77,212 research outputs found
A Model of Low-lying States in Strongly Interacting Electroweak Symmetry-Breaking Sector
It is proposed that, in a strongly-interacting electroweak sector, besides
the Goldstone bosons, the coexistence of a scalar state () and vector
resonances such as [)], [] and
[] is required by the proper Regge behavior of the
forward scattering amplitudes. This is a consequence of the following
well-motivated assumptions: (a). Adler-Weisberger-type sum rules and the
superconvergence relations for scattering amplitudes hold in this strongly
interacting sector; (b). the sum rules at are saturated by a minimal set
of low-lying states with appropriate quantum numbers. It therefore suggests
that a complete description should include all these resonances. These states
may lead to distinctive experimental signatures at future colliders.Comment: revised version, to appear in Modern Physics Letters A; file also
available via anonymous ftp at ftp://ucdhep.ucdavis.edu/han/sews/lowlying.p
A Generic Dynamical Model of Gamma-ray Burst Remnants
The conventional generic model is deemed to explain the dynamics of
-ray burst remnants very well, no matter whether they are adiabatic or
highly radiative. However, we find that for adiabatic expansion, the model
could not reproduce the Sedov solution in the non-relativistic phase, thus the
model needs to be revised. In the present paper, a new differential equation is
derived. The generic model based on this equation has been shown to be correct
for both radiative and adiabatic fireballs, and in both ultra-relativistic and
non-relativistic phase.Comment: 10 pages, LaTeX, 4 postscript figures, accepted for publication in
MNRA
Gamma-ray bursts: postburst evolution of fireballs
The postburst evolution of fireballs that produce -ray bursts is
studied, assuming the expansion of fireballs to be adiabatic and relativistic.
Numerical results as well as an approximate analytic solution for the evolution
are presented. Due to adoption of a new relation among , and
(see the text), our results differ markedly from the previous studies.
Synchrotron radiation from the shocked interstellar medium is attentively
calculated, using a convenient set of equations. The observed X-ray flux of GRB
afterglows can be reproduced easily. Although the optical afterglows seem much
more complicated, our results can still present a rather satisfactory approach
to observations. It is also found that the expansion will no longer be highly
relativistic about 4 days after the main GRB. We thus suggest that the
marginally relativistic phase of the expansion should be investigated so as to
check the afterglows observed a week or more later.Comment: 17 pages, 4 figures, MNRAS in pres
Optical Flashes and Very Early Afterglows in Wind Environments
The interaction of a relativistic fireball with its ambient medium is
described through two shocks: a reverse shock that propagates into the
fireball, and a forward shock that propagates into the medium. The observed
optical flash of GRB 990123 has been considered to be the emission from such a
reverse shock. The observational properties of afterglows suggest that the
progenitors of some GRBs may be massive stars and their surrounding media may
be stellar winds. We here study very early afterglows from the reverse and
forward shocks in winds. An optical flash mainly arises from the relativistic
reverse shock while a radio flare is produced by the forward shock. The peak
flux densities of optical flashes are larger than 1 Jy for typical parameters,
if we do not take into account some appropriate dust obscuration along the line
of sight. The radio flare always has a long lasting constant flux, which will
not be covered up by interstellar scintillation. The non-detections of optical
flashes brighter than about 9th magnitude may constrain the GRBs isotropic
energies to be no more than a few ergs and wind intensities to be
relatively weak.Comment: 21 pages, 6 figures, accepted by MNRAS on March 7, 200
Magnetoresistance in the superconducting state at the (111) LaAlO/SrTiO interface
Condensed matter systems that simultaneously exhibit superconductivity and
ferromagnetism are rare due the antagonistic relationship between conventional
spin-singlet superconductivity and ferromagnetic order. In materials in which
superconductivity and magnetic order is known to coexist (such as some
heavy-fermion materials), the superconductivity is thought to be of an
unconventional nature. Recently, the conducting gas that lives at the interface
between the perovskite band insulators LaAlO (LAO) and SrTiO (STO) has
also been shown to host both superconductivity and magnetism. Most previous
research has focused on LAO/STO samples in which the interface is in the (001)
crystal plane. Relatively little work has focused on the (111) crystal
orientation, which has hexagonal symmetry at the interface, and has been
predicted to have potentially interesting topological properties, including
unconventional superconducting pairing states. Here we report measurements of
the magnetoresistance of (111) LAO/STO heterostructures at temperatures at
which they are also superconducting. As with the (001) structures, the
magnetoresistance is hysteretic, indicating the coexistence of magnetism and
superconductivity, but in addition, we find that this magnetoresistance is
anisotropic. Such an anisotropic response is completely unexpected in the
superconducting state, and suggests that (111) LAO/STO heterostructures may
support unconventional superconductivity.Comment: 6 Pages 4 figure
Anisotropic, multi-carrier transport at the (111) LaAlO/SrTiO interface
The conducting gas that forms at the interface between LaAlO and
SrTiO has proven to be a fertile playground for a wide variety of physical
phenomena. The bulk of previous research has focused on the (001) and (110)
crystal orientations. Here we report detailed measurements of the
low-temperature electrical properties of (111) LAO/STO interface samples. We
find that the low-temperature electrical transport properties are highly
anisotropic, in that they differ significantly along two mutually orthogonal
crystal orientations at the interface. While anisotropy in the resistivity has
been reported in some (001) samples and in (110) samples, the anisotropy in the
(111) samples reported here is much stronger, and also manifests itself in the
Hall coefficient as well as the capacitance. In addition, the anisotropy is not
present at room temperature and at liquid nitrogen temperatures, but only at
liquid helium temperatures and below. The anisotropy is accentuated by exposure
to ultraviolet light, which disproportionately affects transport along one
surface crystal direction. Furthermore, analysis of the low-temperature Hall
coefficient and the capacitance as a function of back gate voltage indicates
that in addition to electrons, holes contribute to the electrical transport.Comment: 11 pages, 9 figure
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