13,794 research outputs found
Cement-rock interaction : infiltration of a high-pH solution into a fractured granite core
Within the framework of the HPF project (Hyperalkaline Plume in Fractured Rock) at the Grimsel Test Site (Switzerland), a small scale core infiltration experiment was performed at the University of Bern. A high-pH solution was continuously injected, under a constant pressure gradient, into a cylindrical core of granite containing a fracture. This high-pH solution was a synthetic version of solutions characteristic of early stages in the degradation of cement. The interaction between the rock and the solutions was reflected by significant changes in the composition of the injected solution, despite the negligible pH-buffering capacity, and a decrease in the permeability of the rock. Changes in the mineralogy and porosity of the fault gouge filling the fracture were only minor. Within the new LCS (Long-Term Cement Studies) project at Grimsel, new one-dimensional reactive transport modeling using CrunchFlow has been used to improve the interpretation of the experimental results. Dispersive and advective solute transport, adsorption processes and mineral reaction kinetics have been taken into account. The evolution of solution composition is mainly controlled by dissolution/precipitation reactions. Adsorption processes (cation exchange, surface complexation) only play a role in the very early stages of the experiment
Ab initio vibrations in nonequilibrium nanowires
We review recent results on electronic and thermal transport in two different
quasi one-dimensional systems: Silicon nanowires (SiNW) and atomic gold chains.
For SiNW's we compute the ballistic electronic and thermal transport properties
on equal footing, allowing us to make quantitative predictions for the
thermoelectric properties, while for the atomic gold chains we evaluate
microscopically the damping of the vibrations, due to the coupling of the chain
atoms to the modes in the bulk contacts. Both approaches are based on a
combination of density-functional theory, and nonequilibrium Green's functions.Comment: 16 pages, to appear in Progress in Nonequilibrium Green's Functions
IV (PNGF4), Eds. M. Bonitz and K. Baltzer, Glasgow, August 200
Prominence seismology using the period ratio of transverse thread oscillations
The ratio of the period of the fundamental mode to that of the first overtone
of kink oscillations, from here on the "period ratio", is a seismology tool
that can be used to infer information about the spatial variation of density
along solar magnetic flux tubes. The period ratio is 2 in longitudinally
homogeneous thin tubes, but it differs from 2 due to longitudinal
inhomogeneity. In this paper we investigate the period ratio in longitudinally
inhomogeneous prominence threads and explore its implications for prominence
seismology. We numerically solve the two-dimensional eigenvalue problem of kink
oscillations in a model of a prominence thread. We take into account three
nonuniform density profiles along the thread. In agreement with previous works
that used simple piecewise constant density profiles, we find that the period
ratio is larger than 2 in prominence threads. When the ratio of the central
density to that at the footpoints is fixed, the period ratio depends strongly
on the form of the density profile along the thread. The more concentrated the
dense prominence plasma near the center of the tube, the larger the period
ratio. However, the period ratio is found to be independent of the specific
density profile when the spatially averaged density in the thread is the same
for all the profiles. An empirical fit of the dependence of the period ratio on
the average density is given and its use for prominence seismology is
discussed.Comment: Accepted for publication in A&
: Confronting theory and lattice simulations
We consider a recent -matrix analysis by Albaladejo {\it et al.}, [Phys.\
Lett.\ B {\bf 755}, 337 (2016)] which accounts for the and
coupled--channels dynamics, and that successfully describes the
experimental information concerning the recently discovered .
Within such scheme, the data can be similarly well described in two different
scenarios, where the is either a resonance or a virtual state. To
shed light into the nature of this state, we apply this formalism in a finite
box with the aim of comparing with recent Lattice QCD (LQCD) simulations. We
see that the energy levels obtained for both scenarios agree well with those
obtained in the single-volume LQCD simulation reported in Prelovsek {\it et
al.} [Phys.\ Rev.\ D {\bf 91}, 014504 (2015)], making thus difficult to
disentangle between both possibilities. We also study the volume dependence of
the energy levels obtained with our formalism, and suggest that LQCD
simulations performed at several volumes could help in discerning the actual
nature of the intriguing state
D-instanton and polyinstanton effects from type I' D0-brane loops
We study non-perturbative D(-1)-instanton corrections to quartic gauge and
curvature couplings in 8d type IIB orientifolds, in terms of a one-loop
computation of BPS D0-branes in T-dual type I' models. The complete
perturbative and non-perturbative results are determined by the BPS
multiplicities of perturbative open strings and D0-brane bound states in the 9d
type I' theory. Its modular properties admit a geometric interpretation by
lifting to Horava-Witten theory. We use the type I' viewpoint to motivate a
proper interpretation of 8d and 4d polyinstanton effects, consistent with
heterotic - type II orientifold duality.Comment: 37 pages, 2 figures, clarifications and references adde
On the properties of surface reconstructed silicon nanowires
We study by means of density-functional calculations the role of lateral
surface reconstructions in determining the electrical properties of
silicon nanowires. The different lateral reconstructions are explored by
relaxing all the nanowires with crystalline bulk silicon structure and all
possible ideal facets that correspond to an average diameter of 1.5 nm. We show
that the reconstruction induces the formation of ubiquitous surface states that
turn the wires into semi-metallic or metallic
Bifurcations in the Lozi map
We study the presence in the Lozi map of a type of abrupt order-to-order and
order-to-chaos transitions which are mediated by an attractor made of a
continuum of neutrally stable limit cycles, all with the same period.Comment: 17 pages, 12 figure
Efficient implementation of a van der Waals density functional: Application to double-wall carbon nanotubes
We present an efficient implementation of the van der Waals density
functional of Dion et al [Phys. Rev. Lett. 92, 246401 (2004)], which expresses
the nonlocal correlation energy as a double spacial integral. We factorize the
integration kernel and use fast Fourier transforms to evaluate the
selfconsistent potential, total energy, and atomic forces, in N log(N)
operations. The resulting overhead in total computational cost, over semilocal
functionals, is very moderate for medium and large systems. We apply the method
to calculate the binding energies and the barriers for relative translation and
rotation in double-wall carbon nanotubes.Comment: 4 pages, 1 figure, 1 tabl
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