A field-induced gap observed in the metal –organic framework [(C₂H₅)₃NH]₂Cu₂(C₂O₄)₃via specific heat measurements

The coordination of metal-linkers in [(C₂H₅)₃NH]₂Cu₂(C₂O₄)₃ forms a three-dimensional hyper-honeycomb lattice. No long-range magnetic order was experimentally observed at low temperatures down to 60 mK in this material [1], suggesting an exotic magnetic ground state. We here report specific heat of the single-crystal [(C₂H₅)₃NH]₂Cu₂(C₂O₄)₃ in magnetic field at low temperatures. The magnetic contribution to the specific heat (C_mag) shows linear-in-temperature behavior in the low temperature region, and C_mag is significantly suppressed with magnetic fields. This behavior can be attributed to weakly-coupled spin-1/2 antiferromagnetic Heisenberg chains proposed by Jacko et al. [2], due to a Jahn−Teller distortion. We can also ascribe the observed field-induced gap to effective staggered fields induced by the Dzyaloshinskii- Moriya interaction in the quasi-one-dimensional chains. Our results strongly suggest that the lack of magnetic order and anomalous specific heat in [(C₂H₅)₃NH]₂Cu₂(C₂O₄)₃ are due to the quasi-one dimensional S=1/2 antiferromagnetic chains.

[1] B.Zhang et al., J. Am. Chem. Soc., 140(1):122–125 (2018)

[2] Jacko et al., Phys. Chem. Chem. Phys., 23, 5012 (2021)