&Bullet; physics 14, 85
When a cloud of cold atoms undergoes a super-solid phase transition, its co-existing superfluid and crystalline phases do not occur simultaneously.
Observing a super-solid – a physical state that has both superfluid and solid properties – has been a challenge for decades. The individual particles of a super solid form a rigid, ordered structure, but can also flow without loss of energy. This paradoxical behavior has only been observed in experiments with cold atoms in the last five years [1–5] . Researchers typically create super solids from cold atoms from superfluid phases, with the question of how super solidity arises from other phases. Now a team led by Francesca Ferlaino from the University of Innsbruck, Austria, has shown that the crystalline and superfluid orders of a super-solid arrive one after the other during the formation and death of a super-solid and decay, assuming a thermal gas  . The results suggest that temperature plays a crucial role in the occurrence of super solidity.
The existence of a super-solid state was first suggested in 1957, long before experimenters knew how to achieve it  . The first experiments focused on realizing super solidity in helium, with researchers attempting to convert a helium sample from a solid, crystalline state to a super solid state. With no clear success, the researchers began looking for alternative systems to obtain a super solid  .
An ensemble of cold atoms is an alternative system. Cold atoms provide an ideal environment in which to create supersolids because researchers can shape their atom-atom interactions in a controlled manner. Cold atom physicists have shown that they can manipulate interactions to create a super solid from a cold atom structure, specifically a dipolar Bose-Einstein condensate (BEC). The properties of a dipolar BEC are determined by short-range repulsive interactions and by long-range dipole-dipole interactions between atoms. Typically, the super-solid phase is reached by canceling the magnitude of the short-range interaction, which induces a density modulation on the pre-existing super-liquid phase of the system.
With this method, super-solid phases were achieved in cold atoms that were originally super-fluid – density modulation added the solid behavior [1, 3, 5] . However, this approach to making a supersolid has a number of problems that limit the study of finite temperature effects. Ferlaino and her group are now showing that they can investigate the influence of temperature on density modulation by extracting a super solid with cold atoms directly from a thermal gas  .
In their experiments, the team used a technique called evaporative cooling. You have a gas of around 10. captured5 Dysprosium atoms with lasers. The lasers created a cigar-shaped optical barrier that the atoms could use to escape if their energy was high enough. As atoms escaped, the temperature of the remaining atoms dropped to several hundred Kelvin. The team then lowered the barrier height (they did so in different experiments at different speeds) and lowered the temperature of the system further until it was low enough that the atoms condensed into a super solid. To examine the system, the team used two imaging modalities: Faraday phase contrast imaging, which captured density modulations in the system, and time of flight (TOF) imaging, which provided information about global phase coherence.
By rapidly lowering the trap height (in this case in just 225 ms) and then developing the resulting atomic cloud, the team was able to investigate how the super-solid formed without the influence of the changing trap height. Under this condition, they found that the density modulations formed looked like droplets – an indication of a super solid. These modulations occurred on a time scale of approximately 150 ms. This time scale corresponds to the equilibration time of the system and is determined by the evaporation rate, which in turn depends on the elastic collision rate between atoms.
By slowly lowering the trap height (in a time interval of 500 ms), the team was able to investigate whether the translational symmetry or the phase symmetry was broken first: did the gas become solid or superfluid first? To answer this question, they used the “phasor function” extracted from the TOF measurements at different time intervals [5, 9] . The value of the pointer gives information about the density modulation, while its phase gives information about the coherence.
Based on this information, they discovered that the density modulations corresponding to the solid phase were established first. About 40 ms later, the superfluid behavior sets in, which they saw in the development of global phase coherence. The 40 ms delay in the onset of superfluity can be traced back to the existence of finite temperature effects which inhibit the global phase coherence of the system.
The appearance of the superfluid behavior marked the entry of the atoms into the super solid phase, which lasted longer than 1 s. After this time, atomic losses from the trap caused the super solid to slowly disappear. At the transition of the system from the super solid, a continuous translational symmetry was first restored, as can be seen from the disappearance of the density modulations. The broken phase symmetry associated with superfluidity persisted much longer, meaning that even after the supersolid died, the two symmetries were not restored at the same time.
The results from Ferlaino and her colleagues help cement the notion that our understanding of supersolids is still in its infancy. So far there is no Supersolid phase diagram with temperature as a parameter, and it is not known how quantitative changes in temperature are related to density modulation, breaking of the continuous translational symmetry in the system and global phase coherence. Previous work has indicated the key role temperature plays in creating the density modulations required to produce a super-solid  . By successfully obtaining a super-solid state from a thermal cloud, the new work contributes to this conclusion, and it will be interesting to see what theoretical models predict on this point. A detailed understanding of density and phase excitations near this transition will deepen our knowledge of supersolids. It is undeniable that exciting times are ahead for researchers working on dipolar BECs and supersolids.
- J.-R. Li et al., “A stripe phase with super-strong properties in spin-orbit-coupled Bose-Einstein condensates”, nature543, 91 (2017).
- J. Leonard et al., “Supersolid formation in a quantum gas that breaks a continuous translational symmetry”, nature543, 87 (2017).
- L. Tanzi et al., “Observation of a dipolar quantum gas with metastable super-solid properties”, Phys. Rev. Lett.122, 130405 (2019).
- F. Cooper et al., “Transient super solid properties in an array of dipolar quantum droplets”, Phys. Rev. X9, 011051 (2019).
- L. Chomaz et al., “Long-lived and transient supersolid behavior in dipolar quantum gases”, Phys. Rev. X9, 021012 (2019).
- M. Sohmen et al., “Birth, Life and Death of a Dipolar Super Solid” Phys. Rev. Lett.126, 233401 (2021).
- EP Gross, “Unified Theory of Interacting Bosons”, Phys. Rev.106, 161 (1957).
- M. Boninsegni and NV Prokof’ev, “Colloquium: Supersolids: What and where are they?” Rev. Mod. Phys.84, 759 (2012).
- P. Ilzhöfer et al., “Phase coherence in non-equilibrium super-solid states of ultra-cold dipolar atoms”, Nat. Phys.17th, 356 (2021).
About the author