Physicists Transform Liquid Metal into Dense Plasma

A team of physicists from the University of Rochester and the Lawrence Livermore National Laboratory has found a way to turn liquid metallic deuterium into a plasma — the fourth fundamental state of matter in the sequence: solid, liquid, gas, and plasma — and to observe the temperature where a liquid under high-density conditions crosses over to a plasma state.

Zaghoo et al transformed liquid metals into plasmas under high-density conditions. Image credit:: Menno van der Krift.

Plasmas consist of a hot soup of free moving electrons and ions that easily conducts electricity.

Although they are not common naturally on Earth, plasmas comprise most of the matter in the observable Universe, such as the surface of the Sun.

Physicists are able to generate artificial plasmas here on Earth, typically by heating a gas to thousands of degrees Fahrenheit, which strips the atoms of their electrons.

University of Rochester’s Dr. Mohamed Zaghoo and colleagues observed, however, there is another way to create a plasma: under high density conditions, heating a liquid metal to very high temperatures will also produce a dense plasma.

“The transition to the latter has not been observed scientifically before and is precisely what we did,” Dr. Zaghoo said.

“One of the unique aspects of this observation is that liquid metals at high densities exhibit quantum properties. However, if they are allowed to cross over to the plasma state at high densities, they will exhibit classical properties.”

Increasing the density to extreme conditions made the liquid enter a state where it exhibited quantum properties; the bottom panel shows the quantum distribution of electrons in a dense liquid metal, where only two electrons can share the same state; however, when the temperature is increased to 0.4. Fermi temperature (90,000 degrees Fahrenheit, or 50,000 degrees Celsius), the electrons rearrange themselves in a random way that resembles a hot soup of plasma and the electrons loose their quantum nature and behave classically (top panel). Image credit: Laboratory for Laser Energetics, University of Rochester / Heather Palmer.

In the 1920s, Enrico Fermi and Paul Dirac introduced the statistical formulation that describes the behavior of matter made out of electrons, neutrons, and protons — normal matter that makes up the objects of Earth.

They hypothesized that at certain conditions — extremely high densities or extremely low temperatures — electrons or protons have to assume certain quantum properties that are not described by classical physics. A plasma, however, does not follow this paradigm.

In order to observe a liquid metal crossing over to a plasma, Dr. Zaghoo and co-authors started off with metallic deuterium, which displayed the classical properties of a liquid.

To increase the density of deuterium, they cooled it to minus 422 degrees Fahrenheit (minus 252 degrees Celsius).

The researchers then used lasers to set off a strong shockwave through ultracool liquid deuterium.

The shockwave compressed deuterium to pressures up to 5 million times greater than atmospheric pressure, while also increasing its temperatures to almost 180,000 degrees Fahrenheit (100,000 degrees Celsius).

The sample started off completely transparent, but as the pressure rose, it transformed into a shiny metal with high optical reflectivity.

“By monitoring the reflectance of the sample as a function of its temperature, we were able to observe the precise conditions where this simple lustrous liquid metal transformed into a dense plasma,” Dr. Zaghoo explained.

The researchers observed that the liquid metal initially exhibited the quantum properties of electrons that would be expected at extreme temperatures and densities.

“However, at about 90,000 degrees Fahrenheit (50,000 degrees Celsius), the reflectance of metallic deuterium started rising with a slope that is expected if the electrons in the system are no longer quantum but classical. This means that the metal had become a plasma,” Dr. Zaghoo said.

That is, the team started off with a simple liquid. Increasing the density to extreme conditions made the liquid enter a state where it exhibited quantum properties.

“Raising the temperature even further made it turn into a plasma, at which point it exhibited classical properties, yet was still under high-density conditions,” said co-author Dr. Suxing Hu, also from the University of Rochester.

“What is remarkable is that the conditions at which this crossover between quantum and classical occurs is different from what most people expected based on plasma textbooks. Furthermore, this behavior could be universal to all other metals.”

The results appear in the journal Physical Review Letters.

_____

M. Zaghoo et al. 2019. Breakdown of Fermi Degeneracy in the Simplest Liquid Metal. Phys. Rev. Lett 122 (8); doi: 10.1103/PhysRevLett.122.085001