1,590 research outputs found
Acceleration of particles by rotating black holes: near-horizon geometry and kinematics
Nowadays, the effect of infinite energy in the centre of mass frame due to
near-horizon collisions attracts much attention.We show generality of the
effect combining two seemingly completely different approaches based on
properties of a particle with respect to its local light cone and calculating
its velocity in the locally nonrotaing frame directly. In doing so, we do not
assume that particles move along geodesics. Usually, a particle reaches a
horizon having the velocity equals that of light. However, there is also case
of "critical" particles for which this is not so. It is just the pair of usual
and critical particles that leads to the effect under discussion. The similar
analysis is carried out for massless particles. Then, critical particles are
distinguishable due to the finiteness of local frequency. Thus, both approach
based on geometrical and kinematic properties of particles moving near the
horizon, reveal the universal character of the effect.Comment: 8 page
Consequences of critical interchain couplings and anisotropy on a Haldane chain
Effects of interchain couplings and anisotropy on a Haldane chain have been
investigated by single crystal inelastic neutron scattering and density
functional theory (DFT) calculations on the model compound SrNiVO.
Significant effects on low energy excitation spectra are found where the
Haldane gap (; where is the intrachain exchange
interaction) is replaced by three energy minima at different antiferromagnetic
zone centers due to the complex interchain couplings. Further, the triplet
states are split into two branches by single-ion anisotropy. Quantitative
information on the intrachain and interchain interactions as well as on the
single-ion anisotropy are obtained from the analyses of the neutron scattering
spectra by the random phase approximation (RPA) method. The presence of
multiple competing interchain interactions is found from the analysis of the
experimental spectra and is also confirmed by the DFT calculations. The
interchain interactions are two orders of magnitude weaker than the
nearest-neighbour intrachain interaction = 8.7~meV. The DFT calculations
reveal that the dominant intrachain nearest-neighbor interaction occurs via
nontrivial extended superexchange pathways Ni--O--V--O--Ni involving the empty
orbital of V ions. The present single crystal study also allows us to
correctly position SrNiVO in the theoretical - phase
diagram [T. Sakai and M. Takahashi, Phys. Rev. B 42, 4537 (1990)] showing where
it lies within the spin-liquid phase.Comment: 12 pages, 12 figures, 3 tables PRB (accepted). in Phys. Rev. B (2015
Acute effect of repeated sprints on inter-limb asymmetries during unilateral jumping
The aim of the present study was to investigate the effects of multiple repeated sprints on unilateral jump performance and inter-limb asymmetries. Eighteen recreationally active males performed three single leg countermovement jumps (SLCMJ) as baseline data. The repeated sprint protocol was 6 x 40 m with 20 seconds of passive rest between each sprint. This protocol was conducted four times, each set separated by four minutes of rest. Within that rest period, subjects performed one SLCMJ on each limb after two minutes of rest. A one-way ANOVA showed significant reductions (p < 0.05; ES = -0.52 to -0.99) in jump height on both limbs after each set relative to baseline. Inter-limb asymmetries increased at each time point and ranged from 7.62-14.67%, with significant increases in asymmetry seen after sets three (p = 0.046) and four (p = 0.002). Significant increases in sprint time were shown between sprints one and six in each set (p ≤ 0.01). A fatigue index (%) was also calculated and showed an exponential increase from 5.74% (set one) to 13.50% (set four), with significant differences between all sets (p < 0.001) with the exception of sets three and four. Results from this study show that a 6 x 40 m repeated sprint protocol is a sufficient dose for implementing acute fatigue in recreationally active subjects. This was manifested by reductions in jump height at all time points and jump height asymmetries after the third and fourth sets. These findings indicate that jump height from unilateral jump testing may be a useful metric to use during the monitoring process in recreationally trained athletes
Spin and charge order in the vortex lattice of the cuprates: experiment and theory
I summarize recent results, obtained with E. Demler, K. Park, A. Polkovnikov,
M. Vojta, and Y. Zhang, on spin and charge correlations near a magnetic quantum
phase transition in the cuprates. STM experiments on slightly overdoped BSCCO
(J.E. Hoffman et al., Science 295, 466 (2002)) are consistent with the
nucleation of static charge order coexisting with dynamic spin correlations
around vortices, and neutron scattering experiments have measured the magnetic
field dependence of static spin order in the underdoped regime in LSCO (B. Lake
et al., Nature 415, 299 (2002)) and LaCuO_4+y (B. Khaykovich et al., Phys. Rev.
B 66, 014528 (2002)). Our predictions provide a semi-quantitative description
of these observations, with only a single parameter measuring distance from the
quantum critical point changing with doping level. These results suggest that a
common theory of competing spin, charge and superconducting orders provides a
unified description of all the cuprates.Comment: 18 pages, 7 figures; Proceedings of the Mexican Meeting on
Mathematical and Experimental Physics, Mexico City, September 2001, to be
published by Kluwer Academic/Plenum Press; (v2) added clarifications and
updated reference
Quantum spin chain as a potential realization of the Nersesyan-Tsvelik model
It is well established that long-range magnetic order is suppressed in
magnetic systems whose interactions are low-dimensional. The prototypical
example is the S-1/2 Heisenberg antiferromagnetic chain (S-1/2 HAFC) whose
ground state is quantum critical. In real S-1/2 HAFC compounds interchain
coupling induces long-range magnetic order although with a suppressed ordered
moment and reduced N\'eel temperature compared to the Curie-Weiss temperature.
Recently, it was suggested that order can also be suppressed if the interchain
interactions are frustrated, as for the Nersesyan-Tsvelik model. Here, we study
the new S-1/2 HAFC, (NO)[Cu(NO3)3]. This material shows extreme suppression of
order which furthermore is incommensurate revealing the presence of frustration
consistent with the Nersesyan-Tsvelik model
Helium, Oxygen, Proton, and Electron (HOPE) Mass Spectrometer for the Radiation Belt Storm Probes Mission
The HOPE mass spectrometer of the Radiation Belt Storm Probes (RBSP) mission (renamed the Van Allen Probes) is designed to measure the in situ plasma ion and electron fluxes over 4π sr at each RBSP spacecraft within the terrestrial radiation belts. The scientific goal is to understand the underlying physical processes that govern the radiation belt structure and dynamics. Spectral measurements for both ions and electrons are acquired over 1 eV to 50 keV in 36 log-spaced steps at an energy resolution ΔE FWHM/E≈15 %. The dominant ion species (H+, He+, and O+) of the magnetosphere are identified using foil-based time-of-flight (TOF) mass spectrometry with channel electron multiplier (CEM) detectors. Angular measurements are derived using five polar pixels coplanar with the spacecraft spin axis, and up to 16 azimuthal bins are acquired for each polar pixel over time as the spacecraft spins. Ion and electron measurements are acquired on alternate spacecraft spins. HOPE incorporates several new methods to minimize and monitor the background induced by penetrating particles in the harsh environment of the radiation belts. The absolute efficiencies of detection are continuously monitored, enabling precise, quantitative measurements of electron and ion fluxes and ion species abundances throughout the mission. We describe the engineering approaches for plasma measurements in the radiation belts and present summaries of HOPE measurement strategy and performance
Adding New Tasks to a Single Network with Weight Transformations using Binary Masks
Visual recognition algorithms are required today to exhibit adaptive
abilities. Given a deep model trained on a specific, given task, it would be
highly desirable to be able to adapt incrementally to new tasks, preserving
scalability as the number of new tasks increases, while at the same time
avoiding catastrophic forgetting issues. Recent work has shown that masking the
internal weights of a given original conv-net through learned binary variables
is a promising strategy. We build upon this intuition and take into account
more elaborated affine transformations of the convolutional weights that
include learned binary masks. We show that with our generalization it is
possible to achieve significantly higher levels of adaptation to new tasks,
enabling the approach to compete with fine tuning strategies by requiring
slightly more than 1 bit per network parameter per additional task. Experiments
on two popular benchmarks showcase the power of our approach, that achieves the
new state of the art on the Visual Decathlon Challenge
Green's function approach to transport through a gate-all-around Si nanowire under impurity scattering
We investigate transport properties of gate-all-around Si nanowires using
non-equilibrium Green's function technique. By taking into account of the
ionized impurity scattering we calculate Green's functions self-consistently
and examine the effects of ionized impurity scattering on electron densities
and currents. For nano-scale Si wires, it is found that, due to the impurity
scattering, the local density of state profiles loose it's interference
oscillations as well as is broaden and shifted. In addition, the impurity
scattering gives rise to a different transconductance as functions of
temperature and impurity scattering strength when compared with the
transconductance without impurity scattering.Comment: 8 pages, 4 figure
High-field spectroscopy of singlet-triplet transitions in the spin-dimer systems Sr3Cr2O8 and Ba3Cr2O8
Magnetic excitations in the isostructural spin-dimer systems Sr3Cr2O8 and
Ba3Cr2O8 are probed by means of high-field electron spin resonance at
sub-terahertz frequencies. Three types of magnetic modes were observed. One
mode is gapless and corresponds to transitions within excited states, while two
other sets of modes are gapped and correspond to transitions from the ground to
the first excited states. The selection rules of the gapped modes are analyzed
in terms of a dynamical Dzyaloshinskii-Moriya interaction, suggesting the
presence of phonon-assisted effects in the low-temperature spin dynamics of
Sr3Cr2O8 and Ba3Cr2O8Comment: 6 pages, 4 figures, all comments are welcome and appreciate
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