8,258 research outputs found
Universal collisionless transport of graphene
The impact of the electron-electron Coulomb interaction on the optical
conductivity of graphene has led to a controversy that calls into question the
universality of collisionless transport in this and other Dirac materials.
Using a lattice calculation that avoids divergences present in previous nodal
Dirac approaches, our work settles this controversy and obtains results in
quantitative agreement with experiment over a wide frequency range. We also
demonstrate that dimensional regularization methods agree, as long as the
scaling properties of the conductivity and the regularization of the theory in
modified dimension are correctly implemented. Tight-binding lattice and nodal
Dirac theory calculations are shown to coincide at low energies even when the
non-zero size of the atomic orbital wave function is included, conclusively
demonstrating the universality of the optical conductivity of graphene.Comment: 4+ pages,4 figures; includes Supplemental Material (18 pages, 2
figures
The Two-Dimensional Square-Lattice S=1/2 Antiferromagnet Cu(pz)(ClO)
We present an experimental study of the two-dimensional S=1/2 square-lattice
antiferromagnet Cu(pz)(ClO) (pz denotes pyrazine - )
using specific heat measurements, neutron diffraction and cold-neutron
spectroscopy. The magnetic field dependence of the magnetic ordering
temperature was determined from specific heat measurements for fields
perpendicular and parallel to the square-lattice planes, showing identical
field-temperature phase diagrams. This suggest that spin anisotropies in
Cu(pz)(ClO) are small. The ordered antiferromagnetic structure is a
collinear arrangement with the magnetic moments along either the
crystallographic b- or c-axis. The estimated ordered magnetic moment at zero
field is m_0=0.47(5)mu_B and thus much smaller than the available single-ion
magnetic moment. This is evidence for strong quantum fluctuations in the
ordered magnetic phase of Cu(pz)(ClO). Magnetic fields applied
perpendicular to the square-lattice planes lead to an increase of the
antiferromagnetically ordered moment to m_0=0.93(5)mu_B at mu_0H=13.5T -
evidence that magnetic fields quench quantum fluctuations. Neutron spectroscopy
reveals the presence of a gapped spin excitations at the antiferromagnetic zone
center, and it can be explained with a slightly anisotropic nearest neighbor
exchange coupling described by J_1^{xy}=1.563(13)meV and
J_1^z=0.9979(2)J_1^{xy}
Neutral Current Production in MiniBooNE
This paper describes the analysis used to determine the neutral current
production in MiniBooNE in bins of momentum. Additionally, a
measurement of the relative coherent production of s is discussed. The
coherent production rate is found to be (19.5 1.1 (stat) 2.5 (sys))%
of the total exclusive neutral current production rate.Comment: Prepared for the Proceedings of Neutrino Interactions 200
Magnetic excitations in the metallic single-layer Ruthenates Ca(2-x)Sr(x)RuO(4) studied by inelastic neutron scattering
By inelastic neutron scattering, we have analyzed the magnetic correlations
in the paramagnetic metallic region of the series Ca(2-x)Sr(x)RuO(4),
0.2<=x<=0.62. We find different contributions that correspond to 2D
ferromagnetic fluctuations and to fluctuations at incommensurate wave vectors
(0.11,0,0), (0.26,0,0) and (0.3,0.3,0). These components constitute the
measured response as function of the Sr-concentration x, of the magnetic field
and of the temperature. A generic model is applicable to metallic
Ca(2-x)Sr(x)RuO(4) close to the Mott transition, in spite of their strongly
varying physical properties. The amplitude, characteristic energy and width of
the incommensurate components vary only little as function of x, but the
ferromagnetic component depends sensitively on concentration, temperature and
magnetic field. While ferromagnetic fluctuations are very strong in
Ca1.38Sr0.62RuO4 with a low characteristic energy of 0.2 meV at T=1.5 K, they
are strongly suppressed in Ca1.8Sr0.2RuO4, but reappear upon the application of
a magnetic field and form a magnon mode above the metamagnetic transition. The
inelastic neutron scattering results document how the competition between
ferromagnetic and incommensurate antiferromagnetic instabilities governs the
physics of this system
Antiferroquadrupolar Order in the Magnetic Semiconductor TmTe
The physical properties of the antiferroquadrupolar state occurring in TmTe
below TQ=1.8 K have been studied using neutron diffraction in applied magnetic
fields. A field-induced antiferromagnetic component k = (1/2,1/2,1/2) is
observed and, from its magnitude and direction for different orientations of H,
an O(2,2) quadrupole order parameter is inferred. Measurements below TN ~= 0.5
K reveal that the magnetic structure is canted, in agreement with theoretical
predictions for in-plane antiferromagnetism. Complex domain repopulation
effects occur when the field is increased in the ordered phases, with
discontinuities in the superstructure peak intensities above 4 T.Comment: 6 pages, 6 figures, Presented at the International Conference on
Strongly Correlated Electrons with Orbital Degrees of Freedom (ORBITAL 2001),
September 11-14, 2001 (Sendai, JAPAN). To appear in: Journal of the Physical
Society of Japan (2002
Discrete element modeling and fibre optical measurements for fluidized bed spray granulation
Spout fluidized beds are frequently used for the production of granules or\ud
particles through granulation. The products find application in a large variety of\ud
applications, for example detergents, fertilizers, pharmaceuticals and food. Spout fluidized\ud
beds have a number of advantageous properties, such as a high mobility of the particles,\ud
which prevents undesired agglomeration and yields excellent heat transfer properties. The\ud
particle growth mechanism in a spout fluidized bed as function of particle-droplet\ud
interaction has a profound influence on the particle morphology and thus on the product\ud
quality. Nevertheless, little is known about the details of the granulation process. This is\ud
mainly due to the fact that the granulation process is not visually accessible. In this work\ud
we use fundamental, deterministic models to enable the detailed investigation of\ud
granulation behaviour in a spout fluidized bed. A discrete element model is used\ud
describing the dynamics of the continuous gas-phase and the discrete droplets and\ud
particles. For each element momentum balances are solved. The momentum transfer\ud
among each of the three phases is described in detail at the level of individual elements.\ud
The results from the discrete element model simulations are compared with local\ud
measurements of particle volume fractions as well as particle velocities by using a novel\ud
fibre optical probe in a fluidized bed of 400 mm I.D. Simulations and experiments were\ud
carried out for two different cases using Geldart B type aluminium oxide particles: a\ud
freely bubbling fluidized bed and a spout fluidized bed with the presence of droplets. It is\ud
demonstrated how the discrete element model can be used to obtain information about the\ud
interaction of the discrete phases, i.e. the growth zone in a spout fluidized bed. Eventually\ud
this kind of information can be used to obtain closure information required in more coarse\ud
grained model
Field-induced paramagnons at the metamagnetic transition in Ca1.8Sr0.2RuO4
The magnetic excitations in Ca1.8Sr0.2RuO4 were studied across the
metamagnetic transition and as a function of temperature using inelastic
neutron scattering. At low temperature and low magnetic field the magnetic
response is dominated by a complex superposition of incommensurate
antiferromagnetic fluctuations. Upon increasing the magnetic field across the
metamagnetic ransition, paramagnon and finally well-defined magnon scattering
is induced, partially suppressing the incommensurate signals. The high-field
phase in Ca1.8Sr0.2RuO4 has, therefore, to be considered as an intrinsically
ferromagnetic state stabilized by the magnetic field
Human-like machines: Transparency and comprehensibility [Commentary]
AbstractArtificial intelligence algorithms seek inspiration from human cognitive systems in areas where humans outperform machines. But on what level should algorithms try to approximate human cognition? We argue that human-like machines should be designed to make decisions in transparent and comprehensible ways, which can be achieved by accurately mirroring human cognitive processes.</jats:p
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