B4 Single crystal growth of tunable quantum spin systems Franz Ritter, Cornelius Krellner (Goethe-Universität Frankfurt) Achievements 2D triangular lattices Project goals and work program 2D kagome lattices Cs2CuCl4 Pnma Growth at 24°C Pnma Growth at 24°C Cs2CuBr4 AF coupled Cu2+-spins Various 2D geometries Pronounced frustration ZnCu3(OH)6Cl2 Cs2CuCl3Br Growth at 50°C Cs2CuCl2Br2 Growth at 50°C A3 A5 A10 B1 B2 B3 Paramagnetic Spin Liquid 3D AFM polarized FM 0 2 4 6 8 10 12 Field (T) 5 4 3 2 1 Temperature (K) Dominant coupling J ~ 4.4 K along b (“chains”) Zigzag coupling J’ along c (“interchain”) Increasing frustration, J’/J ® 1 with x in Cs2Cu(Cl4-xBrx) 2D kagome planes stacked along c Dominant coupling: J1~ 180 K (nn kagome) J3/J1 ~ 0.02 (2nd nn kagome) Interlayer coupling: J2/J1 ~ 0.03 H. Jeschke et al., Phys. Rev. B 88, 075106 (2013) Model system: MCu3(OH)6X2 A3 A5 A8 A10 B1 B2 B3 B7E B13N Achievements Large Pnma-type single crystals for all x S. Streib et al., Phys. Rev. B 91, 041108(R) (2015) M.A. Fayzullin et al., Phys. Rev. B 88, 174421 (2013) Site-selective occupation of Br atoms on halogen sites for x = 1 and 2 Pronounced anisotropic thermal expansion Strong variation of the local Cu environment as function of T and x N. van Well et al., Phys. Rev. B 91, 035124 (2015) x = 0 x = 1 x = 2 x = 4 Setup of a hydrothermal reactor with T = 250°C and up to 200 bar Hydrothermal crystal growth of Herbertsmithite ZnCu3(OH)6Cl2 Synthesis of barlowite, Cu4(OH)6BrF T.-H. Han et al., PRL 113, 227203 (2014) with new synthetic approach: CuBr2 + CuO + NH4F  Cu4(OH)6BrF A8 Barlowite Cu4(OH)6BrF B1 Theoretical proposed correlated Dirac metal in an electron-doped kagome lattice: Nature Commun. 5, 4261 (2014) Ga-substituted herbertsmithite, GaxCu4-x(OH)6Cl2 B2 GaxCu4-x(OH)6Cl2 x = 0.6(2) 50 µm Pronounced T-dependence of the calculated magnetic exchange couplings, from the experimental structure data for Cs2CuCl4 Calculated low-T data match the established J-values from neutron scattering experiments N. van Well et al., Phys. Rev. B 91, 035124 (2015) Project goals and work program B2 Magnetic contribution to the specific-heat for Cs2Cu(Cl4-xBrx) Successful crystal growth of non- magnetic reference Cs2Zn(Cl4-xBrx) Single crystal growth of barlowite, Cu4(OH)6BrF Towards an ideal kagome lattice through non-magnetic substitution on the interlayer position in barlowite-typed systems:  MCu3(OH)6BrF with M = Zn, Sn, Mg, …  MCu3(OH)6ClF with M = Zn, Sn, Mg, … Towards an electron-doped kagome lattice through trivalent substitution on the interlayer position in herbertsmithite-typed systems:  MCu3(OH)6Cl2 with M = Ga, Sc, Y, Al, … Optimization of the crystal growth conditions for the most promising substitution series Structural and chemical characterization of the novel materials Determination of the magnetic ground state using susceptibility and specific heat measurements Cs2 ZnBr4 B2 Structural phase stability at low temperatures Continuing the single crystal growth of x = 0, 1, 2, 4 Determination of the magnetic ground state for x = 1 and 2 Determination of coupling constants for 0 < x < 4 B1 B1 B13N Transregional Collaborative Research Centre SFB/TR 49 Frankfurt / Kaiserslautern / Mainz