8,401 research outputs found
Class of exactly solvable SO(n) symmetric spin chains with matrix product ground states
We introduce a class of exactly solvable SO(n) symmetric Hamiltonians with
matrix product ground states. For an odd case, the ground state is a
translational invariant Haldane gap spin liquid state; while for an even case, the ground state is a spontaneously dimerized state with twofold
degeneracy. In the matrix product ground states for both cases, we identify a
hidden antiferromagnetic order, which is characterized by nonlocal string order
parameters. The ground-state phase diagram of a generalized SO(n) symmetric
bilinear-biquadratic model is discussed.Comment: 11 pages, 5 figure
Noise induced transition from an absorbing phase to a regime of stochastic spatiotemporal intermittency
We introduce a stochastic partial differential equation capable of
reproducing the main features of spatiotemporal intermittency (STI).
Additionally the model displays a noise induced transition from laminarity to
the STI regime. We show by numerical simulations and a mean-field analysis that
for high noise intensities the system globally evolves to a uniform absorbing
phase, while for noise intensities below a critical value spatiotemporal
intermittence dominates. A quantitative computation of the loci of this
transition in the relevant parameter space is presented.Comment: 4 pages, 6 eps figures. Submitted to Phys. Rev. Lett. See for
additional information http://imedea.uib.es
Electronic correlations in iron-pnictide superconductors and beyond; what can we learn from optics
The Coulomb repulsion, impeding electrons' motion, has an important impact on
the charge dynamics. It mainly causes a reduction of the effective metallic
Drude weight (proportional to the so-called optical kinetic energy),
encountered in the optical conductivity, with respect to the expectation within
the nearly-free electron limit (defining the so-called band kinetic energy), as
evinced from band-structure theory. In principle, the ratio between the optical
and band kinetic energy allows defining the degree of electronic correlations.
Through spectral weight arguments based on the excitation spectrum, we provide
an experimental tool, free from any theoretical or band-structure based
assumptions, in order to estimate the degree of electronic correlations in
several systems. We first address the novel iron-pnictide superconductors,
which serve to set the stage for our approach. We then revisit a large variety
of materials, ranging from superconductors, to Kondo-like systems as well as
materials close to the Mott-insulating state. As comparison we also tackle
materials, where the electron-phonon coupling dominates. We establish a direct
relationship between the strength of interaction and the resulting reduction of
the optical kinetic energy of the itinerant charge carriers
Multichannel Photon Counting Lidar Measurements Using USB-based Digital Storage Oscilloscope
We present a simple method of making multichannel photon counting measurements of weak lidar signal from large ranges, using commonly available USB-based digital storage oscilloscopes. The single photon pulses from compact photomultiplier tubes are amplified and stretched so that the pulses are large and broad enough to be sampled efficiently by the USB oscilloscopes. A software interface written in Labview is then used to count the number of photon pulses in each of the prescribed time bins to form the histogram of LIDAR signal. This method presents a flexible alternative to the modular multichannel scalers and facilitate the development of sensitive lidar systems
A Convenient Model of Severe, High Incidence Autoimmune Gastritis Caused by Polyclonal Effector T Cells and without Perturbation of Regulatory T Cells
Autoimmune gastritis results from the breakdown of T cell tolerance to the gastric H+/K+ ATPase. The gastric H+/K+ ATPase is responsible for the acidification of gastric juice and consists of an α subunit (H/Kα) and a ÎČ subunit (H/KÎČ). Here we show that CD4+ T cells from H/Kα-deficient mice (H/Kαâ/â) are highly pathogenic and autoimmune gastritis can be induced in sublethally irradiated wildtype mice by adoptive transfer of unfractionated CD4+ T cells from H/Kαâ/â mice. All recipient mice consistently developed the most severe form of autoimmune gastritis 8 weeks after the transfer, featuring hypertrophy of the gastric mucosa, complete depletion of the parietal and zymogenic cells, and presence of autoantibodies to H+/K+ ATPase in the serum. Furthermore, we demonstrated that the disease significantly affected stomach weight and stomach pH of recipient mice. Depletion of parietal cells in this disease model required the presence of both H/Kα and H/KÎČ since transfer of H/Kαâ/â CD4+ T cells did not result in depletion of parietal cells in H/Kαâ/â or H/KÎČâ/â recipient mice. The consistency of disease severity, the use of polyclonal T cells and a specific T cell response to the gastric autoantigen make this an ideal disease model for the study of many aspects of organ-specific autoimmunity including prevention and treatment of the disease
Quasinormal modes of Schwarzschild black holes in four and higher dimensions
We make a thorough investigation of the asymptotic quasinormal modes of the
four and five-dimensional Schwarzschild black hole for scalar, electromagnetic
and gravitational perturbations. Our numerical results give full support to all
the analytical predictions by Motl and Neitzke, for the leading term. We also
compute the first order corrections analytically, by extending to higher
dimensions, previous work of Musiri and Siopsis, and find excellent agreement
with the numerical results. For generic spacetime dimension number D the
first-order corrections go as . This means that
there is a more rapid convergence to the asymptotic value for the five
dimensional case than for the four dimensional case, as we also show
numerically.Comment: 12 pages, 5 figures, RevTeX4. v2. Typos corrected, references adde
Marginal Fermi liquid analysis of 300 K reflectance of Bi2Sr2CaCu2O8+x
We use 300 K reflectance data to investigate the normal-state electrodynamics
of the high temperature superconductor BiSrCaCuO
over a wide range of doping levels. The data show that at this temperature the
free carriers are coupled to a continuous spectrum of fluctuations. Assuming
the Marginal Fermi Liquid (MFL) form as a first approximation for the
fluctuation spectrum, the doping-dependent coupling constant can
be estimated directly from the slope of the reflectance spectrum. We find that
decreases smoothly with the hole doping level, from underdoped
samples with ( K) where to overdoped
samples with , ( K) where . An analysis of
the intercept and curvature of the reflectance spectrum shows deviations from
the MFL spectrum symmetrically placed at the optimal doping point . The
Kubo formula for the conductivity gives a better fit to the experiments with
the MFL spectrum up to 2000 cm and with an additional Drude component or
an additional Lorentz component up to 7000 cm. By comparing three
different model fits we conclude that the MFL channel is necessary for a good
fit to the reflectance data. Finally, we note that the monotonic variation of
the reflectance slope with doping provides us with an independent measure of
the doping level for the Bi-2212 system.Comment: 11 pages, 11 figure
Noise Filtering Strategies of Adaptive Signaling Networks: The Case of E. Coli Chemotaxis
Two distinct mechanisms for filtering noise in an input signal are identified
in a class of adaptive sensory networks. We find that the high frequency noise
is filtered by the output degradation process through time-averaging; while the
low frequency noise is damped by adaptation through negative feedback. Both
filtering processes themselves introduce intrinsic noises, which are found to
be unfiltered and can thus amount to a significant internal noise floor even
without signaling. These results are applied to E. coli chemotaxis. We show
unambiguously that the molecular mechanism for the Berg-Purcell time-averaging
scheme is the dephosphorylation of the response regulator CheY-P, not the
receptor adaptation process as previously suggested. The high frequency noise
due to the stochastic ligand binding-unbinding events and the random ligand
molecule diffusion is averaged by the CheY-P dephosphorylation process to a
negligible level in E.coli. We identify a previously unstudied noise source
caused by the random motion of the cell in a ligand gradient. We show that this
random walk induced signal noise has a divergent low frequency component, which
is only rendered finite by the receptor adaptation process. For gradients
within the E. coli sensing range, this dominant external noise can be
comparable to the significant intrinsic noise in the system. The dependence of
the response and its fluctuations on the key time scales of the system are
studied systematically. We show that the chemotaxis pathway may have evolved to
optimize gradient sensing, strong response, and noise control in different time
scalesComment: 15 pages, 4 figure
Evidence for the photospheric excitation of incompressible chromospheric waves
Observing the excitation mechanisms of incompressible transverse waves is vital for determining how energy propagates through the lower solar atmosphere. We aim to show the connection between convectively driven photospheric ïŹows and incompressible chromospheric waves. The observations presented here show the propagation of incompressible motion through the quiet lower solar atmosphere, from the photosphere to the chromosphere. We determine photospheric ïŹow vectors to search for signatures of vortex motion and compare results to photospheric ïŹows present in convective simulations. Further, we search for the chromospheric response to vortex motions. Evidence is presented that suggests incompressible waves can be excited by the vortex motions of a strong magnetic ïŹux concentration in the photosphere. A chromospheric counterpart to the photospheric vortex motion is also observed, presenting itself as a quasi-periodic torsional motion. Fine-scale, ïŹbril structures that emanate from the chromospheric counterpart support transverse waves that are driven by the observed torsional motion. A new technique for obtaining details of transverse waves from time-distance diagrams is presented and the properties of transverse waves (e.g., amplitudes and periods) excited by the chromospheric torsional motion are measured
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