812 research outputs found
Electronic band structure and exchange coupling constants in ACr2X4 spinels
We present the results of band structure calculations for ACr2X4 (A=Zn, Cd,
Hg and X=O, S, Se) spinels. Effective exchange coupling constants between Cr
spins are determined by fitting the energy of spin spirals to a classical
Heisenberg model. The calculations reproduce the change of the sign of the
dominant nearest-neighbor exchange interaction J1 from antiferromagnetic in
oxides to ferromagnetic in sulfides and selenides. It is verified that the
ferromagnetic contribution to J1 is due to indirect hopping between Cr t2g and
eg states via X p states. Antiferromagnetic coupling between 3-rd Cr neighbors
is found to be important in all the ACr2X4 spinels studied, whereas other
interactions are much weaker. The results are compared to predictions based on
the Goodenough-Kanamori rules of superexchange.Comment: 15 pages, 10 figures, 3 table
Effective band-structure in the insulating phase versus strong dynamical correlations in metallic VO2
Using a general analytical continuation scheme for cluster dynamical mean
field calculations, we analyze real-frequency self-energies, momentum-resolved
spectral functions, and one-particle excitations of the metallic and insulating
phases of VO2. While for the former dynamical correlations and lifetime effects
prevent a description in terms of quasi-particles, the excitations of the
latter allow for an effective band-structure. We construct an
orbital-dependent, but static one-particle potential that reproduces the full
many-body spectrum. Yet, the ground state is well beyond a static one-particle
description. The emerging picture gives a non-trivial answer to the decade-old
question of the nature of the insulator, which we characterize as a ``many-body
Peierls'' state.Comment: 5 pages, 4 color figure
Spectral (MIR) determination of kaolinite and gibbsite contents in lateritic soils
Kaolinite et gibbsite sont, avec les oxyhydroxydes de fer et le quartz, les constituants de base des sols latéritiques. Les proportions relatives de ces deux minéraux sont des témoins du degré d'évolution de ces sols, d'où l'intérêt de leur détermination quantitative, généralement réalisée par des analyses chimiques ou par diffraction des rayons X, techniques longues et coûteuses. Une procédure nouvelle est proposée; elle est fondée sur la spectroscopie optique en réflectance diffuse et permet une estimation satisfaisante de la teneur en gibbsite et en kaolinite dans les sols étudiés. Pour cette dernière, l'estimation concerne la teneur en kaolinite dans la seule fraction argileuse de la terre fine. En outre, un indice basé sur les intensités d'absorption de la kaolinite et de la gibbsite a été trouvé pour estimer d'une façon précise le rapport Ki = SiO2/Al2O3 dans les sols latéritiques. Ces déterminations peuvent être effectuées directement sur le terrain à l'aide d'un spectromètre portable. (Résumé d'auteur
Effect of Inter-Site Repulsions on Magnetic Susceptibility of One-Dimensional Electron Systems at Quarter-Filling
The temperature dependence of the magnetic susceptibility, \chi (T), is
investigated for one-dimensional interacting electron systems at
quarter-filling within the Kadanoff-Wilson renormalization-group method.
The forward scattering on the same branch (the g_4-process) is examined
together with the backward (g_1) and forward (g_2) scattering amplitudes on
opposite branches.
In connection with lattice models, we show that \chi (T) is strongly enhanced
by the nearest-neighbor interaction, an enhancement that surpasses one of the
next-nearest-neighbor interaction.
A connection between our predictions for \chi (T) and experimental results
for \chi (T) in quasi-one-dimensional organic conductors is presented.Comment: 4 pages, 4 figures, to be published in Journal of the Physical
Society of Japan, vol. 74, No. 1
Polar phonons and intrinsic dielectric response of the ferromagnetic insulating spinel CdCrS from first principles
We have studied the dielectric properties of the ferromagnetic spinel
CdCrS from first principles. Zone-center phonons and Born effective
charges were calculated by frozen-phonon and Berry phase techniques within
LSDA+U. We find that all infrared-active phonons are quite stable within the
cubic space group. The calculated static dielectric constant agrees well with
previous measurements. These results suggest that the recently observed
anomalous dielectric behavior in CdCrS is not due to the softening of a
polar mode. We suggest further experiments to clarify this point
Quantum Phase Transition in Pr2CuO4 to Collinear Spin State in Inclined Magnetic Field: A Neutron Diffraction Observation
In the external field slightly inclined to the - or y-axis of the
frustrated tetragonal atiferromagnet Pr2CuO4, a transition is discovered from
the phase with orthogonal antiferromagnetic spin subsystems along [1,0,0] and
[0,1,0] to the phase with the collinear spins. This phase is shown to be due to
the pseudodipolar interaction, and transforms into the spin-flop phase S perp H
asymptotically at very high field. The discovered phase transition holds at T=0
and is a quantum one, with the transition field being the critical point and
the angle between two subsystems being the order parameter
The Cost of Accumulating Evidence in Perceptual Decision Making
Decision making often involves the accumulation of information over time, but acquiring information typically comes at a cost. Little is known about the cost incurred by animals and humans for acquiring additional information from sensory variables due, for instance, to attentional efforts. Through a novel integration of diffusion models and dynamic programming, we were able to estimate the cost of making additional observations per unit of time from two monkeys and six humans in a reaction time (RT) random-dot motion discrimination task. Surprisingly, we find that the cost is neither zero nor constant over time, but for the animals and humans features a brief period in which it is constant but increases thereafter. In addition, we show that our theory accurately matches the observed reaction time distributions for each stimulus condition, the time-dependent choice accuracy both conditional on stimulus strength and independent of it, and choice accuracy and mean reaction times as a function of stimulus strength. The theory also correctly predicts that urgency signals in the brain should be independent of the difficulty, or stimulus strength, at each trial
Triplet superconducting pairing and density-wave instabilities in organic conductors
Using a renormalization group approach, we determine the phase diagram of an
extended quasi-one-dimensional electron gas model that includes interchain
hopping, nesting deviations and both intrachain and interchain repulsive
interactions. We find a close proximity of spin-density- and
charge-density-wave phases, singlet d-wave and triplet f-wave superconducting
phases. There is a striking correspondence between our results and recent
puzzling experimental findings in the Bechgaard salts, including the
coexistence of spin-density-wave and charge-density-wave phases and the
possibility of a triplet pairing in the superconducting phase.Comment: 4 pages, 5 eps figure
SDW and FISDW transition of (TMTSF)ClO at high magnetic fields
The magnetic field dependence of the SDW transition in (TMTSF)ClO for
various anion cooling rates has been measured, with the field up to 27T
parallel to the lowest conductivity direction . For quenched
(TMTSF)ClO, the SDW transition temperature increases
from 4.5K in zero field up to 8.4K at 27T. A quadratic behavior is observed
below 18T, followed by a saturation behavior. These results are consistent with
the prediction of the mean-field theory. From these behaviors,
is estimated as =13.5K for the perfect nesting case. This
indicates that the SDW phase in quenched (TMTSF)ClO, where is less than 6K, is strongly suppressed by the two-dimensionality of
the system. In the intermediate cooled state in which the SDW phase does not
appear in zero field, the transition temperature for the field-induced SDW
shows a quadratic behavior above 12T and there is no saturation behavior even
at 27T, in contrast to the FISDW phase in the relaxed state. This behavior can
probably be attributed to the difference of the dimerized gap due to anion
ordering.Comment: 4pages,5figures(EPS), accepted for publication in PR
Effect of nearest- and next-nearest neighbor interactions on the spin-wave velocity of one-dimensional quarter-filled spin-density-wave conductors
We study spin fluctuations in quarter-filled one-dimensional
spin-density-wave systems in presence of short-range Coulomb interactions. By
applying a path integral method, the spin-wave velocity is calculated as a
function of on-site (U), nearest (V) and next-nearest (V_2) neighbor-site
interactions. With increasing V or V_2, the pure spin-density-wave state
evolves into a state with coexisting spin- and charge-density waves. The
spin-wave velocity is reduced when several density waves coexist in the ground
state, and may even vanish at large V. The effect of dimerization along the
chain is also considered.Comment: REVTeX, 11 pages, 9 figure
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