Direct mapping of the spin-filtered surface bands of a three-dimensional quantum spin Hall insulator

Spin-polarized band structure of the three-dimensional quantum spin Hall insulator $\rm Bi_{1-x}Sb_{x}$ (x=0.12-0.13) was fully elucidated by spin-polarized angle-resolved photoemission spectroscopy using a high-yield spin polarimeter equipped with a high-resolution electron spectrometer. Between the two time-reversal-invariant points, $\bar{\varGamma}$ and $\bar{M}$, of the (111) surface Brillouin zone, a spin-up band ($\Sigma_3$ band) was found to cross the Fermi energy only once, providing unambiguous evidence for the strong topological insulator phase. The observed spin-polarized band dispersions determine the "mirror chirality" to be -1, which agrees with the theoretical prediction based on first-principles calculations

Spontaneously rotating clusters of active droplets

We report on the emergence of spontaneously rotating clusters in active emulsions. Ensembles of self-propelling droplets sediment and then self-organise into planar, hexagonally ordered clusters which hover over the container bottom while spinning around the plane normal. This effect exists for symmetric and asymmetric arrangements of isotropic droplets and is therefore not caused by torques due to geometric asymmetries. We found, however, that individual droplets exhibit a helical swimming mode in a small window of intermediate activity in a force-free bulk medium. We show that by forming an ordered cluster, the droplets cooperatively suppress their chaotic dynamics and turn the transient instability into a steady rotational state. We analyse the collective rotational dynamics as a function of droplet activity and cluster size and further propose that the stable collective rotation in the cluster is caused by a cooperative coupling between the rotational modes of individual droplets in the cluster

Conductivity of the Si(111)7x7 dangling-bond state

International audienceConductivity of the metallic dangling-bond state of adatoms on Si(111)7x7 clean surface was determined through passivation of the only electrical channel by similar to 0.1 monolayer Na adsorption. Through systematic measurements of electron transport and photoemission spectroscopy during the Na deposition, Si(111)7x7 was found to exhibit a metal-to-insulator transition. The decrease in conductivity through the transition, which is attributed to the conductivity of the dangling-bond state, was 2-4 mu Omega(-1) square(-1). The value is smaller than the two-dimensional Ioffe-Regel limit and the mean-free path is too short to apply the Boltzmann picture

Topological transition in Bi1-xSbx studied as a function of Sb doping

International audienceSpin- and angle-resolved photoemission spectroscopy measurements were performed on Bi1-xSbx samples at x = 0.04, 0.07, and 0.21 to study the change of the surface band structure from nontopological to topological. Energy shift of the T and L-s bulk bands with Sb concentration is quantitatively evaluated. An edge state becomes topologically nontrivial at x = 0.04. An additional trivial edge state appears at the L band gap that forms at x > 0.04 and apparently hybridize with the nontrivial edge state. A scenario for the topological transition mechanism is presented. Related issues of self-energy and temperature dependence of the surface state are also considered