UCLA

The design of molecular machines will depend on incorporating functionalized degrees of freedom into the final product. Toward that end, we examine the physical properties of a crystalline system containing a nearly free rotator synthesized to include an electric dipole moment. Crystals of reorienting 2,2-difluoro-1,4-bicyclo[2.2.2]octane-dicarboxylate (F2-BODCA) acting as linker and rotator in a metal organic framework (MOF) with Zn(II)-nodes and 1,4-diaza-bicyclo[2.2.2]octane (dabco) spacers, revealed emergent order as a result of dipole-dipole interactions. Variable temperature, frequency-dependent dielectric measurements consisting of a relatively sharp maximum in capacitance at Tc = 100 K was observed when a rapidly rotating, dipole-disordered, paraelectric phase, transformed into an ordered, antiferroelectric phase. A frequency-dependent Debye-like dynamic crossover was detected when the rotor dynamics become slower than the frequency of the alternating electric field. The dynamic nature of the F2-BODCA rotators was confirmed by NMR spectroscopy, and the energetics of the rotational profile elucidated with the help of Density Functional Theory (DFT) calculations. Finally, Monte Carlo simulations on a 2D rotary lattice revealed a ground state with an Ising symmetry and the effects of dipole-lattice and dipole-dipole interactions.

In a second project, magnetic resonance methods were used to study the superconductivity of Sr2RuO4. Over the past 20 years, the superconducting state of Sr2RuO4, a material that is quasi-two-dimensional perovskite and strongly correlated, was considered to be the only solid-state analogue to the superfluid 3He-A phase, with an odd-parity order parameter that is unidirectional in spin space for all electron momenta and breaks time-reversal symmetry. A consequence is the expectation of a 'split' transition in a Sr2RuO4 crystal with the presence of in-plane uniaxial strain. Such behavior has not yet been observed in thermodynamic measurements. Instead, on increasing the uniaxial compressive stress, a peak in transition temperature was observed. In this work, we utilised oxygen-17 solid state nuclear magnetic resonance spectroscopy to probe the electron spin degrees of freedom via the hyperfine interaction as a means of constraining the possible order parameter symmetry. Reduction of the Knight shift is observed for all strains at temperatures below the critical temperature, corresponding well to the drop in spin polarization. The result contradicts prior NMR results, and excludes the previously favored topological superconducting state.