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Superfluidity in coherently driven microcavity-polaritons

Juggins, Richard; (2019) Superfluidity in coherently driven microcavity-polaritons. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Superfluidity is a spectacular emergent effect in quantum mechanics where fluids flow without friction, cannot rotate except in quantised vortices, and can form metastable currents that persist for astronomical timescales. First discovered in liquid helium, superfluidity has been extensively studied in thermal equilibrium. Microcavity-polaritons however, which are two-dimensional quasiparticles formed by the strong coupling of cavity photons and quantum well excitons, do not usually equilibrate due to photons leaking from the cavity, and must be pumped with a laser to maintain a steady state. Polaritons are bosonic and have been shown to condense on experimental scales, yet their driven-dissipative nature poses questions regarding whether they may become superfluids. Some associated effects have been observed, and notably the report of nearly dissipationless flow for coherently driven polaritons was taken as evidence of superflow. Here, we use a Keldysh path integral technique to show the superfluid response, given by the difference between the currents induced by longitudinal and transverse forces, is zero for coherently driven polaritons that are continuously and homogeneously pumped, and find this a consequence of the gapped excitation spectrum caused by external phase locking. Furthermore, at zero pump momentum the system forms a rigid state that does not respond to either type of perturbation. It does however exhibit a normal response at finite momenta, due to a coupling between the perturbation and the amplitude of the state rather than a change in its momentum. This response almost vanishes when the real part of the spectrum is linear at low frequency, which was the regime investigated experimentally. These results suggest the observed suppression of scattering should be interpreted as a sign of this new rigid state and not a superfluid, and that driven-dissipative systems exhibit a richer collection of macroscopic quantum phenomena than those in equilibrium.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Superfluidity in coherently driven microcavity-polaritons
Event: UCL
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Copyright © The Author 2019. Original content in this thesis is licensed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) Licence (https://creativecommons.org/licenses/by/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request.
UCL classification: UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/10080025
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