905 research outputs found
Relationships between log N-log S and celestial distribution of gamma-ray bursts
The apparent conflict between log N-log S curve and isotropic celestial distribution of the gamma ray bursts is discussed. A possible selection effect due to the time profile of each burst is examined. It is shown that the contradiction is due to this selection effect of the gamma ray bursts
Nuclear Tetrahedral Symmetry: Possibly Present Throughout the Periodic Table
More than half a century after the fundamental, spherical shell structure in
nuclei has been established, theoretical predictions indicate that the
shell-gaps comparable or even stronger than those at spherical shapes may
exist. Group-theoretical analysis supported by realistic mean-field
calculations indicate that the corresponding nuclei are characterized by the
('double-tetrahedral') group of symmetry, exact or approximate. The
corresponding strong shell-gap structure is markedly enhanced by the existence
of the 4-dimensional irreducible representations of the group in question and
consequently it can be seen as a geometrical effect that does not depend on a
particular realization of the mean-field. Possibilities of discovering the
corresponding symmetry in experiment are discussed.Comment: 4 pages in LaTeX and 4 figures in eps forma
Thermoelectric power quantum oscillations in the ferromagnet UGe
We present thermoelectric power and resistivity measurements in the
ferromagnet UGe as a function of temperature and magnetic field. At low
temperature, huge quantum oscillations are observed in the thermoelectric power
as a function of the magnetic field applied along the axis. The frequencies
of the extreme orbits are determined and an analysis of the cyclotron masses is
performed following different theoretical approaches for quantum oscillations
detected in the thermoelectric power. They are compared to those obtained by
Shubnikov-de Haas experiments on the same crystal and previous de Haas-van
Alphen experiments. The agreement of the different probes confirms
thermoelectric power as an excellent probe to extract simultaneously both
microscopic and macroscopic information on the Fermi-surface properties.
Band-structure calculations of UGe in the ferromagnetic state are compared
to the experiment.Comment: 10 figures, 12 pages, accepted for publication in Phys. Rev.
Lymph node removal enhances corneal graft survival in mice at high risk of rejection
Peer reviewedPublisher PD
Quasiparticle Relaxation Across a Spin Gap in the Itinerant Antiferromagnet UNiGa5
Ultrafast time-resolved photoinduced reflectivity is measured for the
itinerant antiferromagnet UNiGa (85 K) from room
temperature to 10 K. The relaxation time shows a sharp increase at
consistent with the opening of a spin gap. In addition, the temperature
dependence of below is consistent with the opening of a spin gap
leading to a quasiparticle recombination bottleneck as revealed by the
Rothwarf-Taylor model. This contrasts with canonical heavy fermions such as
CeCoIn where the recombination bottleneck arises from the hybridization
gap.Comment: 5 pages, 5 figure
Pressure dependence of the magnetization in the ferromagnetic superconductor UGe_2
The recent discovery that superconductivity occurs in several clean itinerant
ferromagnets close to low temperature magnetic instabilities naturally invites
an interpretation based on a proximity to quantum criticality. Here we report
measurements of the pressure dependence of the low temperature magnetisation in
one of these materials, UGe_2. Our results show that both of the magnetic
transitions observed in this material as a function of pressure are first order
transitions and do not therefore correspond to quantum critical points. Further
we find that the known pressure dependence of the superconducting transition is
not reflected in the pressure dependence of the static susceptibility. This
demonstrates that the spectrum of excitations giving superconductivity is not
that normally associated with a proximity to quantum criticality in weak
itinerant ferromagnets. In contrast our data suggest that instead the pairing
spectrum might be related to a sharp spike in the electronic density of states
that also drives one of the magnetic transitions.Comment: to appear in Phys. Rev. Let
Point symmetries in the Hartree-Fock approach: Symmetry-breaking schemes
We analyze breaking of symmetries that belong to the double point group
D2h(TD) (three mutually perpendicular symmetry axes of the second order,
inversion, and time reversal). Subgroup structure of the D2h(TD) group
indicates that there can be as much as 28 physically different, broken-symmetry
mean-field schemes --- starting with solutions obeying all the symmetries of
the D2h(TD) group, through 26 generic schemes in which only a non-trivial
subgroup of D2h(TD) is conserved, down to solutions that break all of the
D2h(TD) symmetries. Choices of single-particle bases and the corresponding
structures of single-particle hermitian operators are discussed for several
subgroups of D2h(TD).Comment: 10 RevTeX pages, companion paper in nucl-th/991207
Itinerant U 5f band states in the layered compound UFeGa5 observed by soft X-ray angle-resolved photoemission spectroscopy
We have performed angle-resolved photoemission spectroscopy (ARPES)
experiments on paramagnetic UFeGa5 using soft X-ray synchrotron radiation
(hn=500eV) and derived the bulk- and U 5f-sensitive electronic structure of
UFeGa5. Although the agreement between the experimental band structure and the
LDA calculation treating U 5f electrons as being itinerant is qualitative, the
morphology of the Fermi surface is well explained by the calculation,
suggesting that the U 5f states can be essentially understood within the
itinerant-electron model.Comment: 13 pages, 4 figur
Nonaxial-octupole effect in superheavy nuclei
The triaxial-octupole Y correlation in atomic nuclei has long been
expected to exist but experimental evidence has not been clear. We find, in
order to explain the very low-lying 2 bands in the transfermium mass
region, that this exotic effect may manifest itself in superheavy elements.
Favorable conditions for producing triaxial-octupole correlations are shown to
be present in the deformed single-particle spectrum, which is further supported
by quantitative Reflection Asymmetric Shell Model calculations. It is predicted
that the strong nonaxial-octupole effect may persist up to the element 108. Our
result thus represents the first concrete example of spontaneous breaking of
both axial and reflection symmetries in the heaviest nuclear systems.Comment: 6 pages, 3 figures, accepted for publication in Phys. Rev.
Nature of magnetic coupling between Mn ions in as-grown GaMnAs studied by x-ray magnetic circular dichroism
The magnetic properties of as-grown GaMnAs have been
investigated by the systematic measurements of temperature and magnetic field
dependent soft x-ray magnetic circular dichroism (XMCD). The {\it intrinsic}
XMCD intensity at high temperatures obeys the Curie-Weiss law, but residual
spin magnetic moment appears already around 100 K, significantly above Curie
temperature (), suggesting that short-range ferromagnetic correlations are
developed above . The present results also suggest that antiferromagnetic
interaction between the substitutional and interstitial Mn (Mn) ions
exists and that the amount of the Mn affects .Comment: 4 pages, 4 figure
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