1,985 research outputs found
Evidence of Program Quality and Youth Outcomes in the DYCD Out-of-School Time Initiative: Report on the Initiative's First Three Years
Examines New York City's progress in improving out-of-school-time program quality and serving more children and youth, participants' and parents' overall satisfaction with quality and accessibility, and links between programming, quality, and benefits
Optical excitations in hexagonal nanonetwork materials
Optical excitations in hexagonal nanonetwork materials, for example,
Boron-Nitride (BN) sheets and nanotubes, are investigated theoretically. The
bonding of BN systems is positively polarized at the B site, and is negatively
polarized at the N site. There is a permanent electric dipole moment along the
BN bond, whose direction is from the B site to the N site. When the exciton
hopping integral is restricted to the nearest neighbors, the flat band of the
exciton appears at the lowest energy. The higher optical excitations have
excitation bands similar to the electronic bands of graphene planes and carbon
nanotubes. The symmetry of the flat exciton band is optically forbidden,
indicating that the excitons related to this band will show quite long lifetime
which will cause strong luminescence properties.Comment: 4 pages; 3 figures; proceedings of "XVIth International Winterschool
on Electronic Properties of Novel Materials (IWEPNM2002)
Relationship between spiral and ferromagnetic states in the Hubbard model in the thermodynamic limit
We explore how the spiral spin(SP) state, a spin singlet known to accompany
fully-polarized ferromagnetic (F) states in the Hubbard model, is related with
the F state in the thermodynamic limit using the density matrix renormalization
group and exact diagonalization. We first obtain an indication that when the F
state is the ground state the SP state is also eligible as the ground state in
that limit. We then follow the general argument by Koma and Tasaki [J. Stat.
Phys. {\bf 76}, 745 (1994)] to find that: (i) The SP state possesses a kind of
order parameter. (ii) Although the SP state does not break the SU(2) symmetry
in finite systems, it does so in the thermodynamic limit by making a linear
combination with other states that are degenerate in that limit. We also
calculate the one-particle spectral function and dynamical spin and charge
susceptibilities for various 1D finite-size lattices. We find that the
excitation spectrum of the SP state and the F state is almost identical. Our
present results suggest that the SP and the F states are equivalent in the
thermodynamic limit. These properties may be exploited to determine the
magnetic phase diagram from finite-size studies.Comment: 17 figures, to be published in Phys. Rev.
Holocene History of a Portion of Northernmost Ellesmere Island
Radiocarbon dates and glaciological features of the Ward Hunt area along northernmost Ellesmere Island suggest the following chronology, which is consistent with worldwide climatic oscillations: 1) 10,000-4100 B.P.: deglaciation, and development of several marine levels, particularly one now 40 m above sea level, at 7500 ± 300 B.P.; 2) 4100-2400 years B.P.: climatic deterioration, glacial readvance and formation of ice shelves; 3) 2400-1400 years B.P.: general climatic amelioration; development of dust ablation horizon on Ward Hunt Ice Shelf, glacial retreat; 4) 1400 B.P.-present: climatic deterioration, with renewed thickening of Ward Hunt Ice Shelf, and beginnings of growth of ice rises; the last-mentioned experienced maximum growth in the interval between 350-170 years ago; slight glacial readvance. The isostatic rebound curve for northernmost Ellesmere Island differs from that of the Tanquary Fiord area 80 miles (128 km) to the south because of differing Pleistocene ice thicknesses. We estimate these to average at least 600 m for the former area and 1800 m for the latter
Tensor mass and particle number peak at the same location in the scalar-tensor gravity boson star models - an analytical proof
Recently in boson star models in framework of Brans-Dicke theory, three
possible definitions of mass have been identified, all identical in general
relativity, but different in scalar-tensor theories of gravity.It has been
conjectured that it's the tensor mass which peaks, as a function of the central
density, at the same location where the particle number takes its maximum.This
is a very important property which is crucial for stability analysis via
catastrophe theory. This conjecture has received some numerical support. Here
we give an analytical proof of the conjecture in framework of the generalized
scalar-tensor theory of gravity, confirming in this way the numerical
calculations.Comment: 9 pages, latex, no figers, some typos corrected, reference adde
Scaling behaviour of a scalar field model of dark matter halos
Galactic dark matter is modelled by a scalar field. In particular, it is
shown that an analytically solvable toy model with a non-linear
self-interaction potential U(Phi) leads to dark halo models which have the form
of quasi-isothermal spheres. We argue that these fit better the observed
rotation curves of galaxies than the centrally cusped halos of standard cold
dark matter. The scalar field model predicts a proportionality between the
central densities of the dark halos and the inverse of their core radii. We
test this prediction successfully against a set of rotation curves of low
surface brightness galaxies and nearby bright galaxies.Comment: 4 pages, 2 figures, accepted to Mon. Not. R. Astron. So
Calculating critical temperatures of superconductivity from a renormalized Hamiltonian
It is shown that one can obtain quantitatively accurate values for the
superconducting critical temperature within a Hamiltonian framework. This is
possible if one uses a renormalized Hamiltonian that contains an attractive
electron-electron interaction and renormalized single particle energies. It can
be obtained by similarity renormalization or using flow equations for
Hamiltonians. We calculate the critical temperature as a function of the
coupling using the standard BCS-theory. For small coupling we rederive the
McMillan formula for Tc. We compare our results with Eliashberg theory and with
experimental data from various materials. The theoretical results agree with
the experimental data within 10%. Renormalization theory of Hamiltonians
provides a promising way to investigate electron-phonon interactions in
strongly correlated systems.Comment: 6 pages, LaTeX, using EuroPhys.sty, one eps figure included, accepted
for publication in Europhys. Let
Quantum Oscillations in the Underdoped Cuprate YBa2Cu4O8
We report the observation of quantum oscillations in the underdoped cuprate
superconductor YBa2Cu4O8 using a tunnel-diode oscillator technique in pulsed
magnetic fields up to 85T. There is a clear signal, periodic in inverse field,
with frequency 660+/-15T and possible evidence for the presence of two
components of slightly different frequency. The quasiparticle mass is
m*=3.0+/-0.3m_e. In conjunction with the results of Doiron-Leyraud et al. for
YBa2Cu3O6.5, the present measurements suggest that Fermi surface pockets are a
general feature of underdoped copper oxide planes and provide information about
the doping dependence of the Fermi surface.Comment: Contains revisions addressing referees' comments including a
different Fig 1b. 4 pages, 4 figure
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