Maintaining supersolidity from one to two dimensions

Now published in Physical Review A, we theoretically investigate the role of trap geometry plays in determining the dimensionality of dipolar droplet arrays, which range from one-dimensional to zigzag, through to two-dimensional supersolids. Supersolidity is well established in one-dimensional arrays, and may be just as favorable in two-dimensional arrays provided that one appropriately scales the atom number to the trap volume. We develop a tractable variational model—which we benchmark against full numerical simulations—and use it to study droplet crystals and their excitations. We also outline how exotic ring and stripe states may be created with experimentally feasible parameters. Our work paves the way for future studies of two-dimensional dipolar supersolids in realistic settings.

You can see the paper here: E. Poli et al., Phys. Rev. A 104, 063307 (2021) [pdf] [arXiv]

Two-dimensional supersolidity in a circular trap

Figure: Experimental realisation of a two-dimensional seven droplet hexagon state. (a) In-situ image of density profile. (b) Image after 36 ms time-of-flight expansion. (c,d) Corresponding theory result for the same trap. In (d), the time-of-flight expansion is estimated by a Fourier transform of the wavefunction.

Dipolar condensates were recently coaxed into supersolid phases supporting both superfluid and crystal excitations. The first dipolar supersolids consisted of one dimensional droplet arrays, and a recent experiment here achieved two dimensional supersolidity, observing the transition from a linear chain to a zig-zag configuration of droplets.

In this work, in collaboration with Prof. Luis Santos from the Leibniz University Hannover, we show that while one-dimensional supersolids may be prepared from condensates via a roton instability, such a procedure in two dimensions tends to destabilise the supersolid. By evaporatively cooling directly into the supersolid phase–hence bypassing the roton instability–we experimentally produce a 2D supersolid in a near-circular trap, an observation verified through state-of-the-art finite temperature simulations. We show that 2D roton modes have little in common with the supersolid configuration, instead, unstable rotons produce a small number of central droplets, which triggers a nonlinear process of crystal growth. We calculate excitations for a 2D supersolid ground state, and make comparisons with 1D arrays using the static structure factor. These results provide insight into the process of supersolid formation in 2D, and define a realistic path to the formation of large two-dimensional supersolid arrays.

This work is on the arXiv, and a PDF can be found here.

Phase coherence in out-of-equilibrium supersolid states of ultracold dipolar atoms

Now in Nature Physics!

Supersolids are fluid and solid at the same time. In jointly collaboration work, together with Thierry Giamarchi, theoretical physicist from the University of Geneva, we have for the first time investigated what happens when such a state is brought out of balance.

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