Sobre a natureza das excitações de partícula independente em gás de elétrons bidimensional via espectroscopia Raman ressonante

Abstract

The inelastic light scattering has been widely used in the study of the semiconductor materials and it has become an indispensable technique for the understanding of fundamental physical processes. The effects of the electronic interactions on quantized electronic systems as two-dimensional electron gas are investigated through means of Raman scattering which allows understand the nature of collective excitations which are known as charge density excitations (CDE) and spin density excitations (SDE). CDE are plasmonic oscillations arising from the coupling between charge fluctuations via Coulombian and exchange-correlation interactions while SDE occur only when exchange-correlation effects are present. CDE is active when the laser energy is resonant with a semiconductor optical gap (near resonance regime) and the incoming and outgoing light polarizations are parallel to each other and SDE has incoming and outgoing light polarizations perpendicular to each other. Nevertheless, such a picture breaks down when the laser matches interband transitions energies of the material (extreme resonance regime). In addition to the collective excitations emerge anomalous transitions of the electron gas noninteracting-like known as single-particle excitations (SPE). The physics of such transitions is still not completely understood. In this work, were present experimental and theoretical results via electronic Raman scattering originating from the two-dimensional electron gas carried out from single quantum wells with the aim of understand the nature of the single-particle excitations. The experimental measures and calculations present in the thesis consists of the GaAs (gallium arsenide) single quantum well of a 250Å wide with modulation-doped and total electronic density of 8.81 x 1011cm2. It was also considered the coupling of the charge fluctuations with the longitudinal optical phonon of GaAs. In addition, it is performed a theoretical calculation of a structure of the GaAs single quantum well considered narrow of a 100Å wide with modulation-doped and and total electronic density of 1.2 x 1012cm2. However, the goal of this work is to show in a clear and transparent way the physical situation of the existence of electronic excitaions in electron gas. The work showed that, in extreme resonance regime, the plasma oscillations splits into two contributions: a set of renormalized collective excitations (plasmons) and unrenormalized electronic transitions (SPE). In order to accomplish this purpose, the study showed that electronic Raman scattering belongs to the same class of problems such as a set of forced, coupled and damped harmonic oscillators or formation of the superconducting state in BCS theory of normal metals. Comparison between experiment and theory shows an excellent agreement.