Quantum mechanics explains how superfluid helium films grow in carbon nanotubes

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Quantum mechanics explains how superfluid helium films grow in carbon nanotubes

Research by an international team of researchers led by scientists from the ICFO, with the participation of Jordi Boronat, a researcher at the UPC’s Department of Physics, reports on the controlled, layer-by-layer growth process of superfluid helium on the surface of carbon nanotubes. The experiment opens the door to studying new phenomena at the nanoscopic scale and, in particular, in topological phase transitions.

Jul 16, 2019

The study, led by Adrian Bachtold from the Institute of Photonic Sciences (ICFO), a university research institute affiliated to the Universitat Politècnica de Catalunya · BarcelonaTech (UPC), with the collaboration of researchers from the UPC’s Department of Physics, the École Normale Supérieure (ENS, France) and Pablo Olavide University (UPO), shows how superfluid helium grows onto a carbon nanotube layer by layer and not continuously. Whereas carbon nanotube resonators have proved to be excellent sensing devices for the study of new physical phenomena at the nanoscale (for example, in quantum electron transport, surface science and light–matter interaction), superfluid helium is useful for studying phase transitions and, in particular, understanding transitions in two and three dimensions. By combining the two, various phenomena, such as adsorption, supersolidity and superfluidity, can be studied at the nanoscale.

In their experiment, published in Physical Review Letters, the international team of researchers fixed the ends of a carbon nanotube so that it could be stretched and it could oscillate like a guitar string. They placed it inside a chamber and added helium vapour to observe that superfluid helium films were indeed adsorbing onto the surface of the suspended carbon nanotube. The study shows that, as helium accumulated on the nanotube, the frequency of the mechanical vibrations of the nanotube changed as its mass increased. That is, they observed that as helium adsorbed onto the tube the resonating frequency changed abruptly. This indicates that the stacking of helium onto the nanotube was done layer by layer, with discontinuities in both the number of adsorbed atoms and the speed of the third sound in the adsorbed film. In the process, they were able to demonstrate the formation of helium layers up to five atoms thick.

Liquid helium is one of the prominent examples of quantum systems and studying it requires advanced theoretical approaches. The Barcelona Quantum Monte Carlo research group, led by Jordi Boronat, a researcher from the UPC’s Department of Physics, participated in the theoretical analysis of the experiment in collaboration with the UPO researcher Carmen Gordillo. The theoretical modelling of the quantum system predicted the phases that were later observed in the experiment. The data obtained at the ICFO confirm the theoretical predictions and clearly show for the first time that the growth of the layers adsorbed on a nanotube exhibits “jumps”, which is typical of first-order phase transitions.

The combination of experimental and theoretical results shows that the team was able to build helium superfluid films with a number of atomic layers in a controlled manner and that these helium multilayers that adsorbed on a nanotube are of an unprecedented quality compared to previous studies. Such findings open a new pathway into the field of topological phase transitions that will aim for novel research in quantum fluids and solids in reduced geometry.