58,899 research outputs found
Dynamic model for failures in biological systems
A dynamic model for failures in biological organisms is proposed and studied
both analytically and numerically. Each cell in the organism becomes dead under
sufficiently strong stress, and is then allowed to be healed with some
probability. It is found that unlike the case of no healing, the organism in
general does not completely break down even in the presence of noise. Revealed
is the characteristic time evolution that the system tends to resist the stress
longer than the system without healing, followed by sudden breakdown with some
fraction of cells surviving. When the noise is weak, the critical stress beyond
which the system breaks down increases rapidly as the healing parameter is
raised from zero, indicative of the importance of healing in biological
systems.Comment: To appear in Europhys. Let
Persistence of singlet fluctuations in the coupled spin tetrahedra system Cu2Te2O5Br2 revealed by high-field magnetization and 79Br NQR - 125Te NMR
We present high-field magnetization and Br nuclear quadrupole
resonance (NQR) and Te nuclear magnetic resonance (NMR) studies in the
weakly coupled Cu () tetrahedral system CuTeOBr.
The field-induced level crossing effects were observed by the magnetization
measurements in a long-ranged magnetically ordered state which was confirmed by
a strong divergence of the spin-lattice relaxation rate 1/T1 at T0=13.5 K. In
the paramagnetic state, 1/T1 reveals an effective singlet-triplet spin gap much
larger than that observed by static bulk measurements. Our results imply that
the inter- and the intra-tetrahedral interactions compete, but at the same time
they cooperate strengthening effectively the local intratetrahedral exchange
couplings. We discuss that the unusual feature originates from the frustrated
intertetrahedral interactions.Comment: 5 pages, 4 figures, accepted in Phys. Rev. B as a Rapid
Communication
Critical currents for vortex defect motion in superconducting arrays
We study numerically the motion of vortices in two-dimensional arrays of
resistively shunted Josephson junctions. An extra vortex is created in the
ground states by introducing novel boundary conditions and made mobile by
applying external currents. We then measure critical currents and the
corresponding pinning energy barriers to vortex motion, which in the
unfrustrated case agree well with previous theoretical and experimental
findings. In the fully frustrated case our results also give good agreement
with experimental ones, in sharp contrast with the existing theoretical
prediction. A physical explanation is provided in relation with the vortex
motion observed in simulations.Comment: To appear in Physical Review
Affine maps of density matrices
For quantum systems described by finite matrices, linear and affine maps of
matrices are shown to provide equivalent descriptions of evolution of density
matrices for a subsystem caused by unitary Hamiltonian evolution in a larger
system; an affine map can be replaced by a linear map, and a linear map can be
replaced by an affine map. There may be significant advantage in using an
affine map. The linear map is generally not completely positive, but the linear
part of an equivalent affine map can be chosen to be completely positive and
related in the simplest possible way to the unitary Hamiltonian evolution in
the larger system.Comment: 4 pages, title changed, sentence added, reference update
Construction of optimal witness for unknown two-qubit entanglement
Whether entanglement in a state can be detected, distilled, and quantified
without full state reconstruction is a fundamental open problem. We demonstrate
a new scheme encompassing these three tasks for arbitrary two-qubit
entanglement, by constructing the optimal entanglement witness for
polarization-entangled mixed-state photon pairs without full state
reconstruction. With better efficiency than quantum state tomography, the
entanglement is maximally distilled by newly developed tunable polarization
filters, and quantified by the expectation value of the witness, which equals
the concurrence. This scheme is extendible to multiqubit
Greenberger-Horne-Zeilinger entanglement.Comment: Phys. Rev. Lett. 105, 230404 (2010); supplementary information
(OWitness_sup.pdf) is included in source zip fil
Dynamic model of fiber bundles
A realistic continuous-time dynamics for fiber bundles is introduced and
studied both analytically and numerically. The equation of motion reproduces
known stationary-state results in the deterministic limit while the system
under non-vanishing stress always breaks down in the presence of noise.
Revealed in particular is the characteristic time evolution that the system
tends to resist the stress for considerable time, followed by sudden complete
rupture. The critical stress beyond which the complete rupture emerges is also
obtained
Defect Motion and Lattice Pinning Barrier in Josephson-Junction Ladders
We study motion of domain wall defects in a fully frustrated
Josephson-unction ladder system, driven by small applied currents. For small
system sizes, the energy barrier E_B to the defect motion is computed
analytically via symmetry and topological considerations. More generally, we
perform numerical simulations directly on the equations of motion, based on the
resistively-shunted junction model, to study the dynamics of defects, varying
the system size. Coherent motion of domain walls is observed for large system
sizes. In the thermodynamical limit, we find E_B=0.1827 in units of the
Josephson coupling energy.Comment: 7 pages, and to apear in Phys. Rev.
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