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      Origin of Long-Lived Coherences in Light-Harvesting Complexes

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          Abstract

          A vibronic exciton model is applied to explain the long-lived oscillatory features in the two-dimensional (2D) electronic spectra of the Fenna–Matthews–Olson (FMO) complex. Using experimentally determined parameters and uncorrelated site energy fluctuations, the model predicts oscillations with dephasing times of 1.3 ps at 77 K, which is in a good agreement with the experimental results. These long-lived oscillations originate from the coherent superposition of vibronic exciton states with dominant contributions from vibrational excitations on the same pigment. The oscillations obtain a large amplitude due to excitonic intensity borrowing, which gives transitions with strong vibronic character a significant intensity despite the small Huang–Rhys factor. Purely electronic coherences are found to decay on a 200 fs time scale.

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          Rapid planetesimal formation in turbulent circumstellar discs

          Anders Johansen, Jeffrey S. Oishi, Mordecai-Mark Mac Low … (2007)
          The initial stages of planet formation in circumstellar gas discs proceed via dust grains that collide and build up larger and larger bodies (Safronov 1969). How this process continues from metre-sized boulders to kilometre-scale planetesimals is a major unsolved problem (Dominik et al. 2007): boulders stick together poorly (Benz 2000), and spiral into the protostar in a few hundred orbits due to a head wind from the slower rotating gas (Weidenschilling 1977). Gravitational collapse of the solid component has been suggested to overcome this barrier (Safronov 1969, Goldreich & Ward 1973, Youdin & Shu 2002). Even low levels of turbulence, however, inhibit sedimentation of solids to a sufficiently dense midplane layer (Weidenschilling & Cuzzi 1993, Dominik et al. 2007), but turbulence must be present to explain observed gas accretion in protostellar discs (Hartmann 1998). Here we report the discovery of efficient gravitational collapse of boulders in locally overdense regions in the midplane. The boulders concentrate initially in transient high pressures in the turbulent gas (Johansen, Klahr, & Henning 2006), and these concentrations are augmented a further order of magnitude by a streaming instability (Youdin & Goodman 2005, Johansen, Henning, & Klahr 2006, Johansen & Youdin 2007) driven by the relative flow of gas and solids. We find that gravitationally bound clusters form with masses comparable to dwarf planets and containing a distribution of boulder sizes. Gravitational collapse happens much faster than radial drift, offering a possible path to planetesimal formation in accreting circumstellar discs.
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            The Dicke Quantum Phase Transition with a Superfluid Gas in an Optical Cavity

            Ferdinand Brennecke, Tilman Esslinger, Christine Guerlin … (2009)
            A phase transition describes the sudden change of state in a physical system, such as the transition between a fluid and a solid. Quantum gases provide the opportunity to establish a direct link between experiment and generic models which capture the underlying physics. A fundamental concept to describe the collective matter-light interaction is the Dicke model which has been predicted to show an intriguing quantum phase transition. Here we realize the Dicke quantum phase transition in an open system formed by a Bose-Einstein condensate coupled to an optical cavity, and observe the emergence of a self-organized supersolid phase. The phase transition is driven by infinitely long-ranged interactions between the condensed atoms. These are induced by two-photon processes involving the cavity mode and a pump field. We show that the phase transition is described by the Dicke Hamiltonian, including counter-rotating coupling terms, and that the supersolid phase is associated with a spontaneously broken spatial symmetry. The boundary of the phase transition is mapped out in quantitative agreement with the Dicke model. The work opens the field of quantum gases with long-ranged interactions, and provides access to novel quantum phases.
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              Why do ultrasoft repulsive particles cluster and crystallize? Analytical results from density functional theory

              Bianca M Mladek, Dieter Gottwald, Gerhard Kahl … (2007)
              We demonstrate the accuracy of the hypernetted chain closure and of the mean-field approximation for the calculation of the fluid-state properties of systems interacting by means of bounded and positive-definite pair potentials with oscillating Fourier transforms. Subsequently, we prove the validity of a bilinear, random-phase density functional for arbitrary inhomogeneous phases of the same systems. On the basis of this functional, we calculate analytically the freezing parameters of the latter. We demonstrate explicitly that the stable crystals feature a lattice constant that is independent of density and whose value is dictated by the position of the negative minimum of the Fourier transform of the pair potential. This property is equivalent with the existence of clusters, whose population scales proportionally to the density. We establish that regardless of the form of the interaction potential and of the location on the freezing line, all cluster crystals have a universal Lindemann ratio L = 0.189 at freezing. We further make an explicit link between the aforementioned density functional and the harmonic theory of crystals. This allows us to establish an equivalence between the emergence of clusters and the existence of negative Fourier components of the interaction potential. Finally, we make a connection between the class of models at hand and the system of infinite-dimensional hard spheres, when the limits of interaction steepness and space dimension are both taken to infinity in a particularly described fashion.
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                Author and article information

                Journal
                Journal ID (nlm-ta): J Phys Chem B
                Journal ID (iso-abbrev): J Phys Chem B
                Journal ID (publisher-id): jp
                Journal ID (coden): jpcbfk
                Title: The Journal of Physical Chemistry. B
                Publisher: American Chemical Society
                ISSN (Print): 1520-6106
                ISSN (Electronic): 1520-5207
                Publication date (Electronic): 29 May 2012
                Publication date (Print): 28 June 2012
                Volume: 116
                Issue: 25
                Pages: 7449-7454
                Affiliations
                [† ]Faculty of Physics, University of Vienna , Strudlhofgasse 4, 1090 Vienna, Austria
                [‡ ]Department of Chemical Physics, Lund University , P.O. Box 124, SE-22100 Lund, Sweden
                [§ ]Faculty of Mathematics and Physics, Charles University in Prague , Ke Karlovu 5, Prague 121 16, Czech Republic
                [⊥ ]Faculty of Physics, Vienna University of Technology , 1040 Vienna, Austria
                Author notes
                Article
                DOI: 10.1021/jp304649c
                PMC ID: 3789255
                PubMed ID: 22642682
                SO-VID: 53a9af9e-81a8-414e-ae29-fb5e8c88b643
                Copyright © Copyright © 2012 American Chemical Society
                History
                Date received : 14 May 2012
                Categories
                Subject: Article
                Custom metadata
                document-id-old-9 jp304649c
                document-id-new-14 jp-2012-04649c
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                ScienceOpen disciplines: Physical chemistry
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                ScienceOpen disciplines: Physical chemistry

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