526 research outputs found
Spin wave spectrum of the quantum ferromagnet on the pyrochlore lattice Lu2V2O7
Neutron inelastic scattering has been used to probe the spin dynamics of the
quantum (S=1/2) ferromagnet on the pyrochlore lattice Lu2V2O7. Well-defined
spin waves are observed at all energies and wavevectors, allowing us to
determine the parameters of the Hamiltonian of the system. The data are found
to be in excellent overall agreement with a minimal model that includes a
nearest- neighbour Heisenberg exchange J = 8:22(2) meV and a
Dzyaloshinskii-Moriya interaction (DMI) D =1:5(1) meV. The large DMI term
revealed by our study is broadly consistent with the model developed by Onose
et al. to explain the magnon Hall effect they observed in Lu2V2O7 [1], although
our ratio of D=J = 0:18(1) is roughly half of their value and three times
larger than calculated by ab initio methods [2].Comment: 5 pages, 4 figure
Bound states and field-polarized Haldane modes in a quantum spin ladder
The challenge of one-dimensional systems is to understand their physics
beyond the level of known elementary excitations. By high-resolution neutron
spectroscopy in a quantum spin ladder material, we probe the leading
multiparticle excitation by characterizing the two-magnon bound state at zero
field. By applying high magnetic fields, we create and select the singlet
(longitudinal) and triplet (transverse) excitations of the fully spin-polarized
ladder, which have not been observed previously and are close analogs of the
modes anticipated in a polarized Haldane chain. Theoretical modelling of the
dynamical response demonstrates our complete quantitative understanding of
these states.Comment: 6 pages, 3 figures plus supplementary material 7 pages 5 figure
Asperities and barriers on the seismogenic zone in North Chile: state-of-the-art after the 2007 Mw 7.7 Tocopilla earthquake inferred by GPS and InSAR data
The Mw 7.7 2007 November 14 earthquake had an epicentre located close to the city of Tocopilla, at the southern end of a known seismic gap in North Chile. Through modelling of Global Positioning System (GPS) and radar interferometry (InSAR) data, we show that this event ruptured the deeper part of the seismogenic interface (30–50 km) and did not reach the surface. The earthquake initiated at the hypocentre and was arrested ~150 km south, beneath the Mejillones Peninsula, an area already identified as an important structural barrier between two segments of the Peru–Chile subduction zone. Our preferred models for the Tocopilla main shock show slip concentrated in two main asperities, consistent with previous inversions of seismological data. Slip appears to have propagated towards relatively shallow depths at its southern extremity, under the Mejillones Peninsula. Our analysis of post-seismic deformation suggests that small but still significant post-seismic slip occurred within the first 10 d after the main shock, and that it was mostly concentrated at the southern end of the rupture. The post-seismic deformation occurring in this period represents ~12–19 per cent of the coseismic deformation, of which ~30–55 per cent has been released aseismically. Post-seismic slip appears to concentrate within regions that exhibit low coseismic slip, suggesting that the afterslip distribution during the first month of the post-seismic interval complements the coseismic slip. The 2007 Tocopilla earthquake released only ~2.5 per cent of the moment deficit accumulated on the interface during the past 130 yr and may be regarded as a possible precursor of a larger subduction earthquake rupturing partially or completely the 500-km-long North Chile seismic gap
Effect of Long-Term Climbing Training on Cerebellar Ataxia: A Case Series
Background. Efficient therapy for both limb and gait ataxia is required. Climbing, a complex task for the whole motor system involving balance, body stabilization, and the simultaneous coordination of all 4 limbs, may have therapeutic potential. Objective. To investigate whether long-term climbing training improves motor function in patients with cerebellar ataxia. Methods. Four patients suffering from limb and gait ataxia underwent a 6-week climbing training. Its effect on ataxia was evaluated with validated clinical balance and manual dexterity tests and with a kinematic analysis of multijoint arm and leg pointing movements. Results. The patients increased their movement velocity and achieved a more symmetric movement speed profile in both arm and leg pointing movements. Furthermore, the 2 patients who suffered the most from gait ataxia improved their balance and 2 of the 4 patients improved manual dexterity. Conclusion. Climbing training has the potential to serve as a new rehabilitation method for patients with upper and lower limb ataxia
Could dark energy be vector-like?
In this paper I explore whether a vector field can be the origin of the
present stage of cosmic acceleration. In order to avoid violations of isotropy,
the vector has be part of a ``cosmic triad'', that is, a set of three identical
vectors pointing in mutually orthogonal spatial directions. A triad is indeed
able to drive a stage of late accelerated expansion in the universe, and there
exist tracking attractors that render cosmic evolution insensitive to initial
conditions. However, as in most other models, the onset of cosmic acceleration
is determined by a parameter that has to be tuned to reproduce current
observations. The triad equation of state can be sufficiently close to minus
one today, and for tachyonic models it might be even less than that. I briefly
analyze linear cosmological perturbation theory in the presence of a triad. It
turns out that the existence of non-vanishing spatial vectors invalidates the
decomposition theorem, i.e. scalar, vector and tensor perturbations do not
decouple from each other. In a simplified case it is possible to analytically
study the stability of the triad along the different cosmological attractors.
The triad is classically stable during inflation, radiation and matter
domination, but it is unstable during (late-time) cosmic acceleration. I argue
that this instability is not likely to have a significant impact at present.Comment: 28 pages, 6 figures. Uses RevTeX4. v2: Discussion about relation to
phantoms added and additional references cite
Pressure dependence of phonon modes across the tetragonal to collapsed tetragonal phase transition in CaFe2As2
The pressure dependence of a large number of phonon modes in CaFe2As2 with
energies covering the full range of the phonon spectrum has been studied using
inelastic x-ray and neutron scattering. The observed phonon frequency changes
are in general rather small despite the sizable changes of the lattice
parameters at the phase transition. This indicates that the bonding properties
are not profoundly altered by the phase transition. The transverse acoustic
phonons propagating along the c-direction are an exception because they stiffen
very significantly in response to the large contraction of the c-axis. The
lattice parameters are found to change significantly as a function of pressure
before, during and after the first-order phase transition. However, the
frequencies change nearly uniformly with the change in the lattice parameters
due to pressure, with no regard specifically to the first-order phase
transition. Density functional theory describes the frequencies in both the
zero pressure and in the collapsed phase in a satisfactory way if based on the
respective crystal structures
The Algebra of Physical Observables in Nonlinearly Realized Gauge Theories
We classify the physical observables in spontaneously broken nonlinearly
realized gauge theories in the recently proposed loopwise expansion governed by
the Weak Power-Counting (WPC) and the Local Functional Equation. The latter
controls the non-trivial quantum deformation of the classical nonlinearly
realized gauge symmetry, to all orders in the loop expansion. The
Batalin-Vilkovisky (BV) formalism is used. We show that the dependence of the
vertex functional on the Goldstone fields is obtained via a canonical
transformation w.r.t. the BV bracket associated with the BRST symmetry of the
model. We also compare the WPC with strict power-counting renormalizability in
linearly realized gauge theories. In the case of the electroweak group we find
that the tree-level Weinberg relation still holds if power-counting
renormalizability is weakened to the WPC condition.Comment: 20 pages, 1 figur
Particle and Antiparticle sectors in DSR1 and kappa-Minkowski space-time
In this paper we explore the problem of antiparticles in DSR1 and
-Minkowski space-time following three different approaches inspired by
the Lorentz invariant case: a) the dispersion relation, b) the Dirac equation
in space-time and c) the Dirac equation in momentum space. We find that it is
possible to define a map which gives the antiparticle sector from the
negative frequency solutions of the wave equation. In -Poincar\'e, the
corresponding map is the antipodal mapping, which is different from
. The difference is related to the composition law, which is crucial
to define the multiparticle sector of the theory. This discussion permits to
show that the energy of the antiparticle in DSR is the positive root of the
dispersion relation, which is consistent with phenomenological approaches.Comment: 15 pages, no figures, some references added, typos correcte
Review: Perspective on high-performing dairy cows and herds
Milk and dairy products provide highly sustainable concentrations of essential amino acids and other required nutrients for humans; however, amount of milk currently produced per dairy cow globally is inadequate to meet future needs. Higher performing dairy cows and herds produce more milk with less environmental impact per kg than lower performing cows and herds. In 2018, 15.4% of the world\u27s dairy cows produced 45.4% of the world\u27s dairy cow milk, reflecting the global contribution of high-performing cows and herds. In high-performing herds, genomic evaluations are utilized for multiple trait selection, welfare is monitored by remote sensing, rations are formulated at micronutrient levels, health care is focused on prevention and reproduction is managed with precision. Higher performing herds require more inputs and generate more waste products per cow, thus innovations in environmental management on such farms are essential for lowering environmental impacts. Our focus is to provide perspectives on technologies and practices that contribute most to sustainable production of milk from high-performing dairy cows and herds
Spin half fermions with mass dimension one: theory, phenomenology, and dark matter
We provide the first details on the unexpected theoretical discovery of a
spin-one-half matter field with mass dimension one. It is based upon a complete
set of dual-helicity eigenspinors of the charge conjugation operator. Due to
its unusual properties with respect to charge conjugation and parity, it
belongs to a non-standard Wigner class. Consequently, the theory exhibits
non-locality with (CPT)^2 = - I. We briefly discuss its relevance to the
cosmological `horizon problem'. Because the introduced fermionic field is
endowed with mass dimension one, it can carry a quartic self-interaction. Its
dominant interaction with known forms of matter is via Higgs, and with gravity.
This aspect leads us to contemplate the new fermion as a prime dark matter
candidate. Taking this suggestion seriously we study a supernova-like explosion
of a galactic-mass dark matter cloud to set limits on the mass of the new
particle and present a calculation on relic abundance to constrain the relevant
cross-section. The analysis favours light mass (roughly 20 MeV) and relevant
cross-section of about 2 pb. Similarities and differences with the WIMP and
mirror matter proposals for dark matter are enumerated. In a critique of the
theory we bare a hint on non-commutative aspects of spacetime, and
energy-momentum space.Comment: 78 pages [Changes: referee-suggested improvements, additional
important references, and better readability
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