29,466 research outputs found
Fusion of strings vs. percolation and the transition to the quark-gluon plasma
In most of the models of hadronic collisions the number of exchanged colour
strings grows with energy and atomic numbers of the projectile and target. At
high string densities interaction between them should melt them into the
quark-gluon plasma state. It is shown that under certain assumptions about the
the string interaction, a phase transition to the quark gluon plasma indeed
takes place in the system of many colour strings. It may be of the first or
second order (percolation), depending on the particular mechanism of the
interaction. The critical string density is about unity in both cases. The
critical density may have been already reached in central Pb-Pb collisions at
158 A GeV.Comment: 16 pages, 3 Postscript figure
Superconductivity in iron silicide Lu2Fe3Si5 probed by radiation-induced disordering
Resistivity r(T), Hall coefficient RH(T), superconducting temperature Tc, and
the slope of the upper critical field -dHc2/dT were studied in poly- and
single-crystalline samples of the Fe-based superconductor Lu2Fe3Si5 irradiated
by fast neutrons. Atomic disordering induced by the neutron irradiation leads
to a fast suppression of Tc similarly to the case of doping of Lu2Fe3Si5 with
magnetic (Dy) and non-magnetic (Sc, Y) impurities. The same effect was observed
in a novel FeAs-based superconductor La(O-F)FeAs after irradiation. Such
behavior is accounted for by strong pair breaking that is traceable to
scattering at non-magnetic impurities or radiation defects in unconventional
superconductors. In such superconductors the sign of the order parameter
changes between the different Fermi sheets (s+- model). Some relations that are
specified for the properties of the normal and superconducting states in
high-temperature superconductors are also observed in Lu2Fe3Si5. The first is
the relationship -dHc2/dT ~ Tc, instead of the one expected for dirty
superconductors -dHc2/dT ~ r0. The second is a correlation between the
low-temperature linear coefficient a in the resistivity r = r0 + a1T, which
appears presumably due to the scattering at magnetic fluctuations, and Tc; this
correlation being an evidence of a tight relation between the superconductivity
and magnetism. The data point to an unconventional (non-fononic) mechanism of
superconductivity in Lu2Fe3Si5, and, probably, in some other Fe-based
compounds, which can be fruitfully studied via the radiation-induced
disordering.Comment: 7 pages, 8 figure
Fluctuations of the number of participants and binary collisions in AA interactions at fixed centrality in the Glauber approach
In the framework of the classical Glauber approach, the analytical
expressions for the variance of the number of wounded nucleons and binary
collisions in AA interactions at a given centrality are presented. Along with
the optical approximation term, they contain additional contact terms arising
only in the case of nucleus-nucleus collisions. The magnitude of the additional
contributions, e.g., for PbPb collisions at SPS energies, is larger than the
contribution of the optical approximation at some values of the impact
parameter. The sum of the additional contributions is in good agreement with
the results of independent Monte Carlo simulations of this process. Due to
these additional terms, the variance of the total number of participants for
peripheral PbPb collisions and the variance of the number of collisions at all
values of the impact parameter exceed several multiples of the Poisson
variances. The correlator between the numbers of participants in colliding
nuclei at fixed centrality is also analytically calculated.Comment: updated version; as published by Phys. Rev.
Production of light nuclei, hypernuclei and their antiparticles in relativistic nuclear collisions
We present, using the statistical model, an analysis of the production of
light nuclei, hypernuclei and their antiparticles in central collisions of
heavy nuclei. Based on these studies we provide predictions for the production
yields of multiply-strange light nuclei.Comment: 9 pages, 6 figues; v2: final versions accepted for publication (Phys.
Lett. B
A simple conceptual model of abrupt glacial climate events
Here we use a very simple conceptual model in an attempt to reduce essential
parts of the complex nonlinearity of abrupt glacial climate changes (the
so-called Dansgaard-Oeschger events) to a few simple principles, namely (i) a
threshold process, (ii) an overshooting in the stability of the system and
(iii) a millennial-scale relaxation. By comparison with a so-called Earth
system model of intermediate complexity (CLIMBER-2), in which the events
represent oscillations between two climate states corresponding to two
fundamentally different modes of deep-water formation in the North Atlantic, we
demonstrate that the conceptual model captures fundamental aspects of the
nonlinearity of the events in that model. We use the conceptual model in order
to reproduce and reanalyse nonlinear resonance mechanisms that were already
suggested in order to explain the characteristic time scale of
Dansgaard-Oeschger events. In doing so we identify a new form of stochastic
resonance (i.e. an overshooting stochastic resonance) and provide the first
explicitly reported manifestation of ghost resonance in a geosystem, i.e. of a
mechanism which could be relevant for other systems with thresholds and with
multiple states of operation. Our work enables us to explicitly simulate
realistic probability measures of Dansgaard-Oeschger events (e.g. waiting time
distributions, which are a prerequisite for statistical analyses on the
regularity of the events by means of Monte-Carlo simulations). We thus think
that our study is an important advance in order to develop more adequate
methods to test the statistical significance and the origin of the proposed
glacial 1470-year climate cycle
Role of interference in quantum state transfer through spin chains
We examine the role that interference plays in quantum state transfer through
several types of finite spin chains, including chains with isotropic Heisenberg
interaction between nearest neighbors, chains with reduced coupling constants
to the spins at the end of the chain, and chains with anisotropic coupling
constants. We evaluate quantitatively both the interference corresponding to
the propagation of the entire chain, and the interference in the effective
propagation of the first and last spins only, treating the rest of the chain as
black box. We show that perfect quantum state transfer is possible without
quantum interference, and provide evidence that the spin chains examined
realize interference-free quantum state transfer to a good approximation.Comment: 10 figure
Pinning a Domain Wall in (Ga,Mn)As with Focused Ion Beam Lithography
We utilize a focused beam of Ga+ ions to define magnetization pinning sites
in a ferromagnetic epilayer of (Ga,Mn)As. The nonmagnetic defects locally
increase the magneto-crystalline anisotropy energies, by which a domain wall is
pinned at a given position. We demonstrate techniques for manipulating domain
walls at these pinning sites as probed with the giant planar Hall effect
(GPHE). By varying the magnetic field angle relative to the crystal axes, an
upper limit is placed on the local effective anisotropy energy.Comment: 13 pages, 3 figure
Magnetic domain formation in itinerant metamagnets
We examine the effects of long-range dipolar forces on metamagnetic
transitions and generalize the theory of Condon domains to the case of an
itinerant electron system undergoing a first-order metamagnetic transition. We
demonstrate that within a finite range of the applied field, dipolar
interactions induce a spatial modulation of the magnetization in the form of
stripes or bubbles. Our findings are consistent with recent observations in the
bilayer ruthenate SrRuO.Comment: 4 pages, 3 figures, minor changes, references adde
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