663 research outputs found
On the reorientation transition of ultra-thin Ni/Cu(001) films
The reorientation transition of the magnetization of ferromagnetic films is
studied on a microscopic basis within a Heisenberg spin model. Using a modified
mean field formulation it is possible to calculate properties of magnetic thin
films with non-integer thicknesses. This is especially important for the
reorientation transition in Ni/Cu(001), as there the magnetic properties are a
sensitive function of the film thickness. Detailed phase diagrams in the
thickness-temperature plane are calculated using experimental parameters and
are compared with experimental measurements by Baberschke and Farle (J. Appl.
Phys. 81, 5038 (1997)).Comment: 7 pages(LaTeX2e) with one figure(eps), accepted for publication in
JMMM. See also http://www.thp.Uni-Duisburg.DE/Publikationen/Publist_Us_R.htm
Theory of the Spin Reorientation Transition of Ultra-Thin Ferromagnetic Films
The reorientation transition of the magnetization of ferromagnetic films is
studied on a microscopic basis within Heisenberg spin models. Analytic
expressions for the temperature dependent anisotropy are derived from which it
is seen that the reduced magnetization in the film surface at finite
temperatures plays a crucial role for this transition. Detailed phase diagrams
in the temperature-thickness plane are calculated.Comment: 6 pages(LaTeX2e), one figure(eps), accepted for publication in JMM
Strongly bound mesons at finite temperature and in magnetic fields from AdS/CFT
We study mesons in N=4 super Yang-Mills theory with fundamental flavors added
at large 't Hooft coupling using the gauge/gravity correspondence. High-spin
mesons are well described by using semiclassical string configurations. We
determine the meson spectrum at finite temperature and in a background magnetic
field.Comment: 15 pages, 11 figures; v2: references adde
Critical Currents and Vortex States at Fractional Matching Fields in Superconductors with Periodic Pinning
We study vortex states and dynamics in 2D superconductors with periodic
pinning at fractional sub-matching fields using numerical simulations. For
square pinning arrays we show that ordered states form at 1/1, 1/2, and 1/4
filling fractions while only partially ordered states form at other filling
fractions, such as 1/3 and 1/5, in agreement with recent imaging experiments.
For triangular pinning arrays we observe matching effects at filling fractions
of 1/1, 6/7, 2/3, 1/3, 1/4, 1/6, and 1/7. For both square and triangular
pinning arrays we also find that, for certian sub-matching fillings, vortex
configurations depend on pinning strength. For weak pinning, ordering in which
a portion of the vortices are positioned between pinning sites can occur.
Depinning of the vortices at the matching fields, where the vortices are
ordered, is elastic while at the incommensurate fields the motion is plastic.
At the incommensurate fields, as the applied driving force is increased, there
can be a transition to elastic flow where the vortices move along the pinning
sites in 1D channels and a reordering transition to a triangular or distorted
triangular lattice. We also discuss the current-voltage curves and how they
relate to the vortex ordering at commensurate and incommensurate fields.Comment: 14 figure
Global Properties of fp-Shell Interactions in Many-nucleon Systems
Spectral distribution theory, which can be used to compare microscopic
interactions over a broad range of nuclei, is applied in an analysis of two
modern effective interactions based on the realistic CD-Bonn potential for
no-core shell model calculations in the fp shell, as well as in
a comparison of these with the realistic shell-model GXPF1 interaction. In
particular, we explore the ability of these interaction to account for the
development of isovector pairing correlations and collective rotational motion
in the fp shell. Our findings expose the similarities of these two-body
interactions, especially as this relates to their pairing and rotational
characteristics. Further, the GXPF1 interaction is used to determine the
strength parameter of a quadrupole term that can be used to augment an
isovector-pairing model interaction with Sp(4) dynamical symmetry, which in
turn is shown to yield reasonable agreement with the low-lying energy spectra
of Ni and Cu.Comment: 21 pages, 3 figures, accepted in Nuclear Physics
Surface Roughness and Effective Stick-Slip Motion
The effect of random surface roughness on hydrodynamics of viscous
incompressible liquid is discussed. Roughness-driven contributions to
hydrodynamic flows, energy dissipation, and friction force are calculated in a
wide range of parameters. When the hydrodynamic decay length (the viscous wave
penetration depth) is larger than the size of random surface inhomogeneities,
it is possible to replace a random rough surface by effective stick-slip
boundary conditions on a flat surface with two constants: the stick-slip length
and the renormalization of viscosity near the boundary. The stick-slip length
and the renormalization coefficient are expressed explicitly via the
correlation function of random surface inhomogeneities. The effective
stick-slip length is always negative signifying the effective slow-down of the
hydrodynamic flows by the rough surface (stick rather than slip motion). A
simple hydrodynamic model is presented as an illustration of these general
hydrodynamic results. The effective boundary parameters are analyzed
numerically for Gaussian, power-law and exponentially decaying correlators with
various indices. The maximum on the frequency dependence of the dissipation
allows one to extract the correlation radius (characteristic size) of the
surface inhomogeneities directly from, for example, experiments with torsional
quartz oscillators.Comment: RevTeX4, 14 pages, 3 figure
Meson Spectra and Magnetic Fields in the Sakai-Sugimoto Model
We continue our study of the dynamics of the flavour sector of the
Sakai-Sugimoto model in the presence of an external magnetic field, uncovering
several features of the meson spectrum at high and low temperatures. We employ
both analytical and numerical methods to study the coupled non-linear equations
that result from the gravity dual.Comment: 44 pages, multiple figure
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