Supersolid in a new dimension

Quantum matter can be solid and fluid at the same time – a situation known as supersolidity. Researchers led by Italian quantum physicist Francesca Ferlaino have now created for the first time this fascinating property along two dimensions. They now report in the journal Nature on the realization of supersolidity along two axes of an ultracold quantum gas. The experiment offers many possibilities for further investigation of this exotic state of matter.

Quantum gases are very well suited for investigating the microscopic consequences of interactions in matter. Today, scientists can precisely control individual particles in extremely cooled gas clouds in the laboratory, revealing phenomena that cannot be observed in the every-day world. For example, the individual atoms in a Bose-Einstein condensate are completely delocalized. This means that the same atom exists at each point within the condensate at any given time. Two years ago, the research group led by Italian born Francesca Ferlaino from the Department of Experimental Physics at the University of Innsbruck and the Institute of Quantum Optics and Quantum Information at the Austrian Academy of Sciences in Innsbruck managed for the first time to generate supersolid states in ultracold quantum gases of magnetic atoms. The magnetic interaction causes the atoms to self-organize into droplets and arrange themselves in a regular pattern. “Normally, you would think that each atom would be found in a specific droplet, with no way to get between them,” says Matthew Norcia of Francesca Ferlaino’s team. “However, in the supersolid state, each particle is delocalized across all the droplets, existing simultaneously in each droplet. So basically, you have a system with a series of high-density regions (the droplets) that all share the same delocalized atoms.” This bizarre formation enables effects such as frictionless flow despite the presence of spatial order (superfluidity).

New dimensions, new effects to explore

Until now, supersolid states in quantum gases have only ever been observed as a string of droplets (along one dimension). “In collaboration with theorists Luis Santos at Leibniz Universität Hannover and Russell Bisset in Innsbruck we have now extended this phenomenon to two dimensions, giving rise to systems with two or more rows of droplets,” explains Matthew Norcia. This is not only a quantitative improvement, but also crucially broadens the research perspectives. “For example, in a two-dimensional supersolid system, one can study how vortices form in the hole between several adjacent droplets,” he says. “These vortices described in theory have not yet been demonstrated, but they represent an important consequence of superfluidity,” Francesca Ferlaino is already looking into the future. The experiment now reported in the journal Nature creates new opportunities to further investigate the fundamental physics of this fascinating state of matter.

New research field: Supersolids

Predicted 50 years ago, supersolidity with its surprising properties has been investigated extensively in superfluid helium. However, after decades of theoretical and experimental research, a clear proof of supersolidity in this system was still missing. Two years ago, research groups in Pisa, Stuttgart and Innsbruck independently succeeded for the first time in creating so-called supersolids from magnetic atoms in ultracold quantum gases. The basis for the new, growing research field of supersolids is the strong polarity of magnetic atoms, whose interaction characteristics enable the creation of this paradoxical quantum mechanical state of matter in the laboratory.

The research was financially supported by the Austrian Science Fund FWF, the Federal Ministry of Education, Science and Research and the European Union, among others.

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.

Lifecycle of a Supersolid Featured in APS Physics Viewpoint and Physics World

Congratulation to Max, Claudia, Lauritz, Lauriane, Manfred and Matt for their experimental work

on the lifecycle of a supersolid, which has been just published in Phys. Rev. Lett..

The paper got selected as Editors’ Suggestion by APS and featured in Physics Viewpoint and in Physics World.

Read More:

Research News: Physics Viewpoint article by Chinmayee Mishra, entitled  “Cooling a Thermal Cloud to a Supersolid”.

Research News: Physics World article by Oliver Stockdale, entitled “High-resolution imaging sheds light on supersolid formation


Birth, Life, and Death of a Dipolar Supersolid

Maximilian Sohmen, Claudia Politi, Lauritz Klaus, Lauriane Chomaz, Manfred J. Mark, Matthew A. Norcia, and Francesca Ferlaino

Phys. Rev. Lett. 126, 233401 (2021)

Published June 7, 2021

Birth, life, and death of a dipolar supersolid

In the short time since the first observation of supersolid states of ultracold dipolar atoms, substantial progress has been made in understanding the zero-temperature phase diagram and low-energy excitations of these systems. Less is known, however, about their finite-temperature properties, particularly relevant for supersolids formed by cooling through direct evaporation. 

In this new experimental work, we explore this realm by characterizing the evaporative formation and subsequent decay of a dipolar supersolid by combining high-resolution in-trap imaging with time-of-flight observables.  As our atomic system cools towards quantum degeneracy, it first undergoes a transition from thermal gas to a crystalline state with the appearance of periodic density modulation.  This is followed by a transition to a supersolid state with the emergence of long-range phase coherence. Further, we explore the role of temperature in the development of the modulated state.



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