3,469 research outputs found
Mergers of Black Hole -- Neutron Star binaries. I. Methods and First Results
We use a 3-D relativistic SPH (Smoothed Particle Hydrodynamics) code to study
mergers of black hole -- neutron star (BH--NS) binary systems with low mass
ratios, adopting as a representative case. The
outcome of such mergers depends sensitively on both the magnitude of the BH
spin and its obliquity (i.e., the inclination of the binary orbit with respect
to the equatorial plane of the BH). In particular, only systems with
sufficiently high BH spin parameter and sufficiently low orbital
inclinations allow any NS matter to escape or to form a long-lived disk outside
the BH horizon after disruption. Mergers of binaries with orbital inclinations
above lead to complete prompt accretion of the entire NS by the BH,
even for the case of an extreme Kerr BH. We find that the formation of a
significant disk or torus of NS material around the BH always requires a
near-maximal BH spin and a low initial inclination of the NS orbit just prior
to merger.Comment: to appear in ApJ, 54 pages, 19 figure
Physical consequences of PNP and the DMRG-annealing conjecture
Computational complexity theory contains a corpus of theorems and conjectures
regarding the time a Turing machine will need to solve certain types of
problems as a function of the input size. Nature {\em need not} be a Turing
machine and, thus, these theorems do not apply directly to it. But {\em
classical simulations} of physical processes are programs running on Turing
machines and, as such, are subject to them. In this work, computational
complexity theory is applied to classical simulations of systems performing an
adiabatic quantum computation (AQC), based on an annealed extension of the
density matrix renormalization group (DMRG). We conjecture that the
computational time required for those classical simulations is controlled
solely by the {\em maximal entanglement} found during the process. Thus, lower
bounds on the growth of entanglement with the system size can be provided. In
some cases, quantum phase transitions can be predicted to take place in certain
inhomogeneous systems. Concretely, physical conclusions are drawn from the
assumption that the complexity classes {\bf P} and {\bf NP} differ. As a
by-product, an alternative measure of entanglement is proposed which, via
Chebyshev's inequality, allows to establish strict bounds on the required
computational time.Comment: Accepted for publication in JSTA
Transport properties and structures of vortex matter in layered superconductors
In this paper we analyze the structure, phase transitions and some transport
properties of the vortex system when the external magnetic field lies parallel
to the planes in layered superconductors. We show that experimental results for
resistivity are qualitatively consistent with numerical simulations that
describe the melting of a commensurate rotated lattice. However for some
magnetic fields, the structure factor indicates the occurrence of smectic peaks
at an intermediate temperature regime.Comment: 8 pages, 8 eps figure
Compelled to do the right thing
We use a model of opinion formation to study the consequences of some
mechanisms attempting to enforce the right behaviour in a society. We start
from a model where the possible choices are not equivalent (such is the case
when the agents decide to comply or not with a law) and where an imitation
mechanism allow the agents to change their behaviour based on the influence of
a group of partners. In addition, we consider the existence of two social
constraints: a) an external authority, called monitor, that imposes the correct
behaviour with infinite persuasion and b) an educated group of agents that act
upon their fellows but never change their own opinion, i.e., they exhibit
infinite adamancy. We determine the minimum number of monitors to induce an
effective change in the behaviour of the social group, and the size of the
educated group that produces the same effect. Also, we compare the results for
the cases of random social interactions and agents placed on a network. We have
verified that a small number of monitors are enough to change the behaviour of
the society. This also happens with a relatively small educated group in the
case of random interactions.Comment: 8 pages, 9 figures, submitted to EPJ
Universality Classes of Diagonal Quantum Spin Ladders
We find the classification of diagonal spin ladders depending on a
characteristic integer in terms of ferrimagnetic, gapped and critical
phases. We use the finite algorithm DMRG, non-linear sigma model and
bosonization techniques to prove our results. We find stoichiometric contents
in cuprate planes that allow for the existence of weakly interacting
diagonal ladders.Comment: REVTEX4 file, 3 color figures, 1 tabl
- …