Discovery of Pines’ Demon Quasiparticle Confirmed by Scienti

Researchers at the University of Illinois Urbana-Champaign stumbled upon an unexpected phenomenon while investigating a rare superconducting compound. Their data revealed a particle that defied all known classifications.

After extensive study, the scientists confirmed the presence of Pines’ demon, a peculiar quantum excitation initially hypothesized by physicist David Pines in 1956, but never experimentally verified in a stable three-dimensional metal until now. The team’s results were documented in Nature in August 2023, bridging a 67-year gap between theory and experimental evidence.

Theoretical Roots and Challenges in Detection

David Pines proposed over six decades ago that electrons within certain metals could collectively oscillate to form a novel quasiparticle characterized by zero mass, absence of electric charge, and non-interaction with light. He named this entity a “demon,” inspired partly by Maxwell’s thought experiment and partly as an acronym for “distinct electron motion.” Although the concept was widely referenced in theory, experimental proof remained elusive.

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This demon is a special type of plasmon, a collective oscillation of an electron population inside solids. Traditional plasmons interact with light and carry charge, making them easier to observe using optical techniques. However, the demon’s neutral nature renders it invisible to such methods. It appears in metals possessing electrons across multiple energy bands, where electrons moving out of sync cancel their electrical charges, producing a neutral excitation undetectable by standard optical probes.

Strontium ruthenate sample placed on a copper base for electron spectroscopy. Credit: Husain et al., Nature, 2023

Theoretical predictions suggested demons might influence phase transition dynamics in certain semimetals, affect the optical responses of metal nanoparticles, and even play a role in superconductivity seen in substances like metal hydrides. Their massless quality allows demons to form at any energy scale, suggesting they could exist over a wide temperature range. “Most research utilizes light to measure these phenomena,” explained Peter Abbamonte, the lead investigator. “Because these particles don’t couple to light, we needed a totally different approach.”

An Accidental Discovery in Strontium Ruthenate

The team’s breakthrough occurred while studying strontium ruthenate (Sr₂RuO₄), a multiband metal that transitions to a superconducting state at extremely low temperatures. They initially aimed to better understand the mechanisms behind high-temperature superconductivity in related materials but noticed unexpected features in their data.

Key to this discovery was the use of momentum-resolved electron energy-loss spectroscopy (M-EELS), a technique where electrons are directed at the sample with controlled momentum, measuring the energy lost or gained during interaction.

Since M-EELS does not rely on light, it can detect neutral excitations like the demon that evade conventional optical instruments. Sr₂RuO₄’s three electronic bands—α, β, and γ—include two with markedly different speeds, providing the out-of-phase oscillation needed for the demon's existence.

Variation in demon quasiparticle’s energy dispersion across temperatures. Credit: Husain et al., Nature, 2023

Upon scrutinizing the data, former graduate student Ali Husain identified a signal with characteristics unlike any known quasiparticle: a velocity of about 1.065 × 10⁵ meters per second at room temperature, faster than typical acoustic phonons yet slower than surface plasmons, and displaying no mass. Husain noted, “At first, we weren’t sure what it was. The idea of demons seemed far-fetched. But as alternative explanations fell away, we grew confident we had uncovered Pines’ demon.”

Theorist Edwin Huang subsequently performed detailed computational modeling, confirming a quasiparticle arising from β and γ bands oscillating out of phase exactly as Pines theorized.

Verifying Charge Neutrality via Momentum Scaling

To solidify their claim, the researchers needed proof that this mode carried no net electric charge, distinguishing it from other gapless electronic excitations. They measured how the signal intensity varied with momentum transfer.

Charged excitations follow a specific scaling relation linked to electron energy conservation, whereas neutral modes scale differently due to their negligible impact on electrical screening over large distances. The observed scaling exponent of roughly q⁻¹.⁸ far exceeded the −5 exponent expected for charged modes, confirming the excitation’s neutrality. This finding was corroborated through five independent measurements on four distinct strontium ruthenate specimens, eliminating the possibility of experimental error.

The team also observed unexpected phenomena: the demon’s energy loss (“damping”) was less severe than existing theories anticipate, possibly because the quasi-one-dimensional structure of the β band reduces typical Landau damping within the relevant momentum-energy range. Additionally, cooling the material from room temperature to 30 Kelvin decreased the demon’s velocity by 31 percent, a trend not fully explained by current models.

Implications for Multiband Metals and Future Studies

The conditions behind demon formation extend beyond strontium ruthenate. According to the Nature article, this discovery suggests that Pines’ demons could be widespread among multiband metals, a large class of materials in condensed matter physics.

Previous studies failed to detect demons in strontium ruthenate due to insufficient momentum resolution. The precision of the M-EELS apparatus in this work, roughly fivefold finer than earlier measurements, was crucial in revealing the phenomenon.

However, the theoretical understanding remains incomplete. Observed quadratic energy dispersion at very low momentum implies that additional interaction effects, such as local field corrections, may be influential but are not yet incorporated into existing models.

The researchers suggest that advancing technology, such as a meV-resolved scanning transmission electron microscope operated in a defocused mode, could achieve higher resolution measurements. Moreover, exploring other multiband metals might yield further insights into these elusive quasiparticles.