40,901 research outputs found
Theory of inter-edge superexchange in zigzag edge magnetism
A graphene nanoribbon with zigzag edges has a gapped magnetic ground state
with an antiferromagnetic inter-edge superexchange interaction. We present a
theory based on asymptotic properties of the Dirac-model ribbon wavefunction
which predicts and ribbon-width dependencies for the
superexchange interaction strength and the charge gap respectively. We find
that, unlike the case of conventional atomic scale superexchange, opposite
spin-orientations on opposite edges of the ribbon are favored by both kinetic
and interaction energies.Comment: 4 pages 8 figure
Canonically Transformed Detectors Applied to the Classical Inverse Scattering Problem
The concept of measurement in classical scattering is interpreted as an
overlap of a particle packet with some area in phase space that describes the
detector. Considering that usually we record the passage of particles at some
point in space, a common detector is described e.g. for one-dimensional systems
as a narrow strip in phase space. We generalize this concept allowing this
strip to be transformed by some, possibly non-linear, canonical transformation,
introducing thus a canonically transformed detector. We show such detectors to
be useful in the context of the inverse scattering problem in situations where
recently discovered scattering echoes could not be seen without their help.
More relevant applications in quantum systems are suggested.Comment: 8 pages, 15 figures. Better figures can be found in the original
article, wich can be found in
http://www.sm.luth.se/~norbert/home_journal/electronic/v12s1.html Related
movies can be found in www.cicc.unam.mx/~mau
Magneto-electric coupling in zigzag graphene nanoribbons
Zigzag graphene nanoribbons can have magnetic ground states with
ferromagnetic, antiferromagnetic, or canted configurations, depending on
carrier density. We show that an electric field directed across the ribbon
alters the magnetic state, favoring antiferromagnetic configurations. This
property can be used to prepare ribbons with a prescribed spin-orientation on a
given edge.Comment: 4 pages, 5 figure
Symmetry breaking: A tool to unveil the topology of chaotic scattering with three degrees of freedom
We shall use symmetry breaking as a tool to attack the problem of identifying
the topology of chaotic scatteruing with more then two degrees of freedom.
specifically we discuss the structure of the homoclinic/heteroclinic tangle and
the connection between the chaotic invariant set, the scattering functions and
the singularities in the cross section for a class of scattering systems with
one open and two closed degrees of freedom.Comment: 13 pages and 8 figure
Quantum and classical echoes in scattering systems described by simple Smale horseshoes
We explore the quantum scattering of systems classically described by binary
and other low order Smale horseshoes, in a stage of development where the
stable island associated with the inner periodic orbit is large, but chaos
around this island is well developed. For short incoming pulses we find
periodic echoes modulating an exponential decay over many periods. The period
is directly related to the development stage of the horseshoe. We exemplify our
studies with a one-dimensional system periodically kicked in time and we
mention possible experiments.Comment: 7 pages with 6 reduced quality figures! Please contact the authors
([email protected]) for an original good quality pre-prin
Optical Studies of a Layered Manganite La_{1.2}Sr_{1.8}Mn_2O_7 : Polaron Correlation Effect
Optical conductivity spectra of a cleaved ab-plane of a
La_{1.2}Sr_{1.8}Mn_2O_7 single crystal exhibit a small polaron absorption band
in the mid-infrared region at overall temperatures. With decreasing temperature
(T) to Curie temperature (T_C), the center frequency of the small polaron band
moves to a higher frequency, resulting in a gap-like feature, and that it
collapses to a lower frequency below T_C. Interestingly, with decreasing T, the
stretching phonon mode hardens above T_C and softens below T_C. These
concurring changes of lattice and electronic structure indicate that short
range polaron correlation exist above T_C but disappear with a magnetic
ordering.Comment: 4 pages including 5 figures. submitted to Phys. Rev.
Dimensional Crossover driven by Magnetic Ordering in Optical Conductivity of Pr_{1/2}Sr_{1/2}MnO_3
We investigated optical properties of Pr_{0.5}Sr_{0.5}MnO_3, which has the
A-type antiferromagnetic ordering at a low temperature. We found that T-
dependence of spectral weight transfer shows a clear correlation with the
magnetic phase transition. In comparison with the optical conductivity results
of Nd_{0.5}Sr_{0.5}MnO_3, which has the CE-type antiferromagnetic charge
ordering, we showed that optical properties of Pr_{0.5}Sr_{0.5}MnO_3 near the
Neel temperature could be explained by a crossover from 3D to 2D metals.
Details of spectral weight changes are consistent with the polaron picture.Comment: 11 pages, 4 figures, submitted to PRL at June
- …