arXiv subjects

Andrei Manolescu

Publications and source records attributed to Andrei Manolescu.

At least 73 records · Page 4Linked to original sources

arXiv2019

Back-action effects in cavity-coupled quantum conductors

We study the electronic transport through a pair of distant nanosystems ($S_a$ and $S_b$) embedded in a single-mode cavity. Each system is connected to source and drain particle reservoirs and the electron-photon coupling is described by the Tavis-Cummings model. The generalized master equation approach provides the reduced density operator of the double-system in the dressed-states basis. It is shown that the photon-mediated coupling between the two subsystems leaves a signature on their transient and steady-state currents. In particular, a suitable bias applied on subsystem $S_b$ induces a photon-assisted current in the other subsystem $S_a$ which is otherwise in the Coulomb blockade. We also predict that a transient current passing through one subsystem triggers a charge transfer between the optically active levels of the second subsystem even if the latter is not connected to the leads. As a result of back-action, the transient current through the open system develops Rabi oscillations (ROs) whose period depends on the initial state of the closed system.

cond-mat.mes-hall↗

arXiv2019

The interplay of electron-photon and cavity-environment coupling on the electron transport through a quantum dot system

We theoretically investigate the characteristics of the electron transport through a two-dimensionala quantum dot system in the $xy$-plane coupled to a photon cavity and a photon reservoir, the environment. The electron-photon coupling, $g_γ$, and the cavity-reservoir coupling, $κ$, are tuned to study the system in the weak, $g_γ \leq κ$, and the strong coupling regime, $g_γ > κ$. An enhancement of current is both seen with increasing $g_γ$ and $κ$ in the weak coupling regime for both $x$- and $y$-polarization of the photon field. This is a direct consequence of the Purcell effect. The current enhancement is due to the contribution of the photon replica states to the electron transport in which intraband transitions play an important role. The properties of the electron transport are drastically changed in the strong coupling regime with an $x$-polarized photon field in which the current is suppressed with increasing $g_γ$, but it is still increasing with $κ$. This behavior of the current is related to the population of purely electronic states and depopulation of photon replica states.

cond-mat.mes-hall↗

arXiv2019

Generalized Master Equation Approach to Time-Dependent Many-Body Transport

We recall theoretical studies on transient transport through interacting mesoscopic systems. It is shown that a generalized master equation (GME) written and solved in terms of many-body states provides the suitable formal framework to capture both the effects of the Coulomb interaction and electron--photon coupling due to a surrounding single-mode cavity. We outline the derivation of this equation within the Nakajima-Zwanzig formalism and point out technical problems related to its numerical implementation for more realistic systems which can neither be described by non-interacting two-level models nor by a steady-state Markov-Lindblad equation. We first solve the GME for a lattice model and discuss the dynamics of many-body states in a two-dimensional nanowire, the dynamical onset of the current-current correlations in electrostatically coupled parallel quantum dots and transient thermoelectric properties. Secondly, we rely on a continuous model to get the Rabi oscillations of the photocurrent through a double-dot etched in a nanowire and embedded in a quantum cavity. A~many-body Markovian version of the GME for cavity-coupled systems is also presented.

cond-mat.mes-hall↗

arXiv2019

Corner and side localization of electrons in irregular hexagonal semiconductor shells

We discuss the low energy electronic states in hexagonal rings. These states correspond to the transverse modes in core-shell nanowires built of III-V semiconductors which typically have a hexagonal cross section. In the case of symmetric structures the 12 lowest states (including the spin) are localized in the corners, while the next following 12 states are localized mostly on the sides. Depending on the material parameters, in particular the effective mass, the ring diameter and width, the corner and side states may be separated by a considerable energy gap, ranging from few to tens of meV. In a realistic fabrication process geometric asymmetries are unavoidable, and therefore the particles are not symmetrically distributed between all corner and side areas. Possibly, even small deformations may shift the localization of the ground state to one of the sides. The transverse states or the transitions between them may be important in transport or optical experiments. Still, up to date, there are only very few experimental investigations of the localization-dependent properties of core-shell nanowires.

cond-mat.mes-hall↗

arXiv2019

Cavity-photon induced high order transitions between ground states of quantum dots

We show that quantum electromagnetic transitions to high orders are essential to describe the time-dependent path of a nanoscale electron system in a Coulomb blockage regime when coupled to external leads and placed in a three-dimensional rectangular photon cavity. The electronic system consists of two quantum dots embedded asymmetrically in a short quantum wire. The two lowest in energy spin degenerate electron states are mostly localized in each dot with only a tiny probability in the other dot. In the presence of the leads we identify a slow high order transition between the ground states of the two quantum dots. The Fourier power spectrum for photon-photon correlations in the steady state shows a Fano-type of a resonance for the frequency of the slow transition. Full account is taken of the geometry of the multi-level electronic system, and the electron-electron Coulomb interactions together with the para- and diamagnetic electron-photon interactions are treated with step wise exact numerical diagonalization and truncation of appropriate many-body Fock spaces. The matrix elements for all interactions are computed analytically or numerically exactly.

cond-mat.mes-hall↗

arXiv2019

Manifestation of the Purcell effect in current transport through a dot-cavity-QED system

We study the transport properties of a wire-dot system coupled to a cavity and a photon reservoir. Tuning the photon energy, Rabi-resonant states emerge and in turn resonant current peaks are observed. We demonstrate the effects of the cavity-photon reservoir coupling, the mean photon number in the reservoir, the electron-photon coupling and the photon polarization on the intraband transitions occurring between the Rabi-resonant states, and on the corresponding resonant current peaks. The Rabi-splitting can be controlled by the photon polarization and the electron-photon coupling strength. In the selected range of parameters, we observe the results of the Purcell effect enhancing the current peaks through the cavity by increasing the cavity-reservoir coupling, while they decrease with increasing the electron-photon coupling. In addition, the resonant current peaks are also sensitive to the mean number of photons in the reservoir.

cond-mat.mes-hall↗

arXiv2019

Transverse polarization light scattering in tubular semiconductor nanowires

We carry out numerical calculations of the scattering cross section of tubular semiconductor nanocylinders in the optical range. The scattering is investigated for the transversal incidence of light, i.e., along the diameter of the cylinder, with both transverse electric and transverse magnetic polarization. These subwavelength nanostructures support Mie resonances and, when the length of the cylinder is comparable to the wavelength, guided modes that can overlap with the Mie modes giving rise to sharp Fano resonances. We show that a varying internal radius affects each mode differently, allowing for an extra degree of freedom for tuning the spectral position of the resonant peaks.

cond-mat.mes-hall↗

arXiv2019

The photocurrent generated by photon replica states of an off-resonantly coupled dot-cavity system

Transport properties of a quantum dot coupled to a photon cavity are investigated using a quantum master equation in the steady-state regime. In the off-resonance regime, when the photon energy is smaller than the energy spacing between the lowest electron states of the quantum dot, we calculate the current that is generated by photon replica states as the electronic system is pumped with photons.Tuning the electron-photon coupling strength, the photocurrent can be enhanced by the influences of the photon polarization, and the cavity-photon coupling strength of the environment. We show that the current generated through the photon replicas is very sensitive on the photon polarization, but it is not strongly dependent on the average number of photons in the environment.

cond-mat.mes-hall↗

arXiv2019

Oscillations in electron transport caused by multiple resonances in a quantum dot-QED system in the steady-state regime

We model the electron transport current as the photon energy is swept through several resonances of a multi-level quantum dot, embedded in a short quantum wire, coupled to photon cavity. We use a Markovian quantum master equation appropriate for the long-time evolution and include the electron-electron and both the para- and the diamagnetic electron-photon interactions via diagonalization in a truncated many-body Fock space. Tuning the photon energy, several anti-crossings caused by Rabi-splitting in the energy spectrum of the quantum dot system are found. The strength of the Rabi-splittings and the photon-exchange between the resonant states depend on the polarization of the cavity photon field. We observe oscillations of the charge in the system and several resonant transport current peaks for the photon energies corresponding to the resonances in the steady-state regime.

cond-mat.mes-hall↗

arXiv2019

The hysteresis-free behavior of perovskite solar cells from the perspective of the measurement conditions

We investigate in how far the hysteresis-free behavior of perovskite solar cells can be reproduced using particular pre-conditioning and measurement conditions. As there are currently more and more reports of perovskite solar cells without J-V hysteresis it is crucial to distinguish between genuine performance improvements and measurement artifacts. We focus on two of the parameters that influence the dynamic J-V scans, namely the bias scan rate and and the bias poling voltage, and point out measurement conditions for achieving a hysteresis-free behavior. In this context we discuss the suitability of defining a hysteresis index (HI) for the characterization of dynamic J-V scans. Using HI, aging effects are also investigated, establishing a potential connection between the sample degradation and the variation of the maximal hysteresis on one hand, and relaxation time scale of the slow process, on the other hand. Pre-poling induced recombinations effects are identified. In addition, our analysis based on sample pre-biasing reveals potential indication regarding two types of slow processes, with two different relaxation time scales, which provides further insight regarding ionic migration.

physics.app-ph↗

arXiv2018

Thermoelectric inversion in a resonant quantum dot-cavity system in the steady-state regime

The thermoelectric effect in a quantum dot system connected to two electron reservoirs in the presence of a photon cavity is investigated using a quantum master equation in the steady-state regime. If a quantized photon field is applied to the quantum dot system, an extra channel, the photon replica states, are formed leading to a generation of a photon-induced thermoelectric current. We observe that the photon replica states contribute to the transport irrespective of the direction of the thermal gradient. In the off-resonance regime, when the photon energy is smaller than the energy difference between the lowest states of the quantum dot system, a current plateau is seen for strong electron-photon coupling. In the resonant regime, an inversion of thermoelectric current emerges due to the Rabi-splitting. Therefore, the photon field can change both the magnitude and the sign of the thermoelectric current induced by the temperature gradient in the absence of a voltage bias between the leads.

cond-mat.mes-hall↗

arXiv2018

Coexisting spin and Rabi-oscillations at intermediate time in electron transport through a photon cavity

We model theoretically the time-dependent transport through an asymmetric double quantum dot etched in a two-dimensional wire embedded in a FIR photon cavity. For the transient and the intermediate time regimes the currents and the average photon number are calculated by solving a Markovian master equation in the dressed-states picture, with the Coulomb interaction also being taken into account. We predict that in the presence of a transverse magnetic field the interdot Rabi oscillations appearing in the intermediate and transient regime coexist with slower non-equilibrium fluctuations in the occupation of states for opposite spin orientation. The interdot Rabi oscillation induces charge oscillations across the system and a phase difference between the transient source and drain currents. We point out a difference between the steady-state correlation functions in the Coulomb blocking and the photon-assisted transport regimes.

cond-mat.mes-hall↗

arXiv2018

Conductance features of core-shell nanowires determined by the internal geometry

We consider electrons in tubular nanowires with prismatic geometry and infinite length. Such a model corresponds to a core-shell nanowire with an insulating core and a conductive shell. In a prismatic shell the lowest energy states are localized along the edges (corners) of the prism and are separated by a considerable energy gap from the states localized on the prism facets. The corner localization is robust in the presence of a magnetic field longitudinal to the wire. If the magnetic field is transversal to the wire the lowest states can be shifted to the lateral regions of the shell, relatively to the direction of the field. These localization effects should be observable in transport experiments on semiconductor core-shell nanowires, typically with hexagonal geometry. We show that the conductance of the prismatic structures considerably differs from the one of circular nanowires. The effects are observed for sufficiently thin hexagonal wires and become much more pronounced for square and triangular shells. To the best of our knowledge the internal geometry of such nanowires is not revealed in experimental studies. We show that with properly designed nanowires these localization effects may become an important resource of interesting phenomenology.

cond-mat.mes-hall↗

arXiv2018

Radiated fields by polygonal core-shell nanowires

We calculate the electromagnetic field radiated by tubular nanowires with prismatic geometry and infinite length. The polygonal geometry has implications on the electronic localization; the lowest energy states are localized at the edges of the prism and are separated by a considerable energy gap from the states localized on the facets. This localization can be controlled with external electric or magnetic fields. In particular, by applying a magnetic field transverse to the wire the states may become localized on the lateral regions of the shell, relatively to the direction of the field, leading to channels of opposite currents. Because of the prismatic geometry of the nanowire the current distribution, and hence the radiated electromagnetic field, have an anisotropic structure, which can be modified by the external fields. In this work we study hexagonal, square and triangular nanowires.

cond-mat.mes-hall↗

arXiv2018

Thermoelectric current in topological insulator nanowires with impurities

In this paper we consider charge current generated by maintaining a temperature difference over a nanowire at zero voltage bias. For topological insulator nanowires in a perpendicular magnetic field the current can change sign as the temperature of one end is increased. Here we study how this thermoelectric current sign reversal depends on magnetic field and how impurities affect the size of the thermoelectric current. We consider both scalar and magnetic impurities and show that their influence on the current are quite similar, although the magnetic impurities seem to be more effective in reducing the effect. For moderate impurity concentration the sign reversal persists.

cond-mat.mes-hall↗

arXiv2018

Robust topological phase in proximitized core-shell nanowires coupled to multiple superconductors

We consider core-shell nanowires with prismatic geometry contacted with two or more superconductors in the presence of a magnetic field applied parallel to the wire. In this geometry, the lowest energy states are localized on the outer edges of the shell, which strongly inhibits the orbital effects of the longitudinal magnetic field that are detrimental to Majorana physics. Using a tight-binding model of coupled parallel chains, we calculate the topological phase diagram of the hybrid system in the presence of non-vanishing transverse potentials and finite relative phases between the parent superconductors. We show that having finite relative phases strongly enhances the stability of the induced topological superconductivity over a significant range of chemical potentials and reduces the value of the critical field associated with the topological quantum phase transition.

cond-mat.mes-hall↗

arXiv2017

Photon-induced tunability of the thermospin current in a Rashba ring

The goal of this work is to show how the thermospin polarization current in a quantum ring changes in the presence of Rashba spin-orbit coupling and a quantized single photon mode of a cavity the ring is placed in. Employing the reduced density operator and a general master equation formalism, we find that both the Rashba interaction and the photon field can significantly modulate the spin polarization and the thermospin polarization current. Tuning the Rashba coupling constant, degenerate energy levels are formed corresponding to the Aharonov-Casher destructive phase interference in the quantum ring system. Our analysis indicates that the maximum spin polarization can be observed at the points of degenerate energy levels due to spin accumulation in the system without the photon field. The thermospin current is thus suppressed. In the presence of the cavity, the photon field leads to an additional kinetic momentum of the electron. As a result the spin polarization can be enhanced by the photon field.

cond-mat.mes-hall↗

arXiv2017

Effects of photon field on heat transport through a quantum wire attached to leads

We theoretically investigate photo-thermoelectric transport through a quantum wire in a photon cavity coupled to electron reservoirs with different temperatures. Our approach, based on a quantum master equation, allows us to investigate the influence of a quantized photon field on the heat current and thermoelectric transport in the system. We find that the heat current through the quantum wire is influenced by the photon field resulting in a negative heat current in certain cases. The characteristics of the transport are studied by tuning the ratio, $\hbarω_γ / k_{\rm B} ΔT$, between the photon energy, $\hbarω_γ$, and the thermal energy, $k_{\rm B} ΔT$. The thermoelectric transport is enhanced by the cavity photons when $k_{\rm B} ΔT > \hbarω_γ$. By contrast, if $k_{\rm B} ΔT < \hbarω_γ$, the photon field is dominant and a suppression in the thermoelectric transport can be found in the case when the cavity-photon field is close to a resonance with the two lowest one-electron states in the system. Our approach points to a new technique to amplify thermoelectric current in nano-devices.

cond-mat.mes-hall↗