196 research outputs found
Peculiar Velocities of Galaxy Clusters
We investigate the peculiar velocities predicted for galaxy clusters by
theories in the cold dark matter family. A widely used hypothesis identifies
rich clusters with high peaks of a suitably smoothed version of the linear
density fluctuation field. Their peculiar velocities are then obtained by
extrapolating the similarly smoothed linear peculiar velocities at the
positions of these peaks. We test these ideas using large high resolution
N-body simulations carried out within the Virgo supercomputing consortium. We
find that at early times the barycentre of the material which ends up in a rich
cluster is generally very close to a high peak of the initial density field.
Furthermore the mean peculiar velocity of this material agrees well with the
linear value at the peak. The late-time growth of peculiar velocities is,
however, systematically underestimated by linear theory. At the time clusters
are identified we find their rms peculiar velocity to be about 40% larger than
predicted. Nonlinear effects are particularly important in superclusters. These
systematics must be borne in mind when using cluster peculiar velocities to
estimate the parameter combination .Comment: 8 pages, 4 figures; submitted to MNRA
Variational Monte Carlo study of the ground state properties and vacancy formation energy of solid para-H2 using a shadow wave function
A Shadow Wave Function (SWF) is employed along with Variational Monte Carlo
techniques to describe the ground state properties of solid molecular
para-hydrogen. The study has been extended to densities below the equilibrium
value, to obtain a parameterization of the SWF useful for the description of
inhomogeneous phases. We also present an estimate of the vacancy formation
energy as a function of the density, and discuss the importance of relaxation
effects near the vacant site
Singular charge fluctuations at a magnetic quantum critical point
Strange metal behavior is ubiquitous in correlated materials, ranging from cuprate superconductors to bilayer graphene, and may arise from physics beyond the quantum fluctuations of a Landau order parameter. In quantum-critical heavy-fermion antiferromagnets, such physics may be realized as critical Kondo entanglement of spin and charge and probed with optical conductivity. We present terahertz time-domain transmission spectroscopy on molecular beam epitaxyâgrown thin films of YbRhâSiâ, a model strange-metal compound. We observed frequency over temperature scaling of the optical conductivity as a hallmark of beyond-Landau quantum criticality. Our discovery suggests that critical charge fluctuations play a central role in the strange metal behavior, elucidating one of the long-standing mysteries of correlated quantum matter
Substructures in Cold Dark Matter Haloes
We analyse the properties of substructures within dark matter halos
(subhalos) using a set of high-resolution numerical simulations of the
formation of structure in a Lambda-CDM Universe. Our simulation set includes 11
high-resolution simulations of massive clusters as well as a region of mean
density, allowing us to study the spatial and mass distribution of
substructures down to a mass resolution limit of 10^9 h^(-1)Mo. We also
investigate how the properties of substructures vary as a function of the mass
of the `parent' halo in which they are located. We find that the substructure
mass function depends at most weakly on the mass of the parent halo and is well
described by a power-law. The radial number density profiles of substructures
are steeper in low mass halos than in high mass halos. More massive
substructures tend to avoid the centres of halos and are preferentially located
in the external regions of their parent halos. We also study the mass accretion
and merging histories of substructures, which we find to be largely independent
of environment. We find that a significant fraction of the substructures
residing in clusters at the present day were accreted at redshifts z < 1. This
implies that a significant fraction of present-day `passive' cluster galaxies
should have been still outside the cluster progenitor and more active at z~1.Comment: 13 pages, 15 figure. Accepted to MNRA
Singular charge fluctuations at a magnetic quantum critical point
Strange metal behavior is ubiquitous in correlated materials, ranging from cuprate superconductors to bilayer graphene, and may arise from physics beyond the quantum fluctuations of a Landau order parameter. In quantum-critical heavy-fermion antiferromagnets, such physics may be realized as critical Kondo entanglement of spin and charge and probed with optical conductivity. We present terahertz time-domain transmission spectroscopy on molecular beam epitaxyâgrown thin films of YbRh2Si2, a model strange-metal compound. We observed frequency over temperature scaling of the optical conductivity as a hallmark of beyond-Landau quantum criticality. Our discovery suggests that critical charge fluctuations play a central role in the strange metal behavior, elucidating one of the long-standing mysteries of correlated quantum matter
Zero-point vacancies in quantum solids
A Jastrow wave function (JWF) and a shadow wave function (SWF) describe a
quantum solid with Bose--Einstein condensate; i.e. a supersolid. It is known
that both JWF and SWF describe a quantum solid with also a finite equilibrium
concentration of vacancies x_v. We outline a route for estimating x_v by
exploiting the existing formal equivalence between the absolute square of the
ground state wave function and the Boltzmann weight of a classical solid. We
compute x_v for the quantum solids described by JWF and SWF employing very
accurate numerical techniques. For JWF we find a very small value for the zero
point vacancy concentration, x_v=(1.4\pm0.1) x 10^-6. For SWF, which presently
gives the best variational description of solid 4He, we find the significantly
larger value x_v=(1.4\pm0.1) x 10^-3 at a density close to melting. We also
study two and three vacancies. We find that there is a strong short range
attraction but the vacancies do not form a bound state.Comment: 19 pages, submitted to J. Low Temp. Phy
Effects of Pore Walls and Randomness on Phase Transitions in Porous Media
We study spin models within the mean field approximation to elucidate the
topology of the phase diagrams of systems modeling the liquid-vapor transition
and the separation of He--He mixtures in periodic porous media. These
topologies are found to be identical to those of the corresponding random field
and random anisotropy spin systems with a bimodal distribution of the
randomness. Our results suggest that the presence of walls (periodic or
otherwise) are a key factor determining the nature of the phase diagram in
porous media.Comment: REVTeX, 11 eps figures, to appear in Phys. Rev.
Vortex Dynamics in Superfluid Systems: Cyclotron Type Motion
Vortex dynamics in superfluids is investigated in the framework of the
nonlinear Schr\"{o}dinger equation. The natural motion of the vortex is of
cyclotron type, whose frequency is found to be on the order of phonon velocity
divided by the coherence length, and may be heavily damped due to phonon
radiation. Trapping foreign particles into the vortex core can reduce the
cyclotron frequency and make the cyclotron motion underdamped. The density
fluctuations can follow the vortex motion adiabatically within the phonon wave
length at the cyclotron frequency, which results in a further downward
renormalization of the cyclotron frequency. We have also discussed applications
on the dynamics of vortices in superconducting films.Comment: 21 pages, 1 figure include
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