We studied the low-temperature specific heats of two antiferromagnets with the two-dimensional square-kagome structure, i.e., ${\mathrm{KCu}}_6{\mathrm{AlBiO}}_4{\left({\mathrm{SO}}_4\right)}_5\mathrm{Cl}$ $\left({\mathrm{KCu}}_6\right)$ and ${\mathrm{Na}}_6{\mathrm{Cu}}_7{\mathrm{BiO}}_4{\left({\mathrm{PO}}_4\right)}_4{[\mathrm{Cl},(\mathrm{OH}\left)\right]}_3$ $\left({\mathrm{NaCu}}_7\right)$, with the structural difference that there are interlayer ${\mathrm{Cu}}^{2+}$ ions in ${\mathrm{NaCu}}_7$. Both materials show no magnetic ordering down to 50 mK. At zero field, the $C/T$ of ${\mathrm{KCu}}_6$ has a finite value when the temperature is close to 0 K. Under the magnetic field, an apparent $T^2$ dependence appears and its coefficient is progressively suppressed by the field. For ${\mathrm{NaCu}}_7$, the specific heat exhibits the $T^2$ dependence at zero field and under fields. The ratio of the quadratic coefficients of ${\mathrm{KCu}}_6$ and ${\mathrm{NaCu}}_7$ at high fields is inversely proportional to that of the squared Weiss temperatures, which indicates these two compounds host the same ground state under fields. Our results suggest that the interlayer ${\mathrm{Cu}}^{2+}$ ions in ${\mathrm{NaCu}}_7$ play a negligible role in determination of its ground state. We discuss the possible quantum-spin-liquid states in these compounds and further directions to pursue based on our results.

“Ampoule in ampoule” design to grow single crystals of FeSe_1−xS_x.