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This paper presents a detailed study of the temperature dependence of the longitudinal relaxation time T I and of the coherence time T e o[ a population of ionic molecular impurities in silica glass. Electric dipolar echoes taken in a temperature range 4-22 mK and a frequency of 370 MHz show that T I isgoverned by a one-phonon process; consequently T~ vanes hke T ; expertmental data show that Te also vanes hke T and thts m contradtctton wtth predictions which lead to T -e. The relaxation of the spontaneous and stimulated echoes shows that there is a wide distribution of relaxation times (T I and Te); from the amplitude of the signal it is also possible to extract both the longitudinal and the transverse electric moment; the coupling constant of the impurities with the strain is found to be as large as 3 eV.

Measurements of the specific heat of quasi one-dimensional conductors, (TaSe 4 ) 2 I and (NbSe 4 ) 3 I, between 0.1 and 8 K, are reported. Whereas no deviation from the P law occurs at low temperature for (NbSe 4 ) 3 I, we have measured two anomalies in (TaSe^I: an excess heat capacity with a peak at J= 1.75 K and a specific heat which follows a P 2 law below 0.4 K. We ascribe this excess heat capacity to the charge-density-wave phase excitations above their pinning frequency in the insulating Peierls state (rp = 263 K).PACS numbers: 65.40.Em, 63.20.Dj, 63.50, + x, 72.15.Nj It is now well known that many few-dimensional compounds are unstable and undergo a Peierls transition at a critical temperature, T ? } This instability is characterized by the opening of an electronic gap at the Fermi level, by a softening in the phonon spectrum at 0==2/c F with A: F the Fermi wave vector, and by a modulation of the electron charge density with the same periodicity called a charge-density wave (CDW). Below T ? the soft-phonon branch splits into two different modes 2 " 4 : an opticlike amplitude mode and an acousticlike phase mode. For ideal one-dimensional conductors, when Q is incommensurate with the lattice, the phase mode or phason exists at zero frequency and corresponds to the current provided by the sliding motion of the CDW as proposed by Frohlich. 5 In real systems, as shown by Lee, Rice, and Anderson, 3 the gapless character of the phase mode is destroyed through various mechanisms, especially impurity pinning.Recently, the interest has focused on several families of one-dimensional inorganic compounds undergoing Peierls transitions, namely, transition-metal trichalcogenides (NbSe 3 , TaS 3 ), blue bronze, and halogenated transition-metal tetrachalcogenide (TaSe^L 6 In these compounds an extra dc current appears when the applied electric field is higher than a threshold value (typically 0.1-1 V/cm). This extra conductivity has been ascribed to Frohlich-type conduction when the CDW becomes unpinned and moves through the crystal. 6 By complex-impednace measurements in the millimeter-wave range the pinned sliding model for NbSe 3 and TaS 3 is found to be overdamped with a spectral response in the megahertz-gigahertz range, 7 whereas for (TaSe^I this mode is underdamped, indicating a pinning frequency 8 of 35 GHz or 1.7 K. The frequency of this pinned mode is just in the right frequency range to allow for the detection of CDW excitations by thermodynamical measurements.Hereafter we report on heat-capacity measurements on (TaSe^I and, for comparison, on (NbSe 4 ) 3 I. These compounds consist of parallel MSe 4 chains well separated from one another by halogen atoms. In each MSe 4 chain the metal atom is located at the center of a rectangular antiprism of eight Se atoms. The d z2 band filling of the transition metal M can be varied according to the composition. This change in band filling leads to quite different structural and electrical properties: (TaSe 4 ) 2 I and (NbSe 4 )ioI 3 undergo a Peierls transition at T P ...

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2014 Le diagramme de phase de NaNO2 sous champ électrique est exploré par diffraction neutronique. On n'observe pas de phases commensurables induites par le champ. Pour un champ de 3,4 kV cm-1 on observe un point triple où les trois phases paraélectrique, ferroélectrique et modulée coexistent. Les résultats sont discutés sur la base d'un modèle phénoménologique à la Landau.

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Nous présentons les résultats obtenus en diffraction de rayons X sur le nitrite de sodium : nous nous sommes principalement intéressés à la localisation des satellites et de la diffusion diffuse dans la phase incommensurable, ainsi qu'aux effets prétransitionnels dans la phase désordonnée. Ces travaux ont été complétés par des expériences de diffusion élastique de neutrons, qui conduisent à une détermination très précise de l'évolution avec la température de la période de modulation. Ces différentes expériences, auxquelles s'ajoutent des expériences préliminaires de diffusion inélastique des neutrons, permettent d'ébaucher une discussion sur la nature de la modulation dans la phase incommensurable