We compute the electronic structure, momentum resolved spectral function and optical conductivity of the new superconductor LaO1−xFxFeAs within the combination of the Density functional theory and the Dynamical Mean Field Theory. We find that the compound in the normal state is a strongly correlated metal and the parent compound is a bad metal at the verge of the metal insulator transition. We argue that the superconductivity is not phonon mediated.PACS numbers: 71.27.+a,71.30.+h In the Bardeen-Cooper-Schrieffer theory of superconductivity, electrons form Cooper pairs through an interaction mediated by vibrations of the crystal. Like lattice vibrations, antiferromagnetic fluctuations can also produce an attractive interaction creating Cooper pairs, though with spin and angular momentum properties different from those of conventional superconductors. Such interactions was implicated for class of heavy fermion materials based on Ce, U and Pu with rather low transition temperatures, and cuprate superconductors with the highest known transition temperatures. Recently a surprising discovery of superconductivity in iron-based compound LaO 1−x F x FeP [1] with T c ∼ 7 K sparked a new direction to explore superconductivity in a completely new class of materials. Very recently a substitution of P by As raised T c to 26 K [2] becoming already one of the superconductors with highest T c among non-cuprate based materials. Exploring superconductivity in similar iron based compounds holds a lot of promise for increasing T c .In this letter we explore the electronic structure and optical properties of LaO 1−x F x FeAs within Density Functional Theory (DFT) and Dynamical Mean Field Theory (DMFT).LaOFeAs has a layered tetragonal crystal structure shown in Fig.1. Layers of La and O are sandwiched between layers of Fe and As. The Fe atoms form a square two dimensional lattice with Fe-Fe lattice spacing of 2.853Å. To understand the material properties, it is important to identify the character of dominant bands near the Fermi level, their energy and momentum distribution. For this purpose, the first principles density functional theory is the invaluable tool. We used the full-potential augmented plane-wave method as implemented in Wien code [3]. The lattice parameters and internal atomic positions have been determine experimentally (a = 4.035, c = 8.741, z La = 0.142, z As = 0.651). For the exchange correlation potential we used gradient approximation [4] (GGA) in the Perdew-Burke-Ernzerhof variant, and 12 × 12 × 5 k-points.This method predicts that the dominant states at the Fermi level come from Fe 3d atomic states extending roughly between -2 eV and 2 eV as shown in Fig. 2. The partial character of Fe, O and As is shown separately. Due to presence of As, a lot of electronic charge is found in the interstitial regions and can not be assigned an atomic character.The important feature of LaOFeAs compound is that DFT predicts a very steep and negative slope of the density of states (DOS) at the Fermi level. In the rigid band appr...

Thermoelectric energy harvesting-the transformation of waste heat into useful electricity-is of great interest for energy sustainability. The main obstacle is the low thermoelectric efficiency of materials for converting heat to electricity, quantified by the thermoelectric figure of merit, ZT. The best available n-type materials for use in mid-temperature (500-900 K) thermoelectric generators have a relatively low ZT of 1 or less, and so there is much interest in finding avenues for increasing this figure of merit. Here we report a binary crystalline n-type material, In(4)Se(3-delta), which achieves the ZT value of 1.48 at 705 K-very high for a bulk material. Using high-resolution transmission electron microscopy, electron diffraction, and first-principles calculations, we demonstrate that this material supports a charge density wave instability which is responsible for the large anisotropy observed in the electric and thermal transport. The high ZT value is the result of the high Seebeck coefficient and the low thermal conductivity in the plane of the charge density wave. Our results suggest a new direction in the search for high-performance thermoelectric materials, exploiting intrinsic nanostructural bulk properties induced by charge density waves.

Topological semimetals host electronic structures with several band-contact points or lines and are generally expected to exhibit strong topological responses. Up to now, most work has been limited to non-magnetic materials and the interplay between topology and magnetism in this class of quantum materials has been largely unexplored. Here we utilize theoretical calculations, magnetotransport and angle-resolved photoemission spectroscopy to propose FeGeTe, a van der Waals material, as a candidate ferromagnetic (FM) nodal line semimetal. We find that the spin degree of freedom is fully quenched by the large FM polarization, but the line degeneracy is protected by crystalline symmetries that connect two orbitals in adjacent layers. This orbital-driven nodal line is tunable by spin orientation due to spin-orbit coupling and produces a large Berry curvature, which leads to a large anomalous Hall current, angle and factor. These results demonstrate that FM topological semimetals hold significant potential for spin- and orbital-dependent electronic functionalities.

We address the fundamental question of crossover from the localized to the itinerant state of a paradigmatic heavy fermion material: CeIrIn5. The temperature evolution of the one-electron spectra and the optical conductivity are predicted from first-principles calculation. The buildup of coherence in the form of a dispersive many-body feature is followed in detail, and its effects on the conduction electrons of the material are revealed. We find multiple hybridization gaps and link them to the crystal structure of the material. Our theoretical approach explains the multiple peak structures observed in optical experiments and the sensitivity of CeIrIn5 to substitutions of the transition metal element and may provide a microscopic basis for the more phenomenological descriptions currently used to interpret experiments in heavy fermion systems.

In spintronics, two-dimensional van der Waals crystals constitute a most promising material class for long-distance spin transport or effective spin manipulation at room temperature. To realize all-vdW-material–based spintronic devices, however, vdW materials with itinerant ferromagnetism at room temperature are needed for spin current generation and thereby serve as an effective spin source. We report theoretical design and experimental realization of a iron-based vdW material, Fe4GeTe2, showing a nearly room temperature ferromagnetic order, together with a large magnetization and high conductivity. These properties are well retained even in cleaved crystals down to seven layers, with notable improvement in perpendicular magnetic anisotropy. Our findings highlight Fe4GeTe2 and its nanometer-thick crystals as a promising candidate for spin source operation at nearly room temperature and hold promise to further increase Tc in vdW ferromagnets by theory-guided material discovery.