Papers by Keyword: Isotope Effect

Abstract: The effect of 16О → 18О isotope substitution on the properties of Pr0.5Ca0.5MnO3 manganites doped by Cr and Ru is studied. In these compounds, chromium and ruthenium favor (i) the suppression of a charge-ordered state and (ii) the formation of a ferromagnetic metallic phase. The 16О → 18О isotope substitution leads to the growth of the charge-ordering transition temperature (TCO), and to the lowering of ferromagnetic transition temperature (TFM) accompanied by a decrease in the content of ferromagnetic phase. The difference in the behavior of the Cr- and Ru-substituted samples is analyzed.

Abstract: The oxygen isotope effect in ceramic bilayer manganite (La1 zPrz)1.2Sr1.8Mn216 18O7 (z = 0, 0.3, 0.4, 0.6) has been investigated. Real and imaginary parts of ac magnetic susceptibility, dc magnetization and magnetoresistance were measured at temperatures from 4.2 to 320 K in applied magnetic field up to 40 kOe. Substantial decrease of ferromagnetic (FM) transition temperature TFM under oxygen isotope substitution 16O → 18O was found. This positive isotope effect amounts to more than 20 K. A number of additional transitions of ferromagnetic type were observed at T >170 K, which also depend on oxygen overall mass. The obtained results are compared with those observed in other manganites exhibiting the large isotope effect.

Abstract: Since the discovery of bulk metallic glasses there has been considerable research effort on these systems, in particular with respect to mass transport. Now the undercooled melt between the melting temperature and the caloric glass transition temperature, which has not been accessible before due to the rapid onset of crystallization, can be investigated and theories can be tested. Here we report on radiotracer diffusion measurements in metallic bulk-glass-forming Pd-Cu-Ni-P alloys. Serial sectioning was performed by grinding and ion-beam sputtering. The time, temperature as well as the mass dependence, expressed in terms of the isotope effect E, of Co-diffusion were investigated. The Co isotope effect measurements, which have never been carried out near Tc in any material, show atomic transport up to the equilibrium melt to be far away from the hydrodynamic regime of uncorrelated binary collisions. In the glassy state as well as in the deeply supercooled state below the critical temperature Tc, where the mode coupling theory predicts a freezing-in of liquid-like motion, the experimentally determined very small isotope effects indicate a highly collective hopping mechanism involving some ten atoms. Below Tc the temperature dependence shows Arrhenius-type behavior with an effective activation enthalpy of 3.2 eV. Above Tc the onset of liquid-like motion is evidenced by a gradual drop of the effective activation energy and by the validity of the Stokes-Einstein equation, which is found to break down below Tc. Although having strong covalent bonding tendencies, Phosphorous diffusion is only slightly slower than Co diffusion, indicating that it does not determine the overall viscosity below Tc. The Stokes-Einstein equation is presently tested for other constituents of the alloy.

Abstract: Through the review of the correlation on the isotope effects research and superconductivity mechanism research as well as the progress on the study of the isotope effects in the high Tc copper oxide superconductors, we have found the fundamental factors that cause the different changes of the isotope effect value in different dope region in high Tc cupper oxide superconductors. We point the high-temperature superconductivity mechanism possible origin phonons mediation electrons.

Abstract: We report isotope shifts of the no-phonon lines of V_Si+H/D complexes observed in lowtemperature photoluminescence spectra of proton/deuteron implanted and annealed samplesshowing both the hydrogen and deuterium spectra. We propose that the isotope shift of the nophonon lines is related to the zero-point energies of the vibrations in both the electronic ground andexcited states of the complex. The red shift of the deuterium no phonon line with respect to thehydrogen line is consistent with a stiffer spring constant for the potential well associated with theexcited state. We tentatively assign a weak shoulder on the H^s₃ first harmonic line to the ¹³C isotope.