976 research outputs found
Localized ferromagnetic resonance force microscopy in permalloy-cobalt films
We report Ferromagnetic Resonance Force Microscopy (FMRFM) experiments on a
justaposed continuous films of permalloy and cobalt. Our studies demonstrate
the capability of FMRFM to perform local spectroscopy of different
ferromagnetic materials. Theoretical analysis of the uniform resonance mode
near the edge of the film agrees quantitatively with experimental data. Our
experiments demonstrate the micron scale lateral resolution in determining
local magnetic properties in continuous ferromagnetic samples.Comment: 7 pages, 3 figure
Phase Separation of the Two-Dimensional t-J model
The boundary of phase separation of the two-dimensional t-J model is
investigated by the power-Lanczos method and Maxwell construction. The method
is similar to a variational approach and it determines the lower bound of the
phase separation boundary with in the limit . In
the physical interesting regime of high T_c superconductors where
there is no phase separation.Comment: LaTex 5 pages, 4 figure
Self-field effects upon the critical current density of flat superconducting strips
We develop a general theory to account self-consistently for self-field
effects upon the average transport critical current density Jc of a flat
type-II superconducting strip in the mixed state when the bulk pinning is
characterized by a field-dependent depinning critical current density Jp(B),
where B is the local magnetic flux density. We first consider the possibility
of both bulk and edge-pinning contributions but conclude that bulk pinning
dominates over geometrical edge-barrier effects in state-of-the-art YBCO films
and prototype second-generation coated conductors. We apply our theory using
the Kim model, JpK(B) = JpK(0)/(1+|B|/B0), as an example. We calculate Jc(Ba)
as a function of a perpendicular applied magnetic induction Ba and show how
Jc(Ba) is related to JpK(B). We find that Jc(Ba) is very nearly equal to
JpK(Ba) when Ba > Ba*, where Ba* is the value of Ba that makes the net flux
density zero at the strip's edge. However, Jc(Ba) is suppressed relative to
JpK(Ba) at low fields when Ba < Ba*, with the largest suppression occurring
when Ba*/B0 is of order unity or larger.Comment: 9 pages, 4 figures, minor revisions to add four reference
Genomic-Phenomic Reciprocal Illumination: Desyopone hereon gen. et sp. nov., an Exceptional Aneuretine-like Fossil Ant from Ethiopian Amber (Hymenoptera: Formicidae: Ponerinae) â€
Simple Summary: We describe a new species of extinct ants from Miocene-aged Ethiopian amber, based on males that resemble species of the relictual lineage Aneuretinae, but which effectively belong to the Ponerinae, as revealed by advanced 3D-imaging technology (synchrotron radiation micro-computed tomography, SR-µ-CT). We subsequently propose a revision of ant classification at the subfamily level. We also recognize that the new species belongs to a new genus based on recent phylogenomic results that have clarified the generic boundaries of Ponerini ants. Our work, therefore, represents an example of reciprocal illumination between phenomic and genomic data. Abstract: Fossils are critical for understanding the evolutionary diversification, turnover, and morphological disparification of extant lineages. While fossils cannot be sequenced, phenome-scale data may be generated using micro-computed tomography (µ-CT), thus revealing hidden structures and internal anatomy, when preserved. Here, we adduce the male caste of a new fossil ant species from Miocene Ethiopian amber that resembles members of the Aneuretinae, matching the operational definition of the subfamily. Through the use of synchrotron radiation for µ-CT, we critically test the aneuretine-identity hypothesis. Our results indicate that the new fossils do not belong to the Aneuretinae, but rather the Ponerini (Ponerinae). Informed by recent phylogenomic studies, we were able to place the fossils close to the extant genus Cryptopone based on logical character analysis, with the two uniquely sharing absence of the subpetiolar process among all ponerine genera. Consequently, we: (1) revise the male-based key to the global ant subfamilies; (2) revise the definitions of Aneuretinae, Ponerinae, Platythyreini, and Ponerini; (3) discuss the evolution of ant mandibles; and (4) describe the fossils as †Desyopone hereon gen. et sp. nov. Our study highlights the value of males for ant systematics and the tremendous potential of phenomic imaging technologies for the study of ant evolution
Origin of four-fold anisotropy in square lattices of circular ferromagnetic dots
We discuss the four-fold anisotropy of in-plane ferromagnetic resonance (FMR)
field , found in a square lattice of circular Permalloy dots when the
interdot distance gets comparable to the dot diameter . The minimum
, along the lattice axes,
differ by 50 Oe at = 1.1. This anisotropy, not expected in
uniformly magnetized dots, is explained by a non-uniform magnetization
\bm(\br) in a dot in response to dipolar forces in the patterned magnetic
structure. It is well described by an iterative solution of a continuous
variational procedure.Comment: 4 pages, 3 figures, revtex, details of analytic calculation and new
references are adde
Nonlinear Meissner Effect in CuO Superconductors
Recent theories of the NMR in the CuO superconductors are based on a
spin-singlet order parameter. Since this state has nodal lines on
the Fermi surface, nonlinear effects associated with low-energy quasiparticles
become important, particularly at low temperatures. We show that the
field-dependence of the supercurrent, below the nucleation field for vortices,
can be used to locate the positions of the nodal lines of an unconventional gap
in momentum space, and hence test the proposed state.Comment: 5 pages (RevTex), 1 figure (postscript file incl.
35 million-year-old solid-wood-borer beetle larvae support the idea of stressed Eocene amber forests
Eocene amber is an important window into the past about 35 million years ago. The large quantities of resin produced by this forest of the past, resulting in amber, triggered the idea of a forest under stress. Recent findings of higher abundances of hoverfly larvae in Eocene amber, in the modern fauna often associated with wood-borer larvae, provided a hint that wood-borer larvae may have contributed to this stress. Yet, so far only few such larvae have been reported. We have compiled a dozen additional wood-borer larvae in amber, including a giant one of at least 35 mm length in Rovno amber. Heavily damaged fossils furthermore indicate that larger larvae of this type were prone to oxidation and that, at least some, enigmatic tube-like tunnels in larger amber pieces may represent remains of large wood-borer larvae. This find strongly indicates that wood-borer larvae were not rare, but common in the Eocene amber forest, which is compatible with the high abundances of hoverfly larvae and further supports the idea of a forest under stress. Whether the possible higher abundances of wood-borer larvae were the cause of the stress or a symptom of an already stressed forest remains so far unclear
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