Abstract
Fluids of exciton polaritons, excitations of two-dimensional quantum wells in optical cavities, show collective phenomena akin to Bose condensation. However, a fundamental difference from standard condensates stems from the finite lifetime of these excitations, which necessitates continuous driving to maintain a steady state. A basic question is whether a two-dimensional condensate with long-range algebraic correlations can exist under these nonequilibrium conditions. Here, we show that such driven two-dimensional Bose systems cannot exhibit algebraic superfluid order except in low-symmetry, strongly anisotropic systems. Our result implies, in particular, that recent apparent evidence for Bose condensation of exciton polaritons must be an intermediate-scale crossover phenomenon, while the true long-distance correlations fall off exponentially. We obtain these results through a mapping of the long-wavelength condensate dynamics onto the anisotropic Kardar-Parisi-Zhang equation.
- Received 19 May 2014
DOI:https://doi.org/10.1103/PhysRevX.5.011017
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Published by the American Physical Society
Popular Summary
What do a swarm of locusts, a growing snowflake, and light trapped in a semiconductor chip have in common?—a great deal, according to our study, which addresses exciton-polariton superfluidity. Exciton polaritons are particles that can be created in a thin semiconductor by shining light on it; they are a combination of electrons and light itself. For years, researchers have attempted to get these systems to exhibit Bose-Einstein condensation and superfluidity, exotic phenomena in which many particles move in unison like a giant wave.
We demonstrate a surprising connection between this problem and two others: swarming locusts and growing crystals. Using these connections, we show, rather surprisingly, that exciton-polariton superfluidity in two-dimensional semiconductors is only possible if the superconductors are sufficiently anisotropic systems. The superfluidity in such a case is also, surprisingly, “reentrant”: It appears as the driving laser power is increased beyond some threshold power but then disappears again at a second, higher threshold. The key feature that prevents exciton polaritons from forming an isotropic two-dimensional condensate, as conventional interacting particles do, is the need to constantly power the system by light as a source of energy and particles. Herein lies the similarity to flocks of animals, which may also be viewed as powered particles.
Our results may have future applications to ensembles of ultracold atoms.
