611 research outputs found
Constructing genus 3 hyperelliptic Jacobians with CM
Given a sextic CM field , we give an explicit method for finding all genus
3 hyperelliptic curves defined over whose Jacobians are simple and
have complex multiplication by the maximal order of this field, via an
approximation of their Rosenhain invariants. Building on the work of Weng, we
give an algorithm which works in complete generality, for any CM sextic field
, and computes minimal polynomials of the Rosenhain invariants for any
period matrix of the Jacobian. This algorithm can be used to generate genus 3
hyperelliptic curves over a finite field with a given zeta
function by finding roots of the Rosenhain minimal polynomials modulo .Comment: 20 pages; to appear in ANTS XI
Tube Width Fluctuations in F-Actin Solutions
We determine the statistics of the local tube width in F-actin solutions,
beyond the usually reported mean value. Our experimental observations are
explained by a segment fluid theory based on the binary collision approximation
(BCA). In this systematic generalization of the standard mean-field approach
effective polymer segments interact via a potential representing the
topological constraints. The analytically predicted universal tube width
distribution with a stretched tail is in good agreement with the data.Comment: Final version, 5 pages, 4 figure
Magnetic Structure in Fe/Sm-Co Exchange Spring Bilayers with Intermixed Interfaces
The depth profile of the intrinsic magnetic properties in an Fe/Sm-Co bilayer
fabricated under nearly optimal spring-magnet conditions was determined by
complementary studies of polarized neutron reflectometry and micromagnetic
simulations. We found that at the Fe/Sm-Co interface the magnetic properties
change gradually at the length scale of 8 nm. In this intermixed interfacial
region, the saturation magnetization and magnetic anisotropy are lower and the
exchange stiffness is higher than values estimated from the model based on a
mixture of Fe and Sm-Co phases. Therefore, the intermixed interface yields
superior exchange coupling between the Fe and Sm-Co layers, but at the cost of
average magnetization.Comment: 16 pages, 6 figures and 1 tabl
Excitations in confined helium
We design models for helium in matrices like aerogel, Vycor or Geltech from a
manifestly microscopic point of view. For that purpose, we calculate the
dynamic structure function of 4He on Si substrates and between two Si walls as
a function of energy, momentum transfer, and the scattering angle. The
angle--averaged results are in good agreement with the neutron scattering data;
the remaining differences can be attributed to the simplified model used here
for the complex pore structure of the materials. A focus of the present work is
the detailed identification of coexisting layer modes and bulk--like
excitations, and, in the case of thick films, ripplon excitations. Involving
essentially two--dimensional motion of atoms, the layer modes are sensitive to
the scattering angle.Comment: Phys. Rev. B (2003, in press
On the Fredholm property of bisingular pseudodifferential operators
For operators belonging either to a class of global bisingular
pseudodifferential operators on or to a class of bisingular
pseudodifferential operators on a product of two closed smooth
manifolds, we show the equivalence of their ellipticity (defined by the
invertibility of certain associated homogeneous principal symbols) and their
Fredholm mapping property in associated scales of Sobolev spaces. We also prove
the spectral invariance of these operator classes and then extend these results
to the even larger classes of Toeplitz type operators.Comment: 21 pages. Expanded sections 3 and 4. Corrected typos. Added
reference
Quantum sticking, scattering and transmission of 4He atoms from superfluid 4He surfaces
We develop a microscopic theory of the scattering, transmission, and sticking
of 4He atoms impinging on a superfluid 4He slab at near normal incidence, and
inelastic neutron scattering from the slab. The theory includes coupling
between different modes and allows for inelastic processes. We find a number of
essential aspects that must be observed in a physically meaningful and reliable
theory of atom transmission and scattering; all are connected with
multiparticle scattering, particularly the possibility of energy loss. These
processes are (a) the coupling to low-lying (surface) excitations
(ripplons/third sound) which is manifested in a finite imaginary part of the
self energy, and (b) the reduction of the strength of the excitation in the
maxon/roton region
Superconducting triplet pairing in Ni--Ga-bilayer junctions
Ni--Ga bilayers are a versatile platform for exploring the competition
between strongly antagonistic ferromagnetic and superconducting phases. We
characterize the impact of this competition on the transport properties of
highly-ballistic Al/AlO(/EuS)/Ni--Ga tunnel junctions from both
experimental and theoretical points of view. While the conductance spectra of
junctions comprising Ni (3 nm)--Ga (60 nm) bilayers can be well understood
within the framework of earlier results, which associate the emerging main
conductance maxima with the junction films' superconducting gaps, thinner Ni
(1.6 nm)--Ga (30 nm) bilayers entail completely different physics, giving rise
to novel conductance-peak subseries that we term conductance shoulders.
Detecting the paramagnetic Meissner response in Ga from polarized neutron
reflectometry provides the essential experimental hint that these conductance
shoulders are caused by superconducting triplet pairings that Ni's
ferromagnetic exchange interaction induces near thin Ni--Ga bilayers'
interfaces -- most likely owing to inhomogeneously magnetized interface
domains. We further substantiate our findings by means of a phenomenological
theoretical model, clarifying that induced superconducting triplet pairings
around the interface of Ni--Ga bilayers can indeed manifest themselves in the
observed conductance shoulders. Arranging our work in a broader context
demonstrates that Ni--Ga-bilayer junctions have a strong potential for
efficient triplet-pairing engineering in superconducting-spintronics
applications.Comment: 24 pages, 10 figures, 1 tabl
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