A quantum spin liquid (QSL) is a state of matter where fluctuating magnetic spins prevent long-range order, forming a highly entangled phase with fractionalized excitations such as spinons, offering potential for quantum communications. Natural minerals have proven to be a rich playground for exploring this state, while a majority of research has been devoted towards triangular or Kagome spin lattices, less attention has been given to lower-symmetry systems, such as triclinic structures. Contrary to the conventional approaches, we have found a signature of the QSL state in a triclinic natural mineral.
Henmilite (Ca₂Cu(OH)₄[B(OH)₄]₂) is the rare blue-violet natural mineral with triclinic crystallographic structure, where magnetic order is found to be suppressed at low magnetic fields, forming a dome-like shape of the Néel temperatures in an H-T phase diagram [1]. While we employed magnetization and specific heat measurements, it was the μSR technique that played the crucial role in revealing the novel magnetic properties of Henmilite.. In μSR, the longitudinal relaxation rate at H = 0 and 25 mT exhibits a plateau upon cooling to the base temperature of a dilution refrigerator, indicating persistent spin fluctuations which is a hallmark of the spin liquid phase. This QSL pocket extends from zero field up to at least 25 mT, beyond which the system transitions into a long-range ordered (LRO) state. Furthermore, we have found that the dome in the phase diagram is isotropic with respect to crystal orientation. Ongoing neutron diffraction studies aim to further elucidate the magnetic structure within the ordered phase.
Our findings not only demonstrate that reduced symmetry can favor the QSL state but also expand the arena for QSL research into less conventional, triclinic systems.

Reference: [1] H. Yamamoto et. al., PRM 5, 104405 (2021)