Electronic interactions and phase transitions at surfaces and in low dimensions

Low-dimensional and magnetic systems provide particularly clear-cut cases for the study of modified electron states that result from coupling with elementary excitations. Electronic quasiparticles of enhanced mass have been observed on the energy scale of phonons, and very recently on that of spin waves. Such mechanisms have bearing for superconducting pair formation, as spin fluctuations are being considered for high-temperature superconductivity. Phase transitions can occur if these interactions are accompanied by a nesting condition in the Fermi surface, leading to charge- or spin-density waves. The phase transition usually involves a structural distortion and is characterized by the opening of an energy gap. Special cases are the presence of multiple bands at the Fermi surface or the occurrence of thermal fluctuations. Recent progress in photoelectron spectroscopy and synchrotron-radiation instrumentation allow us to study these effects directly and resolved in k-space. Observations on surfaces, in magnetic materials, and in quasi-one-dimensional crystals are presented. PACS 73.20.At; 71.38.-k; 75.30.Ds; 71.30.+h; 79.60.-i     

Functional neural network analysis in frontotemporal dementia and Alzheimer's disease using EEG and graph theory

Background  

Although a large body of knowledge about both brain structure and function has been gathered over the last decades, we still have a poor understanding of their exact relationship. Graph theory provides a method to study the relation between network structure and function, and its application to neuroscientific data is an emerging research field. We investigated topological changes in large-scale functional brain networks in patients with Alzheimer's disease (AD) and frontotemporal lobar degeneration (FTLD) by means of graph theoretical analysis of resting-state EEG recordings. EEGs of 20 patients with mild to moderate AD, 15 FTLD patients, and 23 non-demented individuals were recorded in an eyes-closed resting-state. The synchronization likelihood (SL), a measure of functional connectivity, was calculated for each sensor pair in 0.5–4 Hz, 4–8 Hz, 8–10 Hz, 10–13 Hz, 13–30 Hz and 30–45 Hz frequency bands. The resulting connectivity matrices were converted to unweighted graphs, whose structure was characterized with several measures: mean clustering coefficient (local connectivity), characteristic path length (global connectivity) and degree correlation (network 'assortativity'). All results were normalized for network size and compared with random control networks.     

The electronic structure of KMnO4 investigated by photoemission and electron-energy-loss spectroscopy

From photoemission and electron-energy-loss data the following picture of KMnO₄, with Mn^VII (with a formal charge state Mn⁷⁺ (3d ⁰)) tetrahedrally surrounded by four O^2–-ions, is deduced: strong covalent bonding between Mn^VII and O^2– leads to a considerable occupation of the Mn-3 d shell. The ground state of the (MnO₄)^–1 molecule is an orbital and spin singlet as seen by the absence of any multiplet splitting in the Mn core levels. The valence band shows a four peak structure extending form 4 eV to 8 eV below the Fermi energy. The first peak at 4.2 eV has mainly O-2p character. The remaining peaks are of strongly mixed Mn-3d/O-2p character due to the covalent bonding. This mixing decreases with increasing binding energy. The electron energy loss data show a variety of structures between 2 eV and 10 eV independent of the primary electron energy which defines them as dipole allowed charge-transfer transitions. An additional excitation at 1.8 eV decreases quickly in intensity with increasing electron energy which classifies it as a dipole or spin forbidden transition in the compound. This energy is close to the value of 1.6 eV reported for the activation energy observed in electrical transport data. The results are compared to quantum chemical molecular orbital calculations of the (MnO₄)^–1 molecule.Physics Department, Allahabad University Allahabad 211002, India     

Spatial dependence of the dichroism of photoemission of Fe<Subscript>1/4</Subscript>TiTe<Subscript>2</Subscript> upon excitation with circularly polarized radiation

This paper reports on the results of the experimental investigation into the angular dependence of the photoemission of the valence band (the angle-resolved photoelectron spectrum) for the Fe_1/4TiTe₂ single crystal upon excitation with circularly polarized radiation. The angle-resolved photoelectron spectrum is studied in the vicinity of the Γ point of the Brillouin zone in the two directions ΓK and ΓM with the highest symmetry. The measurements are performed in noncoplanar geometry. The circular dichroism in the photoemission angular distribution is observed over the entire ranges of energies and angles. The magnitude of the dichroism depends on the choice of the band, the binding energy, and the polar angle and varies from 0 at the Γ point to 20%.