SFB Extra Seminar

Kitaev-like bond-directional exchange interactions are novel and quite different from the ubiquitous Heisenberg interactions found in most magnets. These kinds of interactions open up new possibilities in the exploration and designing of new quantum magnets which can host novel quantum ground states like spin-liquids. During my graduate work I have synthesized single crystals of A₂TO₃ (A = Na, Li and T = Ir, Ru) and studied their electrical transport, magnetic, and thermal properties. Our work provides several new results: (i) the first thermodynamic evidence of possible fictionalization of electrons in Na₂IrO₃ because of proximity to the Kitaev spin-liquid state, (ii) evidence through magnetic impurity doping, of fragile magnetic order and importance of nearest neighbour interactions and spin-orbit coupling in deciding the magnetic ground state in Na₂IrO₃, (iii) a novel method (reactive ion etching) of surface doping Na₂IrO₃ and possibly other layered oxides has been discovered. The surface conductivity of Na₂IrO₃ crystals could be increased by 11 orders of magnitude by varying etching times. The samples which turned metallic show transport anomalies consistent with charge density wave or structural instabilities, and (iv) First crystal growth of Li₂RuO₃ is reported where the crystals crystallize in the P21/m structure and show the expected high temperature magneto-structural transition.

The van der Waals compound MnBi₄Te₇ belongs to the family of (Bi₂Te₃)n(MnBi₂Te₄), (n = 0, 1, 2) heterostructures and is a candidate magnetic topological insulator. It is the first magnetic material that features both, the intrinsic net magnetization and a band inversion. Static magnetic susceptibility (????) and magnetization (M) measurements as a function of the applied field (H) on MnBi₄Te₇ single-crystals show an antiferromagnetic state at T_???? = 13 K and a ferromagnetic-like hysteresis occurring upon cooling below 5 K. We performed electron spin resonance (ESR) spectroscopy measurements in wide frequency and temperature ranges to explore the dynamic magnetic properties of MnBi₄Te₇. From high-frequency ESR measurements, we obtain evidence that MnBi₄Te₇ is an easy-axis type ferromagnet and ferromagnetic spin correlations persist up to T = 30 K on the time scale of an ESR experiment (10^-10- 10^-11 s).