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

Pairing interactions and pairing mechanism in high temperature copper oxide superconductors

The polaron binding energy E_{p} in undoped parent cuprates has been determined to be about 1.0 eV from the unconventional oxygen-isotope effect on the antiferromagnetic ordering temperature. The deduced value of E_{p} is in quantitative agreement with that estimated from independent optical data and that estimated theoretically from the measured dielectric constants. The substantial oxygen-isotope effect on the in-plane supercarrier mass observed in optimally doped cuprates suggests that polarons are bound into the Cooper pairs. We also identify the phonon modes that are strongly coupled to conduction electrons from the angle-resolved photoemission spectroscopy, tunneling spectra, and optical data. We consistently show that there is a very strong electron-phonon coupling feature at a phonon energy of about 20 meV along the antinodal direction and that this coupling becomes weaker towards the diagonal direction. We further show that high-temperature superconductivity in cuprates is caused by strong electron-phonon coupling, polaronic effect, and significant coupling with 2 eV Cu-O charge transfer fluctuation.Comment: 11 pages, 7 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

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

Mode d'action des gènes et hétérosis pour le caractère montée à graines dans le croisement de deux lignées fixées de betterave à sucre (Beta vulgaris L.)

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Very high resolution measurement of the penetration depth of superconductors by a novel single-coil inductance technique

We describe a novel single-coil mutual inductance technique for measuring the magnetic penetration depth lambda of superconductors at 2-4 MHz as a function of temperature in the 4-100 K range. We combine a single-coil configuration with a high-stability marginal oscillator; this enables us to measure the absolute value of lambda on both bulk samples and thin films with very high resolution (delta lambda=10 pm) and a precision of 30 nm. As example of application, we report measurements on NbTi bulk samples and Nb films. This contactless technique is suited for probing the superconducting properties of samples over large surfaces

Effect of doping on the linear temperature dependence of the magnetic penetration depth in cuprate superconductors

Consiglio Nazionale delle Ricerche - Biblioteca Centrale - P.le Aldo Moro, 7 Rome / CNR - Consiglio Nazionale delle RichercheSIGLEITItal