Excellent daytime seeing at Dome Fuji on the Antarctic plateau

Context. Dome Fuji, the second highest region on the Antarctic plateau, is expected to have some of the best astronomical seeing on Earth. However, site testing at Dome Fuji is still in its very early stages. Aims. To investigate the astronomical seeing in the free atmosphere above Dome Fuji, and to determine the height of the surface boundary layer. Methods. A Differential Image Motion Monitor was used to measure the seeing in the visible (472 nm) at a height of 11 m above the snow surface at Dome Fuji during the austral summer of 2012/2013. Results. Seeing below 0.2'' has been observed. The seeing often has a local minimum of ~0.3'' near 18 h local time. Some periods of excellent seeing, 0.3'' or smaller, were also observed, sometimes extending for several hours at local midnight. The median seeing is higher, at 0.52''---this large value is believed to be caused by periods when the telescope was within the turbulent boundary layer. Conclusions. The diurnal variation of the daytime seeing at Dome Fuji is similar to that reported for Dome C, and the height of the surface boundary layer is consistent with previous simulations for Dome Fuji. The free atmosphere seeing is ~0.2'', and the height of the surface boundary layer can be as low as ~11 m.Comment: 4 pages, 6 figures, Submitted to Astronomy & Astrophysics (letter

Sliding Density-Wave in Sr_{14}Cu_{24}O_{41} Ladder Compounds

We used transport and Raman scattering measurements to identify the insulating state of self-doped spin 1/2 two-leg ladders of Sr_{14}Cu_{24}O_{41} as a weakly pinned, sliding density wave with non-linear conductivity and a giant dielectric response that persists to remarkably high temperatures

A radiation driven implosion model for the enhanced luminosity of protostars near HII regions

Context. Molecular clouds near the H II regions tend to harbor more luminous protostars. Aims. Our aim in this paper is to investigate whether or not radiation-driven implosion mechanism enhances luminosity of protostars near regions of high-ionizing fluxes. Methods. We performed numerical simulations to model collapse of cores exposed to UV radiation from O stars. We investigated dependence of mass loss rates on the initial density profiles of cores and variation of UV fluxes. We derived simple analytic estimates of accretion rates and final masses of protostars. Results. Radiation-driven implosion mechanism can increase accretion rates of protostars by 1-2 orders of magnitude. On the other hand, mass loss due to photo-evaporation is not large enough to have a significant impact on the luminosity. The increase of accretion rate makes luminosity 1-2 orders higher than those of protostars that form without external triggering. Conclusions. Radiation-driven implosion can help explain the observed higher luminosity of protostars in molecular clouds near H II regions.Comment: 9 pages, 6 figures, accepted for publication in Astronomy and Astrophysic

Manifestation of spin-charge separation in the dynamic dielectric response of one--dimensional Sr2CuO3

We have determined the dynamical dielectric response of a one-dimensional, correlated insulator by carrying out electron energy-loss spectroscopy on Sr2CuO3 single crystals. The observed momentum and energy dependence of the low-energy features, which correspond to collective transitions across the gap, are well described by an extended one-band Hubbard model with moderate nearest neighbor Coulomb interaction strength. An exciton-like peak appears with increasing momentum transfer. These observations provide experimental evidence for spin-charge separation in the relevant excitations of this compound, as theoretically expected for the one-dimensional Hubbard model.Comment: RevTex, 4 pages+2 figures, to appear in PRL (July 13

Microwave and millimeter wave spectroscopy in the slightly hole-doped ladders of Sr14_{14}Cu24_{24}O41_{41}

We have measured the temperature- and frequency dependence of the microwave and millimeter wave conductivity $\sigma_1(T,\omega)$ along both the ladder (c-axis) and the leg (a-axis) directions in Sr$_{14}$Cu$_{24}$O$_{41}$. Below a temperature $T^*$($\sim$170 K), we observed a stronger frequency dependence in $\sigma_1^c(T,\omega)$ than that in $\sigma_1^a(T,\omega)$, forming a small resonance peak developed between 30 GHz and 100 GHz. We also observed nonlinear dc conduction along the c-axis at rather low electric fields below $T^*$. These results suggest some collective excitation contributes to the c-axis charge dynamics of the slightly hole-doped ladders of Sr$_{14}$Cu$_{24}$O$_{41}$ below $T^*$.Comment: 7 pages, 4 figure, to be published in Europhysics Letter

Analysis of the spectral function of Nd1.85Ce0.15CuO4, obtained by angle resolved photoemission spectroscopy

Samples of Nd(2-x)Ce(x)CuO(4), an electron-doped high temperature superconducting cuprate (HTSC), near optimal doping at x = 0.155 were measured via angle resolved photoemission (ARPES). We report a renormalization feature in the self energy ("kink") in the band dispersion at 50 - 60 meV present in nodal and antinodal cuts across the Fermi surface. Specifically, while the kink had previously only been seen in the antinodal region, it is now observed also in the nodal region, reminiscent of what has been observed in hole-doped cuprates.Comment: 4 pages, 4 figure

Ultrafast optical nonlinearity in quasi-one-dimensional Mott-insulator Sr2CuO3{\rm Sr_2CuO_3}

We report strong instantaneous photoinduced absorption (PA) in the quasi-one-dimensional Mott insulator ${\rm Sr_2CuO_3}$ in the IR spectral region. The observed PA is to an even-parity two-photon state that occurs immediately above the absorption edge. Theoretical calculations based on a two-band extended Hubbard model explains the experimental features and indicates that the strong two-photon absorption is due to a very large dipole-coupling between nearly degenerate one- and two-photon states. Room temperature picosecond recovery of the optical transparency suggests the strong potential of ${\rm Sr_2CuO_3}$ for all-optical switching.Comment: 10 pages, 4 figure