EPR Study of Molecular Qubits Based on Lanthanide Nanomagnets

Molecular magnets containing one or more open shell elements are one of the proposed systems for quantum information processing that can be chemically engineered to have well-separated and stable quantum states. This dissertation explores mono- and dinuclear lanthanide nanomagnets for quantum information processing and spintronics by means of continuous-wave and pulsed electron paramagnetic resonance (EPR) spectroscopy. EPR is a powerful and sensitive technique that allows us to probe the fine structure of the ground spin state in molecular magnets. In Chapter 2, we demonstrate that atomic clock transitions (ACTs) can be employed as a means of enhancing the coherence of molecular spin qubits without resorting to extreme dilution, which can be impractical at the stage of device design for multi-qubit gate operations. This approach is illustrated with a holmium molecular nanomagnet in which long coherence times (up to 8.4 microseconds at 5 kelvin) are obtained at unusually high concentrations. ACTs are realized in the vicinity of avoided level crossings within the ground doublet of the holmium compound, at which several sources of decoherence are mitigated. In Chapter 3, we continue the pulsed EPR study presented in Chapter 2 to introduce electro-nuclear atomic clock transitions in a hybrid system with both large electronic and nuclear moment. We demonstrate an enhancement in the coherence time of hybrid transitions that involve coupled dynamics of electron and nuclear spins. This is significant for applications in hybrid magnetic qubits, where manipulation of the nuclear spin is controlled by EPR pulses. In Chapter 4, we report single-crystal and powder high-field EPR (HF-EPR) measurements on a neutral [TbPc2]0 complex for which the organic bis-phthalocyaninato (Pc2) ligand is open shell, i.e., it carries an unpaired electron. A highly anisotropic EPR signal can be attributed to the radical, suggesting an appreciable interaction with the Ising-like Tb(III) ion. Analysis of the results unambiguously demonstrate that the radical-Tb(III) coupling is due to a ferromagnetic exchange interaction. The essential physics is captured via an effective spin Hamiltonian in which the exchange is assumed to be isotropic, while the magnetic anisotropy is folded entirely into the single-ion properties of the terbium ion. In Chapter 4, we investigate lanthanide-radical interactions across the lanthanide (Ln) series in double-decker compounds (LnPc2). We further discuss the effect of free ion anisotropy on the magnetic properties of the lanthanide ion and the exchange coupled radical. Finally, in Chapter 6, we present HF-EPR studies of a series of symmetric and asymmetric triple-decker compounds that are potential candidates for two-qubit gate operations. Triple-decker compounds contain two lanthanide ions in each molecule that are linked by a phthalocyanine (Pc) ring sandwiched by Pc or porphyrin on the top and bottom. We further show that the two inequivalent sites required for two-qubit gates can be chemically engineered by adjusting the coordination symmetry for one of the ions in these dinuclear compounds.