Molecule-based quantum materials under extreme conditions

Molecule-based quantum materials are a class of compounds with competition between the spin, orbitals, charge, and lattice. They feature flexible architectures and structural designs that can be easily modified for different functionalities. As a result of their overall low energy scales, they can be easily tuned with external stimuli like magnetic field or pressure to reveal new states and properties. This dissertation presents a high magnetic field investigation of three different molecule-based quantum materials under extreme conditions revealing insights into their structural, electronic, and magnetic properties.

My initial study analyzes decoherence pathways in spin qubit Na₉[Ho(W₅O₁₈)₂]·35H₂O using magneto-infrared spectroscopy. In this single-molecule magnet, there are two frequency regions where the crystal field excitations from the central Ho³⁺ ion interact with nearby vibrational modes. This vibronic coupling gives rise to decoherence. We find that this material also has a transparency window in its phonon density of states that helps to separate majority of the vibrational modes from the crystal field excitations limiting the extent of vibronic coupling. Designing materials with limited vibronic coupling increases their coherence time and functionality as a spin qubit.

Subsequently, we explored the development of magnetoelectric coupling in chiral chain magnet [Cu(pym)(H₂O)₄]SiF₆·H₂O. We measured the electric polarization as a function of magnetic field and created a series of magnetic field-temperature phase diagrams. We find a series of phase transitions consistent with symmetry-breaking in combination with long-range magnetic ordering, magnetoelectric coupling, and striction effects. These findings deviate from the anticipated behavior of one-dimensional spin chains, revealing increased complexity in applied fields.

The last project focused on exploring quantum phase transitions in the quasi-one-dimensional magnet [CuL₂(H₂O)₂(pyz)](ClO₄)₂ [L = 5-methyl-2-pyridone]. We combined optical spectroscopy, magnetic circular dichroism spectroscopy, high field magnetization, and complementary first-principle calculations to reveal the orbital characteristics as they relate to the magnetic properties across the transition to the fully saturated spin state at 20 T. Magnetic circular dichroism revealed the individual contribution of different electronic excitations to bulk magnetism and showed that all excitations play a role in magnetic properties with the Cu²⁺ to pyrazine charge transfer excitation being the most important.