45,256 research outputs found
Equilibrium spin-glass transition of magnetic dipoles with random anisotropy axes on a site diluted lattice
We study partially occupied lattice systems of classical magnetic dipoles
which point along randomly oriented axes. Only dipolar interactions are taken
into account. The aim of the model is to mimic collective effects in disordered
assemblies of magnetic nanoparticles. From tempered Monte Carlo simulations, we
obtain the following equilibrium results. The zero temperature entropy
approximately vanishes. Below a temperature T_c, given by k_B T_c= (0.95 +-
0.1)x e_d, where e_d is a nearest neighbor dipole-dipole interaction energy and
x is the site occupancy rate, we find a spin glass phase. In it, (1) the mean
value , where q is the spin overlap, decreases algebraically with system
size N as N increases, and (2) D|q| = 0.5 (T/x)^1/2, independently of N,
where D|q| is the root mean square deviation of |q|.Comment: 7 LaTeX pages, 7 eps figures. Submitted to PRB on 30 December 200
Spin-transfer torque on a single magnetic adatom
We theoretically show how the spin orientation of a single magnetic adatom
can be controlled by spin polarized electrons in a scanning tunneling
microscope configuration. The underlying physical mechanism is spin assisted
inelastic tunneling. By changing the direction of the applied current, the
orientation of the magnetic adatom can be completely reversed on a time scale
that ranges from a few nanoseconds to microseconds, depending on bias and
temperature. The changes in the adatom magnetization direction are, in turn,
reflected in the tunneling conductance.Comment: 5 pages, 3 figure
A multiquark description of the and
Within a theoretical framework that accounts for all open-charm mesons,
including the , the and the , we
analyze the structure and explore possible quantum number assignments for the
and the mesons reported by BABAR and Belle
Collaborations. The open-charm sector is properly described if considered as a
combination of conventional quark-antiquark states and four--quark components.
All negative parity and states can be understood in terms only of components, however the description of the and mesons is
improved whenever the mixing between two-- and four--quarks configurations is
included. We analyze all possible quantum number assignments for the
in terms of both and configurations.
We discuss the role played by the electromagnetic and strong decay widths as
basic tools to distinguish among them. The broad structure reported by BABAR
near 2.7 GeV is also analyzed.Comment: 10 pages, 5 table
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