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Supersolid: Physicists Create New State of Matter

Two teams of physicists have independently created a mysterious new state of matter. The state is known as a supersolid and it combines the properties of both solid and superfluid states.

Illustration of a supersolid state, in which the properties of a frictionless fluid and a crystalline state coincide. Image credit: Julian Léonard, ETH Zurich.

Illustration of a supersolid state, in which the properties of a frictionless fluid and a crystalline state coincide. Image credit: Julian Léonard, ETH Zurich.

Solid, liquid or gas — we encounter these three classic states of matter every day. It is difficult to imagine that substances could simultaneously exhibit properties of two of these states.

Yet, precisely such a phenomenon is possible in the realm of quantum physics, where matter can display behaviors that seem mutually exclusive. Supersolidity is one example of such an exotic state.

In a supersolid, atoms are arranged in a crystalline pattern while at the same time behaving like a superfluid, in which particles move without friction.

Until now, supersolidity was merely a theoretical construct.

But in the Mar. 2 issue of the journal Nature, two teams of researchers report the successful production of a supersolid state.

One of the teams, led by MIT Professor of Physics Wolfgang Ketterle, used a combination of laser cooling and evaporative cooling methods to cool atoms of sodium to nanokelvin temperatures.

Atoms of sodium are known as bosons, for their even number of nucleons and electrons. When cooled to near absolute zero, bosons form a superfluid state of dilute gas, called a Bose-Einstein condensate.

Prof. Ketterle and co-authors manipulated the motion of the atoms of the Bose-Einstein condensate using laser beams, introducing ‘spin-orbit coupling.’

In their ultrahigh-vacuum chamber, the researchers used an initial set of lasers to convert half of the condensate’s atoms to a different quantum state, or spin, essentially creating a mixture of two Bose-Einstein condensates.

Additional laser beams then transferred atoms between the two condensates, called a ‘spin flip.’

“These extra lasers gave the ‘spin-flipped’ atoms an extra kick to realize the spin-orbit coupling,” Prof. Ketterle explained.

Physicists had predicted that a spin-orbit coupled Bose-Einstein condensate would be a supersolid due to a spontaneous ‘density modulation.’ Like a crystalline solid, the density of a supersolid is no longer constant and instead has a ripple or wave-like pattern called the ‘stripe phase.’

Currently, the team’s supersolid only exists at extremely low temperatures under ultrahigh-vacuum conditions.

Going forward, Prof. Ketterle and his colleagues plan to carry out further experiments on supersolids and spin-orbit coupling, characterizing and understanding the properties of the new form of matter they created.

“With our cold atoms, we are mapping out what is possible in nature. Now that we have experimentally proven that the theories predicting supersolids are correct, we hope to inspire further research, possibly with unanticipated results,” Prof. Ketterle said.

Another research team, led by Professor Tilman Esslinger of the ETH Institute for Quantum Electronics, used an alternative way of turning a Bose-Einstein condensate into a supersolid with the help of mirrors.

To create the supersolid state, Prof. Esslinger and co-authors introduced a small amount of rubidium gas into a vacuum chamber and cooled it to a temperature of a few billionths of a kelvin above absolute zero.

They then placed the condensate in a device with two intersecting optical resonance chambers, each consisting of two tiny opposing mirrors.

The condensate was then illuminated with laser light, which was scattered into both of these two chambers.

The combination of these two light fields in the resonance chambers caused the atoms in the condensate to adopt a regular, crystal-like structure.

The condensate retained its superfluid properties — the atoms in the condensate were still able to flow without any energy input, at least in one direction, which is not possible in a ‘normal’ solid.

“We were able to produce this special state in the lab thanks to a sophisticated set-up that allowed us to make the two resonance chambers identical for the atoms,” Prof. Esslinger said.

“The simultaneous realization by two groups shows how big the interest is in this new form of matter,” Prof. Ketterle said.

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Jun-Ru Li et al. 2017. A stripe phase with supersolid properties in spin–orbit-coupled Bose–Einstein condensates. Nature 543: 91-94; doi: 10.1038/nature21431

Julian Léonard et al. 2017. Supersolid formation in a quantum gas breaking a continuous translational symmetry. Nature 543: 87-90; doi: 10.1038/nature21067