The dynamics of a mobile quantum impurity in a degenerate Fermi system is a fundamental problem in many-body physics. The interest in this field has been renewed due to recent ground-breaking experiments with ultracold Fermi gases 1-5 . Optical creation of an exciton or a polariton in a twodimensional electron system embedded in a microcavity constitutes a new frontier for this field due to an interplay between cavity coupling favouring ultralow-mass polariton formation 6 and exciton-electron interactions leading to polaron or trion formation 7,8 . Here, we present cavity spectroscopy of gatetunable monolayer MoSe 2 (ref. 9) exhibiting strongly bound trion and polaron resonances, as well as non-perturbative coupling to a single microcavity mode 10,11 . As the electron density is increased, the oscillator strength determined from the polariton splitting is gradually transferred from the higher-energy repulsive exciton-polaron resonance to the lower-energy attractive exciton-polaron state. Transition metal dichalcogenide (TMD) monolayers represent a new class of two-dimensional (2D) semiconductors exhibiting features such as strong Coulomb interactions 14 , locking of spin and valley degrees of freedom due to large spin-orbit coupling 9 and finite electron/exciton Berry curvature with novel transport and optical signatures 15,16 . Unlike quantum wells or 2D electron systems (2DES) in III-V semiconductors, TMD monolayers exhibit an ultralarge exciton binding energy E exc of order 0.5 eV (ref. 14) and strong trion peaks in photoluminescence (PL) that are redshifted from the exciton line by E T ∼ 30 meV (refs 9,17). These features provide a unique opportunity to investigate many-body physics associated with trion 18 formation as well as coupling of excitons to a 2DES 19 and to cavity photons 20,21 , provided that the experimental set-up allows for varying the electron density n e and light-matter coupling strength g c .Here, we carry out an investigation of Fermi polarons 1 in a charge-tunable MoSe 2 monolayer embedded in an open microcavity structure (Fig. 1a,b). Since E exc is much larger than all other relevant energy scales, such as the normal mode splitting (2g c ), E T and the Fermi energy (E F ), an optically generated exciton in a TMD monolayer can be considered as a robust mobile bosonic impurity embedded in a fermionic reservoir (Fig. 1c). The Hamiltonian describing the system iswhere the first line describes the coupling of 2D excitons, described by the exciton annihilation operator x k to a 0D cavity mode c 0 whose resonance frequency ω c can be tuned by applying a voltage (u p ) to a piezoelectric actuator that changes the cavity length. This part of the Hamiltonian corresponds to the elementary building block of the recent ground-breaking experiments based on coupled 0D-polariton systems 22 . The second line of the Hamiltonian describes the Feshbach-like physics associated with the bound-molecular (trion) channel and the corresponding effective interactions between the excitons and the electrons 1 . Thi...

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, which leads to long-lived deviations from the steady state as the system is driven towards the transition. Here, we show that photon correlation measurements can be used to characterize the corresponding critical slowing down of non-equilibrium dynamics. We focus on the extensively studied phenomenon of optical bistability in GaAs cavity polaritons 10,11 , which can be described as a first-order dissipative phase transition [12][13][14] . Increasing the excitation strength towards the bistable range results in an increasing photon-bunching signal along with a decay time that is prolonged by more than nine orders of magnitude as compared with that of single polaritons. In the limit of strong polariton interactions leading to pronounced quantum fluctuations, the mean-field bistability threshold is washed out. Nevertheless, the functional form with which the Liouvillian gap closes as the thermodynamic limit is approached provides a signature of the emerging dissipative phase transition. Our results establish photon correlation measurements as an invaluable tool for studying dynamical properties of dissipative phase transitions without requiring phase-sensitive interferometric measurements.Bistability, the coexistence of two stable states with different photon number under the same driving conditions, is a general signature of driven-dissipative nonlinear systems, and is commonly observed in a broad range of experimental realizations 10,11,[15][16][17] . This phenomenon can be well understood by treating the system in a mean-field description, that is, replacing the system's creation and annihilation operators by their corresponding expectation values. Neglecting quantum fluctuations in this way leads to diverging lifetimes of the two (otherwise metastable) states and results in hysteretic behaviour when the excitation power is ramped. A full quantum treatment, on the other hand, predicts a unique steady state with no indication of the underlying bistability 12,18 . These two pictures can be reconciled by investigating quantum non-equilibrium dynamics, described by a Liouvillian superoperator. In analogy with the spectrum of a Hamiltonian, the real part of the Liouvillian eigenvalues corresponds to the decay rate of the respective eigenmodes. The steady state of the driven-dissipative evolution is the eigenmode of the Liouvillian with zero eigenvalue. In the bistable regime, the Liouvillian excitation gap, or the asymptotic decay rate of its first excited state, vanishes in the thermodynamic limit (where the number of system excitations/particles N → ∞ ), corresponding to a diverging lifetime of a second eigenmode. The latter can exceed all intrinsic timescales by many orders of magnitude and is the reason for the successful description of bistability experiments by meanfield theory 19 . In general, a vanishing Liouvillian excitation gap signals a dissipative phase transition (DPT) 9,13,14,20,21 . Here, we demonstrate that the asymptotic decay rate of the Liouvillian can be determined by measuring photon correlati...

Computer simulations of flow and sediment transport in the swash zone on a beach demonstrate that a model that couples local flow acceleration and alongshore surface gradient is sufficient to produce uniformly spaced beach cusps. The characteristics of the simulated cusps and the conditions under which they form are in reasonable agreement with observations of natural cusps. The self-organization mechanism in the model is incompatible with an accepted model in which standing alongshore waves drive the regular pattern of erosion and deposition that gives rise to beach cusps. Because the models make similar predictions, it is concluded that currently available observational data are insufficient for discrimination between them.

The human neutrophil and monocyte-derived serine protease homologues neutrophil elastase (NE), proteinase 3 (PR3), and azurocidin (AZU) are involved in a variety of immune defense reactions. NE and PR3 assist in the destruction of phagocytosed microorganisms, cleave the important connective-tissue protein elastin, and generate chemotactic activities by forming alpha 1-proteinase inhibitor complexes and elastin peptides. AZU is cytotoxic to certain microorganisms and chemotactic for monocytes. All three proteins are produced and packaged into azurophil granules in large quantities during neutrophil differentiation. We have isolated several cosmid clones each of which contains the functional genes for AZU, PR3, and NE in this order. The PR3 gene is separated by 8 kilobases from the 3' end of the AZU gene and by 3 kilobases from the 5' end of the NE gene. We report a physical map of the gene cluster, its location on chromosome 19pter, and the exon-intron organization of the AZU and PR3 genes. Our fluorescence in situ hybridization studies disprove the previous chromosomal assignment of the human NE gene to 11q14. The five exons of AZU and PR3 are organized like those of NE and other granule-associated serine proteases of hematopoietic cells. NE, PR3, and AZU are coordinately downregulated in the premonocytic cell line U937 during induced terminal differentiation. The cluster-like physical organization of these genes and concerted regulation during hematopoietic differentiation suggests that they are located in a developmentally activated chromatin domain promoting high-level, cell-specific expression in the monocyte-myelocyte lineage.