Fractionation effects in phase equilibria of polydisperse hard sphere colloids

The equilibrium phase behaviour of hard spheres with size polydispersity is studied theoretically. We solve numerically the exact phase equilibrium equations that result from accurate free energy expressions for the fluid and solid phases, while accounting fully for size fractionation between coexisting phases. Fluids up to the largest polydispersities that we can study (around 14%) can phase separate by splitting off a solid with a much narrower size distribution. This shows that experimentally observed terminal polydispersities above which phase separation no longer occurs must be due to non-equilibrium effects. We find no evidence of re-entrant melting; instead, sufficiently compressed solids phase separate into two or more solid phases. Under appropriate conditions, coexistence of multiple solids with a fluid phase is also predicted. The solids have smaller polydispersities than the parent phase as expected, while the reverse is true for the fluid phase, which contains predominantly smaller particles but also residual amounts of the larger ones. The properties of the coexisting phases are studied in detail; mean diameter, polydispersity and volume fraction of the phases all reveal marked fractionation. We also propose a method for constructing quantities that optimally distinguish between the coexisting phases, using Principal Component Analysis in the space of density distributions. We conclude by comparing our predictions to perturbative theories for near-monodisperse systems and to Monte Carlo simulations at imposed chemical potential distribution, and find excellent agreement.Comment: 21 pages, 23 figures, 2 table

Convergence improvement for coupled cluster calculations

Convergence problems in coupled-cluster iterations are discussed, and a new iteration scheme is proposed. Whereas the Jacobi method inverts only the diagonal part of the large matrix of equation coefficients, we invert a matrix which also includes a relatively small number of off-diagonal coefficients, selected according to the excitation amplitudes undergoing the largest change in the coupled cluster iteration. A test case shows that the new IPM (inversion of partial matrix) method gives much better convergence than the straightforward Jacobi-type scheme or such well-known convergence aids as the reduced linear equations or direct inversion in iterative subspace methods.Comment: 7 pages, IOPP styl

The adult mouse hippocampal progenitor is neurogenic but not a stem cell

The aim of this investigation was to characterize the proliferative precursor cells in the adult mouse hippocampal region. Given that a very large number of new hippocampal cells are generated over the lifetime of an animal, it is predicted that a neural stem cell is ultimately responsible for maintaining this genesis. Although it is generally accepted that a proliferative precursor resides within the hippocampus, contradictory reports exist regarding the classification of this cell. Is it a true stem cell or a more limited progenitor? Using a strict functional definition of a neural stem cell and a number of in vitro assays, we report that the resident hippocampal precursor is a progenitor capable of proliferation and multipotential differentiation but is unable to self-renew and thus proliferate indefinitely. Furthermore, the mitogen FGF-2 stimulates proliferation of these cells to a greater extent than epidermal growth factor ( EGF). In addition, we found that BDNF was essential for the production of neurons from the hippocampal progenitor cells, being required during proliferation to trigger neuronal fate. In contrast, a bona fide neural stem cell was identified in the lateral wall of the lateral ventricle surrounding the hippocampus. Interestingly, EGF proved to be the stronger mitogenic factor for this cell, which was clearly a different precursor from the resident hippocampal progenitor. These results suggest that the stem cell ultimately responsible for adult hippocampal neurogenesis resides outside the hippocampus, producing progenitor cells that migrate into the neurogenic zones and proliferate to produce new neurons and glia

Electrochemical immobilisation of enzymes. Part 4.—Co-immobilisation of glucose oxidase and ferro/ferricyanide in poly(N-methylpyrrole) films

Glucose oxidase and ferro/ferricyanide can be co-immobilised in poly(N-methylpyrrole) films. The two species compete for incorporation into the films so that on increasing the concentration of enzyme in the growth solution less ferro/ferricyanide is incorporated into the film, and vice versa. The ferro/ferricyanide entrapped within the film acts as a redox mediator for the oxidation of the enzyme at 0.45 V vs. SCE. Studies of the effect of variation of film thickness, glucose concentration, enzyme loading and ferro/ferricyanide loading are consistent with a model in which diffusion and enzyme-catalysed reaction within the film are coupled. Analysis of data from these experiments allows the rate parameters for these processes to be determined. These data are compared with previous results for oxygen mediation in poly(N-methylpyrrole) films

MACiE: a database of enzyme reaction mechanisms.

SUMMARY: MACiE (mechanism, annotation and classification in enzymes) is a publicly available web-based database, held in CMLReact (an XML application), that aims to help our understanding of the evolution of enzyme catalytic mechanisms and also to create a classification system which reflects the actual chemical mechanism (catalytic steps) of an enzyme reaction, not only the overall reaction. AVAILABILITY: http://www-mitchell.ch.cam.ac.uk/macie/.EPSRC (G.L.H. and J.B.O.M.), the BBSRC (G.J.B. and J.M.T.—CASE studentship in association with Roche Products Ltd; N.M.O.B. and J.B.O.M.—grant BB/C51320X/1), the Chilean Government’s Ministerio de Planificacio´n y Cooperacio´n and Cambridge Overseas Trust (D.E.A.) for funding and Unilever for supporting the Centre for Molecular Science Informatics.application note restricted to 2 printed pages web site: http://www-mitchell.ch.cam.ac.uk/macie

Production of perovskite-like La(1-x)Ca(x)MnO(3+delta) ferromagnetic thin films by electrochemical reduction

Ferromagnetic and perovskite-like thin films (< 1 micron) of La(1-x)Ca(x)MnO(3+delta) have been routinely prepared by heat treatment of an amorphous La Ca Mn precursor. The precursor was electrodeposited cathodically in the absence of oxygen and water onto polished silver substrates from a non-aqueous solution of the components’ nitrates. Analysis by X-Ray diffraction and SQUID magnetometry shows these materials exhibit the appropriate structural and magnetic phases indicative of Colossal Magnetoresistance

Efficient Classical Simulation of Optical Quantum Circuits

We identify a broad class of physical processes in an optical quantum circuit that can be efficiently simulated on a classical computer: this class includes unitary transformations, amplification, noise, and measurements. This simulatability result places powerful constraints on the capability to realize exponential quantum speedups as well as on inducing an optical nonlinear transformation via linear optics, photodetection-based measurement and classical feedforward of measurement results, optimal cloning, and a wide range of other processes.Comment: 4 pages, published versio

The finite size effect of galaxies on the cosmic virial theorem and the pairwise peculiar velocity dispersions

We discuss the effect of the finite size of galaxies on estimating small-scale relative pairwise peculiar velocity dispersions from the cosmic virial theorem (CVT). Specifically we evaluate the effect by incorporating the finite core radius $r_c$ in the two-point correlation function of mass, i.e. $\xi_\rho(r) \propto (r+r_c)^{-\gamma}$ and the effective gravitational force softening $r_s$ on small scales. We analytically obtain the lowest-order correction term for $\gamma <2$ which is in quantitative agreement with the full numerical evaluation. With a nonzero $r_s$ and/or $r_c$ the cosmic virial theorem is no longer limited to the case of $\gamma<2$. We present accurate fitting formulae for the CVT predicted pairwise velocity dispersion for the case of $\gamma>2$. Compared with the idealistic point-mass approximation ($r_s=r_c=0$), the finite size effect can significantly reduce the small-scale velocity dispersions of galaxies at scales much larger than $r_s$ and $r_c$. Even without considering the finite size of galaxies, nonzero values for $r_c$ are generally expected, for instance, for cold dark matter (CDM) models with a scale-invariant primordial spectrum. For these CDM models, a reasonable force softening r_s\le 100 \hikpc would have rather tiny effect. We present the CVT predictions for the small-scale pairwise velocity dispersion in the CDM models normalized by the COBE observation. The implication of our results for confrontation of observations of galaxy pair-wise velocity dispersions and theoretical predictions of the CVT is also discussed.Comment: 18 pages. LaTeX text and 8 postcript figures. submitted to Ap

Non-Gaussian states for continuous variable quantum computation via Gaussian maps

We investigate non-Gaussian states of light as ancillary inputs for generating nonlinear transformations required for quantum computing with continuous variables. We consider a recent proposal for preparing a cubic phase state, find the exact form of the prepared state and perform a detailed comparison to the ideal cubic phase state. We thereby identify the main challenges to preparing an ideal cubic phase state and describe the gates implemented with the non-ideal prepared state. We also find the general form of operations that can be implemented with ancilla Fock states, together with Gaussian input states, linear optics and squeezing transformations, and homodyne detection with feed forward, and discuss the feasibility of continuous variable quantum computing using ancilla Fock states.Comment: 8 pages, 6 figure