1,135 research outputs found
A Signal-To-Noise Ratio Comparison fo Ultrasonic Transducers for C-Scan Imaging in Titanium
Digital data acquisition and the C-scan imaging of ultrasonic data offer improvements over analog recording techniques, such as strip-chart recording. As a result, peak-detected C-scan imaging is becoming the preferred method for the inspection of large titanium parts such as those found in the aircraft engine industry. The effectiveness of the inspection, however, still depends on the transducer. For this reason, a study of the effect of different transducer parameters on the sensitivity for detection of simulated defects in titanium specimens was conducted. Due to the increased emphasis on C-scan imaging, sensitivity is measured as an image-based signal-to-noise ratio
Neutron scattering study of a quasi-2D spin-1/2 dimer system Piperazinium Hexachlorodicuprate under hydrostatic pressure
We report inelastic neutron scattering study of a quasi-two-dimensional S=1/2
dimer system Piperazinium Hexachlorodicuprate under hydrostatic pressure. The
spin gap {\Delta} becomes softened with the increase of the hydrostatic
pressure up to P= 9.0 kbar. The observed threefold degenerate triplet
excitation at P= 6.0 kbar is consistent with the theoretical prediction and the
bandwidth of the dispersion relation is unaffected within the experimental
uncertainty. At P= 9.0 kbar the spin gap is reduced to 0.55 meV from 1.0 meV at
ambient pressure.Comment: 4 pages, 5 figure
Rotational Dynamics of Organic Cations in CH3NH3PbI3 Perovskite
Methylammonium lead iodide (CH3NH3PbI3) based solar cells have shown
impressive power conversion efficiencies of above 20%. However, the microscopic
mechanism of the high photovoltaic performance is yet to be fully understood.
Particularly, the dynamics of CH3NH3+ cations and their impact on relevant
processes such as charge recombination and exciton dissociation are still
poorly understood. Here, using elastic and quasi-elastic neutron scattering
techniques and group theoretical analysis, we studied rotational modes of the
CH3NH3+ cation in CH3NH3PbI3. Our results show that, in the cubic (T > 327K)
and tetragonal (165K < T < 327K) phases, the CH3NH3+ ions exhibit four-fold
rotational symmetry of the C-N axis (C4) along with three-fold rotation around
the C-N axis (C3), while in orthorhombic phase (T < 165K) only C3 rotation is
present. Around room temperature, the characteristic relaxation times for the
C4 rotation is found to be ps while for the C3 rotation ps. The -dependent
rotational relaxation times were fitted with Arrhenius equations to obtain
activation energies. Our data show a close correlation between the C4
rotational mode and the temperature dependent dielectric permittivity. Our
findings on the rotational dynamics of CH3NH3+ and the associated dipole have
important implications on understanding the low exciton binding energy and slow
charge recombination rate in CH3NH3PbI3 which are directly relevant for the
high solar cell performance
Gapped and gapless short range ordered magnetic states with wavevectors in the pyrochlore magnet TbTiO
Recent low temperature heat capacity (C) measurements on polycrystalline
samples of the pyrochlore antiferromagnet TbTiO
have shown a strong sensitivity to the precise Tb concentration , with a
large anomaly exhibited for at K and no such
anomaly and corresponding phase transition for . We have grown single
crystal samples of TbTiO, with approximate
composition , and , where the single
crystal exhibits a large C anomaly at =0.45 K, but neither the
nor the single crystals display any such anomaly. We
present new time-of-flight neutron scattering measurements on the
and the samples which show strong
quasi-Bragg peaks at low
temperatures characteristic of short range antiferromagnetic spin ice (AFSI)
order at zero magnetic field but only under field-cooled conditions, as was
previously observed in our single crystal. These results show that
the strong quasi-Bragg peaks
and gapped AFSI state at low temperatures under field cooled conditions are
robust features of TbTiO, and are not correlated with the presence
or absence of the C anomaly and phase transition at low temperatures.
Further, these results show that the ordered state giving rise to the C
anomaly is confined to for
TbTiO, and is not obviously connected with
conventional order of magnetic dipole degrees of freedom.Comment: 7 pages, 3 figure
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