3,495 research outputs found
Spin Berry phase in anisotropic topological insulators
Three-dimensional topological insulators are characterized by the presence of
protected gapless spin helical surface states. In realistic samples these
surface states are extended from one surface to another, covering the entire
sample. Generally, on a curved surface of a topological insulator an electron
in a surface state acquires a spin Berry phase as an expression of the
constraint that the effective surface spin must follow the tangential surface
of real space geometry. Such a Berry phase adds up to pi when the electron
encircles, e.g., once around a cylinder. Realistic topological insulators
compounds are also often layered, i.e., are anisotropic. We demonstrate
explicitly the existence of such a pi Berry phase in the presence and absence
(due to crystal anisotropy) of cylindrical symmetry, that is, regardless of
fulfilling the spin-to-surface locking condition. The robustness of the spin
Berry phase pi against cylindrical symmetry breaking is confirmed numerically
using a tight-binding model implementation of a topological insulator nanowire
penetrated by a pi-flux tube.Comment: 9 pages, 4 figures (6 panels
Radiation hydrodynamics simulations of wide-angle outflows from super-critical accretion disks around black holes
By performing two-dimensional radiation hydrodynamics simulations with large
computational domain of 5000 Schwarzschild radius, we revealed that wide-angle
outflow is launched via the radiation force from the super-critical accretion
flows around black holes. The angular size of the outflow, of which the radial
velocity (v_r) is over the escape velocity (v_esc), increases with an increase
of the distance from the black hole. As a result, the mass is blown away with
speed of v_r > v_esc in all direction except for the very vicinity of the
equatorial plane, theta=0-85^circ, where theta is the polar angle. The mass
ejected from the outer boundary per unit time by the outflow is larger than the
mass accretion rate onto the black hole, ~150L_Edd/c^2, where L_Edd and c are
the Eddington luminosity and the speed of light. Kinetic power of such
wide-angle high-velocity outflow is comparable to the photon luminosity and is
a few times larger than the Eddington luminosity. This corresponds to
~10^39-10^40 erg/s for the stellar mass black holes. Our model consistent with
the observations of shock excited bubbles observed in some ultra-luminous X-ray
sources (ULXs), supporting a hypothesis that ULXs are powered by the
super-critical accretion onto stellar mass black holes.Comment: 9 pages, 8 figures, accepted for publication in PAS
Weak topological insulator with protected gapless helical states
A workable model for describing dislocation lines introduced into a
three-dimensional topological insulator is proposed. We show how fragile
surface Dirac cones of a weak topological insulator evolve into protected
gapless helical modes confined to the vicinity of dislocation line. It is
demonstrated that surface Dirac cones of a topological insulator (either strong
or weak) acquire a finite-size energy gap, when the surface is deformed into a
cylinder penetrating the otherwise surface-less system. We show that when a
dislocation with a non-trivial Burgers vector is introduced, the finite-size
energy gap play the role of stabilizing the one-dimensional gapless states.Comment: 8 pages, 17 figure
Parallel Imports, Drag Price Control and Pharmaceutical Innovation
This paper examines how parallel importation influences pharmaceutical innovation and the welfare of the economy, when crossnational drug price differentials occur not only because of demand elasticity based factors, but also governmental drug price control based factors. By explicitly considering the governmental drug price control baaed factors, this paper shows that parallel importation may enhance pharmaceutical innovation, when the bargaining power of a foreign government is strong and the price elasticity of demand in the foreign market is small. We also show that the increase in R&D induced by parallel imports may even increase the consumer surplus of a country with high demand elasticities and which could face relatively low drug prices, if parallel imports were not allowed.Parallel Imports, Pharmaceutical Innovation, Drug Price Control
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