Regensburg 2019 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 53: Frustrated Magnets - Strong Spin-Orbit Coupling (joint session TT/MA)

TT 53.8: Talk

Thursday, April 4, 2019, 11:30–11:45, Theater

Bilayer Kitaev models: Phase diagrams and novel phases — •Urban F. P. Seifert¹, Julian Gritsch², Erik Wagner³, Darshan G. Joshi⁴, Wolfram Brenig^3,1, Matthias Vojta¹, and Kai P. Schmidt² — ¹Institut für Theoretische Physik, Technische Universität Dresden, Germany — ²Institut für Theoretische Physik I, Universität Erlangen-Nürnberg, Germany — ³Institut für Theoretische Physik, Technische Universität Braunschweig, Germany — ⁴Max-Planck-Institut für Festkörperforschung, Stuttgart, Germany

We study the fate of Z₂ spin liquid phases in differently stacked bilayer versions of Kitaev’s honeycomb model. Increasing the inter-layer Heisenberg coupling J_⊥ (at fixed Kitaev couplings K^x,y,z) eventually destroys the topological spin liquid in favor of a paramagnetic dimer phase. We establish phase diagrams as a function of J_⊥/K and Kitaev coupling anisotropies using Majorana-fermion mean-field theory, and employ different expansion techniques in the limits of small and large J_⊥/K. For strong anisotropies, we use effective models for the different layer stackings to discuss the quantum phase transition out of the Kitaev phase. The phase diagrams depend sensitively on the nature of the stacking and anisotropy strength. In some stackings and at strong anisotropies we find a single transition between the Kitaev and dimer phases. Importantly, for other stackings we prove the existence of two novel macro-spin phases which can be understood in terms of Ising chains which can be either coupled ferromagnetically, or remain degenerate, thus realizing a classical spin liquid. We also suggest the existence of a flux phase with spontaneous inter-layer coherence.