Coherently driven microcavity-polaritons and the question of superfluidity
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Due to their driven-dissipative nature, photonic quantum fluids present new challenges in understanding superfluidity. Some associated effects have been observed, and notably the report of nearly dissipationless flow for coherently driven microcavity-polaritons was taken as a 'smoking gun' for superflow. Here we show that the superfluid response - the difference between responses to longitudinal and transverse forces - is zero for coherently driven polaritons. This is a direct consequence of the gapped excitation spectrum caused by external phase locking. Furthermore, while a normal component exists at finite pump momentum, the remainder forms a rigid state that does not respond to either longitudinal or transverse perturbations. Interestingly, the total response almost vanishes when the real part of the excitation spectrum has a linear dispersion at low frequency, characteristic of equilibrium bosonic superfluids, which was the regime investigated experimentally. These results suggest that the observed suppression of scattering should be interpreted as a sign of this new rigid state and not of a superfluid.
Juggins , R T , Keeling , J & Szymańska , M H 2018 , ' Coherently driven microcavity-polaritons and the question of superfluidity ' , Nature Communications , vol. 9 , 4062 . https://doi.org/10.1038/s41467-018-06436-2
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DescriptionM.H.S. acknowledges financial support from EPSRC (Grants no. EP/I028900/2 and no. EP/K003623/2) and J.K. from EPSRC program Hybrid Polaritonics (EP/M025330/1).
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