Unmeltable Nature

Israeli and American physicists have created a thought-provoking model that breaks the accepted notion of order in nature. Physicists tend to believe that order will always be lost at high temperatures, just like the loss of crystal structure when matter melts. The new findings suggest the laws of nature permit an unmeltable material that maintains its ordered phase at any temperature. The research was published in Physical Review Letters and has vast implications from supermaterials to the early universe.

Melting Cube
Melting Cube. Image Credit: Atomic Imagery/Getty Images

Here is a thought experiment for you – take a cup full of water and heat the cup above 100 Celsius, what will happen? Any kid will tell you that the water will evaporate. What if the cup was cooled below 0 Celsius? We all know that the water in the cup will freeze. Although it is a simple thought experiment, it raised an interesting question about critical behavior – is it possible to increase the melting point so significantly that it will be at infinity? The answer is YES! this thought-provoking conclusion was published by a group of high-energy physicists from the Hebrew University of Jerusalem and the University of Kentucky in Physical Review Letters.

To appreciate their findings, we need to dive deeper into the atomic structure of materials. Like ice, many solids are made of atoms arranged in a crystal form. Those who learned thermodynamics know that atoms are not steady at all, they love to jiggle around. As we pump heat into the system the particles will jiggle more and in consequence, the temperature will increase. From this pictorial description, the temperature is strongly linked to the random motions of particles and the amount of heat energy it gains. In other words, the temperature is usually described as a statistical parameter that characterizes the amount of energy. To our knowledge, all known materials have a critical temperature in which the jiggling is stronger than the bond between atoms. In that case, the material goes through a phase transition, just like a solid turning into a liquid.

Visualisation of atoms in a crystalline structure.
Visualisation of atoms in a crystalline structure. Image Credit: Getty Images/iStockphoto

Although the pictorial description is pretty convenient, it is limiting, mainly because phase transition generally describes a change in the particle's collective behavior and it is not limited to a spatial arrangement. When physicists describe a phase transition, they use a different language - they ask themselves what symmetries are in the system. The reason is that each phase has different symmetries it expresses. For instance, a two-dimensional cubic crystal has a finite number of points of view in which it looks the same (by rotating in 90 degrees). But as it vaporizes, the atoms fill the space randomly and have infinite transformations in which the randomness remains. So as a material is heated, it increases the degree of symmetry it expresses. What about the opposite scenario? You have probably heard about the term 'Symmetry Breaking' - this process is related to a phase transition in which the degree of symmetries decreases, just like when liquid water turns into an ice cube. On the other hand, when an ice cube turns into liquid water, the crystal order disappears. This means that the order disappears when the material is heated. We conclude by saying that as temperature increases, we expect to observe less order and more symmetries. So the original question can be formulated differently – Is it possible to create a material that sustains in its ordered phase?

In the last century, the great physicist, Lev Landau, thought it was not a thermodynamics law. He was quoted saying that “In the great majority of the known instances of phase transitions, the more symmetrical phase corresponds to higher temperatures and the less symmetrical one to lower temperatures. In particular, a transition of the second kind from an ordered to a disordered state always occurs with increasing temperature. This is not a law of thermodynamics, however, and exceptions are therefore possible.” And the exception was Rochelle salt. This salt is special because it decreases the degree of symmetries when temperature increases, opposite to the expected behavior! Sadly, it doesn’t answer our question, because it happens only in a limited range of temperatures and eventually the crystal will melt. But if this is not a law of thermodynamics, it might be possible to construct at least a theoretical model that manifests unmeltable nature!

Potassium Sodium Tartrate Crystal
Potassium Sodium Tartrate or Rochelle Salt Crystal. Image Credit: Dmitry Shintyakov

In the last month, a group of Israeli and American high-energy physicists have found a way to construct (mathematically) such "material" and published their recent findings in Physical Review Letters. This model has similar properties as superfluids (contain a similar interaction between particles, although it has a more complicated structure). The researchers showed that symmetry is broken and remains broken at any finite non-zero temperature. This means that the particles are held in their order. The order in their paper is not spatial (like in a crystal) but analogously, it means that an ordered phase is maintained no matter what the temperature - as if a crystal has unmeltable properties. They based their model on simple assumptions in quantum field theory, the same language in the standard model is written.

There are several consequences of this research, first, it might suggest that a supermaterial, like a superconductor, is possible at room temperature. The reason is that superconductors are considered a phase just like a solid crystal and are characterized by a quantum order. In order to reach this phase, physicists need to cool special metals to around minus 196 degrees Celsius. Superconductors are important because they are perfect conductors. They can conduct electricity with no resistance, i.e., with no heat loss. If a room-temperature superconductor is possible without extreme pressures, it will mean that we can replace all electric cables with super cables to save energy (a green solution to the energy crisis in the world). Currently, room-temperature superconductors are still a wild dream, but if this quantum-ordered phase exists in principle at any temperature, that would mean it is worth trying to look for it!

Jets of high-energy particles coming out of the active center of the nearby galaxy Centaurus A
X-ray image of jets of high-energy particles coming out of the active center of the nearby galaxy Centaurus A. Image Credit: NASA/CXC/CfA/R. Kraft et al.

A second implication is on the early universe. There are reasons to believe that all known forces behaved the same at the early moments of the universe when it was extremely hot and dense. In other words, their interaction strength and their spatial dependence are the same. If two particles interact it will be impossible to distinguish which force caused this interaction. This hypothesis, known as force unification, was proved partially in particle colliders when they found that the electromagnetic force and the weak force can be united above a critical energy scale. The theory of the electroweak force was established by Steven Weinberg, Abdus Salam, and Sheldon Glashow who were awarded the Nobel Prize in 1979. There is theoretical evidence that all other forces unite at high energy scales but we do not have experimental proof. The unification or separation of forces is caused by symmetry increase or symmetry breaking, respectively, similarly to a phase transition. If their paper is correct, unification is not a must. It might be that this hypothesis is wrong, and all forces remained separated in the early universe; however, the mechanism for the 'non-unified forces hypothesis' is unknown.

The complete research paper can be accessed in the Physical Review Letters

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