Superconductivity in Pseudo-Binary Silicide SrNixSi2-x with AlB2-Type Structure

We demonstrate the emergence of superconductivity in pseudo-binary silicide SrNixSi2-x. The compound exhibits a structural phase transition from the cubic SrSi2-type structure (P4132) to the hexagonal AlB2-type structure (P6/mmm) upon substituting Ni for Si at approximately x = 0.1. The hexagonal structure is stabilized in the range of 0.1 < x < 0.7. The superconducting phase appears in the vicinity of the structural phase boundary. Ni acts as a nonmagnetic dopant, as confirmed by the Pauli paramagnetic behavior.Comment: 12 pages, 5 figure

Generalized Landau-Pollak Uncertainty Relation

The Landau-Pollak uncertainty relation treats a pair of rank one projection valued measures and imposes a restriction on their probability distributions. It gives a nontrivial bound for summation of their maximum values. We give a generalization of this bound (weak version of the Landau-Pollak uncertainty relation). Our generalization covers a pair of positive operator valued measures. A nontrivial but slightly weak inequality that can treat an arbitrary number of positive operator valued measures is also presented.Comment: Simplified the proofs. To be published in Phys.Rev.

The 3-D kinematics of water masers around the semiregular variable RT Virginis

We report observations of water masers around the semiregular variable RT Virginis (RT Vir), which have been made with the Very Long Baseline Array (VLBA) of the National Radio Astronomy Observatory (NRAO) at five epochs, each separated by three weeks of time. We detected about 60 maser features at each epoch. Overall, 61 features, detected at least twice, were tracked by their radial velocities and proper motions. The 3-D maser kinematics exhibited a circumstellar envelope that is expanding roughly spherically with a velocity of about 8 km/s. Asymmetries in both the spatial and velocity distributions of the maser features were found in the envelope, but less significant than that found in other semiregular variables. Systematic radial-velocity drifts of individual maser features were found with amplitudes of <= 2 km/s/yr. For one maser feature, we found a quadratic position shift with time along a straight line on the sky. This apparent motion indicates an acceleration with an amplitude of 33 km/s/yr, implying the passage of a shock wave driven by the stellar pulsation of RT Vir. The acceleration motion is likely seen only on the sky plane because of a large velocity gradient formed in the accelerating maser region. We estimated the distance to RT Vir to be about 220 pc on the basis of both the statistical parallax and model-fitting methods for the maser kinematics.Comment: 18 pages, 8 figures. Accepted to appear in the Astrophysical Journa

The spatiokinematical structure of H_2O and OH masers in the "water fountain" source IRAS 18460-0151

Using the Very Long Baseline Array and the European VLBI Network, we have observed 22.2 GHz H_2O and 1612 MHz OH masers in the "water fountain" source IRAS 18460-0151. The H_2O maser spectrum has a very wide line-of-sight velocity range (~310 km/s) and consists of three groups of emission features at the blue-shifted (-68 km/s <~ V_LSR <~ -17 km/s) and red-shifted (V_LSR ~= 240 km/s) edges as well as around the systemic velocity (112 km/s <~ V_LSR <~ 133 km/s). The first two H_2O spectral components exhibit a highly-collimated high-velocity bipolar jet on the sky, with an angular separation of ~120 milliarcseconds (mas) (240 AU in linear length) and a three-dimensional flow velocity of ~160 km/s. The flow dynamical age is estimated to be only ~6 yr (at the time of the observation epochs of 2006--2007). Interestingly, the systemic velocity component clearly exhibits a spherically-expanding outflow with a radius of ~36 AU and a flow velocity of ~9 km/s. On the other hand, the OH maser spectrum shows double peaks with a velocity separation of ~25 km/s (V_LSR=$111--116 and 138--141 km/s), as typically seen in circumstellar envelopes of OH/IR stars. The angular offset between the velocity-integrated brightness peaks of the two high-velocity H_2O components is ~25 mas (50 AU). The offset direction and the alignment of the red-shifted maser spots are roughly perpendicular to the axis of the H_2O maser flow. High-accuracy astrometry for the H_2O and OH masers demonstrates that the collimated fast jet and the slowly expanding outflow originate from a single or multiple sources which are located within 15 mas (30 AU). On the other hand, the estimated systemic velocity of the collimated jet (V_sys ~87--113 km/s) has a large uncertainty. This makes it difficult to provide strong constraints on models of the central stellar system of IRAS 18460-0151.Comment: 25 pages, 5 figures, 8 tables, accepted for publication in Ap

Conditional sampling for barrier option pricing under the LT method

We develop a conditional sampling scheme for pricing knock-out barrier options under the Linear Transformations (LT) algorithm from Imai and Tan (2006). We compare our new method to an existing conditional Monte Carlo scheme from Glasserman and Staum (2001), and show that a substantial variance reduction is achieved. We extend the method to allow pricing knock-in barrier options and introduce a root-finding method to obtain a further variance reduction. The effectiveness of the new method is supported by numerical results