1,280 research outputs found
Exact Study of the Effect of Level Statistics in Ultrasmall Superconducting Grains
The reduced BCS model that is commonly used for ultrasmall superconducting
grains has an exact solution worked out long ago by Richardson in the context
of nuclear physics. We use it to check the quality of previous treatments of
this model, and to investigate the effect of level statistics on pairing
correlations. We find that the ground state energies are on average somewhat
lower for systems with non-uniform than uniform level spacings, but both have
an equally smooth crossover from the bulk to the few-electron regime. In the
latter, statistical fluctuations in ground state energies strongly depend on
the grain's electron number parity.Comment: 4 pages, 3 eps figs, RevTe
BOLLOCKS!! Designing pervasive games that play with the social rules of built environments
We propose that pervasive games designed with mechanics that are specifically in
opposition with, or disruptive of, social rules of the environment in which they are played, have
unique potential to provide interesting, provocative experiences for players. We explore this
concept through the design and evaluation of an experimental game prototype, Shhh!, inspired
by the juvenile game Bollocks, and implemented on Android mobile devices, which challenges
players to make loud noises in libraries. Six participants played the game before engaging in
semi-structured interviews, explored through inductive thematic analysis. Results suggest that
the game provoked in players a heightened awareness of social rules, as well as a complex
social dilemma of whether or not to act. We conclude by presenting a model for designing
games that play with the social, as well as physical, rules of the environments in which they are
set
Collective behavior in nuclear interactions and shower development
The mechanism of hadronic interactions at very high energies is still
unclear. Available accelerator data constrain weakly the forward rapidity
region which determines the development of atmospheric showers. This ignorance
is one of the main sources of uncertainty in the determination of the energy
and composition of the primary in hadron-induced atmospheric showers. In this
paper we examine the effect on the shower development of two kinds of
collective effects in high-energy hadronic interactions which modify the
production of secondary particles. The first mechanism, modeled as string
fusion, affects strongly the central rapidity region but only slightly the
forward region and is shown to have very little effect on the shower
development. The second mechanism implies a very strong stopping; it affects
modestly the profile of shower maximum but broadens considerably the number
distribution of muons at ground. For the latter mechanism, the development of
air showers is faster mimicking a heavier projectile. On the other hand, the
number of muons at ground is lowered, resembling a shower generated by a
lighter primary.Comment: 17 pages, 10 figure
Phases of QCD, Thermal Quasiparticles and Dilepton Radiation from a Fireball
We calculate dilepton production rates from a fireball adapted to the
kinematical conditions realized in ultrarelativistic heavy ion collisions over
a broad range of beam energies. The freeze-out state of the fireball is fixed
by hadronic observables. We use this information combined with the initial
geometry of the collision region to follow the space-time evolution of the
fireball. Assuming entropy conservation, its bulk thermodynamic properties can
then be uniquely obtained once the equation of state (EoS) is specified. The
high-temperature (QGP) phase is modelled by a non-perturbative quasiparticle
model that incorporates a phenomenological confinement description, adapted to
lattice QCD results. For the hadronic phase, we interpolate the EoS into the
region where a resonance gas approach seems applicable, keeping track of a
possible overpopulation of the pion phase space. In this way, the fireball
evolution is specified without reference to dilepton data, thus eliminating it
as an adjustable parameter in the rate calculations. Dilepton emission in the
QGP phase is then calculated within the quasiparticle model. In the hadronic
phase, both temperature and finite baryon density effects on the photon
spectral function are incorporated. Existing dilepton data from CERES at 158
and 40 AGeV Pb-Au collisions are well described, and a prediction for the
PHENIX setup at RHIC for sqrt(s) = 200 AGeV is given.Comment: 31 pages, 15 figures, final versio
Lectures on Chiral Disorder in QCD
I explain the concept that light quarks diffuse in the QCD vacuum following
the spontaneous breakdown of chiral symmetry. I exploit the striking analogy to
disordered electrons in metals, identifying, among others, the universal regime
described by random matrix theory, diffusive regime described by chiral
perturbation theory and the crossover between these two domains.Comment: Lectures given at the Cargese Summer School, August 6-18, 200
Deep Inelastic Scattering and Gauge/String Duality
We study deep inelastic scattering in gauge theories which have dual string
descriptions. As a function of we find a transition. For small , the
dominant operators in the OPE are the usual ones, of approximate twist two,
corresponding to scattering from weakly interacting partons. For large ,
double-trace operators dominate, corresponding to scattering from entire
hadrons (either the original `valence' hadron or part of a hadron cloud.) At
large we calculate the structure functions. As a function of Bjorken
there are three regimes: of order one, where the scattering produces only
supergravity states; small, where excited strings are produced; and,
exponentially small, where the excited strings are comparable in size to the
AdS space. The last regime requires in principle a full string calculation in
curved spacetime, but the effect of string growth can be simply obtained from
the world-sheet renormalization group.Comment: 52 pages, 10 figure
Enhanced Spin Dependent Shot Noise in Magnetic Tunnel Barriers
We report the observation of enhanced spin dependent shot noise in magnetic
tunnel barriers, suggesting transport through localized states within the
barrier. This is supported by the existence of negative magnetoresistance and
structure in the differential conductance curves. A simple model of tunneling
through two interacting localized states with spin dependent tunneling rates is
used to explain our observations.Comment: 8 pages, 8 figures, submitted to Physica E (proceedings of the
seminar on Quantum Coherence, Noise and Decoherence in Nanostructures
Magnetic Field Effect for Two Electrons in a Two Dimensional Random Potential
We study the problem of two particles with Coulomb repulsion in a
two-dimensional disordered potential in the presence of a magnetic field. For
the regime, when without interaction all states are well localized, it is shown
that above a critical excitation energy electron pairs become delocalized by
interaction. The transition between the localized and delocalized regimes goes
in the same way as the metal-insulator transition at the mobility edge in the
three dimensional Anderson model with broken time reversal symmetry.Comment: revtex, 7 pages, 6 figure
Dynamics of the Hubbard model: a general approach by time dependent variational principle
We describe the quantum dynamics of the Hubbard model at semi-classical
level, by implementing the Time-Dependent Variational Principle (TDVP)
procedure on appropriate macroscopic wavefunctions constructed in terms of
su(2)-coherent states. Within the TDVP procedure, such states turn out to
include a time-dependent quantum phase, part of which can be recognized as
Berry's phase. We derive two new semi-classical model Hamiltonians for
describing the dynamics in the paramagnetic, superconducting, antiferromagnetic
and charge density wave phases and solve the corresponding canonical equations
of motion in various cases. Noticeably, a vortex-like ground state phase
dynamics is found to take place for U>0 away from half filling. Moreover, it
appears that an oscillatory-like ground state dynamics survives at the Fermi
surface at half-filling for any U. The low-energy dynamics is also exactly
solved by separating fast and slow variables. The role of the time-dependent
phase is shown to be particularly interesting in the ordered phases.Comment: ReVTeX file, 38 pages, to appear on Phys. Rev.
Response to Letter: Intrapatient Comparison of the Hepatobiliary Phase of Gd-BOPTA and Gd-EOB-DTPA in the Differentiation of HCA From FNH
Level of Evidence: 5. Technical Efficacy Stage: 3
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