnetoresistance occurs, as function of both polarization and coupling asymmetry. The resonant magnetoresistance may have single or double-dip aspect, depending on the mentioned parameters.

122. Tunnelling magnetoresistance of a T-shaped Fano dot

Authors: Tolea, M; Bulka, BR

Published: JAN 2006, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 243, 254, DOI: 10.1002/pssb.200562462

We show the role of the Fano interference on the magnetoresistance properties of a T-shaped double-dot placed between ferromagnetic electrodes. In the linear voltage regime, the magnetoresistance is a non-monotonic function of the gate voltage. We find high values of the magnetoresistance at the Fano dip and far from it. For resonant transmission, the magnetoresistance is lower and it can be negative for asymmetric couplings. Particularities of the system in nonequilibrium transport are also analysed. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

123. Mesoscopic Fano effect in Aharonov-Bohm rings with an embedded double dot

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

Published: 2006, LOW TEMPERATURE PHYSICS, PTS A AND B, 850, +, DOI:

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-Buttiker 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.

124. Fano interference in mesoscopic rings with quantum dots

Authors: Moldoveanu, V; Aldea, A; Tolea, M

Published: 2006, PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 3, NO 2, 3, +, DOI: 10.1002/pssc.200562736

We propose a general formalism for describing the coexistence of coherent and resonant transport in hybrid mesoscopic structures. The approach is based on Landauer- Buttiker formula for the electronic transmittance and on an old formula of Feshbach. The latter gives the complete Green function of coupled subsystems in terms of effective Green functions of the disconnected parts and provides informations about the individual contributions of each subsystem to transport. Motivated by the experiments of Kobayashi et al.(Phys. Rev. Lett. 88, 256806 (2002)) and Holleitner et al. (Phys. Rev. Lett. 87, 256802 (2001)) we apply the formalism to study transport in Aharonov-Bohm interferometers (ABI) containing one or two coupled two-dimensional quantum dots (QD). In the single dot case, we reproduce and explain the magnetic field control of the Fano interference and investigate the interaction effects in a self-consistent approach. In the double dot case, we obtain the charging diagrams and establish precise criteria for the observation of mesoscopic Fano effect and AB oscillations. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

125. Fano regime of one-dot Aharonov-Bohm interferometers

Authors: Moldoveanu, V; Tolea, M; Gudmundsson, V; Manolescu, A

Published: AUG 2005, PHYSICAL REVIEW B, 72, DOI: 10.1103/PhysRevB.72.085338

We use the Landauer-Buttiker formalism to study the mesoscopic Fano effect in Aharonov-Bohm rings with an embedded two-dimensional noninteracting dot. The magnetic field dependence of the dot levels leads to a global shift of the Fano lines which becomes important for small ring/dot area ratios. As the magnetic field is varied the Fano dips move periodically from one side of the peak to the other, as reported by Kobayashi [Phys. Rev. Lett. 88, 256806 (2002)]. We show that this effect appears due to a specific magnetic control of the difference between the phase of the single nonresonant path via the free arm of the ring and the global phase of all trajectories involving resonant tunnelings through the dot.

126. Spin magnetoconductance in the mesoscopic spin-interferometer

Authors: Aldea, A; Tolea, M; Zittartz, J

Published: JUL 2005, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 28, 202, DOI: 10.1016/j.physe.2005.03.013

We investigate the properties of the electron spin-transmission through an Aharonov-Bohm interferometer with an embedded two-dimensional (2D) multilevel quantum dot containing magnetic impurities. A suitable formalism is developed. The amplitude and the phase of the flip- and non-flip-transmittances are calculated numerically as function of the magnetic field. The effects induced by the exchange interaction and those arising from the 2D character of the dot are shown. (c) 2005 Elsevier B.V. All rights reserved.

127. Transport through a quantum ring, dot, and barrier embedded in a nanowire in magnetic field

Authors: Gudmundsson, V; Lin, YY; Tang, CS; Moldoveanu, V; Bardarson, JH; Manolescu, A

Published: JUN 2005, PHYSICAL REVIEW B, 71, DOI: 10.1103/PhysRevB.71.235302

We investigate the transport through a quantum ring, a dot, and a barrier embedded in a nanowire in a homogeneous perpendicular magnetic field. To be able to treat scattering potentials of finite extent in a magnetic field we use a mixed momentum-coordinate representation to obtain an integral equation for the multiband scattering matrix. For a large embedded quantum ring we are able to obtain Aharonov-Bohm type of oscillations with superimposed narrow resonances caused by interaction with quasibound states in the ring. We also employ the scattering matrix approach to calculate the conductance through a semiextended barrier or well in the wire. The numerical implementations we resort to in order to describe the cases of weak and intermediate magnetic field allow us to produce high resolution maps of the "near field" scattering wave functions, which are used to shed light on the underlying scattering processes.

128. The dynamics of one-dimensional Bloch electrons in constant electric fields

Authors: Bentosela, F; Duclos, P; Moldoveanu, V; Nenciu, G

Published: APR 2005, JOURNAL OF MATHEMATICAL PHYSICS, 46, DOI: 10.1063/1.1870732

We study the dynamics of a one-dimensional Bloch electron subjected to a constant electric field. The periodic potential is supposed to be less singular than the delta-like potential (Dirac comb). We give a rigorous proof of Ao's result that for a large class of initial conditions (high momentum regime) there is no localization in momentum space. The proof is based on the mathematical substantiation of the two simplifying assumptions made in physical literature: the transitions between far away bands can be neglected and the transitions at the quasicrossing can be described by Landau-Zener-type formulas. Using the connection between the above model and the driven quantum ring (DQR) shown by Avron and Nemirovski, our results imply the increase of energy for weakly singular such DQR and appropiate initial conditions. (C) 2005 American Institute of Physics.

129. Resonant and coherent transport through Aharonov-Bohm interferometers with coupled quantum dots

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

Published: MAR 2005, PHYSICAL REVIEW B, 71, DOI: 10.1103/PhysRevB.71.125338

A 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-Buttiker 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.

130. Net current generation in a 1D quantum ring at zero magnetic field

Authors: Gylfadottir, SS; Nita, M; Gudmundsson, V; Manolescu, A

Published: MAR 2005, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 27, 283, DOI: 10.1016/j.physe.2004.12.002

We study a non-adiabatic excitation of an electron system in a 1D quantum ring radiated by a short terahertz pulse. The response of two models, a continuous and a discrete, is explored. By introducing a spatial asymmetry in the external perturbation, a net current can be generated in the ring at a zero magnetic field. Effect of impurities and ratchets are investigated in combination with symmetric and asymmetric external excitation. (c) 2005 Elsevier B.V. All rights reserved.