Switching in Driven Microscopic Systems

Michigan State University, East Lansing MI

Investigators

Abstract

This combined experiment and theory project will explore thermally activated switching in model physical systems that are driven by time-dependent fields. The underlying theoretical idea is that a field interferes with switching dynamics and can therefore strongly affect switching probabilities. Since experimental understanding of this process requires a well-characterized physical system, the use of a mesoscopic particle trapped in an optically-induced double well potential is adopted. This system exhibits behavior analogous to the hysteretic response of a single-domain nanomagnet in a magnetic field or a current-biased resistively-shunted Josephson junction. However, the 3-dimensional confining potential can be readily tuned and modulated by controlling the laser beams comprising the trap. The damping rate can be varied by changing the particle properties or by controlling the environmental coupling. Theoretical studies can provide deeper understanding of fluctuation processes in systems driven far from thermal equilibrium. Results can be directly transferred to areas of current technological importance, for example, controlled switching in nanomagnetic random access memory devices. Graduate and postdoctoral students involved with this research will acquire understanding of lasers, optical methods, real-time computing, theory of transport phenomena and non-linear dynamical systems, preparing them for careers in teaching and/or research in science and engineering. %%% Switching events play a pivotal role in many physical and biological processes. On computer hard disks, information is written into billions of tiny regions with 1's and 0's expressed as the orientations of nanomagnets. Switching between these two states should be easy to do when information is stored but it should not happen spontaneously, which would lead to a loss of memory. In this investigation, a model of a two-state switch is created by two laser beams. A single beam can confine a transparent sub-micron particle to a small region in space. With two parallel beams separated by less than the particle diameter, the particle can switch between the two beams if it acquires enough energy. By modulating the energy of the particle, the transition rates can be controlled and the conditions under which switching occurs studied quantitatively, even in a noisy environment. The experiments are analyzed by statistical theories that will be developed to predict the particle's behavior under extreme conditions, i.e., when the particle is moved rapidly and with large amplitude. These studies should lead to optimal methods for controlling random switching processes with applications not only to magnetism but also to chemistry and biology. Students will receive training in optics, real-time computing, condensed matter and statistical physics.