Electronic structure and FIRM group

The work of the Electronic structure group was focused on three general topics: spin-state transitions, strong spin-orbit coupling and crystal fields on rare earth ions. The spin-state transition was investigated in materials of the LaCoO3 family. In a series of DMFT calculations we were able to reproduce the thermally and doping induced spin-state transitions and explain their microscopic mechanism [1]. The example of the calculated density of states is shown in the Fig. 1. Simultaneously we have studied a much simplified model of spin-state transition materials – the two-band Hubbard model [2]. We have found that close to the transition the system, depending on its details, may become unstable towards long-range order (LRO). A particular type of LRO that can be described as a condensate of magnetic excitons has many peculiar and unexplored properties.

Fig. 1. On the left, the spectral density of states of the magneticperovskite La_0.7Sr_0.3CoO₃: Co-3d t_2g(red), Co-3d e_g (black), O p (blue). Right, the same spectral density by symmetry directions in reciprocal space. The upper and lower panels in both cases correspond to two spin projections.

Spin-orbit related phenomena have become very popular in the past ten years leading to a great interest in materials with 5d elements, iridium oxides in particular. We have participated in some of the first calculations that provided microscopic understanding of some of the physics of Na₂IrO₃ and Sr₂IrO₄ [3]. These studies have addressed two basic questions: How does spin-orbit coupling affect the topological properties of the band structures of the studied materials? To what extend are spin-orbit induced Mott insulators similar to cuprate high-temperature superconductors?

In most materials the 4f shells of rare-earth ions couple on weakly to their environment, which is thus essentially unaffected by their presence. This makes the rare-earth ions unique local probes as their optical or magnetic response carry information about the electronic structure of their surroundings and its changes with temperature or external fields. However, in order to extract this information one has to know to a great accuracy the coupling between the rare-earth ion and the rest of the crystal encoded in the so called crystal field. Calculation of the crystal fields from first principles has been a long standing problem. We have developed a method to calculate the crystal fields and demonstrated its accuracy and capabilities on series of diverse materials [4]. As the necessity to address new theoretical phenomena is mandatory conditioned by the development of novel computational methods, we must mention the development of three theoretical tools [5] that stand out in the past five years: (i) Wannier functions with Wien2k, (ii) Bethe-Salpeter formalism for linear response in DMFT and (iii) new measurement technique in quantum Monte-Carlo algorithm.

Far-InfraRed Magnetospectroscopy

The research of the Far-InfraRed Magnetospectroscopy (FIRM) laboratory (see Fig. 2), was concentrated on study of high frequency vortex dynamics in superconducting materials.

Fig. 2: The FIRM laboratory

The participation in the COST Action MP1201 enabled to study high quality NbN films. The terahertz thermal spectroscopy measurements [6] have been supplemented by a time-domain terahertz spectroscopy method in collaboration with the Terahertz Spectroscopy Group from our institute [7]. As an important result a full quantitative agreement between the experimental data, spanning broad ranges of temperature and frequency and the fundamental BCS-based microscopic theory, was reached without use of any fitting parameter. The experiments outlined, however, some contradictions in the state of art theory and thus motivated its improvements [8].

Literature

1. J. Kuneš, V. Křápek, N. Parragh, G. Sangiovanni, A. Toschi, and A. V. Kozhevnikov
   Spin State of Negative Charge-Transfer Material SrCoO₃
   Phys. Rev. Lett. 109 (2012) 117206(1) - 117206(5).
2. J. Kuneš, V. Křápek
   Disproportionation and Metallization at Low-Spin to High-Spin Transition in Multiorbital Mott Systems
   Phys. Rev. Lett. 106 (2011) 256401(1) - 256401(4).
3. R. Arita et al.,
   Ab initio Studies on the Interplay between Spin-Orbit Interaction and Coulomb Correlation in Sr₂IrO₄ and Ba₂IrO₄
   Phys. Rev. Lett. 108 (2012) 086403.
4. P. Novák, K. Knížek, J. Kuneš
   Crystal field parameters with Wannier functions: Application to rare-earth aluminates
   Phys. Rev. B 87 (2013) 205139(1) - 205139(7).
5. P. Augustinský and J. Kuneš,
   Improved Green’s function measurement for hybridization expansion quantum Monte Carlo
   Comput. Phys. Commun. 184 (2013) 2119;
   J. Kuneš,
   Efficient treatment of two-particle vertices in dynamical mean-field theory
   Phys. Rev. B 83 (2011) 085102;
   J. Kuneš et al.,
   Wien2wannier: From linearized augmented plane waves to maximally localized Wannier functions
   Comput. Phys. Commun. 181 (2010) 1888.
6. R. Tesař, M. Šindler, K. Il’in, J. Koláček, M. Siegel and L. Skrbek
   Terahertz thermal spectroscopy of a NbN superconductor
   Phys. Rev. B 84 (2011) 132506(1) - 132506(4).
7. M. Šindler, R. Tesař, J. Koláček, P. Szabó, P. Samuely, V. Hašková, C. Kadlec, F. Kadlec, P. Kužel
   Far-infrared electrodynamics of thin superconducting NbN film in magnetic fields
   Supercond. Sci. Tech. 27 (2014) 05500(1) - 05500(8).
8. P. Lipavský, J. Bok, J. Koláček
   Time-dependent Ginzburg-Landau equations for rotating and accelerating superconductors
   Physica C 492 (2013) 144 - 151.