Commensurate and Incommensurate Vortex Lattice Melting in Periodic Pinning Arrays

We examine the melting of commensurate and incommensurate vortex lattices interacting with square pinning arrays through the use of numerical simulations. For weak pinning strength in the commensurate case we observe an order-order transition from a commensurate square vortex lattice to a triangular floating solid phase as a function of temperature. This floating solid phase melts into a liquid at still higher temperature. For strong pinning there is only a single transition from the square pinned lattice to the liquid state. For strong pinning in the incommensurate case, we observe a multi-stage melting in which the interstitial vortices become mobile first, followed by the melting of the entire lattice, consistent with recent imaging experiments. The initial motion of vortices in the incommensurate phase occurs by an exchange process of interstitial vortices with vortices located at the pinning sites. We have also examined the vortex melting behavior for higher matching fields and find that a coexistence of a commensurate pinned vortex lattice with an interstitial vortex liquid occurs while at higher temperatures the entire vortex lattice melts. For triangular arrays at incommensurate fields higher than the first matching field we observe that the initial vortex motion can occur through a novel correlated ring excitation where a number of vortices can rotate around a pinned vortex. We also discuss the relevance of our results to recent experiments of colloidal particles interacting with periodic trap arrays.Comment: 8 figure

Yrast structures in the neutron-rich isotopes Fe59,60 and the role of the g9/2 orbital

The structure of the neutron-rich isotopes Fe59,60 has been studied with the Gammasphere detector array using fusion-evaporation reactions. Level schemes for these nuclei are presented which have been extended to spins of ∼20. Both isotopes exhibit regular, near-yrast γ-decay sequences which are generated by the intrusion of the g9/2 orbital into the fp shell-model space. Lower-spin, natural-parity levels are discussed within the context of shell-model calculations using the GXPF1A interaction in the full fp model space. Experimental features of the high-spin bands are compared with total Routhian surface calculations

High-spin structures in the neutron-rich isotopes Mn57-60

Excited states in the neutron-rich isotopes Mn57-60 have been studied with fusion-evaporation reactions induced by Ca48 beams at 130 MeV on C13,14 targets. Level schemes have been deduced reaching spins of ∼16□ and ∼27□/2 in the odd-odd and odd-even isotopes, respectively. States with natural parity within an fp model space are compared to the predictions of large-scale shell-model calculations using the recently developed GXPF1A effective interaction. Quasirotational structures are evident in all of the isotopes and are discussed in terms of the deformation-driving potential of the ν1g9/2 intruder orbital. It is apparent that an enlarged model space, incorporating at least the 1g9/2 intruder state, is necessary to reproduce the observed experimental systematics in a more satisfactory manner

Magnetic rotation and quasicollective structures in 58Fe: Influence of the νg9/2 orbital

The structure of 58Fe was investigated at Gammasphere using 48Ca(13 ,14C,xn) fusion-evaporation reactions at a beam energy of 130 MeV. The level scheme has been revised and extended to J∼17 and an excitation energy of 16.6 MeV. Regular band structures consisting of low-energy ΔJ=1 transitions have been observed at moderate spin (J∼8-15) and are candidates for magnetic rotational bands. Self-consistent tilted-axis-cranking calculations within a relativistic mean-field theory were applied to investigate these bands and were found to reproduce the experimental results well. In other parts of the level scheme, quasirotational bands composed of stretched-E2 transitions have been extended to high spin, and other new bands have been identified. Positive-parity experimental states were compared to predictions of the spherical shell model using the GXPF1A, KB3G, and FPD6 effective interactions in the fp model space. The projected shell model, with a deformed quasiparticle basis including the neutron νg9/2 orbital, was applied to interpret regular ΔJ=2 band structures that extend beyond the maximum spin available for π[(f7/2)-2]- ν[(p3/2f 5/2p1/2)4] configurations and exhibit features characteristic of rotational alignment. It is clear that the νg9/2 intruder orbital plays a crucial role in describing the quasirotational structures in this nucleus, even starting as low as J∼5