24,535 research outputs found
Valley-dependent Brewster angles and Goos-Hanchen effect in strained graphene
We demonstrate theoretically how local strains in graphene can be tailored to
generate a valley polarized current. By suitable engineering of local strain
profiles, we find that electrons in opposite valleys (K or K') show different
Brewster-like angles and Goos-H\"anchen shifts, exhibiting a close analogy with
light propagating behavior. In a strain-induced waveguide, electrons in K and
K' valleys have different group velocities, which can be used to construct a
valley filter in graphene without the need for any external fields.Comment: 5 pages, 4 figure
The effects of a family non-universal Z-prime boson on B--->pipi decays
Motivated by the measured large branching ratio of
(the so-called "" puzzle), we investigate the effects of a family
non-universal model on the tree-dominated decays. We
find that the coupling parameter with a
nontrivial new weak phase , which is relevant to the
contributions to the QCD penguin sector , is needed
to reconcile the observed discrepancy. Combined with the recent fitting results
from , and decays, the
parameter spaces are severely reduced but still not excluded entirely, implying
that both the "" and "" puzzles could be accommodated
simultaneously within such a family non-universal model.Comment: 28 pages, 4 figures. References and discussions added. To appear in
IJMP
On the symbolic manipulation and code generation for elasto-plastic material matrices
A computerized procedure for symbolic manipulations and FORTRAN code generation of an elasto-plastic material matrix for finite element applications is presented. Special emphasis is placed on expression simplifications during intermediate derivations, optimal code generation, and interface with the main program. A systematic procedure is outlined to avoid redundant algebraic manipulations. Symbolic expressions of the derived material stiffness matrix are automatically converted to RATFOR code which is then translated into FORTRAN statements through a preprocessor. To minimize the interface problem with the main program, a template file is prepared so that the translated FORTRAN statements can be merged into the file to form a subroutine (or a submodule). Three constitutive models; namely, von Mises plasticity, Drucker-Prager model, and a concrete plasticity model, are used as illustrative examples
Spin current diode based on an electron waveguide with spin-orbit interaction
We propose a spin current diode which can work even in a small applied bias
condition (the linear-response regime). The prototypal device consists of a
hornlike electron waveguide with Rashba spin-orbit interaction, which is
connected to two leads with different widths. It is demonstrated that when
electrons are incident from the narrow lead, the generated spin conductance
fluctuates around a constant value in a wide range of incident energy. When the
transport direction is reversed, the spin conductance is suppressed strongly.
Such a remarkable difference arises from spin-flipped transitions caused by the
spin-orbit interaction.Comment: 3 pages, 2 figure
Fast production of Bose-Einstein condensates of metastable Helium
We report on the Bose-Einstein condensation of metastable Helium-4 atoms
using a hybrid approach, consisting of a magnetic quadrupole and a crossed
optical dipole trap. In our setup we cross the phase transition with 2x10^6
atoms, and we obtain pure condensates of 5x10^5 atoms in the optical trap. This
novel approach to cooling Helium-4 provides enhanced cycle stability, large
optical access to the atoms and results in production of a condensate every 6
seconds - a factor 3 faster than the state-of-the-art. This speed-up will
dramatically reduce the data acquisition time needed for the measurement of
many particle correlations, made possible by the ability of metastable Helium
to be detected individually
Stabilization of charge ordering in La_(1/3)Sr_(2/3)FeO_(3-d) by magnetic exchange
The magnetic exchange energies in charge ordered La_(1/3)Sr_(2/3)FeO_(3-d)
(LSFO) and its parent compound LaFeO_(3) (LFO) have been determined by
inelastic neutron scattering. In LSFO, the measured ratio of ferromagnetic
exchange between Fe3+ - Fe5+ pairs (J_F) and antiferromagnetic exchange between
Fe3+ - Fe3+ pairs (J_AF) fulfills the criterion for charge ordering driven by
magnetic interactions (|J_F/J_AF| > 1). The 30% reduction of J_AF as compared
to LFO indicates that doped holes are delocalized, and charge ordering occurs
without a dominant influence from Coulomb interactions.Comment: 18 pages, 4 color figure
Raman spectroscopic determination of the length, strength, compressibility, Debye temperature, elasticity, and force constant of the C-C bond in graphene
From the perspective of bond relaxation and vibration, we have reconciled the
Raman shifts of graphene under the stimuli of the number-of-layer,
uni-axial-strain, pressure, and temperature in terms of the response of the
length and strength of the representative bond of the entire specimen to the
applied stimuli. Theoretical unification of the measurements clarifies that:
(i) the opposite trends of Raman shifts due to number-of-layer reduction
indicate that the G-peak shift is dominated by the vibration of a pair of atoms
while the D- and the 2D-peak shifts involves z-neighbor of a specific atom;
(ii) the tensile strain-induced phonon softening and phonon-band splitting
arise from the asymmetric response of the C3v bond geometry to the C2v
uni-axial bond elongation; (iii) the thermal-softening of the phonons
originates from bond expansion and weakening; and (iv) the pressure- stiffening
of the phonons results from bond compression and work hardening. Reproduction
of the measurements has led to quantitative information about the referential
frequencies from which the Raman frequencies shift, the length, energy, force
constant, Debye temperature, compressibility, elastic modulus of the C-C bond
in graphene, which is of instrumental importance to the understanding of the
unusual behavior of graphene
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