What is a supersolid?

The names that scientists, especially physicists, give to things has been a source of humour or irritation, depending on your point of view. Despite observatories named the Very Large Telescope, succeeded by the Extremely Large Telescope, and in spite of Murray Gell-Mann naming the quarks after a word that appears only once in James Joyce’s Finnegans Wake, I’m firmly on the irritated side. There is no reason that the colour charge, for example, should be called that considering it has nothing to do with colour. Nor should the theory describing how subatomic particles are affected by the colour charge be called quantum chromodynamics.

For another example, a superfluid is a quantum phase of matter that flows without any resistance – so the name makes sense. But a supersolid is a quantum phase of matter that has the ordered structure of a solid but can flow like a superfluid! (A ‘quantum phase’ is a phase of matter that exists at extremely low temperatures, in quantum systems – i.e. systems where quantum-mechanical forces dominate.) Supersolids are clearly inappropriately named, but we can say the same thing about its properties. In the 1960s, scientists worked out the math and concluded that supersolids should exist – but they weren’t able to create them in the lab until the last decade or so.

A crystal is a solid whose constituent atoms are arranged in a fixed, repeating pattern. The grid of atoms is called the lattice and each point occupied by an atom is called a node. When a node is empty, i.e. when an atom isn’t present, the site is called a vacancy. When you cool a substance to a lower and lower temperature, you take away energy from it until, at absolute zero, it has no energy. But in quantum systems, the material retains some energy even at absolute zero. This is the energy implicit to the system and is called zero-point energy. This energy allows atoms to move from occupied nodes in the lattice to nearby vacancies.

Sometimes there could be a cluster of such vacancies. When this cluster moves as a group through the material, it is equivalent to a group of atoms in the lattice moving in the opposite direction. (If you’re sitting at spot 1 on the couch and move to spot 2, it’s equivalent to the vacancy on the couch moving from spot 2 to spot 1.) When this happens, the cloud of vacancies is called a supersolid: the cluster maintains its fixed structure, defined by the lattice, yet it move without resistance through the material.

The first several attempts to create a supersolid succeeded but they were confined to one dimension. This is because many of them used a common method: to assemble a bunch of atoms of a particular element, “turn them into a superfluid and then add a crystalline structure by altering the interactions between the atoms” (source). This technique doesn’t work well to create two-dimensional supersolids because the “add a crystalline structure” step weakens the fragile superfluid state.

In 2021, a group of physicists from Austria and Germany attempted to overcome this barrier by using magnetic atoms that formed small clumps with each other as well as allowing the clumps to repel each other and arrange themselves in a two-dimensional array. The jump from one dimension to two dimensions is significant because it allows physicists to explore the presence of other features of supersolids in the system. For example, theoretical calculations say that supersolids can have vortices on their surface. A one-dimensional supersolid doesn’t have a surface per se but a two-dimensional one does. Physicists can also study other features depending on the number of atoms involved. This said, the researchers’ method was cumbersome and didn’t produce a supersolid of sufficient quality.

In a new study, published on May 13, 2022 (preprint paper), some members of the 2021 group reported that they were able to create a supersolid disc. This is also a two-dimensional supersolid but with a different geometry (the 2021 effort had produced a roughly rhombus-shaped supersolid). More importantly, the researchers devised a new method to synthesise it. While the previous method first introduced the superfluidity and then the crystallinity, in the new method, the physicists introduced both together.

When you sweat in warm weather, water gets on your skin and then evaporates. When it does, it takes away some heat to change from liquid to vapour, thus cooling your skin. This is called evaporative cooling. When you start with a cloud of atoms, proceed to take away their energy and then progressively remove the most energetic atoms at each stage, you also progressively reduce the average energy of the system, and thus the overall temperature. This is evaporative cooling with atoms. In their study, the research team developed a theory to explain how this form of cooling could be used to create a supersolid inside a circular trap. Then they tested the theory to create a roughly hexagonal supersolid of a few tens of thousands of dysprosium atoms. (Dysprosium is highly magnetic, so its atoms can be clustered by modifying the magnetic field.)

(a) The almost-circular supersolid of dysprosium atoms, in seven clusters (or droplets) in a hexagonal shape with one cluster at the centre. (b) The research team conducted 68 trials of the same experiment and in each case photographed the dysprosium supersolid after it had moved for 36 milliseconds. This is the ‘averaged’ shot. Credit: arXiv:2107.06680v2

Considering the way in which physicists have achieved this new supersolid, the name seems all the more confusing.