arXiv subjects

Nelson Studart

Publications and source records attributed to Nelson Studart.

At least 19 recordsLinked to original sources

arXiv2016

Lifetime enhancement for multi-photon absorption in intermediate band solar cells

A semiconductor structure consisting of two coupled quantum wells embedded into the intrinsic region of a {\it p-i-n} junction is proposed to be implemented as an intermediate band solar cell with ratchet state. The localized conduction subband of the right-hand side quantum well is thought as the intermediated band, while the excited conduction subband of the right-hand side quantum well, coupled to right-rand side one, is thought to acts as the ratchet state. The photo-excited electron in the intermediate band can tunnel out the thin barrier separating the wells and accumulate into ratchet subband. This might raise the electron probability of being hit by a second photon and exiting out to the continuum, increasing solar cell current. Is presented a temporal rate model for describing the charge transport properties of the cell. Calculations are carried out by solving the time-dependent Schrödinger equation applying the time evolution operator within a pertinent choice of the non-commuting kinetic and potential operators. The efficiency in the generation of current is analyzed directly by studying the occupation of the subbands wells in the p-i-n junction, taking into account the injection and draining dynamic provided by the electrical contacts connected to the cell. As a result, the efficiency in the generation of current was found to be directly correlated to the relationship between optical generation and recombination rates regarding to the scattering to the ratchet state rate. This suggests that a good coupling between the intermediate band and the additional band is a key point to be analyzed when developing an efficient solar cell.

cond-mat.mes-hall↗

arXiv2013

Negative photoconductance in a biased multiquantum well with filter barriers

In this paper the photon-assisted electron motion in a multiquantum well (MQW) semiconductor heterostructure in the presence of an electric field is investigated. The time-dependent Schrodinger equation is solved by using the split-operator technique to determine the photocurrent generated by the electron movement through the biased MQW system. An analysis of the energy shifts in the photocurrent spectra reveals interesting features coming from the contributions of localized and extended states on the MQW system. The photocurrent signal is found to increase for certain values of electric field, leading to the analogue of the negative-conductance in resonant tunneling diodes. The origin of this enhancement is traced to the mixing of localized states in the QWs with those in the continuum. This mixing appears as anticrossings between the localized and extended states and the enhanced photocurrent can be related to the dynamically induced Landau-Zener-Stuckelberg-Majorana transition between two levels at the anticrossing.

cond-mat.mes-hall↗

arXiv2013

Generation and control of spin-polarized photocurrents in GaMnAs heterostructures

Photocurrents are calculated for a specially designed GaMnAs semiconductor heterostructure. The results reveal regions in the infrared range of the energy spectrum in which the proposed structure is remarkably spin-selective. For such photon energies, the generated photocurrents are strongly spin-polarized. Application of a relatively small static bias in the growth direction of the structure is predicted to efficiently reverse the spin-polarization for some photon energies. This behavior suggests the possibility of conveniently simple switching mechanisms. The physics underlying the results is studied and understood in terms of the spin-dependent properties emerging from the particular potential profile of the structure.

cond-mat.mes-hall↗

arXiv2006

Dispersion of confined optical phonons in semiconductor nanowires in the framework of acontinuum approach

Confined optical phonons are discussed for a semiconductor nanowire of the Ge (Si)prototype on the basis of a theory developed some years ago. In the present work this theory is adapted to a non polar material and generalized to the case when the phonon dispersion law involves both linear and quadratic terms in the wave vector. The treatment is considered along the lines of a continuous medium model and leads to a system of coupled differential equations describing oscillations of mixed nature. The nanowire is modelled in the form of an infinite circular cylinder and the solutions of the fundamental equations are found. We are thus led to a description of long wavelength optical phonons, which should show a closer agreement with experimental data and with calculations along atomistic models. The presented theory is applied to the calculation of optical phonons in a Ge nanowire. We have found the dispersion curves for various optical phonon modes. We also normalize the modes and discuss the electron-phonon interaction within the deformation potential approximation.

cond-mat.mtrl-sci↗

arXiv2006

Surface electron mobility over a helium film

The mobility of surface electrons localized over helium films underlying solid substrates has been evaluated by solving the Boltzmann equation in the time relaxation approximation and the force balance equation in which an effective mobility is obtained in terms of the dynamical structure factor of the nondegenerate electron liquid. The essential processes of electron scattering by gas atoms, ripplons, and film-solid interface roughness are taken into account. The electron mobility dependence on the film thickness and temperature is determined and compared with experimental data available. We find that the interface-roughness scattering is the dominant process for explaining the experimental results. We estimate the extended defect sizes of the underlying substrate within the Gaussian correlated model for interface roughness.

cond-mat.mes-hall↗

arXiv2005

Many-body interactions in a quantum wire in the integer quantum Hall regime: suppression of exchange-enhanced g factor

The collapse of Hall gaps in the integer quantum Hall liquid in a quantum wire is investigated. Motivated by recent experiment [Pallecchi et al. PRB 65, 125303 (2002)] previous approaches are extended to treat confinement effects and the exchanged enhanced g-factor in quantum wires. Two scenarios for the collapse of the $ν=1$ state are discussed. In the first one the $ν=1$ state becomes unstable at $B_{cr}^{(1)}$, due to the exchange interaction and correlation effects, coming from the edge-states screening. In the second scenario, a transition to the $ν=2$ state occurs at $B_{cr}^{(2)}$, with a smaller effective channel width, caused by the redistribution of the charge density. This effect turns the Hartree interaction essential in calculating the total energy and changes $B_{cr}^{(2)}$ drastically. In both scenarios, the exchange enhanced g-factor is suppressed for magnetic fields lower than $B_{cr}$. Phase diagrams for the Hall gap collapse are determined. The critical fields, activation energy, and optical $g$-factor obtained are compared with experiments. Within the accuracy of the available data, the first scenario is most probable to be realized.

cond-mat.mes-hall↗

arXiv2004

Exchange interaction effects in the thermodynamic properties of quantum dots

We study electron-electron interaction effects in the thermodynamic properties of quantum-dot systems. We obtain the direct and exchange contributions to the specific heat C_v in the self-consistent Hartree-Fock approximation at finite temperatures. An exchange-induced phase transition is observed and the transition temperature is shown to be inversely proportional to the size of the system. The exchange contribution to C_v dominates over the direct and kinetic contributions in the intermediate regime of interaction strength (r_s ~ 1). Furthermore, the electron-electron interaction modifies both the amplitude and the period of magnetic field induced oscillations in C_v.

cond-mat.mes-hall↗

arXiv2004

Selective coherent destruction of tunneling in a quantum-dot array

The coherent manipulation of quantum states is one of the main tasks required in quantum computation. In this paper we demonstrate that it is possible to control coherently the electronic position of a particle in a quantum-dot array. By tuning an external ac electric field we can selectively suppress the tunneling between dots, trapping the particle in a determined region of the array. The problem is treated non-perturbatively by a time-dependent Hamiltonian in the effective mass approximation and using Floquet theory. We find that the quasienergy spectrum exhibits crossings at certain field intensities that result in the selective suppression of tunneling.

cond-mat.mes-hall↗

arXiv2003

Orbital magnetic properties of quantum dots: the role of electron-electron interactions

We study the magnetic orbital response of a system of N interacting electrons confined in a two-dimensional geometry and subjected to a perpendicular magnetic field in the finite temperature Hartree-Fock approximation. The electron-electron interaction is modelled by a short-range Yukawa-type potential. We calculate the ground state energy, magnetization, and magnetic susceptibility as a function of the temperature, the potential range, and the magnetic field. We show that the amplitude and period of oscillations in the magnetic susceptibility are strongly affected by the electron-electron interaction as evidenced in experimental results. The zero-field susceptibility displays both paramagnetic and diamagnetic phases as a function of temperature and the number of confined electrons.

cond-mat.mes-hall↗

arXiv2003

Magnetoresistance of nondegenerate quantum electron channels formed on the surface of superfluid helium

Transport properties of quasi-one-dimensional nondegenerate quantum wires formed on the surface of liquid helium in the presence of a normal magnetic field are studied using the momentum balance equation method and the memory function formalism. The interaction with both kinds of scatterers available (vapor atoms and capillary wave quanta) is considered. We show that unlike classical wires, quantum nondegenerate channels exhibit strong magnetoresistance which increases with lowering the temperature.

cond-mat.str-el↗

arXiv2003

Electron-phonon interaction in quantum-dot/quantum-well semiconductor heterostructures

Polar optical phonons are studied in the framework of the dielectric continuum approach for a prototypical quantum-dot/quantum-well (QD/QW) heterostructure, including the derivation of the electron-phonon interaction Hamiltonian and a discussion of the effects of this interaction on the electronic energy levels. The particular example of the CdS/HgS QD/QW is addressed and the system is modelled according to the spherical geometry, considering a core sphere of material "1" surrounded by a spherically concentric layer of material "2", while the whole structure is embedded in a host matrix assumed as an infinite dielectric medium. The strength of the electron-LO phonon coupling is discussed in details and the polaronic corrections to both ground state and excited state electron energy levels are calculated. Interesting results concerning the dependence of polaronic corrections with the QD/QW structure size are analyzed.

cond-mat.mes-hall↗

arXiv2002

Non-perturbative electron dynamics in crossed fields

Intense AC electric fields on semiconductor structures have been studied in photon-assisted tunneling experiments with magnetic field applied either parallel (B_par) or perpendicular (B_per) to the interfaces. We examine here the electron dynamics in a double quantum well when intense AC electric fields F, and tilted magnetic fields are applied simultaneously. The problem is treated non-perturbatively by a time-dependent Hamiltonian in the effective mass approximation, and using a Floquet-Fourier formalism. For B_par=0, the quasi-energy spectra show two types of crossings: those related to different Landau levels, and those associated to dynamic localization (DL), where the electron is confined to one of the wells, despite the non-negligible tunneling between wells. B_par couples parallel and in-plane motions producing anti-crossings in the spectrum. However, since our approach is non-perturbative, we are able to explore the entire frequency range. For high frequencies, we reproduce the well known results of perfect DL given by zeroes of a Bessel function. We find also that the system exhibits DL at the same values of the field F, even as B_par non-zero, suggesting a hidden dynamical symmetry in the system which we identify with different parity operations. The return times for the electron at various values of field exhibit interesting and complex behavior which is also studied in detail. We find that smaller frequencies shifts the DL points to lower field F, and more importantly, yields poorer localization by the field. We analyze the explicit time evolution of the system, monitoring the elapsed time to return to a given well for each Landau level, and find non-monotonic behavior for decreasing frequencies.

cond-mat.mes-hall↗

arXiv2001

Interface optical phonons in spheroidal dots: Raman selection rules

The contribution of interface phonons to the first order Raman scattering in nanocrystals with non spherical geometry is analyzed. Interface optical phonons in the spheroidal geometry are discussed and the corresponding Frohlich-like electron-phonon interaction is reported in the framework of the dielectric continuum approach. It is shown that the interface phonon modes are strongly dependent on the nanocrystal geometry, particularly on the ellipsoid's semi-axis ratio. The new Raman selection rules have revealed that solely interface phonon modes with even angular momentum are allowed to contribute to the first order phonon-assisted scattering of light. On this basis we are able to give an explanation for the observed low frequency shoulders present in the Raman cross-section of several II-VI semiconductor nanostructures.

cond-mat.mes-hall↗

arXiv2001

Temperature effects on edge-state properties in the integer quantum Hall regime

The edge and bulk structure of Landau levels (LLs) in a wide channel at the $ ν=1$ quantum Hall regime is calculated for not-too-low temperatures, $\hbar ω_{c} \gg k_{B}T\gg \hbar v_{g}/2\ell_{0}$, where $v_{g}$ is the group velocity of the edge states and $\ell_{0}=\sqrt{\hbar c/|e|B}$ is the magnetic length. Edge-states correlations essentially modify the spatial behavior of the lowest spin-up LL, which is occupied, compared to the lowest spin-down LL, which is empty. The influence of many-body interactions on the spatially inhomogeneous spin-splitting between the two lowest LLs is studied within the generalized local density approximation. Temperature effects on the enhanced spin-splitting, the position of the Fermi level within the exchange enhanced gap and the renormalization of edge-states group velocity by edge states screening are considered. It is shown that the maximum activation energy $G$ in the bulk of the channel is determined by the gap between the Fermi level and the bottom of the spin-down LL, because the gap between the Fermi level and the spin-up LL is much larger. For the maximum value of $G$, it is shown that the renormalized group velocity $v_{g}\propto T$ for $T\to 0$ and, in particular, the condition of not-too-low $T$ can be satisfied for $4.2\agt T\agt0.3$ K. In other words, the regime of not-too-low temperatures regime can be achieved even for rather low $T$.

cond-mat.mes-hall↗

arXiv2001

Renormalization approach for quantum-dot structures under strong alternating fields

We develop a renormalization method for calculating the electronic structure of single and double quantum dots under intense ac fields. The nanostructures are emulated by lattice models with a clear continuum limit of the effective-mass and single-particle approximations. The coupling to the ac field is treated non-perturbatively by means of the Floquet Hamiltonian. The renormalization approach allows the study of dressed states of the nanoscopic system with realistic geometries as well arbitrary strong ac fields. We give examples of a single quantum dot, emphasizing the analysis of the effective-mass limit for lattice models, and double-dot structures, where we discuss the limit of the well used two-level approximation.

cond-mat.mes-hall↗

arXiv2001

Electron transport in a quasi-one dimensional channel on suspended helium films

Quasi-one dimensional electron systems have been created using a suspended helium film on a structured substrate. The electron mobility along the channel is calculated by taking into account the essential scattering processes of electrons by helium atoms in the vapor phase, ripplons, and surface defects of the film substrate. It is shown that the last scattering mechanism may dominate the electron mobility in the low temperature limit changing drastically the temperature dependence of the mobility in comparison with that controlled by the electron-ripplon scattering.

cond-mat.mes-hall↗

arXiv2001

The invention of the quantum energy concept according to Planck

A hundred years ago, the quantum concept provoked a revolution in science and the search of a new conceptual basis for whole physics, as emphasized by Einstein. In this paper, I discuss the essential features of Planck's works in 1900 on the blackbody radiation and the hypothesis of energy quantization.

physics.hist-ph↗

arXiv2001

Polaron effects in electron channels on a helium film

Using the Feynman path-integral formalism we study the polaron effects in quantum wires above a liquid helium film. The electron interacts with two-dimensional (2D) surface phonons, i.e. ripplons, and is confined in one dimension (1D) by an harmonic potential. The obtained results are valid for arbitrary temperature ($T$), electron-phonon coupling strength ($α$), and lateral confinement ($ω_{0}$). Analytical and numerical results are obtained for limiting cases of $T$, $α$, and $ω_{0}$. We found the surprising result that reducing the electron motion from 2D to quasi-1D makes the self-trapping transition more continuous.

cond-mat.str-el↗