Onset of the nonlinear dielectric response of glasses in the two-level system model

We have calculated the real part $\chi'$ of the nonlinear dielectric susceptibility of amorphous insulators in the kHz range, by using the two-level system model and a nonperturbative numerical quantum approach. At low temperature $T$, it is first shown that the standard two-level model should lead to a \textit{decrease} of $\chi'$ when the measuring field $E$ is raised, since raising $E$ increases the population of the upper level and induces Rabi oscillations canceling the ones induced from the ground level. This predicted $E$-induced decrease of $\chi'$ is at \textit{odds} with experiments. However, a \textit{good agreement} with low-frequency experimental nonlinear data is achieved if, in our fully quantum simulations, interactions between defects are taken into account by a new relaxation rate whose efficiency increases as $\sqrt{E}$, as was proposed recently by Burin \textit{et al.} (Phys. Rev. Lett. {\bf 86}, 5616 (2001)). In this approach, the behavior of $\chi'$ at low $T$ is mainly explained by the efficiency of this new relaxation channel. This new relaxation rate could be further tested since it is shown that it should lead: \textit{i)} to a completely new nonlinear behavior for samples whose thickness is $\simeq 10$ nm; \textit{ii)} to a decrease of nonequilibrium effects when $E$ is increased.Comment: latex Sept02.tex, 5 files, 4 figures, 17 pages, submitted to Eur. Phys. J. B. Text change

Dielectric constant of glasses: first observation of a two-dimensional behavior

The 1kHz real part $\chi'$ of the dielectric constant of a structural glass was measured at low temperature $T$ down to 14 mK. Reducing the sample thickness $h$ to 10 nm suppresses the usual minimum of $\chi'$ for measuring fields $E<.5$ MV/m. This contradicts the Two Level System (TLS) model but is well accounted for by including TLS-TLS interactions where excitations delocalize between TLS's through a $E$-induced mechanism recently designed: for small $h$'s this interaction is reduced, which explains the two-dimensional behavior of $\chi'(T)$. Hence, interactions play a key role in standard thick samples.Comment: latex finesse3.tex, 5 files, 4 figures, 4 pages [SPEC-S02/050], submitted to Phys. Rev. Let

Electrodynamic response of MgB2 sintered pellets and thin films

We present a study of the electrodynamic response of MgB2 pellets and thin film samples exhibiting critical temperatures ranging between 26 and 38 K. We have performed accurate measurements of the surface impedance ZS =RS+iXS as a function of the temperature and of the magnitude of the electromagnetic field. The temperature variation and the field dependence of ZS was measured by a dielectric resonator cavity technique in the microwave region. In particular, the temperature variation of the magnetic penetration depth was also determined in the RF region by a single coil mutual inductance method. In the case of the films, for T<TC/2 a clear exponential behavior of the penetration depth is observed, which can be explained by a simple BCS s-wave model with a reduced value of the energy gap. On the contrary, pellets show no evidence of saturation, and the experimental results strictly follow a quadratic dependence down to the lowest temperatures. This behavior can be induced by the presence of metallic Mg inclusions that may locally depress the gap. The analysis of the field dependence of the surface impedance in the microwave region confirms that the electrodynamic response of MgB2 is dominated by different sources of dissipation, depending on the sample history, likely to be ascribed to the predominance of grain boundaries or normal regions on its surface.Comment: To appear as a chapter in "Studies of High Temperature Superconductors", Vol. 41, A.V. Narlikar ed., Nova Sci. Publ., New York. (submitted October 5, 2001). 21 pages, 16 figure

Magnetic field effect on the dielectric constant of glasses: Evidence of disorder within tunneling barriers

The magnetic field dependence of the low frequency dielectric constant $e_r$(H) of a structural glass a - SiO2 + xCyHz was studied from 400 mK to 50 mK and for H up to 3T. Measurement of both the real and the imaginary parts of $e_r$ is used to eliminate the difficult question of keeping constant the temperature of the sample while increasing H: a non-zero $e_r$(H) dependence is reported in the same range as that one very recently reported on multicomponent glasses. In addition to the recently proposed explanation based on interactions, the reported $e_r$(H) is interpreted quantitatively as a consequence of the disorder lying within the nanometric barriers of the elementary tunneling systems of the glass.Comment: latex Bcorrige1.tex, 5 files, 4 figures, 7 pages [SPEC-S02/009

Granularity-induced gapless superconductivity in NbN films: evidence of thermal phase fluctuations

Using a single coil mutual inductance technique, we measure the low temperature dependence of the magnetic penetration depth in superconducting NbN films prepared with similar critical temperatures around 16 K but with different microstructures. Only (100) epitaxial and weakly granular (100) textured films display the characteristic exponential dependence of conventional BCS s-wave superconductors. More granular (111) textured films exhibit a linear dependence, indicating a gapless state in spite of the s-wave gap. This result is quantitatively explained by a model of thermal phase fluctuations favored by the granular structure.Comment: 10 pages, 4 figures, to appear in Phys. Rev.

Effect of Nuclear Quadrupole Interaction on the Relaxation in Amorphous Solids

Recently it has been experimentally demonstrated that certain glasses display an unexpected magnetic field dependence of the dielectric constant. In particular, the echo technique experiments have shown that the echo amplitude depends on the magnetic field. The analysis of these experiments results in the conclusion that the effect seems to be related to the nuclear degrees of freedom of tunneling systems. The interactions of a nuclear quadrupole electrical moment with the crystal field and of a nuclear magnetic moment with magnetic field transform the two-level tunneling systems inherent in amorphous dielectrics into many-level tunneling systems. The fact that these features show up at temperatures $T<100mK$, where the properties of amorphous materials are governed by the long-range $R^{-3}$ interaction between tunneling systems, suggests that this interaction is responsible for the magnetic field dependent relaxation. We have developed a theory of many-body relaxation in an ensemble of interacting many-level tunneling systems and show that the relaxation rate is controlled by the magnetic field. The results obtained correlate with the available experimental data. Our approach strongly supports the idea that the nuclear quadrupole interaction is just the key for understanding the unusual behavior of glasses in a magnetic field.Comment: 18 pages, 9 figure

Penetration of Josephson vortices and measurement of the c-axis penetration depth in Bi2Sr2CaCu2O8+δBi_{2}Sr_{2}CaCu_{2}O_{8+\delta}: Interplay of Josephson coupling, surface barrier and defects

The first penetration field H_{J}(T) of Josephson vortices is measured through the onset of microwave absorption in the locked state, in slightly overdoped $\rm{Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta}}$ single crystals (T_{c} ~ 84 K). The magnitude of H_{J}(T) is too large to be accounted for by the first thermodynamic critical field H_{c1}(T). We discuss the possibility of a Bean-Livingston barrier, also supported by irreversible behavior upon flux exit, and the role of defects, which relates H_{J}(T) to the c-axis penetration depth $\lambda_{c}(T)$. The temperature dependence of the latter, determined by a cavity perturbation technique and a theoretical estimate of the defect-limited penetration field are used to deduce from H_{J}(T) the absolute value of $\lambda_{c}(0)=(35 \pm 15) \mu m$.Comment: 9 pages, 6 figure

Variabilité génétique, héritabilités et corrélations de 15 caractères d'une population de clones de topinambour (Helianthus tuberosus L)

Quinze caractères en relation avec les stades de développement de la plante et le rendement ont été observés sur 16 clones de topinambour représentatifs de la variabilité existant dans une collection de clones. Pour tous les caractères, les différences entre clones sont très significatives et indiquent l'existence d'une variabilité importante. Les taux d'héritabilité au sens large sont compris entre 19,5 et 96,1%. Ils sont très bas pour les caractères liés au rendement et très élevés pour le pourcentage de matière sèche des tubercules et les caractères en relation avec le développement de la plante. Les gains attendus en sélection, exprimés en proportion (%) de la moyenne, varient de 9,5% pour la durée de la levée à 30,7% pour la hauteur des tiges. L'analyse des path-coefficients de 5 caractères a mis en évidence un effet direct et indirect important du poids sec de la plante entière sur le rendement en matière sèche des tubercules. Les valeurs moyennes des clones ont été analysées suivant la méthode des composantes principales en utilisant 4 axes de référence qui représentent 90% de la variabilité totale. La dispersion des clones le long du premier axe principal montre l'existence de 2 groupes morphologiques distincts : les clones précoces (en nombre limité) et les clones tardifs. Les corrélations génétiques et les corrélations environnementales ont été calculées pour les 15 caractères. Il existe une corrélation génétique positive élevée entre le rendement en matière sèche des tubercules d'une part et le poids frais des tubercules, le poids sec de la plante entière, la durée de vie des feuilles et la durée de la période plantation-floraison d'autre part. Ce dernier caractère apparaît le plus efficace à associer au rendement pour augmenter la réponse à la sélection de celui-ci. Cependant, les possibilités de progrès en utilisant cette relation semblent limitées en raison de la tardivité des floraisons.Genetic variability, heritabilities and correlations of 15 characters in a Jerusalem artichoke clone population (Helianthus tuberosus L). Fifteen developmental and yield component characters were recorded from 16 different Jerusalem Artichoke clones, sampled among a large clone collection. The clone differences were highly significant, showing a wide range of variation in respect of all characters under study (table I). Broad sense heritability estimates ranged between 19.5 and 96.1%. They were very low for yield related characters and very high for tuber dry matter content and developmental characters. Expected genetic advance with selection of the highest 5%, expressed as the percentage of the mean, varied from 9.5 for emerging time to 30.7 for stalk height (table II). Path analysis was used to partition the genetic correlations between 5 characters into direct and indirect effects. Whole plant dry matter had high positive direct and indirect effects on tuber dry matter yield (tables IV and V). The mean clone effects for the 15 characters were submitted to a principal component analysis. The first 4 components explained 90% of the total variation. The dispersion of clones along the first principal component showed the existence of 2 separate morphologic clusters: early clones (a limited number) and late clones (table VI, fig 1). Genetic and environmental correlations between characters were evaluated (table III). Tuber dry matter yield had high positive genetic correlations with fresh tuber weight, whole plant dry matter, green leaf duration and flowering time. Flowering time appeared to be the most efficient character to be associated in a selection for the improvement of tuber yield. However the possibilities for increasing tuber yield by selecting for a later flowering time appeared limited and uncertain

Hétérosis et hérédité de quatre caractères agronomiques dans les croisements de lignées fixées de betterave fourragère et sucrière (Beta vulgaris L)

Les parents et les générations F1, F2, B1, B2, issues du croisement d'une lignée fixée P1 de betterave fourragère à faible teneur en matière sèche et d'une lignée P2 de betterave sucrière ont été testés dans 2 croisements différents pour les caractères poids frais des racines, teneur en matière sèche et poids sec des racines et dans un croisement pour le caractère de coloration des racines. Les variations observées pour les caractères poids frais et poids sec des racines dans les différentes générations ont été expliquées principalement par des effets de dominance et d'épistasie digénique. Toutefois, dans un croisement, les variations du poids frais des racines ne peuvent s'expliquer qu'en faisant intervenir des effets d'épistasie trigénique (ou d'un ordre supérieur) et de linkage entre loci. Pour le caractère teneur en matière sèche, il y a prédominance des effets d'additivité avec, néanmoins, dans chaque croisement, des effets de dominance et d'épistasie très significatifs. Ce caractère apparaît conditionné par au moins 5 paires de gènes. Les manifestations d'hétérosis, très significatives, sont dues à des effets de dominance, de superdominance et d'épistasie pour les 3 caractères dans les 2 croisements. Les taux d'héritabilité au sens large sont bas ou très bas (0,25-0,49) pour les poids frais et sec des racines et élevés (0,62-0,89) pour la teneur en matière sèche. Les corrélations du poids frais et de la teneur en matière sèche sont négatives et généralement prononcées. Les relations entre le poids sec et la teneur en matière sèche sont faibles et inconsistantes. La coloration des racines est déterminée par des allèles présents à 2 loci non liés R et Y. Les résultats de cette étude mettent en évidence l'importance, la fréquence et la diversité des effets d'épistasie dans l'expression des caractères quantitatifs étudiés. L'hétérosis apparaît, pour l'essentiel, comme le résultat d'effets d'épistasie d'ordre élevé au stade juvénile.Heterosis and inheritance of four agronomic characters in sugar beet and mangel (Beta vulgaris L) inbred line crosses. Parents, F1, F2 and backcross generations of 2 crosses between sugar beet and mangel inbred lines were evaluated for root weight, percentage dry matter and dry matter yield characters. The root colour character was observed in one cross. Variation among generation means for root weight and dry matter yield characters were mainly explained by dominance and digenic epistatic effects. However, in one cross, variation for root weight character could not be explained without trigenic (or higher order) interactions and linkage between the pairs of loci. For the character percentage dry matter, additivity effects were predominant with very significant dominance and epistatic effects. The minimum number of pairs of genes affecting this character was 4 in one cross and 5 in the other cross (tables I, II, III). Phenotypic dominance and significant heterosis due to dominance, overdominance and epistasis occurred in the 2 crosses and the 3 characters. Broad sense heritability estimates were low (0.25-0.49) for the root weight and dry matter yield and high (0.62-0.89) for dry matter percent (table IV). The correlations for weight of roots and dry matter percent were negative and frequently pronounced. Correlations for dry matter yield and dry matter percent were weak and inconsistent (table V). Root colour was determined by alleles at two loci, R and Y, and these two loci were not linked (table VI). The results showed the importance, frequency and diversity of epistatic effects in the expression of the quantitative characters under study. Heterosis resulted primarily from high order epistatic effects at a young stage