Magnetohydrostatic equilibrium in starspots: dependences on color (T_{eff}) and surface gravity (g)

Temperature contrasts and magnetic field strengths of sunspot umbrae broadly follow the thermal-magnetic relationship obtained from magnetohydrostatic equilibrium. Using a compilation of recent observations, especially in molecular bands, of temperature contrasts of starspots in cool stars, and a grid of Kurucz stellar model atmospheres constructed to cover layers of sub-surface convection zone, we examine how the above relationship scales with effective temperature T_{eff}, surface gravity g and the associated changes in opacity of stellar photospheric gas. We calculate expected field strengths in starpots and find that a given relative reduction in temperatures (or the same darkness contrasts) yield increasing field strengths against decreasing T_{eff} due to a combination of pressure and opacity variations against T_{eff}.Comment: 4 pages, 3 figures, to appear in the Proceedings of IAUS 273: "Physics of Sun and Star Spots", eds. D.P. Choudhary and K. Strassmeier 2010, Cambridge University Pres

Dynamics and Heating of the Magnetic Network on the Sun: Efficiency of mode transformation

We aim to identify the physical processes which occur in the magnetic network of the chromosphere and which contribute to its dynamics and heating. Specifically, we study the propagation of transverse (kink) MHD waves which are impulsively excited in flux tubes through footpoint motions. When these waves travel upwards, they get partially converted to longitudinal waves through nonlinear effects (mode coupling). By solving the nonlinear, time-dependent MHD equations we find that significant longitudinal wave generation occurs in the photosphere typically for Mach numbers as low as 0.2 and that the onset of shock formation occurs at heights of about 600 km above the photospheric base. We also investigate the compressional heating due to longitudinal waves and the efficiency of mode coupling for various values of the plasma $\beta$, that parameterises the magnetic field strength in the network. We find that this efficiency is maximum for field strengths corresponding to $\beta\approx 0.2$, when the kink and tube wave speeds are almost identical. This can have interesting observational implications. Furthermore, we find that even when the two speeds are different, once shock formation occurs, the longitudinal and transverse shocks exhibit strong mode coupling.Comment: 8 pages, 3 figure

Selective coupling of optical energy into the fundamental diffusion mode of a scattering medium

We demonstrate experimentally that optical wavefront shaping selectively couples light into the fundamental diffusion mode of a scattering medium. The total energy density inside a scattering medium of zinc oxide (ZnO) nanoparticles was probed by measuring the emitted fluorescent power of spheres that were randomly positioned inside the medium. The fluorescent power of an optimized incident wave front is observed to be enhanced compared to a non-optimized incident front. The observed enhancement increases with sample thickness. Based on diffusion theory, we derive a model wherein the distribution of energy density of wavefront-shaped light is described by the fundamental diffusion mode. The agreement between our model and the data is striking not in the least since there are no adjustable parameters. Enhanced total energy density is crucial to increase the efficiency of white LEDs, solar cells, and of random lasers, as well as to realize controlled illumination in biomedical optics.Comment: 5 pages, 5 figure

Exciton energy transfer in nanotube bundles

Photoluminescence is commonly used to identify the electronic structure of individual nanotubes. But, nanotubes naturally occur in bundles. Thus, we investigate photoluminescence of nanotube bundles. We show that their complex spectra are simply explained by exciton energy transfer between adjacent tubes, whereby excitation of large gap tubes induces emission from smaller gap ones via Forster interaction between excitons. The consequent relaxation rate is faster than non-radiative recombination, leading to enhanced photoluminescence of acceptor tubes. This fingerprints bundles with different compositions and opens opportunities to optimize them for opto-electronics.Comment: 5 pages, 5 figure

Ballisticity of nanotube FETs: Role of phonon energy and gate bias

We investigate the role of electron-phonon scattering and gate bias in degrading the drive current of nanotube MOSFETs. Our central results are: (i) Optical phonon scattering significantly decreases the drive current only when gate voltage is higher than a well-defined threshold. It means that elastic scattering mechanisms are most detrimental to nanotube MOSFETs. (ii) For comparable mean free paths, a lower phonon energy leads to a larger degradation of drive current. Thus for semiconducting nanowire FETs, the drive current will be more sensitive than carbon nanotube FETs because of the smaller phonon energies in semiconductors. (iii) Radial breathing mode phonons cause an appreciable reduction in drive current.Comment: 16 pages, 1 table, 4 figure

Single-Dirac-Cone topological surface states in TlBiSe2 class of Topological Insulators

We have investigated several strong spin-orbit coupling ternary chalcogenides related to the (Pb,Sn)Te series of compounds. Our first-principles calculations predict the low temperature rhombohedral ordered phase in TlBiTe2, TlBiSe2, and TlSbX2 (X=Te, Se, S) to be topologically Kane-Mele Z2 = -1 nontrivial. We identify the specific surface termination that realizes the single Dirac cone through first-principles surface state computations. This termination minimizes effects of dangling bonds making it favorable for photoemission (ARPES) experiments. Our analysis predicts that thin films of these materials would harbor novel 2D quantum spin Hall states, and support odd-parity topological superconductivity. For a related work also see arXiv:1003.2615v1. Experimental ARPES results will be published elsewhere.Comment: Accepted for publication in Phys. Rev. Lett. (2010). Submitted March 201

Hubungan Keberadaan Jentik Aedes Aegypti dan Pelaksanaan 3m Plus dengan Kejadian Penyakit Dbd di Lingkungan XVIII Kelurahan Binjai Kota Medan Tahun 2012

The relation of Aedes aegypti larva existence and 3M Plus implementation with DHF disease in Area XVIII Binjai District Medan. Medan is one of DHF endemic area and the most endemic subdistrict is Medan Denai Subdistrict. The most effective way to eradicate DHF disease is with eradicating mosquito nest or PSN DBD, so that is important to know the Aedes aegypti larva existence and effectiveness of 3M Plus implementation.This research aims to know the relation of Aedes aegypti larva existence and 3M Plus implementation with DHF disease. The research location is in Area XVIII Binjai District Medan Denai Subdistrict. This research samples are 100 housewive, that is taken by purposive sampling technique. This research is analytic survey with cross sectional design study using Exact Fisher test.Results showed that the House Index value is 5%, Container Index is 4%. 3M Plus implementation that includes to good category 78% and bad 22% . The relation of Aedes aegypti larva existence with DHF disease has p=0,002. The relation of 3M Plus implementation with DHF disease has p=0,047. From the results, it can be known that there is relation of Aedes aegypti larva existence and 3M Plus implementation with DHF disease in Area XVIII Binjai District. It is suggested to Medan Health Department and Desa Binjai Health Service Center to socialize to do eradicating Aedes aegypti mosquito nest or PSN DBD regularly to decrease the number of DHF disease in Binjai District especially in Area XVIII

Solar physics at the Kodaikanal Observatory: A Historical Perspective

This article traces the birth and growth of solar physics at the Kodaikanal Observatory of the Indian Institute of Astrophysics, Bangalore, India. A major discovery took place here in 1909 by John Evershed who detected radial outflow of matter in the penumbra of sunspots. Major developments at the Observatory since its inception in 1899 as well as the scientific results are highlighted.Comment: 26 pages, 7 figures To appear in "Magnetic Coupling between the Interior and the Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200