Observation of an oxygen isotope effect inYBa2Cu3

Abstract

A small decrease in Tc of 0.3 to 0.5 K is observed when as much as 90% of the '^O in YBa2Cu307 is substituted with ^^O. This result is consistent with our observation that there is an oxygen isotope effect in Lai.85Sro.i5Cu04, but in contrast with previous reports that there is no isotope effect for YBa2Cu307. This new result suggests that phonons play an important role in the electron-pairing mechanism in YBa2Cu307. PACS numbers: 74.70.Ya Since the discovery of superconductivity above 90 K in YBa2Cu307,^''^ it has been debated whether the superconductivity in this material is phonon mediated as expected from conventional BCS theory,^ or whether a different mechanism is involved. Support for a nonphonon mechanism was provided when our group, ^ Batlogg et al.,^ reported that the superconducting transition temperature, Tc, does not change significantly when ^^O is replaced with ^^O in YBaiCuaOy. However, more recently we have observed an oxygen isotope effect in samples of Lai.85Sro.i5Cu04,^ Lai.85Cao.i5Cu04,^ and BaPbo.75Bio.2503.^ The shift in Tc to lower temperatures observed for Lai.85Sro.i5Cu04 when 75% of the ^^O was replaced with ^^O varied between 0.3 and 1.0 K depending on the sample. This shift is smaller than what is predicted by conventional BCS theory. The small decrease in Tc for Lai.85Sro.i5Cu04 prompted us to reexamine the effect of ^^O substitution on the transition temperature of YBa2Cu307. In this paper, we report the observation of a drop in Tc to lower temperatures by 0.3 to 0.5 K in three different samples of YBaiCuaOy when as much as 90% of the ^^O is replaced by ^^O. This shift in the onset of the diamagnetic transition due to ^^O substitution was reversible by resubstitution of ^^O for ^^O.According to the conventional BCS treatment,^ Tc should vary as M ~", where M is the oxygen mass. This assumes that the frequency of the important phonon mode depends only on the mass of oxygen. For a value of a =0.5, a shift in Tc to lower temperatures by approximately 5 K is predicted for a 90-K superconductor (on the assumption of complete substitution of ^^O for '^O). We note that this is the maximum shift expected and that because of the complexity of the structure, i.e., both chains and layers of Cu-O, this prediction is at best tenuous. Although the 0.3-to 0.5-K shift in Tc for YBaiCusOv is smaller than the predicted shift, the presence of an oxygen isotope effect demonstrates that phonons play an important role in the electron-pairing mechanism in this material.Three samples of YBa2Cu307, referred to below as samples I, II, and III, were synthesized via a nitrate route which has been described previously.^ This route involved our dissolving stoichiometric ratios of Y2O3, BaC03, and CuO in nitric acid and evaporating the solution to dryness. The resulting powders were decomposed near 800 °C in AI2O3 crucibles. To ensure homogeneous products, these powders were then ground and fired in air for 48 h at temperatures ranging from 750 to 950 °C, Sample I 11 11-18^ III III-

Anisotropic superconductivity mediated by phonons in layered compounds with weak screening effects

“…This result is consistent with the observed large shifts of T c as absolute values due to the isotope effect [10][11][12][13][14][15][16].…”

“…However, experimental results of isotope effect suggest that there are contributions to the superconductivity from phonon-mediated interactions in many high-T c cuprates [10][11][12][13][14][15][16]. Absolute values of shifts of T c are very large (0.2K ∼ 0.7K), but isotope effect exponents α are small because of the high transition temperature.…”

Isotope effect in the superconducting high-pressure simple cubic phase of calcium from first principles

“…Moreover, the analysis of the isotope effect is still a very active field in more complex superconductors such as cuprates and Fe-based high-temperature superconductors, since the role of lattice vibrations in their coupling mechanism is yet to be fully understood. [2][3][4][5][6][7][8][9] The value given by BCS theory for the isotope coefficient is a BCS ¼ 0:5. In simple metals and alloys, a ranges typically between 0.2 and 0.5 in reasonable agreement with the theory.…”

Isotope effect in impure high-Tcsuperconductors

“…In chemically complex multicomponent systems one usually defines the partial coefficients α i = − ∂ ln Tc ∂ ln Mi , where M i is the isotope mass of the i-th component. In high temperature superconductors (HTS) typically O 18 is replaced [3,4,5] by O 16 . This defines so called oxygen isotope coefficient α O .…”

“…Similarly the systematic studies of various isotopic substitutions in HTS are important to understand the role of electronphonon interaction in these materials. After early experiments with contradictory conclusions [3,4,5,10] it has later been unequivocally established that the oxygen isotope shift α is non-zero, and takes smallest value for optimally doped materials. It increases when one moves into underdoped region.…”