100,548 research outputs found
Oriented gap opening in the magnetically ordered state of Iron-pnicitides: an impact of intrinsic unit cell doubling on the square lattice by atoms
We show that the complicated band reconstruction near Fermi surfaces in the
magnetically ordered state of iron-pnictides observed by angle-resolved
photoemission spectroscopies (ARPES) can be understood in a meanfield level if
the \emph{intrinsic unit cell doubling} due to As atoms is properly considered
as shown in the recently constructed S microscopic effective model. The
(0,) or (,0) col-linear antiferromagnetic (C-AFM) order does not open
gaps between two points at Fermi surfaces linked by the ordered wave vector but
forces a band reconstruction involving four points in unfolded Brillouin zone
(BZ) and gives rise to small pockets or hot spots. The S symmetry naturally
chooses a staggered orbital order over a ferro-orbital order to coexist with
the C-AFM order. These results strongly suggest that the kinematics based on
the S symmetry captures the essential low energy physics of iron-based
superconductors.Comment: 5 figures, 5 page
A Model for Structure Formation Seeded by Gravitationally Produced Matter
This model assumes the baryons, radiation, three families of massless
neutrinos, and cold dark matter were mutually thermalized before the baryon
number was fixed, primeval curvature fluctuations were subdominant, and
homogeneity was broken by scale-invariant fluctuations in a new dark matter
component that behaves like a relativistic ideal fluid. The fluid behavior
could follow if this new component were a single scalar field that interacts
only with gravity and with itself by a pure quartic potential. The initial
energy distribution could follow if this component were gravitationally
produced by inflation. The power spectra of the present distributions of mass
and radiation in this model are not inconsistent with the measurements but are
sufficiently different from the adiabatic cold dark matter model to allow a
sharp test in the near future.Comment: 4 pages, 2 figures submitted to ApJ Letter
Two-spin relaxation of P-dimers in Silicon
We study two-electron singlet-triplet relaxation of donor-bound electrons in
Silicon. Hyperfine interaction of the electrons with the phosphorus (P) nuclei,
in combination with the electron-phonon interaction, lead to relaxation of the
triplet states. Within the Heitler-London and effective mass approximations, we
calculate the triplet relaxation rates in the presence of an applied magnetic
field. This relaxation mechanism affects the resonance peaks in current
Electron Spin Resonance (ESR) experiments on P-dimers. Moreover, the estimated
time scales for the spin decay put an upper bound on the gate pulses needed to
perform fault-tolerant two-qubit operations in donor-spin-based quantum
computers (QCs).Comment: 3 figures, 1 tabl
Relativistic description of magnetic moments in nuclei with doubly closed shells plus or minus one nucleon
Using the relativistic point-coupling model with density functional PC-PK1,
the magnetic moments of the nuclei Pb, Pb, Tl and
Bi with a closed-shell core Pb are studied on the basis of
relativistic mean field (RMF) theory. The corresponding time-odd fields, the
one-pion exchange currents, and the first- and second-order corrections are
taken into account. The present relativistic results reproduce the data well.
The relative deviation between theory and experiment for these four nuclei is
6.1% for the relativistic calculations and somewhat smaller than the value of
13.2% found in earlier non-relativistic investigations. It turns out that the
meson is important for the description of magnetic moments, first by
means of one-pion exchange currents and second by the residual interaction
provided by the exchange.Comment: 11 pages, 7 figure
Flux-lattice melting in LaOFFeAs: first-principles prediction
We report the theoretical study of the flux-lattice melting in the novel
iron-based superconductor and
. Using the Hypernetted-Chain closure and an
efficient algorithm, we calculate the two-dimensional one-component plasma pair
distribution functions, static structure factors and direct correlation
functions at various temperatures. The Hansen-Verlet freezing criterion is
shown to be valid for vortex-liquid freezing in type-II superconductors.
Flux-lattice meting lines for and
are predicted through the combination of the density
functional theory and the mean-field substrate approach.Comment: 5 pages, 4 figures, to appear in Phys. Rev.
Gluon GPDs and Exclusive Photoproduction of a Quarkonium in Forward Region
Forward photoproduction of can be used to extract Generalized Parton
Distributions(GPD's) of gluons. We analyze the process at twist-3 level and
study relevant classifications of twist-3 gluon GPD's. At leading power or
twist-2 level the produced is transversely polarized. We find that at
twist-3 the produced is longitudinally polarized. Our study shows that
in high energy limit the twist-3 amplitude is only suppressed by the inverse
power of the heavy quark mass relatively to the twist-2 amplitude. This
indicates that the power correction to the cross-section of unpolarized
can have a sizeable effect. We have also derived the amplitude of the
production of at twist-3, but the result contains end-point
singularities. The production of other quarkonia has been briefly discussed.Comment: Discussions of results are adde
Phonon Squeezed States Generated by Second Order Raman Scattering
We study squeezed states of phonons, which allow a reduction in the quantum
fluctuations of the atomic displacements to below the zero-point quantum noise
level of coherent phonon states. We investigate the generation of squeezed
phonon states using a second order Raman scattering process. We calculate the
expectation values and fluctuations of both the atomic displacement and the
lattice amplitude operators, as well as the effects of the phonon squeezed
states on macroscopically measurable quantities, such as changes in the
dielectric constant. These results are compared with recent experiments.Comment: 4 pages, REVTE
Interacting dark energy, holographic principle and coincidence problem
The interacting and holographic dark energy models involve two important
quantities. One is the characteristic size of the holographic bound and the
other is the coupling term of the interaction between dark energy and dark
matter. Rather than fixing either of them, we present a detailed study of
theoretical relationships among these quantities and cosmological parameters as
well as observational constraints in a very general formalism. In particular,
we argue that the ratio of dark matter to dark energy density depends on the
choice of these two quantities, thus providing a mechanism to change the
evolution history of the ratio from that in standard cosmology such that the
coincidence problem may be solved. We investigate this problem in detail and
construct explicit models to demonstrate that it may be alleviated provided
that the interacting term and the characteristic size of holographic bound are
appropriately specified. Furthermore, these models are well fitted with the
current observation at least in the low red-shift region.Comment: 20 pages, 3 figure
Delayed Recombination
Under the standard model for recombination of the primeval plasma, and the
cold dark matter model for structure formation, recent measurements of the
first peak in the angular power spectrum of the cosmic microwave background
temperature indicate the spatial geometry of the universe is nearly flat. If
sources of Lya resonance radiation, such as stars or active galactic nuclei,
were present at z ~ 1000 they would delay recombination, shifting the first
peak to larger angular scales, and producing a positive bias in this measure of
space curvature. It can be distinguished from space curvature by its
suppression of the secondary peaks in the spectrum.Comment: submitted to ApJ
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