The phaseless auxiliary-field quantum Monte Carlo ͑AF QMC͒ method ͓S. Zhang and H. Krakauer, Phys. Rev. Lett. 90, 136401 ͑2003͔͒ is used to carry out a systematic study of the dissociation and ionization energies of second-row group 3A-7A atoms and dimers: Al, Si, P, S, and Cl. In addition, the P 2 dimer is compared to the third-row As 2 dimer, which is also triply bonded. This method projects the many-body ground state by means of importance-sampled random walks in the space of Slater determinants. The Monte Carlo phase problem, due to the electron-electron Coulomb interaction, is controlled via the phaseless approximation, with a trial wave function ͉⌿ T ͘. As in previous calculations, a mean-field single Slater determinant is used as ͉⌿ T ͘. The method is formulated in the Hilbert space defined by any chosen one-particle basis. The present calculations use a plane wave basis under periodic boundary conditions with norm-conserving pseudopotentials. Computational details of the plane wave AF QMC method are presented. The isolated systems chosen here allow a systematic study of the various algorithmic issues. We show the accuracy of the plane wave method and discuss its convergence with respect to parameters such as the supercell size and plane wave cutoff. The use of standard norm-conserving pseudopotentials in the many-body AF QMC framework is examined.

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14High-field magnetization of the spin-1/2 antiferromagnet α-Cu2V2O7 was measured in pulsed magnetic fields of up to 56 T in order to study its magnetic phase diagram. When the field was applied along the easy axis (the a-axis), two distinct transitions were observed at Hc1 = 6.5 T and Hc2 = 18.0 T. The former is a spin-flop transition typical for a collinear antiferromagnet and the latter is believed to be a spin-flip transition of canted moments. The canted moments, which are induced by the Dzyaloshinskii-Moriya interactions, anti-align for Hc1 < H < Hc2 due to the anisotropic exchange interaction that favors the antiferromagnetic arrangement along the a-axis. Above Hc2, the Zeeman energy of the applied field overcomes the antiferromagnetic anisotropic interaction and the canted moments are aligned along the field direction. Density functional theory was employed to compute the exchange interactions, which were used as inputs for quantum Monte Carlo calculations and then further refined by fitting to the magnetic susceptibility data. Contrary to our previous report in Phys. Rev. B 92, 024423, the dominant exchange interaction is between the third nearest-neighbor spins, which form zigzag spin-chains that are coupled with one another through an intertwining network of the nonnegligible nearest and second nearest-neighbor interactions. In addition, elastic neutron scattering under the applied magnetic fields of up to 10 T reveals the incommensurate helical spin structure in the spin-flop state.

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Hamiltonian coordinate, guiding center code calculations of the confinement of suprathermal ions in quasi-axisymmetric stellarator (QAS) designs have been carried out to evaluate the atractiveness of compact configurations which are optimized for ballooning stability. A new stellarator particle following code is used to predict ion loss rates and particle confinement for thermal and neutral beam ions in a small experiment with R=145 cm, B= 1-2 T and for alpha particles in a reactor size device. In contrast to tokamaks, it is found that high edge poloidal flux has limited value in improving ion confinement in QAS, since collisional pitch angle scattering drives ions into ripple wells and stochastic field regions, where they are quickly lost. The necessity for reduced stellarator ripple fields is emphasized. The high neutral beam ion loss predicted for these configurations suggests that more interesting physics could be explored with an experiment of less constrained size and magnetic field geometry.PACS numbers 52.20.Dq, 52.55.Hc,52.65.Cc,52.65.-y 2

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Density functional calculations of the atomic coordinates in crsytalline lead pyrophosphate ͑Pb 2 P 2 O 7 ͒ are reported. These calculations yield atomic positions differing from a prior x-ray refinement by up to 0.2 Å. The main difference is a change in the orientation of the ͑P 2 O 7 ͒ 4− units to bring certain O ions closer to Pb. An analysis of the resulting structure in terms of pair distribution functions is presented. These show that the most significant changes are in the local Pb coordination. The electronic structure is not significantly affected by the change in crystal structure.

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