Proposed 67 years later, Scientists find Pines’ demon!

Lurking for decades: Researchers have discovered a "Pines' demon" - a collection of electrons in a metal that behaves like massless waves.

For nearly seven decades, plasmas known as Pines' demon have been a purely hypothetical feature in solid-state systems. This unusual quasiparticle, massless, neutral and unable to interact with light, is thought to play a key role in some superconductors and semi-metals. Now, scientists in the United States and Japan have finally detected it while studying strontium ruthenate materials using specialized electron spectrometers.

Pines’ demon observed as a 3D acoustic plasmon in Sr₂RuO₄

Proposed by physicists David Pines and David Bohm in 1952, protons are the quantum of collective electron density fluctuations in plasmas. They are similar to phonons - quanta of sound, but unlike phonons, their frequency does not go to zero when they have no momentum. This is because a finite amount of energy is needed to overcome the coulomb attraction between electrons and ions in the plasma, resulting in oscillations, which require a finite frequency of oscillation (zero momentum).

Today, plasmas are often studied in metals and semiconductors because they have conducting electrons that behave like plasmas. Protons, phonons, and other quantized fluctuations are called quasiparticles because they have the same properties as elementary particles such as photons.

In 1956, Pines hypothesized the existence of a plasma that, like sound, does not require an initial burst of energy. In honor of James Clerk Maxwell's famous thermodynamic demon (or Maxwell's demon), he called the new quasi particles "monsters". Pines' demon is formed when electrons in different bands of a metal shift from one another, keeping the overall charge static. In fact, the "demon" is the collective motion of neutral quasiparticles whose charge is shielded by electrons from another band (band).

However, this age-old prediction has been difficult to confirm in experiments. Since the two electron streams are not in phase, they cancel out and eliminate the long-range coulomb interaction. This rules out the "demon" signal in the properties of metallic dielectrics, meaning that quasiparticles do not interact with light.

Now, Peter Abbamonte and colleagues at the University of Illinois at Urbana-Champaign (UIUC) have demonstrated how to overcome this difficulty by using a non-standard technique to study strontium ruthenate. As they have publicly explained, their findings were accidental. They were not so much looking for the Pines demon as exploring the electronic properties of strontium ruthenate in order to use the material as an alternative to high-temperature superconductors, which have similar properties.

The technique they used is called electron energy-loss spectroscopy. This involves firing a beam of electrons with a notoriously narrow energy range and recording how much energy is lost at which moments after the electrons pass through the target material: this technique is well suited to studying protons because electrons are very sensitive to fluctuations in charge density.

Using millimeter-scale strontium ruthenate single crystals grown by Yoshiteru Maeno and his collaborators at Kyoto University in Japan, the researchers recorded very different spectra using low-energy and high-energy electrons. In the latter case, they found that the energy loss peaked at around 1.2 eV, which they believe is due to the interaction with a typical (charged) plasma. At lower energies, on the other hand, they observed an oscillation with a small energy gap at zero momentum: less than 8 meV.

The "demon" excitation concept diagram in Sr₂RuO₄

High-energy M-EELS spectra from Sr₂RuO₄

The second feature is an acoustic pattern that travels about 100 times faster than the speed of sound, which is too different from the speed of phonons. But at the same time, the speed of the mode is three orders of magnitude lower than that of the surface plasma. However, this number is less than 10% off the value predicted by Edwin Huang, a theorist at the University of California, Los Angeles, for the "Pines demon", a quastiparticle produced by two electron bands oscillating in reverse phase with each other in strontium ruthenate.

To make sure they had actually found this "demon," the researchers tested its neutrality by studying how its intensity changes with momentum-that is, how the scattering Angle of the electrons changes. They calculated that the strength of a conventional plasma should vary inversely with the fifth power of momentum. They say that the strength of a neutral plasma should also vary inversely with momentum, but with smaller powers. This is what they found: They determined that the new quasiparticle is characterized by an inverse power of only 1.83.

Properties of "demon" excitation in Sr₂RuO₄.

"We conclude that this acoustic pattern is Pines' demon, which was predicted in 1956 but has only now been observed in three-dimensional materials," they wrote in their paper published in Nature.

They believe that by conducting more experiments using scanning transmission electron microscopy, as well as developing a fluid mechanics theory of quasiparticles, this demon can be better understood. But Abbamonte added that such research need not be limited to strontium ruthenate, explaining that this "demon" should exist in other metals with sufficiently different electron bands, including some superconductors such as magnesium diboride or iron-based superconductors.

-- "This shouldn't be a rare or mysterious effect."

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