In 3D topological insulators achieving a genuine bulk-insulating state is an important research topic. Recently, the material system (Bi,Sb)2(Te,Se)3 (BSTS) has been proposed as a topological ...insulator with high resistivity and a low carrier concentration (Ren et al 2011 Phys. Rev. B 84 165311). Here we present a study to further refine the bulk-insulating properties of BSTS. We have synthesized BSTS single crystals with compositions around x = 0.5 and y = 1.3. Resistance and Hall effect measurements show high resistivity and record low bulk carrier density for the composition Bi Sb Te Se . The analysis of the resistance measured for crystals with different thicknesses within a parallel resistor model shows that the surface contribution to the electrical transport amounts to 97% when the sample thickness is reduced to 1 m. The magnetoconductance of exfoliated BSTS nanoflakes shows 2D weak antilocalization with as expected for transport dominated by topological surface states.
We describe the design and fabrication of novel all-magnetic atom chips for use in ultracold atom trapping. The considerations leading to the choice of nanocrystalline exchange coupled FePt as best ...material are discussed. Using stray field calculations, we designed patterns that function as magnetic atom traps. These patterns were realized by spark erosion of FePt foil and e-beam lithography of FePt film. A mirror magneto-optical trap (MMOT) was obtained using the stray field of the foil chip.
At normal conditions crystalline polyethylene (PE) has an orthorhombic structure, whereas at high pressure (above 3200
bar) a phase with a hexagonal structure occurs. We investigate the effect of ...high pressure (up to about 5000
bar) on the chain motion in PE in the orthorhombic structure and in the vicinity of the transformation to the hexagonal phase. From proton and deuteron NMR spectra and spin–lattice relaxation rates we conclude that, apart from high-frequency mobility at defects, the rigid orthorhombic structure only allows small-angle reorientation of the CH
2 groups on a time scale of the order of 10
−7
s. The application of high pressure results in a considerable reduction of the amplitude of this motion. The degree of crystallinity is found to increase spectacularly by solidification from the hexagonal phase.
At high pressure (>3300
bar) polyethylene shows a hexagonal phase between the orthorhombic phase and the liquid phase. This first NMR investigation of polyethylene in its hexagonal phase shows that ...the mobility of the molecular chains is very high. Deuteron quadrupole echo spectra have been used to determine the character and rate of local reorientations of the molecular chains. The chains perform rapid axial reorientations, similar to those observed in the rotator phases occurring in the long normal alkanes. From proton rotating frame relaxation data we derive a high rate of translational diffusion in the chain direction. These observations contribute to the understanding of the important role of the hexagonal phase in the growth of chain-extended polyethylene crystals.
In 3D topological insulators achieving a genuine bulk-insulating state is an important research topic. Recently, the material system (Bi,Sb) sub(2)(Te,Se) sub(3) (BSTS) has been proposed as a ...topological insulator with high resistivity and a low carrier concentration (Ren et al 2011 Phys. Rev. B 84 165311). Here we present a study to further refine the bulk-insulating properties of BSTS. We have synthesized BSTS single crystals with compositions around x = 0.5 and y = 1.3. Resistance and Hall effect measurements show high resistivity and record low bulk carrier density for the composition Bi sub(1.46)Sb sub(0.54)Te sub(1.7)Se sub(1.3). The analysis of the resistance measured for crystals with different thicknesses within a parallel resistor model shows that the surface contribution to the electrical transport amounts to 97% when the sample thickness is reduced to 1 mu m. The magnetoconductance of exfoliated BSTS nanoflakes shows 2D weak antilocalization with alpha Asymptotically = to -1 as expected for transport dominated by topological surface states.
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