271 research outputs found
UV (IUE) spectra of the central stars of high latitude planetary nebulae Hb7 and Sp3
We present an analysis of the UV (IUE) spectra of the central stars of Hb7
and Sp3. Comparison with the IUE spectrum of the standard star HD 93205 leads
to a spectral classification of O3V for these stars, with an effective
temperature of 50,000 K. From the P-Cygni profiles of CIV (1550 A), we derive
stellar wind velocities and mass loss rates of -1317 km/s +/- 300 km/s and
2.9X10^{-8} solar mass yr^{-1} and -1603 km/s +/- 400 km/s and 7X10^{-9} solar
mass yr^{-1} for Hb7 and Sp3 respectively. From all the available data, we
reconstruct the spectral energy distribution of Hb7 and Sp3.Comment: 4 pages, 3 figures, latex, accepted for publication in Astronomy &
Astrophysic
High-Field Quasiparticle Tunneling in Bi_2Sr_2CaCu_2O_8+delta: Negative Magnetoresistance in the Superconducting State
We report on the c-axis resistivity rho_c(H) in Bi_2Sr_2CaCu_2O_{8+\delta}
that peaks in quasi-static magnetic fields up to 60 T. By suppressing the
Josephson part of the two-channel (Cooper pair/quasiparticle) conductivity
\sigma_c (H), we find that the negative slope of \rho_c(H) above the peak is
due to quasiparticle tunneling conductivity \sigma_q(H) across the CuO_2 layers
below H_{c2}. At high fields (a) \sigma_q(H) grows linearly with H, and (b)
\rho_c(T) tends to saturate (sigma_c \neq 0) as T->0, consistent with the
scattering at the nodes of the d-gap. A superlinear sigma_q(H) marks the normal
state above T_c.Comment: 4p., 5 fig. (.eps), will be published in Phys. Rev. Let
Knots in the outer shells of the planetary nebulae IC 2553 and NGC 5882
We present images and high-resolution spectra of the planetary nebulae IC
2553 and NGC 5882. Spatio-kinematic modeling of the nebulae shows that they are
composed of a markedly elongated inner shell, and of a less aspherical outer
shell expanding at a considerably higher velocity than the inner one. Embedded
in the outer shells of both nebulae are found several low-ionization knots. In
IC 2553, the knots show a point-symmetric distribution with respect to the
central star: one possible explanation for their formation is that they are the
survivors of pre-existing point-symmetric condensations in the AGB wind, a fact
which would imply a quite peculiar mass-loss geometry from the giant
progenitor. In the case of NGC 5882, the lack of symmetry in the distribution
of the observed low-ionization structures makes it possible that they are the
result of in situ instabilities.Comment: 20 pages including 1 table and 6 figures. ApJ accepted. Also
available at http://andromeda.roque.ing.iac.es/~sanchez/ingpub/index2000.htm
Effect of Magnetic field on the Pseudogap Phenomena in High-Tc Cuprates
We theoretically investigate the effect of magnetic field on the pseudogap
phenomena in High-Tc cuprates.
The obtained results well explain the experimental results including their
doping dependences.
In our previous paper (J. Phys. Soc. Jpn. 68 (1999) 2999.), we have shown
that the pseudogap phenomena observed in High-Tc cuprates are naturally
understood as a precursor of the strong coupling superconductivity. On the
other hand, there is an interpretation for the recent high field NMR
measurements to be an evidence denying the pairing scenarios for the pseudogap.
In this paper, we investigate the magnetic field dependence of NMR
on the basis of our formalism and show the interpretation to be inappropriate.
The results indicate that the value of the characteristic magnetic field
is remarkably large in case of the strong coupling
superconductivity, especially near the pseudogap onset temperature .
Therefore, the magnetic field dependences can not be observed and does
not vary when the strong pseudogap anomaly is observed. On the other hand,
is small in the comparatively weak coupling case and
varies when the weak pseudogap phenomena are observed.
These results properly explain the high magnetic field NMR experiments
continuously from under-doped to over-doped cuprates.
Moreover, we discuss the transport phenomena in the pseudogap phase. The
behaviors of the in-plane resistivity, the Hall coefficient and the c-axis
resistivity in the pseudogap phase are naturally understood by considering the
d-wave pseudogap
Different Pattern of Immunoglobulin Gene Usage by HIV-1 Compared to Non-HIV-1 Antibodies Derived from the Same Infected Subject
A biased usage of immunoglobulin (Ig) genes is observed in human anti-HIV-1 monoclonal antibodies (mAbs) resulting probably from compensation to reduced usage of the VH3 family genes, while the other alternative suggests that this bias usage is due to antigen requirements. If the antigen structure is responsible for the preferential usage of particular Ig genes, it may have certain implications for HIV vaccine development by the targeting of particular Ig gene-encoded B cell receptors to induce neutralizing anti-HIV-1 antibodies. To address this issue, we have produced HIV-1 specific and non-HIV-1 mAbs from an infected individual and analyzed the Ig gene usage. Green-fluorescence labeled virus-like particles (VLP) expressing HIV-1 envelope (Env) proteins of JRFL and BaL and control VLPs (without Env) were used to select single B cells for the production of 68 recombinant mAbs. Ten of these mAbs were HIV-1 Env specific with neutralizing activity against V3 and the CD4 binding site, as well as non-neutralizing mAbs to gp41. The remaining 58 mAbs were non-HIV-1 Env mAbs with undefined specificities. Analysis revealed that biased usage of Ig genes was restricted only to anti-HIV-1 but not to non-HIV-1 mAbs. The VH1 family genes were dominantly used, followed by VH3, VH4, and VH5 among anti-HIV-1 mAbs, while non-HIV-1 specific mAbs preferentially used VH3 family genes, followed by VH4, VH1 and VH5 families in a pattern identical to Abs derived from healthy individuals. This observation suggests that the biased usage of Ig genes by anti-HIV-1 mAbs is driven by structural requirements of the virus antigens rather than by compensation to any depletion of VH3 B cells due to autoreactive mechanisms, according to the gp120 superantigen hypothesis
Superconducting Fluctuation and Pseudogap in Disordered Short Coherence Length Superconductor
We investigate the role of disorder on the superconducting (SC) fluctuation
in short coherence length d-wave superconductors. The particular intetest is
focused on the disorder-induced microscopic inhomogeneity of SC fluctuation and
its effect on the pseudogap phenomena. We formulate the self-consistent 1-loop
order theory for the SC fluctuation in inhomogeneous systems and analyze the
disordered -- model. The SC correlation function, electronic DOS and
the critical temperature are estimated. The SC fluctuation is localized like a
nanoscale granular structure when the coherence length is short, namely the
transition temperature is high. This is contrasted to the long coherence length
superconductors where the order parameter is almost uniform in the microscopic
scale. In the former case, the SC fluctuation is enhanced by the disorder in
contrast to the Abrikosov-Gorkov theory. These results are consistent with the
STM, NMR and transport measurements in high- cuprates and illuminate
the essential role of the microscopic inhomogeneity. We calculate the spacial
dependence of DOS around the single impurity and discuss the consistency with
the NMR measurements
Superconducting Fluctuations and the Pseudogap in the Slightly-overdoped High-Tc Superconductor TlSr2CaCu2O6.8: High Magnetic Field NMR Studies
From measurements of the ^{63}Cu Knight shift (K) and the nuclear
spin-lattice relaxation rate (1/T_{1}) under magnetic fields from zero up to 28
T in the slightly overdoped superconductor TlSr_{2}CaCu_{2}O_{6.8} (T_{c}=68
K), we find that the pseudogap behavior, {\em i.e.}, the reductions of 1/T_{1}T
and K above T_{c} from the values expected from the normal state at high T, is
strongly field dependent and follows a scaling relation. We show that this
scaling is consistent with the effects of the Cooper pair density fluctuations.
The present finding contrasts sharply with the pseudogap property reported
previously in the underdoped regime where no field effect was seen up to 23.2
T. The implications are discussed.Comment: 10 pages, 4 GIF figures, to be published in Phys. Rev. Let
Theory of vortex excitation imaging via an NMR relaxation measurement
The temperature dependence of the site-dependent nuclear spin relaxation time
T_1 around vortices is studied in s-wave and d-wave superconductors.Reflecting
low energy electronic excitations associated with the vortex core, temperature
dependences deviate from those of the zero-field case, and T_1 becomes faster
with approaching the vortex core. In the core region, T_1^{-1} has a new peak
below T_c. The NMR study by the resonance field dependence may be a new method
to prove the spatial resolved vortex core structure in various superconductors.Comment: 5 pages, 3 figure
Superconducting phase coherence in the presence of a pseudogap: Relation to specific heat, tunneling and vortex core spectroscopies
In this paper we demonstrate how, using a natural generalization of BCS
theory, superconducting phase coherence manifests itself in phase insensitive
measurements, when there is a smooth evolution of the excitation gap \Delta
from above to below Tc. In this context, we address the underdoped cuprates.
Our premise is that just as Fermi liquid theory is failing above Tc, BCS theory
is failing below. The order parameter \Delta_{sc} is different from the
excitation gap \Delta. Equivalently there is a (pseudo)gap in the excitation
spectrum above Tc which is also present in the underlying normal state of the
superconducting phase, and can be directly inferred from specific heat and
vortex core experiments. At the same time many features of BCS theory, e.g.,
fermionic quasiparticles below Tc, are clearly present. These observations can
be reconciled by a natural extension of BCS theory, which includes finite
center-of-mass momentum pair excitations, in addition to the usual fermionic
quasiparticles. Applying this theory we find that the Bose condensation of
Cooper pairs, which is reflected in \Delta_{sc}, leads to sharp peaks in the
spectral function once . These are manifested in ARPES spectra as
well as in specific heat jumps, which become more like the behavior in a
\lambda transition as the pseudogap develops. We end with a discussion of
tunneling experiments and condensation energy issues. Comparison between
theoretical and experimental plots of C_v, and of tunneling and vortex core
spectroscopy measurements is good.Comment: 12 pages, 8 figures, ReVTeX 3.
Magnetic Field Effects in the Pseudogap Phase: A Competing Energy Gap Scenario for Precursor Superconductivity
We study the sensitivity of T_c and T^* to low fields, H, within the
pseudogap state using a BCS-based approach extended to arbitrary coupling. We
find that T^* and T_c, which are of the same superconducting origin, have very
different H dependences. This is due to the pseudogap, \Delta_{pg}, which is
present at the latter, but not former temperature. Our results for the
coherence length \xi fit well with existing experiments.We predict that very
near the insulator \xi will rapidly increase.Comment: 4 pages, 4 figures, RevTe
- …