Ending Laminations and Cannon-Thurston Maps

In earlier work, we had shown that Cannon-Thurston maps exist for Kleinian surface groups. In this paper we prove that pre-images of points are precisely end-points of leaves of the ending lamination whenever the Cannon-Thurston map is not one-to-one. In particular, the Cannon-Thurston map is finite-to-one. This completes the proof of the conjectural picture of Cannon-Thurston maps for surface groups.Comment: v4: Final version 22pgs 2figures. Includes the main theorem of the appendix arXiv:1002.2090 by Shubhabrata Das and Mahan Mj. To appear in Geometric and Functional Analysi

Ba+^+ Quadrupole Polarizabilities: Theory versus Experiment

Three different measurements have been reported for the ground state quadrupole polarizability in the singly ionized barium (Ba$^+$) which disagree with each other. Our calculation of this quantity using the relativistic coupled-cluster method disagrees with two of the experimental values and is within the error bars of the other. We discuss the issues related to the accuracy of our calculations and emphasize the need for further experiments to measure the quadrupole polarizability for this state and/or the 5D states.Comment: 6 pages, 3 table

Federal Copyright Law in the Computer Era: Protection for the Authors of Video Games

This Comment analyzes both the manner and scope of copyright protection currently afforded computer video games. It then discusses the means available under federal copyright laws to protect the underlying computer program and concludes that the game should be regarded as a unit. The effect of treating the game as a unit of audiovisual and computer elements—as opposed to considering only the audiovisual display—will be to raise certain appropriations to the level of copyright infringement

Mott Insulator to Superfluid transition in Bose-Bose mixtures in a two-dimensional lattice

We perform a numeric study (Worm algorithm Monte Carlo simulations) of ultracold two-component bosons in two-dimensional optical lattices. We study how the Mott insulator to superfluid transition is affected by the presence of a second superfluid bosonic species. We find that, at fixed interspecies interaction, the upper and lower boundaries of the Mott lobe are differently modified. The lower boundary is strongly renormalized even for relatively low filling factor of the second component and moderate (interspecies) interaction. The upper boundary, instead, is affected only for large enough filling of the second component. Whereas boundaries are renormalized we find evidence of polaron-like excitations. Our results are of interest for current experimental setups.Comment: 4 pages, 3 figures, accepted as PRA Rapid Communicatio

Federal Copyright Law in the Computer Era: Protection for the Authors of Video Games

This Comment analyzes both the manner and scope of copyright protection currently afforded computer video games. It then discusses the means available under federal copyright laws to protect the underlying computer program and concludes that the game should be regarded as a unit. The effect of treating the game as a unit of audiovisual and computer elements—as opposed to considering only the audiovisual display—will be to raise certain appropriations to the level of copyright infringement

Dielectric function and plasmons in graphene

The electromagnetic response of graphene, expressed by the dielectric function, and the spectrum of collective excitations are studied as a function of wave vector and frequency. Our calculation is based on the full band structure, calculated within the tight-binding approximation. As a result, we find plasmons whose dispersion is similar to that obtained in the single-valley approximation by Dirac fermions. In contrast to the latter, however, we find a stronger damping of the plasmon modes due to inter-band absorption. Our calculation also reveals effects due to deviations from the linear Dirac spectrum as we increase the Fermi energy, indicating an anisotropic behavior with respect to the wave vector of the external electromagnetic field

Tunneling between edge states in a quantum spin Hall system

We analyze a quantum spin Hall (QSH) device with a point contact connecting two of its edges. The contact supports a net spin tunneling current that can be probed experimentally via a two-terminal resistance measurement. We find that the low-bias tunneling current and the differential conductance exhibit scaling with voltage and temperature that depend nonlinearly on the strength of the electron-electron interaction.Comment: 4 pages, 3 figures; published versio

Quantum-enhanced capture of photons using optical ratchet states

Natural and artificial light harvesting systems often operate in a regime where the flux of photons is relatively low. Besides absorbing as many photons as possible it is therefore paramount to prevent excitons from annihilation via photon re-emission until they have undergone an irreversible energy conversion process. Taking inspiration from photosynthetic antenna structures, we here consider ring-like systems and introduce a class of states we call ratchets: excited states capable of absorbing but not emitting light. This allows our antennae to absorb further photons whilst retaining the excitations from those that have already been captured. Simulations for a ring of four sites reveal a peak power enhancement by up to a factor of 35 under ambient conditions owing to a combination of ratcheting and the prevention of emission through dark-state population. In the slow extraction limit the achievable power enhancement due to ratcheting alone exceeds 20%.Comment: major revision with improved model (all data and figures updated

Suppression of electron relaxation and dephasing rates in quantum dots caused by external magnetic fields

An external magnetic field has been applied in laterally coupled dots (QDs) and we have studied the QD properties related to charge decoherence. The significance of the applied magnetic field to the suppression of electron-phonon relaxation and dephasing rates has been explored. The coupled QDs have been studied by varing the magnetic field and the interdot distance as other system parameters. Our numerical results show that the electron scattering rates are strongly dependent on the applied external magnetic field and the details of the double QD configuration.Comment: 13 pages, 6 figure

Thermopower of gapped bilayer graphene

We calculate thermopower of clean and impure bilayer graphene systems. Opening a band gap through the application of an external electric field is shown to greatly enhance the thermopower of bilayer graphene, which is more than four times that of the monolayer graphene and gapless bilayer graphene at room temperature. The effect of scattering by dilute charged impurities is discussed in terms of the self-consistent Born approximation. Temperature dependence of the thermopower is also analyzed.Comment: 8 pages, 5 figures; An inconsistency in the definitions of Eq.(17) and (18) in version 1 is found and correcte