Roton quasiparticles observed in quantum gas

An Innsbruck team of experimental physicists, in collaboration with theorists from Innsbruck and Hannover, has for the first time observed so-called roton quasiparticles in a quantum gas. Empirically introduced by Landau to explain the bizarre properties of superfluid liquid Helium, these quasiparticles reflect an "energy softening" in the system as precursor of a crystallization instability. The new work published demonstrate similar phenomena in the quantum-gas phase thanks to magnetic interactions, paving the way for a novel understanding of paradigmatic states of quantum fluids, such as supersolids. Discovered in liquid helium about 80 years ago, superfluidity is a counterintuitive phenomenon, in which quantum physics and particle-wave duality manifest at the macroscopic level. Since then, it has yielded many advances in understanding quantum matter, yet leaving mysterious some of its features. A hallmark of superfluidity is the existence of so-called "quasi-particles", i.e. elementary excitations dressed by interactions. The behavior of such a special fluid is mainly dictated by two types of excitations at low temperature, as their moderate energy cost allows to easily excite them.