A review of the basic ideas and techniques of the spectral density-functional theory is presented. This method is currently used for electronic structure calculations of strongly correlated materials where the one-electron description breaks down. The method is illustrated with several examples where interactions play a dominant role: systems near metal-insulator transitions, systems near volume collapse transitions, and systems with local moments.

The iron pnictide and chalcogenide compounds are a subject of intensive investigations due to their high temperature superconductivity.[1] They all share the same structure, but there is significant variation in their physical properties, such as magnetic ordered moments, effective masses, superconducting gaps and T c . Many theoretical techniques have been applied to individual compounds but no consistent description of the trends is available [2]. We carry out a comparative theoretical study of a large number of iron-based compounds in both their magnetic and paramagnetic states. We show that the nature of both states is well described by our method and the trends in all the calculated physical properties such as the ordered moments, effective masses and Fermi surfaces are in good agreement with experiments across the compounds. The variation of these properties can be traced to variations in the key structural parameters, rather than changes in the screening of the Coulomb interactions. Our results provide a natural explanation of the strongly Fermi surface dependent superconducting gaps observed in experiments [3]. We propose a specific optimization of the crystal structure to look for higher T c superconductors.The iron pnictides are Hund's metals [4], where the interaction between the electrons is not strong enough to fully localize them, but it significantly slows them down, so that the low energy quasiparticles have much enhanced mass. These quasiparticles are composites of charge and a fluctuating magnetic moment originating in the Hund's rule interactions which tend to align electrons with the same spin and different orbital quantum numbers when they find themselves on the same iron atom.A central puzzle in this field is posed by the variation of the ordered magnetic moment across the iron pnictides/chalcogenides series. In the fully localized picture the atom resides in a single valence, therefore the ordered moment is equal to the atomic moment (4µ B per iron), possibly reduced by quantum fluctuations. This picture is realized in cuprate superconductors where quantum fluctuations reduce the Cu 2+ moment by 20%. In the fully itinerant weak coupling picture, such as spin density wave (SDW) in chromium metal, the ordered moment is related to the degree of Fermi surface nesting. It is by now clear that the iron pnictides are not well described by either fully localized or fully itinerant picture, nor by the density functional theory (DFT), which greatly overestimates the ordered magnetic moments. It has been advocated that the shortcomings of DFT can be circumvented by incorporating the physics of long wavelength fluctuations [5]. Here we take the opposite perspective. While critical long-wavelength fluctuations certainly play a role near the phase transition lines, we will show that the local fluctuations on the iron atom can account for the correct trend of magnetic moments and correlation strength in iron pnictides/chalcogenide layered compounds.Using the combination of density functional theory and d...

Studies of the electromagnetic response of various classes of correlated electron materials including transition-metal oxides, organic and molecular conductors, intermetallic compounds with d and f electrons, as well as magnetic semiconductors are reviewed. Optical inquiry into correlations in all these diverse systems is enabled by experimental access to the fundamental characteristics of an ensemble of electrons including their self-energy and kinetic energy. Steady-state spectroscopy carried out over a broad range of frequencies from microwaves to UV light and fast optics timeresolved techniques provides complimentary prospectives on correlations. Because the theoretical understanding of strong correlations is still evolving, the review is focused on the analysis of the universal trends that are emerging out of a large body of experimental data augmented where possible with insights from numerical studies.

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...

A new class of high temperature superconductors based on iron and arsenic was recently discovered [1], with superconducting transition temperature as high as 55 K [2]. Here we show, using microscopic theory, that the normal state of the iron pnictides at high temperatures is highly anomalous, displaying a very enhanced magnetic susceptibility and a linear temperature dependence of the resistivity. Below a coherence scale T * , the resistivity sharply drops and susceptibility crosses over to Pauli-like temperature dependence. Remarkably, the coherence-incoherence crossover temperature is a very strong function of the strength of the Hund's rule coupling J Hund . On the basis of the normal state properties, we estimate J Hund to be ∼ 0.35 eV. In the atomic limit, this value of J Hund leads to the critical ratio of the exchange constants J 1 /J 2 ∼ 2. While normal state incoherence is in common to all strongly correlated superconductors, the mechanism for emergence of the incoherent state in iron-oxypnictides, is unique due to its multiorbital electronic structure. arXiv:0805.0722v2 [cond-mat.supr-con]Coherence-incoherence crossover in the normal state of iron-oxypnictides and importance of the Hund's ruleThe unusually high superconducting critical temperatures in iron-oxypnictides together with unusual normal state properties, which do not fit within the standard framework of the Fermi liquid theory of solids, place the iron pnictides in the broad category of strongly correlated superconductors, such as κ organics, cerium and plutonium based heavy fermions, and cuprate high temperature superconductors. In all these materials, superconductivity emerges in close proximity to an incoherent state with unconventional spin dynamics, that cannot be describe in terms of weakly interacting quasiparticles. Describing the normal state of these materials, is one of the grand challenges in condensed matter theory and has resulted in numerous controversies in the context of the cuprates.Iron pnictides show very high resistivity and very large uniform susceptibility in the normal state [1]. Superconductivity emerges from a state of matter with highly enhanced Pauli like type of susceptibility [3]. As a function of F − doping, the susceptibility is peaked around 5% doping reaching a value 25 times bigger than Pauli susceptibility given by LDA [3,4]. In the parent compound, the resistivity exhibits a peak at 150 K [1] followed by a sharp drop, which is due to a structural transition from tetragonal (space group P4/nmm) to orthorhombic structure (space group Cmma) [7, 3] followed by a spin density wave transition [5,6] at lower temperature. Only a 5% doping completely suppress the specific heat anomaly [8], while resistivity still shows a very steep drop below a characteristic temperature T * [8,9]. While there are suggestions that the drop of resistivity is due to opening of a pseudogap and proximity to a quantum critical point in samarium compound [9], other measurements in lanthanum compound seem to suggest less exotic and more F...

The discovery of correlated electronic phases, including Mott-like insulators and superconductivity, in twisted bilayer graphene (TBLG) near the magic angle 1-4 , and the intriguing similarity of their phenomenology to that of the high-temperature superconductors, has spurred a surge of research to uncover the underlying physical mechanism 5-9 . Local spectroscopy, which is capable of accessing the symmetry and spatial distribution of the spectral function, can provide essential clues towards unraveling this puzzle. Here we use scanning tunneling microscopy (STM) and spectroscopy (STS) in magic angle TBLG to visualize the local density of states (DOS) and charge distribution. Doping the sample to partially fill the flat band, where low temperature transport measurements revealed the emergence of correlated electronic phases, we find a pseudogap phase accompanied by a global stripe charge-order whose similarity to high-temperature superconductors 10-16 provides new evidence of a deeper link underlying the phenomenology of these systems.

We generalized the recently introduced new impurity solver 1 based on the diagrammatic expansion around the atomic limit and Quantum Monte Carlo summation of the diagrams. We present generalization to the cluster of impurities, which is at the heart of the cluster Dynamical Mean-Field methods, and to realistic multiplet structure of a correlated atom, which will allow a high precision study of actinide and lanthanide based compounds with the combination of the Dynamical MeanField theory and band structure methods. The approach is applied to both, the two dimensional Hubbard and t-J model within Cellular Dynamical Mean Field method. The efficient implementation of the new algorithm, which we describe in detail, allows us to study coherence of the system at low temperature from the underdoped to overdoped regime. We show that the point of maximal superconducting transition temperature coincides with the point of maximum scattering rate although this optimal doped point appears at different electron densities in the two models. The power of the method is further demonstrated on the example of the Kondo volume collapse transition in Cerium. The valence histogram of the DMFT solution is presented showing the importance of the multiplet splitting of the atomic states.

We address the nature of the Mott transition in the Hubbard model at half-filling using cluster dynamical mean field theory (DMFT). We compare cluster-DMFT results with those of single-site DMFT. We show that inclusion of the short-range correlations on top of the on-site correlations does not change the order of the transition between the paramagnetic metal and the paramagnetic Mott insulator, which remains first order. However, the short range correlations reduce substantially the critical U and modify the shape of the transition lines. Moreover, they lead to very different physical properties of the metallic and insulating phases near the transition point. Approaching the transition from the metallic side, we find an anomalous metallic state with very low coherence scale. The insulating state is characterized by the narrow Mott gap with pronounced peaks at the gap edge.