Superfluidity is a counter-intuitive occurrence where quantum physics and particle-wave duality arise at the macroscopic level. A hallmark of the phenomenon is the presence of quasiparticles, and for the first time, scientists in Austria have observed one of these excitations - the roton - in a dipolar quantum gas.
Rotons are a type of quasiparticle – an elementary excitation – first predicted by Soviet physicist Lev Landau in the 1940s while he was studying superfluid helium. He introduced the idea of quasiparticles to describe how the quantum fluid becomes excited. The quasiparticles correspond to collective states of excitation, behaving as particles with a well-defined momentum and energy. They are characterized by the relationship between their momentum and energy – the so-called dispersion relation. Landau predicted two types of quasiparticles – the roton and the now well-known phonon – which he distinguished by their differing dispersion relations.
Phonons exhibit long-wavelength sound-wave quanta, and as expected, energy typically increases with momentum, but rotons are different – they exhibit a large momentum, but minimal energy. This odd behavior – the result of correlations between particles due to the extremely high density of the fluids - points to the inclination of the superfluid to accumulate a short-wavelength density modulation in space, a precursor to crystallization instability. This suggests the system prefers excitation with the modulation of the wavelength corresponding to an energy minimum, the so-called roton wavelength.
Superfluidity research was boosted by the creation of gaseous Bose-Einstein condensates in the laboratory - ultracold quantum gases which present an alternative model of superfluidity. Due to the much lower densities in such condensates, the roton mode is absent. It was suggested in 2003, that roton excitations might occur in gaseous condensates for special types of interactions among particles, with theoreticians believing magnetic atoms with long-range anisotropic dipole-dipole interactions could lead to remarkable correlations between particles and a roton dispersion relation.
Researchers from the Institute of Quantum Optics and Quantum Information of the Austrian Academy of Sciences, the University of Hannover and the University of Innsbruck have built on previous work using Bose-Einstein condensate with erbium atoms to successfully observe roton quasiparticles in quantum gas. Erbium atoms have strong magnetic characteristics and lead to an extreme dipolar behavior in the quantum system, and researchers have successfully prepared a Bose-Einstein condensate of approximately 100,000 erbium atoms in such a way that a roton mode occurs.
“We use a cigar-shaped trap of laser light and orient the atomic dipoles transversely to it thanks to a magnetic field," explains Lauriane Chomaz, from the Institute of Quantum Optics and Quantum Information of the Austrian Academy of Sciences, and first author of the paper published in Nature Physics.
In this geometry that atomic dipoles attract each other when positioned along the short axis if the cigar and repel each other when they sit along the longer 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.
The roton excitation occurs because this orientation energetically favors the modulations of the cloud along the long axis of the cigar, with a wavelength matching the cigar’s shorter length.
"By additionally quenching the strength of the interparticle interactions, we can populate the roton mode," Chomaz adds.
The successful detection of this quasiparticles has been long-awaiting and now opens the door to further research into superfluidity. It also offers the chance to explore a paradoxical state of matter which exhibits both the properties of solids and superfluids at the same time, with researchers convinced that magnetic atoms could offer a different means to directly access the supersolid phase of matter.
This breakthrough research also confirms the budding opportunities offered by dipolar gases towards new paradigms of quantum fluids.
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