17,398 research outputs found
Constraints on a new alternative model to dark energy
The recent type Ia supernova data suggest that the universe is accelerating
now and decelerated in recent past. This may provide the evidence that the
standard Friedmann equation needs to be modified. We analyze in detail a new
model in the context of modified Friedmann equation using the supernova data
published by the High- Supernova Search Team and the Supernova Cosmology
Project. The new model explains recent acceleration and past deceleration.
Furthermore, the new model also gives a decelerated universe in the future.Comment: 12 pages, 5 figures, use ws-ijmpd, minor changes made. In the new
version, a detailed derivation of the model is give
Interface Facilitated Reorientation of Mg Nanolayers in Mg-Nb Nanolaminates
Mg/Nb nanolaminates synthesized through vapor deposition techniques exhibit high flow strength without conventional twinning in Mg. In this work, we investigated the influence of laminated microstructures on deformation mechanisms of Mg nanolayers. Using molecular dynamics simulations, we explored that (0001)-oriented Mg layers transform or re-orient to {10¯10}-oriented Mg layers through nucleation and growth of {10¯12} twins by atomic shuffling, instead of conventional {10¯12} twinning shear. Such a reorientation accommodates in-plane compressive strain and out-of-plane tensile strain when Mg/Nb laminates are subjected to compression parallel to the Mg/Nb interfaces. The nucleation of {10¯12} twins is promoted at the Mg/Nb interface due to the structural change associated with the glide of interface dislocations. The growth of {10¯12} twins is accomplished through migration of basal–prismatic boundaries via nucleation and glide of one-layer and two-layer disconnections associated with atomic shuffling. The results shed light on improving mechanical properties of hexagonal close-packed metals employing laminated structures
Strain relaxation in InGaN/GaN micro-pillars evidenced by high resolution cathodoluminescence hyperspectral imaging
A size-dependent strain relaxation and its effects on the optical properties of InGaN/GaN multiple quantum wells (QWs) in micro-pillars have been investigated through a combination of high spatial resolution cathodoluminescence (CL) hyperspectral imaging and numerical modeling. The pillars have diameters (d) ranging from 2 to 150 μm and were fabricated from a III-nitride light-emitting diode (LED) structure optimized for yellow-green emission at ∼560 nm. The CL mapping enables us to investigate strain relaxation in these pillars on a sub-micron scale and to confirm for the first time that a narrow (≤2 μm) edge blue-shift occurs even for the large InGaN/GaN pillars (d > 10 μm). The observed maximum blue-shift at the pillar edge exceeds 7 nm with respect to the pillar centre for the pillars with diameters in the 2–16 μm range. For the smallest pillar (d = 2 μm), the total blue-shift at the edge is 17.5 nm including an 8.2 nm “global” blue-shift at the pillar centre in comparison with the unetched wafer. By using a finite element method with a boundary condition taking account of a strained GaN buffer layer which was neglected in previous simulation works, the strain distribution in the QWs of these pillars was simulated as a function of pillar diameter. The blue-shift in the QWs emission wavelength was then calculated from the strain-dependent changes in piezoelectric field, and the consequent modification of transition energy in the QWs. The simulation and experimental results agree well, confirming the necessity for considering the strained buffer layer in the strain simulation. These results provide not only significant insights into the mechanism of strain relaxation in these micro-pillars but also practical guidance for design of micro/nano LEDs
Charm quark system at the physical point of 2+1 flavor lattice QCD
We investigate the charm quark system using the relativistic heavy quark
action on 2+1 flavor PACS-CS configurations previously generated on lattice. The dynamical up-down and strange quark masses are set to
the physical values by using the technique of reweighting to shift the quark
hopping parameters from the values employed in the configuration generation. At
the physical point, the lattice spacing equals GeV and the
spatial extent fm. The charm quark mass is determined by the
spin-averaged mass of the 1S charmonium state, from which we obtain m_{\rm
charm}^{\msbar}(\mu = m_{\rm charm}^{\msbar}) = 1.260(1)(6)(35) GeV, where the
errors are due to our statistics, scale determination and renormalization
factor. An additional systematic error from the heavy quark is of order
, which is estimated to be a percent
level if the factor analytic in is of order unity. Our
results for the charmed and charmed-strange meson decay constants are
MeV, MeV, again up to the heavy quark
errors of order . Combined with the CLEO
values for the leptonic decay widths, these values yield , , where the last error is on
account of the experimental uncertainty of the decay widths.Comment: 16 pages, 12 figure
Robust Quantum State Transfer in Random Unpolarized Spin Chains
We propose and analyze a new approach for quantum state transfer between
remote spin qubits. Specifically, we demonstrate that coherent quantum coupling
between remote qubits can be achieved via certain classes of random,
unpolarized (infinite temperature) spin chains. Our method is robust to
coupling strength disorder and does not require manipulation or control over
individual spins. In principle, it can be used to attain perfect state transfer
over arbitrarily long range via purely Hamiltonian evolution and may be
particularly applicable in a solid-state quantum information processor. As an
example, we demonstrate that it can be used to attain strong coherent coupling
between Nitrogen-Vacancy centers separated by micrometer distances at room
temperature. Realistic imperfections and decoherence effects are analyzed.Comment: 4 pages, 2 figures. V2: Modified discussion of disorder, added
references - final version as published in Phys. Rev. Let
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