73 research outputs found
Gamma Radiation Assisted Grafting of Low Surface Energy Long Chain Molecule on Various Types of Polyolefin Elastomers (POE)
Surface modification of polymers can lead to changes of properties such as surface wettability, adhesion, bioactivity and biocompatibility. Here, a long chain, low surface energy monomer, docecyl methacrylate (DMA), which is difficult to graft by chemical mean was grafted onto three different types of ‘Engages’ (a polyolefin elastomer, POE) in ethanol/water medium by gamma radiation. Total dose and DMA concentration were optimized for the desired amount of low degree grafting (less than 15wt%). The surface wetting properties towards water were decreased for all Engage surfaces after grafting. Mechanical properties of the base polymers remained almost unchanged after grafting
Dynamic correlations of the Coulomb Luttinger liquid
The dynamic density response function, form-factor, and spectral function of
a Luttinger liquid with Coulomb electron-electron interaction are studied with
the emphasis on the short-range electron correlations. The Coulomb interaction
changes dramatically the density response function as compared to the case of
the short-ranged interaction. The form of the density response function is
smoothing with time, and the oscillatory structure appears. However, the
spectral functions remain qualitatively the same. The dynamic form-factor
contains the -peak in the long-wave region, corresponding to one-boson
excitations. Besides, the multi-boson-excitations band exists in the
wave-number region near to . The dynamic form-factor diverges at the
edges of this band, while the dielectric function goes to zero there, which
indicates the appearance of a soft mode. We develop a method to analyze the
asymptotics of the spectral functions near to the edges of the
multi-boson-excitations band.Comment: 11 pages, 3 figures, submitted to PR
Continuous-distribution puddle model for conduction in trilayer graphene
An insulator-to-metal transition is observed in trilayer graphene based on
the temperature dependence of the resistance under different applied gate
voltages. At small gate voltages the resistance decreases with increasing
temperature due to the increase in carrier concentration resulting from thermal
excitation of electron-hole pairs. At large gate voltages excitation of
electron-hole pairs is suppressed, and the resistance increases with increasing
temperature because of the enhanced electron-phonon scattering. We find that
the simple model with overlapping conduction and valence bands, each with
quadratic dispersion relations, is unsatisfactory. Instead, we conclude that
impurities in the substrate that create local puddles of higher electron or
hole densities are responsible for the residual conductivity at low
temperatures. The best fit is obtained using a continuous distribution of
puddles. From the fit the average of the electron and hole effective masses can
be determined.Comment: 18 pages, 5 figure
Ultrarelativistic electron-hole pairing in graphene bilayer
We consider ground state of electron-hole graphene bilayer composed of two
independently doped graphene layers when a condensate of spatially separated
electron-hole pairs is formed. In the weak coupling regime the pairing affects
only conduction band of electron-doped layer and valence band of hole-doped
layer, thus the ground state is similar to ordinary BCS condensate. At strong
coupling, an ultrarelativistic character of electron dynamics reveals and the
bands which are remote from Fermi surfaces (valence band of electron-doped
layer and conduction band of hole-doped layer) are also affected by the
pairing. The analysis of instability of unpaired state shows that s-wave
pairing with band-diagonal condensate structure, described by two gaps, is
preferable. A relative phase of the gaps is fixed, however at weak coupling
this fixation diminishes allowing gapped and soliton-like excitations. The
coupled self-consistent gap equations for these two gaps are solved at zero
temperature in the constant-gap approximation and in the approximation of
separable potential. It is shown that, if characteristic width of the pairing
region is of the order of magnitude of chemical potential, then the value of
the gap in the spectrum is not much different from the BCS estimation. However,
if the pairing region is wider, then the gap value can be much larger and
depends exponentially on its energy width.Comment: 13 pages with 8 figures; accepted to Eur. Phys. J.
Analytic theory of ground-state properties of a three-dimensional electron gas at varying spin polarization
We present an analytic theory of the spin-resolved pair distribution
functions and the ground-state energy of an electron gas
with an arbitrary degree of spin polarization. We first use the Hohenberg-Kohn
variational principle and the von Weizs\"{a}cker-Herring ideal kinetic energy
functional to derive a zero-energy scattering Schr\"{o}dinger equation for
. The solution of this equation is implemented
within a Fermi-hypernetted-chain approximation which embodies the Hartree-Fock
limit and is shown to satisfy an important set of sum rules. We present
numerical results for the ground-state energy at selected values of the spin
polarization and for in both a paramagnetic and a fully
spin-polarized electron gas, in comparison with the available data from Quantum
Monte Carlo studies over a wide range of electron density.Comment: 13 pages, 8 figures, submitted to Phys. Rev.
Double-Layer Systems at Zero Magnetic Field
We investigate theoretically the effects of intralayer and interlayer
exchange in biased double-layer electron and hole systems, in the absence of a
magnetic field. We use a variational Hartree-Fock-like approximation to analyze
the effects of layer separation, layer density, tunneling, and applied gate
voltages on the layer densities and on interlayer phase coherence. In agreement
with earlier work, we find that for very small layer separations and low layer
densities, an interlayer-correlated ground state possessing spontaneous
interlayer coherence (SILC) is obtained, even in the absence of interlayer
tunneling. In contrast to earlier work, we find that as a function of total
density, there exist four, rather than three, distinct noncrystalline phases
for balanced double-layer systems without interlayer tunneling. The newly
identified phase exists for a narrow range of densities and has three
components and slightly unequal layer densities, with one layer being spin
polarized, and the other unpolarized. An additional two-component phase is also
possible in the presence of sufficiently strong bias or tunneling. The
lowest-density SILC phase is the fully spin- and pseudospin-polarized
``one-component'' phase discussed by Zheng {\it et al.} [Phys. Rev. B {\bf 55},
4506 (1997)]. We argue that this phase will produce a finite interlayer Coulomb
drag at zero temperature due to the SILC. We calculate the particle densities
in each layer as a function of the gate voltage and total particle density, and
find that interlayer exchange can reduce or prevent abrupt transfers of charge
between the two layers. We also calculate the effect of interlayer exchange on
the interlayer capacitance.Comment: 35 pages, 19 figures included. To appear in PR
Faraday rotation in graphene
We study magneto--optical properties of monolayer graphene by means of
quantum field theory methods in the framework of the Dirac model. We reveal a
good agreement between the Dirac model and a recent experiment on giant Faraday
rotation in cyclotron resonance. We also predict other regimes when the effects
are well pronounced. The general dependence of the Faraday rotation and
absorption on various parameters of samples is revealed both for suspended and
epitaxial graphene.Comment: 10 pp; v2: typos corrected and references added, v3, v4: small
changes and more reference
Optical Properties of III-Mn-V Ferromagnetic Semiconductors
We review the first decade of extensive optical studies of ferromagnetic,
III-Mn-V diluted magnetic semiconductors. Mn introduces holes and local moments
to the III-V host, which can result in carrier mediated ferromagnetism in these
disordered semiconductors. Spectroscopic experiments provide direct access to
the strength and nature of the exchange between holes and local moments; the
degree of itineracy of the carriers; and the evolution of the states at the
Fermi energy with doping. Taken together, diversity of optical methods reveal
that Mn is an unconventional dopant, in that the metal to insulator transition
is governed by the strength of the hybridization between Mn and its p-nictogen
neighbor. The interplay between the optical, electronic and magnetic properties
of III-Mn-V magnetic semiconductors is of fundamental interest and may enable
future spin-optoelectronic devices.Comment: Topical Revie
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