382 research outputs found
Electric field gradients in MgB synthesized at high pressure: Cd TDPAC study and ab initio calculation
We report the high-pressure synthesis of novel superconductor MgB and
some related compounds. The superconducting transition temperature of our
samples of MgB is equal to 36.6 K. The MgB lattice parameters
determined via X-ray diffraction are in excellent agreement with results of our
ab initio calculations. The time-differential perturbed angular correlation
(TDPAC) experiments demonstrate a small increase in quadrupole frequency of
Cd probe with decreasing temperature from 293 to 4.2 K. The electric
field gradient (EFG) at the B site calculated from first principles is in fair
agreement with EFG obtained from B NMR spectra of MgB reported in the
literature. It is also very close to EFG found in our Cd TDPAC
measurements, which suggests that the Cd probe substitutes for boron in
the MgB lattice.Comment: 10 pages, 3 figure
Response function analysis of excited-state kinetic energy functional constructed by splitting k-space
Over the past decade, fundamentals of time independent density functional
theory for excited state have been established. However, construction of the
corresponding energy functionals for excited states remains a challenging
problem. We have developed a method for constructing functionals for excited
states by splitting k-space according to the occupation of orbitals. In this
paper we first show the accuracy of kinetic energy functional thus obtained. We
then perform a response function analysis of the kinetic energy functional
proposed by us and show why method of splitting the k-space could be the method
of choice for construction of energy functionals for excited states.Comment: 11 page
Ab initio study of ferroelectric domain walls in PbTiO3
We have investigated the atomistic structure of the 180-degree and 90-degree
domain boundaries in the ferroelectric perovskite compound PbTiO3 using a
first-principles ultrasoft-pseudopotential approach. For each case we have
computed the position, thickness and creation energy of the domain walls, and
an estimate of the barrier height for their motion has been obtained. We find
both kinds of domain walls to be very narrow with a similar width of the order
of one to two lattice constants. The energy of the 90-dergree domain wall is
calculated to be 35 mJ/m^2, about a factor of four lower than the energy of its
180-degree counterpart, and only a miniscule barrier for its motion is found.
As a surprising feature we detected a small offset of 0.15-0.2 eV in the
electrostatic potential across the 90-degree domain wall.Comment: 12 pages, with 9 postscript figures embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/bm_dw/index.htm
Chemical Protection Studies of Activated Carbon Spheres based Permeable Protective Clothing Against Sulfur Mustard, a Chemical Warfare Agent
Technological advancements in the field of chemical threat have made it possible to create extremely dangerous chemical warfare agents (CWA). Hence, the effective protection of personnel is very important in a chemical warfare scenario amidst the current climate of terrorism awareness. In particular, body protection plays a substantial role in the chemical defence considering the urgency of situation in the nuclear, biological and chemical environment. Activated carbon spheres (ACS) based permeable chemical protective clothing (coverall) was developed for protection against CWA. The adsorbent material i.e, ACS used in this protective clothing provided higher adsorption capacity (1029 mg/g in terms of iodine) and low thermal burden (34 °C WBGT index) compared to earlier indigenously developed NBC suit. This article focuses on the extensive evaluation of chemical protective clothing against sulfur mustard (HD), a CWA. The results revealed that the developed protective clothing provided more than 24 h protection against HD. This chemical protective suit is light weight (< 2.75 kg for XL size). It also has higher air permeability (> 30 cm3/s/cm2) as well as less water vapour resistance (< 9.6 m2Pa/W). With continued innovations in materials and attention to key challenges it is expected that advanced, multifunction chemical protective suit will play a pivotal role in the CWA protection scenario
Muon-Spin Rotation Spectra in the Mixed Phase of High-T_c Superconductors : Thermal Fluctuations and Disorder Effects
We study muon-spin rotation (muSR) spectra in the mixed phase of highly
anisotropic layered superconductors, specifically Bi_2+xSr_2-xCaCu_2O_8+delta
(BSCCO), by modeling the fluid and solid phases of pancake vortices using
liquid-state and density functional methods. The role of thermal fluctuations
in causing motional narrowing of muSR lineshapes is quantified in terms of a
first-principles theory of the flux-lattice melting transition. The effects of
random point pinning are investigated using a replica treatment of liquid state
correlations and a replicated density functional theory. Our results indicate
that motional narrowing in the pure system, although substantial, cannot
account for the remarkably small linewidths obtained experimentally at
relatively high fields and low temperatures. We find that satisfactory
agreement with the muSR data for BSCCO in this regime can be obtained through
the ansatz that this ``phase'' is characterized by frozen short-range
positional correlations reflecting the structure of the liquid just above the
melting transition. This proposal is consistent with recent suggestions of a
``pinned liquid'' or ``glassy'' state of pancake vortices in the presence of
pinning disorder. Our results for the high-temperature liquid phase indicate
that measurable linewidths may be obtained in this phase as a consequence of
density inhomogeneities induced by the pinning disorder. The results presented
here comprise a unified, first-principles theoretical treatment of muSR spectra
in highly anisotropic layered superconductors in terms of a controlled set of
approximations.Comment: 50 pages Latex file, including 10 postscript figure
Spectral and transport properties of doped Mott-Hubbard systems with incommensurate magnetic order
We present spectral and optical properties of the Hubbard model on a
two-dimensional square lattice using a generalization of dynamical mean-field
theory to magnetic states in finite dimension. The self-energy includes the
effect of spin fluctuations and screening of the Coulomb interaction due to
particle-particle scattering. At half-filling the quasiparticles reduce the
width of the Mott-Hubbard `gap' and have dispersions and spectral weights that
agree remarkably well with quantum Monte Carlo and exact diagonalization
calculations. Away from half-filling we consider incommensurate magnetic order
with a varying local spin direction, and derive the photoemission and optical
spectra. The incommensurate magnetic order leads to a pseudogap which opens at
the Fermi energy and coexists with a large Mott-Hubbard gap. The quasiparticle
states survive in the doped systems, but their dispersion is modified with the
doping and a rigid band picture does not apply. Spectral weight in the optical
conductivity is transferred to lower energies and the Drude weight increases
linearly with increasing doping. We show that incommensurate magnetic order
leads also to mid-gap states in the optical spectra and to decreased scattering
rates in the transport processes, in qualitative agreement with the
experimental observations in doped systems. The gradual disappearence of the
spiral magnetic order and the vanishing pseudogap with increasing temperature
is found to be responsible for the linear resistivity. We discuss the possible
reasons why these results may only partially explain the features observed in
the optical spectra of high temperature superconductors.Comment: 22 pages, 18 figure
Search for the standard model Higgs boson decaying into two photons in pp collisions at sqrt(s)=7 TeV
A search for a Higgs boson decaying into two photons is described. The
analysis is performed using a dataset recorded by the CMS experiment at the LHC
from pp collisions at a centre-of-mass energy of 7 TeV, which corresponds to an
integrated luminosity of 4.8 inverse femtobarns. Limits are set on the cross
section of the standard model Higgs boson decaying to two photons. The expected
exclusion limit at 95% confidence level is between 1.4 and 2.4 times the
standard model cross section in the mass range between 110 and 150 GeV. The
analysis of the data excludes, at 95% confidence level, the standard model
Higgs boson decaying into two photons in the mass range 128 to 132 GeV. The
largest excess of events above the expected standard model background is
observed for a Higgs boson mass hypothesis of 124 GeV with a local significance
of 3.1 sigma. The global significance of observing an excess with a local
significance greater than 3.1 sigma anywhere in the search range 110-150 GeV is
estimated to be 1.8 sigma. More data are required to ascertain the origin of
this excess.Comment: Submitted to Physics Letters
Search for a W' boson decaying to a bottom quark and a top quark in pp collisions at sqrt(s) = 7 TeV
Results are presented from a search for a W' boson using a dataset
corresponding to 5.0 inverse femtobarns of integrated luminosity collected
during 2011 by the CMS experiment at the LHC in pp collisions at sqrt(s)=7 TeV.
The W' boson is modeled as a heavy W boson, but different scenarios for the
couplings to fermions are considered, involving both left-handed and
right-handed chiral projections of the fermions, as well as an arbitrary
mixture of the two. The search is performed in the decay channel W' to t b,
leading to a final state signature with a single lepton (e, mu), missing
transverse energy, and jets, at least one of which is tagged as a b-jet. A W'
boson that couples to fermions with the same coupling constant as the W, but to
the right-handed rather than left-handed chiral projections, is excluded for
masses below 1.85 TeV at the 95% confidence level. For the first time using LHC
data, constraints on the W' gauge coupling for a set of left- and right-handed
coupling combinations have been placed. These results represent a significant
improvement over previously published limits.Comment: Submitted to Physics Letters B. Replaced with version publishe
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