Jiří Chaloupka

Intense paramagnon excitations in a large family of high-temperature superconductors

In the copper oxide superconductors, spin fluctuations might be involved in the electronic pairing mechanism. The case for such magnetically mediated superconductivity is now strengthened by the discovery of high-energy magnetic excitations that are not affected by chemical doping levels within several cuprates.

Zigzag magnetic order in the iridium oxide Na2IrO3.

We explore the phase diagram of spin-orbit Mott insulators on a honeycomb lattice, within the Kitaev-Heisenberg model extended to its full parameter space. Zigzag-type magnetic order is found to occupy a large part of the phase diagram of the model, and its physical origin is explained as due to interorbital t2g − eg hopping. Magnetic susceptibility and spin wave spectra are calculated and compared to the experimental data, obtaining thereby the spin coupling constants in Na2IrO3 and Li2IrO3.

Evidence for two separate energy gaps in underdoped high-temperature cuprate superconductors from broadband infrared ellipsometry

Li Yu, D. Munzar, A.V. Boris, P. Yordanov, J. Chaloupka, Th. Wolf, C.T. Lin, B. Keimer, and C. Bernhard 1.) University of Fribourg, Department of Physics and Fribourg, Center for Nanomaterials (FriMat), Chemin du Musee 3, CH-1700 Fribourg, Switzerland 2.) Max-Planck-Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany 3.) Institute of Condensed Matter Physics, Masaryk University, Kotlárská 2, CZ-61137 Brno, Czech Republic 4.) Forschungszentrum Karlsruhe, IFP, D-76021 Karlsruhe, Germany (Dated: February 5, 2008)

Hidden symmetries of the extended Kitaev-Heisenberg model: Implications for the honeycomb-lattice iridates A2IrO3

We have explored the hidden symmetries of a generic four-parameter nearest-neighbor spin model, allowed in honeycomb-lattice compounds under trigonal compression. Our method utilizes a systematic algorithm to identify all dual transformations of the model that map the Hamiltonian on itself, changing the parameters and providing exact links between different points in its parameter space. We have found the complete set of points of hidden SU(2) symmetry at which a seemingly highly anisotropic model can be mapped back on the Heisenberg model and inherits therefore its properties such as the presence of gapless Goldstone modes. The procedure used to search for the hidden symmetries is quite general and may be extended to other bond-anisotropic spin models and other lattices, such as the triangular, kagome, hyperhoneycomb, or harmonic-honeycomb lattices. We apply our findings to the honeycomb-lattice iridates Na2IrO3 and Li2IrO3, and illustrate how they help to identify plausible values of the model parameters that are compatible with the available experimental data.

Magnetic anisotropy in the Kitaev model systems Na2IrO3 and RuCl3

We study the ordered moment direction in the extended Kitaev-Heisenberg model relevant to honeycomb lattice magnets with strong spin-orbit coupling. We utilize numerical diagonalization and analyze the exact cluster ground states using a particular set of spin-coherent states, obtaining thereby quantum corrections to the magnetic anisotropy beyond conventional perturbative methods. It is found that the quantum fluctuations strongly modify the moment direction obtained at a classical level and are thus crucial for a precise quantification of the interactions. The results show that the moment direction is a sensitive probe of the model parameters in real materials. Focusing on the experimentally relevant zigzag phases of the model, we analyze the currently available neutron-diffraction and resonant x-ray-diffraction data on

Doping-Induced Ferromagnetism and Possible Triplet Pairing in d(4) Mott Insulators

{\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}

Raman Scattering from Higgs Mode Oscillations in the Two-Dimensional Antiferromagnet Ca2RuO4

and

Phase diagram and spin correlations of the Kitaev-Heisenberg model: Importance of quantum effects

{\mathrm{RuCl}}_{3}

Spin-orbital resonating valence bond liquid on a triangular lattice: Evidence from finite-cluster diagonalization

and discuss the parameter regimes plausible in these Kitaev-Heisenberg model systems.

Spin-State Crossover Model for the Magnetism of Iron Pnictides

We study the effects of electron doping in Mott insulators containing d(4) ions such as Ru(4+), Os(4+), Rh(5+), and Ir(5+) with J=0 singlet ground state. Depending on the strength of the spin-orbit coupling, the undoped systems are either nonmagnetic or host an unusual, excitonic magnetism arising from a condensation of the excited J=1 triplet states of t(2g)(4). We find that the interaction between J excitons and doped carriers strongly supports ferromagnetism, converting both the nonmagnetic and antiferromagnetic phases of the parent insulator into a ferromagnetic metal, and further to a nonmagnetic metal. Close to the ferromagnetic phase, the low-energy spin response is dominated by intense paramagnon excitations that may act as mediators of a triplet pairing.