21 research outputs found
Lanczos algorithm with Matrix Product States for dynamical correlation functions
The density-matrix renormalization group (DMRG) algorithm can be adapted to
the calculation of dynamical correlation functions in various ways which all
represent compromises between computational efficiency and physical accuracy.
In this paper we reconsider the oldest approach based on a suitable
Lanczos-generated approximate basis and implement it using matrix product
states (MPS) for the representation of the basis states. The direct use of
matrix product states combined with an ex-post reorthogonalization method
allows to avoid several shortcomings of the original approach, namely the
multi-targeting and the approximate representation of the Hamiltonian inherent
in earlier Lanczos-method implementations in the DMRG framework, and to deal
with the ghost problem of Lanczos methods, leading to a much better convergence
of the spectral weights and poles. We present results for the dynamic spin
structure factor of the spin-1/2 antiferromagnetic Heisenberg chain. A
comparison to Bethe ansatz results in the thermodynamic limit reveals that the
MPS-based Lanczos approach is much more accurate than earlier approaches at
minor additional numerical cost.Comment: final version 11 pages, 11 figure
Two-dimensional discrete solitons in dipolar Bose-Einstein condensates
We analyze the formation and dynamics of bright unstaggered solitons in the
disk-shaped dipolar Bose-Einstein condensate, which features the interplay of
contact (collisional) and long-range dipole-dipole (DD) interactions between
atoms. The condensate is assumed to be trapped in a strong optical-lattice
potential in the disk's plane, hence it may be approximated by a
two-dimensional (2D) discrete model, which includes the on-site nonlinearity
and cubic long-range (DD) interactions between sites of the lattice. We
consider two such models, that differ by the form of the on-site nonlinearity,
represented by the usual cubic term, or more accurate nonpolynomial one,
derived from the underlying 3D Gross-Pitaevskii equation. Similar results are
obtained for both models. The analysis is focused on effects of the DD
interaction on fundamental localized modes in the lattice (2D discrete
solitons). The repulsive isotropic DD nonlinearity extends the existence and
stability regions of the fundamental solitons. New families of on-site,
inter-site and hybrid solitons, built on top of a finite background, are found
as a result of the interplay of the isotropic repulsive DD interaction and
attractive contact nonlinearity. By themselves, these solutions are unstable,
but they evolve into robust breathers which exist on an oscillating background.
In the presence of the repulsive contact interactions, fundamental localized
modes exist if the DD interaction (attractive isotropic or anisotropic) is
strong enough. They are stable in narrow regions close to the anticontinuum
limit, while unstable solitons evolve into breathers. In the latter case, the
presence of the background is immaterial
Activation of myosin Vâbased motility and F-actinâdependent network formation of endoplasmic reticulum during mitosis
It is widely believed that microtubule- and F-actinâbased transport of cytoplasmic organelles and membrane fusion is down-regulated during mitosis. Here we show that during the transition of Xenopus egg extracts from interphase to metaphase myosin Vâdriven movement of small globular vesicles along F-actin is strongly inhibited. In contrast, the movement of ER and ER network formation on F-actin is up-regulated in metaphase extracts. Our data demonstrate that myosin Vâdriven motility of distinct organelles is differently controlled during the cell cycle and suggest an active role of F-actin in partitioning, positioning, and membrane fusion of the ER during cell division
Extreme events in two dimensional disordered nonlinear lattices
Spatiotemporal complexity is induced in a two dimensional nonlinear
disordered lattice through the modulational instability of an initially weakly
perturbed excitation. In the course of evolution we observe the formation of
transient as well as persistent localized structures, some of which have
extreme magnitude. We analyze the statistics of occurrence of these extreme
collective events and find that the appearance of transient extreme events is
more likely in the weakly nonlinear regime. We observe a transition in the
extreme events recurrence time probability from exponential, in the
nonlinearity dominated regime, to power law for the disordered one.Comment: 5 figures, 5 page
Myosin5a tail associates directly with Rab3A-containing compartments in neurons
Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of American Society for Biochemistry and Molecular Biology. The definitive version was published in Journal of Biological Chemistry, 286 (2011): 14352-14361, doi:10.1074/jbc.M110.187286.Myosin-Va (Myo5a) is a motor protein
associated with synaptic vesicles (SVs) but the
mechanism by which it interacts has not yet
been identified. A potential class of binding
partners are Rab GTPases and Rab3A is known
to associate with SVs and is involved in SV
trafficking. We performed experiments to
determine whether Rab3A interacts with
Myo5a and whether it is required for transport
of neuronal vesicles. In vitro motility assays
performed with axoplasm from the squid giant
axon showed a requirement for a Rab GTPase
in Myo5a-dependent vesicle transport.
Furthermore, mouse recombinant Myo5a tail
revealed that it associated with Rab3A in rat
brain synaptosomal preparations in vitro and
the association was confirmed by
immunofluorescence imaging of primary
neurons isolated from the frontal cortex of
mouse brains. Synaptosomal Rab3A was
retained on recombinant GST-tagged Myo5a
tail affinity columns in a GTP-dependent
manner. Finally, the direct interaction of
Myo5a and Rab3A was determined by
sedimentation v e l o c i t y analytical
ultracentrifugation using recombinant mouse
Myo5a tail and human Rab3A. When both
proteins were incubated in the presence of 1
mM GTPÎłS, Myo5a tail and Rab3A formed a
complex and a direct interaction was observed.
Further analysis revealed that GTP-bound
Rab3A interacts with both the monomeric and
dimeric species of the Myo5a tail. However, the
interaction between Myo5a tail and nucleotidefree
Rab3A did not occur. Thus, our results
show that Myo5a and Rab3A are direct binding
partners and interact on SVs and that the
Myo5a/Rab3A complex is involved in transport
of neuronal vesicles
Spin polarized electron-positron pair production via elliptical polarized laser fields
We study nonperturbative multiphoton electron-positron pair creation in ultrastrong electromagnetic fields formed by two counterpropagating pulses with elliptic polarization. Our numerical approach allows us to take into account the temporal as well as the spatial variation of the standing electromagnetic field. The spin and momentum resolved pair creation probabilities feature characteristic Rabi oscillations and resonance spectra. Therefore, each laser frequency features a specific momentum distribution of the created particles. We find that, depending on the relative polarization of both pulses, the created electrons may be spin polarized along the direction of field propagation
Multi-pair states in electron-positron pair creation
Ultra strong electromagnetic fields can lead to spontaneous creation of single or multiple electronâpositron pairs. A quantum field theoretical treatment of the pair creation process combined with numerical methods provides a description of the fermionic quantum field state, from which all observables of the multiple electronâpositron pairs can be inferred. This allows to study the complex multi-particle dynamics of electronâpositron pair creation in-depth, including multi-pair statistics as well as momentum distributions and spin. To illustrate the potential benefit of this approach, it is applied to the intermediate regime of pair creation between nonperturbative Schwinger pair creation and perturbative multiphoton pair creation where the creation of multi-pair states becomes nonnegligible but cascades do not yet set in. Furthermore, it is demonstrated how spin and helicity of the created electrons and positrons are affected by the polarization of the counterpropagating laser fields, which induce the creation of electronâpositron pairs
Modulation Instability of Two-Dimensional Dipolar Bose-Einstein Condensate in a Deep Optical Lattice
We study the stability of the continuous waves in the pancake shaped dipolar Bose-Einstein condensate trapped in the strong optical lattice potential with the coexisting local (the short-range s-wave) interaction and nonlocal (the dipole-dipole) interactions between the condensate atoms. The system is modeled by two two-dimensional discrete models derived from the Gross-Pitaevskii equation accounting the dipole-dipole interactions: discrete nonlinear Schrödinger equation with cubic nonlinearity and nonpolynomial Schrödinger equation. The corresponding dispersion relations are calculated analytically and the regions of the modulation instability in the parametric space are summarized into the stability diagrams