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- Universidade do Minho. Centro de Ciências Históricas e Sociais
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Resultados filtrados por Publicador: Universidade Cornell

## ‣ Pseudo-magnetoexcitons in strained graphene bilayers without external magnetic fields

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 01/06/2012
Português

Relevância na Pesquisa
17.136553%

The structural and electronic properties of graphene leads its charge
carriers to behave like relativistic particles, which is described by a
Dirac-like Hamiltonian. Since graphene is a monolayer of carbon atoms, the
strain due to elastic deformations will give rise to so-called `pseudomagnetic
fields (PMF)' in graphene sheet, and that has been realized experimentally in
strained graphene sample. Here we propose a realistic strained graphene bilayer
(SGB) device to detect the pseudo-magnetoexcitons (PME) in the absence of
external magnetic field. The carriers in each graphene layer suffer different
strong PMFs due to strain engineering, which give rise to Landau quantization.
The pseudo-Landau levels (PLLs) of electron-hole pair under inhomogeneous PMFs
in SGB are analytically obtained in the absence of Coulomb interactions. Based
on the general analytical optical absorption selection rule for PME, we show
that the optical absorption spectrums can interpret the corresponding formation
of Dirac-type PME. We also predict that in the presence of inhomogeneous PMFs,
the superfluidity-normal phase transition temperature of PME is greater than
that under homogeneous PMFs.}; Comment: 16 pages, 6 figures

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## ‣ On two mass estimators for clusters of galaxies

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 10/02/1999
Português

Relevância na Pesquisa
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The two most common mass estimators that use velocity dispersions and
positions of objects in a stellar system, the virial mass estimator (VME) and
the projected mass estimator (PME), are revisited and tested using N-body
experiments. We consider here only spherical, isolated, and isotropic velocity
dispersion systems. We have found that the PME can overestimate masses by ~
20%, for realistic cluster mass profiles, if applied only to regions around the
total system's "effective" radius. The VME can yield a correct mass at
different radii provided that the total potential energy term is correctly
taken into account and the system is completely sampled; otherwise, it may lead
to similar errors as the PME. A surface pressure (3PV) term recently alluded to
be usually neglected when using the VME and therefore required as a correction
term is here found not necessarily required, although it can be used to yield a
reasonable correction term. The preferred method here, however, is the virial
theorem due to its simplicity and better agreement with N-body experiments. The
possible reasons for the mass discrepancies found when using the PME and the
VME in some N-body simulations are also briefly discussed.; Comment: 10 pages & 6 figures, uses epsf.sty. Accepted for publication in A&A

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## ‣ Gradient estimates for $u_t=\Delta F(u)$ on manifolds and some Liouville-type theorems

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

Relevância na Pesquisa
17.136553%

In this paper, we first prove a localized Hamilton-type gradient estimate for
the positive solutions of Porous Media type equations: $$u_t=\Delta F(u),$$
with $F'(u) > 0$, on a complete Riemannian manifold with Ricci curvature
bounded from below. In the second part, we study Fast Diffusion Equation (FDE)
and Porous Media Equation (PME): $$u_t=\Delta (u^p),\qquad p>0,$$ and obtain
localized Hamilton-type gradient estimates for FDE and PME in a larger range of
$p$ than that for Aronson-B\'enilan estimate, Harnack inequalities and Cauchy
problems in the literature. Applying the localized gradient estimates for FDE
and PME, we prove some Liouville-type theorems for positive global solutions of
FDE and PME on noncompact complete manifolds with nonnegative Ricci curvature,
generalizing Yaus celebrated Liouville theorem for positive harmonic functions.; Comment: 24 pages, this is a revised version

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## ‣ Quantum algorithm for universal implementation of projective measurement of energy

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

Relevância na Pesquisa
17.136553%

A projective measurement of energy (PME) on a quantum system is a quantum
measurement, determined by the Hamiltonian of the system. PME protocols exist
when the Hamiltonian is given in advance. Unknown Hamiltonians can be
identified by quantum tomography, but the time cost to achieve a given accuracy
increases exponentially with the size of the quantum system. In this letter, we
improve the time cost by adapting quantum phase estimation, an algorithm
designed for computational problems, to measurements on physical systems. We
present a PME protocol without quantum tomography for Hamiltonians whose
dimension and energy scale are given but otherwise unknown. Our protocol
implements a PME to arbitrary accuracy without any dimension dependence on its
time cost. We also show that another computational quantum algorithm may be
used for efficient estimation of the energy scale. These algorithms show that
computational quantum algorithms have applications beyond their original
context with suitable modifications.; Comment: 4 pages with 9-page supplemental, 4 figures. Comments welcome

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