17,210 research outputs found
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
Transverse momentum distributions and their forward- backward correlations in the percolating colour string approach
The forward-backward correlations in the distributions, which present a
clear signature of non-linear effects in particle production, are studied in
the model of percolating colour strings. Quantitative predictions are given for
these correlations at SPS, RHIC and LHC energies. Interaction of strings also
naturally explains the flattening of distributions and increase of
with energy and atomic number for nuclear collisionsComment: 6 pages in LaTex, 3 figures in Postscrip
BELLE Data on the Form Factor: A Game Changer?
We extend our analysis of the form factor by including
a comparison with the new BELLE data. The necessity of new precision
measurements in a broad interval of momentum transfers is emphasized.Comment: 4 pages, 3 figures. Addendum to Phys. Rev. D 83, 054020 (2011
Static friction on the fly: velocity depinning transitions of lubricants in motion
The dragging velocity of a model solid lubricant confined between sliding
periodic substrates exhibits a phase transition between two regimes,
respectively with quantized and with continuous lubricant center-of-mass
velocity. The transition, occurring for increasing external driving force F_ext
acting on the lubricant, displays a large hysteresis, and has the features of
depinning transitions in static friction, only taking place on the fly.
Although different in nature, this phenomenon appears isomorphic to a static
Aubry depinning transition in a Frenkel-Kontorova model, the role of particles
now taken by the moving kinks of the lubricant-substrate interface. We suggest
a possible realization in 2D optical lattice experiments.Comment: 5 pages, 4 figures, revtex, in print in Phys. Rev. Let
Transverse momentum fluctuations and percolation of strings
The behaviour of the transverse momentum fluctuations with the centrality of
the collision shown by the Relativistic Heavy Ion Collider data is naturally
explained by the clustering of color sources. In this framework, elementary
color sources --strings-- overlap forming clusters, so the number of effective
sources is modified. These clusters decay into particles with mean transverse
momentum that depends on the number of elementary sources that conform each
cluster, and the area occupied by the cluster. The transverse momentum
fluctuations in this approach correspond to the fluctuations of the transverse
momentum of these clusters, and they behave essentially as the number of
effective sources.Comment: 16 pages, RevTex, 4 postscript figures. Enhanced version. New figure
Effect of continuous gamma-ray exposure on performance of learned tasks and effect of subsequent fractionated exposures on blood-forming tissue
Sixteen monkeys trained to perform continuous and discrete-avoidance and fixed-ratio tasks with visual and auditory cues were performance-tested before, during, and after 10-day gamma-ray exposures totaling 0, 500, 750, and 1000 rads. Approximately 14 months after the performance-test exposures, surviving animals were exposed to 100-rad gamma-ray fractions at 56-day intervals to observe injury and recovery patterns of blood-forming tissues. The fixed-ratio, food-reward task performance showed a transient decline in all dose groups within 24 hours of the start of gamma-ray exposure, followed by recovery to normal food-consumption levels within 48 to 72 hours. Avoidance tasks were performed successfully by all groups during the 10-day exposure, but reaction times of the two higher dose-rate groups in which animals received 3 and 4 rads per hour or total doses of 750 and 1000 rads, respectively, were somewhat slower
Virtual Processes and Superradiance in Spin-Boson Models
We consider spin-boson models composed by a single bosonic mode and an
ensemble of identical two-level atoms. The situation where the coupling
between the bosonic mode and the atoms generates real and virtual processes is
studied, where the whole system is in thermal equilibrium with a reservoir at
temperature . Phase transitions from ordinary fluorescence to
superradiant phase in three different models is investigated. First a model
where the coupling between the bosonic mode and the atom is via the
pseudo-spin operator is studied. Second, we investigate the
generalized Dicke model, introducing different coupling constants between the
single mode bosonic field and the environment, and for rotating
and counter-rotating terms, respectively. Finally it is considered a modified
version of the generalized Dicke model with intensity-dependent coupling in the
rotating terms. In the first model the zero mode contributes to render the
canonical entropy a negative quantity for low temperatures. The last two models
presents phase transitions, even when only Hamiltonian terms which generates
virtual processes are considered
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