Far-infrared measurements of oxygen-doped polycrystalline La2CuO4.0315 superconductor under slow-cooled and fast-cooled conditions

We have studied the far-infrared (far-IR) charge dynamics of an equilibrated pure oxygen doped La2CuO4+0.0315 under slow-cooled and fast-cooled conditions. The superconducting transition temperature (Tc) for the slow-cooled and that for the fast-cooled processes were respectively found to be close to the two intrinsic Tc's: One at 30 K and the other at 15 K. Direct comparison with our previous results and other far-IR and Raman studies on single crystalline La2-xSrxCuO4, we conclude that the topology of the pristine electronic phases that are responsible for the two intrinsic Tc's is holes arranged into two-dimensional (2D) square lattices.Comment: Submitted to PR

Nonlinear optical probe of tunable surface electrons on a topological insulator

We use ultrafast laser pulses to experimentally demonstrate that the second-order optical response of bulk single crystals of the topological insulator Bi$_2$Se$_3$ is sensitive to its surface electrons. By performing surface doping dependence measurements as a function of photon polarization and sample orientation we show that second harmonic generation can simultaneously probe both the surface crystalline structure and the surface charge of Bi$_2$Se$_3$. Furthermore, we find that second harmonic generation using circularly polarized photons reveals the time-reversal symmetry properties of the system and is surprisingly robust against surface charging, which makes it a promising tool for spectroscopic studies of topological surfaces and buried interfaces

ELECTROMYOGRAPHIC ANALYSIS IN ABDOMINAL MUSLCES DURING CURL-UP EXERCISES

Trunk muscle strength is necessary to maintain the core stability. Several abdominal machine exercises have been investigated through electromyographic (EMG) analysis. Escamilla, et al. (2008) studied numerous abdominal exercises and found that Ab Slide and Torso Track were the most effective exercises in activating abdominal muscles with the relative high EMG activities during abdominal exercise. High internal oblique activities and low rectus abdominis activities in bridging stabilization exercise were also found in EMG analysis (Stevens et al., 2006). One of the most common and convenient ways to strengthen abdominal muscle is the sit-up or curl exercise, which could even be performed at home in healthy populations. However, little research has been conducted to study the curl-up exercise. Therefore, the purpose of this study was to investigate the relative EMG activation levels in rectus abdominis and external oblique during the curl-up exercise. The abdominal strengthening technique with higher EMG activities would be revealed in this study

Charge collective modes in an incommensurately modulated cuprate

We report the first measurement of collective charge modes of insulating Sr14Cu24O41 using inelastic resonant x-ray scattering over the complete Brillouin zone. Our results show that the intense excitation modes at the charge gap edge predominantly originate from the ladder-containing planar substructures. The observed ladder modes (E vs. Q) are found to be dispersive for momentum transfers along the "legs" but nearly localized along the "rungs". Dispersion and peakwidth characteristics are similar to the charge spectrum of 1D Mott insulators, and we show that our results can be understood in the strong coupling limit (U >> t_{ladder}> t_{chain}). The observed behavior is in marked contrast to the charge spectrum seen in most two dimensional cuprates. Quite generally, our results also show that momentum-tunability of inelastic scattering can be used to resolve mode contributions in multi-component incommensurate systems.Comment: 4+ pages, 5 figure

Anisotropic softening of collective charge modes in the vicinity of critical doping in a doped Mott insulator

Momentum resolved inelastic resonant x-ray scattering is used to map the evolution of charge excitations over a large range of energies, momenta and doping levels in the electron doped Mott insulator class Nd$_{2-x}$Ce$_x$CuO$_4$. As the doping induced AFM-SC (antiferromagnetic-superconducting) transition is approached, we observe an anisotropic softening of collective charge modes over a large energy scale along the Gamma to (\pi,\pi)-direction, whereas the modes exhibit broadening ($\sim$ 1 eV) with relatively little softening along Gamma to (\pi,0) with respect to the parent Mott insulator (x=0). Our study indicates a systematic collapse of the gap consistent with the scenario that the system dopes uniformly with electrons even though the softening of the modes involves an unusually large energy scale.Comment: 5 pages + 5 Figure

The Ultraluminous X-ray Sources near the Center of M82

We report the identification of a recurrent ultraluminous X-ray source (ULX), a highly absorbed X-ray source (possibly a background AGN), and a young supernova remnant near the center of the starburst galaxy M82. From a series of Chandra observations taken from 1999 to 2005, we found that the transient ULX first appeared in 1999 October. The source turned off in 2000 January, but later reappeared and has been active since then. The X-ray luminosity of this source varies from below the detection level (~2.5e38 erg/s) to its active state in between ~7e39 erg/s and 1.3e40 erg/s (in the 0.5-10 keV energy band) and shows unusual spectral changes. The X-ray spectra of some Chandra observations are best fitted with an absorbed power-law model with photon index ranging from 1.3 to 1.7. These spectra are similar to those of Galactic black hole binary candidates seen in the low/hard state except that a very hard spectrum was seen in one of the observations. By comparing with near infrared images taken with the Hubble Space Telescope, the ULX is found to be located within a young star cluster. Radio imaging indicates that it is associated with a H II region. We suggest that the ULX is likely to be a > 100 solar mass intermediate-mass black hole in the low/hard state. In addition to the transient ULX, we also found a highly absorbed hard X-ray source which is likely to be an AGN and an ultraluminous X-ray emitting young supernova remnant which may be related to a 100-year old gamma-ray burst event, within 2 arcsec of the transient ULX.Comment: 9 pages, 8 figures. Accepted for publication in Ap