What is the 7th state of matter

"An exotic state of matter"

In our everyday life, we encounter matter in three different aggregate states - solid, liquid and gaseous. According to the laws of quantum physics, however, more exotic states are also possible. One of them - a super solid - was theoretically predicted over 50 years ago. Since then, scientists have been trying to create this state of matter. Now, for the first time, researchers have unequivocally demonstrated a super solid in the laboratory. How this succeeded, explains Tilman Pfau from the University of Stuttgart in an interview with Welt der Physik.

World of physics: what is a super solid?

Tilman Pfau: A super solid is a body that is liquid and solid at the same time. This contradicts our everyday experience, but is possible according to the laws of quantum physics. In classical physics, too, there are mixtures of different states of matter. For example, partially melted ice has a solid and a liquid part. But unlike in such classical mixtures - in which the two states exist side by side - they overlap in the super solid in a quantum mechanical way.

How can one imagine this overlay?

Solid and liquid differ in the behavior of their components - that is, of the atoms or molecules. While the components of a solid are arranged regularly - in a so-called crystal structure - they move in a liquid without a solid structure. In a super solid, the individual particles are part of both the crystal structure of a solid and the free movement of a liquid. In addition, the liquid part of the super solid is superfluid. This means that the atoms or molecules - due to quantum physics - move without friction.

Can you find super solids in nature?

Such a superimposed state of superfluids and solids has not yet been found in nature. Since the theoretical prediction more than 50 years ago, there have been repeated attempts to produce the exotic state of matter in the laboratory - but initially without success. The difficulty lies in controlling the forces in a body in such a way that a solid and a liquid state develop at the same time. But we have now succeeded.

How was that possible?

Tilman peacock

A superfluid is produced, for example, with helium atoms, which are cooled down to just above zero temperature. However, in order to create a superposition of a superfluid and solid state, atoms with special properties are necessary. We used dysprosium atoms which, like bar magnets, either repel or attract each other, depending on their position in relation to one another. As a result of this direction-dependent force, a superfluid and an ordered solid structure are formed at the same time as it cools.

And how do you know that it is actually a super solid?

The detection of a super solid is divided into three parts: First, we looked at the state of matter produced under a microscope. So we checked whether a periodic crystal structure had really formed. Then we had to make sure that our system is not a classic mixture of states, but has quantum mechanical properties. With the help of interference experiments we have shown that all atoms of the system can be described by a common quantum mechanical state. The final step, which was previously missing for the unequivocal detection of a super solid, was the detection of sound waves in the super solid.

How can a super solid be detected with sound?

If you bump into a body, its components vibrate in a certain way - depending on the internal structure of the body. This oscillation then spreads in the form of sound waves of different speeds. When we excited our super solid to vibrate, we discovered a very special vibration mode. One of these so-called Goldstone modes occurs when a crystal structure and a liquid oscillate in opposite directions. By demonstrating this very slowly oscillating sound wave, we were able to prove beyond doubt for the first time that both a superfluid and a solid state actually exist in a superposition in our system.

What applications does this state of matter offer?

With our experiment we are mainly doing basic research. We wanted to prove that super solids not only exist in theory, but that they can also be produced in the laboratory. This gives us the opportunity to research many previously unknown properties of super solids. The discovery of other states of matter is also conceivable. Whether this will lead to specific applications at some point, however, is still in the stars.