6,577 research outputs found
Spinwave damping in the two-dimensional ferromagnetic XY model
The effect of damping of spinwaves in a two-dimensional classical
ferromagnetic XY model is considered. The damping rate is
calculated using the leading diagrams due to the quartic-order deviations from
the harmonic spin Hamiltonian. The resulting four-dimensional integrals are
evaluated by extending the techniques developed by Gilat and others for
spectral density types of integrals. is included into the memory
function formalism due to Reiter and Solander, and Menezes, to determine the
dynamic structure function . For the infinite sized system, the
memory function approach is found to give non-divergent spinwave peaks, and a
smooth nonzero background intensity (``plateau'' or distributed intensity) for
the whole range of frequencies below the spinwave peak. The background
amplitude relative to the spinwave peak rises with temperature, and eventually
becomes higher than the spinwave peak, where it appears as a central peak. For
finite-sized systems, there are multiple sequences of weak peaks on both sides
of the spinwave peaks whose number and positions depend on the system size and
wavevector in integer units of . These dynamical finite size effects
are explained in the memory function analysis as due to either spinwave
difference processes below the spinwave peak or sum processes above the
spinwave peak. These features are also found in classical Monte Carlo --
Spin-Dynamics simulations.Comment: 20 two-column page
Universality of weakly bound dimers and Efimov trimers close to Li-Cs Feshbach resonances
We study the interspecies scattering properties of ultracold Li-Cs mixtures
in their two energetically lowest spin channels in the magnetic field range
between 800 G and 1000 G. Close to two broad Feshbach resonances we create
weakly bound LiCs dimers by radio-frequency association and measure the
dependence of the binding energy on the external magnetic field strength. Based
on the binding energies and complementary atom loss spectroscopy of three other
Li-Cs s-wave Feshbach resonances we construct precise molecular singlet and
triplet electronic ground state potentials using a coupled-channels
calculation. We extract the Li-Cs interspecies scattering length as a function
of the external field and obtain almost a ten-fold improvement in the precision
of the values for the pole positions and widths of the s-wave Li-Cs Feshbach
resonances as compared to our previous work [Pires \textit{et al.}, Phys. Rev.
Lett. \textbf{112}, 250404 (2014)]. We discuss implications on the Efimov
scenario and the universal geometric scaling for LiCsCs trimers
XMM-Newton reveals a candidate period for the spin of the "Magnificent Seven" neutron star RX J1605.3+3249
The group of thermally emitting isolated neutron stars (INSs) known as the
"Magnificent Seven" (M7) is unique among the various neutron star populations.
Crustal heating by means of magnetic field decay and an evolutionary link with
magnetars may explain why these objects rotate more slowly and have higher
thermal luminosities and magnetic field intensities than standard pulsars of
similar age. The third brightest INS, RX J1605.3+3249, is the only object
amidst the seven still lacking a detected periodicity. We observed the source
with the XMM-Newton Observatory for 60 ks aiming at unveiling the neutron star
rotation rate and investigating its spectrum in detail. A periodic signal at
P=3.387864(16) s, most likely the neutron star spin period, is detected at the
4-sigma confidence level. The coherent combination of the new data with a past
XMM-Newton EPIC-pn observation of the source constrains the pulsar spin-down
rate at the 2-sigma confidence level, implying a dipolar magnetic field of
B~7.4e13 G. If confirmed, RX J1605.3+3249 would be the neutron star with the
highest dipolar field amongst the M7. The spectrum of the source shows evidence
of a cool blackbody component, as well as for the presence of two broad
absorption features. Furthermore, high-resolution spectroscopy with the RGS
cameras confirms the presence of a narrow absorption feature at energy 0.57 keV
in the co-added spectrum of the source, also seen in other thermally emitting
isolated neutron stars. Phase-resolved spectroscopy, as well as a dedicated
observing campaign aimed at determining a timing solution, will give invaluable
constraints on the neutron star geometry and will allow one to confirm the high
value of spin down, which would place the source closer to a magnetar than any
other M7 INS.Comment: 12 pages, 6 figures; accepted for publication in A&A (revised version
after language editing; results unchanged
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