3,534 research outputs found
The influence of normal stress and sliding velocity on the frictional behaviour of calcite at room temperature. Insights from laboratory experiments and microstructural observations
The presence of calcite in and near faults, as the dominant material, cement, or vein fill,
indicates that the mechanical behaviour of carbonate-dominated material likely plays an important role in shallow- and mid-crustal faulting. To better understand the behaviour of calcite,
under loading conditions relevant to earthquake nucleation, we sheared powdered gouge of
Carrara Marble, >98 per cent CaCO3, at constant normal stresses between 1 and 100 MPa
under water-saturated conditions at room temperature. We performed slide-hold-slide tests,
1–3000 s, to measure the amount of static frictional strengthening and creep relaxation, and
velocity-stepping tests, 0.1–1000 µm s–1, to evaluate frictional stability. We observe that the
rates of frictional strengthening and creep relaxation decrease with increasing normal stress
and diverge as shear velocity is increased from 1 to 3000 µm s–1 during slide-hold-slide experiments. We also observe complex frictional stability behaviour that depends on both normal
stress and shearing velocity. At normal stresses less than 20 MPa, we observe predominantly
velocity-neutral friction behaviour. Above 20 MPa, we observe strong velocity-strengthening
frictional behaviour at low velocities, which then evolves towards velocity-weakening friction
behaviour at high velocities. Microstructural analyses of recovered samples highlight a variety
of deformation mechanisms including grain size reduction and localization, folding of calcite grains and fluid-assisted diffusion mass transfer processes promoting the development of
calcite nanograins in the highly deformed portions of the experimental fault. Our combined
analyses indicate that calcite fault gouge transitions from brittle to semi-brittle behaviour at
high normal stress and slow sliding velocities. This transition has important implications for
earthquake nucleation and propagation on faults in carbonate-dominated lithologies
Properties of Dark Matter Haloes in Clusters, Filaments, Sheets and Voids
Using a series of high-resolution N-body simulations of the concordance
cosmology we investigate how the formation histories, shapes and angular
momenta of dark-matter haloes depend on environment. We first present a
classification scheme that allows to distinguish between haloes in clusters,
filaments, sheets and voids in the large-scale distribution of matter. This
method is based on a local-stability criterion for the orbits of test particles
and closely relates to the Zel'dovich approximation. Applying this scheme to
our simulations we then find that: i) Mass assembly histories and formation
redshifts strongly depend on environment for haloes of mass M<M* (haloes of a
given mass tend to be older in clusters and younger in voids) and are
independent of it for larger masses; ii) Low-mass haloes in clusters are
generally less spherical and more oblate than in other regions; iii) Low-mass
haloes in clusters have a higher median spin than in filaments and present a
more prominent fraction of rapidly spinning objects; we identify recent major
mergers as a likely source of this effect. For all these relations, we provide
accurate functional fits as a function of halo mass and environment. We also
look for correlations between halo-spin directions and the large-scale
structures: the strongest effect is seen in sheets where halo spins tend to lie
within the plane of symmetry of the mass distribution. Finally, we measure the
spatial auto-correlation of spin directions and the cross-correlation between
the directions of intrinsic and orbital angular momenta of neighbouring haloes.
While the first quantity is always very small, we find that spin-orbit
correlations are rather strong especially for low-mass haloes in clusters and
high-mass haloes in filaments.Comment: 13 pages, 13 figures. Version accepted for publication in MNRAS
(references added). Version with high-resolution figures available at
http://www.exp-astro.phys.ethz.ch/hahn/pub/HPCD06.pd
Quantum squeezing generation versus photon localization in a disordered microcavity
We investigate theoretically the nonlinear dynamics induced by an intense
pump field in a disordered planar microcavity. Through a self-consistent
theory, we show how the generation of quantum optical noise squeezing is
affected by the breaking of the in-plane translational invariance and the
occurrence of photon localization. We find that the generation of single-mode
Kerr squeezing for the ideal planar case can be prevented by disorder as a
result of multimode nonlinear coupling, even when the other modes are in the
vacuum state. However, the excess noise is a non-monotonous function of the
disorder amplitude. In the strong localization limit, we show that the system
becomes protected with respect to this fundamental coupling mechanism and that
the ideal quadrature squeezing generation can be obtained
Waldschmidt constants for Stanley-Reisner ideals of a class of graphs
In the present note we study Waldschmidt constants of Stanley-Reisner ideals
of a hypergraph and a graph with vertices forming a bipyramid over a planar
n-gon. The case of the hypergraph has been studied by Bocci and Franci. We
reprove their main result. The case of the graph is new. Interestingly, both
cases provide series of ideals with Waldschmidt constants descending to 1. It
would be interesting to known if there are bounded ascending sequences of
Waldschmidt constants.Comment: 7 pages, 2 figure
Current distribution in a parallel configuration superconducting strip-line detector
Superconducting detectors based on parallel microscopic strip-lines are promising candidates for single molecule detection in time-of-flight mass spectrometry. The device physics of this configuration is complex. In this letter, we employ nano-optical techniques to study the variation of current density, count rate, and pulse amplitude transversely across the parallel strip device. Using the phenomenological London theory, we are able to correlate our results to a non-uniform current distribution between the strips, governed by the London magnetic penetration depth. This fresh perspective convincingly explains anomalous behaviour in large area parallel superconducting strip-line detectors reported in previous studies
Viscoelasticity and Stokes-Einstein relation in repulsive and attractive colloidal glasses
We report a numerical investigation of the visco-elastic behavior in models
for steric repulsive and short-range attractive colloidal suspensions, along
different paths in the attraction-strength vs packing fraction plane. More
specifically, we study the behavior of the viscosity (and its frequency
dependence) on approaching the repulsive glass, the attractive glass and in the
re-entrant region where viscosity shows a non monotonic behavior on increasing
attraction strength. On approaching the glass lines, the increase of the
viscosity is consistent with a power-law divergence with the same exponent and
critical packing fraction previously obtained for the divergence of the density
fluctuations. Based on mode-coupling calculations, we associate the increase of
the viscosity with specific contributions from different length scales. We also
show that the results are independent on the microscopic dynamics by comparing
newtonian and brownian simulations for the same model. Finally we evaluate the
Stokes-Einstein relation approaching both glass transitions, finding a clear
breakdown which is particularly strong for the case of the attractive glass.Comment: 12 pages; sent to J. Chem. Phy
John Maynard Keynes: The Economist as Investor
John Maynard Keynes: The Economist as Investor
Abstract
Keynes the investor has recently attracted the attention of several scholars and quite a few articles have come out in the last five years. A description of Keynes’s dealings has emerged, assessing his performance as an investor as superior but not as stellar as had previously been believed. However, overall evaluation of Keynes’s performance is still lacking. This paper contributes to this growing literature by filling some of the gaps, especially in relation to Keynes’s investment philosophy and economic theory, and by undertaking a more comprehensive review of the available evidence, drawing on some unpublished sources which have not as yet been fully exploite
The influence of bond-rigidity and cluster diffusion on the self-diffusion of hard spheres with square-well interaction
Hard spheres interacting through a square-well potential were simulated using
two different methods: Brownian Cluster Dynamics (BCD) and Event Driven
Brownian Dynamics (EDBD). The structure of the equilibrium states obtained by
both methods were compared and found to be almost the identical. Self diffusion
coefficients () were determined as a function of the interaction strength.
The same values were found using BCD or EDBD. Contrary the EDBD, BCD allows one
to study the effect of bond rigidity and hydrodynamic interaction within the
clusters. When the bonds are flexible the effect of attraction on is
relatively weak compared to systems with rigid bonds. increases first with
increasing attraction strength, and then decreases for stronger interaction.
Introducing intra-cluster hydrodynamic interaction weakly increases for a
given interaction strength. Introducing bond rigidity causes a strong decrease
of which no longer shows a maximum as function of the attraction strength
Quantum vacuum properties of the intersubband cavity polariton field
We present a quantum description of a planar microcavity photon mode strongly
coupled to a semiconductor intersubband transition in presence of a
two-dimensional electron gas. We show that, in this kind of system, the vacuum
Rabi frequency can be a significant fraction of the intersubband
transition frequency . This regime of ultra-strong light-matter
coupling is enhanced for long wavelength transitions, because for a given
doping density, effective mass and number of quantum wells, the ratio
increases as the square root of the intersubband
emission wavelength. We characterize the quantum properties of the ground state
(a two-mode squeezed vacuum), which can be tuned {\it in-situ} by changing the
value of , e.g., through an electrostatic gate. We finally point out
how the tunability of the polariton quantum vacuum can be exploited to generate
correlated photon pairs out of the vacuum via quantum electrodynamics phenomena
reminiscent of the dynamic Casimir effect.Comment: Final version accepted in PR
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