Synthesis of phosphatidic acids via cobalt(salen) catalyzed epoxide ring-opening with dibenzyl phosphate

With a CoIII(salen)OTs catalyst, dibenzyl phosphate ring-opens a variety of terminal epoxides with excellent regio-selectively and yields up to 85%. The reaction is used in a highly efficient synthesis of enantiopure mixed-diacyl phosphatidic acids, including a photoswitchable phosphatidic acid mimic

The correlations between the spin frequencies and kHz QPOs of Neutron Stars in LMXBs

We studied the correlations between spin frequencies and kilohertz quasi-periodic oscillations (kHz QPOs) in neutron star low mass X-ray binaries. The updated data of kHz QPOs and spin frequencies are statistically analyzed. We found that when two simultaneous kHz QPOs are present in the power spectrum, the minimum frequency of upper kHz QPO is at least 1.3 times larger than the spin frequency, i.e. \nu_{s}<\nu_{2min}/1.3. We also found that the average kHz QPO peak separation in 6 Atoll sources anti-correlates with the spin frequency in the form \lan\dn\ran = -(0.19\pm0.05)\ns+(389.40\pm21.67)Hz. If we shifted this correlation in the direction of the peak separation by a factor of 1.5, this correlation matches the data points of the two accretion powered millisecond X-ray pulsars, SAX J1808.4-3658 and XTE J1807-294.Comment: 4 pages, 2 figures, accepted for publication in A&

Spin Dynamics of the Magnetoresistive Pyrochlore Tl_2Mn_2O_7

Neutron scattering has been used to study the magnetic order and spin dynamics of the colossal magnetoresistive pyrochlore Tl_2Mn_2O_7. On cooling from the paramagnetic state, magnetic correlations develop and appear to diverge at T_C (123 K). In the ferromagnetic phase well defined spin waves are observed, with a gapless ($\Delta <0.04$ meV) dispersion relation E=Dq^{2} as expected for an ideal isotropic ferromagnet. As T approaches T_C from low T, the spin waves renormalize, but no significant central diffusive component to the fluctuation spectrum is observed in stark contrast to the La$_{1-x}$(Ca,Ba,Sr)$_x$MnO$_3$ system. These results argue strongly that the mechanism responsible for the magnetoresistive effect has a different origin in these two classes of materials.Comment: 4 pages (RevTex), 4 figures (encapsulated postscript), to be published in Phys. Rev. Let

Structure and Spin Dynamics of La0.85_{0.85}Sr0.15_{0.15}MnO3_3

Neutron scattering has been used to study the structure and spin dynamics of La$_{0.85}$Sr$_{0.15}$MnO$_3$. The magnetic structure of this system is ferromagnetic below T_C = 235 K. We see anomalies in the Bragg peak intensities and new superlattice peaks consistent with the onset of a spin-canted phase below T_{CA} = 205 K, which appears to be associated with a gap at q = (0, 0, 0.5) in the spin-wave spectrum. Anomalies in the lattice parameters indicate a concomitant lattice distortion. The long-wavelength magnetic excitations are found to be conventional spin waves, with a gapless (< 0.02 meV) isotropic dispersion relation $E = Dq^2$. The spin stiffness constant D has a $T^{5/2}$ dependence at low T, and the damping at small q follows $q^4T^{2}$. An anomalously strong quasielastic component, however, develops at small wave vector above 200 K and dominates the fluctuation spectrum as T -> T_C. At larger q, on the other hand, the magnetic excitations become heavily damped at low temperatures, indicating that spin waves in this regime are not eigenstates of the system, while raising the temperature dramatically increases the damping. The strength of the spin-wave damping also depends strongly on the symmetry direction in the crystal. These anomalous damping effects are likely due to the itinerant character of the $e_g$ electrons.Comment: 8 pages (RevTex), 9 figures (encapsulated postscript

Modeling Micro-Porous Surfaces for Secondary Electron Emission Control to Suppress Multipactor

This work seeks to understand how the topography of a surface can be engineered to control secondary electron emission (SEE) for multipactor suppression. Two unique, semi-empirical models for the secondary electron yield (SEY) of a micro-porous surface are derived and compared. The first model is based on a two-dimensional (2D) pore geometry. The second model is based on a three-dimensional (3D) pore geometry. The SEY of both models is shown to depend on two categories of surface parameters: chemistry and topography. An important parameter in these models is the probability of electron emissions to escape the surface pores. This probability is shown by both models to depend exclusively on the aspect ratio of the pore (the ratio of the pore height to the pore diameter). The increased accuracy of the 3D model (compared to the 2D model) results in lower electron escape probabilities with the greatest reductions occurring for aspect ratios less than two. In order to validate these models, a variety of micro-porous gold surfaces were designed and fabricated using photolithography and electroplating processes. The use of an additive metal-deposition process (instead of the more commonly used subtractive metal-etch process) provided geometrically ideal pores which were necessary to accurately assess the 2D and 3D models. Comparison of the experimentally measured SEY data with model predictions from both the 2D and 3D models illustrates the improved accuracy of the 3D model. For a micro-porous gold surface consisting of pores with aspect ratios of two and a 50% pore density, the 3D model predicts that the maximum total SEY will be one. This provides optimal engineered surface design objectives to pursue for multipactor suppression using gold surfaces

Attentional Bias for Cues Signaling Punishment and Reward in Adolescents:Cross-Sectional and Prognostic Associations with Symptoms of Anxiety and Behavioral Disorders

Heightened reward sensitivity has been proposed as a risk factor for developing behavioral disorders whereas heightened punishment sensitivity has been related to the development of anxiety disorders in youth. Combining a cross-sectional (n = 696, mean age = 16.14) and prospective (n = 598, mean age = 20.20) approach, this study tested the hypotheses that an attentional bias for punishing cues is involved in the development of anxiety disorders and an attentional bias for rewarding cues in the development of behavioral disorders. A spatial orientation task was used to examine the relation between an attentional bias for punishing cues and an attentional bias for rewarding cues with anxiety and behavioral problems in a subsample of a large prospective population cohort study. Our study indicates that attentional biases to general cues of punishment and reward do not seem to be important risk factors for the development of anxiety or behavioral problems respectively. It might be that attentional biases play a role in the maintenance of psychological problems. This remains open for future research

Electronic and magnetic states in doped LaCoO_3

The electronic and magnetic states in doped perovskite cobaltites, (La, Sr)CoO_3, are studied in the numerically exact diagonalization method on Co_2O_{11} clusters. For realistic parameter values, it is shown that a high spin state and an intermediate spin state coexist in one-hole doped clusters due to strong p-d mixing. The magnetic states in the doped cobaltites obtained in the calculation explain various experimental results.Comment: 4 pages, 2 figures, epsfj.st

Spin versus Lattice Polaron: Prediction for Electron-Doped CaMnO3

CaMnO3 is a simple bi-partite antiferromagnet(AF) which can be continuously electron-doped up to LaMnO3. Electrons enter the doubly degenerate E_g subshell with spins aligned to the S=3/2 core of Mn^4+ (T_2g^3)$. We take the Hubbard and Hund energies to be effectively infinite. Our model Hamiltonian has two E_g orbitals per Mn atom, nearest neighbor hopping, nearest neighbor exchange coupling of the S=3/2 cores, and electron-phonon coupling of Mn orbitals to adjacent oxygen atoms. We solve this model for light doping. Electrons are confined in local ferromagnetic (FM) regions (spin polarons) where there proceeds an interesting competition between spin polarization (spin polarons) which enlarges the polaron, and lattice polarization (Jahn-Teller polarons) which makes it smaller. A symmetric 7-atom ferromagnetic cluster (Mn_7^27+) is the stable result, with net spin S=2 relative to the undoped AF. The distorted oxygen positions around the electron are predicted. The model also predicts a critical doping x_c=0.045 where the polaronic insulator becomes unstable relative to a FM metal.Comment: 9 pages with 7 embedded postscript figures and 2 table

Gravitational waves from Sco X-1: A comparison of search methods and prospects for detection with advanced detectors

The low-mass X-ray binary Scorpius X-1 (Sco X-1) is potentially the most luminous source of continuous gravitational-wave radiation for interferometers such as LIGO and Virgo. For low-mass X-ray binaries this radiation would be sustained by active accretion of matter from its binary companion. With the Advanced Detector Era fast approaching, work is underway to develop an array of robust tools for maximizing the science and detection potential of Sco X-1. We describe the plans and progress of a project designed to compare the numerous independent search algorithms currently available. We employ a mock-data challenge in which the search pipelines are tested for their relative proficiencies in parameter estimation, computational efficiency, robust- ness, and most importantly, search sensitivity. The mock-data challenge data contains an ensemble of 50 Scorpius X-1 (Sco X-1) type signals, simulated within a frequency band of 50-1500 Hz. Simulated detector noise was generated assuming the expected best strain sensitivity of Advanced LIGO and Advanced VIRGO ($4 \times 10^{-24}$ Hz$^{-1/2}$). A distribution of signal amplitudes was then chosen so as to allow a useful comparison of search methodologies. A factor of 2 in strain separates the quietest detected signal, at $6.8 \times 10^{-26}$ strain, from the torque-balance limit at a spin frequency of 300 Hz, although this limit could range from $1.2 \times 10^{-25}$ (25 Hz) to $2.2 \times 10^{-26}$ (750 Hz) depending on the unknown frequency of Sco X-1. With future improvements to the search algorithms and using advanced detector data, our expectations for probing below the theoretical torque-balance strain limit are optimistic.Comment: 33 pages, 11 figure