3,609 research outputs found
Radiative interactions in laminar duct flows
Analyses and numerical procedures are presented for infrared radiative energy transfer in gases when other modes of energy transfer occur simultaneously. Two types of geometries are considered, a parallel plate duct and a circular duct. Fully developed laminar incompressible flows of absorbing-emitting species in black surfaced ducts are considered under the conditions of uniform wall heat flux. The participating species considered are OH, CO, CO2, and H2O. Nongray as well as gray formulations are developed for both geometries. Appropriate limiting solutions of the governing equations are obtained and conduction-radiation interaction parameters are evaluated. Tien and Lowder's wide band model correlation was used in nongray formulation. Numerical procedures are presented to solve the integro-differential equations for both geometries. The range of physical variables considered are 300 to 2000 K for temperature, 0.1 to 100.0 atm for pressure, and 0.1 to 100 cm spacings between plates/radius of the tube. An extensive parametric study based on nongray formulation is presented. Results obtained for different flow conditions indicate that the radiative interactions can be quite significant in fully developed incompressible flows
Weak Mott insulators on the triangular lattice: possibility of a gapless nematic quantum spin liquid
We study the energetics of Gutzwiller projected BCS states of various
symmetries for the triangular lattice antiferromagnet with a four particle ring
exchange using variational Monte Carlo methods. In a range of parameters the
energetically favored state is found to be a projected paired
state which breaks lattice rotational symmetry. We show that the properties of
this nematic or orientationally ordered paired spin liquid state as a function
of temperature and pressure can account for many of the experiments on organic
materials. We also study the ring-exchange model with ferromagnetic Heisenberg
exchange and find that amongst the studied ans\"atze, a projected wave
state is the most favorable.Comment: Longer version, 7+ pages, 5 figure
Antiferromagnetism and phase separation in the t-J model at low doping: a variational study
Using Gutzwiller-projected wave functions, I estimate the ground-state energy
of the t-J model for several variational states relevant for high-temperature
cuprate superconductors. The results indicate antiferromagnetism and phase
separation at low doping both in the superconducting state and in the
staggered-flux normal state proposed for the vortex cores. While phase
separation in the underdoped superconducting state may be relevant for the
stripe formation mechanism, the results for the normal state suggest that
similar charge inhomogeneities may also appear in vortex cores up to relatively
high doping values.Comment: 4 pages, 3 figures, reference adde
Particle-hole character of the Higgs and Goldstone modes in strongly-interacting lattice bosons
We study the low-energy excitations of the Bose-Hubbard model in the
strongly-interacting superfluid phase using a Gutzwiller approach and extract
the single-particle and single-hole excitation amplitudes for each mode. We
report emergent mode-dependent particle-hole symmetry on specific arc-shaped
lines in the phase diagram connecting the well-known Lorentz-invariant limits
of the Bose-Hubbard model. By tracking the in-phase particle-hole symmetric
oscillations of the order parameter, we provide an answer to the long-standing
question about the fate of the pure amplitude Higgs mode away from the
integer-density critical point. Furthermore, we point out that out-of-phase
oscillations are responsible for a full suppression of the condensate density
oscillations of the gapless Goldstone mode. Possible detection protocols are
also discussed.Comment: 6 pages, 3 figure
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Encapsulation of water insoluble drugs in mesoporous silica nanoparticles using supercritical carbon dioxide
Mesoporous silica nanoparticles MCM – 41 were synthesized with two dimensional hexagonal p6mm symmetry, high specific surface area(~ 980m2/g) narrow pore size and an average particle size of 186 nm. The produced nanoparticles were used to encapsulate carbamazepine through a supercritical carbon dioxide process combined with various organic solvents. Supercritical processing was found to provide increased drug encapsulation. The loaded MCM - 41 nanoparticles were analyzed using X–ray diffraction and differential scanning calorimetry (DSC) to investigate the crystalline state of the encapsulated carbamazepine and it was found to be dependent on the nature of the organic solvent. Carbamazepine showed increased dissolution rates under sink conditions. Viability studies of Caco – 2 cells demonstrated negligible cytotoxicity for the MCM–41 nanoparticles
Spectral weight redistribution in strongly correlated bosons in optical lattices
We calculate the single-particle spectral function for the one-band
Bose-Hubbard model within the random phase approximation (RPA). In the strongly
correlated superfluid, in addition to the gapless phonon excitations, we find
extra gapped modes which become particularly relevant near the superfluid-Mott
quantum phase transition (QPT). The strength in one of the gapped modes, a
precursor of the Mott phase, grows as the QPT is approached and evolves into a
hole (particle) excitation in the Mott insulator depending on whether the
chemical potential is above (below) the tip of the lobe. The sound velocity of
the Goldstone modes remains finite when the transition is approached at a
constant density, otherwise, it vanishes at the transition. It agrees well with
Bogoliubov theory except close to the transition. We also calculate the spatial
correlations for bosons in an inhomogeneous trapping potential creating
alternating shells of Mott insulator and superfluid. Finally, we discuss the
capability of the RPA approximation to correctly account for quantum
fluctuations in the vicinity of the QPT.Comment: 14 pages, 12 figure
Particle-Hole Symmetry and the Effect of Disorder on the Mott-Hubbard Insulator
Recent experiments have emphasized that our understanding of the interplay of
electron correlations and randomness in solids is still incomplete. We address
this important issue and demonstrate that particle-hole (ph) symmetry plays a
crucial role in determining the effects of disorder on the transport and
thermodynamic properties of the half-filled Hubbard Hamiltonian. We show that
the low-temperature conductivity decreases with increasing disorder when
ph-symmetry is preserved, and shows the opposite behavior, i.e. conductivity
increases with increasing disorder, when ph-symmetry is broken. The Mott
insulating gap is insensitive to weak disorder when there is ph-symmetry,
whereas in its absence the gap diminishes with increasing disorder.Comment: 4 pages, 4 figure
Mystery of Excess Low Energy States in a Disordered Superconductor in a Zeeman Field
Tunneling density of states measurements of disordered superconducting (SC)
Al films in high Zeeman fields reveal a significant population of subgap states
which cannot be explained by standard BCS theory. We provide a natural
explanation of these excess states in terms of a novel disordered
Larkin-Ovchinnikov (dLO) phase that occurs near the spin-paramagnetic
transition at the Chandrasekhar-Clogston critical field. The dLO superconductor
is characterized by a pairing amplitude that changes sign at domain walls.
These domain walls carry magnetization and support Andreev bound states, which
lead to distinct spectral signatures at low energy.Comment: 5 pages, 4 figures, plus supplementary section describing methods (2
pages
A Representation of Symmetry Generators for the Type IIB Superstring on a Plane Wave in the U(4) Formalism
We calculate the symmetry currents for the type IIB superstring on a
maximally supersymmetric plane wave background using the N=(2,2)
superconformally covariant U(4) formulation developed by Berkovits, Maldacena
and Maoz. An explicit realization of the U(4) generators together with 16
fermionic generators is obtained in terms of the N=(2,2) worldsheet fields.
Because the action is no longer quadratic, we use a light-cone version to
display the currents in terms of the covariant worldsheet variables.Comment: 9 pages, harvmac, Corrected some typographical errors, Added
reference
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