Sc2Ga2CuO7: A possible quantum spin liquid near the percolation threshold

Sc2Ga2CuO7 (SGCO) crystallizes in a hexagonal structure (space group: P63/mmc), which can be seen as an alternating stacking of single and double triangular layers. Combining neutron, x-ray, and resonant x-ray diffraction, we establish that the single triangular layers are mainly populated by nonmagnetic Ga3+ ions (85% Ga and 15% Cu), while the bilayers have comparable population of Cu2+ and Ga3+ ions (43% Cu and 57% Ga). Our susceptibility measurements in the temperature range 1.8–400 K give no indication of any spin-freezing or magnetic long-range order (LRO). We infer an effective paramagnetic moment μeff=1.79±0.09μB and a Curie-Weiss temperature θCW of about −44 K, suggesting antiferromagnetic interactions between the Cu2+(S=1/2) ions. Low-temperature neutron powder diffraction data showed no evidence for LRO down to 1.5 K. In our specific heat data as well, no anomalies were found down to 0.35 K, in the field range 0–140 kOe. The magnetic specific heat Cm, exhibits a broad maximum at around 2.5 K followed by a nearly power law Cm∝Tα behavior at lower temperatures, with α increasing from 0.3 to 1.9 as a function of field for fields up to 90 kOe and then remaining at 1.9 for fields up to 140 kOe. Our results point to a disordered ground state in SGCO.