Recent multicolor multiphoton microscopy schemes are opening the way to high quality fluorescence color imaging at near-micrometric resolution over virtually unlimited volumes of brain tissue. However, unavoidable artifacts related to depth, chromatic aberration or anisotropic resolution affect the resulting images and can bias measurements and quantifications. Here, we prove the possibility of performing precise automated quantitative measurements on such tridimensional multicolor images, taking as a model fluorescently labeled axons belonging to an auditory tract in the mouse brainstem. We present an analysis pipeline to compute the diameter of these axons based on the calculation of distance in HSV color space, its binarization using a random walker algorithm and skeleton extraction by distance transform. We measure a biologically meaningful difference of about 500 nm before and after the axons enter their target nucleus where they form synaptic connections, proving the robustness of our pipeline with respect to the aforementioned limitations. This demonstrates the ability of sub-micrometric measurements across tissue depths of 0.5 mm with multicolor multiphoton microscopy.

Analyzing temperature dependent photoemission (PM) data of the ferromagnetic kondo-lattice (KL) system YbNiSn in the light of the periodic Anderson model (PAM) we show that the KL behavior is not limited to temperatures below a temperature (T) over bar (K), defined empirically from resistivity and specific heat measurements. As characteristic for weakly hybridized Ce and Yb systems, the PE spectra reveal a 4f -derived Fermi level peak, which reflects contributions from the Kondo resonance and its crystal electric field (CEF) satellites. In YbNiSn this peak has an unusual temperature dependence: With decreasing temperature a steady linear increase of intensity is observed which extends over a large interval ranging from 100 K down to 1 K without showing any peculiarities in the region of (T) over bar (K) similar to T-C = 5.6 K. In the light of the single-impurity Anderson model (SIAM) this intensity variation reflects a linear increase of 4f occupancy with decreasing temperature, indicating an onset of Kondo screening at temperatures above 100 K. Within the PAM this phenomenon could be described by a non-Fermi-liquid-like T - linear damping of the self-energy which accounts phenomenologically for the feedback from the closely spaced CEF states.

Strongly correlated electron systems are one of the central topics in contemporary solid-state physics. Prominent examples for such systems are Kondo lattices, i.e., intermetallic materials in which below a critical temperature, the Kondo temperature T-K, the magnetic moments become quenched and the effective masses of the conduction electrons approach the mass of a proton. In Ce-and Yb-based systems, this so-called heavy-fermion behavior is caused by interactions between the strongly localized 4f and itinerant electrons. A major and very controversially discussed issue in this context is how the localized electronic degree of freedom gets involved in the Fermi surface (FS) upon increasing the interaction between both kinds of electrons or upon changing the temperature. In this paper, we show that the FS of a prototypic Kondo lattice, YbRh2Si2, does not change its size or shape in a wide temperature range extending from well below to far above the single-ion Kondo temperature T-K similar to 25 K of this system. This experimental observation, obtained by means of angle-resolved photoemission spectroscopy, is in remarkable contrast to the widely believed evolution from a large FS, including the 4f degrees of freedom, to a small FS, without the 4f's, upon increasing temperature. Our results explicitly demonstrate a need to further advance in theoretical approaches based on the periodic Anderson model in order to elucidate the temperature dependence of Fermi surfaces in Kondo lattices.

In this paper we use a generic form for the Green function G(k, ω) in a correlated metal, already proven successful in describing ARPES line shapes [1]. The associated many body self-energy function has only a single pole. We now investigate, whether this generic model can be used all the way to the limit of strong correlations and, when applied to ARPES intensities, whether it is able to explain some of the ubiquitous dispersive crossover phenomena that have been attributed to dynamical, i.e.: ω-dependent effects. We argue that a quantitative interpretation of experimental data requires to calculate extrema not only in the momentum distribution curve but also in the energy distribution curve. In passing, we give a formula for the extrema in the latter distribution that is valid for the general G(k, ω) in a many body system. To our knowledge, this is a new formula, not found in the literature. The investigation of the generic model proceeds on two levels: on the one hand, we explore the rich variety of crossovers that can be predicted and linked to well defined features in the complex ω-plain. On the other hand, we show that the generic one-pole self-energy can be viewed as a projection on the low energy sector of a microscopic solution, belonging to a lattice model of interacting fermions. To obtain approximate microscopic solutions, we use our continued fraction method [2], [3]. As an explicit example, we study the projection for the case of a hole doped Hubbard model in infinite dimension. A discussion section gives examples, how the generic model is able to cope with the ubiquity of the crossover phenomena, also in finite dimension and beyond the Hubbard model.

We present the continued fraction method (CFM) as a new microscopic approximation to the spectral density of the Hubbard model in the correlated metal phase away from half filling. The quantity expanded as a continued fraction is the single particle Green function. Leading spectral moments are taken into account through a set of real expansion coefficients, as known from the projection technique. The new aspect is to add further stages to the continued fraction, with complex coefficients, thus defining a terminator function. This enables us to treat the entire spectral range of the Green function on equal footing and determine the energy scale of the Fermi liquid quasiparticles by minimizing the total energy. The solution is free of phenomenological parameters and remains well defined in the strong coupling limit, near the doping controlled metal-insulator transition. Our results for the density of states agree reasonably with several variants of the dynamical mean field theory. The CFM requires minimal numerical effort and can be generalized in several ways that are interesting for applications to real materials.

The exact Dyson equation for a transition orbital with strong interactions, coupled with an arbitrary number of ligands, is evaluated with a phenomenological interpolation formula that combines microscopic input from the high and low energy sectors. Explicit expressions for momentum-resolved fermionic excitation spectra and photoemission spectra are derived for the case of one ligand. The two types of spectra differ by observable interference effects.

An interpolation formula, connecting the high and low energy expansions of a momentum resolved Green function G(k, ω) was first outlined in: K. Matho, J. Phys. Chem. Solids 56 (1995) 1735. Here, the algorithm is presented in detail, allowing to calculate the interpolated spectrum for all energies. The high energy part is given by a Padé approximant of arbitrary order. The low energy scenario, with a large Fermi surface, is either backed up by microscopic manybody theory or suggested by phenomenological considerations. The algorithm itself decides, whether the two scenarios to be interpolated are compatible with each other. As an application, we give an in depth discussion of a correlated photohole, as observable in photoemission experiments. The emphasis is on experimental and theoretical exploration of generic low energy features, in agreement with the overall electronic structure of real materials. The context of doped, metallic Mott–Hubbard systems is chosen to indicate ways of modeling correlated spectra in the presence of strong onsite repulsion U and in finite dimension d=2 or d=3.

ARPES integrals are related to the momentum distribution nk. In case of a metal, points k(F) on the Fermi surface were identified by: (i) a change of sign in the temperature variation of the ARPES integrals or (ii) maximum slope in its angular variation. These criteria are based on the assumption of particle-hole symmetry in the vicinity of the Fermi edge. Here, we check (i) and (ii) on the level of the momentum distribution, for an electronic structure with most of its incoherent weight below the Fermi edge. Evaluating n(k)(T) up to T approximate to Delta*, a Fermi liquid coherence energy, we find: criterion (i) remains stable, while (ii) deviates from k(F) proportional to m*T/k(F). Published data on the hole doped t-J model are examined in this light.

We present a continued fraction analysis of the many-body self-energy for holes in a cuprate above Tc. The ansatz terminates in a phenomenological low energy power law. As shown by quantitative fits to ARPES lines in optimally doped Bi-2212, this is a possible answer to the question posed by P.W. Anderson: “How to have your Fermi surface without a Fermi liquid?” A connection of this scenario with the 1D Luttinger liquid is not evident.

A Fermi liquid type self-energy is introduced into a class of hybridized two-band models, with interactions confined to one of the bands. After calculating the k-resolved single particle spectra, it is shown that the photoemission signals can be strongly influenced by matrix element effects.

The electron removal spectrum and momentum distribution of a many-body system are calculated, combining low energy FL phenomenology and high energy Padé approximants. The influence of non-FL power laws at low energy is also studied.

This paper presents an improved Fermi liquid lineshape analysis of the ARPES spectra of 1T-TiTe(2), and a brief overview of recent efforts to identify Luttinger liquid type behavior in photoemission spectra of quasi one-dimensional materials such as K(0.3)MoO(3).

The characteristics asymmetry of angle-resolved photoemission signals near the Fermi edge is shown to be due to inference between the singular part of the hole propagator and an incoherent background. The Fermi liquid case with quasiparticle weight Z → 0 is analysed in detail.

Possible connections between magnetism and atomic structure in AlMn and AlSiMn icosahedral and amorphous phases are critically reviewed. The magnetic entropy, when analyzed in an RKKY model, indicates a strong anticlustering of the spins. This is interpreted in terms of symmetry induced moments with an icosahedral environment around each magnetic site, extending to ≈ three atomic shells. This structural unit is called a “magnetic amorphon”. The role of Si is also discussed in this context.

A crystal field (CF) of axial symmetry is incorporated in the resonance model of Schotte and Schotte for magnetic multiplets of angular momentum j. The free energy for j = 52 and arbitrary level schemes is explicity evaluated and the competition between CF-splitting and Kondo effect in the susceptibility tensor is discussed.

A so-called Hubbard sum rule determines the weight of a satellite in fermionic single-particle excitations with strong local repulsion (U → ∞). Together with the Luttinger sum rule, this imposes two different energy scales on the remaining finite excitations. In the Hubbard chain, this has been identified microscopically as being due to a separation of spin and charge.

The free energy and static spin-correlation function of a two site resonance model akin to the single site Schotte model are calculated. Thermodynamic functions are discussed and model parameters are linked to those of the microscopic two site Kondo model.

The resistivity ϱ and the susceptibility χ of very small single crystals of CeAl3 have been measured. The anisotropy of χ reflects the magnetic property of the crystal field doublet ground state |Jz = ± 3/2〉. At low temperature, both the resistivity and the magnetoresistance provide evidence that some kind of magnetic order starts to develop at 1.6 K.