5,182 research outputs found
Separability of a Low-Momentum Effective Nucleon-Nucleon Potential
A realistic nucleon-nucleon potential is transformed into a low-momentum
effective one (LMNN) using the Okubo theory. The separable potentials are
converted from the LMNN with a universal separable expansion method and a
simple Legendre expansion. Through the calculation of the triton binding
energies, the separability for the convergence of these ranks is evaluated. It
is found that there is a tendency for the lower momentum cutoff parameter
of LMNN to gain good separability.Comment: 6 pages, 1 tabl
SNARE proteins: zip codes in vesicle targeting?
Membrane traffic in eukaryotic cells is mediated by transport vesicles that bud from a precursor compartment and are transported to their destination compartment where they dock and fuse. To reach their intracellular destination, transport vesicles contain targeting signals such as Rab GTPases and polyphosphoinositides that are recognized by tethering factors in the cytoplasm and that connect the vesicles with their respective destination compartment. The final step, membrane fusion, is mediated by SNARE proteins. SNAREs are connected to targeting signals and tethering factors by multiple interactions. However, it is still debated whether SNAREs only function downstream of targeting and tethering or whether they also participate in regulating targeting specificity. Here, we review the evidence and discuss recent data supporting a role of SNARE proteins as targeting signals in vesicle traffic
Closed Spaces in Cosmology
This paper deals with two aspects of relativistic cosmologies with closed
(compact and boundless) spatial sections. These spacetimes are based on the
theory of General Relativity, and admit a foliation into space sections S(t),
which are spacelike hypersurfaces satisfying the postulate of the closure of
space: each S(t) is a 3-dimensional, closed Riemannian manifold. The discussed
topics are: (1) A comparison, previously obtained, between Thurston's
geometries and Bianchi-Kantowski-Sachs metrics for such 3-manifolds is here
clarified and developed. (2) Some implications of global inhomogeneity for
locally homogeneous 3-spaces of constant curvature are analyzed from an
observational viewpoint.Comment: 20 pages, 6 figures, revised version of published paper. In version
2: several misprints corrected, 'redshifting' in figures improved. Version 3:
a few style corrections; couple of paragraphs in subsection 2.4 rewritten.
Version 4: figures 5 and 6 corrrecte
Probing and manipulating intracellular membrane traffic by microinjection of artificial vesicles.
There is still a large gap in our understanding between the functional complexity of cells and the reconstruction of partial cellular functions in vitro from purified or engineered parts. Here we have introduced artificial vesicles of defined composition into living cells to probe the capacity of the cellular cytoplasm in dealing with foreign material and to develop tools for the directed manipulation of cellular functions. Our data show that protein-free liposomes, after variable delay times, are captured by the Golgi apparatus that is reached either by random diffusion or, in the case of large unilamellar vesicles, by microtubule-dependent transport via a dynactin/dynein motor complex. However, insertion of early endosomal SNARE proteins suffices to convert liposomes into trafficking vesicles that dock and fuse with early endosomes, thus overriding the default pathway to the Golgi. Moreover, such liposomes can be directed to mitochondria expressing simple artificial affinity tags, which can also be employed to divert endogenous trafficking vesicles. In addition, fusion or subsequent acidification of liposomes can be monitored by incorporation of appropriate chemical sensors. This approach provides an opportunity for probing and manipulating cellular functions that cannot be addressed by conventional genetic approaches. We conclude that the cellular cytoplasm has a remarkable capacity for self-organization and that introduction of such macromolecular complexes may advance nanoengineering of eukaryotic cells
Magnetic domain walls in constrained geometries
Magnetic domain walls have been studied in micrometer-sized Fe20Ni80 elements
containing geometrical constrictions by spin-polarized scanning electron
microscopy and numerical simulations. By controlling the constriction
dimensions, the wall width can be tailored and the wall type modified. In
particular, the width of a 180 degree Neel wall can be strongly reduced or
increased by the constriction geometry compared with the wall in unconstrained
systems.Comment: 4 pages, 6 figure
Time complexity and gate complexity
We formulate and investigate the simplest version of time-optimal quantum
computation theory (t-QCT), where the computation time is defined by the
physical one and the Hamiltonian contains only one- and two-qubit interactions.
This version of t-QCT is also considered as optimality by sub-Riemannian
geodesic length. The work has two aims: one is to develop a t-QCT itself based
on physically natural concept of time, and the other is to pursue the
possibility of using t-QCT as a tool to estimate the complexity in conventional
gate-optimal quantum computation theory (g-QCT). In particular, we investigate
to what extent is true the statement: time complexity is polynomial in the
number of qubits if and only if so is gate complexity. In the analysis, we
relate t-QCT and optimal control theory (OCT) through fidelity-optimal
computation theory (f-QCT); f-QCT is equivalent to t-QCT in the limit of unit
optimal fidelity, while it is formally similar to OCT. We then develop an
efficient numerical scheme for f-QCT by modifying Krotov's method in OCT, which
has monotonic convergence property. We implemented the scheme and obtained
solutions of f-QCT and of t-QCT for the quantum Fourier transform and a unitary
operator that does not have an apparent symmetry. The former has a polynomial
gate complexity and the latter is expected to have exponential one because a
series of generic unitary operators has a exponential gate complexity. The time
complexity for the former is found to be linear in the number of qubits, which
is understood naturally by the existence of an upper bound. The time complexity
for the latter is exponential. Thus the both targets are examples satisfyng the
statement above. The typical characteristics of the optimal Hamiltonians are
symmetry under time-reversal and constancy of one-qubit operation, which are
mathematically shown to hold in fairly general situations.Comment: 11 pages, 6 figure
Model building by coset space dimensional reduction in ten-dimensions with direct product gauge symmetry
We investigate ten-dimensional gauge theories whose extra six-dimensional
space is a compact coset space, , and gauge group is a direct product of
two Lie groups. We list up candidates of the gauge group and embeddings of
into them. After dimensional reduction of the coset space,we find fermion and
scalar representations of with
and which accomodate all of the standard
model particles. We also discuss possibilities to generate distinct Yukawa
couplings among the generations using representations with a different
dimension for and models.Comment: 14 pages; added local report number, added refferenc
Possible solution to the Li problem by the long lived stau
Modification of standard big-bang nucleosynthesis is considered in the
minimal supersymmetric standard model to resolve the excessive theoretical
prediction of the abundance of primordial lithium 7. We focus on the stau as a
next-lightest superparticle, which is long lived due to its small mass
difference with the lightest superparticle. It provides a number of additional
decay processes of and . A particularly
important process is the internal conversion in the stau-nucleus bound state,
which destroys the and effectively. We show
that the modification can lead to a prediction consistent with the observed
abundance of .Comment: 6 pages, 5 figure
Enhancement of pairing due to the presence of resonant cavities
A correlated fermion system is considered surrounding a finite cavity with
virtual levels. The pairing properties are calculated and the influence of the
cavity is demonstrated. To this end the Gell-Mann and Goldberger formula is
generalized to many-body systems. We find a possible enhancement of pairing
temperature if the Fermi momentum times the cavity radius fulfills a certain
resonance condition which suggests an experimental realization.Comment: 4 pages 2 figure
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