Many-body theory of spin-orbit coupled materials and novel spin drag effects

University Of Missouri-Columbia, Columbia MO

Investigators

Abstract

Technical Summary This award supports theoretical research and education on materials with strong spin-orbit interaction. These materials are potentially important in a new generation of electronic devices, based on the electron spin. Spin-orbit interactions are traditionally studied within a one-electron context, but there are instances in which electron-electron interactions latch-on to spin-orbit interactions to produce intriguing collective effects. Examples of this are the spin Hall drag and the inhomogeneous Gilbert damping of spin waves. The PI aims: (i) to identify and study novel many-body effects in spin-orbit coupled systems, and (ii) to study Coulomb drag effects under novel conditions, e.g. in dilute magnetic semiconductors close to the magnetic ordering transition, in semiconductor bi-layers close to an electron-hole pairing transition, and in semiconductors with strong spin-orbit interactions. (i) Many-body effects in spin-orbit-coupled systems - Spin-orbit coupling in solid-state systems modifies the electron-electron interaction, producing an effective interaction similar to the Breit interaction of quantum electrodynamics, but considerably stronger. The PI will investigate the effects of this interaction in Coulomb-coupled bilayer systems and in single-layer two-dimensional electron liquids. The PI will further develop the many-body theory of coupled current-spin excitations and the optical conductivity in the single-layer systems, both in the normal and in the superconducting state and in the presence of a magnetic field. In particular, he will study these effects in systems with Bychkov-Rashba and Dresselhaus spin-orbit coupling, in the vicinity of the 'neutrality point' where the two interactions balance each other. (ii) Coulomb-drag effects - The PI will extend his investigations of spin Coulomb drag to electronic systems on the verge of phase transitions, such as magnetic ordering in dilute magnetic semiconductors and electron-hole pairing in semiconductor bilayers. In both cases, a strong enhancement of the drag effect is expected due to the contribution of spin fluctuations or pairing fluctuations to the effective electron-electron or electron-hole interaction. In this context the PI will study the behavior of the Dyakonov-Perel spin relaxation time near the magnetic ordering transition. In collaboration with experimentalists, the PI will calculate spin Coulomb drag parameters of optically excited electrons in hole-doped magnetic semiconductors near the ferromagnetic transition. The PI also plans to investigate the transport and spin relaxation dynamics of optically induced spin gratings in quantum wells with Bychkov-Rashba and Dresselhaus spin-orbit coupling, near the neutrality point. A recently introduced spin-to-density dynamics mapping will be used to connect the spin relaxation time in the presence of spin-orbit coupling to the spin Coulomb drag time without spin-orbit coupling. Research on interacting spin-orbit coupled systems can have a large impact on the field of spintronics and contributes to the intellectual foundations upon which possible new electronic device technologies rest. This award and the supported research contribute to the education of graduate students and postdoctoral researchers. Non-Technical Summary This award supports theoretical research and education to study materials in which electrons experience strong spin-orbit interactions. Electrons have an intrinsic property called spin where it appears as if the electron spins like a tiny top. The spin of the electron is also connected to its intrinsic magnetic properties; it behaves as though it was a tiny bar magnet. As an electron moves through a solid the theory of relativity says that it will experience a magnetic field from the atomic cores in the lattice. The interaction of the electron with this magnetic field gives rise to the spin-orbit interaction. Materials where spin-orbit interactions are particularly strong have recently gained the spotlight as key actors in a possible new generation of electronic devices, spintronic devices, which use the electron spin just as ordinary electronic devices use the electron charge. This interest has largely been fueled by the hope to realize the spintronic analog to a transistor, the 'spin transistor,' in which the on/off state would be achieved through control of the electron's spin in a way that does not use a magnetic field but exploits our ability to control the electron's motion and the spin-orbit interaction. Basically, the spin-orbit interaction allows us to manipulate the electron spin using an electric field. Spin-orbit interactions are traditionally studied ignoring the interaction of electrons with each other. But, there are instances in which interactions between electrons latch-on to spin-orbit interactions to produce interesting effects. For example, in a system of two closely spaced but clearly separated electron layers, a current flowing in one of the two layers can induce a spin accumulation in the other layer. This effect, known as spin Hall drag, is caused by the Coulomb interaction between electrons in different layers, coupled with spin-orbit interaction of electrons in each layer. This effect is one instance of a broader class of 'Coulomb drag effects' in which the motion of one group of electrons is used to drag along a second group of electrons and, in so doing, induces a desired property. This award supports research with the aim to discover new effects in materials with strong spin-orbit interactions, with an emphasis on Coulomb drag effects. The PI will also study Coulomb drag effects under novel conditions, for example in novel materials close that are close to becoming magnetic. Research on interacting spin-orbit coupled systems can have a large impact on the field of spintronics and contributes to the intellectual foundations upon which possible new electronic device technologies rest. This award and the supported research contribute to the education of graduate students and postdoctoral researchers.