1,146 research outputs found
Josephson array of mesoscopic objects. Modulation of system properties through the chemical potential
The phase diagram of a two-dimensional Josephson array of mesoscopic objects
is examined. Quantum fluctuations in both the modulus and phase of the
superconducting order parameter are taken into account within a lattice boson
Hubbard model. Modulating the average occupation number of the sites in
the system leads to changes in the state of the array, and the character of
these changes depends significantly on the region of the phase diagram being
examined. In the region where there are large quantum fluctuations in the phase
of the superconducting order parameter, variation of the chemical potential
causes oscillations with alternating superconducting (superfluid) and normal
states of the array. On the other hand, in the region where the bosons interact
weakly, the properties of the system depend monotonically on . Lowering
the temperature and increasing the particle interaction force lead to a
reduction in the width of the region of variation in within which the
system properties depend weakly on the average occupation number. The phase
diagram of the array is obtained by mapping this quantum system onto a
classical two-dimensional XY model with a renormalized Josephson coupling
constant and is consistent with our quantum Path-Integral Monte Carlo
calculations.Comment: 12 pages, 8 Postscript figure
Tagging High Energy Photons in the H1 Detector at HERA
Measures taken to extend the acceptance of the H1 detector at HERA for
photoproduction events are described. These will enable the measurement of
electrons scattered in events in the high y range 0.85 < y < 0.95 in the 1998
and 1999 HERA run period. The improvement is achieved by the installation of an
electromagnetic calorimeter, the ET8, in the HERA tunnel close to the electron
beam line 8 m downstream of the H1 interaction point in the electron direction.
The ET8 will allow the study of tagged gamma p interactions at centre-of-mass
energies significantly higher than those previously attainable. The calorimeter
design and expected performance are discussed, as are results obtained using a
prototype placed as close as possible to the position of the ET8 during the
1996 and 1997 HERA running.Comment: 13 pages, 13 figure
A New High Energy Photon Tagger for the H1 - Detector at HERA
The H1 detector at HERA has been upgraded by the addition of a new
electromagnetic calorimeter. This is installed in the HERA tunnel close to the
electron beam line at a position 8m from the interaction point in the electron
beam direction. The new calorimeter extends the acceptance for tagged
photoproduction events to the high y range, 0.85 < y < 0.95, and thus
significantly improves the capability of H1 to study high energy gamma-p
processes. The calorimeter design, performance and first results obtained
during the 1996-1999 HERA running are described.Comment: 17 pages, 16 figure
Rift zone reorganization through flank instability in ocean island volcanoes: an example from Tenerife, Canary Islands
Muon-Spin Rotation Spectra in the Mixed Phase of High-T_c Superconductors : Thermal Fluctuations and Disorder Effects
We study muon-spin rotation (muSR) spectra in the mixed phase of highly
anisotropic layered superconductors, specifically Bi_2+xSr_2-xCaCu_2O_8+delta
(BSCCO), by modeling the fluid and solid phases of pancake vortices using
liquid-state and density functional methods. The role of thermal fluctuations
in causing motional narrowing of muSR lineshapes is quantified in terms of a
first-principles theory of the flux-lattice melting transition. The effects of
random point pinning are investigated using a replica treatment of liquid state
correlations and a replicated density functional theory. Our results indicate
that motional narrowing in the pure system, although substantial, cannot
account for the remarkably small linewidths obtained experimentally at
relatively high fields and low temperatures. We find that satisfactory
agreement with the muSR data for BSCCO in this regime can be obtained through
the ansatz that this ``phase'' is characterized by frozen short-range
positional correlations reflecting the structure of the liquid just above the
melting transition. This proposal is consistent with recent suggestions of a
``pinned liquid'' or ``glassy'' state of pancake vortices in the presence of
pinning disorder. Our results for the high-temperature liquid phase indicate
that measurable linewidths may be obtained in this phase as a consequence of
density inhomogeneities induced by the pinning disorder. The results presented
here comprise a unified, first-principles theoretical treatment of muSR spectra
in highly anisotropic layered superconductors in terms of a controlled set of
approximations.Comment: 50 pages Latex file, including 10 postscript figure
Smooth Loops and Fiber Bundles: Theory of Principal Q-bundles
A nonassociative generalization of the principal fiber bundles with a smooth
loop mapping on the fiber is presented. Our approach allows us to construct a
new kind of gauge theories that involve higher ''nonassociative'' symmetries.Comment: 20 page
Measurements of Transverse Energy Flow in Deep-Inelastic Scattering at HERA
Measurements of transverse energy flow are presented for neutral current
deep-inelastic scattering events produced in positron-proton collisions at
HERA. The kinematic range covers squared momentum transfers Q^2 from 3.2 to
2,200 GeV^2, the Bjorken scaling variable x from 8.10^{-5} to 0.11 and the
hadronic mass W from 66 to 233 GeV. The transverse energy flow is measured in
the hadronic centre of mass frame and is studied as a function of Q^2, x, W and
pseudorapidity. A comparison is made with QCD based models. The behaviour of
the mean transverse energy in the central pseudorapidity region and an interval
corresponding to the photon fragmentation region are analysed as a function of
Q^2 and W.Comment: 26 pages, 8 figures, submitted to Eur. Phys.
Condensed Matter Theory of Dipolar Quantum Gases
Recent experimental breakthroughs in trapping, cooling and controlling
ultracold gases of polar molecules, magnetic and Rydberg atoms have paved the
way toward the investigation of highly tunable quantum systems, where
anisotropic, long-range dipolar interactions play a prominent role at the
many-body level. In this article we review recent theoretical studies
concerning the physics of such systems. Starting from a general discussion on
interaction design techniques and microscopic Hamiltonians, we provide a
summary of recent work focused on many-body properties of dipolar systems,
including: weakly interacting Bose gases, weakly interacting Fermi gases,
multilayer systems, strongly interacting dipolar gases and dipolar gases in 1D
and quasi-1D geometries. Within each of these topics, purely dipolar effects
and connections with experimental realizations are emphasized.Comment: Review article; submitted 09/06/2011. 158 pages, 52 figures. This
document is the unedited author's version of a Submitted Work that was
subsequently accepted for publication in Chemical Reviews, copyright American
Chemical Society after peer review. To access the final edited and published
work, a link will be provided soo
Searches at HERA for Squarks in R-Parity Violating Supersymmetry
A search for squarks in R-parity violating supersymmetry is performed in e^+p
collisions at HERA at a centre of mass energy of 300 GeV, using H1 data
corresponding to an integrated luminosity of 37 pb^(-1). The direct production
of single squarks of any generation in positron-quark fusion via a Yukawa
coupling lambda' is considered, taking into account R-parity violating and
conserving decays of the squarks. No significant deviation from the Standard
Model expectation is found. The results are interpreted in terms of constraints
within the Minimal Supersymmetric Standard Model (MSSM), the constrained MSSM
and the minimal Supergravity model, and their sensitivity to the model
parameters is studied in detail. For a Yukawa coupling of electromagnetic
strength, squark masses below 260 GeV are excluded at 95% confidence level in a
large part of the parameter space. For a 100 times smaller coupling strength
masses up to 182 GeV are excluded.Comment: 32 pages, 14 figures, 3 table
Multiplicity Structure of the Hadronic Final State in Diffractive Deep-Inelastic Scattering at HERA
The multiplicity structure of the hadronic system X produced in
deep-inelastic processes at HERA of the type ep -> eXY, where Y is a hadronic
system with mass M_Y< 1.6 GeV and where the squared momentum transfer at the pY
vertex, t, is limited to |t|<1 GeV^2, is studied as a function of the invariant
mass M_X of the system X. Results are presented on multiplicity distributions
and multiplicity moments, rapidity spectra and forward-backward correlations in
the centre-of-mass system of X. The data are compared to results in e+e-
annihilation, fixed-target lepton-nucleon collisions, hadro-produced
diffractive final states and to non-diffractive hadron-hadron collisions. The
comparison suggests a production mechanism of virtual photon dissociation which
involves a mixture of partonic states and a significant gluon content. The data
are well described by a model, based on a QCD-Regge analysis of the diffractive
structure function, which assumes a large hard gluonic component of the
colourless exchange at low Q^2. A model with soft colour interactions is also
successful.Comment: 22 pages, 4 figures, submitted to Eur. Phys. J., error in first
submission - omitted bibliograph
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