1,196 research outputs found
Reduced dimensionality in layered quantum dimer magnets: Frustration vs. inhomogeneous condensates
Motivated by recent experiments on BaCuSi2O6, we investigate magnetic
excitations and quantum phase transitions of layered dimer magnets with
inter-layer frustration. We consider two scenarios, (A) a lattice with one
dimer per unit cell and perfect inter-layer frustration, and (B) an enlarged
unit cell with inequivalent layers, with and without perfect frustration. In
all situations, the critical behavior at asymptotically low temperatures is
three-dimensional, but the corresponding crossover scale may be tiny. Magnetic
ordering in case (B) can be discussed in terms of two condensates; remarkably,
perfect frustration renders the proximity effect ineffective. Then, the
ordering transition will be generically split, with clear signatures in
measurable properties. Using a generalized bond-operator method, we calculate
the low-temperature magnetic properties in the paramagnetic and
antiferromagnetic phases. Based on the available experimental data on
BaCuSi2O6, we propose that scenario (B) with inequivalent layers and imperfect
frustration is realized in this material, likely with an additional modulation
of the inter-layer couling along the c axis.Comment: 23 pages, 18 figs, (v2) new fig for bandwidths, (v3) triplon binding
energy discussed, (v4) small changes for clarification, accepted (PRB
Dynamic fracture of icosahedral model quasicrystals: A molecular dynamics study
Ebert et al. [Phys. Rev. Lett. 77, 3827 (1996)] have fractured icosahedral
Al-Mn-Pd single crystals in ultrahigh vacuum and have investigated the cleavage
planes in-situ by scanning tunneling microscopy (STM). Globular patterns in the
STM-images were interpreted as clusters of atoms. These are significant
structural units of quasicrystals. The experiments of Ebert et al. imply that
they are also stable physical entities, a property controversially discussed
currently. For a clarification we performed the first large scale fracture
simulations on three-dimensional complex binary systems. We studied the
propagation of mode I cracks in an icosahedral model quasicrystal by molecular
dynamics techniques at low temperature. In particular we examined how the shape
of the cleavage plane is influenced by the clusters inherent in the model and
how it depends on the plane structure. Brittle fracture with no indication of
dislocation activity is observed. The crack surfaces are rough on the scale of
the clusters, but exhibit constant average heights for orientations
perpendicular to high symmetry axes. From detailed analyses of the fractured
samples we conclude that both, the plane structure and the clusters, strongly
influence dynamic fracture in quasicrystals and that the clusters therefore
have to be regarded as physical entities.Comment: 10 pages, 12 figures, for associated avi files, see
http://www.itap.physik.uni-stuttgart.de/~frohmut/MOVIES/emitted_soundwaves.avi
and
http://www.itap.physik.uni-stuttgart.de/~frohmut/MOVIES/dynamic_fracture.av
Fracture of complex metallic alloys: An atomistic study of model systems
Molecular dynamics simulations of crack propagation are performed for two
extreme cases of complex metallic alloys (CMAs): In a model quasicrystal the
structure is determined by clusters of atoms, whereas the model C15 Laves phase
is a simple periodic stacking of a unit cell. The simulations reveal that the
basic building units of the structures also govern their fracture behaviour.
Atoms in the Laves phase play a comparable role to the clusters in the
quasicrystal. Although the latter are not rigid units, they have to be regarded
as significant physical entities.Comment: 6 pages, 4 figures, for associated avi file, see
http://www.itap.physik.uni-stuttgart.de/~frohmut/MOVIES/C15.LJ.011.100.av
Apparent electron-phonon interaction in strongly correlated systems
We study the interaction of electrons with phonons in strongly correlated
solids, having high-T_c cuprates in mind. Using sum-rules, we show that the
apparent strength of this interaction strongly depends on the property studied.
If the solid has a small fraction (doping) delta of charge carriers, the
influence of the interaction on the phonon self-energy is reduced by a factor
delta, while there is no corresponding reduction of the coupling seen in the
electron self-energy. This supports the interpretation of recent photoemission
experiments, assuming a strong coupling to phonons.Comment: 4 pages, RevTeX, 2 eps figure
Superconducting d-wave stripes in cuprates: Valence bond order coexisting with nodal quasiparticles
We point out that unidirectional bond-centered charge-density-wave states in
cuprates involve electronic order in both s- and d-wave channels, with
non-local Coulomb repulsion suppressing the s-wave component. The resulting
bond-charge-density wave, coexisting with superconductivity, is compatible with
recent photoemission and tunneling data and as well as neutron-scattering
measurements, once long-range order is destroyed by slow fluctuations or glassy
disorder. In particular, the real-space structure of d-wave stripes is
consistent with the scanning-tunneling-microscopy measurements on both
underdoped Bi2Sr2CaCu2O8+x and Ca2-xNaxCuO2Cl2 of Kohsaka et al. [Science 315,
1380 (2007), arXiv:cond-mat/0703309].Comment: 5 pages, 3 figs, (v2) final version to be published in PR
When resources collide: Towards a theory of coincidence in information spaces
This paper is an attempt to lay out foundations for a general theory of coincidence in information spaces such as the World Wide Web, expanding on existing work on bursty structures in document streams and information cascades. We elaborate on the hypothesis that every resource that is published in an information space, enters a temporary interaction with another resource once a unique explicit or implicit reference between the two is found. This thought is motivated by Erwin Shroedingers notion of entanglement between quantum systems. We present a generic information cascade model that exploits only the temporal order of information sharing activities, combined with inherent properties of the shared information resources. The approach was applied to data from the world's largest online citizen science platform Zooniverse and we report about findings of this case study
dacl10k: Benchmark for Semantic Bridge Damage Segmentation
Reliably identifying reinforced concrete defects (RCDs)plays a crucial role
in assessing the structural integrity, traffic safety, and long-term durability
of concrete bridges, which represent the most common bridge type worldwide.
Nevertheless, available datasets for the recognition of RCDs are small in terms
of size and class variety, which questions their usability in real-world
scenarios and their role as a benchmark. Our contribution to this problem is
"dacl10k", an exceptionally diverse RCD dataset for multi-label semantic
segmentation comprising 9,920 images deriving from real-world bridge
inspections. dacl10k distinguishes 12 damage classes as well as 6 bridge
components that play a key role in the building assessment and recommending
actions, such as restoration works, traffic load limitations or bridge
closures. In addition, we examine baseline models for dacl10k which are
subsequently evaluated. The best model achieves a mean intersection-over-union
of 0.42 on the test set. dacl10k, along with our baselines, will be openly
accessible to researchers and practitioners, representing the currently biggest
dataset regarding number of images and class diversity for semantic
segmentation in the bridge inspection domain.Comment: 23 pages, 6 figure
- …