Since the advent of quantum mechanics, the world of physics has been divided into classical and quantum physics. Classical physics deals with the motion of objects we normally see every day in the macroscopic world, while quantum physics explains the exotic behavior of elementary particles in the microscopic world.
Many solids and liquids are composed of particles that interact with each other over close distances, sometimes resulting in “quasi-particles”. Quasiparticles are long-lived excitations that effectively behave as weakly interacting particles. The idea of ​​quasiparticles was introduced by the Soviet physicist Lev Landau in 1941 and has been very fruitful in quantum matter research ever since. Some examples of quasiparticles include Bogaliubov quasiparticles (ie, “broken Cooper pairs”) in superconductivity, excitons in semiconductors, and phonons.
The study of emergent collective phenomena from the perspective of quasiparticles has allowed us to understand a wide range of physical settings, most notably in superconductivity and superfluidity, and more recently in the famous example of Dirac quasiparticles in graphene. But until now, the observation and use of quasiparticles has been limited to quantum physics: in classical condensed matter, the frequency of collisions is usually too high to allow long-lived particle-like excitations.
However, a group of researchers at the Center for Soft and Living Matter (CSLM) at the Institute of Basic Science (IBS) in South Korea recently challenged the standard view that quasiparticles are exclusively quantum matter. They investigated a classical system of microparticles moving in a viscous flow in a thin microfluidic channel. As the particles are drawn into 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 into pairs. This is because the hydrodynamic interaction violates Newton’s third law, which states that the forces between two particles must be equal in magnitude and the opposite in the direction. Instead, the forces are “anti-Newtonian” because they are equal and relative the same direction, thereby stabilizing the pair.
A large population of particles bound in pairs hinted that these are long-lived elementary excitations in the system – its quasiparticles. This hypothesis was confirmed when the researchers modeled a large two-dimensional crystal consisting of thousands of particles and studied its motion. Hydrodynamic forces between the particles cause the crystal to vibrate, similar to thermal phonons in a vibrating solid.
These pairs of quasiparticles propagate through the crystal, stimulating the creation of other pairs through a chain reaction. Quasiparticles travel faster than the speed of phonons, and thus each pair leaves behind an avalanche of newly formed pairs, much like a Mach cone is created behind a supersonic jet. Finally, all these pairs collide with each other, which eventually leads to the melting of the crystal (film).
Vapor-induced melting is observed in all crystal symmetries except for one special case: the hexagonal crystal. Here, the threefold symmetry of the hydrodynamic interaction coincides with the crystal symmetry and, as a result, the elementary excitations are very slow low-frequency phonons (rather than pairs, as usual). A “flat band” can be seen in the spectrum where these ultraslow phonons condense. The interaction between planar phonons is highly collective and correlated, which manifests itself in a much sharper, different class of melting transition.
It is noteworthy that when analyzing the spectrum of phonons, the researchers found conical structures characteristic of Dirac quasiparticles, just like the structure found in the electronic spectrum of graphene. In the case of a hydrodynamic crystal, Dirac quasiparticles are simply pairs of particles that form due to flow-mediated “anti-Newtonian” interactions. This shows that the system can serve as a classical analogue of the particles found in graphene.
“The work is a first-of-its-kind demonstration that fundamental concepts of quantum matter – especially quasiparticles and flat bands – can help us understand the many-body physics of classical dissipative systems,” explains Zvi Tlusty, one of the paper’s corresponding authors.
Moreover, quasiparticles and planar stripes are of particular interest in condensed-state physics. For example, planar bands have recently been observed in graphene bilayers twisted at a certain “magic angle,” and the hydrodynamic system studied at IBS CSLM reveals a similar planar band in a much simpler 2D crystal.
“Overall, these results suggest that other emergent collective phenomena that have so far only been measured in quantum systems can manifest themselves in different classical dissipative environments, such as active and living matter,” says Hyuk Kyu Pak, one from the corresponding authors of the paper.
 "The bills exclude most students with waivers from graduation credits")
