9,578 research outputs found
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
A Web como fonte de informação para a cadeia do leite: um estudo exploratório do Portal Milkpoint.
Chaos and Synchronized Chaos in an Earthquake Model
We show that chaos is present in the symmetric two-block Burridge-Knopoff
model for earthquakes. This is in contrast with previous numerical studies, but
in agreement with experimental results. In this system, we have found a rich
dynamical behavior with an unusual route to chaos. In the three-block system,
we see the appearance of synchronized chaos, showing that this concept can have
potential applications in the field of seismology.Comment: To appear in Physical Review Letters (13 pages, 6 figures
Two-dimensional quantum spin-1/2 Heisenberg model with competing interactions
We study the quantum spin-1/2 Heisenberg model in two dimensions, interacting
through a nearest-neighbor antiferromagnetic exchange () and a ferromagnetic
dipolar-like interaction (), using double-time Green's function, decoupled
within the random phase approximation (RPA). We obtain the dependence of as a function of frustration parameter , where is the
ferromagnetic (F) transition temperature and is the ratio between the
strengths of the exchange and dipolar interaction (i.e., ). The
transition temperature between the F and paramagnetic phases decreases with
, as expected, but goes to zero at a finite value of this parameter,
namely . At T=0 (quantum phase transition), we
analyze the critical parameter for the general case of an
exchange interaction in the form , where ferromagnetic
and antiferromagnetic phases are present.Comment: 4 pages, 1 figur
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