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 (12,12,12)(\frac{1}{2},\frac{1}{2},\frac{1}{2}) wavevectors in the pyrochlore magnet Tb2+x_{2+x}Ti2−x_{2-x}O7+δ_{7+\delta}

Recent low temperature heat capacity (C$_P$) measurements on polycrystalline samples of the pyrochlore antiferromagnet Tb$_{2+x}$Ti$_{2-x}$O$_{7+\delta}$ have shown a strong sensitivity to the precise Tb concentration $x$, with a large anomaly exhibited for $x \sim 0.005$ at $T_C \sim 0.5$ K and no such anomaly and corresponding phase transition for $x \le 0$. We have grown single crystal samples of Tb$_{2+x}$Ti$_{2-x}$O$_{7+\delta}$, with approximate composition $x=-0.001, +0.0042$, and $+0.0147$, where the $x=0.0042$ single crystal exhibits a large C$_P$ anomaly at $T_C$=0.45 K, but neither the $x=-0.001$ nor the $x=+0.0147$ single crystals display any such anomaly. We present new time-of-flight neutron scattering measurements on the $x=-0.001$ and the $x=+0.0147$ samples which show strong $\left(\frac{1}{2},\frac{1}{2},\frac{1}{2}\right)$ 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 $x = 0.0042$ single crystal. These results show that the strong $\left(\frac{1}{2},\frac{1}{2},\frac{1}{2}\right)$ quasi-Bragg peaks and gapped AFSI state at low temperatures under field cooled conditions are robust features of Tb$_2$Ti$_2$O$_7$, and are not correlated with the presence or absence of the C$_P$ anomaly and phase transition at low temperatures. Further, these results show that the ordered state giving rise to the C$_P$ anomaly is confined to $0 \leq x \leq 0.01$ for Tb$_{2+x}$Ti$_{2-x}$O$_{7+\delta}$, and is not obviously connected with conventional order of magnetic dipole degrees of freedom.Comment: 7 pages, 3 figure