Enhancement of ferromagnetism by oxygen isotope substitution in strontium ruthenate SrRuO3.

The oxygen isotope effect of the ferromagnetic transition in itinerant ferromagnet strontium ruthenate SrRuO3 with a Curie temperature Tc of 160 K is studied. We observed for the first time a shift of ∆Tc ~ 1 K by oxygen isotope substitution of 16O → 18O in SrRuO3 by precise measurements of DC and AC magnetizations. The results surprisingly lead to the noteworthy inverse isotope effect with negative coefficient α = -∂ lnTc/∂ lnM. The Raman spectra indicate that the main vibration frequency of 16O at 363 cm-1 shifts to 341 cm-1 following oxygen isotope substitution 18O. This shift is remarkably consistent with the Debye frequency being proportional to ∝ 1√M where M is the mass of an oxygen atom. The positive isotope shift of ∆Tc can be understood by taking account of the electron-phonon interaction.

Persistence of the superconducting condensate far above the critical temperature of YBa2(Cu,Zn)3O(y) revealed by c-axis optical conductivity measurements for several Zn concentrations and carrier doping levels.

The superconductivity precursor phenomena in high temperature cuprate superconductors is studied by direct measurements of the superconducting condensate with the use of the c-axis optical conductivity of YBa2(Cu1-xZnx)3Oy for several doping levels (p) as well as for several Zn concentrations. Both the real and imaginary parts of the optical conductivity clearly show that the superconducting carriers persist up to the high temperatures Tp that is higher than the critical temperature Tc but lower than the pseudogap temperature T*. Tp increases with reducing doping level like T*, but decreases with Zn substitution unlike T*.

Superconductors: unusual oxygen isotope effects in cuprates?

The possibility that a pairing boson might act as the 'glue' to bind electrons into a Cooper pair in superconductors with a high critical temperature (T(c)) is being actively pursued in condensed-matter physics. Gweon et al. claim that there is a large and unusual oxygen-isotope effect on the electronic structure, indicating that phonons have a special importance in high-temperature superconductors. However, we are unable to detect this unusual oxygen-isotope effect in new data collected under almost identical material and experimental conditions. Our findings point towards a more conventional influence of phonons in these materials.