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We report that the finite thickness of three-dimensional topological insulator (TI) thin films produces an observable magnetoresistance (MR) in phase coherent transport in parallel magnetic fields. The MR data of Bi2 Se3 and (Bi1-xSbx)2Te3 thin films are compared with existing theoretical models of parallel field magnetotransport. We conclude that the TI thin films bring parallel field transport into a unique regime in which the coupling of surface states to bulk and to opposite surfaces is indispensable for understanding the observed MR. The β parameter extracted from parallel field MR can in principle provide a figure of merit for searching TI compounds with more insulating bulk than existing materials. [ABSTRACT FROM AUTHOR]

Atomically thin MoS2 crystals have been recognized as quasi-two-dimensional semiconductors with remarkable physical properties. We report our Raman scattering measurements on multilayer and monolayer MoS2, especially in the low-frequency range (<50 cm-1). We find two low-frequency Raman modes with a contrasting thickness dependence. When increasing the number of MoS2 layers, one mode shows a significant increase in frequency while the other decreases following a 1/N (N denotes the number of unit layers) trend. With the aid of first-principles calculations we assign the former as the shear mode E2g2 . The latter is distinguished as the compression vibrational mode, similar to the surface vibration of other epitaxial thin films. The opposite evolution of the two modes with thickness demonstrates vibrational modes in an atomically thin crystal as well as a more precise way to characterize the thickness of atomically thin MoS2 films. In addition, we observe a broad feature around 38 cm-1 (5 meV) which is visible only under near-resonance excitation and pinned at a fixed energy, independent of thickness. We interpret the feature as an electronic Raman scattering associated with the spin-orbit coupling induced splitting in a conduction band at K points in their Brillouin zone. [ABSTRACT FROM AUTHOR]

We have performed Raman-scattering measurements on high-quality single crystals of the superconductors K0.8Fe1.6Se2 (Tc = 32 K), Tl0.5K0.3Fe1.6Se2 (Tc = 29 K), and Tl0.5Rb0.3Fe1.6Se2 (Tc = 31 K) as well as of the insulating compound KFe1.5Se2. To interpret our results, we have made first-principles calculations for the phonon modes in the ordered iron-vacancy structure of K0.8Fe1.6Se2. The modes we observe can be assigned very well from our symmetry analysis and calculations, allowing us to compare Raman-active phonons in the AFeSe compounds. We find a clear frequency difference in most phonon modes between the superconducting and nonsuperconducting potassium crystals, indicating the fundamental influence of iron content. By contrast, substitution of ? by TI or Rb in A0.8Fe1.6Se2 causes no substantial frequency shift for any modes above 60 cm-1, demonstrating that the alkali-type metal has little effect on the microstructure of the FeSe layer. Several additional modes appear below 60 cm-1 in TI- and Rb-substituted samples, which are vibrations of heavier TI and Rb ions. Finally, our calculations reveal the presence of "chiral" phonon modes, whose origin lies in the chiral nature of the K0.8Fe1.6Se2 structure. [ABSTRACT FROM AUTHOR]

We performed Raman-scattering measurements on high-quality single crystals of A0.8Fe1.6Se2 superconductors of several compositions. We found a broad, asymmetric peak around 1600 cm-1 (200 meV), which we identify as a two-magnon process involving optical magnons. The intensity of the two-magnon peak falls sharply on entering the superconducting phase. This effect, which is entirely absent in the non-superconducting system KFe1.5Se2, requires a strong mutual exclusion between antiferromagnetism and superconductivity arising from proximity effects within regions of microscale phase separation. [ABSTRACT FROM AUTHOR]