This award supports research into spin and charge current generation relevant to future "spintronics" technologies, educating wider student populations about the quantum mechanics of spin, and outreach to local communities on issues pertaining to quantum science. Familiar electronics is based on a current of charges. Spintronics works, instead, on a current of magnetic spins, which one can think of as infinitesimal bar magnets. Efficiently generating and measuring spin currents is a condition for many proposed spin-based devices. The research effort will theoretically investigate a mechanism of spin-current generation and measurement that involves non-uniform magnetic fields coming from atomic nuclei embedded in a semiconductor. A further goal of this activity will be to compare figures of merit of this new mechanism to traditional spin-orbit coupling for spin-current generation and detection. Students will take part in all aspects of the project: learning and applying analytic and numerical methods, drafting articles, and presenting results at meetings. The principal investigator (PI) will develop course materials for an introductory class in the quantum mechanics of spin that will be geared toward a wide swath of students: those who have not taken traditional quantum mechanics or even many upper-level physics courses. The curriculum will be designed to be accessible to chemistry and engineering/computer science students in an effort to mainstream quantum mechanical and quantum computing concepts. Quantum technologies are expected to bring about a quantum revolution. The PI will engage the campus and local community in discussions (through public presentations) of quantum science, the directions quantum technology will lead, and the importance of quantum education for the nation.
This award supports theoretical investigations of spin-current-to-charge-current and charge-current-to-spin-current conversion by means of gradients in nuclear field. The spin-Hall effect and its inverse effect are presently the chief methods by which spin/charge currents are converted. Spin-Hall conversions require materials with strong spin-orbit interactions, which shorten spin diffusion lengths. The research thrust here is to tap into alternative materials for charge/spin current conversion where strong spin-orbit interactions are not necessary or desirable; instead, strong hyperfine interactions are relied upon. Nuclear field or hyperfine gradients offer a mechanism by which either spin or charge current can be generated. Spin separation via this nuclear gradient channel is reminiscent of the Stern-Gerlach effect, but since the nuclear field is not a true magnetic field in the sense of a Lorentz force being operational, spins are able to separate more effectively than they would in a true magnetic field gradient (the nuclear field - at the level of the Fermi contact potential - acts only on the spin and not on the orbital degree of freedom). The project goal is to develop and solve spin and charge drift-diffusion equations with the gradient effect included. Analytic solutions are possible for some configurations but in general the coupled equations are nonlinear and necessitate numerical methods. Furthermore, two possible experimental realizations of the effect will be modeled: electrical spin injection and optical spin injection. In both situations, a nuclear gradient is feasible through non-uniform dynamic nuclear polarization by injected spin polarized carriers. By modeling and evaluating these scenarios, a guide will be provided to experimentalists who wish to observe the spin and charge separating effects. Ultimately, the spin/charge drift diffusion formalism developed will be able to incorporate other types of field gradients as well, namely, spin-orbit fields such as those due to the Rashba interaction.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
