Supersolidity
A quantum phase of the matter which displays different types of order. Supersolid matter is characterized by both crystalline and superfluid properties. Atoms have periodicity of a crystal, together with frictionless flow. The first attempts to find this effect were held in superfluid helium. Although up to the moment no evident observation has been conducted. Two years ago (2017) two groups observed superfluid properties of BEC. Creation of density modulation is restricted by the atom-atom interaction and requires a long-range dipolar coupling. Atoms forming BEC are dilute and have only short-range(contact) interactions. To overcome this difficulty by spin-orbit coupling [1] or use cavity enhancement [2]. Authors proved that two continuous symmetries were broken, although the periodicity was set by the laser wavelength.
Three works were published this year (2019) with so called dipolar gases. This is a natural way to use atoms with strong magnetic moments.These atoms are distinct for their long-range dipole-dipole interactions. In these works periodicity follow from the intrinsic interactions, in contrast to previously used schemes, where the cause were the periodic optical potentials. [3][4][5].
Pfau's group investigated an appearence of droplets in dilute superfluid BEC of 162Dy. It happens in a small region of interaction strengths. This regime is surrounded by the regular BEC and isolated droplets phases. They are preparing BEC cloud and probe in situ with a far-detuned phase contrast imaging or time of flight absorption imaging. In situ measurement reveals density modulation. The time of flight expansion demonstrates an interference pattern depending on the expansion time.
The last group (Ferlaino) have compared creation of the phase in 166Er and 164Dy. At first they perform numerical simulation, when they solved an extended Gross-Pitaevskii equation. They noticed that at high scattering lengths (as>as*) density profile corresponds to a regular dilute BEC. For the scattering lengths below this critical value ground state has periodically modulated density (see Fig.1). They prepare ultra-cold atoms in a cigar-shape trap, they ramp scattering length via Feischbach resonance to a different values and observe three distinct phases.
In conclusion, in the latest works groups theoretically have proven an existence of three phases. They experimentally demonstrated self-organized density modulation and phase coherence between separate droplets. The more rigorous proof of the superfluidity would be an observation of collective excitations (phononic and rotonic modes). As well exemplary experiments would be quantized vortices or measuring the transport properties through the narrow aperture.
References
- ↑ Li, Jun-Ru, et al. "A stripe phase with supersolid properties in spin–orbit-coupled Bose–Einstein condensates." Nature 543.7643 (2017): 91
- ↑ J. Léonard, A. Morales, P. Zupancic, T. Esslinger, and T. Donner, “Supersolid formation in a quantum gas breaking a continuous translational symmetry,” Nature 543, 87 (2017)
- ↑ L. Tanzi, E. Lucioni, F. Famà, J. Catani, A. Fioretti, C. Gabbanini, R. N. Bisset, L. Santos, and G. Modugno, “Observation of a dipolar quantum gas with metastable supersolid properties,” Phys. Rev. Lett. 122, 130405 (2019).
- ↑ Böttcher, Fabian, et al. "Transient supersolid properties in an array of dipolar quantum droplets." Physical Review X 9.1 (2019): 011051.
- ↑ Chomaz, L., et al. "Long-lived and transient supersolid behaviors in dipolar quantum gases." arXiv preprint arXiv:1903.04375 (2019).
