451 research outputs found
The study of frequency-scan photothermal reflectance technique for thermal diffusivity measurement
A frequency scan photothermal reflectance technique to measure thermal diffusivity of bulk samples is studied in this manuscript. Similar to general photothermal reflectance methods, an intensity-modulated heating laser and a constant intensity probe laser are used to determine the surface temperature response under sinusoidal heating. The approach involves fixing the distance between the heating and probe laser spots, recording the phase lag of reflected probe laser intensity with respect to the heating laser frequency modulation, and extracting thermal diffusivity using the phase lag–(frequency)1/2 relation. The experimental validation is performed on three samples (SiO2, CaF2, and Ge), which have a wide range of thermal diffusivities. The measured thermal diffusivity values agree closely with the literature values. Compared to the commonly used spatial scan method, the experimental setup and operation of the frequency scan method are simplified, and the uncertainty level is equal to or smaller than that of the spatial scan method
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Coherent Control of Optically Generated and Detected Picosecond Surface Acoustic Phonons
Coherent control of elementary optical excitations is a key issue in ultrafast materials science. Manipulation of electronic and vibronic excitations in solids as well as chemical and biological systems on ultrafast time scales has attracted a great deal of attention recently. In semiconductors, coherent control of vibronic excitations has been demonstrated for bulk acoustic and optical phonons generated in superlattice structures. The bandwidth of these approaches is typically fully utilized by employing a 1-D geometry where the laser spot size is much larger than the superlattice repeat length. In this presentation we demonstrate coherent control of optically generated picosecond surface acoustic waves using sub-optical wavelength absorption gratings. The generation and detection characteristics of two material systems are investigated (aluminum absorption gratings on Si and GaAs substrates)
Optical pulse induced ultrafast antiferrodistortive transition in SrTiO3
The ultrafast dynamics of the antiferrodistortive (AFD) phase transition in
perovskite SrTiO3 is monitored via time-domain Brillouin scattering. Using
femtosecond optical pulses, we induce a thermally driven tetragonal-to-cubic
structural transformation and detect notable changes in the frequency of
Brillouin oscillations (BO) induced by propagating acoustic phonons. First, we
establish a fingerprint frequency of different regions across the temperature
phase diagram of the AFD transition characterized by tetragonal and cubic
phases in the low and high temperature sides, respectively. Then, we
demonstrate that in a sample nominally kept in tetragonal phase, deposition of
sufficient thermal energy induces an instantaneous transformation of the
heat-affected region to the cubic phase. Coupling the measured depth-resolved
BO frequency with a time and depth-resolved heat diffusion model, we detect a
reverse cubic-to-tetragonal phase transformation occurring on a time scale of
hundreds of picoseconds. We attribute this ultrafast phase transformation in
the perovskite to a structural resemblance between atomic displacements of the
R-point soft optic mode of the cubic phase and the tetragonal phase, both
characterized by anti-phase rotation of oxygen octahedra. Evidence of such a
fast structural transition in perovskites can open up new avenues in the field
of information processing and energy storage.Comment: 15 Pages, 4 Figure
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Simultaneous Microscopic Imaging of Elastic and Thermal Anisotropy
Simultaneous imaging of elastic and thermal properties of anisotropic materials with micron (lateral) and nanometer (depth) resolution is presented. This approach employs an ultrafast laser for the generation and detection of thermal and acoustic waves. Demonstrations involving the visualization of thermal waves and surface acoustic waves are presented for single crystal quartz and fused silica substrates supper coated with chromium films. These images dramatically reveal and contrast the symmetry of thermal and elastic properties and compare favorably with theoretical prediction. This hybrid approach shows great promise to investigate fundamental properties of materials and interfacts on both a low-frequency (elastic wave) and a high-frequency (phonon diffusion) scale
Spatially localized measurement of thermal conductivity using a hybrid photothermal technique
A photothermal technique capable of measuring thermal conductivity with micrometer lateral resolution is presented. This technique involves measuring separately the thermal diffusivity, D, and thermal effusivity, e, to extract the thermal conductivity, k = (e2/D)1/2. To generalize this approach, sensitivity analysis is conducted for materials having a range of thermal conductivities. Application to nuclear fuel is consider by performing experimental validation using two materials (CaF2 and SiO2) having thermal properties representative of fresh and high burnup nuclear fuel. The measured conductivities compare favorably with literature values
Kapitza Resistance of Si/SiOâ‚‚ Interface
A phonon wave packet dynamics method is used to characterize the Kapitza resistance of a Si/SiO2 interface in a Si/SiO2/Si heterostructure. By varying the thickness of SiO2 layer sandwiched between two Si layers, we determine the Kapitza resistance for the Si/SiO 2 interface from both wave packet dynamics and a direct, non-equilibrium molecular dynamics approach. The good agreement between the two methods indicates that they have each captured the anharmonic phonon scatterings at the interface. Moreover, detailed analysis provides insights as to how individual phonon mode scatters at the interface and their contribution to the Kapitza resistance
Immunocompromise in Gnotobiotic Pigs Induced by Verotoxin-Producing \u3ci\u3eEscherichia coli\u3c/i\u3e (O111:NM)
A verotoxin-producing Escherichia coli serotype O111:NM strain (strain 10049; verotoxin 1 positive) persistently infected experimentally inoculated gnotobiotic pigs, causing attaching-effacing intestinal lesions and chronic diarrhea. Experiments were performed to determine whether persistent infection might be associated with immunocompromise of the host by this organism. Pigs inoculated with this strain had a significant reduction in peripheral blood lymphocytes and lower antibody titers to sheep erythrocytes compared with control pigs. Compared with pigs given a verotoxin-negative pathogenic strain of the same serotype (O111:NM, strain 2430), pigs inoculated with the verotoxin-positive strain had lower peripheral lymphocyte counts and proliferative responses to concanavalin A, phytohemagglutinin, and pokeweed mitogens. The results of this study suggest that strain 10049 has an immunocompromising effect on gnotobiotic pigs
Immunocompromise in Gnotobiotic Pigs Induced by Verotoxin-Producing \u3ci\u3eEscherichia coli\u3c/i\u3e (O111:NM)
A verotoxin-producing Escherichia coli serotype O111:NM strain (strain 10049; verotoxin 1 positive) persistently infected experimentally inoculated gnotobiotic pigs, causing attaching-effacing intestinal lesions and chronic diarrhea. Experiments were performed to determine whether persistent infection might be associated with immunocompromise of the host by this organism. Pigs inoculated with this strain had a significant reduction in peripheral blood lymphocytes and lower antibody titers to sheep erythrocytes compared with control pigs. Compared with pigs given a verotoxin-negative pathogenic strain of the same serotype (O111:NM, strain 2430), pigs inoculated with the verotoxin-positive strain had lower peripheral lymphocyte counts and proliferative responses to concanavalin A, phytohemagglutinin, and pokeweed mitogens. The results of this study suggest that strain 10049 has an immunocompromising effect on gnotobiotic pigs
First-principles determination of the phonon-point defect scattering and thermal transport due to fission products in ThO2
This work presents the first principles calculations of the lattice thermal
conductivity degradation due to point defects in thorium dioxide using an
alternative solution of the Pierels-Boltzmann transport equation. We have used
the non-perturbative Green's function methodology to compute the phonon point
defect scattering rates that consider the local distortion around the point
defect, including the mass difference changes, interatomic force constants and
structural relaxation near the point defects. The point defects considered in
the work include the vacancy of thorium and oxygen, substitution of helium,
krypton, zirconium, iodine, xenon, in the thorium site, and the three different
configuration of the Schottky defects. The results of the phonon-defect
scattering rate reveals that among the considered intrinsic defects, the
thorium vacancy and helium substitution in the thorium site scatter the phonon
most due to substantial changes in the force constant and structural
distortions. The scattering of phonons due to the substitutional defects
unveils that the zirconium atom scatters phonons the least, followed by xenon,
iodine, krypton, and helium. This is contrary to the intuition that the
scattering strength follows HeTh > KrTh > ZrTh > ITh > XeTh based on the mass
difference. This striking difference in the zirconium phonon scattering is due
to the local chemical environment changes. Zirconium is an electropositive
element with valency similar to thorium and, therefore, can bond with the
oxygen atoms, thus creating less force constant variance compared to iodine, an
electronegative element, noble gas helium, xenon, and krypton. These results
can serve as the benchmark for the analytical models and help the
engineering-scale modeling effort for nuclear design.Comment: 10 page
Spin-dynamics of the low-dimensional magnet (CH3)2NH2CuCl3
Dimethylammonium copper (II) chloride (also known as DMACuCl3 or MCCL) is a
low dimensional S=1/2 quantum spin system proposed to be an alternating
ferro-antiferromagnetic chain with similar magnitude ferromagnetic (FM) and
antiferromagnetic (AFM) exchange interactions. Subsequently, it was shown that
the existing bulk measurements could be adequately modeled by considering
DMACuCl3 as independent AFM and FM dimer spin pairs. We present here new
inelastic neutron scattering measurements of the spin-excitations in single
crystals of DMACuCl3. These results show significant quasi-one-dimensional
coupling, however the magnetic excitations do not propagate along the expected
direction. We observe a band of excitations with a gap of 0.95 meV and a
bandwidth of 0.82 meV.Comment: 3 pages, 2 figures included in text, submitted to proceedings of
International Conference on Neutron Scattering, December 200
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