Electron correlations in solids from the Dynamical Mean Field perspective. Origin of anomalous state in iron pnictides and chalchogenides

Electron correlations in solids from the Dynamical Mean Field perspective. Origin of anomalous state in iron pnictides and chalchogenides
Kristjan Haule, Rutgers
Date and time: Thu, Nov 06, 2014 - 11:21am
Refreshments at 11:06am
Location: LGRT 1033
Category: Condensed Matter Seminar
Abstract:

Electron correlations in solids from the Dynamical Mean Field perspective and the origin anomalous state of matter in iron pnictides and chalchogenides Materials with strong electronic correlations have long resisted abinitio modeling due to their complexity arising from non-perturbative strength of the interaction. The Dynamical Mean Field Theory in combination with the Density Functional Theory has recently changed this position, and enabled detailed modeling of the electronic structure of complex heavy fermions, transition metal oxides, chalchogenides and arsenides.

A new class of high temperature superconductors based on iron was recently discovered, and their complex multi-band nature makes the interplay of superconductivity with spin and orbital dynamics very intriguing, leading to very material dependent magnetic excitations, and pairing symmetries. In iron superconductors, the Coulomb interaction among the electrons is not strong enough to localize electrons, but it significantly slows them down, such that low-energy emerging quasiparticles have a substantially enhanced mass, and at intermediate temperature and intermediate energy scale show strong deviations from the Fermi liquid theory. This enhanced mass emerges not because of the Hubbard interaction U, but because of the Hund's rule interactions J that tends to align electrons with the same spin but different orbital quantum numbers when they find themselves on the same atom.

The ab-initio simulations with the Dynamical Mean Field Theory not only uncover the origin of anomalous properties, but also successfully explains the key properties of these material: such as the mass renormalizations and anisotropy of quasiparticles, the crossover into an incoherent regime above a low temperature scale, and the magnetic excitations in energy and momentum space. The ab-initio simulations of the two particle verte function allows us to study the spin dynamics and superconducting pairing symmetry in a large number of iron-based superconductors. We predicted a novel orbital antiphase s+− symmetry of superconducting pairing, which was not found by either weak coupling nor strong coupling approaches, as it is driven by Hund's coupling in metallic systems. This orbital-antiphase pairing symmetry explains the puzzling variation of the experimentally observed superconducting gaps on all the Fermi surfaces of LiFeAs, and may be realized in other iron superconductors.