Field-Induced Quasiparticle Excitation in Ca(Al0.5_{0.5}Si0.5_{0.5})2_2: Evidence for unconventional Superconductivity

The temperature ($T$) and magnetic field ($H$) dependence of the magnetic penetration depth, $\lambda(T,H)$, in Ca(Al$_{0.5}$Si$_{0.5}$)$_2$ exhibits significant deviation from that expected for conventional BCS superconductors. In particular, it is inferred from a field dependence of $\lambda(H)$ ($\propto H$) at 2.0 K that the quasiparticle excitation is strongly enhanced by the Doppler shift. This suggests that the superconducting order parameter in Ca(Al$_{0.5}$Si$_{0.5}$)$_2$ is characterized by a small energy scale $\Delta_S/k_B\le 2$ K originating either from anisotropy or multi-gap structure.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jp

New detections of isotopic molecular absorption lines: a low ¹²C:¹³C ratio in nearby gas

Molecular absorption line observations towards the background source Sgr B2 `M' are presented. Previous observations have shown that there are ~9 foreground clouds of moderate density along this line of sight, which produce absorption lines that are well spaced in velocity. In two of these clouds, first detections have now been made of the rare isotopomers 12CS, HN13C, HC15N and HC18O+. For a feature at lsr velocities of -4 to +18km s-1, the isotopic ratio 12C:13C has been estimated, from the relative intensities of 12CS and 13CS J=1-0 lines, and also by comparing the strength of the 13CS line with that of C34S J=1-0 observed previously. A convergent solution for the two methods is found if 12CS is optically thick but the isotopomer lines are optically thin. In this case 12C:13C is 24±11, which is surprisingly low if the gas lies near the Sun, as indicated by its velocity. However, it has been suggested that parts of this feature may in fact arise in hot gas close to the Sgr B2 cloud, where a low isotope ratio is expected. If this region of the line is excluded, the 12C:13C ratio for the remaining lsr velocities of +11 to +18kms-1 is only slightly changed, with a value of 22±13. This is the true carbon isotope ratio in some nearby gas, if effects such as peculiar velocities and isotopic fractionation are unimportant. The value found here is well below the local average of ~60-70 in the solar neighbourhood, which suggests that some of the nearby absorbing gas has been recently isotopically enriched by stellar ejecta. This moderate density absorbing gas is then more likely to be material left over after star-formation, rather than a pre-star-for

Atomic Carbon and CO Isotope Emission in the Vicinity of DR15

We present observations of the 3P1-3P0 fine structure transition of atomic carbon [CI], the J=3-2 transition of CO, as well as of the J=1-0 transitions of 13CO and C18O toward DR15, an HII region associated with two mid-infrared dark clouds (IRDCs). The 13CO and C18O J=1-0 emissions closely follow the dark patches seen in optical wavelength, showing two self-gravitating molecular cores with masses of 2000 Msun and 900 Msun, respectively, at the positions of the catalogued IRDCs. Our data show a rough spatial correlation between [CI] and 13CO J=1-0. Bright [CI] emission occurs in relatively cold gas behind the molecular cores, neither in highly excited gas traced by CO J=3-2 emission nor in HII region/molecular cloud interface. These results are inconsistent with those predicted by standard photodissociation region (PDR) models, suggesting an origin for interstellar atomic carbon unrelated to photodissociation processes.Comment: 11 pages Latex, 6 figures, Accepted for publication in The Astrophysical Journa

Observation of the first gravitational microlensing event in a sparse stellar field : the Tago event

We report the observation of the first gravitational microlensing event in a sparse stellar field, involving the brightest (V=11.4 mag) andclosest (~ 1 kpc) source star to date. This event was discovered by an amateurastronomer, A. Tago, on 2006 October 31 as a transient brightening, by ~4.5 mag during a ~15 day period, of a normal A-type star (GSC 3656-1328) in the Cassiopeia constellation. Analysis of both spectroscopic observations and the light curve indicates that this event was caused by gravitational microlensing rather than an intrinsically variable star. Discovery of this single event over a 30 year period is roughly consistent with the expected microlensing rate for the whole sky down to V = 12 mag stars. However, the probability for finding events with such a high magnification (~ 50) is much smaller, by a factor ~1/50, which implies that the true event rate may be higher than expected. This discovery indicates the potential of all sky variability surveys, employing frequent sampling by telescopes with small apertures and wide fields of view, for finding such rare transient events, and using the observations to explore galactic disk structure and search for exo-planets.Comment: 13 pages, 2 tables, 3 figures, accepted by Ap

Muonium as a shallow center in GaN

A paramagnetic muonium (Mu) state with an extremely small hyperfine parameter was observed for the first time in single-crystalline GaN below 25 K. It has a highly anisotropic hyperfine structure with axial symmetry along the [0001] direction, suggesting that it is located either at a nitrogen-antibonding or a bond-centered site oriented parallel to the c-axis. Its small ionization energy (=< 14 meV) and small hyperfine parameter (--10^{-4} times the vacuum value) indicate that muonium in one of its possible sites produces a shallow state, raising the possibility that the analogous hydrogen center could be a source of n-type conductivity in as-grown GaN.Comment: 4 figures, to be published in Phys. Rev. Letter