1,077 research outputs found
Internal localized eigenmodes on spin discrete breathers in antiferromagnetic chains with on-site easy axis anisotropy
We investigate internal localized eigenmodes of the linearized equation
around spin discrete breathers in 1D antiferromagnets with on-site easy axis
anisotropy. The threshold of occurrence of the internal localized eigenmodes
has a typical structure in parameter space depending on the frequency of the
spin discrete breather. We also performed molecular dynamics simulation in
order to show the validity of our linear analysis.Comment: 4 pages including 5 figure
Probing magnetic order in EELS of chromite spinels using both multiple scattering (FEFF8.2) and DFT (WIEN2k)
The electron energy loss near edge structure on the O K-edge from chromite spinels contains fine structure from the hybridisation of the O p-orbitals and the Cr d-orbitals. Unlike the aluminates, a non-spin polarised calculation of this fine structure differs significantly from experimental observations. This is due to the large magnetic moment on the Cr. Calculations using simplified collinear ordering of the spins and the local spin density approximation give much improved agreement. A real space multiple scattering formalism and a reciprocal space density functional formalism give results in substantial agreement. In general, the actual spin arrangement of these chromites is not known since they are typically frustrated magnetic systems with ordering temperatures in the 10–20 K range. The calculations are based on the hypothesis that dynamic short range order persists to room temperature over the time scale of the interaction with the fast electron. However, it is possible that the observed effects are due to the strong paramagnetism present at room temperatures but which it is not possible to simulate accurately at present
Nanofriction mechanisms derived from the dependence of friction on load and sliding velocity from air to UHV on hydrophilic silicon
This paper examines friction as a function of the sliding velocity and
applied normal load from air to UHV in a scanning force microscope (SFM)
experiment in which a sharp silicon tip slides against a flat Si(100) sample.
Under ambient conditions, both surfaces are covered by a native oxide, which is
hydrophilic. During pump-down in the vacuum chamber housing the SFM, the
behavior of friction as a function of the applied normal load and the sliding
velocity undergoes a change. By analyzing these changes it is possible to
identify three distinct friction regimes with corresponding contact properties:
(a) friction dominated by the additional normal forces induced by capillarity
due to the presence of thick water films, (b) higher drag force from ordering
effects present in thin water layers and (c) low friction due to direct
solid-solid contact for the sample with the counterbody. Depending on
environmental conditions and the applied normal load, all three mechanisms may
be present at one time. Their individual contributions can be identified by
investigating the dependence of friction on the applied normal load as well as
on the sliding velocity in different pressure regimes, thus providing
information about nanoscale friction mechanisms
Calculating the Prepotential by Localization on the Moduli Space of Instantons
We describe a new technique for calculating instanton effects in
supersymmetric gauge theories applicable on the Higgs or Coulomb branches. In
these situations the instantons are constrained and a potential is generated on
the instanton moduli space. Due to existence of a nilpotent fermionic symmetry
the resulting integral over the instanton moduli space localizes on the
critical points of the potential. Using this technology we calculate the one-
and two-instanton contributions to the prepotential of SU(N) gauge theory with
N=2 supersymmetry and show how the localization approach yields the prediction
extracted from the Seiberg-Witten curve. The technique appears to extend to
arbitrary instanton number in a tractable way.Comment: 24 pages, JHEP.cls, more references and extra discussion on N_F=2N
cas
On the study of jamming percolation
We investigate kinetically constrained models of glassy transitions, and
determine which model characteristics are crucial in allowing a rigorous proof
that such models have discontinuous transitions with faster than power law
diverging length and time scales. The models we investigate have constraints
similar to that of the knights model, introduced by Toninelli, Biroli, and
Fisher (TBF), but differing neighbor relations. We find that such knights-like
models, otherwise known as models of jamming percolation, need a ``No Parallel
Crossing'' rule for the TBF proof of a glassy transition to be valid.
Furthermore, most knight-like models fail a ``No Perpendicular Crossing''
requirement, and thus need modification to be made rigorous. We also show how
the ``No Parallel Crossing'' requirement can be used to evaluate the provable
glassiness of other correlated percolation models, by looking at models with
more stable directions than the knights model. Finally, we show that the TBF
proof does not generalize in any straightforward fashion for three-dimensional
versions of the knights-like models.Comment: 13 pages, 18 figures; Spiral model does satisfy property
Pattern formation and localization in the forced-damped FPU lattice
We study spatial pattern formation and energy localization in the dynamics of
an anharmonic chain with quadratic and quartic intersite potential subject to
an optical, sinusoidally oscillating field and a weak damping. The
zone-boundary mode is stable and locked to the driving field below a critical
forcing that we determine analytically using an approximate model which
describes mode interactions. Above such a forcing, a standing modulated wave
forms for driving frequencies below the band-edge, while a ``multibreather''
state develops at higher frequencies. Of the former, we give an explicit
approximate analytical expression which compares well with numerical data. At
higher forcing space-time chaotic patterns are observed.Comment: submitted to Phys.Rev.
Itinerant electron metamagnetism in LaCoSi
The strongly exchange enhanced Pauli paramagnet LaCoSi is found to
exhibit an itinerant metamagnetic phase transition with indications for
metamagnetic quantum criticality. Our investigation comprises magnetic,
specific heat, and NMR measurements as well as ab-initio electronic structure
calculations. The critical field is about 3.5 T for and 6 T for , which is the lowest value ever found for rare earth intermetallic
compounds. In the ferromagnetic state there appears a moment of about 0.2
/Co at the Co-sites, but sigificantly smaller moments at the 4d
and Co-sites.Comment: 11 pages, 5 figures, PRB Rapid Communication, in prin
The Uncertainty in Newton's Constant and Precision Predictions of the Primordial Helium Abundance
The current uncertainty in Newton's constant, G_N, is of the order of 0.15%.
For values of the baryon to photon ratio consistent with both cosmic microwave
background observations and the primordial deuterium abundance, this
uncertainty in G_N corresponds to an uncertainty in the primordial 4He mass
fraction, Y_P, of +-1.3 x 10^{-4}. This uncertainty in Y_P is comparable to the
effect from the current uncertainty in the neutron lifetime, which is often
treated as the dominant uncertainty in calculations of Y_P. Recent measurements
of G_N seem to be converging within a smaller range; a reduction in the
estimated error on G_N by a factor of 10 would essentially eliminate it as a
source of uncertainty in the calculation of the primordial 4He abundance.Comment: 3 pages, no figures, fixed typos, to appear in Phys. Rev.
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