Valency of rare earths in RIn3 and RSn3: Ab initio analysis of electric-field gradients

In RIn3 and RSn3 the rare earth (R) is trivalent, except for Eu and Yb, which are divalent. This was experimentally determined in 1977 by perturbed angular correlation measurements of the electric-field gradient on a 111Cd impurity. At that time, the data were interpreted using a point charge model, which is now known to be unphysical and unreliable. This makes the valency determination potentially questionable. We revisit these data, and analyze them using ab initio calculations of the electric-field gradient. From these calculations, the physical mechanism that is responsible for the influence of the valency on the electric-field gradient is derived. A generally applicable scheme to interpret electric-field gradients is used, which in a transparent way correlates the size of the field gradient with chemical properties of the system.Comment: 10 page

Studies Of Vibrational Surface Modes In Ionic Crystals. I. Detailed Shell-model Studies For The Unrelaxed (001) Face Of Seven Crystals Having The Rocksalt Structure

We present a comprehensive, unified account of the lattice dynamics of unrelaxed (001) faces of seven crystals having the rock salt structure, three of which have an absolute gap between the acoustical and optical bulk bands (RbF, RbCl, and NaI) and four of which have overlap between these bulk bands (NaF, NaCl, LiF, and MgO). The results are obtained from calculations of the normal modes of a thin crystal film in which the ionic interactions are described by the rigid-shell model. An outline of the formulation of the shell model for the film is given, and the rapid convergence of surface-phonon frequencies with increasing film thickness is demonstrated. Surface- and pseudo surface-phonon dispersion curves and bulk bands are displayed in the form of the dispersion curves of a 15-layer film over the two-dimensional surface Brillouin zone. The surface-excess phonon density of states fs(ω) is given, and its peaks and valleys are correlated with features in the film dispersion curves. The domains of existence and the vibrational character of the more prominent surface-phonon bands are discussed. We establish the relation between the surface modes of the lattice models and the macroscopic surface waves of the dielectric and elastic continuum theories (Fuchs-Kliewer and Rayleigh waves, respectively). Comparison with experiments is made; the agreement with low-energy atom scattering from single-crystal surfaces of LiF and with inelastic neutron scattering from micro crystallite samples of MgO is particularly notable, although the latter comparison also displays some significant discrepancies. © 1977 The American Physical Society

Report on an all-sky LIGO search for periodic gravitational waves in the S4 data

We report on an all-sky search with the LIGO detectors for periodic gravitational waves in the frequency range 50-1000 Hz and having a negative frequency time derivative with magnitude between zero and $10^{-8}$ Hz/s. Data from the fourth LIGO science run have been used in this search. Three different semi-coherent methods of summing strain power were applied. Observing no evidence for periodic gravitational radiation, we report upper limits on strain amplitude and interpret these limits to constrain radiation from rotating neutron stars.Comment: 5 pages, 1 figure, presented at Amaldi7, Sydney (July 2007

Possible Competing Order-Induced Fermi Arcs in Cuprate Superconductors

We investigate the scenario of competing order (CO) induced Fermi arcs and pseudogap in cuprate superconductors. For hole-type cuprates, both phenomena as a function of temperature and doping level can be accounted for if the CO vanishes at $T^{\ast}$ above the superconducting transition $T_c$ and the CO wave-vector Q is parallel to the antinodal direction. In contrast, the absence of these phenomena and the non-monotonic d-wave gap in electron-type cuprates may be attributed to $T^{\ast} < T_c$ and a CO wave-vector Q parallel to the nodal direction.Comment: 6 pages and 5 figures, with one figure added in the updated version. Accepted for publication in Solid State Communications, Fast Communications. Corresponding author: Nai-Chang Yeh ([email protected]

Hierarchical Hough all-sky search for periodic gravitational waves in LIGO S5 data

We describe a new pipeline used to analyze the data from the fifth science run (S5) of the LIGO detectors to search for continuous gravitational waves from isolated spinning neutron stars. The method employed is based on the Hough transform, which is a semi-coherent, computationally efficient, and robust pattern recognition technique. The Hough transform is used to find signals in the time-frequency plane of the data whose frequency evolution fits the pattern produced by the Doppler shift imposed on the signal by the Earth's motion and the pulsar's spin-down during the observation period. The main differences with respect to previous Hough all-sky searches are described. These differences include the use of a two-step hierarchical Hough search, analysis of coincidences among the candidates produced in the first and second year of S5, and veto strategies based on a $\chi^2$ test.Comment: 7 pages, 2 figures, Amaldi08 proceedings, submitted to JPC

Implications of the measured parameters of PSR J1903+0327 for its progenitor neutron star

Using the intrinsic PSR J1903+0327 parameters evaluated from radio observations (mass, rotation period and dipole magnetic field deduced from the timing properties) we calculate the mass of its neutron star progenitor, M_i, at the onset of accretion. Simultaneously, we derive constraints on average accretion rate Mdot and the pre-accretion magnetic field B_i. Spin-up is modelled by accretion from a thin disk, using the magnetic-torque disk-pulsar coupling model proposed by Kluzniak and Rappaport (2007), improved for the existence of relativistic marginally-stable circular orbit. Orbital parameters in the disk are obtained using the space-time generated by a rotating neutron star in the framework of General Relativity. We employ an observationally motivated model of the surface magnetic field decay. We also seek for the imprint of the poorly known equation of state of dense matter on the spin-up tracks - three equations of state of dense matter, consistent with the existence of 2 Msun neutron star, are considered. We find that the minimum average accretion rate should be larger than 2-8 10^(-10) Msun/yr, the highest lower bound corresponding to the stiffest equation of state. We conclude that the influence of magnetic field in the "recycling" process is crucial - it leads to a significant decrease of spin-up rate and larger accreted masses, in comparison to the B=0 model. Allowed B_i-dependent values of M_i are within 1.0-1.4 Msun, i.e., much lower than an oversimplified but widely used B=0 result, where one gets M_i>1.55 Msun. Estimated initial neutron-star mass depends on the assumed dense-matter equation of state.Comment: 11 pages, 10 figures; A&A accepte

First LIGO search for gravitational wave bursts from cosmic (super)strings

We report on a matched-filter search for gravitational wave bursts from cosmic string cusps using LIGO data from the fourth science run (S4) which took place in February and March 2005. No gravitational waves were detected in 14.9 days of data from times when all three LIGO detectors were operating. We interpret the result in terms of a frequentist upper limit on the rate of gravitational wave bursts and use the limits on the rate to constrain the parameter space (string tension, reconnection probability, and loop sizes) of cosmic string models.Comment: 11 pages, 3 figures. Replaced with version submitted to PR

All-sky LIGO Search for Periodic Gravitational Waves in the Early S5 Data

We report on an all-sky search with the LIGO detectors for periodic gravitational waves in the frequency range 50--1100 Hz and with the frequency's time derivative in the range -5.0E-9 Hz/s to zero. Data from the first eight months of the fifth LIGO science run (S5) have been used in this search, which is based on a semi-coherent method (PowerFlux) of summing strain power. Observing no evidence of periodic gravitational radiation, we report 95% confidence-level upper limits on radiation emitted by any unknown isolated rotating neutron stars within the search range. Strain limits below 1.E-24 are obtained over a 200-Hz band, and the sensitivity improvement over previous searches increases the spatial volume sampled by an average factor of about 100 over the entire search band. For a neutron star with nominal equatorial ellipticity of 1.0E-6, the search is sensitive to distances as great as 500 pc--a range that could encompass many undiscovered neutron stars, albeit only a tiny fraction of which would likely be rotating fast enough to be accessible to LIGO. This ellipticity is at the upper range thought to be sustainable by conventional neutron stars and well below the maximum sustainable by a strange quark star.Comment: 6 pages, 1 figur