14,994 research outputs found
Dangling-bond spin relaxation and magnetic 1/f noise from the amorphous-semiconductor/oxide interface: Theory
We propose a model for magnetic noise based on spin-flips (not
electron-trapping) of paramagnetic dangling-bonds at the
amorphous-semiconductor/oxide interface. A wide distribution of spin-flip times
is derived from the single-phonon cross-relaxation mechanism for a
dangling-bond interacting with the tunneling two-level systems of the amorphous
interface. The temperature and frequency dependence is sensitive to three
energy scales: The dangling-bond spin Zeeman energy delta, as well as the
minimum (E_min) and maximum (E_max) values for the energy splittings of the
tunneling two-level systems. We compare and fit our model parameters to a
recent experiment probing spin coherence of antimony donors implanted in
nuclear-spin-free silicon [T. Schenkel {\it et al.}, Appl. Phys. Lett. 88,
112101 (2006)], and conclude that a dangling-bond area density of the order of
10^{14}cm^{-2} is consistent with the data. This enables the prediction of
single spin qubit coherence times as a function of the distance from the
interface and the dangling-bond area density in a real device structure. We
apply our theory to calculations of magnetic flux noise affecting SQUID devices
due to their Si/SiO_2 substrate. Our explicit estimates of flux noise in SQUIDs
lead to a noise spectral density of the order of 10^{-12}Phi_{0}^{2} {Hz}^{-1}
at f=1Hz. This value might explain the origin of flux noise in some SQUID
devices. Finally, we consider the suppression of these effects using surface
passivation with hydrogen, and the residual nuclear-spin noise resulting from a
perfect silicon-hydride surface.Comment: Final published versio
Site-dependent hydrogenation on graphdiyne
Graphene is one of the most important materials in science today due to its
unique and remarkable electronic, thermal and mechanical properties. However in
its pristine state, graphene is a gapless semiconductor, what limits its use in
transistor electronics. In part due to the revolution created by graphene in
materials science, there is a renewed interest in other possible graphene-like
two-dimensional structures. Examples of these structures are graphynes and
graphdiynes, which are two-dimensional structures, composed of carbon atoms in
sp2 and sp-hybridized states. Graphdiynes (benzenoid rings connecting two
acetylenic groups) were recently synthesized and some of them are intrinsically
nonzero gap systems. These systems can be easily hydrogenated and the relative
level of hydrogenation can be used to tune the band gap values. We have
investigated, using fully reactive molecular dynamics (ReaxFF), the structural
and dynamics aspects of the hydrogenation mechanisms of graphdiyne membranes.
Our results showed that the hydrogen bindings have different atom incorporation
rates and that the hydrogenation patterns change in time in a very complex way.
The formation of correlated domains reported to hydrogenated graphene is no
longer observed in graphdiyne cases.Comment: Submitted to Carbo
Exponential Distributions in a Mechanical Model for Earthquakes
We study statistical distributions in a mechanical model for an earthquake
fault introduced by Burridge and Knopoff [R. Burridge and L. Knopoff, {\sl
Bull. Seismol. Soc. Am.} {\bf 57}, 341 (1967)]. Our investigations on the size
(moment), time duration and number of blocks involved in an event show that
exponential distributions are found in a given range of the paramenter space.
This occurs when the two kinds of springs present in the model have the same,
or approximately the same, value for the elastic constants. Exponential
distributions have also been seen recently in an experimental system to model
earthquake-like dynamics [M. A. Rubio and J. Galeano, {\sl Phys. Rev. E} {\bf
50}, 1000 (1994)].Comment: 11 pages, uuencoded (submitted to Phys. Rev. E
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