Ionic and electronic properties of the topological insulator Bi2_2Te2_2Se investigated using β\beta-detected nuclear magnetic relaxation and resonance of 8^8Li

We report measurements on the high temperature ionic and low temperature electronic properties of the 3D topological insulator Bi$_2$Te$_2$Se using ion-implanted $^8$Li $\beta$-detected nuclear magnetic relaxation and resonance. With implantation energies in the range 5-28 keV, the probes penetrate beyond the expected range of the topological surface state, but are still within 250 nm of the surface. At temperatures above ~150 K, spin-lattice relaxation measurements reveal isolated $^8$Li$^{+}$ diffusion with an activation energy $E_{A} = 0.185(8)$ eV and attempt frequency $\tau_{0}^{-1} = 8(3) \times 10^{11}$ s$^{-1}$ for atomic site-to-site hopping. At lower temperature, we find a linear Korringa-like relaxation mechanism with a field dependent slope and intercept, which is accompanied by an anomalous field dependence to the resonance shift. We suggest that these may be related to a strong contribution from orbital currents or the magnetic freezeout of charge carriers in this heavily compensated semiconductor, but that conventional theories are unable to account for the extent of the field dependence. Conventional NMR of the stable host nuclei may help elucidate their origin.Comment: 17 pages, 12 figures, submitted to Phys. Rev.

Nuclear magnetic resonance of ion implanted 8^8Li in ZnO

We report on the stability and magnetic state of ion implanted $^8$Li in single crystals of the semiconductor ZnO using $\beta$-detected nuclear magnetic resonance. At ultradilute concentrations, the spectra reveal distinct Li sites from 7.6 to 400 K. Ionized shallow donor interstitial Li is stable across the entire temperature range, confirming its ability to self-compensate the acceptor character of its (Zn) substitutional counterpart. Above 300 K, spin-lattice relaxation indicates the onset of correlated local motion of interacting defects, and the spectra show a site change transition from disordered configurations to substitutional. Like the interstitial, the substitutional shows no resolved hyperfine splitting, indicating it is also fully ionized above 210 K. The electric field gradient at the interstitial $^8$Li exhibits substantial temperature dependence with a power law typical of non-cubic metals.Comment: 15 pages and 11 figure

City Documentation: Creation and Visualization of High Resolution Panoramic Image Mosaics

We present a novel system for 2D city documentation utilizing panoramic image mosaics. Current panoramic stitching methods often produce poor results and available visualization tools are not able to deal with high resolution panoramic image mosaics. For the construction of a full view high resolution panorama, we introduce a rotational mosaic representation, which includes the rotation parameters, the focal length and the principal point with each input image. A local registration algorithm is developed to quickly align two images exclusively from the image data. In order to reduce accumulated errors a global registration algorithm is performed to simultaneously minimize the misregistration between all overlapping pairs of images. By combining both registration techniques, we improve the quality of full view panoramic image mosaics. Finally we introduce a weighted blending algorithm to generate spherical or cylindrical panoramas. Furthermore we present an efficient visualization tool to display our produced high resolution panoramas. We developed a method to warp panoramas by utilizing the OpenGL graphic system to overcome the restriction of common panorama viewers, which are only allowing the visualization of small panoramas. The panoramic image is digitally warped on-the-fly to simulate camera panning and zooming. Walking through the virtual environment is currently accomplished by jumping to different panoramic points. The presented documentation system provides the creation as well as the efficient visualization of high resolution panoramic image mosaics to document important areas in urban environments.

Exploring the Dynamics of Glasses Using Beta Detected NMR

We report 8Li spin lattice relaxation in two forms of the molecular glass TPD, one a normal glass and one prepared in an ultrastable configuration. The relaxation is remarkably fast, similar to 8Li relaxation in other organic materials and shows a stretched exponential behavior typical of glasses with a small stretching exponent β = 0.3. At low temperature, there is little or no difference between the two glasses, while above 250 K, the relaxation in the denser ultrastable form is faster. In addition, in this temperature range, the data shows a significant thermal hysteresis

Comparison of 8Li and 9Li spin relaxation in SrTiO3 and Pt: A means to distinguish magnetic and electric quadrupolar sources of relaxation

We demonstrate that the ratio of 1/T1 spin-lattice relaxation rates (SLR) of two different isotopes (here 8Li and 9Li) can be used in order to identify the predominant relaxation mechanism within the β-NMR technique. We studied with both isotopes a Pt foil and two samples of single crystal SrTiO3. In Pt, the ratio of the SLR rates was 6.82(29), which is close to but less than the theoretical limit of ∼7.68 for pure magnetic relaxation. In SrTiO3, the weighted averaged SLR ratio of the two samples was 2.7(3), which is close but larger than the theoretical limit of ∼2.14 expected for pure electric quadrupolar relaxation. This indicates that the primary source of relaxation in SrTiO3 is electric quadrupolar in origin and that magnetic effects have a much smaller importance in this material. The low effective asymmetry of 9Li was the dominant cause of uncertainty of the measurements in this study. We propose a system of tagging the beta-decays in coincidence with the alpha decays that take place in two of the three primary decay channels of 9Li as a way of increasing the effective asymmetry and enhance the signal of 9Li β-NMR

Beta Detected NMR of LaAlO3

Using 8Li β-NMR we studied the temperature dependence of the spin-lattice relaxation in LaAlO3 in high magnetic field. The relaxation in LAO is composed of two components: a fast-relaxing signal with a small amplitude, and a large amplitude, slow relaxing component. The data was fit using a highly-constrained model which assumes that the temperature dependence of the slow and fast component is the same at all temperatures. The 8Li relaxation suggests a T2 temperature dependence, indicating that the relaxation mechanism could be due to a coupling between the quadrupole moment of 8Li and the soft-mode, low energy phonons in LAO