Dr. Radu DRAGOMIR

Using the master equation approach, we look for fingerprints of the interaction between the localized spin S of a nanomagnet coupled to spin-polarized leads and its quantized vibrational modes. We find that the stationary and transient currents are sensitive to vibron-assisted transitions of the molecular spin on both sides of the anisotropy barrier. Such transitions are associated with vibron-dressed states and triggered under resonant conditions. Transport calculations are presented for two antiparallel configurations of the spin-polarized electrodes. In the first configuration, and far from a resonance point, a blockade is imposed on both the electronic and molecular spins via their exchange interaction. When sweeping the magnetic field through resonance, the spin-vibron interaction removes this blockade and allows the indirect reading of resonant transitions as the molecular spin climbs the left side of the anisotropy barrier. In the second configuration, the anisotropy barrier is overcome but the vibron-assisted transitions on the right side of the anisotropy barrier "delocalize" the molecular spin and do not allow the complete current-induced magnetic switching -S & RARR; S. In both configurations, the stationary current increases on resonance, due to additional transport channels triggered by the spin-vibron coupling. Therefore, the switching of the spin-vibron coupling could be detected in future transport experiments.

We present a theoretical analysis of the interplay between the spin-vibron and electron-vibron interactions in a hybrid system made of a single-molecule magnet and a suspended conductor. The latter is coupled to particle reservoirs and supports quantized vibrational modes which, once activated, interact with the localized magnetic moment S of the nanomagnet. The dynamics of the molecular spin, the average vibron number, and the transient currents are calculated from the reduced density operator of the hybrid system. We focus on the effect of the vibron-assisted transitions from the lowest energy spin doublet S-z = +/- S to higher energy excited states. The numerical simulations performed for the simplest case S = 2 prove that the vibron-assisted spin transitions and dynamics can be described in terms of a three-level Lambda model borrowed from quantum optics. In particular we predict the existence of Rabi oscillations of the transient currents as fingerprints of the spin-vibron coupling. The role of symmetric or asymmetric bias configurations in setting different mixtures of molecular spin states in the steady-state regime is also emphasized.

The 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 s(v). associated to an electronic configuration nu. 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.

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.

Using the kp theory and configuration interaction method, we analyze the effect of heavy hole-light hole (HH-LH) mixing in CdTe quantum dots (QDs) with a single manganese (Mn) ion. We find that the hole-Mn exchange switches the coupling between two excitons whose Luttinger spinors have both HH and LH components. If the magnetic dopant is off-centered and the QD is subjected to a single pulse the system periodically bounces between bright and dark mostly HH excitons with opposite HH spins. A pump-and-probe setup allows to estimate the efficiency of this HH spin unpinning from the biexciton response. The biexciton absorption spectrum is also discussed.

When a quantum dot is embedded into a superconducting environment, it leads to the formations of localized Shiba states inside the gap. If the Coulomb interaction is sufficiently small, the Shiba states consist of a pair of singlet states and a doublet, that compete for the ground state and induce a quantum phase transition. In the presence of an external microwave field, the Shiba energy spectrum is significantly modified. Moreover, the transmission of the cavity inherits features that can pinpoint the exact location of the quantum critical point. In terms of methods used, our analytical calculations are supplemented by state of the art numerical renormalization group calculations.

We 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.

We study the generation of p-shell excitons in optically active disk-shaped quantum dots subjected to ultrafast optical pulses. The single-particle spectral properties are obtained from the four-band kp theory, whereas the Coulomb interaction is taken into account within the configuration-interaction approach, particular attention being payed to configuration mixing due to electron-hole correlations. The effect of intraband relaxation processes is included in the non-unitary dynamics derived from the von-Neumann Lindblad equation. We find that the fast hole relaxation processes drive the p-shell excitons to intermediate states which eventually evolve to s-shell excitons via slower electron relaxation.

We study the effects of intraband relaxation processes on optical manipulation protocols for sp biexcitons hosted by CdTe disk-shaped quantum dots. The many-body states are calculated within the configuration interaction method starting from single-particle states given by the k.p theory. The time-dependent occupations of relevant many-body states are extracted from the von Neumann-Lindblad equation for the density operator. We mainly investigate the generation of sp biexcitons with two pulses of different polarizations sigma(+) and sigma(-). The fast hole relaxation processes prevent a high-fidelity controlled operation on sp biexcitons and lead to the occupation of some transient states which can be optically probed. More importantly, the many-body structure of the transient states consists of two holes on the s shell and antiparallel sp triplet states for electrons. Our simulations show that these triplet states are more stable against decoherence as they can only be damaged through slow electron relaxation. The configuration mixing due to correlation effects is also discussed.