Two-dimensional superexchange-mediated magnetization dynamics in an optical lattice

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Science  01 May 2015:
Vol. 348, Issue 6234, pp. 540-544
DOI: 10.1126/science.aaa1385

Simulating magnetism out of equilibrium

A major goal of quantum simulation is to help us understand problems that are difficult to describe analytically or solve with conventional computers. This goal has been very challenging to reach experimentally, requiring, for example, extremely low temperatures to determine the effects of quantum magnetism in equilibrium. Brown et al. studied a nonequilibrium system of ultracold 87Rb atoms in a two-dimensional optical lattice. They monitored the atoms' dynamics after a sudden change in the lattice parameters and were able to reach a regime where the magnetic interactions dominated the dynamics.

Science, this issue p. 540


The interplay of magnetic exchange interactions and tunneling underlies many complex quantum phenomena observed in real materials. We study nonequilibrium magnetization dynamics in an extended two-dimensional (2D) system by loading effective spin-1/2 bosons into a spin-dependent optical lattice and use the lattice to separately control the resonance conditions for tunneling and superexchange. After preparing a nonequilibrium antiferromagnetically ordered state, we observe relaxation dynamics governed by two well-separated rates, which scale with the parameters associated with superexchange and tunneling. With tunneling off-resonantly suppressed, we observe superexchange-dominated dynamics over two orders of magnitude in magnetic coupling strength. Our experiment will serve as a benchmark for future theoretical work as the detailed dynamics of this 2D, strongly correlated, and far-from-equilibrium quantum system remain out of reach of current computational techniques.

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