$\mathbf{q}=\mathbf{0}$ long-range magnetic order in centennialite CaCu$_3$(OD)$_6$Cl$_2$$\cdot$0.6D$_2$O: A spin-1/2 perfect kagome antiferromagnet with $J_1$-$J_2$-$J_d$

Abstract

Crystal and magnetic structures of the mineral centennialite CaCu3(OH)6Cl2 • 0.6H2O are investigated by means of synchrotron x-ray diffraction and neutron diffraction measurements complemented by density functional theory (DFT) and pseudofermion functional renormalization group (PFFRG) calculations. CaCu3(OH)6Cl2 • 0.6H2O crystallizes in the P 3m1 space group and Cu 2+ ions form a geometrically perfect kagome network with antiferromagnetic J1. No intersite disorder between Cu 2+ and Ca 2+ ions is detected. CaCu3(OH)6Cl2 • 0.6H2O enters a magnetic long-range ordered state below TN = 7.2 K, and the q = 0 magnetic structure with negative vector spin chirality is obtained. The ordered moment at 0.3 K is suppressed to 0.58(2)µB. Our DFT calculations indicate the presence of antiferromagnetic J2 and ferromagnetic J d superexchange couplings of a strength which places the system at the crossroads of three magnetic orders (at the classical level) and a spin-1 2 PFFRG analysis shows a dominance of q = 0 type magnetic correlations, consistent with and indicating proximity to the observed q = 0 spin structure. The results suggest that this material is located close to a quantum critical point and is a good realization of a J1-J2-J d kagome antiferromagnet.