2,014 research outputs found
Antiferromagnetic order in multi-band Hubbard models for iron-pnictides
We investigate multi-band Hubbard models for the three iron 3-
bands and the two iron 3- bands in by means of the
Gutzwiller variational theory. Our analysis of the paramagnetic ground state
shows that neither Hartree--Fock mean-field theories nor effective spin models
describe these systems adequately. In contrast to Hartree--Fock-type
approaches, the Gutzwiller theory predicts that antiferromagnetic order
requires substantial values of the local Hund's-rule exchange interaction. For
the three-band model, the antiferromagnetic moment fits experimental data for a
broad range of interaction parameters. However, for the more appropriate
five-band model, the iron electrons polarize the electrons and
they substantially contribute to the ordered moment.Comment: 4 pages, 4 figure
Auditory dominance in motor-sensory temporal recalibration
Perception of synchrony between one’s own action (e.g. a finger tap) and the sensory feedback thereof (e.g. a flash or click) can be shifted after exposure to an induced delay (temporal recalibration effect, TRE). It remains elusive, however, whether the same mechanism underlies motor-visual (MV) and motor-auditory (MA) TRE. We examined this by measuring crosstalk between MV- and MA-delayed feedbacks. During an exposure phase, participants pressed a mouse at a constant pace while receiving visual or auditory feedback that was either delayed (+150 ms) or subjectively synchronous (+50 ms). During a post-test, participants then tried to tap in sync with visual or auditory pacers. TRE manifested itself as a compensatory shift in the tap–pacer asynchrony (a larger anticipation error after exposure to delayed feedback). In experiment 1, MA and MV feedback were either both synchronous (MV-sync and MA-sync) or both delayed (MV-delay and MA-delay), whereas in experiment 2, different delays were mixed across alternating trials (MV-sync and MA-delay or MV-delay and MA-sync). Exposure to consistent delays induced equally large TREs for auditory and visual pacers with similar build-up courses. However, with mixed delays, we found that synchronized sounds erased MV-TRE, but synchronized flashes did not erase MA-TRE. These results suggest that similar mechanisms underlie MA- and MV-TRE, but that auditory feedback is more potent than visual feedback to induce a rearrangement of motor-sensory timing
Semiflexible Filamentous Composites
Inspired by the ubiquity of composite filamentous networks in nature we
investigate models of biopolymer networks that consist of interconnected floppy
and stiff filaments. Numerical simulations carried out in three dimensions
allow us to explore the microscopic partitioning of stresses and strains
between the stiff and floppy fractions c_s and c_f, and reveal a non-trivial
relationship between the mechanical behavior and the relative fraction of stiff
polymer: when there are few stiff polymers, non-percolated stiff ``inclusions``
are protected from large deformations by an encompassing floppy matrix, while
at higher fractions of stiff material the stiff network is independently
percolated and dominates the mechanical response.Comment: Phys. Rev. Lett, to appear (4 pages, 2 figures
Evidence for a temperature-induced spin-state transition of Co3+ in La2-xSrxCoO4
We study the magnetic susceptibility of mixed-valent La2-xSrxCoO4 single
crystals in the doping range of 0.5<= x <= 0.8 for temperatures up to 1000 K.
The magnetism below room temperature is described by paramagnetic Co2+ in the
high-spin state and by Co3+ in the non-magnetic low-spin state. Above room
temperature, an increase in susceptibility compared to the behavior expected
from Co2+ is seen, which we attribute to a spin-state transition of Co3+. The
susceptibility is analyzed by comparison to full-multiplet calculations for the
thermal population of the high- and intermediate-spin states of Co3+
Electrical Control of Dynamic Spin Splitting Induced by Exchange Interaction as Revealed by Time Resolved Kerr Rotation in a Degenerate Spin-Polarized Electron Gas
The manipulation of spin degree of freedom have been demonstrated in spin
polarized electron plasma in a heterostructure by using exchange-interaction
induced dynamic spin splitting rather than the Rashba and Dresselhaus types, as
revealed by time resolved Kerr rotation. The measured spin splitting increases
from 0.256meV to 0.559meV as the bias varies from -0.3V to -0.6V. Both the sign
switch of Kerr signal and the phase reversal of Larmor precessions have been
observed with biases, which all fit into the framework of
exchange-interaction-induced spin splitting. The electrical control of it may
provide a new effective scheme for manipulating spin-selected transport in spin
FET-like devices.Comment: 8 pages, 3 figures ; added some discussion
Spin Dynamics in the Second Subband of a Quasi Two Dimensional System Studied in a Single Barrier Heterostructure by Time Resolved Kerr Rotation
By biasing a single barrier heterostructure with a 500nm-thick GaAs layer as
the absorption layer, the spin dynamics for both of the first and second
subband near the AlAs barrier are examined. We find that when simultaneously
scanning the photon energy of both the probe and pump beams, a sign reversal of
the Kerr rotation (KR) takes place as long as the probe photons break away the
first subband and probe the second subband. This novel feature, while stemming
from the exchange interaction, has been used to unambiguously distinguish the
different spin dynamics ( and ) for the first and second
subbands under the different conditions by their KR signs (negative for
and positive for ). In the zero magnetic field, by scanning
the wavelength towards the short wavelength, decreases in accordance
with the D'yakonov-Perel' (DP) spin decoherence mechanism. At 803nm,
(450ps) becomes ten times longer than (50ps). However, the
value of at 803nm is roughly the same as the value of at
815nm. A new feature has been disclosed at the wavelength of 811nm under the
bias of -0.3V (807nm under the bias of -0.6V) that the spin coherence times
( and ) and the effective factors ( and
) all display a sudden change, due to the "resonant" spin exchange
coupling between two spin opposite bands.Comment: 9pages, 3 figure
Hubbard U and Hund's Exchange J in Transition Metal Oxides: Screening vs. Localization Trends from Constrained Random Phase Approximation
In this work, we address the question of calculating the local effective
Coulomb interaction matrix in materials with strong electronic Coulomb
interactions from first principles. To this purpose, we implement the
constrained random phase approximation (cRPA) into a density functional code
within the linearized augmented plane wave (LAPW) framework.
We apply our approach to the 3d and 4d early transition metal oxides SrMO3
(M=V, Cr, Mn) and (M=Nb, Mo, Tc) in their paramagnetic phases. For these
systems, we explicitly assess the differences between two physically motivated
low-energy Hamiltonians: The first is the three-orbital model comprising the
t2g states only, that is often used for early transition metal oxides. The
second choice is a model where both, metal d- and oxygen p-states are retained
in the construction of Wannier functions, but the Hubbard interactions are
applied to the d-states only ("d-dp Hamiltonian"). Interestingly, since -- for
a given compound -- both U and J depend on the choice of the model, so do their
trends within a family of these compounds. In the 3d perovskite series SrMO3
the effective Coulomb interactions in the t2g Hamiltonian decrease along the
series, due to the more efficient screening. The inverse -- generally expected
-- trend, increasing interactions with increasing atomic number, is however
recovered within the more localized "d-dp Hamiltonian". Similar conclusions are
established in the layered 4d perovskites series Sr2MO4 (M=Mo, Tc, Ru, Rh).
Compared to their isoelectronic and isostructural 3d analogues, the 4d 113
perovskite oxides SrMO3 (M=Nb, Mo, Tc) exhibit weaker screening effects.
Interestingly, this leads to an effectively larger U on 4d shells than on 3d
when a t2g model is constructed.Comment: 21 pages, 7 figure
Coulombic Energy Transfer and Triple Ionization in Clusters
Using neon and its dimer as a specific example, it is shown that excited
Auger decay channels that are electronically stable in the isolated monomer can
relax in a cluster by electron emission. The decay mechanism, leading to the
formation of a tricationic cluster, is based on an efficient energy-transfer
process from the excited, dicationic monomer to a neighbor. The decay is
ultrafast and expected to be relevant to numerous physical phenomena involving
core holes in clusters and other forms of spatially extended atomic and
molecular matter.Comment: 5 pages, 1 figure, to be published in PR
Controlling orbital moment and spin orientation in CoO layers by strain
We have observed that CoO films grown on different substrates show dramatic
differences in their magnetic properties. Using polarization dependent x-ray
absorption spectroscopy at the Co L edges, we revealed that the
magnitude and orientation of the magnetic moments strongly depend on the strain
in the films induced by the substrate. We presented a quantitative model to
explain how strain together with the spin-orbit interaction determine the 3d
orbital occupation, the magnetic anisotropy, as well as the spin and orbital
contributions to the magnetic moments. Control over the sign and direction of
the strain may therefore open new opportunities for applications in the field
of exchange bias in multilayered magnetic films
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