1,354 research outputs found
Avalanche analysis from multi-electrode ensemble recordings in cat, monkey and human cerebral cortex during wakefulness and sleep
Self-organized critical states are found in many natural systems, from
earthquakes to forest fires, they have also been observed in neural systems,
particularly, in neuronal cultures. However, the presence of critical states in
the awake brain remains controversial. Here, we compared avalanche analyses
performed on different in vivo preparations during wakefulness, slow-wave sleep
and REM sleep, using high-density electrode arrays in cat motor cortex (96
electrodes), monkey motor cortex and premotor cortex and human temporal cortex
(96 electrodes) in epileptic patients. In neuronal avalanches defined from
units (up to 160 single units), the size of avalanches never clearly scaled as
power-law, but rather scaled exponentially or displayed intermediate scaling.
We also analyzed the dynamics of local field potentials (LFPs) and in
particular LFP negative peaks (nLFPs) among the different electrodes (up to 96
sites in temporal cortex or up to 128 sites in adjacent motor and pre-motor
cortices). In this case, the avalanches defined from nLFPs displayed power-law
scaling in double log representations, as reported previously in monkey.
However, avalanche defined as positive LFP (pLFP) peaks, which are less
directly related to neuronal firing, also displayed apparent power-law scaling.
Closer examination of this scaling using more reliable cumulative distribution
functions (CDF) and other rigorous statistical measures, did not confirm
power-law scaling. The same pattern was seen for cats, monkey and human, as
well as for different brain states of wakefulness and sleep. We also tested
other alternative distributions. Multiple exponential fitting yielded optimal
fits of the avalanche dynamics with bi-exponential distributions. Collectively,
these results show no clear evidence for power-law scaling or self-organized
critical states in the awake and sleeping brain of mammals, from cat to man.Comment: In press in: Frontiers in Physiology, 2012, special issue "Critical
Brain Dynamics" (Edited by He BY, Daffertshofer A, Boonstra TW); 33 pages, 13
figures. 3 table
Direct observation of lattice symmetry breaking at the hidden-order transition in URu2Si2
Since the 1985 discovery of the phase transition at K in
the heavy-fermion metal URuSi, neither symmetry change in the crystal
structure nor magnetic ordering have been observed, which makes this "hidden
order" enigmatic. Some high-field experiments have suggested electronic
nematicity which breaks fourfold rotational symmetry, but direct evidence has
been lacking for its ground state at zero magnetic field. Here we report on the
observation of lattice symmetry breaking from the fourfold tetragonal to
twofold orthorhombic structure by high-resolution synchrotron X-ray diffraction
measurements at zero field, which pins down the space symmetry of the order.
Small orthorhombic symmetry-breaking distortion sets in at with a
jump, uncovering the weakly first-order nature of the hidden-order transition.
This distortion is observed only in ultrapure sample, implying a highly unusual
coupling nature between the electronic nematicity and underlying lattice.Comment: 17 pages, 3 figures. Submitted version. Revisions have been made
through the review process. See the published version in Nature
Communication
Dynamical structure factors of two-leg spin ladder systems
We investigate dynamical properties of two-leg spin ladder systems.
In a strong coupling region, an isolated mode appears in the lowest excited
states, while in a weak coupling region, an isolated mode is reduced and the
lowest excited states become a lower bound of the excitation continuum. We find
in the system with equal intrachain and interchain couplings that due to a
cyclic four-spin interaction, the distribution of the weights for the dynamical
structure factor and characteristics of the lowest excited states are strongly
influenced. The dynamical properties of two systems proposed for are also discussed.Comment: 5 pages, 6 figure
Current control system of the power supplies for LHD superconducting coils
The LHD is a fusion experimental facility using a large-scale superconducting coil system. The coil system includes six sets of superconducting coils, and six DC power supplies are used to charge them. For the current controllers of these power supplies, high accuracy of current control, fast response and robustness of the system are required. This paper describes the current control system for the LHD DC power supplies. First, the outline of the power system is presented, and then, the current controllers for the LHD are described. Finally, experimental results are presented and discussed in case of coil excitation using these control systems. The results show the various characteristics for each control system and indicate its possibility to control the system according to the requirements from a plasma experiment
Microstructural evolution under low shear rates during Rheo processing of LM25 alloy
© ASM InternationalMicrostructural features of LM25 alloy processed by two different routes: (1) conventional casting, and(2)shear casting based on inclined heated surface are studied. The microstructures of the primary phase for the shear-cast samples show rosette or ellipsoidal morphologies. Heat transfer of contacting melt with the
inclined tube surface and shear stress exerted on the layers of the melt as result of gravitational force are crucial parameters for the microstructural evolution. Compared to those produced by conventional casting, shear-cast samples have a much improved tensile strength and ductility due to globular microstructure
Unique Spin Dynamics and Unconventional Superconductivity in the Layered Heavy Fermion Compound CeIrIn_5:NQR Evidence
We report measurements of the ^{115}In nuclear spin-lattice relaxation rate
(1/T_1) between T=0.09 K and 100 K in the new heavy fermion (HF) compound
CeIrIn_5. At 0.4 K < T < 100 K, 1/T_1 is strongly T-dependent, which indicates
that CeIrIn_5 is much more itinerant than known Ce-based HFs. We find that
1/T_1T, subtracting that for LaIrIn_5, follows a 1/(T+\theta)^{3/4} variation
with \theta=8 K. We argue that this novel feature points to anisotropic, due to
a layered crystal structure, spin fluctuations near a magnetic ordering. The
bulk superconductivity sets in at 0.40 K below which the coherence peak is
absent and 1/T_1 follows a T^3 variation, which suggests unconventional
superconductivity with line-node gap.Comment: minor changes, appeared in PRL (4 pages, 4 figures
- …