4,724 research outputs found
Probing large distance higher dimensional gravity from lensing data
The modifications induced in the standard weak-lensing formula if Newtonian
gravity differs from inverse square law at large distances are studied. The
possibility of putting bounds on the mass of gravitons from lensing data is
explored. A bound on graviton mass, esitmated to be about 100 Mpc is
obtained from analysis of some recent data on gravitational lensing.Comment: 6 pages, 1 figure, added reference
Microscopic origin of local moments in a zinc-doped high- superconductor
The formation of a local moment around a zinc impurity in the high-
cuprate superconductors is studied within the framework of the bosonic
resonating-valence-bond (RVB) description of the model. A topological
origin of the local moment has been shown based on the phase string effect in
the bosonic RVB theory. It is found that such an moment distributes
near the zinc in a form of staggered magnetic moments at the copper sites. The
corresponding magnetic properties, including NMR spin relaxation rate, uniform
spin susceptibility, and dynamic spin susceptibility, etc., calculated based on
the theory, are consistent with the experimental measurements. Our work
suggests that the zinc substitution in the cuprates provide an important
experimental evidence for the RVB nature of local physics in the original (zinc
free) state.Comment: The topological reason of local moment formation is given. One figure
is adde
Effect of Nonmagnetic Impurity in Nearly Antiferromagnetic Fermi Liquid: Magnetic Correlations and Transport Phenomena
In nearly antiferromagnetic (AF) metals such as high-Tc superconductors
(HTSC's), a single nonmagnetic impurity frequently causes nontrivial widespread
change of the electronic states. To elucidate this long-standing issue, we
study a Hubbard model with a strong onsite impurity potential based on an
improved fluctuation-exchange (FLEX) approximation, which we call the GV^I-FLEX
method. This model corresponds to the HTSC with dilute nonmagnetic impurity
concentration. We find that (i) both local and staggered susceptibilities are
strongly enhanced around the impurity. By this reason, (ii) the quasiparticle
lifetime as well as the local density of states (DOS) are strongly suppressed
in a wide area around the impurity (like a Swiss cheese hole), which causes the
``huge residual resistivity'' beyond the s-wave unitary scattering limit. We
stress that the excess quasiparticle damping rate caused by impurities has
strong momentum-dependence due to non-s-wave scatterings induced by many-body
effects, so the structure of the ``hot spot/cold spot'' in the host system
persists against impurity doping. This result could be examined by the ARPES
measurements. In addition, (iii) only a few percent of impurities can causes a
``Kondo-like'' upturn of resistivity () at low temperatures when
the system is very close to the AF quantum critical point (QCP). The results
(i)-(iii) obtained in the present study, which cannot be derived by the simple
FLEX approximation, naturally explains the main impurity effects in HTSC's. We
also discuss the impurity effect in heavy fermion systems and organic
superconductors.Comment: 22 pages, to be published in PR
Electronic structure of spin 1/2 Heisenberg antiferromagnetic systems: Ba_2Cu(PO_4)_2 and Sr_2Cu(PO_4)_2
We have employed first principles calculations to study the electronic
structure and magnetic properties of the low-dimensional phosphates,
Ba2Cu(PO4)2 and Sr2Cu(PO4)2. Using the self-consistent tight-binding lin-
earized muffin-tin orbital method and the Nth order muffin-tin orbital method,
we have calculated the various intrachain as well as the interchain hopping
parameters between the magnetic ions Cu2+ for both the com- pounds. We find
that the nearest-neighbor intrachain hopping t is the dominant interaction
suggesting the compounds to be indeed one dimensional. Our analysis of the band
dispersion, orbital projected band struc- tures, and the hopping parameters
confirms that the Cu2+-Cu2+ super-super exchange interaction takes place along
the crystallographic b direction mediated by O-P-O. We have also analyzed in
detail the origin of short-range exchange interaction for these systems. Our ab
initio estimate of the ratio of the exchange inter- action of Sr2Cu(PO4)2 to
that of Ba2Cu(PO4)2 compares excellently with available experimental results.Comment: 6 pages, 4 figure
Spin-gap behaviour in the 2-leg spin-ladder BiCu2PO6
We present magnetic suscceptibility and heat capacity data on a new S=1/2
two-leg spin ladder compound BiCu2PO6. From our susceptibility analysis, we
find that the leg coupling J1/k_B is ~ 80 K and the ratio of the rung to leg
coupling J2/J1 ~ 0.9. We present the magnetic contribution to the heat capacity
of a two-leg ladder. The spin-gap Delta/k_B =3 4 K obtained from the heat
capacity agrees very well with that obtained from the magnetic susceptibility.
Significant inter-ladder coupling is suggested from the susceptibility
analysis. The hopping integrals determined using Nth order muffin tin orbital
(NMTO) based downfolding method lead to ratios of various exchange couplings in
agreement with our experimental data. Based on our band structure analysis, we
find the inter-ladder coupling in the bc-plane J2 to be about 0.75J1 placing
the compound presumably close to the quantum critical limit.Comment: 8 pages, 5 figure
Erythropoetin receptor expression in the human diabetic retina
<p>Abstract</p> <p>Background</p> <p>Recent evidence suggests erythropoietin (<it>EPO</it>) and the erythropoietin receptor (<it>EPOR</it>) may play a direct role in the pathogenesis of diabetic retinopathy. Better characterization of the <it>EPO-EPOR </it>signaling system in the ischemic retina may offer a new therapeutic modality for ischemic ophthalmic diseases. This study was performed to identify <it>EPOR </it>mRNA expression in the human diabetic eye.</p> <p>Findings</p> <p><it>EPOR </it>antisense RNA probes were validated on human pancreas tissue. In the normal eye, <it>EPOR </it>was expressed in the retinal ganglion cell layer. Minimal expression was observed in the inner and outer nuclear layer. Under conditions of diabetic retinopathy, <it>EPOR </it>expression shifted to photoreceptor cells. Increased expression was also observed in the peripheral retina.</p> <p>Conclusion</p> <p><it>EPOR </it>expression may be a biomarker or contribute to disease mechanisms in diabetic retinopathy.</p
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