Hamiltonian Engineering in Molecular Lanthanide Spin Qubits

This presentation will highlight several recent examples involving optimization of coherence in molecular lanthanide (Ln) spin qubits through systematic engineering of so-called clock-transitions (CTs) [1-4] – avoided Zeeman level crossings at which the spin dynamics become desensitized to fluctuations (noise) in the local magnetic field. In this way, CTs provide optimal operating points at which the electron spin dynamics decouples from the magnetic environment leading to enhanced coherence [5]. The CT frequency can be controlled via crystal-field engineering in the case of integer spin-orbital moments, and the on-site hyperfine interaction in the half-integer case. In the first example, we describe a family of Ho^III complexes based on an octadentate cage-like ligand that wraps around the metal center, resulting in a <i>pseudo</i>-fourfold molecular geometry, while leaving open an axial coordination site that permits tuning of the CT frequency, Δ [1]. This approach leads to dramatic increases in Δ relative to prior examples [5], thus reducing 2^nd-order sensitivity to magnetic noise (which scales as 1/Δ). Meanwhile, in the half-integer spin case, we describe several molecular complexes in which the Ln ion adopts the rare 2+ oxidation state such that a lone electron occupies a mixed 5d/6s orbital. Again, ligand design principles enable realization of a doublet ground state with tunable 6s character, resulting in the possibility of engineering the hyperfine CT gap [2,3]. This latter approach has the advantage that the unpaired spin resides in an orbital with significantly reduced spin-orbit coupling relative to the 4f shell, giving rise to relatively long spin-lattice relaxation times.<br/><br/>[1] Stewart et al., J. Am. Chem. Soc. <b>146</b>, 11083 (2024); https://doi.org/10.1021/jacs.3c09353.<br/>[2] Smith et al., J. Am. Chem. Soc. <b>146</b>, 5781 (2024); https://doi.org/10.1021/jacs.3c12725.<br/>[3] Ngo et al. (submitted, 2024).<br/>[4] Kundu et al., Nat. Chem. <b>14</b>, 392 – 397 (2022); https://doi.org/10.1038/s41557-022-00894-4.<br/>[5] A. Gaita-Ariño et al., Nat. Chem. <b>11</b>, 301 (2019); https://doi.org/10.1038/s41557-019-0232-y.