THEORY OF ELECTRONIC CORRELATIONS AND COLLECTIVE PHENOMENA

Group of Jan Kuneš

LDA+DMFT method to X-ray spectroscopy

We have developed a package for Opens external link in new windowcalculation of (core-level) X-ray absorption and photoemission spectra within LDA+DMFT. The core of the method is a quantum impurity solver based on truncated configuration interaction method. Compared to the conventional cluster-model approach, The present method is (almost) parameter free and can access fine spectral features in recent high-energy resolution experiments. We have used this method to study the non-local screening effects in series of transition metal oxides.

Linear response

Evolution of competing instabilities (susceptibility modes) of two-band Hubbard model in 2D Brilouin zone with decreasing temperature (from the left to the right box)

We have developed a package for Opens external link in new windowlinear response (suscpetibilities) calculations within DMFT. This represents a non-trivial post-processing step, which requires calculation of two-particle (four-point) correlation functions and solution of a pair of Bethe-Salpeter equations. Our main goal is to calculate static suseptibilities and analyze their divergencies, in order to allow for an unbiased search for ordering instabilities. Our first non-trivial application of the approach was investigation of the two-band Hubbard model with the instability towards Opens internal link in current windowexcitonic condensation.

 

 

Dynamical mean-field theory

Our main approach to interacting electron problems is the Opens external link in new windowdynamical mean-field theory (DMFT). We use our own implementation of the self-consistency cycle, which allows treatment of multiple (inequivalent) impurities. Our main impurity solver is hybridization-expansion continuous-time Quantum Monte-Carlo (CT-HYB). We use several implementation of the algorithm: Opens external link in new windowALPS, Opens external link in new windoww2dynamics and our own. We also use exact diagonalization solver. We have developed or are developing extentions of these solvers to allow treatment of off-diagonal (and complex off-diagonal hybridzation).

 

 

Density functional methods

Spinor Wannier function with 5d j=1/2 character of Ir in Sr2IrO4 (blue and rainbow are the two z-axis spin projections)

Density functional theory allows to effectively reduce a many-body problem to the a one-particle problem. With this methodological simplification is it possible to study real materials, i.e. models that work in continuous space and use only positions of atomic nuclei as an input. In our work we use the Opens external link in new windowWien2k code based using LAPW basis set. In the past we have contributed to several packages of this code:

  • LAPWSO (spin-orbit coupling)
  • LAPWDM, ORB (LDA+U and other orbital potentials)
  • QTL (projected densities of states)
  • Opens external link in new windowWIEN2WANNIER, WPLOT, WOPTIC (interface to Opens external link in new windowwannier90 code)