Spin Dependent Transport in Materials with Multivalley Band Structures

University Of Rochester, Rochester NY

Investigators

Abstract

NONTECHNICAL SUMMARY This award supports fundamental theoretical research and education to understand how electron spin behaves in materials contributing to the foundations of possible future electronic devices which function by manipulating electron spin as well as its charge. In addition to carrying an electric charge, an electron is also in a sense like a spinning top that behaves according to the rules of quantum mechanics. There are two senses of spin to an electron and it is natural to explore using the electron to physically represent information, like the familiar '0' and '1' in digital electronics. Spintronics research aims to introduce the quantum mechanical spin into electronic devices by making use of the interactions between the electron spin and its environment in a material. Because charge-based field effect transistors are rapidly approaching their physical limits, the spin-charge computation scheme has recently become an active research frontier. The PI will develop theoretical models that quantify spin transport in materials that include silicon and elements vertically above and below silicon in the periodic table of elements. The PI will also investigate crystals that are essentially a single layer of atoms such as transition-metal dichalcogenides which are made of atoms from the transition-metal block of the periodic table and elements from the periodic table vertically below oxygen. This modeling is aimed to enable the design of spintronic devices in which the time needed for manipulating and processing information is prolonged by strain engineering and proper application of external electric fields. The primary research focus will be on quantifying this spin relaxation time in doped materials where the conductivity is improved either by intentionally adding foreign atoms that donate/accept electrons to/from the host crystal, or by applying a gate-voltage that brings charge carriers into atomically thin materials from surrounding materials. This project presents ideas that are fascinating to a wide range of students. Through collaboration of materials science, physics and engineering graduate students, each learns from the other concepts of quantum mechanics or device function. The PI will actively promote this research among K-12 students by delivering interactive lectures/demos that highlight the use of future technology in devices. The PI aims to utilize the potential of this research for societal impact to help attract students from under-represented groups to the project. TECHNICAL SUMMARY Understanding of the interactions between the electron's spin and its solid-state environment in metallic material systems has spurred immense development in information storage technologies. Contrary to metals, silicon holds complete sway over logic circuits. Fortunately, this and other group IV materials are promising candidates for spintronic logic devices owing to their crystalline inversion symmetry which suppresses precession of spins about randomly changing intrinsic magnetic fields, and the zero nuclear spin of their naturally abundant isotopes which suppresses spin relaxation by hyperfine interactions. A major objective in this project will be to fill a longstanding gap in the theory of spin relaxation processes in multi-valley semiconductors such as silicon and germanium. The spin relaxation in these materials shows a strong dependence on the identity of the donor atom. By invoking analytical and numerical methods, the PI will quantify the donor-driven spin relaxation by establishing a connection with the induced changes in the donor ground state due to the spin orbit coupling. The effect will be investigated as a function of strain, temperature, donor concentration, and donor identity. Apart from group IV materials, monolayer transition-metal dichalcogenides and oxides present another important class of materials potentially useful for spintronics. These two-dimensional monolayer semiconductors put together exotic charge, spin and valley electronic phenomena. The strong binding between electrons and holes, a result of the impeded Coulomb screening in genuine 2D systems, enables exceptionally strong light-matter interaction that persists up to room temperature. The PI will systematically quantify the momentum scattering and spin relaxation due to electron-phonon interaction in two-dimensional crystals such as graphene and transition-metal dichalcogenides. Transport effects will include signatures of electrical fields via piezoelectricity and symmetry breaking by the gate voltage. The PI will also analyze the often-discussed topological insulator phase in two-dimensional crystals from the perspective of relaxation. The PI aims to recruit qualified undergraduate students to participate in the research, and teach a new course on spintronics which explains how quantum mechanics and magnetism can be applied into new logic and memory architectures. The PI will try to recruit McNair scholars and will work with the Kearns Center at the University of Rochester to attract students from under-represented minorities to this research.