158 research outputs found
Fluid dynamics of bacterial turbulence
Self-sustained turbulent structures have been observed in a wide range of
living fluids, yet no quantitative theory exists to explain their properties.
We report experiments on active turbulence in highly concentrated 3D
suspensions of Bacillus subtilis and compare them with a minimal fourth-order
vector-field theory for incompressible bacterial dynamics. Velocimetry of
bacteria and surrounding fluid, determined by imaging cells and tracking
colloidal tracers, yields consistent results for velocity statistics and
correlations over two orders of magnitude in kinetic energy, revealing a
decrease of fluid memory with increasing swimming activity and linear scaling
between energy and enstrophy. The best-fit model parameters allow for
quantitative agreement with experimental data.Comment: 5 pages, 4 figure
Dynamical Signatures of Symmetry Broken and Liquid Phases in an Heisenberg Antiferromagnet on the Triangular Lattice
We present the dynamical spin structure factor of the antiferromagnetic
spin- Heisenberg model on a triangular lattice obtained
from large-scale matrix-product state simulations. The high frustration due to
the combination of antiferromagnetic nearest and next-to-nearest neighbour
interactions yields a rich phase diagram. We resolve the low-energy excitations
both in the -ordered phase and in the putative spin liquid phase
at . In the ordered phase, we observe an avoided decay of the
lowest magnon-branch, demonstrating the robustness of this phenomenon in the
presence of gapless excitations. Our findings in the spin-liquid phase chime
with the field-theoretical predictions for a gapless Dirac spin liquid, in
particular the picture of low-lying monopole excitations at the corners of the
Brillouin zone. We comment on possible practical difficulties of distinguishing
proximate liquid and solid phases based on the dynamical structure factor
Sensitivity of the correlation between the depth of shower maximum and the muon shower size to the cosmic ray composition
The composition of ultra-high energy cosmic rays is an important issue in
astroparticle physics research, and additional experimental results are
required for further progress. Here we investigate what can be learned from the
statistical correlation factor r between the depth of shower maximum and the
muon shower size, when these observables are measured simultaneously for a set
of air showers. The correlation factor r contains the lowest-order moment of a
two-dimensional distribution taking both observables into account, and it is
independent of systematic uncertainties of the absolute scales of the two
observables. We find that, assuming realistic measurement uncertainties, the
value of r can provide a measure of the spread of masses in the primary beam.
Particularly, one can differentiate between a well-mixed composition (i.e., a
beam that contains large fractions of both light and heavy primaries) and a
relatively pure composition (i.e., a beam that contains species all of a
similar mass). The number of events required for a statistically significant
differentiation is ~ 200. This differentiation, though diluted, is maintained
to a significant extent in the presence of uncertainties in the phenomenology
of high energy hadronic interactions. Testing whether the beam is pure or
well-mixed is well motivated by recent measurements of the depth of shower
maximum.Comment: Accepted for publication in Astroparticle Physics, LA-UR-12-2008
Spin-Peierls instability of the U(1) Dirac spin liquid
Quantum spin liquids are tantalizing phases of frustrated quantum magnets. A
complicating factor in their realization and observation in materials is the
ubiquitous presence of other degrees of freedom, in particular lattice
distortion modes (phonons). These provide additional routes for relieving
magnetic frustration, thereby possibly destabilizing spin-liquid ground states.
In this work, we focus on triangular-lattice Heisenberg antiferromagnets, where
recent numerical evidence suggests the presence of an extended U(1) Dirac spin
liquid phase which is described by compact quantum electrodynamics in 2+1
dimensions (QED), featuring gapless spinons and monopoles as gauge
excitations. Its low energy theory is believed to flow to a strongly-coupled
fixed point with conformal symmetries. Using complementary perturbation theory
and scaling arguments, we show that a symmetry-allowed coupling between
(classical) finite-wavevector lattice distortions and monopole operators of the
U(1) Dirac spin liquid generally induces a spin-Peierls instability towards a
(confining) 12-site valence-bond solid state. We support our theoretical
analysis with state-of-the-art density matrix renormalization group
simulations. Away from the limit of static distortions, we demonstrate that the
phonon energy gap establishes a parameter regime where the spin liquid is
expected to be stable.Comment: 23 pages, 10 figure
Molecular quantum spin network controlled by a single qubit
Scalable quantum technologies will require an unprecedented combination of
precision and complexity for designing stable structures of well-controllable
quantum systems. It is a challenging task to find a suitable elementary
building block, of which a quantum network can be comprised in a scalable way.
Here we present the working principle of such a basic unit, engineered using
molecular chemistry, whose control and readout are executed using a nitrogen
vacancy (NV) center in diamond. The basic unit we investigate is a synthetic
polyproline with electron spins localized on attached molecular sidegroups
separated by a few nanometers. We demonstrate the readout and coherent
manipulation of very few () of these electronic spin systems
and access their direct dipolar coupling tensor. Our results show, that it is
feasible to use spin-labeled peptides as a resource for a molecular-qubit based
network, while at the same time providing simple optical readout of single
quantum states through NV-magnetometry. This work lays the foundation for
building arbitrary quantum networks using well-established chemistry methods,
which has many applications ranging from mapping distances in single molecules
to quantum information processing.Comment: Author name typ
Holzreste von spĂ€tglazialen Kiefern aus der tiefgrĂŒndigen und tonreichen Permanentrutschung âSpiegelbergâ, Kanton Schwyz (Schweiz)
Die Hauptstrasse Nr. 8 zwischen Schwyz und Sattel verlĂ€uft durch aktive und tiefgrĂŒndige Permanentrutschungen. Im lehmigen, matrix-gestĂŒtzten GehĂ€ngeschutt der Rutschung âSpiegelbergâ wurde zwischen 1979â81 die GĂŒtschbrĂŒcke erstellt (LK: 690.314/211.943; 670 m ĂŒ.M.) und unter Anwendung von GrĂŒndungsschutzschĂ€chten im unterlagernden Fels fundiert. Beim Aushub des Schutzschachtes fĂŒr den Pfeiler WL-Nord wurden in der Tiefe von 25 m bzw. 38 m unter Oberkante Terrain zwei Nadelbaumfragmente gefunden. Letzterer Holzfund lag wenige Meter ĂŒber der FelsoberflĂ€che. Die 14C-Altersdatierung der Holzfunde (beide Pinus sylvestris) ergaben kalibrierte Altersspannen zwischen 11.690â11.270 cal. a BP (2s) am Ăbergang vom Grönland Stadial 1 (GS-1; âJĂŒngere Dryasâ) zum HolozĂ€n bzw. 13.830â13.640 cal. a BP (2s) zu Beginn der spĂ€tglazialen WĂ€rmeschwankung GI-1c (Grönland Interstadial 1c; âAllerĂždâ). Die vorliegenden Daten zeigen, dass die HanginstabilitĂ€ten bei âSpiegelbergâ nach dem Zerfall des letzteiszeitlichen Muota/Reussgletschers zu Beginn des SpĂ€tglazials eingesetzt haben mussten, und die Waldkiefer schon kurz nach den KĂ€lterĂŒckschlĂ€gen des GI-1d (âAegelsee-Schwankungâ) bzw. des GS-1 am nördlichen Alpenrand prĂ€sent war.researc
Introducing LoCo, a Logic for Configuration Problems
In this paper we present the core of LoCo, a logic-based high-level
representation language for expressing configuration problems. LoCo shall allow
to model these problems in an intuitive and declarative way, the dynamic
aspects of configuration notwithstanding. Our logic enforces that
configurations contain only finitely many components and reasoning can be
reduced to the task of model construction.Comment: In Proceedings LoCoCo 2011, arXiv:1108.609
Minimizing attosecond CEP jitter by carrier envelope phase tuning
Minimizing the CEP jitter of isolated attosecond pulses (IAP) will be important for future applications. This jitter is experimentally and theoretically investigated and can be minimized when the driving pulse is near its Fourier limit but with slightly negative chirp. Thus, understanding and characterization of the CEP jitter of IAPs is a first step towards exact control of the electric field of IAP pulses
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