32 research outputs found
Thickness-dependent magnetic structure of ultrathin Fe/Ir(001) films: from spin-spiral states towards ferromagnetic order
We present a detailed study of the ground-state magnetic structure of
ultrathin Fe films on the surface of fcc Ir(001). We use the spin-cluster
expansion technique in combination with the relativistic disordered local
moment scheme to obtain parameters of spin models and then determine the
favored magnetic structure of the system by means of a mean field approach and
atomistic spin dynamics simulations. For the case of a single monolayer of Fe
we find that layer relaxations very strongly influence the ground-state spin
configurations, whereas Dzyaloshinskii-Moriya (DM) interactions and biquadratic
couplings also have remarkable effects. To characterize the latter effect we
introduce and analyze spin collinearity maps of the system. While for two
monolayers of Fe we find a single-q spin spiral as ground state due to DM
interactions, for the case of four monolayers the system shows a noncollinear
spin structure with nonzero net magnetization. These findings are consistent
with experimental measurements indicating ferromagnetic order in films of four
monolayers and thicker.Comment: 9 pages, 7 figure
Theory of anisotropic Rashba splitting of surface states
We investigate the surface Rashba effect for a surface of reduced in-plane
symmetry. Formulating a k.p perturbation theory, we show that the Rashba
splitting is anisotropic, in agreement with symmetry-based considerations. We
show that the anisotropic Rashba splitting is due to the admixture of bulk
states of different symmetry to the surface state, and it cannot be explained
within the standard theoretical picture supposing just a normal-to-surface
variation of the crystal potential. Performing relativistic ab initio
calculations we find a remarkably large Rashba anisotropy for an
unreconstructed Au(110) surface that is in the experimentally accessible range.Comment: 4 pages, 5 figure
Local Charge Excesses in Metallic Alloys: a Local Field Coherent Potential Approximation Theory
Electronic structure calculations performed on very large supercells have
shown that the local charge excesses in metallic alloys are related through
simple linear relations to the local electrostatic field resulting from
distribution of charges in the whole crystal.
By including local external fields in the single site Coherent Potential
Approximation theory, we develop a novel theoretical scheme in which the local
charge excesses for random alloys can be obtained as the responses to local
external fields. Our model maintains all the computational advantages of a
single site theory but allows for full charge relaxation at the impurity sites.
Through applications to CuPd and CuZn alloys, we find that, as a general rule,
non linear charge rearrangements occur at the impurity site as a consequence of
the complex phenomena related with the electronic screening of the external
potential. This nothwithstanding, we observe that linear relations hold between
charge excesses and external potentials, in quantitative agreement with the
mentioned supercell calculations, and well beyond the limits of linearity for
any other site property.Comment: 11 pages, 1 table, 7 figure
Disease burden of psoriatic arthritis compared to rheumatoid arthritis, Hungarian experiment
Parallel Computational Subunits in Dentate Granule Cells Generate Multiple Place Fields
A fundamental question in understanding neuronal computations is how dendritic events influence the output of the neuron. Different forms of integration of neighbouring and distributed synaptic inputs, isolated dendritic spikes and local regulation of synaptic efficacy suggest that individual dendritic branches may function as independent computational subunits. In the present paper, we study how these local computations influence the output of the neuron. Using a simple cascade model, we demonstrate that triggering somatic firing by a relatively small dendritic branch requires the amplification of local events by dendritic spiking and synaptic plasticity. The moderately branching dendritic tree of granule cells seems optimal for this computation since larger dendritic trees favor local plasticity by isolating dendritic compartments, while reliable detection of individual dendritic spikes in the soma requires a low branch number. Finally, we demonstrate that these parallel dendritic computations could contribute to the generation of multiple independent place fields of hippocampal granule cells
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Population based models of cortical drug response: insights from anaesthesia
A great explanatory gap lies between the molecular pharmacology of psychoactive agents and the neurophysiological changes they induce, as recorded by neuroimaging modalities. Causally relating the cellular actions of psychoactive compounds to their influence on population activity is experimentally challenging. Recent developments in the dynamical modelling of neural tissue have attempted to span this explanatory gap between microscopic targets and their macroscopic neurophysiological effects via a range of biologically plausible dynamical models of cortical tissue. Such theoretical models allow exploration of neural dynamics, in particular their modification by drug action. The ability to theoretically bridge scales is due to a biologically plausible averaging of cortical tissue properties. In the resulting macroscopic neural field, individual neurons need not be explicitly represented (as in neural networks). The following paper aims to provide a non-technical introduction to the mean field population modelling of drug action and its recent successes in modelling anaesthesia
Dendritic Spikes Amplify the Synaptic Signal to Enhance Detection of Motion in a Simulation of the Direction-Selective Ganglion Cell
The On-Off direction-selective ganglion cell (DSGC) in mammalian retinas responds most strongly to a stimulus moving in a specific direction. The DSGC initiates spikes in its dendritic tree, which are thought to propagate to the soma with high probability. Both dendritic and somatic spikes in the DSGC display strong directional tuning, whereas somatic PSPs (postsynaptic potentials) are only weakly directional, indicating that spike generation includes marked enhancement of the directional signal. We used a realistic computational model based on anatomical and physiological measurements to determine the source of the enhancement. Our results indicate that the DSGC dendritic tree is partitioned into separate electrotonic regions, each summing its local excitatory and inhibitory synaptic inputs to initiate spikes. Within each local region the local spike threshold nonlinearly amplifies the preferred response over the null response on the basis of PSP amplitude. Using inhibitory conductances previously measured in DSGCs, the simulation results showed that inhibition is only sufficient to prevent spike initiation and cannot affect spike propagation. Therefore, inhibition will only act locally within the dendritic arbor. We identified the role of three mechanisms that generate directional selectivity (DS) in the local dendritic regions. First, a mechanism for DS intrinsic to the dendritic structure of the DSGC enhances DS on the null side of the cell's dendritic tree and weakens it on the preferred side. Second, spatially offset postsynaptic inhibition generates robust DS in the isolated dendritic tips but weak DS near the soma. Third, presynaptic DS is apparently necessary because it is more robust across the dendritic tree. The pre- and postsynaptic mechanisms together can overcome the local intrinsic DS. These local dendritic mechanisms can perform independent nonlinear computations to make a decision, and there could be analogous mechanisms within cortical circuitry