Collective charge and spin dynamics in low-dimensional itinerant electron systems with spin-orbit coupling

Abstract

This research project in theoretical condensed-matter physics aims to study the collective charge and spin excitations of itinerant quasi-two-dimensional electronic systems such as doped semiconductor nanostructures and carbon-based materials, in the presence of impurities and spin-orbit coupling. In previous work under DOE support, we had discovered an interesting interplay between many-body effects and Rashba and Dresselhaus spin-orbit interactions: contrary to many situation in spintronics where spin-orbit interactions lead to loss of spin coherence, collective spin modes are robust against such coherence losses, and behave as macroscopic quantum objects with a collective magnetic moment precessing about an effective spin-orbit magnetic field. Experimental data for GaAs and CdMnTe quantum wells, obtained by our collaborators at the University Pierre et Marie Curie in Paris, confirmed our predictions and suggested that the observed effect is universal. However, a full theoretical explanation of the effect is still missing, and a wealth of new experimental data remains to be analyzed. We therefore plan to address the following three interrelated objectives: 1. Theory of collective modes in chiral systems. We will explore the various ways in which spin-orbit interactions affect collective spin-density and spin-flip excitations in semiconductor quantum wells. The goal will be to develop a universal formalism for chiral collective modes, with a particular emphasis on analytic results for dispersion relations, which will be compared with experimental results from inelastic light scattering. We will calculate the linewidths of spin plasmons and spin-flip waves due to impurity scattering and spin Coulomb drag from first principles. 2.Many-body effects in noncollinear magnetism. We will describe electronic many-body effects using time-dependent density-functional theory (TDDFT) in the linear-response regime. Quantum wells with spin-orbit coupling are noncollinear itinerant magnetic systems at the crossover between two and three dimensions, which poses subtle challenges for standard exchange-correlation (xc) functionals such as the local-density approximation (LDA). We will develop new orbital-dependent xc functionals for noncollinear magnetic systems by generalizing the so-called STLS approach. 3. Beyond single semiconductor quantum wells. We will consider quantum well bilayer or multilayer systems with spin-orbit coupling, where the coupling between the quasi-2D electron systems leads to acoustic and optical modes (in phase and out of phase). How are these modes affected if the two layers have the same or different chirality? Plasmon modes have been widely studied in graphene and other topological materials, and we will study collective spin excitations in graphene and other carbon-based materials in the presence of spin-orbit coupling and magnetic fields. The potential impact of the proposed research will be both on a fundamental and a practical level. A universal theory of chiral collective modes will advance our understanding of the interplay between many-body effects and spin-orbit interactions in a wide variety of materials and systems. A generalized STLS approach for noncollinear magnetism will be of wide interest beyond the systems studied here. This work may also point towards new ways of manipulating and controlling spin waves in low-dimensional itinerant electron systems: this could be used to encode, transport and process information, which would be of interest to the field of magnon spintronics.