4,036 research outputs found
Synthesis and Characterizations of Strontium Cerium Oxide Phosphor
A new Strontium Cerium Oxide (Sr4Ce2O7) blue phosphor with, orthorhombic structure was synthesized via standard solid state reaction method using strontium oxide and cerium oxide as a raw materials. The samples were characterized by Thermo analytical techniques (TG, DTA, and DTG), Fourier transformation infrared (FTIR) spectroscopy, Raman spectroscopy, and Photoluminescence studies at room temperature. In excitation spectra two excitation bands were located at 361 and 391 nm respectively.   The emission spectrum was a broad band peaking at 474 nm, which was suitable for the doping of rare earth ions
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
Dark Energy and the Statistical Study of the Observed Image Separations of the Multiply Imaged Systems in the CLASS Statistical Sample
The present day observations favour a universe which is flat, accelerated and
composed of matter (baryonic + dark) and of a negative
pressure component, usually referred to as dark energy or quintessence. The
Cosmic Lens All Sky Survey (CLASS), the largest radio-selected galactic mass
scale gravitational lens search project to date, has resulted in the largest
sample suitable for statistical analyses. In the work presented here, we
exploit observed image separations of the multiply imaged lensed radio sources
in the sample. We use two different tests: (1) image separation distribution
function of the lensed radio sources and (2)
{\dtheta}_{\mathrm{pred}} vs {\dtheta}_{\mathrm{obs}} as observational
tools to constrain the cosmological parameters and \Om. The results are
in concordance with the bounds imposed by other cosmological tests.Comment: 20 pages latex; Modified " Results and Discussion " section, new
references adde
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
Quantum Monte Carlo study of a nonmagnetic impurity in the two-dimensional Hubbard model
In order to investigate the effects of nonmagnetic impurities in strongly
correlated systems, Quantum Monte Carlo (QMC) simulations have been carried out
for the doped two-dimensional Hubbard model with one nonmagnetic impurity.
Using a bare impurity potential which is onsite and attractive, magnetic and
single-particle properties have been calculated. The QMC results show that
giant oscillations develop in the Knight shift response around the impurity
site due to the short-range antiferromagnetic correlations. These results are
useful for interpreting the NMR data on Li and Zn substituted layered cuprates.Comment: 10 pages, 7 figure
Sc2Ga2CuO7: A possible quantum spin liquid near the percolation threshold
Sc2Ga2CuO7 (SGCO) crystallizes in a hexagonal structure (space group: P63/mmc), which can be seen as an alternating
stacking of single and double triangular layers. Combining neutron, x-ray, and resonant x-ray diffraction we establish that
the single triangular layers are mainly populated by non-magnetic Ga3+ ions (85% Ga and 15% Cu), while the bi-layers have comparable population of Cu2+ and Ga3+ ions (43% Cu and 57% Ga). Our susceptibility measurements in the temperature range 1.8 - 400 K give no indication of any spin-freezing or magnetic long-range order (LRO).We infer an effective paramagnetic moment μeff = 1.79±0.09 μB and a Curie-Weiss temperature �CW of about −44 K, suggesting antiferromagnetic interactions between the Cu2+(S = 1/2) ions. Low-temperature neutron powder diffraction data showed no evidence for LRO down to 1.5
K. In our specific heat data as well, no anomalies were found down to 0.35 K, in the field range 0-140 kOe. The magnetic
specific heat, Cm, exhibits a broad maximum at around 2.5 K followed by a nearly power law Cm/ T� behavior at lower
temperatures, with � increasing from 0.3 to 1.9 as a function of field for fields upto 90 kOe and then remaining at 1.9 for fields
upto 140 kOe. Our results point to a disordered ground state in SGCO
Direct simulations of helical Hall-MHD turbulence and dynamo action
Direct numerical simulations of turbulent Hall dynamos are presented. The
evolution of an initially weak and small scale magnetic field in a system
maintained in a stationary turbulent regime by a stirring force at a
macroscopic scale is studied to explore the conditions for exponential growth
of the magnetic energy. Scaling of the dynamo efficiency with the Reynolds
numbers is studied, and the resulting total energy spectra are found to be
compatible with a Kolmogorov type law. A faster growth of large scale magnetic
fields is observed at intermediate intensities of the Hall effect.Comment: 13 pages, 17 figures, ApJ (in press
Rebounce and Black hole formation in a Gravitational Collapse Model with Vanishing Radial Pressure
We examine spherical gravitational collapse of a matter model with vanishing
radial pressure and non-zero tangential pressure. It is seen analytically that
the collapsing cloud either forms a black hole or disperses depending on values
of the initial parameters which are initial density, tangential pressure and
velocity profile of the cloud. A threshold of black hole formation is observed
near which a scaling relation is obtained for the mass of black hole, assuming
initial profiles to be smooth. The similarities in the behaviour of this model
at the onset of black hole formation with that of numerical critical behaviour
in other collapse models are indicated.Comment: 15 pages, To be published in Gen.Rel.Gra
- …