Estudo das propriedades termoelétricas e ópticas não lineares de pontos quânticos semicondutores

Abstract

The objective of this work is to study the thermoelectric properties and nonlinear optical properties of semiconductor quantum dots, for this purpose the work is divided into two parts. In the first part, we analyze the effects of quantum interference and electronic interaction on the thermoelectric properties and the thermoelectric efficiency of a system composed of two ferromagnetic quantum wires coupled to a semiconductor quantum dot, which exhibits Rashba spin-orbit interaction and Coulomb interaction between its electrons, on the influence of an external magnetic field. The system is described by a tight-binding approximation and the Coulomb interaction between the electrons in the quantum dot is described by the Hubbard model. For the calculation of the thermoelectric properties of the system, the formalism of Landauer-Büttiker is used, which depends on the function of transmission, calculated through Green’s functions. In addition, using the Sommerfeld expansion, we obtain analytical expressions for the electric G and thermal K conductances, thermopower S, as well as for the Lorenz ratio L and the dimensionless figure of merit ZT. It is considered that the electron can tunnel through the system without changing the polarization of its spin. On the other hand, the Rashba spin-orbit interaction creates a second path to the electron passage through the quantum dot, in which the electron spin is inverted. The interference between both paths gives rise to the Fano-Rashba effect, consequently, the transmission function, in addition to showing a symmetrical peak like Breit-Wigner, also, shows a Fano-type antisymmetric peak. The results show that in the antiresonant regions, in the transmission function of the studied system, there is a violation of the Wiedemann-Franz law; and an improvement of both thermopower and thermoelectric efficiency. When the Rashba spin-orbit interaction values are close to the direct coupling value, high efficiency is reported, ZT ą 1, whose value increases when the electronic interaction is considered. In the second part, we study the exciton contribution to the third-order electric susceptibility of one-dimensional semiconductor quantum dots with a semi-parabolic confining potential and show that the screening of the electron-hole interaction has a strong influence on the nonlinear optical properties in the weak confinement regime. Based on a density matrix formulation, we estimate the spectrum of the third-order electric susceptibility and its contribution to the refraction index and absorption coefficient. In particular, we show that the multi-peaked spectrum of the nonlinear susceptibility, which results from the hydrogenoid character of the exciton eigenstates for a purely Coulombian electron-hole coupling, is reverted towards a single-peaked structure as the interaction becomes strongly screened, thus leading to a substantial enhancement of the nonlinear optical properties of semiconductor quantum dots.