title: Dark High Density Dipolar Liquid of Excitons.
abstract: The possible phases and the nanoscale particle correlations of two-dimensional interacting dipolar particles is a long-sought problem in many-body physics. Here we observe a spontaneous condensation of trapped two-dimensional dipolar excitons with internal spin degrees of freedom from an interacting gas into a high density, closely packed liquid state made mostly of dark dipoles. Another phase transition, into a bright, highly repulsive plasma, is observed at even higher excitation powers. The dark liquid state is formed below a critical temperature Tc ≈ 4.8 K, and it is manifested by a clear spontaneous spatial condensation to a smaller and denser cloud, suggesting an attractive part to the interaction which goes beyond the purely repulsive dipole-dipole forces. Contributions from quantum mechanical fluctuations are expected to be significant in this strongly correlated, long living dark liquid. This is a new example of a two-dimensional atomic-like interacting dipolar liquid, but where the coupling of light to its internal spin degrees of freedom plays a crucial role in the dynamical formation and the nature of resulting condensed dark ground state.
authors.affiliation: Racah Institute of Physics, The Hebrew University of Jerusalem , Jerusalem 9190401, Israel.
authors.forename: Kobi
authors.initials: K
authors.lastname: Cohen
authors.affiliation: Racah Institute of Physics, The Hebrew University of Jerusalem , Jerusalem 9190401, Israel.
authors.forename: Yehiel
authors.initials: Y
authors.lastname: Shilo
authors.affiliation: Department of Electrical Engineering, Princeton University , Princeton, New Jersey 08544, United States.
authors.forename: Ken
authors.initials: K
authors.lastname: West
authors.affiliation: Department of Electrical Engineering, Princeton University , Princeton, New Jersey 08544, United States.
authors.forename: Loren
authors.initials: L
authors.lastname: Pfeiffer
authors.affiliation: Racah Institute of Physics, The Hebrew University of Jerusalem , Jerusalem 9190401, Israel.
authors.forename: Ronen
authors.initials: R
authors.lastname: Rapaport
keywords: Indirect excitons; cold dipolar gas; dark excitons; dipolar excitons; exciton condensation; quantum fluids