310 research outputs found
Highly Sensitive Metal Mesh Sensors
AbstractMetal mesh devices were developed with a greater sensitivity at 10 THz than at 1 THz. Detection of a biotin– streptavidin combination was tested using these devices operating at 40 THz to evaluate their high-sensitivity capability. Experimental results showed that the combination was clearly detected label-free by a red spectral shift, indicating the possibility of using these metal mesh devices for medical inspections
Anisotropic dielectric function in polar nano-regions of relaxor ferroelectrics
The paper suggests to treat the infrared reflectivity spectra of single
crystal perovskite relaxors as fine-grained ferroelectric ceramics: locally
frozen polarization makes the dielectric function strongly anisotropic in the
phonon frequency range and the random orientation of the polarization at
nano-scopic scale requires to take into account the inhomogeneous
depolarization field. Employing a simple effective medium approximation
(Bruggeman symmetrical formula) to dielectric function describing the polar
optic modes as damped harmonic oscillators turns out to be sufficient for
reproducing all principal features of room temperature reflectivity of PMN. One
of the reflectivity bands is identified as a geometrical resonance entirely
related to the nanoscale polarization inhomogeneity. The approach provides a
general guide for systematic determination of the polar mode frequencies split
by the inhomogeneous polarization at nanometer scale.Comment: 5 pages, 2 figure
Magnetic and dielectric properties of multiferroic Eu0.5Ba0.25Sr0.25TiO3 ceramics
Dielectric and magnetic properties of Eu0.5Ba0.25Sr0.25TiO3 are investigated
between 10 K and 300 K in the frequency range from 10 Hz to 100 THz. A peak in
permittivity revealed near 130 K and observed ferroelectric hysteresis loops
prove the ferroelectric order below thistemperature. The peak in permittivity
is given mainly by softening of the lowest frequency polar phonon (soft mode
revealed in THz and IR spectra) that demonstrates displacive character of the
phase transition. Room-temperature X-ray diffraction analysis reveals cubic
structure, but the IR reflectivity spectra give evidence of a lower crystal
structure, presumably tetragonal I4/mcm with tilted oxygen octahedra as it has
been observed in EuTiO3. The magnetic measurements show that the
antiferromagnetic order occurs below 1.8 K. Eu0.5Ba0.25Sr0.25TiO3 has three
times lower coercive field than Eu0.5Ba0.5TiO3, therefore we propose this
system for measurements of electric dipole moment of electron.Comment: Phase Transitions, in pres
Analysis of Control Measures for Vector-borne Diseases Using a Multistage Vector Model with Multi-Host Sub-populations
We propose and analyze an epidemiological model for vector borne diseases
that integrates a multi-stage vector population and several host
sub-populations which may be characterized by a variety of compartmental model
types: subpopulations all include Susceptible and Infected compartments, but
may or may not include Exposed and/or Recovered compartments. The model was
originally designed to evaluate the effectiveness of various prophylactic
measures in malaria-endemic areas, but can be applied as well to other
vector-borne diseases. This model is expressed as a system of several
differential equations, where the number of equations depends on the particular
assumptions of the model. We compute the basic reproduction number , and show that if , the disease free equilibrium
(DFE) is globally asymptotically stable (GAS) on the nonnegative orthant. If
, the system admits a unique endemic equilibrium (EE) that is
GAS. We analyze the sensitivity of and the EE to different system
parameters, and based on this analysis we discuss the relative effectiveness of
different control measures.Comment: 42 pages, 3 figures. arXiv admin note: substantial text overlap with
arXiv:1808.0757
Ferroelectric and Incipient Ferroelectric Properties of a Novel Sr_(9-x)PbxCe2Ti2O36 (x=0-9) Ceramic System
Sr_(9-x)PbxCe2Ti12O36 system is derived from the perovskite SrTiO3 and its
chemical formula can be written as (Sr_(1-y)Pby)0.75Ce0.167TiO3. We
investigated dielectric response of Sr_(9-x)PbxCe2Ti12O36 ceramics (x = 0-9)
between 100 Hz and 100 THz at temperatures from 10 to 700 K using low- and
high-frequency dielectric, microwave (MW), THz and infrared spectroscopy. We
revealed that Sr9Ce2Ti12O36 is an incipient ferroelectric with the R-3c
trigonal structure whose relative permittivity e' increases from 167 at 300 K
and saturates near 240 below 30 K. The subsequent substitution of Sr by Pb
enhances e' to several thousands and induces a ferroelectric phase transition
to monoclinic Cc phase for x>=3. Its critical temperature Tc linearly depends
on the Pb concentration and reaches 550 K for x=9. The phase transition is of
displacive type. The soft mode frequency follows the Barrett formula in samples
with x=3.
The MW dispersion is lacking and quality factor Q is high in samples with low
Pb concentration, although the permittivity is very high in some cases.
However, due to the lattice softening, the temperature coefficient of the
permittivity is rather high. The best MW quality factor was observed for x=1:
Q*f=5800 GHz and e'=250. Concluding, the dielectric properties of Sr_(9-
x)PbxCe2Ti12O36 are similar to those of Ba_(1-x)SrxTiO3 so that this system can
be presumably used as an alternative for MW devices or capacitors.Comment: subm. to Chem. Mate
Magnetodielectric coupling and phonon properties of compressively strained EuTiO3 thin films deposited on LSAT
Compressively strained epitaxial (001) EuTiO3 thin films of tetragonal
symmetry have been deposited on (001) (LaAlO3)_0.29-(SrAl_{1/2}Ta_{1/2}O3)_0.71
(LSAT) substrates by reactive molecular-beam epitaxy. Enhancement of the Neel
temperature by 1 K with 0.9% compressive strain was revealed. The polar phonons
ofthe films have been investigated as a function of temperature and magnetic
field by means of infrared reflectance spectroscopy. All three infrared active
phonons show strongly stiffened frequencies compared to bulk EuTiO3 in
accordance with first principles calculations. The phonon frequencies exhibit
gradual softening on cooling leading to an increase in static permittivity. A
new polar phonon with frequency near the TO1 soft mode was detected below 150
K. The new mode coupled with the TO1 mode was assigned as the optical phonon
from the Brillouin zone edge, which is activated in infrared spectra due to an
antiferrodistortive phase transition and due to simultaneous presence of polar
and/or magnetic nanoclusters. In the antiferromagnetic phase we have observed a
remarkable softening of the lowest-frequency polar phonon under an applied
magnetic field, which qualitatively agrees with first principles calculations.
This demonstrates the strong spin-phonon coupling in EuTiO3, which is
responsible for the pronounced dependence of its static permittivity on
magnetic field in the antiferromagnetic phase.Comment: Submitted to Phys. Rev.
Origin of Low-Energy Excitations in Charge-Ordered Manganites
The low-energy excitations in the charge-ordered phase of polycrystalline
La0.25Ca0.75MnO3 are explored by frequency-domain terahertz spectroscopy. In
the frequency range from 4 cm^-1 to 700 cm^-1 (energies 0.4 meV to 90 meV) and
at temperatures down to 5 K, we do not detect any feature that can be
associated with the collective response of the spatially modulated charge
continuum. In the antiferromagnetically ordered phase, broad absorption bands
appear in the conductivity and permittivity spectra around 30 cm^-1 and 100
cm^-1 which are assigned to former acoustic phonons optically activated due to
a fourfold superstructure in the crystal lattice. Our results indicate that
characteristic energies of collective excitations of the charge-ordered phase
in La0:25Ca0:75MnO3, if any, lie below 1 meV. At our lowest frequencies of only
few wavenumbers a strong relaxation is observed above 100 K connected to the
formation of the charge-ordered state.Comment: 5 pages, 3 figure
Polar phonons in some compressively stressed epitaxial and polycrystalline SrTiO3 thin films
Several SrTiO3 (STO) thin films without electrodes processed by pulsed laser
deposition, of thicknesses down to 40 nm, were studied using infrared
transmission and reflection spectroscopy. The complex dielectric responses of
polar phonon modes, particularly ferroelectric soft mode, in the films were
determined quantitatively. The compressed epitaxial STO films on (100)
La0.18Sr0.82Al0.59-Ta0.41O3 substrates (strain 0.9%) show strongly stiffened
phonon responses, whereas the soft mode in polycrystalline film on (0001)
sapphire substrate shows a strong broadening due to grain boundaries and/or
other inhomogeneities and defects. The stiffened soft mode is responsible for a
much lower static permittivity in the plane of the compressed film than in the
bulk samples.Comment: 11 page
Spin-phonon coupling in epitaxial Sr0.6Ba0.4MnO3 thin films
Spin-phonon coupling is investigated in epitaxially strained Sr1-xBaxMnO3 thin films with perovskite structure by means of microwave (MW) and infrared (IR) spectroscopy. In this work we focus on the Sr0.6Ba0.4MnO3 composition grown on (LaAlO3)0.3(Sr2AlTaO6)0.7 substrate. The MW complex electromagnetic response shows a decrease in the real part and a clear anomaly in the imaginary part around 150 K. Moreover, it coincides with a 17% hardening of the lowest-frequency polar phonon seen in IR reflectance spectra. In order to further elucidate this phenomenon, low-energy muon-spin spectroscopy was carried out, signaling the emergence of antiferromagnetic order with NĂ©el temperature (TN) around 150 K. Thus, our results confirm that epitaxial Sr0.6Ba0.4MnO3 thin films display strong spin-phonon coupling below TN, which may stimulate further research on tuning the magnetoelectric coupling by controlling the epitaxial strain and chemical pressure in the Sr1-xBaxMnO3 system
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