Российский Квантовый Центр Russian Quantum Center

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Open RQC colloquium. Lecture 11


The eleventh lecture "Superfluidity of two-dimensional Bose gases" in "Open RQC colloquium" will be held in National University of Science and Technology MISIS (Moscow, Leninskiy prospekt 4), lecture hall № Б-536 (5th floor) on 11th of November at 10:00 a.m by Hélène Perrin, CNRS fellow at Laser Physics Laboratory, Paris 13 (North) University.

Abstract

"Superfluidity is an intriguing property of certain quantum fluids, which is characterized by specific manifestations in its dynamics: absence of viscosity, existence of a critical velocity for the appearance of excitations, vanishing moment of inertia, hydrodynamic behavior including irrotational flow, quantum vortices and collective oscillations. Widely studied in the context of liquid helium, it has been extended to three-dimensional quantum degenerate weakly interacting Bose gases which appear to present a superfluid character as Bose-Einstein condensation (BEC) is reached.

The case of two-dimensional quantum gases is very specific in this respect. In homogeneous gases, BEC is absent while a superfluid transition still occurs at low temperature when local phase fluctuations are reduced by vortex-antivortex pairing, as described by Berezinskii, Kosterlitz and Thouless (BKT). Superfluidity then exists even in the absence of long range order. 

In trapped Bose gases, the situation is more complex. BEC is recovered, but the BKT superfluidity mechanism still holds and is responsible for the superfluid character of the sample. As the density is inhomogeneous in a trapped gas, the central core is expected to present a superfluid dynamics while the outer region of the sample is still normal. At equilibrium at a given temperature, the gas properties (density, chemical potential) can be defined locally, which is know as the local density approximation (LDA). The criterion for the BKT transition could then be applied locally, giving rise to the existence of a boundary in the sample between a normal component and a superfluid component. 

In this lecture, I will describe recent results obtained in the study these quasi two-dimensional systems of ultra cold atoms, from the first evidence of the BKT phase to the experiments evidencing the dynamical manifestations of superfluidity."

Biography

Hélène Perrin is formation at Ecole Polytechnique, PhD at ENS defended in 1998, supervised by Christophe Salomon in the group of Prof. Claude-Cohen Tannoudji, post-doc at CEA-Saclay on fractional quantum Hall effect. Since 2000 she's CNRS researcher at Laboratoire de physique des lasers, Paris-North University and Head of the BEC group in LPL since 2012. In 2012 she became Co-chair of ICAP conference in Palaiseau and member of various scientific boards (incl. CNRS Physics division).

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