Magnetic ground states in kagome YCu$_3$(OH)$_6$[(Cl$_x$Br$_{1-x}$)$_{3-y}$(OH)$_{y}$]

Abstract: Quantum spin liquids represent exotic states of spin systems characterized by long-range entanglement and emergent fractionalized quasiparticles. It is generally believed that disorder is hostile to quantum spin liquids. In our study, we investigated the magnetic properties of a kagome system, YCu$3$(OH)$_6$[(Cl$_x$Br${1-x}$)${3-y}$(OH)${y}$]. Within this system, some of the hexagons exhibit alternate bonds along the Cu-O-Cu exchange paths, while others remain uniform. We found that a long-range antiferromagnetic order emerges when uniform hexagons dominate. Conversely, a possible quantum-spin-liquid state arises when the number of alternate-bond hexagons exceeds about 2/3. Therefore, the alternate-bond hexagons, typically considered as disorders, actually serve as the building blocks of the quantum spin liquid in this system. Notably, the low-temperature properties of the quantum spin liquid are directly associated with the height of the out-of-plane yttrium ions, which may be linked to changes in superexchange energies. Our results suggest that understanding the magnetic ground states in this system lies beyond the theoretical framework of the Heisenberg model constructed on the kagome lattice.