2014 Nous décrivons un circuit utilisant une contre-réaction capacitive et un déphasage de 90° pour réduire le temps mort des sondes RMN qui existe après une impulsion de radiofréquence. Le circuit d'amortissement fonctionne seulement pendant et pour un temps bref après l'impulsion, par conséquent l'efficacité et la sensibilité du spectromètre restent inchangées. Nous expliquons aussi l'utilisation d'un transistor à effet de champ à haute tension pour isoler le circuit de transmission de la sonde RMN après l'impulsion. L'amortissement est très efficace pour les basses fréquences et nous avons pu réduire le temps mort d'un circuit de résonance en série (Q = 50) à moins de 7 03BCs à 3 MHz avec cette technique. Abstract 2014 We describe an active damping circuit which uses a 90° phase-shift feedback loop to dampen the ringing of nuclear resonance probes just after the RF excitation. The damping does not operate during the reception of the nuclear signal and there is therefore no loss in efficiency or in sensitivity as would be the case for permanent damping. The use of a high voltage FET as a gated duplexer to isolate the RF transmitter after the pulse is also described. The circuit damper is especially useful at low frequencies and we have been able to reduce the recovery time of a series resonant circuit (Q = 50) to less than 7 03BCs. at 3 MHz using this feedback scheme and the gated duplexer.

NMR pulse techniques have been used to study the orientational ordering of 15 N 2 molecules physisorbed on graphite. A small hysteresis (~2.5%) and small discontinuity in the evolution of the NMR echo amplitudes which depend directly on the local order parameters indicates that the order-disorder transition is first order.PACS numbers: 64.70. Kb, 76.60.Lz We report results of an NMR study of the orientational ordering of monolayers of 15 N 2 molecules physisorbed on exfoliated graphite. This system is a physical realization of the two-dimensional (2D) ordering of electric quadrupoles on a triangular lattice for which topological frustration and critical fluctuations are expected to play major roles. 1 ' 2 The transition from the orientationally disordered phase has stimulated considerable interest since it is believed to belong to the same universality class as the n = 3 component Heisenberg model with face-oriented anisotropy, and it is predicted that critical fluctuations will drive the transition first order. 1,3 ' 4 Although recent low-energy-electron diffraction measurements 5 * 6 have shown that the orientationally ordered phase is a (2x1) herringbone configuration, the nature of the phase transition has not been elucidated. In order to determine whether or not the transition is first order we have used NMR techniques to follow the evolution of the local order parameters. The results indicate that the transition is first order with a small discontinuity in the order parameter and a hysteresis ^ 2.5% on thermal cycling. This agrees qualitatively with recent Monte Carlo simulations. 3 Monolayers of N 2 form a commensurate 2D solid on graphite below 47 K with the molecules occupying the center of every third carbon ring, i.e., a {/ZxV~3)R3Q° triangular net. 7 At high temperatures, the molecular axes are disordered as a result of thermal agitation, but at low temperatures this rotational symmetry is broken by anisotropic forces acting on the molecules. There are two contributions: (1) the strong anisotropic potential of the graphite substrate 8 for which the molecules tend to lie parallel to the graphite, and (2) the anisotropic interaction between the molecules (principally electric quadrupole-quadrupole) which leads to a collective orientational ordering of the axes. 9 The results of en-ergy minimization, 10 Monte Carlo simulations, 11 and mean-field-approximation calculations 1 show that the ground state is a two-sublattice herringbone array.The orientational ordering transition was first seen in heat-capacity studies 12 and later confirmed by neutron scattering. 13 The herringbone structure was identified by low-energy electrondiffraction measurements 5 ' 6 which were able to detect several superlattice reflections. The intensity of the superlattice peaks varied continuously on warming with an apparently smooth transition at approximately 30 K although a weak intensity persisted up to 39 K. It was not possible to conclude from these studies whether the transition was smooth or first order.More detailed...

2014 Nous avons effectué des mesures des raies d'absorption RMN du deuterium solide afin d'établir le diagramme de phase de l'ordre orientationnel pour les concentrations d'espèces J = 1, 42 X 75 %, et pour les températures 0,035 T 2,5 K. Comme dans l'hydrogène solide, on observe trois phases : une phase désordonnée à haute température, une phase présentant un ordre à longue distance pour X > Xc ~ 55 % et pour X Xc une nouvelle phase encore jamais observée dans le deutérium solide. La nature de l'ordre dans cette nouvelle phase a été déterminée grâce à l'analyse des formes de raies de résonance. Il y a une distribution large du paramètre d'ordre local pour X Xc et nous interprétons ce résultat par un verre quadrupolaire analogue à celui observé dans l'hydrogène solide.