61,278 research outputs found
Collective excitations of Dirac electrons in a graphene layer with spin-orbit interaction
Coulomb screening and excitation spectra of electrons in a graphene layer
with spin-orbit interaction (SOI) is studied in the random phase approximation.
The SOI opens a gap between the valence and conduction bands of the semi-metal
Dirac system and reshapes the bottom and top of the bands. As a result, we have
observed a dramatic change in the long-wavelength dielectric function of the
system. An undamped plasmon mode emerges from the inter-band electron-hole
excitation continuum edge and vanishes or merges with a Landau damped mode on
the edge of the intra-band electron-hole continuum. The characteristic
collective excitation of the Dirac gas is recovered in a system with high
carrier density or for a large wave vector
Liquid-Crystal Blazed-Grating Beam Deflector
A transmission-type nonmechanical multiple-angle beam-steering device that uses liquid-crystal blazed grating has been developed. Sixteen steering angles with a contrast ratio of 18 has been demonstrated. A detailed analysis of the liquid-crystal and poly(methyl methacrylate) blazed-grating deflector was carried out to provide guidance during the deflector’s development. A manufacturing offset compensation technique is proposed to improve the device’s performance greatly. A hybrid approach utilizing electrically generated blazed grating combined with the cascading approach described here yields in excess of 500 deflecting angles
A Relation between the Anomalous Dimensions and OPE Coefficients in Asymptotic Free Field Theories
In asymptotic free field theories we show that part of the OPE of the trace
of the stress-energy tensor and an arbitrary composite field is determined by
the anomalous dimension of the composite field. We take examples from the
two-dimensional O(N) non-linear sigma model.Comment: 6 pages, plain TeX, uses harvma
Bouncing cosmologies with dark matter and dark energy
We review matter bounce scenarios where the matter content is dark matter and
dark energy. These cosmologies predict a nearly scale-invariant power spectrum
with a slightly red tilt for scalar perturbations and a small tensor-to-scalar
ratio. Importantly, these models predict a positive running of the scalar
index, contrary to the predictions of the simplest inflationary and ekpyrotic
models, and hence could potentially be falsified by future observations. We
also review how bouncing cosmological space-times can arise in theories where
either the Einstein equations are modified or where matter fields that violate
the null energy condition are included.Comment: 16 pages, 1 figure, v2: Discussion extended and clarifications added.
Invited review for special edition of Univers
Doping driven structural distortion in the bilayer iridate (SrLa)IrO
Neutron single crystal diffraction and rotational anisotropy optical second
harmonic generation data are presented resolving the nature of the structural
distortion realized in electron-doped (SrLa)IrO with
and . Once electrons are introduced into the bilayer
spin-orbit assisted Mott insulator SrIrO, previous studies have
identified the appearance of a low temperature structural distortion and have
suggested the presence of a competing electronic instability in the phase
diagram of this material. Our measurements resolve a lowering of the structural
symmetry from monoclinic to monoclinic and the creation of two
unique Ir sites within the chemical unit cell as the lattice distorts below a
critical temperature . Details regarding the modifications to oxygen
octahedral rotations and tilting through the transition are discussed as well
as the evolution of the low temperature distorted lattice as a function of
carrier substitution.Comment: 8 pages, 4 figure
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