[(C₂H₅)₃NH]₂Cu₂(C₂O₄)_3, is a three-dimensional metal-oxalate framework that forms a hyper-honeycomb lattice structure. In this lattice, Cu atoms form two distinct zigzag chains perpendicular to each other and offer alternating handedness in both left and right. As a result, this lattice contains two crystallographically distinct sub-lattices of Cu atoms [1, 2]. Jacko et al. suggest that one sub-lattice is strongly dimerized while the other forms isolated isotropic antiferromagnetic (AF) Heisenberg chains with Tomonaga-Luttinger spin liquid ground state [2]. Here we report thermodynamic properties of [(C₂H₅)₃NH]₂Cu₂(C₂O₄)₃ to characterize the spin liquid ground state. We find a sizable fermionic contribution in the specific heat at low temperatures, suggestive of the presence of a gapless ground state. We will also discuss the magnetic field dependence of the heat capacity.

[1] B.Zhang et al., J. Am. Chem. Soc., 140(1):122–125 (2018)
[2] Jacko et al., arXiv:1805.05495 (2018)