Open Access

Ina-Marie Pietsch

• The development of the Honeycomb Kitaev Model by A. Kitaev in 2006 [1] has excited the solid state physics community. A. Kitaev showed that an ideal honeycomb lattice with spin-1/2 moments sitting at each corner can show the highly desired quantum spin liquid state as the ground state as well as anyonic excitations. To produce such a state the three neighbors of each magnetic atom need to possess an Ising-like interaction with this atom with their Ising axes being perpendicular to one another. Such a exchange interaction is called the Kitaev interaction. To find a material that shows these characteristics is a long sought goal. Na2IrO3, α-Li2IrO3, α-RuCl3 are three candidates that have been singled out to inhabit the Kitaev interaction - along with other interactions as a Heisenberg interaction or the Dzyaloshinsky-Moriya interaction[2-4]. To widen our understanding about these materials was the purpose of this thesis, with the focus on Na2IrO3. In particular, four questions have beenThe development of the Honeycomb Kitaev Model by A. Kitaev in 2006 [1] has excited the solid state physics community. A. Kitaev showed that an ideal honeycomb lattice with spin-1/2 moments sitting at each corner can show the highly desired quantum spin liquid state as the ground state as well as anyonic excitations. To produce such a state the three neighbors of each magnetic atom need to possess an Ising-like interaction with this atom with their Ising axes being perpendicular to one another. Such a exchange interaction is called the Kitaev interaction. To find a material that shows these characteristics is a long sought goal. Na2IrO3, α-Li2IrO3, α-RuCl3 are three candidates that have been singled out to inhabit the Kitaev interaction - along with other interactions as a Heisenberg interaction or the Dzyaloshinsky-Moriya interaction[2-4]. To widen our understanding about these materials was the purpose of this thesis, with the focus on Na2IrO3. In particular, four questions have been the basis of this work. 1. Can we optimize the crystal growth and enhance the size of Na2IrO3 single crystals and, therefore, widen the range of measurement techniques that they can be used for? 2. Can we measure the transverse magnetization of α-RuCl3 and Na2IrO3? 3. Is there an in-plane anisotropy of Na2IrO3? 4. How does the in-plane vs. out-of-plane anisotropy change with Li-content in (Na1-xLix)2IrO3? In the end, these questions have led to many new results. The crystal growth was optimized and the mass of Na2IrO3 single crystals enhanced by one order of magnitude. These crystals degraded in air much slower than found in earlier studies[5,6]. Further, the presence of three domain types in Na2IrO3 single crystals with a 120° angle between them was identified, as it has been found for α-RuCl3 before[7]. While the signal of the transverse magnetization of Na2IrO3 was too small, the results for α-RuCl3 showed unexpected behavior for the field-dependent measurements, which might be related to the domain redistribution previously reported[7]. [1] A. Kitaev, Ann. Phys. (N. Y.), 321, (2006), 2–111. [2] S. Hwan Chun et al., Nature Phys., 11 (2015). [3] S. C. Williams et al., Phys. Rev. B, 93 (2016), 195158. [4] K. W. Plumb et al., Phys. Rev. B, 90 (2014), 041112. [5] J.W Krizan et al., Mater. Res. Bull., 52 (2014) , 162 – 166. [6] M. J. O’Malley et al., J. Solid State Chem., 181(8) (2008) , 1803–1809. [7] J. A. Sears et al., Phys. Rev. B, 95 (2017), 180411.…