40,148 research outputs found
Temperature-dependent Raman scattering of KTa1-xNbxO3 thin films
We report a Raman scattering investigation of KTa1-xNbxO3 (x = 0.35, 0.5)
thin films deposited on MgO and LaAlO3 as a function of temperature. The
observed phase sequence in the range from 90 K to 720 K is similar to the
structural phase transitions of the end-member material KNbO3. Although similar
in the phase sequence, the actual temperatures observed for phase transition
temperatures are significantly different from those observed in the literature
for bulk samples. Namely, the tetragonal (ferroelectric) to cubic
(paraelectric) phase transition is up to 50 K higher in the films when compared
to bulk samples. This enhanced ferroelectricity is attributed to biaxial strain
in the investigated thin films
Electronic Properties of Strained Si/Ge Core-Shell Nanowires
We investigated the electronic properties of strained Si/Ge core-shell
nanowires along the [110] direction using first principles calculations based
on density-functional theory. The diameter of the studied core-shell wire is up
to 5 nm. We found the band gap of the core-shell wire is smaller than that of
both pure Si and Ge wires with the same diameter. This reduced band gap is
ascribed to the intrinsic strain between Ge and Si layers, which partially
counters the quantum confinement effect. The external strain is further applied
to the nanowires for tuning the band structure and band gap. By applying
sufficient tensile strain, we found the band gap of Si-core/Ge-shell nanowire
with diameter larger than ~3 nm experiences a transition from direct to
indirect gap.Comment: 4 figure
Pulse retrieval and soliton formation in a non-standard scheme for dynamic electromagnetically induced transparency
We examine in detail an alternative method of retrieving the information
written into an atomic ensemble of three-level atoms using electromagnetically
induced transparency. We find that the behavior of the retrieved pulse is
strongly influenced by the relative collective atom-light coupling strengths of
the two relevant transitions. When the collective atom-light coupling strength
for the retrieval beam is the stronger of the two transitions, regeneration of
the stored pulse is possible. Otherwise, we show the retrieval process can lead
to creation of soliton-like pulses.Comment: 11 figure
The scattering of a cylindrical invisibility cloak: reduced parameters and optimization
We investigate the scattering of 2D cylindrical invisibility cloaks with
simplified constitutive parameters with the assistance of scattering
coefficients. We show that the scattering of the cloaks originates not only
from the boundary conditions but also from the spatial variation of the
component of permittivity/permeability. According to our formulation, we
propose some restrictions to the invisibility cloak in order to minimize its
scattering after the simplification has taken place. With our theoretical
analysis, it is possible to design a simplified cloak by using some peculiar
composites like photonic crystals (PCs) which mimic an effective refractive
index landscape rather than offering effective constitutives, meanwhile
canceling the scattering from the inner and outer boundaries.Comment: Accepted for J. Phys.
Connecting the vulcanization transition to percolation
The vulcanization transition is addressed via a minimal
replica-field-theoretic model. The appropriate long-wave-length behavior of the
two- and three-point vertex functions is considered diagrammatically, to all
orders in perturbation theory, and identified with the corresponding quantities
in the Houghton-Reeve-Wallace field-theoretic approach to the percolation
critical phenomenon. Hence, it is shown that percolation theory correctly
captures the critical phenomenology of the vulcanization transition associated
with the liquid and critical states.Comment: 9 pages, 5 figure
Strain Modulated Electronic Properties of Ge Nanowires - A First Principles Study
We used density-functional theory based first principles simulations to study
the effects of uniaxial strain and quantum confinement on the electronic
properties of germanium nanowires along the [110] direction, such as the energy
gap and the effective masses of the electron and hole. The diameters of the
nanowires being studied are up to 50 {\AA}. As shown in our calculations, the
Ge [110] nanowires possess a direct band gap, in contrast to the nature of an
indirect band gap in bulk. We discovered that the band gap and the effective
masses of charge carries can be modulated by applying uniaxial strain to the
nanowires. These strain modulations are size-dependent. For a smaller wire (~
12 {\AA}), the band gap is almost a linear function of strain; compressive
strain increases the gap while tensile strain reduces the gap. For a larger
wire (20 {\AA} - 50 {\AA}), the variation of the band gap with respect to
strain shows nearly parabolic behavior: compressive strain beyond -1% also
reduces the gap. In addition, our studies showed that strain affects effective
masses of the electron and hole very differently. The effective mass of the
hole increases with a tensile strain while the effective mass of the electron
increases with a compressive strain. Our results suggested both strain and size
can be used to tune the band structures of nanowires, which may help in design
of future nano-electronic devices. We also discussed our results by applying
the tight-binding model.Comment: 1 table, 8 figure
The Kagome Antiferromagnet: A Schwinger-Boson Mean-Field Theory Study
The Heisenberg antiferromagnet on the Kagom\'{e} lattice is studied in the
framework of Schwinger-boson mean-field theory. Two solutions with different
symmetries are presented. One solution gives a conventional quantum state with
order for all spin values. Another gives a gapped spin liquid
state for spin and a mixed state with both and
orders for spin . We emphasize that the mixed
state exhibits two sets of peaks in the static spin structure factor. And for
the case of spin , the gap value we obtained is consistent with the
previous numerical calculations by other means. We also discuss the
thermodynamic quantities such as the specific heat and magnetic susceptibility
at low temperatures and show that our result is in a good agreement with the
Mermin-Wagner theorem.Comment: 9 pages, 5 figure
Formation and kinetics of transient metastable states in mixtures under coupled phase ordering and chemical demixing
We present theory and simulation of simultaneous chemical demixing and phase
ordering in a polymer-liquid crystal mixture in conditions where isotropic-
isotropic phase separation is metastable with respect to isotropic-nematic
phase transition. It is found that mesophase formation proceeds by a transient
metastable phase that surround the ordered phase, and whose lifetime is a
function of the ratio of diffusional to orientational mobilities. It is shown
that kinetic phase ordering in polymer-mesogen mixtures is analogous to kinetic
crystallization in polymer solutions.Comment: 17 pages, 5 figures accepted for publication in EP
Review of morphology dependent charge carrier mobility in MEH-PPV
Charge carrier mobility in poly(2-methoxy,5(2'-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) films were measured as a function of temperature and electric field parallel and perpendicular to the substrate for devices prepared from different solvents and under different processing conditions Bulk structural morphology was characterized by various X-ray diffraction measurements such as wide angle, small angle and X-ray reflection. Surface morphology was characterized using various scanning probe microscopic techniques Mobilities were found to follow Gaussian disorder model (GDM) and to be highly anisotropic not only depending on the solvents used but also on the film preparation method such as spin-coating or drop-casting While no direct correlation was found between charge carrier mobility and photoluminescence, charge transport parameters were correlated with structural morpholog
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