3,564 research outputs found

    Air Conditions Close to the Ground and the Effect on Airplane Landings

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    This report presents the results of an investigation undertaken to determine the feasibility of making glide landings in gusty air. Wind velocities were measured at several stations between the ground and a height of 51 feet, and flight tests were made to determine the actual influence of gusts on an airplane gliding close to the ground

    Ultrafast supercontinuum spectroscopy of carrier multiplication and biexcitonic effects in excited states of PbS quantum dots

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    We examine the multiple exciton population dynamics in PbS quantum dots by ultrafast spectrally-resolved supercontinuum transient absorption (SC-TA). We simultaneously probe the first three excitonic transitions over a broad spectral range. Transient spectra show the presence of first order bleach of absorption for the 1S_h-1S_e transition and second order bleach along with photoinduced absorption band for 1P_h-1P_e transition. We also report evidence of the one-photon forbidden 1S_{h,e}-1P_{h,e} transition. We examine signatures of carrier multiplication (multiexcitons for the single absorbed photon) from analysis of the first and second order bleaches, in the limit of low absorbed photon numbers (~ 10^-2), at pump energies from two to four times the semiconductor band gap. The multiexciton generation efficiency is discussed both in terms of a broadband global fit and the ratio between early- to long-time transient absorption signals.. Analysis of population dynamics shows that the bleach peak due to the biexciton population is red-shifted respect the single exciton one, indicating a positive binding energy.Comment: 16 pages, 5 figure

    Stochastic series expansion method with operator-loop update

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    A cluster update (the ``operator-loop'') is developed within the framework of a numerically exact quantum Monte Carlo method based on the power series expansion of exp(-BH) (stochastic series expansion). The method is generally applicable to a wide class of lattice Hamiltonians for which the expansion is positive definite. For some important models the operator-loop algorithm is more efficient than loop updates previously developed for ``worldline'' simulations. The method is here tested on a two-dimensional anisotropic Heisenberg antiferromagnet in a magnetic field.Comment: 5 pages, 4 figure

    The Physical Origins of Entropy Production, Free Energy Dissipation and their Mathematical Representations

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    A complete mathematical theory of nonequilibrium thermodynamics of stochastic systems in terms of master equations is presented. As generalizations of isothermal entropy and free energy, two functions of states play central roles: the Gibbs entropy SS and the relative entropy FF, which are related via the stationary distribution of the stochastic dynamics. SS satisfies the fundamental entropy balance equation dS/dt=ep−hd/TdS/dt=e_p-h_d/T with entropy production rate ep≥0e_p\ge 0 and heat dissipation rate hdh_d, while dF/dt=−fd≤0dF/dt=-f_d\le 0. For closed systems that satisfy detailed balance: Tep(t)=fd(t)Te_p(t)=f_d(t). For open system one has Tep(t)=fd(t)+Qhk(t)Te_p(t)=f_d(t)+Q_{hk}(t) where the housekeeping heat Qhk≥0Q_{hk}\ge 0 was first introduced in the phenomenological nonequilibrium steady state thermodynamics. Entropy production epe_p consists of free energy dissipation associated with spontaneous relaxation, fdf_d, and active energy pumping that sustains the open system QhkQ_{hk}. The amount of excess heat involved in the relaxation Qex=hd−Qhk=fd−T(dS/dt)Q_{ex}=h_d-Q_{hk} = f_d-T(dS/dt).Comment: 4 pages, no figure

    Duality, thermodynamics, and the linear programming problem in constraint-based models of metabolism

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    It is shown that the dual to the linear programming problem that arises in constraint-based models of metabolism can be given a thermodynamic interpretation in which the shadow prices are chemical potential analogues, and the objective is to minimise free energy consumption given a free energy drain corresponding to growth. The interpretation is distinct from conventional non-equilibrium thermodynamics, although it does satisfy a minimum entropy production principle. It can be used to motivate extensions of constraint-based modelling, for example to microbial ecosystems.Comment: 4 pages, 2 figures, 1 table, RevTeX 4, final accepted versio

    Supersymmetry in models with strong on-site Coulomb repulsion - application to t-J model

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    A supersymmetric way of imposing the constraint of no double occupancy in models with strong on-site Coulomb repulsion is presented in this paper. In this formulation the physical operators in the constrainted Hilbert space are invariant under local unitary transformations mixing boson and fermion representations. As an illustration the formulation is applied to the t−Jt-J model. The model is studied in the mean-field level in the J=0 limit where we show how both the slave-boson and slave-fermion formulations are included naturally in the present approach and how further results beyond both approaches are obtained.Comment: 12 pages, Latex file, 1 figur

    The role of winding numbers in quantum Monte Carlo simulations

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    We discuss the effects of fixing the winding number in quantum Monte Carlo simulations. We present a simple geometrical argument as well as strong numerical evidence that one can obtain exact ground state results for periodic boundary conditions without changing the winding number. However, for very small systems the temperature has to be considerably lower than in simulations with fluctuating winding numbers. The relative deviation of a calculated observable from the exact ground state result typically scales as TγT^{\gamma}, where the exponent γ\gamma is model and observable dependent and the prefactor decreases with increasing system size. Analytic results for a quantum rotor model further support our claim.Comment: 5 pages, 5 figure
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