National Institute Of Materials Physics - Romania

The usual experimental set-up for measuring the wave function phase shift of electrons tunneling through a quantum dot (QD) embedded in a ring (i.e., the transmittance phase) is the so-called 'open' interferometer as first proposed by Schuster et al. in 1997, in which the electrons back-scattered at source and the drain contacts are absorbed by additional leads in order to exclude multiple interference. While in this case one can conveniently use a simple two-path interference formula to extract the QD transmittance phase, the open interferometer has also a number of draw-backs, such as a reduced signal and some uncertainty regarding the effects of the extra leads. Here we present a meaningful theoretical study of the QD transmittance phase in 'closed' interferometers (i.e., connected only to source and drain leads). By putting together data from existing literature and giving some new proofs, we show both analytically and by numerical simulations that the existence of phase lapses between consecutive resonances of the 'bare' QD is related to the signs of the corresponding Fano parameters - of the QD + ring system. More precisely, if the Fano parameters have the same sign, the transmittance phase of the QD exhibits a I lapse. Therefore, closed mesoscopic interferometers can be used to address the 'universal phase lapse' problem. Moreover, the data from already existing Fano interference experiments from Kobayashi et al. in 2003 can be used to infer the phase lapses.

We investigate the connection between the asymmetry of the Fano resonances in a mesoscopic interferometer with an embedded quantum dot and the pi 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 pi 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 20 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. (C) 2012 Elsevier B.V. All rights reserved.

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 and 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. (c) 2012 Elsevier B.V. All rights reserved.

We use a generalized master equation (GME) formalism to describe the nonequilibrium time-dependent transport of Coulomb interacting electrons through a short quantum wire connected to semi-infinite biased leads. The contact strength between the leads and the wire is modulated by out-of-phase time-dependent potentials that simulate a turnstile device. We explore this setup by keeping the contact with one lead at a fixed location at one end of the wire, whereas the contact with the other lead is placed on various sites along the length of the wire. We study the propagation of sinusoidal and rectangular pulses. We find that the current profiles in both leads depend not only on the shape of the pulses, but also on the position of the second contact. The current reflects standing waves created by the contact potentials, like in a wind musical instrument (for example, a flute), but occurring on the background of the equilibrium charge distribution. The number of electrons in our quantum "flute" device varies between two and three. We find that for rectangular pulses the currents in the leads may flow against the bias for short time intervals, due to the higher harmonics of the charge response. The GME is solved numerically in small time steps without resorting to the traditional Markov and rotating wave approximations. The Coulomb interaction between the electrons in the sample is included via the exact diagonalization method. The system (leads plus sample wire) is described by a lattice model.

Carbon nanowalls (CNWs) functionalization allows the tailoring of some of their properties after they have been synthesized. Herewith we report on post-synthesis CNWs functionalization by plasma treatments. Radiofrequency plasmas generated in hydrogen, oxygen, nitrogen, tetrafluoroethane, and sulfur hexafluoride admixed to argon, were used. The changes of morphology, structure, chemical composition were determined by SEM, FTIR, Raman Spectroscopy, specifically for each gas mixture. The results point out to the surface erosion, the functional groups attachment, or to thin film coverage of CNWs material. Such processes strongly influence the material properties, as exemplified by the wettability: superhydrophilic or superhydrophobic surfaces are obtained starting from the same base CNWs material.

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 by 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 us 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 similar to 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.

We 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. (C) 2012 Elsevier B.V. All rights reserved.

When subjected to a linearly polarized terahertz pulse, a mesoscopic ring endowed with spin-orbit interaction (SOT) of the Rashba-Dresselhaus type exhibits non-uniform azimuthal charge and spin distributions. Both types of SOT 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 0, 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.

We study the conductance G and Seebeck coefficient S of a side-coupled double quantum dot system. The transport properties are the result of the competition between the Fano interference and Kondo correlations, being also controlled by the Coulomb blockade of the multilevel side-dot. The external parameters are the gate potential V-g applied on the side-dot and the temperature. When V-g is varied continuously the blockade is switched on and off periodically, resulting in oscillations of the transport coefficients. The profile of the oscillations and the temperature dependence are studied. An extended Anderson model and the Keldysh transport formalism are used.

The zero bias anomaly of the differential conductance of mesoscopic systems is a fingerprint of the electron-electron interaction. The common example is the peak of the differential conductance in Kondo mesoscopic systems. We show that in complex mesoscopic systems, in particular in the side-coupled double dot, the dip of the differential conductance at zero bias is also possible due to simultaneous effects of interference and correlations. The external parameters that control this effect are the temperature and the gate potential on the lateral dot. We argue that even in single dots, in a multiple lead configuration, the shift from suppression to enhancement of dI/dV with increasing bias is allowed if the bias applied on the leads is not symmetric. The differential conductance exhibits a peak-dip crossover, the effect being controlled by the strength of the asymmetry and the ratio of the dot-lead couplings. The scaling of the differential conductance for the double-dot system is also discussed. The results are obtained using an extended Anderson model, the Keldysh transport formalism and the equation of motion technique extended to the non-equilibrium.