We report thermodynamic and neutron diffraction measurements on a $S=1/2$ metal-organic kagome compound ${\left\{\right[{\mathrm{Cu}}_3\left({\mathrm{CO}}_3\right){}_2{\left(\mathrm{bpe}\right)}_3]·{2\mathrm{ClO}}_4\}}_n$ [bpe=1,2-bis(4-pyridyl)ethane] with a Curie-Weiss temperature of $\sim -2.9\pm 0.9\mathrm{K}$. The dc and ac magnetization measurements as well as neutron diffraction study show no long-range magnetic order down to 1.5 K. The temperature dependence of specific heat also shows no signature of long-range magnetic order down to 86 mK, as expected for an ideal Heisenberg kagome lattice with a nearest-neighbor antiferromagnetic exchange interaction. The frequency-independent flattening behavior observed in the real part of the ac susceptibility in a zero applied magnetic field below $\sim 6$ K rules out the presence of a spin glass state and indicates a slowing down of the spin dynamics at low temperatures. Our results suggest that strong quantum fluctuations enhanced by geometrical frustration suppress the long-range magnetic order and a ground state with quasistatic short-range canted antiferromagnetic-type order is realized in the ${\left\{\right[{\mathrm{Cu}}_3\left({\mathrm{CO}}_3\right){}_2{\left(\mathrm{bpe}\right)}_3]·{2\mathrm{ClO}}_4\}}_n$ [bpe=1,2-bis(4-pyridyl)ethane]. The ground state is fragile against external magnetic fields. An applied magnetic field of 125 Oe (and higher) induces ferromagneticlike spin-spin correlations.

• Received 22 January 2019
• Revised 2 April 2020
• Accepted 13 April 2020

DOI:https://doi.org/10.1103/PhysRevB.101.140413