744 research outputs found
A Two-dimensional Superconductor in a Tilted Magnetic Field - new states with finite Cooper-pair momentum
Varying the angle Theta between applied field and the conducting planes of a
layered superconductor in a small interval close to the plane-parallel field
direction, a large number of superconducting states with unusual properties may
be produced. For these states, the pair breaking effect of the magnetic field
affects both the orbital and the spin degree of freedom. This leads to pair
wave functions with finite momentum, which are labeled by Landau quantum
numbers 0<n<\infty. The stable order parameter structure and magnetic field
distribution for these states is found by minimizing the quasiclassical free
energy near H_{c2} including nonlinear terms. One finds states with coexisting
line-like and point-like order parameter zeros and states with coexisting
vortices and antivortices. The magnetic response may be diamagnetic or
paramagnetic depending on the position within the unit cell. The structure of
the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states at Theta=0 is reconsidered.
The transition n->\infty of the paramagnetic vortex states to the FFLO-limit is
analyzed and the physical reason for the occupation of higher Landau levels is
pointed out.Comment: 24 pages, 11 figure
Influence of orbital pair breaking on paramagnetically limited states in clean superconductors
Paramagnetic pair breaking is believed to be of increasing importance in many
layered superconducting materials such as cuprates and organic compounds.
Recently, strong evidence for a phase transition to the
Fulde-Ferrell-Larkin-Ovchinnikov(FFLO) state has been obtained for the first
time. We present a new theory of competing spin and orbital pair breaking in
clean superconducting films or layers. As a general result, we find that the
influence of orbital pair breaking on the paramagnetically limited phase
boundary is rather strong, and its neglect seldom justified. This is
particularly true for the FFLO state which can be destroyed by a very small
orbital contribution. We discuss the situation in YBa_2Cu_3O_7 which has two
coupled conducting Cu-O layers per unit cell. As a consequence, an intrinsic
orbital pair breaking component might exist even for applied field exactly
parallel to the layers.Comment: 19 pages, 5 figures, submitted to PR
Vortex dynamics and states of artificially layered superconducting films with correlated defects
Linear resistances and -characteristics have been measured over a wide
range in the parameter space of the mixed phase of multilayered a-TaGe/Ge
films. Three films with varying interlayer coupling and correlated defects
oriented at an angle from the film normal were investigated.
Experimental data were analyzed within vortex glass models and a second order
phase transition from a resistive vortex liquid to a pinned glass phase.
Various vortex phases including changes from three to two dimensional behavior
depending on anisotropy have been identified. Careful analysis of
-characteristics in the glass phases revealed a distinctive and
-dependence of the glass exponent . The vortex dynamics in the
Bose-glass phase does not follow the predicted behavior for excitations of
vortex kinks or loops.Comment: 16 pages, 10 figures, 3 table
Decoupling and decommensuration in layered superconductors with columnar defects
We consider layered superconductors with a flux lattice perpendicular to the
layers and random columnar defects parallel to the magnetic field B. We show
that the decoupling transition temperature Td, at which the Josephson coupling
vanishes, is enhanced by columnar defects by an amount ~B^2 relative to Td.
Decoupling by increasing field can be followed by a reentrant recoupling
transition for strong disorder. We also consider a commensurate component of
the columnar density and show that its pinning potential is renormalized to
zero above a critical long wavelength disorder. This decommnesuration
transition may account for a recently observed kink in the melting line.Comment: 5 pages, Revte
Forward pi^0 Production and Associated Transverse Energy Flow in Deep-Inelastic Scattering at HERA
Deep-inelastic positron-proton interactions at low values of Bjorken-x down
to x \approx 4.10^-5 which give rise to high transverse momentum pi^0 mesons
are studied with the H1 experiment at HERA. The inclusive cross section for
pi^0 mesons produced at small angles with respect to the proton remnant (the
forward region) is presented as a function of the transverse momentum and
energy of the pi^0 and of the four-momentum transfer Q^2 and Bjorken-x.
Measurements are also presented of the transverse energy flow in events
containing a forward pi^0 meson. Hadronic final state calculations based on QCD
models implementing different parton evolution schemes are confronted with the
data.Comment: 27 pages, 8 figures and 3 table
Search for Doubly-Charged Higgs Boson Production at HERA
A search for the single production of doubly-charged Higgs bosons H^{\pm \pm}
in ep collisions is presented. The signal is searched for via the Higgs decays
into a high mass pair of same charge leptons, one of them being an electron.
The analysis uses up to 118 pb^{-1} of ep data collected by the H1 experiment
at HERA. No evidence for doubly-charged Higgs production is observed and mass
dependent upper limits are derived on the Yukawa couplings h_{el} of the Higgs
boson to an electron-lepton pair. Assuming that the doubly-charged Higgs only
decays into an electron and a muon via a coupling of electromagnetic strength
h_{e \mu} = \sqrt{4 \pi \alpha_{em}} = 0.3, a lower limit of 141 GeV on the
H^{\pm\pm} mass is obtained at the 95% confidence level. For a doubly-charged
Higgs decaying only into an electron and a tau and a coupling h_{e\tau} = 0.3,
masses below 112 GeV are ruled out.Comment: 15 pages, 3 figures, 1 tabl
Low Q^2 Jet Production at HERA and Virtual Photon Structure
The transition between photoproduction and deep-inelastic scattering is
investigated in jet production at the HERA ep collider, using data collected by
the H1 experiment. Measurements of the differential inclusive jet
cross-sections dsigep/dEt* and dsigmep/deta*, where Et* and eta* are the
transverse energy and the pseudorapidity of the jets in the virtual
photon-proton centre of mass frame, are presented for 0 < Q2 < 49 GeV2 and 0.3
< y < 0.6. The interpretation of the results in terms of the structure of the
virtual photon is discussed. The data are best described by QCD calculations
which include a partonic structure of the virtual photon that evolves with Q2.Comment: 20 pages, 5 Figure
Origins of the Ambient Solar Wind: Implications for Space Weather
The Sun's outer atmosphere is heated to temperatures of millions of degrees,
and solar plasma flows out into interplanetary space at supersonic speeds. This
paper reviews our current understanding of these interrelated problems: coronal
heating and the acceleration of the ambient solar wind. We also discuss where
the community stands in its ability to forecast how variations in the solar
wind (i.e., fast and slow wind streams) impact the Earth. Although the last few
decades have seen significant progress in observations and modeling, we still
do not have a complete understanding of the relevant physical processes, nor do
we have a quantitatively precise census of which coronal structures contribute
to specific types of solar wind. Fast streams are known to be connected to the
central regions of large coronal holes. Slow streams, however, appear to come
from a wide range of sources, including streamers, pseudostreamers, coronal
loops, active regions, and coronal hole boundaries. Complicating our
understanding even more is the fact that processes such as turbulence,
stream-stream interactions, and Coulomb collisions can make it difficult to
unambiguously map a parcel measured at 1 AU back down to its coronal source. We
also review recent progress -- in theoretical modeling, observational data
analysis, and forecasting techniques that sit at the interface between data and
theory -- that gives us hope that the above problems are indeed solvable.Comment: Accepted for publication in Space Science Reviews. Special issue
connected with a 2016 ISSI workshop on "The Scientific Foundations of Space
Weather." 44 pages, 9 figure
Hadron Production in Diffractive Deep-Inelastic Scattering
Characteristics of hadron production in diffractive deep-inelastic
positron-proton scattering are studied using data collected in 1994 by the H1
experiment at HERA. The following distributions are measured in the
centre-of-mass frame of the photon dissociation system: the hadronic energy
flow, the Feynman-x (x_F) variable for charged particles, the squared
transverse momentum of charged particles (p_T^{*2}), and the mean p_T^{*2} as a
function of x_F. These distributions are compared with results in the gamma^* p
centre-of-mass frame from inclusive deep-inelastic scattering in the
fixed-target experiment EMC, and also with the predictions of several Monte
Carlo calculations. The data are consistent with a picture in which the
partonic structure of the diffractive exchange is dominated at low Q^2 by hard
gluons.Comment: 16 pages, 6 figures, submitted to Phys. Lett.
Energy Flow in the Hadronic Final State of Diffractive and Non-Diffractive Deep-Inelastic Scattering at HERA
An investigation of the hadronic final state in diffractive and
non--diffractive deep--inelastic electron--proton scattering at HERA is
presented, where diffractive data are selected experimentally by demanding a
large gap in pseudo --rapidity around the proton remnant direction. The
transverse energy flow in the hadronic final state is evaluated using a set of
estimators which quantify topological properties. Using available Monte Carlo
QCD calculations, it is demonstrated that the final state in diffractive DIS
exhibits the features expected if the interaction is interpreted as the
scattering of an electron off a current quark with associated effects of
perturbative QCD. A model in which deep--inelastic diffraction is taken to be
the exchange of a pomeron with partonic structure is found to reproduce the
measurements well. Models for deep--inelastic scattering, in which a
sizeable diffractive contribution is present because of non--perturbative
effects in the production of the hadronic final state, reproduce the general
tendencies of the data but in all give a worse description.Comment: 22 pages, latex, 6 Figures appended as uuencoded fil
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