6,439 research outputs found
Effect of spin-orbit interaction on a magnetic impurity in the vicinity of a surface
We propose a new mechanism for surface-induced magnetic anisotropy to explain
the thickness-dependence of the Kondo resistivity of thin films of dilute
magnetic alloys. The surface anisotropy energy, generated by spin-orbit
coupling on the magnetic impurity itself, is an oscillating function of the
distance d from the surface and decays as 1/d^2. Numerical estimates based on
simple models suggest that this mechanism, unlike its alternatives, gives rise
to an effect of the desired order of magnitude.Comment: 4 pages, 4 figure
Tuning surface metallicity and ferromagnetism by hydrogen adsorption at the polar ZnO(0001) surface
The adsorption of hydrogen on the polar Zn-ended ZnO(0001) surface has been
investigated by density functional {\it ab-initio} calculations. An on top
H(1x1) ordered overlayer with genuine H-Zn chemical bonds is shown to be
energetically favorable. The H covered surface is metallic and spin-polarized,
with a noticeable magnetic moment at the surface region. Lower hydrogen
coverages lead to strengthening of the H-Zn bonds, corrugation of the surface
layer and to an insulating surface. Our results explain experimental
observations of hydrogen adsorption on this surface, and not only predict a
metal-insulator transition, but primarily provide a method to reversible switch
surface magnetism by varying the hydrogen density on the surface.Comment: 4 pages, 3 figure
Level and length of cyclic solar activity during the Maunder minimum as deduced from the active day statistics
The Maunder minimum (MM) of greatly reduced solar activity took place in
1645-1715, but the exact level of sunspot activity is uncertain as based, to a
large extent, on historical generic statements of the absence of spots on the
Sun. Here we aim, using a conservative approach, to assess the level and length
of solar cycle during the Maunder minimum, on the basis of direct historical
records by astronomers of that time. A database of the active and inactive days
(days with and without recorded sunspots on the solar disc respectively) is
constructed for three models of different levels of conservatism (loose ML,
optimum MO and strict MS models) regarding generic no-spot records. We have
used the active day fraction to estimate the group sunspot number during the
MM. A clear cyclic variability is found throughout the MM with peaks at around
1655--1657, 1675, 1684 and 1705, and possibly 1666, with the active day
fraction not exceeding 0.2, 0.3 or 0.4 during the core MM, for the three
models. Estimated sunspot numbers are found very low in accordance with a grand
minimum of solar activity.
We have found, for the core MM (1650-1700), that: (1) A large fraction of
no-spot records, corresponding to the solar meridian observations, may be
unreliable in the conventional database. (2) The active day fraction remained
low (below 0.3-0.4) throughout the MM, indicating the low level of sunspot
activity. (3) The solar cycle appears clearly during the core MM. (4) The
length of the solar cycle during the core MM appears years, but there
is an uncertainty in that. (5) The magnitude of the sunspot cycle during MM is
assessed to be below 5-10 in sunspot numbers;
A hypothesis of the high solar cycles during the MM is not confirmed.Comment: Accepted to Astron. Astrophy
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