Quantum Magnetism Beyond Spin Up and Spin Down

William Marsh Rice University, Houston TX

Investigators

Abstract

Order arises spontaneously in complex systems, such as the motions of the planets and the periodic structure of crystals. Understanding how such order arises is a central pursuit of science. A great deal of this effort now focuses on the behavior of electrons and atoms at temperatures near absolute zero, where weak interactions can dominate and new states of matter emerge. These new states are often exotic and unexpected, such as magnetic states that are characterized by specific alignments of particles, or super fluids that flow through tiny constrictions with no resistance because of a long-range synchronicity of particle motion within a fluid. Such phenomena are originally investigated for fundamental interest that satisfies a human drive to understand the world in which we live, but they often lead to technological advances such as computer hard-drives or superconductivity. The particular project funded by this grant aims to improve our fundamental understanding of how order arises to produce magnetism. At a microscopic level, a magnet sticks to a refrigerator because a significant fraction of the electrons in the magnet are spinning or rotating in the same direction. To understand magnetism, one needs to understand why the electrons orient themselves, and what types of correlated motions they display. Because these phenomena involve many interacting particles, a full understanding of this problem has eluded us so far. This group is developing new methods to understand problems like this. A powerful trend in physics research today is to use ultracold atoms as analogs for electrons in solids in order to study magnetism. Atomic gases are much easier to study than solids because they can be prepared without any impurities and the atom-atom interactions can be tuned, for example. In the work funded by this award, atoms will be confined in a corrugated cage of light, analogous to an egg crate. By simultaneously using multiple lasers which emit light of different colors, the geometric details of the "egg crate" can be smoothly morphed into a variety of patterns, changing the shapes of the pockets into which the atoms are nestled, and even merging several pockets into one larger pocket, for example. This allows the scientists involved in this work to explore the richness of theories that have been developed to understand real solid materials, such as the "Fermi-Hubbard model." It is possible that in atomic gases one might discover new phenomena that lead to practical applications, and this improved understanding of the behavior of interacting collections of many atoms or electrons may give us new tools for designing materials with desired properties. The specific system at the heart of this project is the fermionic isotope of strontium, Sr-87, which has a nuclear spin of 9/2. This nuclear spin provides the analog of the electron spin in conventional magnetic materials, but with ten possible spin orientations rather than the conventional two for an electron. Because strontium is a closed-shell atom, interactions between Sr-87 atoms are independent of the orientation of the nucleus. This creates a huge degeneracy in the ground state of the system, which is predicted to display a rich collection of new phenomena related to the alignment and correlation of nuclear spins. The experimental plan is to study the thermodynamics of a large-spin gas in a bulk system and to probe the alignment of nuclear spins when individual atoms are confined to sites of an optical-lattice potential formed by standing waves of light.