The interplay between the quantum effects from low-dimensionality and the spin-orbit coupling leads to exotic ground states with unusual excitations. We report the structural, magnetic, heat capacity, and electronic structure studies of ${\mathrm{Bi}}_2{\mathrm{YbO}}_4\mathrm{Cl}$, which constitutes a structurally perfect 2D square lattice with rare-earth magnetic ${\mathrm{Yb}}^{3+}$ ions. The magnetization and heat capacity data analysis confirms that the ${\mathrm{Yb}}^{3+}$ ion hosts the spin-orbit driven ${\mathit{J}}_{\text{eff}}=\frac{1}{2}$ state at low temperatures. From the fit to the Curie-Weiss law on the magnetic susceptibility data in the low-temperature region, the observed Curie-Weiss temperature is about −1 K, implying an antiferromagnetic (AFM) coupling between the ${\mathrm{Yb}}^{3+}$ moments. The heat capacity data show the presence of a broad maximum at 0.3 K and the absence of any sharp magnetic anomaly down to 0.09 K, indicating the onset of short-range correlations. Our first-principles calculations based on density functional theory provide further insight into the role of the microscopic parameters. In particular, it points out the crucial role of spin-orbit coupling in driving both the ${\mathit{J}}_{\text{eff}}=\frac{1}{2}$ state as well as the antiferromagnetic interaction between the nearest-neighbor ${\mathrm{Yb}}^{3+}$ moments that is consistent with experimental results. The total energy calculations suggest an easy-axis (out-of-plane) anisotropy of the spins.

• Received 29 October 2023
• Revised 12 January 2024
• Accepted 23 January 2024

DOI:https://doi.org/10.1103/PhysRevB.109.075128