Landau levels, edge states and magneto-conductance in GaAs/AlGaAs core-shell nanowires

Magnetic states of the electron gas confined in modulation-doped core-shell nanowires are calculated for a transverse field of arbitrary strength and orientation. Magneto-conductance is predicted within the Landauer approach. The modeling takes fully into account the radial material modulation, the prismatic symmetry and the doping profile of realistic GaAs/AlGaAs devices within an envelope-function approach, and electron-electron interaction is included in a mean-field self-consistent approach. Calculations show that in the low free-carrier density regime, magnetic states can be described in terms of Landau levels and edge states, similar to planar two-dimensional electron gases in a Hall bar. However, at higher carrier density the dominating electron-electron interaction leads to a strongly inhomogeneous localization at the prismatic heterointerface. This gives rise to a complex band dispersion, with local minima at finite values of the longitudinal wave vector, and a region of negative magneto-resistance. The predicted marked anisotropy of the magneto-conductance with field direction is a direct probe of the inhomogeneous electron gas localization of the conductive channel induced by the prismatic geometry

X-band UAV-SAR optimization and validation

This project consists on the improvement of an already validated System, as it is the work of a finished PhD Thesis. Due to the wide range of that thesis, many sub blocs need to be revaluated and optimized. That is the main motivation of this project as the original prototype admitted improvements in many ways; such as in weight reduction or the use of other electronic components and new materials that, when finished, have led to a outperform of the System. The Base Band circuitry Works wrongly and the High Frequency generates oscillations that caused several problems that led to a not working System. Furthermore, the design of new prototypes is greatly limited by the high consumption of power of the amplifiers, over 10 Watts. Throughout this project, a profound analysis is carried out for each bloc and improved as much as possible until a robust, well-performing system is reached, fulfilling many of the initial goals.Este proyecto consiste en la mejora de un sistema que ya ha estado validado, puesto que es el propósito de una tesis doctoral terminada. Debido a la gran extensión de la tesis, muchos de los subsistemas han quedado pendientes de una revisión i optimización. La motivación de este se re sume en: la reducción de peso, la utilización de otros componentes alternativos i nuevos materiales que, una vez terminado, han permitido un mejor comportamiento del sistema. Nos encontramos con que la banda base tiene problemas en cuanto a comportamiento y la parte de alta frecuencia genera harmónicos que impiden el funcionamiento correcto del sistema. Hay que añadir que el diseño de los prototipos está muy limitado por el alto consumo de les etapas amplificadoras, de más de 10 Vatios. A lo largo de este proyecto, analizo uno a uno los bloques del sistema y los mejoro en la medida de lo posible hasta conseguir un sistema robusto que funciona correctamente compliendo muchos de los requisitos iniciales.Aquest projecte consisteix en la millora d'un sistema que ja ha estat validat, ja que és el propòsit d'una tesi doctoral ja acabada. Degut a la gran extensió de la tesi, molts dels blocs han quedat pendents d'una revisió i optimització. És aquesta la motivació d'aquest projecte ja que el prototip original permetia millores en molts sentits; tan en la reducció de pes com en la utilització d'altres components alternatius i nous materials que, un cop acabat aquest projecte, han permès un millor comportament del sistema. Ens trobem amb que la banda base té problemes en quant a comportament i la part d'alta freqüència genera molts harmònics que impedien el funcionament del sistema. A més, el disseny dels prototips està molt limitat per l'alt consum de les etapes amplificadores, de més de 10 Watts. Al llarg d'aquest projecte, analitzo un a un els blocs del sistema i els milloro en la mesura del possible, fins a aconseguir un sistema robust que funciona correctament complint molts dels requisits inicials

Symmetries in the collective excitations of an electron gas in core-shell nanowires

We study the collective excitations and inelastic light scattering cross-section of an electron gas confined in a GaAs/AlGaAs coaxial quantum well. These system can be engineered in a core-multi-shell nanowire and inherit the hexagonal symmetry of the underlying nanowire substrate. As a result, the electron gas forms both quasi 1D channels and quasi 2D channels at the quantum well bents and facets, respectively. Calculations are performed within the RPA and TDDFT approaches. We derive symmetry arguments which allow to enumerate and classify charge and spin excitations and determine whether excitations may survive to Landau damping. We also derive inelastic light scattering selection rules for different scattering geometries. Computational issues stemming from the need to use a symmetry compliant grid are also investigated systematically

Magneto-photoluminescence in GaAs/AlAs core-multishell nanowires: a theoretical investigation

The magneto-photoluminescence in modulation doped core-multishell nanowires is predicted as a function of photo-excitation intensity in non-perturbative transverse magnetic fields. We use a self-consistent field approach within the effective mass approximation to determine the photoexcited electron and hole populations, including the complex composition and anisotropic geometry of the nano-material. The evolution of the photoluminescence is analyzed as a function of i) photo-excitation power, ii) magnetic field intensity, iii) type of doping, and iv) anisotropy with respect to field orientation.Comment: 11 pages, 11 figures, accepted for publication in Physical Review

First-principles theory of spatial dispersion: Dynamical quadrupoles and flexoelectricity

Density-functional perturbation theory (DFPT) is nowadays the method of choice for the accurate computation of linear and non-linear response properties of materials from first principles. A notable advantage of DFPT over alternative approaches is the possibility of treating incommensurate lattice distortions with an arbitrary wavevector, ${\bf q}$, at essentially the same computational cost as the lattice-periodic case. Here we show that ${\bf q}$ can be formally treated as a perturbation parameter, and used in conjunction with established results of perturbation theory (e.g. the "2n+1" theorem) to perform a long-wave expansion of an arbitrary response function in powers of the wavevector components. This provides a powerful, general framework to accessing a wide range of spatial dispersion effects that were formerly difficult to calculate by means of first-principles electronic-structure methods. In particular, the physical response to the spatial gradient of any external field can now be calculated at negligible cost, by using the response functions to $\mathit{uniform}$ perturbations (electric, magnetic or strain fields) as the only input. We demonstrate our method by calculating the flexoelectric and dynamical quadrupole tensors of selected crystalline insulators and model systems

Aharonov-Bohm oscillations and electron gas transitions in hexagonal core-shell nanowires with an axial magnetic field

We use spin-density-functional theory within an envelope function approach to calculate electronic states in a GaAs/InAs core-shell nanowire pierced by an axial magnetic field. Our fully 3D quantum modeling includes explicitly the description of the realistic cross-section and composition of the sample, and the electrostatic field induced by external gates in two different device geometries, gate-all-around and back-gate. At low magnetic fields, we investigate Aharonov-Bohm oscillations and signatures therein of the discrete symmetry of the electronic system, and we critically analyze recent magnetoconductance observations. At high magnetic fields we find that several charge and spin transitions occur. We discuss the origin of these transitions in terms of different localization and Coulomb regimes and predict their signatures in magnetoconductance experiments

Tuning thermal transport in Si nanowires by isotope engineering

We study thermal transport in isotopically disordered Si nanowires, discussing the feasibility of phonon engineering for thermoelectric applications within these systems. To this purpose, we carry out atomistic molecular dynamics and nonequilibrium Green's function calculations to characterize the dependence of the thermal conductance as a function of the isotope concentration, isotope radial distribution and temperature. We show that a reduction of the conductivity of up to 20% can be achieved with suitable isotope blends at room temperature and approximately 50% at low temperature. Interestingly, precise control of the isotope composition or radial distribution is not needed. An isotope disordered nanowire roughly behaves like a low-pass filter, as isotope impurities are transparent for long wave-length acoustic phonons, while only mid- and high-frequency optical phonons undergo significant scattering.We acknowledge financial support from the Ministerio de Economía y Competitividad (MINECO) under grant FEDER-MAT2013-40581-P and the Severo Ochoa Centres of Excellence Program under Grant SEV-2015-0496 and from the Generalitat de Catalunya under grants no. 2014 SGR 301 and through the Beatriu de Pinós fellowship program (2014 BP_B 00101). The calculations were performed at the Barcelona Supercomputing Center (BSC-CNS) within the project “Thermal transport in isotopically disordered Si nanowires (FI-2016-1-0022)”. We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI) ReferencesPeer Reviewe