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Bond order solid of two-dimensional dipolar fermions

Phase diagram for magnetic atoms loaded into a two-dimensional optical lattice. Depending on the strength and orientation of an external magnetic field, various phases can be achieved, including a checkerboard (cb) or striped pattern (st) of atoms, or a bond order solid (BOS).

New PFC-supported work shows how a simple “joystick” consisting of an adjustable magnetic field can create several new phases of atomtronic matter, many of them never seen before. The field is used to tune the interaction---giving the researcher force on demand, causing the atoms to assume phases, which can be mapped on a diagram. Two phases correspond to charge-density waves in a checkerboard or striped pattern. A third phase is labeled BCS. Two other phases seen in the study were totally unexpected. These consist of pairs---an atom yoked with a neighboring vacancy---distributing themselves in a checkerboard or striped pattern. The authors call this new phase a “bond-order solid” (BOS) since in a sense the bonds between the atom-vacancy couplets seem to be forming the patterns rather than the atoms themselves. Bond-order phases have been conjectured previously in idealized one-dimensional models, but this is the first report of their presence in a realizable physical system. These phases are associated with the presence of strong long-range dipole interactions between ultracold atoms, a feature that does not exist for electrons in solids or in the first generation of ultracold atomic and molecular systems. Recent experimental developments show prospects for implementing ultracold dipole fermion systems in the laboratory.

S. G. Bhongale, L. Mathey, S.-W. Tsai, C. W. Clark, and E. Zhao
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