Scientists first observed “quasiparticles” in the classical system

Starting with the emergence of quantum mechanics, the world of physics was divided between classical physics and quantum physics. Classical physics deals with the motions of objects we see every day in the macroscopic world, whereas quantum physics explains the strange behavior of elementary particles in the microscopic world.

Many solids or liquids are composed of particles that interact with each other in close proximity, sometimes resulting in the appearance of “pseudo-particles”. Quasiparticles are long-lived exciters that behave effectively as weakly interacting particles. Soviet physicist Lev Landau introduced the idea of ​​quasiparticles in 1941, and it has been of great use in research of quantum matter ever since. Some examples of quasiparticles include Bogolyubov quasiparticles (i.e., “broken Cooper pairs”) in superconductivity, excitations in semiconductors, and phonons.

Examining the collective phenomena that arise in terms of quasiparticles has provided insights into various physical arrangements, especially superconductivity, superfluidity and more recently in the famous example of Dirac’s quasiparticles in graphene. But so far, the observation and use of quasiparticles is still limited Quantum physics: In classical condensed matter, the collision rate is usually too high to allow the excitation of such long-lived particles.

However, the standard view that quasiparticles are confined to quantum matter was recently challenged by a group of researchers at the Center for Soft and Living Materials (CSLM) within the Institute of Basic Sciences (IBS), South Korea. They examined a classical system made of microparticles driven by a viscous flow in a microfluidic channel. When the particles are pulled by the flow, they disturb the fluidity around them, thereby exerting hydrodynamic forces on one another.

Amazingly, the researchers found that this long-distance force causes the particles to pair up. This is because hydrodynamic interactions violate Newton’s third law, which states that the force between two particles must be equal and opposite in direction. On the other hand, the forces are “anti-Newtonian” because they are equal and unidirectional, thereby stabilizing the pair.

The large number of paired particles suggests that this is a long-lived primary excitation in the quasiparticle system. This hypothesis was proven correct when the researchers simulated large two-dimensional crystals made of thousands of particles and observed their motion. Hydrodynamic forces between molecules make crystals vibrate, much like thermal phonons in a vibrating solid.

The quasiparticle pair diffuses through the crystal, catalyzing the formation of another pair by a chain reaction. Quasiparticles travel faster than the phonon speed, so each pair leaves behind an avalanche of newly formed pairs, much like the resulting Mach cones behind a supersonic jet. Eventually, all of these pairs collide with each other, which eventually causes the crystals to melt.

news/tmb/2023/scientists-observe-qua-1.jpg" data-src="https://scx2.b-cdn.net/gfx/news/2023/scientists-observe-qua-1.jpg" data-sub-html="The spectrum of phonons in a hydrodynamic crystal exhibits Dirac cones, manifesting the generation of quasiparticle pairs. The zoom shows one of the Dirac double cones. Credit: Institute for Basic Science">

The fusion produced by the pair is observed in all crystal symmetries except for one particular case: hexagonal crystals. Here, the 3D symmetry of the hydrodynamic interactions matches the symmetry of the crystal, and consequently, the initial excitation is a very slow (unpaired) low-frequency phonon as usual. In the spectrum, one sees “flat bands” where these ultraslow phonons condense. The interactions between flatband phonons are highly collective and coherent, which is evident in the most prominent and distinct classes of fusion transitions.

Specifically, when analyzing the phonon spectra, the researchers identified a typical conical structure of Dirac’s quasiparticles, like the structures found in graphene’s electronic spectrum. In the case of hydrodynamic crystals, Dirac’s quasiparticles are simply pairs of particles, formed by flow-mediated “anti-Newtonian” interactions. This indicates that the system can function as a classical pair for the particles detected in graphene.

explains Tsvi Tusty, co-author of the corresponding paper.

In addition, quasiparticles and flat bands are of great interest in condensed matter physics. For example, flat bands were recently observed in graphene bilayers twisted at certain ‘magic angles’, and coincidentally the hydrodynamic systems studied at IBS CSLM exhibit similar flat bands in much simpler 2D crystals.

Taken together, these results suggest that other emerging collective phenomena measured so far only in quantum systems can be detected in a variety of classical dissipative settings, such as energetic and living mattersaid Hyuk Kyu-bak, one of the corresponding newspaper writers.