by Beginning with the advent of quantum mechanics, the world of physics was divided between classical and quantum physics. Classical physics deals with the movements of objects that we usually see every day in the macroscopic world, while quantum physics explains the exotic behavior of elementary particles in the microscopic world.
Many solids or liquids are made up of particles that interact with each other at close intervals, sometimes leading to the emergence of ‘quasiparticles’. Quasiparticles are long-lived excitations that effectively behave like weakly interacting particles. The idea of quasiparticles was introduced by the Soviet physicist Lev Landau in 1941 and has been extremely fruitful in quantum matter research ever since. Some examples of quasiparticles include Bogoliubov quasiparticles (i.e. “broken Cooper pairs”) in superconductivity, excitons in semiconductors, and phonons.
The study of emerging collective phenomena related to quasiparticles has provided insights into a variety of physical relationships, particularly superconductivity and superfluidity, and recently the famous example of Dirac quasiparticles in graphene. So far, however, the observation and use of quasiparticles has been limited to quantum physics: in classical condensed matter, the collision rate is typically far too high to enable long-lived particle-like excitations.
However, the standard view that quasiparticles are exclusively quantum matter has recently been challenged by a group of researchers at the Center for Soft and Living Matter (CSLM) within the Institute for Basic Science (IBS), South Korea. They studied a classic system of microparticles driven by viscous flow in a thin microfluidic channel. As the particles are entrained in the flow, they disrupt the streamlines around them, thereby exerting hydrodynamic forces on each other.
Remarkably, the researchers found that these long-range forces cause the particles to organize in pairs. Because the hydrodynamic interaction breaks Newton’s third law, which states that the forces between two particles must be equal and equal opposite in the direction. Instead, the forces are “anti-Newtonian” because they are equal and in the even direction, stabilizing the pair.
The large population of pairwise coupled particles suggested that these are the long-lived elementary excitations in the system – its quasiparticles. This hypothesis proved correct when the researchers simulated a large two-dimensional crystal made up of thousands of particles and studied its motion. The hydrodynamic forces between the particles cause the crystal to oscillate, similar to the thermal phonons in an oscillating solid.
These paired quasiparticles propagate through the crystal and stimulate the formation of other pairs through a chain reaction. The quasiparticles travel faster than the speed of phonons, and so each pair leaves behind an avalanche of newly formed pairs, just like the Mach cone created behind a supersonic jet plane. Eventually, all these pairs will collide with each other, eventually causing the crystal to melt (movie).
Pair-induced melting is observed in all crystal symmetries except in one special case: the hexagonal crystal. Here the three-fold symmetry of the hydrodynamic interaction agrees with the crystal symmetry and as a result the elementary excitations are extremely slow low-frequency phonons (and not pairs as usual). A “flat band” is seen in the spectrum where these ultra-slow phonons condense. The interaction between the flatband phonons is highly collective and correlated, as reflected in the much sharper, distinct class of melting transition.
In particular, when analyzing the spectrum of the phonons, the researchers identified conical structures typical of Dirac quasiparticles, just like the structure found in the electronic spectrum of graphene. In the case of the hydrodynamic crystal, the Dirac quasiparticles are simply pairs of particles that form thanks to the flow-mediated “anti-Newtonian” interaction. This shows that the system can serve as a classical analogue of the particles discovered in graphene.
“The work is a unique demonstration that fundamental concepts of quantum matter – in particular quasiparticles and flat ribbons – can help us to understand the many-body physics of classical dissipative systems,” explains Tsvi Tlusty, one of the paper’s corresponding authors.
In addition, quasiparticles and flat ribbons are of particular interest in condensed matter physics. For example, flat ribbons in graphene bilayers twisted by a certain “magic angle” have recently been observed, and the hydrodynamic system studied at IBS CSLM happens to show an analogous flat ribbon in a much simpler 2D crystal.
“Taken together, these results suggest that other emergent collective phenomena, previously measured only in quantum systems, can be uncovered in a variety of classical dissipative environments, such as active and living matter,” says Hyuk Kyu Pak, one of the corresponding authors of the papers.
