In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

We present a review of the basic ideas and techniques of the spectral density functional theory which are currently used in electronic structure calculations of strongly{correlated materials where the one{electron description breaks down. We illustrate the method with several examples where interactions play a dominant role: systems near metal{insulator transition, systems near volume collapse transition, and systems with local moments.

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Electronic structure calculations with dynamical mean-field theory

The proximity effect at superconductor-ferromagnet interfaces produces damped oscillatory behavior of the Cooper pair wave function within the ferromagnetic medium. This is analogous to the inhomogeneous superconductivity, predicted long ago by Fulde and Ferrell (P. Fulde and R. A. Ferrell, 1964, ``Superconductivity in a strong spin-exchange field,'' Phys. Rev. 135, A550--A563), and by Larkin and Ovchinnikov (A. I. Larkin and Y. N. Ovchinnikov, 1964, ``Inhomogeneous state of superconductors,'' Zh. Eksp. Teor. Fiz. 47, 1136--1146 [Sov. Phys. JETP 20, 762--769 (1965)]), and sought by condensed-matter experimentalists ever since. This article offers a qualitative analysis of the proximity effect in the presence of an exchange field and then provides a description of the properties of superconductor-ferromagnet heterostructures. Special attention is paid to the striking nonmonotonic dependence of the critical temperature of multilayers and bilayers on the ferromagnetic layer thickness as well as to the conditions under which ``$\ensuremath{\pi}$'' Josephson junctions are realized. Recent progress in the preparation of high-quality hybrid systems has permitted the observation of many interesting experimental effects, which are also discussed. Finally, the author analyzes the phenomenon of domain-wall superconductivity and the influence of superconductivity on the magnetic structure in superconductor-ferromagnet bilayers.

Proximity effects in superconductor-ferromagnet heterostructures

Hydrogen fuel is considered as the cleanest renewable resource and the primary alternative to fossil fuels for future energy supply. Sustainable hydrogen generation is the major prerequisite to realize future hydrogen economy. The electrocatalytic hydrogen evolution reaction (HER), as the vital step of water electrolysis to H2 production, has been the subject of extensive study over the past decades. In this comprehensive review, we first summarize the fundamentals of HER and review the recent state-of-the-art advances in the low-cost and high-performance catalysts based on noble and non-noble metals, as well as metal-free HER electrocatalysts. We systemically discuss the insights into the relationship among the catalytic activity, morphology, structure, composition, and synthetic method. Strategies for developing an effective catalyst, including increasing the intrinsic activity of active sites and/or increasing the number of active sites, are summarized and highlighted. Finally, the challenges, perspectives, and research directions of HER electrocatalysis are featured.

Recent Advances in Electrocatalytic Hydrogen Evolution Using Nanoparticles.

Recent studies have revealed that single-layer transition-metal oxides and dichalcogenides (MX2) might offer properties superior to those of graphene. So far, only very few MX2 compounds have been synthesized as suspended single layers, and some of them have been exfoliated as thin sheets. Using first-principles structure optimization and phonon calculations based on density functional theory, we predict that, out of 88 different combinations of MX2 compounds, several of them can be stable in free-standing, single-layer honeycomb-like structures. These materials have two-dimensional hexagonal lattices and have top-view appearances as if they consisted of either honeycombs or centered honeycombs. However, their bonding is different from that of graphene; they can be viewed as a positively charged plane of transition-metal atoms sandwiched between two planes of negatively charged oxygen or chalcogen atoms. Electron correlation in transition-metal oxides was treated by including Coulomb repulsion through LDA...

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Stable, Single-Layer MX2 Transition-Metal Oxides and Dichalcogenides in a Honeycomb-Like Structure

Measurements of the specific heat as a function of temperature at varying magnetic fields for the Kondo lattice systems Ce${\mathrm{Cu}}_{2}$${\mathrm{Si}}_{2}$ and Ce${\mathrm{Al}}_{3}$ reveal that, on an energy scale $\ensuremath{\sim}0.5 \mathrm{K}\ensuremath{\ll}{T}_{K}$, there is structure which is due to the periodicity and is qualitatively changed by disorder in the alloys ${\mathrm{Ce}}_{0.9}$${\mathrm{Y}}_{0.1}$${\mathrm{Cu}}_{2}$${\mathrm{Si}}_{2}$, ${\mathrm{Ce}}_{0.8}$${\mathrm{Y}}_{0.2}$${\mathrm{Cu}}_{2}$${\mathrm{Si}}_{2}$, and ${\mathrm{Ce}}_{0.8}$${\mathrm{La}}_{0.2}$${\mathrm{Cu}}_{2}$${\mathrm{Si}}_{2}$. The results support the Fermi-liquid theory of Kondo lattices.

Low-Temperature Specific Heat of CeCu2Si2 and CeAl3: Coherence Effects in Kondo Lattice Systems

Pattern recognition approach to identify natural clusters of acoustic emission signals

Ce${\mathrm{Cu}}_{2}$${\mathrm{Si}}_{2}$ single crystals prepared with an excess of Cu exhibit low residual resistivity ${\ensuremath{\rho}}_{0}$, low susceptibility $\ensuremath{\chi}$, small unit-cell volume $V$, and bulk "heavy-fermion" superconductivity below 0.65 K, while crystals grown from stoichiometric melt show higher ${\ensuremath{\rho}}_{0}$ and $\ensuremath{\chi}$, larger $V$, but no superconductivity. Ce${\mathrm{Cu}}_{2}$${\mathrm{Si}}_{2}$ behaves as an $s$-state superconductor with strongly temperature-dependent pair breaking and almost isotropic Fermi surface.

Superconductivity in CeCu2Si2 Single Crystals

Quantification of failure mechanisms in mode-I loading of fiber reinforced plastics utilizing acoustic emission analysis

The simulation of acoustic emission waveforms resulting from failure during mechanical loading of carbon fiber reinforced plastic structures is investigated using a finite element simulation approach. For this investigation we focus on the dominant failure mechanisms in fiber reinforced structures consisting of matrix cracking, fiber breakage and fiber-matrix interface failure. To simulate the failure process accurately, we present a new acoustic emission source model that is based on the microscopic source geometry and micromechanical properties of fiber and resin. We demonstrate that based on this microscopic source model these failure mechanisms result in excitation of macroscopic plate waves. The propagation of these plate waves is described using a macroscopic three-dimensional model geometry which includes contributions of reflections from the specimen boundaries. We further present a model of the acoustic emission sensors used in experiments to simulate the influence of aperture effects. To enhance the understanding of correlation between macroscopically detectable acoustic emission signals and microscopic failure mechanisms we simulate the response to different source excitation times, crack surface displacements and displacement directions. The results obtained show good agreement with fundamental assumptions about the crack process reported by various other authors. The simulated acoustic emission signals obtained are compared to experimentally measured waveforms during four-point bending experiments of carbon fiber reinforced plastic structures. The simulated signals of fiber-breakage, matrix-cracking and fiber-matrix interface failure show systematic agreement with the respective experimental signals.

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Siegfried Horn

Papers

Low-Temperature Specific Heat of CeCu2Si2 and CeAl3: Coherence Effects in Kondo Lattice Systems

Pattern recognition approach to identify natural clusters of acoustic emission signals

Superconductivity in CeCu2Si2 Single Crystals

Quantification of failure mechanisms in mode-I loading of fiber reinforced plastics utilizing acoustic emission analysis

Simulation of Acoustic Emission in Planar Carbon Fiber Reinforced Plastic Specimens