Antiferromagnetism in the Hubbard Model with Ultracold Atoms

Department Colloquia

Speaker: Randall Hulet, Rice University Texas, USA
Date & Time: September 11, 2014 16:00 - 17:00
Location: UBC, Hennings 201
Local Contact: Ian Affleck
Intended Audience: Undergraduate

Ultracold atoms on optical lattices form a versatile platform for studying many-body physics. We have realized the Hubbard model, a “standard model” of strongly-correlated matter. The Hubbard model consists of a cubic lattice with on-site interactions and kinetic energy arising from tunneling to nearest neighbors. Notably, it may contain the essential ingredients of high-temperature superconductivity. While the Hamiltonian has only two terms it cannot be numerically solved for arbitrary density of spin-½ fermions due to exponential growth in the basis size. At a density of one particle per site, however, the Hubbard model is known to exhibit antiferromagnetism at temperatures below the Néel temperature T_N, a property shared by most of the undoped parent compounds of high-T_c superconductors. The realization of antiferromagnetism in a 3D optical lattice with atomic fermions has been impeded by the inability to attain sufficiently low temperatures. We have detected antiferromagnetic correlations by spin-sensitive Bragg scattering of light. This was enabled by the development of a compensated optical lattice that facilitates evaporative cooling by compensating the confinement envelope of the infrared optical lattice beams with blue-detuned laser beams. Comparison with quantum Monte Carlo constrains the temperature in the center of the lattice to 1.4 T_N, a temperature 2 times lower than achieved in previous work. The prospects for attaining even lower temperatures are good, and open up a number of exciting directions.