In the Spring of 2013, NMR spectroscopists convened at the Weizmann Institute in Israel to brainstorm on approaches to improve the sensitivity of NMR experiments, particularly when applied in biomolecular settings. This multi-author interdisciplinary Review presents a state-of-the-art description of the primary approaches that were considered. Topics discussed included the future of ultrahigh-field NMR systems, emerging NMR detection technologies, new approaches to nuclear hyperpolarization, and progress in sample preparation. All of these are orthogonal efforts, whose gains could multiply and thereby enhance the sensitivity of solid- and liquid-state experiments. While substantial advances have been made in all these areas, numerous challenges remain in the quest of endowing NMR spectroscopy with the sensitivity that has characterized forms of spectroscopies based on electrical or optical measurements. These challenges, and the ways by which scientists and engineers are striving to solve them, are also addressed.

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Im Frühling 2013 versammelten sich NMR‐Spektroskopiker am Weizmann‐Institut in Israel, um neue Ansätze zur Verbesserung der Empfindlichkeit von NMR‐Experimenten zu diskutieren, besonders im Hinblick auf Experimente an biomolekularen Systemen. Der vorliegende Aufsatz ist von vielen Autoren mit unterschiedlichem fachlichem Hintergrund verfasst worden; er beschreibt den Stand der Forschung auf diesem Gebiet, wie er im Rahmen des Treffens diskutiert wurde. Es werden Ansätze für Ultra‐Hochfeld‐NMR‐Spektroskopie, für neuartige Techniken der NMR‐Detektion, für Prinzipien und Methoden der Kernhyperpolarisation, aber auch für neue Probenpräparationsmethoden diskutiert. Diese Ansätze verbessern die Empfindlichkeit von NMR‐Experimenten im Festkörper und in Lösung, sind voneinander unabhängig und können sich in ihren Auswirkungen multiplizieren. Auch wenn in allen genannten Bereichen bereits substanzielle Verbesserungen erzielt wurden, ist doch noch eine weite Strecke zu überwinden, um an die Empfindlichkeiten von optischen und elektronischen Spektroskopien heranzureichen. Diese Probleme und erste Lösungsansätze werden ebenfalls diskutiert.

We have investigated the mechanisms of electrical conduction of polymercopper-particle composites in the low-particle-concentration limit (dilute limit) by electrical-noise measurements. The l/f flicker noise, observed close to the percolation threshold, has been studied as a function of the current through the sample, frequency (10 -10 Hz), and resistance (10'-10 0). In this paper we relate the total resistance noise measured on the samples to the local noise due to the different contacts between the particles. The resistance and noise power of the inhomogeneous medium is modeled by an extended effective-medium theory that includes a transition between two different conduction mechanisms near the percolation threshold. This model gives support to our hypothesis that the noise is produced by small electrical contacts between the particles, as well as the node-link picture introduced to characterize the conductive backbone preexisting at the percolation transition in the continuous percolation models. I. I /f NOISE IN COMPOSITE SYSTEMSAlthough many experiments have recently shown that impurities can give rise to 1/f electric field fluctuations in metals, ' ' there is at present no general microscopic theory for the electrical noise in various solid-state physical systems. This noise, different from the thermal noise that, at low frequency, only depends on resistance and temperature, is also sensitive to the dc current passing through the sample and the frequency. In a disordered system, as in the case of the conducting polymerparticle systems we are considering, the total resistance R is a moment of order 2 of the currents i flowing through different conduction paths a, each characterized by the value of its resistance r, ' I2 where I is the total current through the sample. In contrast, the resistance fluctuations are moment of order 4 of the current i 4(g 2)The noise power is therefore a very sensitive probe of inhomogeneities in the conductor. " ' It will be shown that, although the noise power and the electrical resistance of the samples are rapidly diverging when the percolation limit is approached, study of the noise power at the percolation threshold versus the electrical resistance of the same samples allow us to get rid of the unavoidable local inhomogeneities in particle distribution which otherwise could only be eliminated by a prohibitive number of experiments.In 1958, Holm' published his trend-setting work in which he related the total I/f-noise power in carbonmembrane microphones to the supposedly uncorrelated noise at the particle interfaces. Later, Williams and Burdett' and Celasco and co-workers studied the noise spectra of evaporated thin gold films. Chen and Chou ' have more recently investigated systems very similar to the ones discussed in this paper, carbon-black-loaded resin, and have observed a divergence of the noise power as the particle concentration approaches the percolation limit at p, .(with K = 5.By substituting the concentration dependence of R near the percolation threshold, this ...

Microwave cyclotron resonance measurements on (ET)2NH4Hg(NCS)4 have revealed a series of harmonics in addition to the fundamental absorption. An attempt is made to account for the associated cyclotron effective masses in terms of a renormalised band of quasiparticles. IntroductionOrganic charge transfer salts of tile form (ET)2+X (ET is short for BEDT-TTF or bis(ethylenedithio)tetrathiafulvalene) have attracted much interesl recently due to file discovery of superconductivity in a number of what is now a large class of materials [1]. The mechanisms for superconductivity in these quasi-2D conductors is not well understood, although it has been suggested that an enhanced density of states (DOS) at the Fermi energy (ev), brought about by electron-electron interactions (EEI), plays a key role [2]. A comprehensive study of the effective masses (a direct measure of the DOS) in these materials is therefore essential in order to test any potential theories. Cyclotron resonance (CR) is one technique which is used to measure effective mass and may also be used to detect any deviations from a simple parabolic band dispersion.In a previous publication [3], we investigated CR in a series of ET salts (X=Cu(NCS)2, NHAtg(NCS)4 and KHg(NCS)4). Cyclotron masses (mcR)were measured ranging from 0.4 to 1.2m, in contrast to the effective masses obtained from the temperature dependence of the Shubnikov-de Haas (SdH) effect which were 2-5 times greater.At this point, one needs to distinguish between different types of quasiparticle masses [4]. The effective mass m °, which occurs in the thermodynamic density of states includes contributions arising from quasiparticles displacing each other as they move through a medium. The dynamical mass mx, is the mass which would be observed in the absence of quasiparticle interactions, and therefore represents the band mass renormalised only by electron-phonon interactions. The SdH effect examines oscillations in the electronic properties of a system as Landau levels cross the Fermi level. The effective mass measured in this way will therefore include effects due to EEIs, since it is at ~v that these are strongest. In contrast, CR probes the energy separation between Landau levels at and above ~v. In the limit where the cyclotron energy (¢cR) is much larger than the typical EEI bandwidth, mcR may be quite different from m ° (discussed below). Indeed, this is thought to be the reason for the difference in measured masses; inert corresponds to ran, while the mass measured from the SdH effect is m" [4].