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A new variant of the superconducting fulleride $\mathrm{Rb_{3}C_{60}}$ is presented, with $\mathrm{^{3}He}$ atoms encapsulated in the $\mathrm{C_{60}}$ cages. The $\mathrm{^{3}He}$ nuclei act as sensitive NMR probes embedded in the material. The superconducting and normal states are characterised by $\mathrm{^{3}He}$ NMR. Evidence is found for co-existing vortex liquid and vortex solid phases below the superconducting transition temperature. A strong dependence of the spin-lattice relaxation time constant on spectral frequency is observed in the superconducting state, as revealed by two-dimensional NMR utilising an inverse Laplace transform. Surprisingly, this phenomenon persists, in attenuated form, at temperatures well above the superconducting transition., Comment: 20 pages, 15 figures

The NMR spectroscopy of spin-1/2 nuclei with low gyromagnetic ratio is challenging, due to the low NMR signal strength. Methodology for the rapid acquisition of $^{103}$Rh NMR parameters is demonstrated for the case of the rhodium formate "paddlewheel" complex $\mathrm{Rh_2(HCO_2)_4}$. A scheme is described for enhancing the $^{103}$Rh signal strength by polarization transfer from $^{1}$H nuclei and which also greatly reduces the interference from ringing artifacts, a common hurdle for the direct observation of low-$\gamma$ nuclei. The $^{103}$Rh relaxation time constants $T_1$ and $T_2$ are measured within 20 minutes using $^{1}$H-detected experiments. The field-dependence of the $^{103}$Rh $T_1$ is measured. The high-field relaxation is dominated by the chemical shift anisotropy (CSA) mechanism. The $^{103}$Rh shielding anisotropy is found to be very large: $|\Delta\sigma|=9900\pm540\mathrm{\,ppm}$. This estimate is compared with density functional theory calculations., Comment: submitted to JCP

At the magnetic fields of common NMR instruments, electron Zeeman frequencies are too high for efficient electron-nuclear dipolar cross-relaxation to occur in solution. The rate of that process fades with the electron Zeeman frequency as omega^{-2} - in the absence of isotropic hyperfine couplings, liquid state dynamic nuclear polarisation (DNP) in high-field magnets is therefore impractical. However, contact coupling and dipolar cross-relaxation are not the only mechanisms that can move electron magnetisation to nuclei in liquids: multiple cross-correlated (CC) relaxation processes also exist, involving various combinations of interaction tensor anisotropies. The rates of some of those processes have more favourable high-field behaviour than dipolar cross-relaxation, but due to the difficulty of their numerical - and particularly analytical - treatment, they remain largely uncharted. In this communication, we report analytical evaluation of every rotationally driven relaxation process in liquid state for 1e1n and 2e1n spin systems, as well as numerical optimisations of the steady-state DNP with respect to spin Hamiltonian parameters. A previously unreported cross-correlation DNP (CCDNP) mechanism was identified for the 2e1n system, involving multiple relaxation interference effects and inter-electron exchange coupling. Using simulations, we found realistic spin Hamiltonian parameters that yield stronger nuclear polarisation at high magnetic fields than dipolar cross-relaxation.

Dynamic nuclear polarization (DNP) enhances nuclear magnetic resonance (NMR) signals by transferring electron spin polarization to nuclei. As DNP requires microwave magnetic fields B1 strong enough to saturate electron spins, microwave resonators are generally used to achieve a sufficient B1, at the expense of restricting the sample size. Higher fields improve NMR sensitivity and resolution. However, resonators at 9 T for example can only hold nano-liters (nL). Larger volumes are possible by avoiding resonators, but the higher power needed to reach B1 is likely to evaporate the sample. Here, we demonstrate a breakthrough in liquid state DNP at 9 T, boosting the sample size to the microliter range. We could use high-power (70 W) microwaves thanks to a planar probe designed to alleviate dielectric heating. We enhanced the 1H NMR signal intensity of 2 uL of liquid water by a factor of 14, while maintaining the water temperature below 40 degree Celsius.