Local Environmental Effects in Magnetic Alloys and Multilayers: Calculations of the Static Response Functions

1989 ◽  
Vol 166 ◽  
Author(s):  
D. D. Johnson ◽  
J. B. Staunton ◽  
B. L. Györffy ◽  
F. J. Pinski ◽  
G. M. Stocks
Keyword(s):  
Diffuse Scattering ◽  
Mean Field ◽  
Chemical Environment ◽  
Response Functions ◽  
Static Response ◽  
Magnetic Alloys ◽  
Multilayer Systems ◽  
Local Moments ◽  
Statistical Mechanical ◽  
Bulk Alloys

ABSTRACTWe have developed an ab-initio method for calculating the static response functions in substitutional alloys. For magnetic alloys, in addition to the nuclear diffuse scattering, a contribution to the alloy diffuse scattering intensities results from the response of the local moments to changes in the ‘local’ chemical environment (i.e. ∂μi/∂cj). We present results of firstprinciples calculations of these ‘local’ response functions in magnetic alloys. These response functions, which may be directly compared to neutron-scattering and Mößbauer experiments, are derived via a mean-field statistical mechanical description of compositional fluctuations in alloys. The statistical averages are performed via the Korringa-Kohn-Rostoker coherent potential approximation, which incorporates the electronic structure of the high-temperature, chemically disordered state. As a first application of the theory, we have investigated the environmental dependence of the moments in NiFe alloys and FeV alloys and multilayers. We compare our results with experiments on bulk alloys and multilayers. Also, a comparison is made to a set of first-principle ‘supercell’ calculations. Although preliminary, the results demonstrate the utility of these response functions for investigating the effects of changes in the chemical environment on the alloy moments and for aiding experimental interpretation in other multilayer systems that are less experimentally amenable than FeV.

scholarly journals THE EXTENDED VARIATIONAL PRINCIPLE FOR MEAN-FIELD, CLASSICAL SPIN SYSTEMS

2005 ◽  
Vol 17 (10) ◽  
pp. 1209-1239
Author(s):  
E. KRITCHEVSKI ◽  
S. STARR
Keyword(s):  
Variational Principle ◽  
Optimization Problems ◽  
Mean Field ◽  
Spin Systems ◽  
Field Equations ◽  
Convex Optimization Problems ◽  
Mean Field Equations ◽  
Statistical Mechanical ◽  
Thermodynamic Pressure ◽  
Extended Variational Principle

The purpose of this article is to obtain a better understanding of the extended variational principle (EVP). The EVP is a formula for the thermodynamic pressure of a statistical mechanical system as a limit of a sequence of minimization problems. It was developed for disordered mean-field spin systems, spin systems where the underlying Hamiltonian is itself random, and whose distribution is permutation invariant. We present the EVP in the simpler setting of classical mean-field spin systems, where the Hamiltonian is non-random and symmetric. The EVP essentially solves these models. We compare the EVP with another method for mean-field spin systems: the self-consistent mean-field equations. The two approaches lead to dual convex optimization problems. This is a new connection, and it permits a generalization of the EVP.

scholarly journals What Can we Learn from off-Specular Neutron Reflection?

1994 ◽  
Vol 376 ◽  
Author(s):  
Roger Pynn
Keyword(s):  
Born Approximation ◽  
Diffuse Scattering ◽  
Organic Molecules ◽  
Distorted Wave ◽  
Distorted Wave Born Approximation ◽  
Multilayer Systems ◽  
Remarkable Feature ◽  
Neutron Reflection ◽  
The Subject ◽  
Diffuse Neutron

ABSTRACTThis paper provides a brief introduction to the subject of off-specular neutron scattering by rough interfaces. Such scattering can be calculated using the distorted-wave Born approximation (DWBA) in which the neutron wavefunctions for a system with smooth interfaces are used as the basis for perturbation theory. Although it has been applied successfully to model the scattering from rough surfaces, the DWBA has several quantitative and qualitative limitations, some of which are described in this paper. When it is applied to multilayer systems, the DWBA correctly predicts a number of qualitative features of the diffuse scattering, including fringes of scattering that result from correlations between the roughnesses of neighbouring interfaces. A remarkable feature of diffuse neutron reflection is the range of length scales within a surface or interface that is probed. This is illustrated here by data obtained with a self-assembled multilayer of organic molecules where distances between 500 Å and 3 μm parallel to the layers are probed.

scholarly journals Predicting mind wandering episodes quarter hours in advances from EEG

2021 ◽  
Author(s):  
Chie Nakatani ◽  
Hannah Bernhard ◽  
Cees van Leeuwen
Keyword(s):  
Cognitive Control ◽  
Dynamic Response ◽  
Mind Wandering ◽  
Directed Network ◽  
Response Functions ◽  
Static Response ◽  
Negative Effects ◽  
Counting Task ◽  
Spontaneous Thought ◽  
Basic Level

Mind wandering occurs when spontaneous thought generation is increased while cognitive control is decreased, presumably in response to antagonistic effects of default mode- (DMN) and goal-directed network (GDN) activity. Previous studies assumed that DMN or GDN activity manifests in behavior with minimal delays and has rigid effects, i.e., DMN always facilitates generation of spontaneous thought and inhibits cognitive control. With those static response functions, the antagonism of DMN and GDN could explain the alternation of task focused and mind wandering states. But it rules out others, such as multi-tasking, where spontaneous thought and cognitive control are both increased, or rumination, where both are decreased. However, shifts from task focused to mind wandering have been shown to occur via a multi-tasking state. We therefore propose dynamic response functions, allowing delay or reversal of DMN and GDN effects. Dynamic response functions enabled us to stochastically predict mind wandering up to at least 30 min in advance from EEG measures of DMN and GDN activity in a tone counting task. Experimental results show both considerable delays and switches between task-positive and negative effects. Mind wandering was initiated by a barrage of DMN activity more than 20 minutes prior a report of mind wandering, directing the mental focus inward. Critically, facilitation of spontaneous thoughts occurred several minutes prior to the report. These thoughts sustained up to 10 min, after which a rebound to task-focused behavior was effectuated, notably without intervention from the GDN. Repeated GDN interventions, however, secured a basic level of task performance.

scholarly journals Neutrino-pair interactions in astrophysical systems

2020 ◽  
Vol 29 (09) ◽  
pp. 2050080
Author(s):  
María Paula Colombi ◽  
Osvaldo Civitarese ◽  
Ana V. Penacchioni
Keyword(s):  
Eigenvalue Problem ◽  
Body Problem ◽  
Mean Field ◽  
Response Functions ◽  
Many Body ◽  
Quasi Particle ◽  
Finite Density ◽  
Neutrino Pair ◽  
Neutrino Fluxes ◽  
Pair Interactions

We study the effects produced by interactions among neutrinos upon extra-galactic neutrino-fluxes. We have assumed a separable type of pair interactions and performed a transformation to a quasi-particle mean field followed by a Tamm–Damcoff diagonalization. In doing so, we have adopted techniques originated in the quantum many-body problem, and adapted them to this specific case. The solutions of the associated eigenvalue problem provide us with energies and amplitudes which are then used to construct the neutrino response functions at finite density and temperature. The formalism is applied to the description of neutrinos produced in a SN environment.

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