Using an ultracold quantum gas of dysprosium atoms, it was possible for the first time to obtain a two-dimensional “droplet” lattice that possesses both the properties of a solid and a superfluid. We talk about it with the scientist at the head of the team of experimental physicists who conducted the experiment, Francesca Ferlaino of the University of Innsbruck
It’s called supersolidity and it’s a state of matter chased by physicists for decades. A quantum state with almost magical properties: a supersolid, in fact, simultaneously possesses the properties of a solid – with the atoms arranged in an ordered way, forming a crystalline structure – and those of a superfluid – in particular, the absence of friction. A bit like an ice cube sliding on water, or water sliding on an ice cube, wanting to look for an analogy, but with the individual atoms that are each and at the same time water andice. A state that is difficult even to imagine, let alone to achieve. Yet in the laboratory of the Department of Experimental Physics of the University of Innsbruck, Austria, they have succeeded.
They succeeded by using an ultra-cold quantum gas – we are talking about just a hundred nanokelvin, therefore on the border with absolute zero – of dysprosium atoms : an element of the rare earth group which, at low temperatures, is strongly magnetic. And what they managed to produce represents an absolute novelty: not a single row of atoms – therefore a one-dimensional supersolid , already made two years ago – but a matrix, or a two-dimensional supersolid (see opening image). A lattice of what scientists call droplets: “Droplets” thickened in a sea of ultra-cold quantum gas. With a purely quantum peculiarity, and completely counterintuitive, which establishes its supersolidity: despite the densification, the atoms of dysprosium are indistinguishable, as each of them is spread over the entire lattice. “Usually, one would think that every atom is in a given droplet , and that there is no way for this atom to move between them,” says the first author of the study published today in Nature , Matthew Norcia , of Institute of Quantum Optics and Quantum Information ( Iqoqi) of the Austrian Academy of Sciences in Innsbruck. On the contrary, in the supersolid state, each particle is delocalized on all the droplets , being simultaneously in each droplet. Thus, the system forms a series of high-density regions ( droplets ) that share the same delocalized atoms ».
But how did they get there? To understand this, it is better to go back to the origins of the studies on supersolidity, explains to Media Inaf the scientist at the head of the group that carried out the experiment , Francesca Ferlaino , who after a degree in physics from Federico II in Naples and a doctorate from Lens in Florence is today full professor at the University of Innsbruck, Austria, as well as managing and research director of Iqoqi, also in Innsbruck.
“If we have to choose a year from which to start this story, I would say 1957, when the great founding fathers of quantum mechanics started wondering what were the most paradoxical states that this theory could support, and superfluidity was starting to be an increasingly better phenomenon. known. First Eugene Gross , then Andreev, Lifshitz and Chester, wondered if it was possible to create a state of matter that on the one hand shows the rigidity of a solid, but on the other hand flows like a liquid “
And was it possible?
«From the point of view of the rules of quantum mechanics, yes: this type of state seemed to exist. Of course it is a paradoxical state, even just imagining a crystal flowing is quite difficult. However, this opened up a whole series of theoretical debates in the 1960s and 1970s, with scientists like Oliver Penrose and Lars Onsager saying no: superfluidity, with a fluid that flows without friction, and localization are two types of orders of matter that contradict each other – one prevents the formation of the other, so this super-solid state cannot exist ».
On the other hand, there were those who said that yes, maybe it existed?
“Well, there were those who wondered: but if it existed, what system could it be seen in? Which system in nature? In this sense, a great contribution came from Nobel laureate Anthony Leggett, theoretical physicist who in 1970 published an article – now considered a milestone on the subject – entitled “ Can a solid be superfluid? “. An article in which he discusses various possibilities ».
«The experimental approaches – or theoretical simulations – started from the idea of creating a solid that at some point, if cooled enough, could have superfluid properties. A material in which there were no two components – there were no solid and superfluid – but a single particle, indistinguishable as much as mechanically, which behaved at the same time as localized and diffused ».
Which material? Was there any candidate?
“From an experimental point of view, we started looking for this state in solid helium. A great result was then published in Nature in 2004 by two scientists at Penn State University – E. Kim and MHW Chan – in an article entitled ” Probable observation of a supersolid in helium “. A result that has attracted the attention of the whole condensed matter community: there have been great debates, there have been many theoretical groups that have tried to reproduce it by doing calculations, numerical simulations, etc. And the community began to split into two parts: those who believed in that result and those who tended to be a little skeptical. On the other hand, even just from the title of the paper – “ probableobservation… ”- it was obvious that not everything was completely under control at that time. The debate was very heated, but very heated within this community ».
How did it turn out?
«There has been an intense scientific work, some experimental groups have repeated the experiment of Kim and Chan obtaining different results, thus increasing even more the mystery around this state. Subsequently the same Kim and Chan repeated their experiment, this time on the basis of the calculations and the understanding of the system developed over the years, ending with a comment on their own work, published in 2012 in Physical Review Letters , entitled ” Absence of supersolidity in solid helium “”.
A tombstone …
«Let’s say that in the condensed matter community the question has become: game over , is it all over, or is this system still possible? A part of the research – above all theoretical, but also experimental – continues to develop considering these solids as the mother platform for the observation of supersolidity. Other theorists, however, have shifted attention to another type of platform: that of “cold atoms”. Because in cold atoms, in fact, various ingredients necessary for supersolidity are, let’s say, innate ».
What is special about cold atoms?
“In an ultra-cold gas, such as a Bose-Einstein condensate , with long-range interactions – therefore with the atoms that are” seen “from a distance – and not spherical, therefore anisotropic – such that if the atoms are seen in a direction they attract but if they look in another direction they repel – the system tends to be somewhat unstable. The system wants to crystallize: it is a gas, it never becomes a true crystalline structure, but it wants to spontaneously organize itself into a structure like a wave of matter – like a regular, crystalline wave. The problem is that this wantsof the system is too strong: only with this “desire” of theirs – because we know that in physics all systems seek a lower energy state – in a direction that decreases energy, the desire to crystallize would make them collapse. What was lacking in understanding – and this is where the experiments really mattered – was to find a mechanism that could stabilize , that is to say: you can crystallize up to a certain point, but then you have to stop. In this way the system undergoes a phase transition into a new state, which is precisely this state of minimum energy which is supersolid: the wave of matter forms and remains ».
And what is this mechanism capable of stabilizing the system?
“It was discovered, first by a group from Stuttgart and then quantitatively confirmed by our group, that in the dipolar atoms the system is stabilized by its own quantum fluctuations. The system, therefore, does create a crystal, but it cannot completely “explode” with infinitely high peaks, let’s say, because there is this stabilization system that tells it: you can’t create density peaks that much. It’s a kind of “quantum pressure”. All these ingredients together then allowed the observation of the fact that the system, by itself, spontaneously enters a new fundamental state: a completely coherent state, in which each atom is identical to the others – therefore they are indistinguishable – and they are all both localized than widespread. And this is precisely the supersolid state ».
State that you have made with an ultracold gas of dysprosium atoms, we said. But how did you get there, using dysprosium? Do you take the table of elements and choose a few at random – elements, by the way, which for us are absolutely exotic – until you find the right one to achieve the effect you are looking for? Or do you go without fail, already knowing more or less what region of the periodic table is to go fishing in order to achieve such a state?
«The first condensate was made with simple atoms, the alkalis , then atoms from the first column of the periodic table, which have only one valence electron. More or less there and in the next column, that of alkaline earths , the search for cold atoms had stopped a bit. Then, as we began to understand these simpler systems, we took the courage to go further and ask ourselves: I want a certain atomic property, which atom can give it to me?
And you made the leap from helium to dysprosium …
“Yes, but with a series of steps. We tried ytterbium, but it looks a lot like alkaline earthy. Then with chromium, which already showed dipolar effects, but needed an even higher magnetic moment. So we went to see which were the most magnetic atoms of the periodic table, and we arrived at dysprosium ».
Supersolids aside, among all the elements of the periodic table which is the one that gives you the most satisfaction? The one to which, by dint of using it in the laboratory, are you most fond of?
«I am very attached to erbium, because we were the first to condense it, but also to find a“ recipe ”that is now used in many laboratories . But it must be said that the erbium its lanthanide brothers are almost the same: you learn one and you have learned them all. So yes, I feel a certain sense of “motherhood” – so to speak – towards the erbium, the firstborn, but also for the other children ».
Featured image: Supersolid in 2D created by an ultracold gas. The colors represent the density, from black (low) to yellow (high). Source: Matthew A. Norcia et al., Nature, 2021
To know more:
- Read in Nature the article ” Two-dimensional supersolidity in a dipolar quantum gas “, by Matthew A. Norcia, Claudia Politi, Lauritz Klaus, Elena Poli, Maximilian Sohmen, Manfred J. Mark, Russell N. Bisset, Luis Santos and Francesca Ferlaino
Provided by INAF