M. V. Sadovskii

High-temperature superconductivity in iron-based layered iron compounds

Basic experimental data are presented for a new class of high-temperature superconductors — iron-based layered compounds of the types REOFeAs (RE = La, Ce, Nd, Pr, Sm, ...), AFe2As2 (A = Ba, Sr, ...), AFeAs (A = Li, ...), and FeSe(Te). The structure of electronic spectra in these compounds is discussed, including the correlation effects, as is the spectrum and role of collective excitations (phonons and spin waves). Basic models for describing various types of magnetic ordering and Cooper pairing are reviewed.

Pseudogap in high-temperature superconductors

This review describes the main experimental facts and a number of theoretical models concerning the pseudogap state in high - temperature superconductors. On the phase diagram of HTSC - cuprates the pseudogap state is observed in the region of current carrier concentrations less than optimal and leads to a number of anomalies of electronic properties. These pseudogap anomalies apparently are due to fluctuations of antiferromagnetic short - range order developing as system moves towards antiferromagnetic region on the phase diagram. Electron interaction with these fluctuations leads to anisotropic renormalization of electronic spectrum and non - Fermi liquid behavior on certain parts of the Fermi surface. We also discuss some simple theoretical models describing basic properties of the pseudogap state, including the anomalies of superconducting state due to pseudogap renormalization of electronic spectrum.

Electronic structure of prototype AFe2As2 and ReOFeAs high-temperature superconductors: A comparison

We have performed ab initio LDA calculations of the electronic structure of newly discovered prototype high-temperature superconductors AFe2As2 (A = Ba, Sr) and compared it with the previously calculated electronic spectra of ReOFeAs (Re = La, Ce, Pr, Nd, Sm). In all cases, we obtain almost identical densities of states in a rather wide energy interval (up to 1 eV) around the Fermi level. Energy dispersions are also very similar and almost two dimensional in this energy interval, leading to the same basic (minimal) model of the electronic spectra, determined mainly by Fe d orbitals of the FeAs layers. The other constituents, such as A ions or rare-earth Re (or oxygen states) are more or less irrelevant for superconductivity. LDA Fermi surfaces for AFe2As2 are also very similar to that of ReOFeAs. This makes the more simple AFe2As2 a generic system to study the high-temperature superconductivity in FeAs-layered compounds.

Generalized dynamical mean-field theory in the physics of strongly correlated systems

This review discusses the generalization of dynamical mean-field theory (DMFT) for strongly correlated electronic systems to include additional interactions necessary for the correct description of physical effects in such systems. Specifically, the additional interactions include: (1) the interaction of electrons with antiferromagnetic (or charge) order-parameter fluctuations in high-temperature superconductors leading to the formation of a pseudogap state; (2) scattering on static disorder and its role in the general picture of the Anderson-Hubbard metal-insulator transition, and (3) electron-phonon interaction and the features of electronic spectra in strongly correlated systems. The proposed DMFT+ approach incorporates the above interactions by introducing into the general DMFT model an additional (generally momentum-dependent) self-energy which is calculated in a self-consistent way without violating the general structure of the DMFT iteration cycle. The paper formulates a general calculational scheme for both one-particle (spectral functions and densities of states) and two-particle (optical conductivity) properties. The problem of pseudogap formation is examined, including Fermi arc formation and partial destruction of the Fermi surface, as are the metal-insulator transition in the disordered Anderson-Hubbard model, and the general picture of kink formation in the electronic spectra of strongly correlated systems. A generalization of the DMFT+ approach to realistic materials with strong electron-electron correlations is presented based on the LDA+DMFT method. The general model of the LDA+DMFT method is reviewed, as are some of its applications to real systems. The generalized LDA+DMFT+ approach is employed to calculate pseudogap states in electron- and hole-doped HTSC cuprates. Comparisons with angle-resolved photoemission spectroscopy (ARPES) results are presented.

Electronic structure of new LiFeAs high-Tc superconductor

We present results of ab initio LDA calculations of electronic structure of “next generation” layered iron-pnictide high-Tc superconductor LiFeAs (Tc = 18 K). Obtained electronic structure of LiFeAs very similar to recently studied ReOFeAs (Re = La, Ce, Pr, Nd, Sm) and AFe2As2 (A = Ba, Sr) compounds. Namely close to the Fermi level its electronic properties are also determined mainly by Fe 3d-orbilats of FeAs4 two-dimensional layers. Band dispersions of LiFeAs are very similar to the LaOFeAs and BaFe2As2 systems as well as the shape of the Fe-3d density of states and Fermi surface.

High-temperature superconductivity in transition metal oxypnictides: A rare-earth puzzle?

Extensive ab initio LDA and LSDA + U calculations of an electronic structure of newly discovered high-temperature superconducting series ReO1 − xFxFeAs (Re = La, Ce, Pr, Nd, and Sm and the hypothetical case of Re = Y) have been performed. In all cases, almost identical electronic spectrum (both energy dispersions and the densities of states) has been obtained in a rather wide energy interval (about 2 eV) around the Fermi level. This fact is unlikely to be changed by strong correlations. This leads inevitably to the same critical temperature Tc of a superconducting transition in any theoretical BCS-like mechanism of the Cooper pair formation. The experimentally observed variations of the Tc for different rare-earth substitutions are either due to the disorder effects or less probably because of possible changes in the spin-fluctuation spectrum of FeAs layers caused by magnetic interactions with rare-earth spins in ReO layers.

Electronic structure, topological phase transitions and superconductivity in (K, Cs)xFe2Se2

We present LDA band structure of novel hole doped high temperature superconductors (Tc ∼ 30 K) KxFe2Se2 and CsxFe2Se2 and compare it with previously studied electronic structure of isostructural FeAs superconductor BaFe2As2 (Ba122). We show that stoichiometric KFe2Se2 and CsFe2Se2 have rather different Fermi surfaces as compared with Ba122. However at about 60% of hole doping Fermi surfaces of novel materials closely resemble those of Ba122. In between these dopings we observe a number of topological Fermi surface transitions near the Γ point in the Brillouin zone. Superconducting transition temperature Tc of new systems is apparently governed by the value of the total density of states (DOS) at the Fermi level.

Destruction of the fermi surface due to pseudogap fluctuations in strongly correlated systems

We generalize the dynamical-mean field theory (DMFT) by including into the DMFT equations dependence on the correlation length of the pseudogap fluctuations via the additional (momentum dependent) self-energy Σk. This self-energy describes nonlocal dynamical correlations induced by short-ranged collective SDW-like antiferromagnetic spin (or CDW-like charge) fluctuations. At high enough temperatures, these fluctuations can be viewed as a quenched Gaussian random field with finite correlation length. This generalized DMFT + Σk approach is used for the numerical solution of the weakly doped one-band Hubbard model with repulsive Coulomb interaction on a square lattice with nearest and next nearest neighbor hopping. The effective single impurity problem is solved by using a numerical renormalization group (NRG). Both types of strongly correlated metals, namely, (i) doped Mott insulator and (ii) the case of the bandwidth W ≲ U (U-value of local Coulomb interaction) are considered. By calculating profiles of the spectral densities for different parameters of the model, we demonstrate the qualitative picture of Fermi surface destruction and formation of Fermi arcs due to pseudogap fluctuations in qualitative agreement with the ARPES experiments. Blurring of the Fermi surface is enhanced with the growth of the Coulomb interaction.

Anion height dependence of T c and the density of states in iron-based superconductors

Systematic ab initio LDA calculations were performed for all the typical representatives of recently discovered class of iron-based high-temperature superconductors: REOFe(As,P) (RE = La, Ce, Nd, Sm, Tb), Ba2Fe2As, (Sr,Ca)FFeAs, Sr4Sc2O6Fe2P2, LiFeAs and Fe(Se,Te). Non-monotonic behavior of total density of states at the Fermi level is observed as a function of anion height relative to Fe layer with maximum at about Δza ∼ 1.37 Å, attributed to changing Fe-As (P, Se, Te) hybridization. This leads to a similar dependence of superconducting transition temperature Tc as observed in the experiments. The fit of this dependence to elementary BCS theory produces semiquantitative agreement with experimental data for Tc for the whole class of iron-based superconductors. The similar fit to Allen-Dynes formula underestimates Tc in the vicinity of the maximum, signifying the possible importance of non-phonon pairing in this region. These results unambiguously demonstrate that the main effect of Tc variation between different types of iron-based superconductors is due to the corresponding variation of the density of states at the Fermi level.

Iron based superconductors: Pnictides versus chalcogenides

Abstract We present a brief review of the present day situation with studies of high-temperature superconductivity in iron pnictides and chalcogenides. Recent discovery of superconductivity with T c > 30 K in AxFe2−x/2Se2 (A=K, Cs, Tl, etc) represents the major new step in the development of new concepts in the physics of Fe-based high-temperature superconductors. We compare LDA and ARPES data on the band structure and Fermi surfaces of novel superconductors and those of the previously studied FeAs superconductors, especially isostructural 122-superconductors like BaFe2As2. It appears that electronic structure of new superconductors is rather different from that of FeAs 122-systems. In particular, no nesting properties of electron and hole-like Fermi surfaces is observed, casting doubts on most popular theoretical schemes of Cooper pairing for these systems. Doping of novel materials is extremely important as a number of topological transitions of Fermi surface near the Γ point in the Brillouin zone are observed for different doping levels. The discovery of Fe vacancies ordering and antiferromagnetic (AFM) ordering at pretty high temperatures ( T N > 500 K ), much exceeding superconducting Tc makes these systems unique antiferromagnetic superconductors with highest TN observed up to now. This poses very difficult problems for theoretical understanding of superconductivity. We discuss the role of both vacancies and AFM ordering in transformations of band structure and Fermi surfaces, as well as their importance for superconductivity. In particular, we show that system remains metallic with unfolded Fermi surfaces quite similar to that in paramagnetic state. Superconducting transition temperature Tc of new superconductors is discussed within the general picture of superconductivity in multiple band systems. It is demonstrated that both in FeAs-superconductors and in new FeSe-systems the value of Tc correlates with the value of the total density of states (DOSs) at the Fermi level.