Institute for Solid State Physics and Optics

Atoms, ions, and molecules crystal field theory Mott transition and Hubbard model Mott insulators Heisenberg magnets itinerant electron magnetism ferromagnetism in Hubbard models the Gutzwiller variational method the correlated metallic state mixed valence and heavy fermions quantum hall effect hydrogen atom single-spin-flip ansatz Gutzwiller approximation Schrieffer-Wolff transformation.

The rate of the nuclear spin-lattice relaxation due to the hyperfine interaction is shown to be enhanced by a repulsive electron-electron interaction. The relaxation time is expressed in terms of the wavelength and frequency dependent magnetic susceptibilities of the conduction electrons. The magnetic susceptibilities are calculated taking account of the electron-electron interaction of the δ-function type with a random phase approximation. The Korringa relation between the frequency shift and the relaxation time is shown to be altered by the electron-electron interaction. The theory applies also to the electron spin relaxation of localized magnetic atoms in certain alloys.

Microglia are the main immune cells in the brain and have roles in brain homeostasis and neurological diseases. Mechanisms underlying microglia-neuron communication remain elusive. Here, we identified an interaction site between neuronal cell bodies and microglial processes in mouse and human brain. Somatic microglia-neuron junctions have a specialized nanoarchitecture optimized for purinergic signaling. Activity of neuronal mitochondria was linked with microglial junction formation, which was induced rapidly in response to neuronal activation and blocked by inhibition of P2Y12 receptors. Brain injury-induced changes at somatic junctions triggered P2Y12 receptor-dependent microglial neuroprotection, regulating neuronal calcium load and functional connectivity. Thus, microglial processes at these junctions could potentially monitor and protect neuronal functions.

The effect of electron correlation on itinerant electron ferromagnetism, or the correction to the Stoner (Hartree-Fock) theory, is studied. The additional free energy (correction to the Hartree-Fock contribution) as a function of magnetization is expressed exactly in terms of the transversal spin fluctuations or the transversal dynamical susceptibilities and is actually calculated by using a modified random phase approximation for the dynamical susceptibilities. This contribution turns out to be predominant in describing the thermodynamical properties of itinerant ferromagnets, leading to serious modification of the Stoner theory; the Curie temperature is lowered and the magnetic susceptibility shows approximate Curie-Weiss behavior.

The ordinary single-channel Kondo model consists of one or more spin-½ local moments interacting antiferromagnetically with conduction electrons in a metal. This model has provided a paradigm for understanding many phenomena of strongly correlated electronic materials, ranging from the formation of heavyfermion Fermi liquids to the mapping of a one-band model in the cuprate superconductors. The simplest extension of this ordinary Kondo model in metals which yields exotic non-Fermi-liquid physics is the multichannel Kondo impurity model in which the conduction electrons are given an extra quantum label known as the channel or flavour index. In the overcompensated regime of this model, nonFermi-liquid physics is possible, in contrast with the single-channel model. We overview here the multichannel Kondo impurity model candidates most extensively studied for explaining real materials, specifically the two-level system Kondo model relevant for metallic glasses, nanoscale devices and some doped semiconductors, and the quadrupolar and magnetic two-channel Kondo models developed for rare-earth and actinide ions with crystal-field splittings in metals. We provide an extensive justification for the derivation of the theoretical models, noting that, whenever the local impurity degree of freedom is non-magnetic, a two-channel Kondo model must follow by virtue of the magnetic spin degeneracy of the conduction electrons. We carefully delineate all energy and symmetry restrictions on the applicability of these models. We describe the various methods used to study these models along with their results and limitations (multiplicative renormalization group, numerical renormalization group, non-crossing approximation, conformal field theory and Abelian bosonization), all of which provide differing and useful views of the physics. We pay particular attention to the role that scale invariance plays in all these theoretical approaches. We point out in each case how various perturbing fields (magnetic, crystalline electric, electric field gradients and uniaxial stress) may destabilize the non-Fermi-liquid fixed point. We then provide an extensive discussion of the experimental evidence for the relevance of the two-level system Kondo model to metallic glasses and nanoscale devices, and of the quadrupolar and magnetic two-channel models to a number of heavyfermion-based alloys and compounds. We close with a discussion of the extension of the single-impurity models which comprise the main focus of this review to other systems (Coulomb blockade), multiple impurities and lattice models. In the latter case, we provide an overview of the relevance of the two-channel Kondo lattice model to non-Fermi-liquid behaviour and exotic superconductivity in heavy-fermion compounds and to the theoretical possibility of odd-frequency superconductivity, which is realized (for the first time) in the limit of infinite spatial dimensions for this model.

Optical thin-film structures exhibiting high reflectivity and a nearly constant negative group-delay dispersion over frequency ranges as broad as 80 THz are presented. This attractive combination makes these coatings well suited for intracavity dispersion control in broadband femtosecond solid-state lasers. We address design issues and the principle of operation of these novel devices.

In this paper, an extremely simple structure solution method termed charge flipping is presented. It works ab initio on high-resolution X-ray diffraction data in the manner of Fourier recycling. The real-space modification simply changes the sign of charge density below a threshold, while in reciprocal space the moduli F(obs) are retained resulting in an F(obs) map without weighting. The algorithm is tested using synthetic data for a wide range of structures, the solution statistics are analysed and the quality of reconstruction is checked. Finally, mathematical aspects of the algorithm are considered in detail, and these show that in this chaotic iteration process the solution is a limit cycle and not a fixed point.

Abstract Using the approach of Finnis and Sinclair, N-body potentials for copper, silver, gold and nickel have been constructed. The total energy is regarded as consisting of a pair-potential part and a many body cohesive part. Both these parts are functions of the atomic separations only and are represented by cubic splines, fitted to various bulk properties. For the noble metals, the pair-potentials were fitted at short range to pressure-volume relationships calculated by Christensen and Heine so that interactions at separations smaller than that of the first-nearest neighbours can be treated in this scheme. Using these potentials, point defects, surfaces (including the surface reconstructions) and grain boundaries have been studied and satisfactory agreement with available experimental data has been found.

We present several entanglement criteria in terms of the quantum Fisher information that help to relate various forms of multipartite entanglement to the sensitivity of phase estimation. We show that genuine multipartite entanglement is necessary to reach the maximum sensitivity in some very general metrological tasks using a two-arm linear interferometer. We also show that it is needed to reach the maximum average sensitivity in a certain combination of such metrological tasks.

Abstract An ambient pressure superconductivity of (BEDT-TTF)2Cu(SCN)2 was observed by d.c. magnetic susceptibility and electrical conductivity measurements. The superconducting critical temperature is the highest (TC=10.4 K) among the organic superconductors so far obtained, even though the anion has a positional disorder in the crystal.

Many structural materials (metal alloys, polymers, minerals, etc.) are formed by quenching liquids into crystalline solids. This highly nonequilibrium process often leads to polycrystalline growth patterns that are broadly termed "spherulites" because of their large-scale average spherical shape. Despite the prevalence and practical importance of spherulite formation, only rather qualitative concepts of this phenomenon exist. It is established that phase field methods naturally account for diffusional instabilities that are responsible for dendritic single-crystal growth. However, a generalization of this model is required to describe spherulitic growth patterns, and in the present paper we propose a minimal model of this fundamental crystal growth process. Our calculations indicate that the diversity of spherulitic growth morphologies arises from a competition between the ordering effect of discrete local crystallographic symmetries and the randomization of the local crystallographic orientation that accompanies crystal grain nucleation at the growth front [growth front nucleation (GFN)]. This randomization in the orientation accounts for the isotropy of spherulitic growth at large length scales and long times. In practice, many mechanisms can give rise to GFN, and the present work describes and explores three physically prevalent sources of disorder that lead to this kind of growth. While previous phase field modeling elucidated two of these mechanisms--disorder created by particulate impurities or other static disorder or by the dynamic heterogeneities that spontaneously form in supercooled liquids (even pure ones)--the present paper considers an additional mechanism, crystalline branching induced by a misorientation-dependent grain boundary energy, which can significantly affect spherulite morphology. We find the entire range of observed spherulite morphologies can be reproduced by this generalized phase field model of polycrystalline growth.

By using the effective-mass theory for exciton, the allowed and forbidden direct interband transitions in extremely anisotropic semiconductors are discussed. In this case, the problem is reduced to solving the Schrödinger equation for a hypothetical two-dimensional hydrogen atom. The bound and unbound solutions of the equation are obtained, and the absorption intensities in the discrete, quasi-continuous, and continuous spectral regions are calculated. It is shown that, in the allowed transitions, a small peak may appear just above the absorption edge because of the Coulomb interaction between an excited electron and a hole. This result is compared with the experimental curves of the absorption in layer-type semiconductors, CaS, GaSe, and GaTe.

The dynamical properties of a third-order nonlinear optical process, in which two incident light pulses with wave vectors k 1 and k 2 generate a light wave with 2 k 2 - k 1 in resonant media, are discussed with particular attention to the effect of transverse relaxation time T 2 . It is shown that the correlation trace (output light energy vs incident pulse separation) for this process provides a general means of obtaining information on ultrashort T 2 for both the homogeneously and inhomogeneously broadened transitions, and for both t p (pulse width)< T 2 and t p > T 2 . An experimental demonstration for t p > T 2 is given by using a dye molecule and a picosecond light source.

Americanae nace como un proyecto conjunto que surge dentro de la Red Europea de Información y Documentación sobre América Latina (REDIAL), y que ha afrontado la Biblioteca de la Agencia Española de Cooperación Internacional para el Desarrollo (AECID). Esta nueva biblioteca virtual hace más accesibles los libros digitales de tema americanista a los investigadores y usuarios interesados de cualquier parte del mundo.

Nonlinear time evolution of the condensate wave function in superfluid films is studied on the basis of a Schrödinger equation, which incorporates van der Waals potential due to substrate in its fully nonlinear form, and a surface tension term. In the weak nonlinearity limit, our equation reduces to the ordinary (cubic) nonlinear Schrödinger equation for which exact soliton solutions are known. It is demonstrated by numerical analysis that even under strong nonlinearity , where our equation is far different from cubic Schrödinger equation, there exist quite stable composite “quasi-solitons" . These quasi-solitons are bound states of localized excitations of amplitude and phase of the condensate (superfluid thickness and superfluid velocity, in more physical terms). Thus the present work shows the persistence of the solitonic behavior of superfluid films in the fully nonlinear situation.

Despite exceptional experimental efforts to map out the human interactome, the continued data incompleteness limits our ability to understand the molecular roots of human disease. Computational tools offer a promising alternative, helping identify biologically significant, yet unmapped protein-protein interactions (PPIs). While link prediction methods connect proteins on the basis of biological or network-based similarity, interacting proteins are not necessarily similar and similar proteins do not necessarily interact. Here, we offer structural and evolutionary evidence that proteins interact not if they are similar to each other, but if one of them is similar to the other's partners. This approach, that mathematically relies on network paths of length three (L3), significantly outperforms all existing link prediction methods. Given its high accuracy, we show that L3 can offer mechanistic insights into disease mechanisms and can complement future experimental efforts to complete the human interactome.

The negatively charged silicon vacancy (SiV) color center in diamond has recently proven its suitability for bright and stable single photon emission. However, its electronic structure so far has remained elusive. We here explore the electronic structure by exposing single SiV defects to a magnetic field where the Zeeman effect lifts the degeneracy of magnetic sublevels. The similar responses of single centers and a SiV ensemble in a low strain reference sample prove our ability to fabricate almost perfect single SiVs, revealing the true nature of the defect's electronic properties. We model the electronic states using a group-theoretical approach yielding a good agreement with the experimental observations. Furthermore, the model correctly predicts polarization measurements on single SiV centers and explains recently discovered spin selective excitation of SiV defects.

Propagation of electromagnetic wave packets through 1D photonic band gap materials has been studied using 12 fs optical pulses. The measured transit time is found to be paradoxically short (implying superluminal tunneling) and independent of the barrier thickness for opaque barriers, in analogy to the behavior of electrons tunneling through potential barriers. Shortening of Fourier-limited incident wave packets is observed upon transmission through these linear systems. Although in apparent conflict with causality and the uncertainty principle, neither of these general principles is violated because of the strong attenuation suffered by the transmitted signals.

Possible patterns of vibration-induced splitting in the absorption band of localized electrons in solid are investigated within the Condon approximation. The modes of lattice vibrations around the imperfection which give rise to the level splitting of degenerate excited states are classified into active and potentially active modes according as they cause the absorption band to split or not. The result is applied to the analysis of the absorption bands of Tl + - like ions in alkali halides. One can explain the triplet structure of the C band and the doublet structure of the A band with its temperature sensitive asymmetry, by introducing the coupling constant c between the p -electron and the trigonal lattice vibrations as a single adjustable parameter. The dependences of c upon the host alkali halide and on the charge of the impurity ion are explained by a point-charge plus point-dipole model for the neighboring halide ions.

Theory for the Urbach-Martienssen (U-M) rule on the low energy tail of the fundamental absorption edge of insulators is proposed. The exciton propagator solved for an adiabatic lattice is averaged for lattice vibrations at finite temperature, and the self-energy of exciton is obtained. It describes two characters of exciton in the lattice; the one is the mobile nature of exciton in the undeformed lattice, and the other is the localized nature of exciton trapped momentarily by the lattice deformation due to thermal vibrations. The interplay of the two natures results in the U-M tail below the exciton absorption peak. The result can be interpreted in terms of the Franck-Condon principle where the mobile nature is incorporated in the giant oscillator strength of the momentarily trapped exciton. Emission from this trapped state is discussed in connection with the U-M rule.