193 research outputs found
Ground state of a double-exchange system containing impurities: bounds of ferromagnetism
We study the boundary between ferromagnetic and non-ferromagnetic ground
state of a double-exchange system with quenched disorder for arbitrary relation
between Hund exchange coupling and electron band width. The boundary is found
both from the solution of the Dynamical Mean Field Approximation equations and
from the comparison of the energies of the saturated ferromagnetic and
paramagnetic states. Both methods give very similar results. To explain the
disappearance of ferromagnetism in part of the parameter space we derive from
the double-exchange Hamiltonian with classical localized spins in the limit of
large but finite Hund exchange coupling the model (with classical
localized spins).Comment: 5 pages, 8 eps figures, latex; minor typos correcte
An Overview of the NASA FAP Hypersonics Project Airbreathing Propulsion Research
The propulsion research portfolio of the National Aeronautics and Space Administration Fundamental Aeronautics Program Hypersonics Project encompasses a significant number of technical tasks that are aligned to achieve mastery and intellectual stewardship of the core competencies in the hypersonic-flight regime. An overall coordinated programmatic and technical effort has been structured to advance the state-of-the-art, via both experimental and analytical efforts. A subset of the entire hypersonics propulsion research portfolio is presented in this overview paper. To this end, two programmatic research disciplines are discussed; namely, (1) the Propulsion Discipline, including three associated research elements: the X-51A partnership, the HIFiRE-2 partnership, and the Durable Combustor Rig, and (2) the Turbine-Based Combine Cycle Discipline, including three associated research elements: the Combined Cycle Engine Large Scale Inlet Mode Transition Experiment, the small-scale Inlet Mode Transition Experiment, and the High-Mach Fan Rig
Numerical Investigation of Shock-Train Response to Inflow Boundary-Layer Variations
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/143113/1/1.J055333.pd
Ferromagnetic transition in a double-exchange system
We study ferromagnetic transition in three-dimensional double-exchange model.
The influence of strong spin fluctuations on conduction electrons is described
in coherent potential approximation. In the framework of thermodynamic approach
we construct for the system "electrons (in a disordered spin configuration) +
spins" the Landau functional, from the analysis of which critical temperature
of ferromagnetic transition is calculated.Comment: 4 pages, 1 eps figure, LaTeX2e, RevTeX. References added, text
change
Nonquasiparticle states in half-metallic ferromagnets
Anomalous magnetic and electronic properties of the half-metallic
ferromagnets (HMF) have been discussed. The general conception of the HMF
electronic structure which take into account the most important correlation
effects from electron-magnon interactions, in particular, the spin-polaron
effects, is presented. Special attention is paid to the so called
non-quasiparticle (NQP) or incoherent states which are present in the gap near
the Fermi level and can give considerable contributions to thermodynamic and
transport properties. Prospects of experimental observation of the NQP states
in core-level spectroscopy is discussed. Special features of transport
properties of the HMF which are connected with the absence of one-magnon
spin-flip scattering processes are investigated. The temperature and magnetic
field dependences of resistivity in various regimes are calculated. It is shown
that the NQP states can give a dominate contribution to the temperature
dependence of the impurity-induced resistivity and in the tunnel junction
conductivity. First principle calculations of the NQP-states for the prototype
half-metallic material NiMnSb within the local-density approximation plus
dynamical mean field theory (LDA+DMFT) are presented.Comment: 27 pages, 9 figures, Proceedings of Berlin/Wandlitz workshop 2004;
Local-Moment Ferromagnets. Unique Properties for Moder Applications, ed. M.
Donath, W.Nolting, Springer, Berlin, 200
Effect of a nonuniform distribution of voids on the plastic response of voided materials: a computational and statistical analysis
This study investigates the overall and local response of porous media composed of a perfectly plastic matrix weakened by stress-free voids. Attention is focused on the specific role played by porosity fluctuations inside a representative volume element. To this end, numerical simulations using the Fast Fourier Transform (FFT) are performed on different classes of microstructure corresponding to different spatial distributions of voids. Three types of microstructures are investigated: random microstructures with no void clustering, microstructures with a connected cluster of voids and microstructures with disconnected void clusters. These numerical simulations show that the porosity fluctuations can have a strong effect on the overall yield surface of porous materials. Random microstructures without clusters and microstructures with a connected cluster are the hardest and the softest configurations, respectively, whereas microstructures with disconnected clusters lead to intermediate responses. At a more local scale, the salient feature of the fields is the tendency for the strain fields to concentrate in specific bands. Finally, an image analysis tool is proposed for the statistical characterization of the porosity distribution. It relies on the distribution of the ‘distance function’, the width of which increases when clusters are present. An additional connectedness analysis allows us to discriminate between clustered microstructures
Observations of Shock Diffusion and Interactions in Supersonic Freestreams with Counterflowing Jets
One of the technical challenges in long-duration space exploration and interplanetary missions is controlled entry and re-entry into planetary and Earth atmospheres, which requires the dissipation of considerable kinetic energy as the spacecraft decelerates and penetrates the atmosphere. Efficient heat load management of stagnation points and acreage heating remains a technological challenge and poses significant risk, particularly for human missions. An innovative approach using active flow control concept is proposed to significantly modify the external flow field about the spacecraft in planetary atmospheric entry and re-entry in order to mitigate the harsh aerothermal environments, and significantly weaken and disperse the shock-wave system to reduce aerothermal loads and wave drag, as well as improving aerodynamic performance. To explore the potential benefits of this approach, we conducted fundamental experiments in a trisonic blow down wind tunnel to investigate the effects of counterflowing sonic and supersonic jets against supersonic freestreams to gain a better understanding of the flow physics of the interactions of the opposing flows and the resulting shock structure
Generalized kinetic equations for charge carriers in graphene
A system of generalized kinetic equations for the distribution functions of
two-dimensional Dirac fermions scattered by impurities is derived in the Born
approximation with respect to short-range impurity potential. It is proven that
the conductivity following from classical Boltzmann equation picture, where
electrons or holes have scattering amplitude reduced due chirality, is
justified except for an exponentially narrow range of chemical potential near
the conical point. When in this range, creation of infinite number of
electron-hole pairs related to quasi-relativistic nature of electrons in
graphene results in a renormalization of minimal conductivity as compared to
the Boltzmann term and logarithmic corrections in the conductivity similar to
the Kondo effect.Comment: final version, Phys. Rev. B, accepte
Spin Wave Theory of Double Exchange Ferromagnets
We construct the 1/S spin-wave expansion for double exchange ferromagnets at
T=0. It is assumed that the value of Hund's rule coupling, J_H, is sufficiently
large, resulting in a fully saturated, ferromagnetic half-metallic ground
state. We evaluate corrections to the magnon dispersion law, and we also find
that, in contrast to earlier statements in the literature, magnon-electron
scattering does give rise to spin wave damping. We analyse the momentum
dependence of these quantities and discuss the experimental implications for
colossal magnetoresistance compounds.Comment: 4 pages, Latex-Revtex, 2 PostScript figures. Minor revisions,
references added. See also cond-mat/990921
Quantum effects in the quasiparticle structure of the ferromagnetic Kondo lattice model
A new ``Dynamical Mean-field theory'' based approach for the Kondo lattice
model with quantum spins is introduced. The inspection of exactly solvable
limiting cases and several known approximation methods, namely the second-order
perturbation theory, the self-consistent CPA and finally a moment-conserving
decoupling of the equations of motion help in evaluating the new approach. This
comprehensive investigation gives some certainty to our results: Whereas our
method is somewhat limited in the investigation of the J<0-model, the results
for J>0 reveal important aspects of the physics of the model: The energetically
lowest states are not completely spin-polarized.A band splitting, which occurs
already for relatively low interaction strengths, can be related to distinct
elementary excitations, namely magnon emission (absorption) and the formation
of magnetic polarons. We demonstrate the properties of the ferromagnetic Kondo
lattice model in terms of spectral densities and quasiparticle densities of
states.Comment: 19 pages, 4 figure
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