An optical atomic clock based on a highly charged ion.

Optical atomic clocks are the most accurate measurement devices ever constructed and have found many applications in fundamental science and technology1-3. The use of highly charged ions (HCI) as a new class of references for highest-accuracy clocks and precision tests of fundamental physics4-11 has long been motivated by their extreme atomic properties and reduced sensitivity to perturbations from external electric and magnetic fields compared with singly charged ions or neutral atoms. Here we present the realization of this new class of clocks, based on an optical magnetic-dipole transition in Ar13+. Its comprehensively evaluated systematic frequency uncertainty of 2.2 × 10-17 is comparable with that of many optical clocks in operation. From clock comparisons, we improve by eight and nine orders of magnitude on the uncertainties for the absolute transition frequency12 and isotope shift (40Ar versus 36Ar) (ref. 13), respectively. These measurements allow us to investigate the largely unexplored quantum electrodynamic (QED) nuclear recoil, presented as part of improved calculations of the isotope shift, which reduce the uncertainty of previous theory14 by a factor of three. This work establishes forbidden optical transitions in HCI as references for cutting-edge optical clocks and future high-sensitivity searches for physics beyond the standard model.

Moderate MAS enhances local (1)H spin exchange and spin diffusion.

Proton NMR spin-diffusion experiments are often combined with magic-angle spinning (MAS) to achieve higher spectral resolution of solid samples. Here we show that local proton spin diffusion can indeed become faster at low (<10 kHz) spinning rates as compared to static conditions. Spin diffusion under static conditions can thus be slower than the often referred value of 0.8 nm(2)/ms, which was determined using slow MAS (Clauss et al., 1993). The enhancement of spin diffusion by slow MAS relies on the modulation of the orientation-dependent dipolar couplings during sample rotation and goes along with transient level crossings in combination with dipolar truncation. The experimental finding and its explanation is supported by density matrix simulations, and also emphasizes the sensitivity of spin diffusion to the local coupling topology. The amplification of spin diffusion by slow MAS cannot be explained by any model based on independent spin pairs; at least three spins have to be considered.

Basic principles of static proton low-resolution spin diffusion NMR in nanophase-separated materials with mobility contrast.

We review basic principles of low-resolution proton NMR spin diffusion experiments, relying on mobility differences in nm-sized phases of inhomogeneous organic materials such as block-co- or semicrystalline polymers. They are of use for estimates of domain sizes and insights into nanometric dynamic inhomogeneities. Experimental procedures and limitations of mobility-based signal decomposition/filtering prior to spin diffusion are addressed on the example of as yet unpublished data on semicrystalline poly(ϵ-caprolactone), PCL. Specifically, we discuss technical aspects of the quantitative, dead-time free detection of rigid-domain signals by aid of the magic-sandwich echo (MSE), and magic-and-polarization-echo (MAPE) and double-quantum (DQ) magnetization filters to select rigid and mobile components, respectively. Such filters are of general use in reliable fitting approaches for phase composition determinations. Spin diffusion studies at low field using benchtop instruments are challenged by rather short (1)H T1 relaxation times, which calls for simulation-based analyses. Applying these, in combination with domain sizes as determined by small-angle X-ray scattering, we have determined spin diffusion coefficients D for PCL (0.34, 0.19 and 0.032nm(2)/ms for crystalline, interphase and amorphous parts, respectively). We further address thermal-history effects related to secondary crystallization. Finally, the state of knowledge concerning the connection between D values determined locally at the atomic level, using (13)C detection and CP- or REDOR-based "(1)H hole burning" procedures, and those obtained by calibration experiments, is summarized. Specifically, the non-trivial dependence of D on the magic-angle spinning (MAS) frequency, with a minimum under static and a local maximum under moderate-MAS conditions, is highlighted.