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Electronic structure of the n-type doped AgInO2 and CuAlO2 delafossites: similarities and differences

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      Abstract

      We performed the first-principles band-structure calculations for a pure and a Cd-doped AgInO2 delafossite compound. The results are carefully analyzed and compared with the results obtained for a pure and a Cd-doped CuAlO2 compound, calculated previously by our group. The electronic structures of both systems are found to be similar in many details, being characterized by the same hybridization scheme that occurs at both Cu and Ag positions. Introduction of Cd impurity into the Cu site produces a shallow band within the CuAlO2 gap, while the Cd presence at the Ag site creates a wide band pinned at the Fermi level in the AgInO2 spectrum. In both cases, however, the n-type conductivities are predicted.

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      Rapid planetesimal formation in turbulent circumstellar discs

      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

        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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          Configurational entropy of Wigner crystals

          We present a theoretical study of classical Wigner crystals in two- and three-dimensional isotropic parabolic traps aiming at understanding and quantifying the configurational uncertainty due to the presence of multiple stable configurations. Strongly interacting systems of classical charged particles confined in traps are known to form regular structures. The number of distinct arrangements grows very rapidly with the number of particles, many of these arrangements have quite low occurrence probabilities and often the lowest-energy structure is not the most probable one. We perform numerical simulations on systems containing up to 100 particles interacting through Coulomb and Yukawa forces, and show that the total number of metastable configurations is not a well defined and representative quantity. Instead, we propose to rely on the configurational entropy as a robust and objective measure of uncertainty. The configurational entropy can be understood as the logarithm of the effective number of states; it is insensitive to the presence of overlooked low-probability states and can be reliably determined even within a limited time of a simulation or an experiment.
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            Author and article information

            Affiliations
            [1 ] Instituto de Pesquisas Energéticas e Nucleares Brazil
            Contributors
            Role: ND
            Role: ND
            Role: ND
            Role: ND
            Journal
            bjp
            Brazilian Journal of Physics
            Braz. J. Phys.
            Sociedade Brasileira de Física (São Paulo )
            1678-4448
            June 2004
            : 34
            : 2b
            : 611-613
            S0103-97332004000400019 10.1590/S0103-97332004000400019

            http://creativecommons.org/licenses/by/4.0/

            Product
            Product Information: SciELO Brazil
            Categories
            PHYSICS, MULTIDISCIPLINARY

            General physics

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