In the realm of solid-state physics, a theoretical quasiparticle known as "Pines' demon" has remained elusive for nearly seven decades. This peculiar entity, massless, neutral, and unresponsive to light, was hypothesized to play a pivotal role in specific superconductors and semimetals. Now, scientists from the United States and Japan claim to have successfully detected this enigmatic quasiparticle while conducting specialized electron spectroscopy on strontium ruthenate, marking a significant breakthrough.
Plasmons, proposed by physicists David Pines and David Bohm in 1952, are quantized fluctuations in collective electron density within a plasma. They bear resemblance to phonons, which represent sound quanta, with a notable distinction – plasmons do not approach zero frequency in the absence of momentum. This unique property arises from the energy required to overcome the Coulomb attraction between electrons and ions in a plasma, thereby establishing a finite oscillation frequency even at zero momentum.
Presently, plasmons are routinely studied in metals and semiconductors that possess conduction electrons behaving akin to a plasma. These quantized fluctuations, along with phonons and others, are categorized as quasiparticles due to their shared characteristics with fundamental particles like photons.
In 1956, David Pines conjectured the existence of a particular plasmon, one that, unlike traditional plasmons, did not demand an initial energy burst. He aptly named this new quasiparticle "Pines' demon" in homage to James Clerk Maxwell's renowned thermodynamic demon. Pines' demon arises when electrons in different bands of a metal fall out of phase with each other, preserving an overall static charge. In essence, it represents the collective motion of neutral quasiparticles whose charge remains screened by electrons from another band.
However, experimental confirmation of this long-standing hypothesis has proven immensely challenging. The opposing phase shifts of the two electron currents effectively cancel each other out, nullifying long-range Coulomb interactions. Consequently, no discernible signature from the demon exists in the metal's dielectric properties, rendering it non-reactive to light.
Nonetheless, a team led by Peter Abbamonte from the University of Illinois Urbana-Champaign (UIUC), in collaboration with Japanese researchers, found a novel approach to surmount this obstacle. Serendipitously, while studying the electronic properties of strontium ruthenate for potential use as a surrogate in high-temperature superconductors, they made their groundbreaking discovery. The technique employed was electron energy-loss spectroscopy, involving the emission of electrons with specific, narrow energy ranges and the subsequent measurement of energy loss concerning momentum as the electrons traverse a target material. This technique proves ideal for studying plasmons, as electrons exhibit high sensitivity to charge density fluctuations.
Using millimeter-sized single crystals of strontium ruthenate, the researchers observed distinct spectra with low- and high-energy electrons. At higher energies, they detected energy loss peaking at approximately 1.2 eV, associated with a conventional charged plasmon interaction. However, at lower energies, they identified an oscillation featuring a minuscule energy gap, less than 8 meV, at zero momentum.
This second feature, characterized by a velocity approximately 100 times that of sound, exceeded the possible association with phonons. Nevertheless, it remained significantly lower than the velocity of a surface plasmon, yet within 10% of the velocity predicted by UIUC theorist Edwin Huang for a quasiparticle consisting of two electron bands in strontium ruthenate oscillating out of phase – precisely the description of Pines' demon.
To verify the demon's existence, the researchers scrutinized its neutrality by examining intensity variations concerning momentum or changes in electron scattering angles. Their analysis revealed that the intensity of a conventional plasmon should vary inversely with momentum raised to the power of five. Remarkably, they found that the intensity of the neutral plasmon followed a similar inverse variation but with a smaller power of 1.83.
In conclusion, the researchers confidently assert that the observed acoustic mode is indeed Pines' demon, a theoretical concept predicted in 1956 but only now observed in a 3D material. This groundbreaking discovery opens avenues for further research and exploration of the demon's properties, potentially through advanced techniques like scanning transmission electron microscopy and the development of a hydrodynamic theory for the quasiparticle. Moreover, this phenomenon is likely not exclusive to strontium ruthenate, as it should manifest in other metals with differing electron bands, including certain superconductors. Pines' demon is poised to become a fascinating subject of study with broader implications in the field of solid-state physics.