58,413 research outputs found
Intrinsic Charm in the Nucleon
The quark spin and flavor structure of the nucleon is discussed in the SU(4)
symmetry breaking chiral quark model. The spin and flavor contents for charm
quarks and anti-charm quarks are predicted and compared with the results given
by other models. The intrinsic charm quark contribution to the Ellis-Jaffe sum
rule is discussed.Comment: 10 pages, 3 tables and 1 figure
Substructure of the Nucleon in the Chiral Quark Model
The spin and orbital angular momentum carried by different quark flavors in
the nucleon are calculated in the SU(3) chiral quark model with
symmetry-breaking. The similar calculation is also performed for other octet
and decuplet baryons. Furthermore, the flavor and spin contents for charm and
anti-charm quarks are predicted in the SU(4) symmetry breaking chiral quark
model.Comment: 9 pages, 3 tables, 3 ps figures, plenary talk presented at the
Circum-Pan-Pacific Riken Symposium on ``High Energy Spin Physics'', RIKEN,
Wako, Japan, November 3-6, 199
Tests of SU(3) Symmetry-Breaking for Baryonic Beta Transitions in the Standard Model
It is generally assumed that deviations from flavor SU(3) symmetry arise
entirely from quark mass-differences, reflected in the mass splittings between
strange and nonstrange members of the same SU(3) multiplet. Under this
assumption, a parametrization is proposed which expresses the ratios of
Gamow-Teller to Fermi matrix elements for nucleon and hyperon beta decays
entirely in terms of two SU(3)-invariant coupling constants and and two
parameters and representing SU(3) breaking effects.
Therefore, in principle, measurement of this ratio for any four
beta-transitions should yield all four parameters. Available data do not show
any evidence for SU(3) breaking. Improved measurements, also for transitions
not previously measured, would provide more stringent tests.Comment: 9 pages, 2 tables, a few minor corrections in reference
A Study of S doped ZnSb
We report on S-doping of ZnSb for S concentrations ranging from 0.02 at% to
2.5 at%. There are no previous reports on S-doping. ZnSb is a thermoelectric
material with some advantages for the temperature range 400 K - 600 K. The
solid solubility of S in ZnSb was estimated to be lower than 0.1% from
observations of precipitates by scanning microscopy. Hall and Seebeck
measurements were performed as a function of temperature from 6K to 623 K. The
temperature dependence of the electrical properties suggests that S introduces
neutral scattering centers for holes in the p-type material. An increase in
hole concentration by S is argued by defect reactions involving Zn vacancies
How to Evolve Safe Control Strategies
Autonomous space vehicles need adaptive control strategies that can
accommodate unanticipated environmental conditions. The evaluation of new
strategies can often be done only by actually trying them out in the real
physical environment. Consequently, a candidate control strategy must be deemed
safe--i.e., it won't damage any systems--prior to being tested online. How to
do this efficiently has been a challenging problem.
We propose using evolutionary programming in conjunction with a formal
verification technique (called model checking) to evolve candidate control
strategies that are guaranteed to be safe for implementation and evaluation.Comment: 3 pages, 1 figur
Geometric phase and phase diagram for non-Hermitian quantum XY model
We study the geometric phase for the ground state of a generalized
one-dimensional non-Hermitian quantum XY model, which has
transverse-field-dependent intrinsic rotation-time reversal symmetry. Based on
the exact solution, this model is shown to have full real spectrum in multiple
regions for the finite size system. The result indicates that the phase diagram
or exceptional boundary, which separates the unbroken and broken symmetry
regions corresponds to the divergence of the Berry curvature. The scaling
behaviors of the groundstate energy and Berry curvature are obtained in an
analytical manner for a concrete system.Comment: 6 pages, 3 figure
Momentum-independent reflectionless transmission in the non-Hermitian time-reversal symmetric system
We theoretically study the non-Hermitian systems, the non-Hermiticity of
which arises from the unequal hopping amplitude (UHA) dimers. The
distinguishing features of these models are that they have full real spectra if
all of the eigenvectors are time-reversal (T) symmetric rather than
parity-time-reversal (PT) symmetric, and that their Hermitian counterparts are
shown to be an experimentally accessible system, which have the same
topological structures as that of the original ones but modulated hopping
amplitudes within the unbroken region. Under the reflectionless transmission
condition, the scattering behavior of momentum-independent reflectionless
transmission (RT) can be achieved in the concerned non-Hermitian system. This
peculiar feature indicates that, for a certain class of non-Hermitian systems
with a balanced combination of the RT dimers, the defects can appear fully
invisible to an outside observer.Comment: 9 pages, 4 figures. arXiv admin note: text overlap with
arXiv:1008.5306 by other author
Non-Hermitian anisotropic XY model with intrinsic rotation-time reversal symmetry
We systematically study the non-Hermitian version of the one-dimensional
anisotropic XY model, which in its original form, is a unique exactly solvable
quantum spin model for understanding the quantum phase transition. The
distinguishing features of this model are that it has full real spectrum if all
the eigenvectors are intrinsic rotation-time reversal (RT) symmetric rather
than parity-time reversal (PT) symmetric, and that its Hermitian counterpart is
shown approximately to be an experimentally accessible system, an isotropic XY
spin chain with nearest neighbor coupling. Based on the exact solution,
exceptional points which separated the unbroken and broken symmetry regions are
obtained and lie on a hyperbola in the thermodynamic limit. It provides a nice
paradigm to elucidate the complex quantum mechanics theory for a quantum spin
system.Comment: 7 pages, 3 figure
Partial topological Zak phase and dynamical confinement in non-Hermitian bipartite system
Unlike a Chern number in D and D topological system, Zak phase takes a
subtle role to characterize the topological phase in D. On the one hand, it
is not a gauge invariant, on the other hand, the Zak phase difference between
two quantum phases can be used to identify the topological phase transitions. A
non-Hermitian system may inherit some characters of a Hermitian system, such as
entirely real spectrum, unitary evolution, topological energy band, etc. In
this paper, we study the influence of non-Hermitian term on the Zak phase for a
class of non-Hermitian systems. We show exactly that the real part of the Zak
phase remains unchanged in a bipartite lattice. In a concrete example, D
Su-Schrieffer-Heeger (SSH) model, we find that the real part of Zak phase can
be obtained by an adiabatic process. To demonstrate this finding, we
investigate a scattering problem for a time-dependent scattering center, which
is a magnetic-flux-driven non-Hermitian SSH ring. Owing to the nature of the
Zak phase, the intriguing features of this design are the wave-vector
independence and allow two distinct behaviors, perfect transmission or
confinement, depending on the timing of a flux impulse threading the ring. When
the flux is added during a wavepacket travelling within the ring, the
wavepacket is confined in the scatter partially. Otherwise, it exhibits perfect
transmission through the scatter. Our finding extends the understanding and
broaden the possible application of geometric phase in a non-Hermitian system.Comment: 11 pages, 7 figure
EPR pairing dynamics in Hubbard model with resonant
We study the dynamics of the collision between two fermions in Hubbard model
with on-site interaction strength . The exact solution shows that the
scattering matrix for two-wavepacket collision is separable into two
independent parts, operating on spatial and spin degrees of freedom,
respectively. The S-matrix for spin configuration is equivalent to that of
Heisenberg-type pulsed interaction with the strength depending on and
relative group velocity . This can be applied to create distant
EPR pair, through a collision process for two fermions with opposite spins in
the case of ,\ without the need for
temporal control and measurement process. Multiple collision process for many
particles is also discussed.Comment: 7 pages, 3 figure
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