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. The first ones are the phonon modes, the well-known long-wavelength sound-wave quanta. The second ones, much more bizarre and intriguing, are massive quasi-particles named rotons. They have large momenta, and, contrarily to the common (quasi)particles for which the energy increases with the momentum, the roton dispersion relation exhibits a minimum at a finite momentum, called roton momentum. This unusual behavior expresses the tendency of the fluids to build up short-wavelength density modulation in space, precursor of a crystallization instability. This behavior arises from a remarkable crosstalk (or correlations) between the particles, owing to the extremely high density of the fluid.
Ultracold quantum gases and, in particular, Bose-Einstein condensates, first realized in 1995, offer another paradigm of superfluidity, where, because of the much lower densities, the roton mode is absent. However, in 2003, theoreticians suggested that roton excitations might also occur in gaseous condensates for special types of interactions among particles.
In their view, magnetic atoms with their long-range and anisotropic dipole-dipole interaction would make possible to introduce remarkable correlations between the particles, leading to a roton dispersion relation. Now, thanks to the theory input from Luis Santos's research group at the University of Hannover and from Rick van Bijnen of the Institute of Quantum Optics and Quantum Information of the Austrian Academy of Sciences, the team led by Francesca Ferlaino at the Department of Experimental Physics of the University of Innsbruck and the Institute of Quantum Optics and Quantum Information of the Austrian Academy of Sciences has demonstrated roton excitations in a dipolar quantum gas for the first time.
Roton observed in dipolar quantum gas
In an international first the Innsbruck scientists realized a Bose-Einstein condensate of erbium atoms in 2012. The strong magnetic character of these atoms leads to an extreme dipolar behavior of the quantum system. With this model system, they have already been able to detect several dipolar few- and many-particle effects. The group has now succeeded in preparing a Bose-Einstein condensate of about 100,000 erbium atoms in such a way that a roton mode arises. "We use a cigar-shaped trap of laser light and orient the atomic dipoles transversely to it thanks to a magnetic field," explains first author Lauriane Chomaz. In this geometry, the atomic dipoles attract each other when they sit along the short direction of the cigar and repel when they sit along the long one. "The long-range character of the dipolar interaction introduces a cross-talk between the different directions of the cigar trap and the attractive/repulsive features of the interaction in this trap". This energetically favors a modulation of the cloud along the long direction of the cigar, with a wavelength matching the cigar short length. This is the roton excitation. "By additionally quenching the strength of the interparticle interactions, we can populate the roton mode," says Chomaz.
New focus on supersolidity
The successful detection of this long-awaited quasiparticle paves the way for further research into superfluidity. In addition, it also creates possibilities to explore a paradoxical state of matter that simultaneously shows both the properties of solids and superfluids. The first evidence of supersolid states was presented last year in hybrid systems of atoms and light. Magnetic atoms could offer a different perspective to direct access supersolid phase of matter, the Innsbruck researchers are convinced. Finally, this breakthrough confirms the potentialities offered by dipolar gases toward new paradigms of quantum fluids, as also previously demonstrated with the discovery of dipolar quantum droplets in the group of Tilman Pfau in Stuttgart.
The work by the Innsbruck scientists was financially supported, among others, by the Austrian Science Fund FWF and the European Union.
Publication: Observation of roton mode population in a dipolar quantum gas. Lauriane Chomaz, Rick M. W. van Bijnen, Daniel Petter, Giulia Faraoni, Simon Baier, Jan Hendrik Becher, Manfred J. Mark, Falk Wächtler, Luis Santos, Francesca Ferlaino. Nature Physics 2018 DOI: 10.1038/s41567-018-0054-7
Further information on roton and superfluidity: In the 1940's, when superfluid helium was a fresh puzzle, the Soviet physicist Lev Landau made a big step forward in the understanding of superfluidity when he introduced the concept of quasiparticles to describe how this quantum fluid gets excited. The quasi-particles correspond to collective states of excitations of the fluid and, similarly to the photons being excitations of the electro-magnetic field, they behave as particles, carrying a well-defined momentum and energy. The inverse of the momentum defines a wavelength corresponding to the period of the spatial modulation associated to the excitation. The quasiparticles are mostly characterized by the relation connecting their momentum and their energy, so called dispersion relation. With a remarkable intuition, Landau postulated the existence of two distinct types of quasiparticles, the phonons and the rotons, that he distinguished phenomenologically by the different dispersion relation. While the energy typically increases with the momentum, as it does for the phonons, the rotons are the special excitations that have a large momentum but exhibit a minimum of energy. This indicates that the system is more preferably excited with a modulation of wavelength corresponding to the energy minimum, the roton wavelength. Landau's intuition was topic of an intense debate, in particular with colleague Fritz London, and was confirmed and further explored by numerous experiments since the 60's.
Since the mid-1990s, research into superfluidity has taken a new boost with the creation of gaseous Bose-Einstein condensates in the laboratory. Quantum gases underpin the universality of the superfluid behavior first observed in liquid helium. Liquid helium and ultracold gases share numerous properties, yet, in contrast to dense liquid helium, quantum gases typically only support phonon excitations and no roton. This is because the much dilute character of gases makes the cross-talk between the particles very weak. In helium, the interaction between the particle are so strong that they make the particles tend to separate with a specific distance, giving rise to a privileged wavelength of excitation, the roton wavelength.
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