6,560 research outputs found
Probing gravitational non-minimal coupling with dark energy surveys
We investigate observational constraints on a specific one-parameter
extension to the minimal quintessence model, where the quintessence field
acquires a quadratic coupling to the scalar curvature through a coupling
constant . The value of is highly suppressed in typical tracker
models if the late-time cosmic acceleration is driven at some field values near
the Planck scale. We test in a second class of models in which the field
value today becomes a free model parameter. We use the combined data from
type-Ia supernovae, cosmic microwave background, baryon acoustic oscillations
and matter power spectrum, to weak lensing measurements and find a best-fit
value where is excluded outside the 95 per cent
confidence region. The effective gravitational constant subject
to the hint of a non-zero is constrained to at the same confidence level on cosmological scales, and can be narrowed
down to when combining with Solar
System tests.Comment: Context extended, figures and references added, title changed to
match with accepted version for publicatio
Robust Intrinsic Ferromagnetism and Half Semiconductivity in Stable Two-Dimensional Single-Layer Chromium Trihalides
Two-dimensional (2D) intrinsic ferromagnetic (FM) semiconductors are crucial
to develop low-dimensional spintronic devices. Using density functional theory,
we show that single-layer chromium trihalides (SLCTs) (CrX,X=F, Cl, Br and
I) constitute a series of stable 2D intrinsic FM semiconductors. A
free-standing SLCT can be easily exfoliated from the bulk crystal, due to a low
cleavage energy and a high in-plane stiffness. Electronic structure
calculations using the HSE06 functional indicate that both bulk and
single-layer CrX are half semiconductors with indirect gaps and their
valence bands and conduction bands are fully spin-polarized in the same spin
direction. The energy gaps and absorption edges of CrBr and CrI are
found to be in the visible frequency range, which implies possible
opt-electronic applications. Furthermore, SLCTs are found to possess a large
magnetic moment of 3 per formula unit and a sizable magnetic anisotropy
energy. The magnetic exchange constants of SLCTs are then extracted using the
Heisenberg spin Hamiltonian and the microscopic origins of the various exchange
interactions are analyzed. A competition between a near 90 FM
superexchange and a direct antiferromagnetic (AFM) exchange results in a FM
nearest-neighbour exchange interaction. The next and third nearest-neighbour
exchange interactions are found to be FM and AFM respectively and this can be
understood by the angle-dependent extended Cr-X-X-Cr superexchange interaction.
Moreover, the Curie temperatures of SLCTs are also predicted using Monte Carlo
simulations and the values can further increase by applying a biaxial tensile
strain. The unique combination of robust intrinsic ferromagnetism, half
semiconductivity and large magnetic anisotropy energies renders the SLCTs as
promising candidates for next-generation semiconductor spintronic applications.Comment: 12 pages, 14 figures. published in J. Mater. Chem.
"Teleparallel" Dark Energy
Using the "teleparallel" equivalent of General Relativity as the
gravitational sector, which is based on torsion instead of curvature, we add a
canonical scalar field, allowing for a nonminimal coupling with gravity.
Although the minimal case is completely equivalent to standard quintessence,
the nonminimal scenario has a richer structure, exhibiting quintessence-like or
phantom-like behavior, or experiencing the phantom-divide crossing. The richer
structure is manifested in the absence of a conformal transformation to an
equivalent minimally-coupled model.Comment: 5 pages, 1 figure, Version published in PLB704 (2011) 384-38
Leber's Hereditary Optic Neuropathy: A Case Report
Leber's hereditary optic neuropathy (LHON) is a maternally inherited mitochondrial disease that primarily affects the optic nerve, causing bilateral vision loss in juveniles and young adults. A 12-year-old boy had complained of blurred vision in both eyes for more than 1 year. His best-corrected visual acuity was 0.08 in the right eye and 0.1 in the left. Ophthalmologic examination showed bilateral optic disc hyperemia and margin blurring, peripapillary telangiectasis, and a relative afferent pupil defect in his right eye. Fluorescein angiography showed no stain or leakage around the optic disc in the late phase. Visual field analysis showed central scotoma in the left eye and a near-total defect in the right. Upon examination of the patient's mitochondrial DNA, a point mutation at nucleotide position 11778 was found, and the diagnosis of LHON was confirmed. Coenzyme Q10 was used to treat the patient
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