, as possible quantum spin liquids (QSL’s)*

QSL’s are of great interest due to their unusual ground states; they do not exhibit spin freezing even at 0K, have no conventional order parameter associated with a broken symmetry, and can emerge in insulators containing localized spin degrees of freedom. In this work, single crystals of the low-spin geometrically frustrated 2D lattice pyrochlore compounds Ce₂Zr₂O₇ and Ce₂Hf₂O, prepared by the floating zone method, were investigated for a possible QSL state. Measurements of AC magnetic susceptibility from 100 Hz to 10 kHz and heat capacity up to 9T were performed down to 100 mK. Despite a well-ordered pyrochlore crystal structure and strong magnetic interactions between Ce³⁺ ions in Ce₂Zr₂O₇ and Ce₂Hf₂O₇, no evidence for a transition to a long-range ordered state was found, possibly due to strong quantum fluctuations that prevent spin freezing and lead to QSL ground state. With increasing magnetic field, the Schottky heat capacity anomaly moves to higher temperatures, indicating the presence of a gap in the excitation spectrum, which increases monotonically with increasing field. The behavior for a two-level system with an excited level at energy Δ above the ground state is compared with the heat capacity data.