611 research outputs found
Lattice effects on the current-voltage characteristics of superconducting arrays
The lattice effects on the current-voltage characteristics of two-dimensional
arrays of resistively shunted Josephson junctions are investigated. The lattice
potential energies due to the discrete lattice structure are calculated for
several geometries and directions of current injection. We compare the energy
barrier for vortex-pair unbinding with the lattice pinning potential, which
shows that lattice effects are negligible in the low-current limit as well as
in the high-current limit. At intermediate currents, on the other hand, the
lattice potential becomes comparable to the barrier height and the lattice
effects may be observed in the current-voltage characteristics.Comment: 5 pages including 5 figures in two columns, to appear in Phys. Rev.
Exact Calculation of the Vortex-Antivortex Interaction Energy in the Anisotropic 3D XY-model
We have developed an exact method to calculate the vortex-antivortex
interaction energy in the anisotropic 3D-XY model. For this calculation, dual
transformation which is already known for the 2D XY-model was extended. We
found an explicit form of this interaction energy as a function of the
anisotropic ratio and the separation between the vortex and antivortex
located on the same layer. The form of interaction energy is at the
small limi t but is proportional to at the opposite limit. This form of
interaction energ y is consistent with the upper bound calculation using the
variational method by Cataudella and Minnhagen.Comment: REVTeX 12 pages, In print for publication in Phys. Rev.
Influence of uncorrelated overlayers on the magnetism in thin itinerant-electron films
The influence of uncorrelated (nonmagnetic) overlayers on the magnetic
properties of thin itinerant-electron films is investigated within the
single-band Hubbard model. The Coulomb correlation between the electrons in the
ferromagnetic layers is treated by using the spectral density approach (SDA).
It is found that the presence of nonmagnetic layers has a strong effect on the
magnetic properties of thin films. The Curie temperatures of very thin films
are modified by the uncorrelated overlayers. The quasiparticle density of
states is used to analyze the results. In addition, the coupling between the
ferromagnetic layers and the nonmagnetic layers is discussed in detail. The
coupling depends on the band occupation of the nonmagnetic layers, while it is
almost independent of the number of the nonmagnetic layers. The induced
polarization in the nonmagnetic layers shows a long-range decreasing
oscillatory behavior and it depends on the coupling between ferromagnetic and
nonmagnetic layers.Comment: 9 pages, RevTex, 6 figures, for related work see:
http://orion.physik.hu-berlin.d
Hadronic observables from SIS to SPS energies - anything strange with strangeness ?
We calculate and (+) rapidity
distributions and compare to experimental data from SIS to SPS energies within
the UrQMD and HSD transport approaches that are both based on string, quark,
diquark () and hadronic degrees of freedom. The
two transport models do not include any explicit phase transition to a
quark-gluon plasma (QGP). It is found that both approaches agree rather well
with each other and with the experimental rapidity distributions for protons,
's, and . Inspite of this apparent agreement both
transport models fail to reproduce the maximum in the excitation function for
the ratio found experimentally between 11 and 40 AGeV. A
comparison to the various experimental data shows that this 'failure' is
dominantly due to an insufficient description of pion rapidity distributions
rather than missing 'strangeness'. The modest differences in the transport
model results -- on the other hand -- can be attributed to different
implementations of string formation and fragmentation, that are not
sufficiently controlled by experimental data for the 'elementary' reactions in
vacuum.Comment: 46 pages, including 15 eps figures, to be published in Phys. Rev.
Thermal excitations of frustated XY spins in two dimensions
We present a new variational approach to the study of phase transitions in
frustrated 2D XY models. In the spirit of Villain's approach for the
ferromagnetic case we divide thermal excitations into a low temperature long
wavelength part (LW) and a high temperature short wavelength part (SW). In the
present work we mainly deal with LW excitations and we explicitly consider the
cases of the fully frustrated triangular (FFTXY) and square ( FFSQXY) XY
models. The novel aspect of our method is that it preserves the coupling
between phase (spin angles) and chiral degrees of freedom. LW fluctuations
consist of coupled phase and chiral excitations. As a result, we find that for
frustrated systems the effective interactions between phase variables is long
range and oscillatory in contrast to the unfrustrated problem. Using Monte
Carlo (MC) simulations we show that our analytical calculations produce
accurate results at all temperature ; this is seen at low in the spin
wave stiffness constant and in the staggered chirality; this is also the case
near : transitions are driven by the SW part associated with domain walls
and vortices, but the coupling between phase and chiral variables is still
relevant in the critical region. In that regime our analytical results yield
the correct dependence for bare couplings (given by the LW fluctuations)
such as the Coulomb gas temperature of the frustrated XY models . In
particular we find that tracks chiral rather than phase fluctuations.
Our results provides support for a single phase transition scenario in the
FFTXY and FFSQXY models.Comment: 32 pages, RevTex, 11 eps figures available upon request, article to
appear in Phys. Rev.
The Role of Color Neutrality in Nuclear Physics--Modifications of Nucleonic Wave Functions
The influence of the nuclear medium upon the internal structure of a
composite nucleon is examined. The interaction with the medium is assumed to
depend on the relative distances between the quarks in the nucleon consistent
with the notion of color neutrality, and to be proportional to the nucleon
density. In the resulting description the nucleon in matter is a superposition
of the ground state (free nucleon) and radial excitations. The effects of the
nuclear medium on the electromagnetic and weak nucleon form factors, and the
nucleon structure function are computed using a light-front constituent quark
model. Further experimental consequences are examined by considering the
electromagnetic nuclear response functions. The effects of color neutrality
supply small but significant corrections to predictions of observables.Comment: 37 pages, postscript figures available on request to
[email protected]
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
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