Isotope Effect on Superconducting Transition Temperature of Na_xCoO₂·yH₂O

Abstract

The effect of the 16 O→ 18 O substitution on the superconducting transition temperatures T c of Na x CoO 2 ⋅yH 2 O has been studied for several sets of samples. Each set has two kinds of samples, one with the oxygen sites in the CoO 2 layers filled with 16 O atoms and another with these sites partially substituted with 18 O isotope. These two kinds of samples were prepared in parallel in whole preparation processes. For all the sample sets, we have not found any significant difference of the T c values between the non-substituted and substituted samples, which indicates that the oxygen isotope effect is not appreciable in Na x CoO 2 ⋅yH 2 O. The result, however, does not necessarily exclude the possibility of the phonon mechanism of the superconductivity, because the strong correlation effect suppresses the isotope effect as pointed out previously.

Unconventional superconductivity

“…Specific heat gives (Oeschler et al [664]) a large residual (6.4 or 11.0 mJ/molK 2 depending on aging vs 16.1 mJ/molK 2 for normal), 14. Isotope effect experiments are unclear (Yokoi et al [669]), since the measured changes are a.) expected to be small (substitution for 16 O in the CoO2 planes) and b.)…”

Superconductivity of cobalt oxide hydrate, Na (H3O) CoO2· yH2O

“…The isotope effect on the T C has also been investigated, by substituting 18 O atoms in place of 16 O atoms in the CoO 2 layers [109]. However, no important change was observed.…”

Anomalous isotope effect in iron-based superconductors

“…To reveal the role of phonons, isotope effect in the superconducting temperature T c is commonly used [2,3,[25][26][27][28][29][30]. By detecting the change in T c upon isotope substitutions, the isotope exponent α = −d(log T c )/d(log M ) can be obtained [31,32], where M is the mass of the substituted element.…”