arXiv subjects

Andrei Manolescu

Publications and source records attributed to Andrei Manolescu.

At least 37 records · Page 2Linked to original sources

arXiv2023

Electron-hole coherence in core-shell nanowires with partial proximity induced superconductivity

By solving the Bogoliubov-de Gennes Hamiltonian, the electron-hole coherence within a partially proximitized n-doped semiconductor shell of a core-shell nanowire heterostructure is investigated numerically and compared with the Andreev reflection interpretation of proximity induced superconductivity. Partial proximitization is considered to quantify the effects of a reduced coherence length. Three cases of partial proximitization of the shell are explored: radial, angular and longitudinal. For the radial case, it is found that the boundary conditions impose localization probability maxima in the center of the shell in spite of off-center radial proximitization. The induced superconductivity gap is calculated as a function of the ratio between the proximitized shell thickness and the total shell thickness. In the angular case, the lowest energy state of a hexagonal wire with a single proximitized side is found to display the essence of Andreev reflection, only by lengthwise summation of the localization probability. In the longitudinal case, a clear correspondence with Andreev reflection is seen in the localization probability as a function of length along a half-proximitized wire. The effect of an external magnetic field oriented along the wire is explored.

cond-mat.mes-hall↗

arXiv2023

Magnetic properties of a cavity-embedded square lattice of quantum dots or antidots

We apply quantum electrodynamical density functional theory to obtain the electronic density, the spin polarization, as well as the orbital and the spin magnetization of square periodic arrays of quantum dots or antidots subjected to the influence of a far-infrared cavity photon field. A gradient-based exchange-correlation functional adapted to a two-dimensional electron gas in a transverse homogeneous magnetic field is used in the theoretical framework and calculations. The obtained results predict a non-trivial effect of the cavity field on the electron distribution in the unit cell of the superlattice, as well as on the orbital and the spin magnetization. The number of electrons per unit cell of the superlattice is shown to play a crucial role in the modification of the magnetization via the electron-photon coupling. The calculations show that cavity photons strengthen the diamagnetic effect in the quantum dots structure, while they weaken the paramagnetic effect in an antidot structure. As the number of electrons per unit cell of the lattice increases the electron-photon interaction reduces the exchange forces that would otherwise promote strong spin splitting for both the dot and the antidot array.

cond-mat.mes-hall↗

arXiv2023

Efficient method to calculate energy spectra for analysing magneto-oscillations

Magneto-oscillations in two-dimensional systems with spin-orbit interaction are typically characterized by fast Shubnikov-de~Haas (SdH) oscillations and slower spin-orbit-related beatings. The characterization of the full SdH oscillatory behavior in systems with both spin-orbit interaction and Zeeman coupling requires a time consuming diagonalization of large matrices for many magnetic field values. By using the Poisson summation formula we can explicitly separate the density of states into, fast and slow oscillations, which determine the corresponding fast and slow parts of the magneto-oscillations. We introduce an efficient scheme of partial diagonalization of our Hamiltonian, where only states close to the Fermi energy are needed to obtain the SdH oscillations, thus reducing the required computational time. This allows an efficient method for fitting numerically the SdH data, using the inherent separation of the fast and slow oscillations. We compare systems with only Rashba spin-orbit interaction (SOI) and both Rashba and Dresselhaus SOI with, and without, an in-plane magnetic field. The energy spectra are characterized in terms of symmetries, which have direct and visible consequences in the magneto-oscillations. To highlight the benefits of our methodology, we use it to extract the spin-orbit parameters by fitting realistic transport data.

cond-mat.mes-hall↗

arXiv2023

Mapping confinement potentials and charge densities of interacting quantum systems using conditional generative adversarial networks

Accurate and efficient tools for calculating the ground state properties of interacting quantum systems are essential in the design of nanoelectronic devices. The exact diagonalization method fully accounts for the Coulomb interaction beyond mean field approximations and it is regarded as the gold-standard for few electron systems. However, by increasing the number of instances to be solved, the computational costs become prohibitive and new approaches based on machine learning techniques can provide a significant reduction in computational time and resources, maintaining a reasonable accuracy. Here, we employ {\tt pix2pix}, a general-purpose image-to-image translation method based on conditional generative adversarial network (cGAN), for predicting ground state densities from randomly generated confinement potentials. Other mappings were also investigated, like potentials to non-interacting densities and the translation from non-interacting to interacting densities. The architecture of the cGAN was optimized with respect to the internal parameters of the generator and discriminator. Moreover, the inverse problem of finding the confinement potential given the interacting density can also be approached by the pix2pix mapping, which is an important step in finding near-optimal solutions for confinement potentials.

cond-mat.mes-hall↗

arXiv2023

Effect of Impurities on Charge and Heat Transport in Tubular Nanowires

We calculate the charge and heat currents carried by electrons, originating from a temperature gradient and a chemical potential difference between the two ends of tubular nanowires with different geometries of the cross-sectional areas: circular, square, triangular, and hexagonal. We consider nanowires based on InAs semiconductor material, and use the Landauer-Büttiker approach to calculate the transport quantities. We include impurities in the form of delta scatterers and compare their effect for different geometries. The results depend on the quantum localization of the electrons along the edges of the tubular prismatic shell. For example, the effect of impurities on the charge and heat transport is weaker in the triangular shell than in the hexagonal shell, and the thermoelectric current in the triangular case is several times larger than in the hexagonal case, for the same temperature gradient.

cond-mat.mes-hall↗

arXiv2023

Controlling the excitation spectrum of a quantum dot array with a photon cavity

We use a recently proposed quantum electrodynamical density functional theory (QEDFT) functional in a real-time excitation calculation for a two-dimensional electron gas in a square array of quantum dots in an external constant perpendicular magnetic field to model the influence of cavity photons on the excitation spectra of the system. The excitation is generated by a short elecrical pulse. The quantum dot array is defined in an AlGaAs-GaAs heterostructure, which is in turn embedded in a parallel plate far-infrared photon-microcavity. The required exchange and correlation energy functionals describing the electron-electron and electron-photon interactions have therefore been adapted for a two-dimensional electron gas in a homogeneous external magnetic field. We predict that the energies of the excitation modes activated by the pulse are generally red-shifted to lower values in the presence of a cavity. The red-shift can be understood in terms of the polarization of the electron charge by the cavity photons and depends on the magnetic flux, the number of electrons in a unit cell of the lattice, and the electron-photon interaction strength. We find an interesting interplay of the exchange forces in a spin polarized two-dimensional electron gas and the square lattice structure leading to a small but clear blue-shift of the excitation mode spectra when one electron resides in each dot.

cond-mat.mes-hall↗

arXiv2023

Effect of pressure on the dynamics of iodine defects in MAPI: An atomistic simulation

The diffusion of iodine defects has been considered the most important degradation mechanism of methylammonium lead iodine (MAPI) in solar cells. The present study demonstrates the importance of the pressure inside this material on the dynamics of iodine defects, using molecular dynamics simulations. It is known that the diffusion coefficient of an iodine vacancy is an order of magnitude higher than that of interstitial iodine. We show that this difference systematically increases with increased tensile strain and that both diffusion coefficients tend to zero when a compressive strain is applied. This result suggests that compression of the MAPI can be a good solution to reduce its degradation rate. %Also, different methods of determining the diffusion coefficient, including mean squared displacement (MSD) and velocity auto-correlation function, are discussed and applied to the case of ${\rm V}_{\rm I}$. Besides, the statistical aspect of deriving the diffusion coefficient from the mean squared displacement (MSD) is discussed in terms of the initial conditions (positions and velocities) of the atoms and the simulation time, considering different seeds of the pseudo-random number generator used in the simulations performed with the LAMMPS software.

cond-mat.mtrl-sci↗

arXiv2023

Distinct melting behaviour of partially oxidized Cu nanoparticles and nanowires

While most of the experimental studies are dealing with partially oxidized metallic nanosolids, their behaviour is not well understood theoretically. To this end we utilized molecular dynamics simulation and charge-optimized many-body potential to probe melting of Cu nanoparticles and nanowires with and without a monolayer of Cu$_2$O as a shell. It is shown that partially oxidized nanosolids present a different melting behaviour than the widely accepted picture of melting, and especially different than surface melting. (i) For both types of nanosolids we observed inward diffusion of $\sim$40\% of atomic oxygen into the metallic core at room temperature. (ii) For the nanowire with oxide shell we observed a solid state phase transition in the Cu core that is not present for the case without the oxide. (iii) Prior to melting, the oxide shell shrinks, and this process continues after melting, to form a particulate oxide, for both types of nanosolids.

cond-mat.mtrl-sci↗

arXiv2023

Charge and heat currents in prismatic tubular nanowires

We calculate electronic charge and heat transport in tubular nanowires generated by a temperature gradient or a chemical potential bias. These nanowires correspond to semiconductor core-shell nanowires with insulating (undoped) core and conductive (doped) shell, such that the conduction takes place only in the shell. The cross section of such nanowires is typically polygonal. We study the influence of the cross section shape and shell thickness on the electric and heat conduction of the shell. We use the Landauer-Büttiker approach to calculate the electric and heat currents as a non-linear function of temperature and chemical potential bias beyond the linear regime.

cond-mat.mes-hall↗

arXiv2023

Simulation of short pulse photoemission in a micro-diode with implications for optimal beam brightness

Molecular dynamics simulations, with full Coulomb interaction are used to model short-pulse photoemission from a finite area in a microdiode. We demonstrate three emission regimes, source-limited emission, space-charge limited emission for short-pulses, and space-charge limited emission for the steady state. We show that beam brightness is at a maximum during transition from the source-limited emission regime to the space-charge limited emission regime for short pulses. From our simulations it is apparent that the important factor is the emitter spot size when estimating the critical charge density for short-pulse electron emission.

physics.plasm-ph↗

arXiv2022

Capacitive and inductive effects in perovskite solar cells: the different roles of ionic current and ionic charge accumulation

Dynamic hysteresis effects have been long known to occur in the J-V characteristics of perovskite solar cells (PSCs), with the ionic migration being identified as the primary factor. The hysteretic effects impacted early studies by the uncertainty in the evaluation of the power conversion efficiency, while currently, potential links to degradation mechanisms are in the focus. Therefore, understanding ion migration is a central goal, typically addressed by performing a combined large- and small signal analysis. The reported large capacitive and inductive effects created controversies with respect to the underlying mechanisms, yielding essentially two classes of models, one based on large accumulation capacitances and the other based on ionic modulation of the collected current. We introduce here an equivalent circuit model and interpret these phenomena in terms of recombination current modulation, identifying the distinct contributions from ion current and ionic charge accumulations. These contributions to the recombination current are associated with capacitive and inductive effects, respectively, and we corroborate the numerical simulations with electrochemical impedance spectroscopy (EIS) measurements. These show the role of the recombination currents of photogenerated carriers in producing both capacitive and inductive effects as the illumination is varied. Moreover, we provide a bridging point between the two classes of models and suggest a framework of investigation of defect states based on the observed inductive behavior, which would further aid the mitigation of the degradation effects.

cond-mat.mtrl-sci↗

arXiv2022

Role of planar buckling on the electronic, thermal, and optical properties of Germagraphene nanosheets

We report the electronic, the thermal, and the optical properties of a Germagraphene (GeC) monolayer taking into account buckling effects. The relatively wide direct band gap of a flat GeC nanosheet can be changed by tuning the planar buckling. A GeC monolayer has an sp$^2$ hybridization in which the contribution of an $s$-orbital is half of the contribution of a $p$-orbital leading to stronger $σ\text{-}σ$ bonds compared to the $σ\text{-}π$ bonds. Increasing the planar buckling, the contribution of an $s$-orbital is decreased while the contribution of a $p$-orbital is increased resulting in a sp$^3$-hybridization in which the $σ\text{-}π$ bond becomes stronger than the $σ\text{-}σ$ bond. As a result, the band gap of a buckled GeC is reduced and thus the thermal and the optical properties are significantly modified. We find that the heat capacity of the buckled GeC is decreased at low values of planar buckling, which is caused by the anticrossing of the optical and the acoustic phonon modes affecting phonon scattering processes. The resulting optical properties, such as the dielectric function, the refractive index, the electron energy loss spectra, the absorption, and the optical conductivity show that a buckled GeC nanosheet has increased optical activities in the visible light region compared to a flat GeC. The optical conductivity is red shifted from the near ultraviolet to the visible light region, when the planar buckling is increased. We can thus confirm that the buckling can be seen as another parameter to improve GeC monolayers for optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2022

The effects of a far-infrared photon cavity field on the magnetization of a square quantum dot array

The orbital and spin magnetization of a cavity-embedded quantum dot array defined in a GaAs heterostructure are calculated within quantum-electrodynamical density-functional theory (QEDFT). To this end a gradient-based exchange-correlation functional recently employed for atomic systems is adapted to the hosting two-dimensional electron gas (2DEG) submitted to an external perpendicular homogeneous magnetic field. Numerical results reveal the polarizing effects of the cavity photon field on the electron charge distribution and nontrivial changes of the orbital magnetization. We discuss its intertwined dependence on the electron number in each dot, and on the electron-photon coupling strength. In particular, the calculated dispersion of the photon-dressed electron states around the Fermi energy as a function of the electron-photon coupling strength indicates the formation of magnetoplasmon-polaritons in the dots.

cond-mat.mes-hall↗

arXiv2022

Piezoresistance characterization of silicon nanowires in uniaxial and isostatic pressure variation

Silicon nanowires (SiNWs) are known to exhibit large piezoresistance (PZR) effect, making it suitable for various sensing applications. Here, we report the results of a PZR investigation on randomly distributed and interconnected vertical silicon nanowire arrays as a pressure sensor. The samples were produced from p-type (100) Si wafers using a silver catalysed top-down etching process. The piezoresistance response of these SiNW arrays was analysed by measuring their I-V characteristics under applied uniaxial as well as isostatic pressure. The interconnected SiNWs exhibit increased mechanical stability in comparison with separated or periodic nanowires. The repeatability of the fabrication process and statistical distribution of measurements were also tested on several samples from different batches. A sensing resolution down to roughly \SI{1}{\milli\bar} pressure was observed with uniaxial force application, and more than two orders of magnitude resistance variation was determined for isostatic pressure below atmospheric pressure.

physics.app-ph↗

arXiv2022

Molecular Dynamics Simulations of Mutual Space-Charge Effect between Planar Field Emitters

Molecular dynamics simulations, with full Coulomb interaction and self-consistent field emission, are used to examine mutual space-charge interactions between beams originating from several emitter areas, in a planar infinite diode. The simulations allow observation of the trajectory of each individual electron through the diode gap. Results show that when the center-to-center spacing between emitters is greater than half of the gap spacing the emitters are essentially independent. For smaller spacing the mutual space-charge effect increases rapidly and should not be discounted. A simple qualitative explanation for this effect is given.

physics.comp-ph↗

arXiv2022

Effects of transverse geometry on the thermal conductivity of Si and Ge nanowires

We explore the effects of geometry on the thermal conductivity (kappa) of silicon and germanium nanowires, with lengths between 10-120 nm and diameters up to 5-6 nm. To this end we perform molecular dynamics simulations with the LAMMPS software, using Tersoff interatomic potentials. We consider nanowires with polygonal cross section and we discuss the effect of the transverse geometry on the thermal conductivity. We also consider tubular (hollow) nanowires and core/shell combinations of Si/Ge and Ge/Si, and we compare the heat transport of the core/shell structure with that of the separated core and shell components.

cond-mat.mes-hall↗

arXiv2022

Thermal transport controlled by intra- and inter-dot Coulomb interactions in sequential and cotunneling serially-coupled double quantum dots

We study thermoelectric transport through a serial double quantum dot (DQD) coupled to two metallic leads with different thermal energies. We take into account the electron sequential and cotunneling effects via different master equation approaches. In the absence of intra- and inter-dot Coulomb interactions, a small peak in thermoelectric and heat currents is found for $E_{\rm L} \text{=} E_{\rm R}$ indicating the Coulomb blockade DQD regime, where $E_{\rm L}(E_{\rm R})$ is the energy of the state of the left(right) quantum dot. In the presence of intra- and inter-dot Coulomb interactions with strengths U$_{\rm intra}$, and U$_{\rm inter}$, respectively, avoided crossings or resonance energies between the intra- and the inter-dot two-electron states, 2ES, are found. These resonances induce extra transport channels through the DQD leading to strong side peaks in the thermoelectric and heat currents at $ E_{\rm L} \text{-} E_{\rm R} = \pm (U_{\rm intra} \text{-} U_{\rm inter})$ in addition to the main peak generated at $E_{\rm L} \text{=} E_{\rm R}$. The current side peaks are enhanced by increased strength of the Coulomb interactions. Interestingly, the current side peaks are enhanced when cotunneling terms are considered in which the resonances of the 2ESs assist the electron cotunneling process through the system. Furthermore, the issue of coherences is carefully checked in the DQD-leads system via different approaches to the master equation, which are the Pauli, the Redfield, a first order Lindblad, and the first- and second order von-Neumann methods. We realize that the Pauli method gives a wrong results for the thermoelectric transport when the role of the coherences is relevant.

cond-mat.mes-hall↗

arXiv2021

Unified approach to cyclotron and plasmon resonances in a periodic 2DEG hosting the Hofstadter butterfly

We present theoretical calculations for the cyclotron resonance and various magnetoplasmon modes of a Coulomb interacting two-dimensional GaAs electron gas (2DEG) modulated as a lateral superlattice of quantum dots subjected to an external perpendicular constant magnetic field. We use a real-time excitation approach based on the Liouville-von Neumann equation for the density operator, that can go beyond linear response delivering information of all longitudinal and transverse collective modes of interest to the same order. We perform an extensive analysis of the coexisting collective modes due to the lateral confinement and the magnetic field for a different number of electrons in each dot. In the limit of vanishing dot modulation of the 2DEG we find signs of the structure of the Hofstadter butterfly in the excitation spectra.

cond-mat.mes-hall↗