Random singlets in a quasi-one-dimensional alterating-bond spin-1/2 antiferromagnet

First synthesized in 2014, Sr₂₁Bi₈Cu₂(CO₃)₂O₄₁ is a spin-1/2 linear-chain antiferromagnet in which alternating-bond chains of Cu²⁺ spins form a triangular lattice. Magnetization data in magnetic fields up to 35 T and magnetic entropy in magnetic fields up to 1.5 T indicate that as much as about 60% of spins in our polycrystalline sample of this compound form random singlets. Scaling collapse of low-temperature specific heat at fields between 3.5 T and 14 T points to the probability distribution P(J) of the emergent exchange interaction J proportional to 1/J, whereas specific heat at zero field indicates that P(J) has a logarithmic J dependence, suggestive of critical behavior. Both J dependences are in contrast to 1/J^γ, with the exponent γ ranging from 0.4 to 0.8, typically observed for random singlets in other materials. Calculated in-field specific heat is in qualitative agreement with experiment but gives values that are only about 29% of experimental values. Moreover, the calculated specific heat exhibits an elbow-like anomaly, which is absent in experiment. These discrepancies suggest that emergent Dzyaloshinskii-Moriya interaction may not be the sole player in the scaling collapse of the in-field specific heat.