Daniel Greif, Thomas Uehlinger, Gregor Jotzu, Leticia Tarruell, Tilman Esslinger
Quantum magnetism lies at the heart of many intriguing phenomena in condensed matter physics. Its manifestations range from antiferromagnets to spin-liquids, and it is believed to play a central role in high-temperature superconductivity [1-3]. Remarkably, even simple models of the underlying many-body physics are often intractable with current theoretical and computational methods. The controlled setting of ultracold fermionic atoms in optical lattices is therefore regarded as a promising route to provide new insights [4-6]. Yet, the low temperature scale required for entering the regime of quantum magnetism has hindered progress for optical lattice based systems. So far, superexchange oscillations on isolated double wells, one-dimensional decoupled Ising spin chains and classical magnetism on a triangular lattice were studied with bosonic quantum gases [7-9]. Here we report on the observation of quantum magnetism of a Fermi gas in an optical lattice. The key to obtaining and detecting the short-range magnetic order is a tunable geometry optical lattice. When loading a low-temperature two-component gas with repulsive interactions into either a dimerized or anisotropic simple cubic lattice, we find magnetic correlations on neighbouring sites. The correlations manifest as an excess number of singlets as compared to triplets consisting of two atoms with opposite spins. For the anisotropic lattice, we determine the transverse spin correlator from the singlet-triplet imbalance and observe antiferromagnetic correlations along one spatial axis.
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http://arxiv.org/abs/1212.2634
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