162 research outputs found
Similar glassy features in the NMR response of pure and disordered La1.88Sr0.12CuO4
High Tc superconductivity in La2-xSrxCuO4 coexists with (striped and glassy)
magnetic order. Here, we report NMR measurements of the 139La spin-lattice
relaxation, which displays a stretched-exponential time dependence, in both
pure and disordered x=0.12 single crystals. An analysis in terms of a
distribution of relaxation rates T1^-1 indicates that i) the spin-freezing
temperature is spatially inhomogeneous with an onset at Tg(onset)=20 K for the
pristine samples, and ii) the width of the T1^-1 distribution in the vicinity
of Tg(onset) is insensitive to an ~1% level of atomic disorder in CuO2 planes.
This suggests that the stretched-exponential 139La relaxation, considered as a
manifestation of the systems glassiness, may not arise from quenched disorder.Comment: 7 pages, to be published in Phys. Rev.
Changes in Optical Conductivity due to Readjustments in Electronic Density of States
Within the model of elastic impurity scattering, we study how changes in the
energy dependence of the electronic density of states (EDOS)
around the Fermi energy are reflected in the frequency-dependent
optical conductivity . While conserving the total number of
states in we compute the induced changes in as a
function of and in the corresponding optical scattering rate
. These quantities mirror some aspects of the EDOS
changes but the relationship is not direct. Conservation of optical oscillator
strength is found not to hold, and there is no sum rule on the optical
scattering rate although one does hold for the quasiparticle scattering.
Temperature as well as increases in impurity scattering lead to additional
changes in optical properties not seen in the constant EDOS case. These effects
have their origin in an averaging of the EDOS around the Fermi energy
on an energy scale set by the impurity scattering.Comment: 13 pages, 7 figure
Rat duodenal motility in vitro: Prokinetic effects of DL-homocysteine thiolactone and modulation of nitric oxide mediated inhibition
Homocysteine is a significant but modifiable risk factor for vascular
diseases. As gastrointestinal smooth musculature is similar to blood vessel
muscles, we investigated how elevated homocysteine levels affect nitric
oxide-mediated neurotransmission in the gut. There is accumulated evidence
that a dysfunction of NO neurons in the myenteric plexus may cause various
diseases in the gastrointestinal tract such as achalasia, diabetic
gastroparesis and infantile hypertrophic pyloric stenosis. In the present
study, we aimed to assess the effects of homocysteine on NO-mediated
responses in vitro, and to examine the effects of DL-homocysteine thiolactone
on the spontaneous motility of rat duodenum and nitrergic neurotransmission.
DL-homocysteine thiolactone concentration of 10 μmol/L leads to the immediate
increase in tone, amplitude and frequency of spontaneous movements in
isolated rat duodenum. L-NAME (30 μmol/L) leads to an increase in basal tone,
amplitude and frequency of spontaneous contractions. The relaxations induced
by EFS were significantly reduced in duodenal segments incubated in
DL-homocysteine thiolactone compared with the control group. EFS-induced
relaxations were inhibited by L-NAME in both experimental and control groups.
These results suggest that a high level of homocysteine causes an important
impairment of non-adrenergic non-cholinergic innervation of the rat duodenum.
[Projekat Ministarstva nauke Republike Srbije, br. 175043
Spin glass behavior of frustrated 2-D Penrose lattice in the classical planar model
Via extensive Monte Carlo studies we show that the frustrated XY Hamiltonian
on a 2-D Penrose lattice admits of a spin glass phase at low temperature.
Studies of the Edwards-Anderson order parameter, spin glass susceptibility, and
local (linear) susceptibility point unequivocally to a paramagnetic to spin
glass transition as the temperature is lowered. Specific heat shows a rounded
peak at a temperature above the spin glass transition temperature, as is
commonly observed in spin glasses. Our results strongly suggest that the
critical point exponents are the same as obtained by Bhatt and Young in the
Ising model on a square lattice. However, unlike in the latter case,
the critical temperature is clearly finite (nonzero). The results imply that a
quasiperiodic 2-D array of superconducting grains in a suitably chosen
transverse magnetic field should behave as a superconducting glass at low
temperature.Comment: RevTex, 4 pages Including 4 figures. To appear in the June 1 1996
issue of Phys. Rev. B (Rapid Communications). Revised/replaced edition
contains an erratum at the end of the paper, also to appear in Phys. Rev.
Colossal Positive Magnetoresistance in a Doped Nearly Magnetic Semiconductor
We report on a positive colossal magnetoresistance (MR) induced by
metallization of FeSb, a nearly magnetic or "Kondo" semiconductor with 3d
ions. We discuss contribution of orbital MR and quantum interference to
enhanced magnetic field response of electrical resistivity.Comment: 5 pages, 5 figure
Two-dimensional Vortices in Superconductors
Superconductors have two key characteristics. They expel magnetic field and
they conduct electrical current with zero resistance. However, both properties
are compromised in high magnetic fields which can penetrate the material and
create a mixed state of quantized vortices. The vortices move in response to an
electrical current dissipating energy which destroys the zero resistance
state\cite{And64}. One of the central problems for applications of high
temperature superconductivity is the stabilization of vortices to ensure zero
electrical resistance. We find that vortices in the anisotropic superconductor
BiSrCaCuO (Bi-2212) have a phase transition from
a liquid state, which is inherently unstable, to a two-dimensional vortex
solid. We show that at high field the transition temperature is independent of
magnetic field, as was predicted theoretically for the melting of an ideal
two-dimensional vortex lattice\cite{Fis80,Gla91}. Our results indicate that the
stable solid phase can be reached at any field as may be necessary for
applications involving superconducting magnets\cite{Has04,Sca04,COHMAG}. The
vortex solid is disordered, as suggested by previous studies at lower
fields\cite{Lee93,Cub93}. But its evolution with increasing magnetic field
displays unexpected threshold behavior that needs further investigation.Comment: 5 pages and 4 figures. submitted to Nature Physic
Quasiparticle structure in antiferromagnetism around the vortex and nuclear magnetic relaxation time
On the basis of the Bogoliubov-de Gennes theory for the two-dimensional
extended Hubbard model, the vortex structure in d-wave superconductors is
investigated including the contribution of the induced incommensurate
antiferromagnetism around the vortex core. As the on-site repulsive interaction
increases, the spatial structure of charge and spin changes from the
antiferromagnetic state with checkerboard modulation to that with the stripe
modulation. By the effect of the induced antiferromagnetic moment, the
zero-energy density of states is suppressed, and the vortex core radius
increases. We also study the effect of the local density of states (LDOS)
change on the site-dependent nuclear relaxation rate . These
results are compared with a variety of experiments performed on high
cuprates.Comment: 10pages, 8 figure
Expansion of Vortex Cores by Strong Electronic Correlation in LaSrCuO at Low Magnetic Induction
The vortex core radius \rv, defined as the peak position of the supercurrent
around the vortex, has been determined by muon spin rotation measurements in
the mixed state of \lscox for , 0.15, and 0.19. At lower doping (x=0.13
and 0.15), \rv(T) increases with decreasing temperature T, which is opposite to
the behavior predicted by the conventional theory. Moreover, \rv(T\to0) is
significantly larger than the Ginsburg-Landau coherence length determined by
the upper critical field, and shows a clear tendency to decrease with
increasing the doping x. These features can be qualitatively reproduced in a
microscopic model involving antiferromagnetic electronic correlations.Comment: 6 pages, 4 figures, to be published in Phys. Rev.
Collective emotions online and their influence on community life
E-communities, social groups interacting online, have recently become an
object of interdisciplinary research. As with face-to-face meetings, Internet
exchanges may not only include factual information but also emotional
information - how participants feel about the subject discussed or other group
members. Emotions are known to be important in affecting interaction partners
in offline communication in many ways. Could emotions in Internet exchanges
affect others and systematically influence quantitative and qualitative aspects
of the trajectory of e-communities? The development of automatic sentiment
analysis has made large scale emotion detection and analysis possible using
text messages collected from the web. It is not clear if emotions in
e-communities primarily derive from individual group members' personalities or
if they result from intra-group interactions, and whether they influence group
activities. We show the collective character of affective phenomena on a large
scale as observed in 4 million posts downloaded from Blogs, Digg and BBC
forums. To test whether the emotions of a community member may influence the
emotions of others, posts were grouped into clusters of messages with similar
emotional valences. The frequency of long clusters was much higher than it
would be if emotions occurred at random. Distributions for cluster lengths can
be explained by preferential processes because conditional probabilities for
consecutive messages grow as a power law with cluster length. For BBC forum
threads, average discussion lengths were higher for larger values of absolute
average emotional valence in the first ten comments and the average amount of
emotion in messages fell during discussions. Our results prove that collective
emotional states can be created and modulated via Internet communication and
that emotional expressiveness is the fuel that sustains some e-communities.Comment: 23 pages including Supporting Information, accepted to PLoS ON
Field-Induced Uniform Antiferromagnetic Order Associated with Superconductivity in PrLaCeCuO
Strong correlation between field-induced antiferromagnetic (AF) order and
superconductivity is demonstrated for an electron-doped cuprate superconductor,
PrLaCeCuO (PLCCO). In addition to the specimen with
(which is close to the AF phase boundary, ), we show that
the one with ( K at zero field) also exhibits the
field-induced AF order with a reduced magnitude of the induced moment. The
uniform muon Knight shift at a low magnetic field ( Oe) indicates
that the AF order is not localized within the cores of flux lines, which is in
a marked contrast with theoretical prediction for hole-doped cuprates. The
presence of anomalous non-diagonal hyperfine coupling between muons and Pr ions
is also demonstrated in detail.Comment: 8 pages, 5 figures, to be published in J. Phys. Soc. Jp
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