Observation of an oxygen isotope shift in the superconducting transition temperature ofLa1.85Sr0.15

Abstract

An oxygen isotope shift is observed in superconducting Lai.ssSro.isCuC^ when 18 0 is substituted partially for ,6 0; the superconducting transition temperature T c is lowered by 0.3 to 1.0 K in different samples. We examine these results using conventional phonon-mediated BCS theory and conclude that for Lai.gsSro.isCuO^ phonons play an important role in the pairing mechanism.PACS numbers: 74.70. Ya, The discovery of superconductivity near 37 K, in Lai. 8 5Sro.i5Cu0 4 l and near 92 K in YBa2Cu 3 07 2,3 has generated great interest in the mechanism of superconductivity in these materials. Although the conventional phonon-mediated electron-pairing mechanism 4 may be consistent with superconductivity above 30 K, novel mechanisms are under consideration. One experiment that investigates whether a phonon mechanism is involved is to measure the superconducting transition temperature T c for samples containing different isotopes. 5,6 In YBa2Cu3C>7, the mass of the ion at the yttrium site can be varied by rare-earth (R) substitutions; although T c differs for various 7?Ba2Cu 3 07 compounds, there is no obvious correlation with mass. 7,8 Experiments in which I8 0 was substituted for l6 0 in YBa 2 Cu 3 07 have suggested the absence of an isotope shift. 9~n In addition, for this system, Cu and Ba isotope substitution produces no measurable shift in T c . 12 In contrast to the 92-K superconductor, we report here 13 that we have detected an oxygen isotope effect in the 37-K superconductor Lai.8 5 Sro.i5Cu0 4 .Our Meissner-effect and resistance measurements indicate that T c is shifted to lower temperatures when 18 0 is substituted partially for 16 0 in Lai.8sSr 0 .isCuO 4 . The magnitude of the shift is between 0.3 and 1.0 K, depending on the sample. If we use the original BCS treatment 4 of T c ozM~a, where M is the oxygen mass and a =0.5, the shift predicted for a 37-K superconductor is approximately 2.1 K. This is an upper limit since it assumes that the frequency of the phonon mode depends only on the mass of oxygen.In order to provide convincing evidence that the shift in T c is due to 18 0 substitution, rather than sample inhomogeneity or other sample-to-sample variations, five samples of Lai.8sSro.isCu04 were isotopically exchanged. These samples were synthesized independently with starting materials obtained from different sources. Isotopic exchange was accomplished by the annealing of each sample in 18 02. The resistance and magnetic susceptibility of these samples were compared with control samples annealed in l6 0 2 under nearly identical conditions.The two best characterized samples of La1.85Sro.15-Cu0 4 , referred to below as samples I and II, were prepared by our dissolving stoichiometric ratios of La 2 03, SrC0 3 , and CuO in concentrated nitric acid, adding oxalic acid, and evaporating to dryness. Sample I was fired in a platinum crucible in air for 14 h at 800 °C with one intermediate grinding. The powder was then reground, pressed into a pellet, and fired for 12 h at 900°C, 2 h at 1100°C, 6 h at 900°C, and f...

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.…”

Unified picture of the oxygen isotope effect in cuprate superconductors

“…Such a difference is also believed to be a result of the electronic structure effect. Currently, La 2Ϫx Sr x CuO 4 is the only monolayer system whose doping dependence of ␣ is well known over the whole doping regime (2,3,9). Note that the experimentally observed ␣ behavior in this system can be reproduced by the present d-wave BCS-like model, except in the region near x ϭ 0.125.…”

“…When hightemperature superconductivity was discovered in copper oxides, the oxygen isotope exponents, defined by ␣ ' ϪdlnT c /dlnM, with M being the isotopic mass, were promptly measured. The initial finding of the small value of ␣ in La 1.85 Sr 0.15 CuO 4 (2,3) and near zero value in YBa 2 Cu 3 O 7Ϫ␦ (␦ ϳ 0) (4-6) was taken to be convincing evidence against electron-phonon interaction as the dominant mechanism in these materials. However, later experiments have revealed a much richer and more complex situation (7).…”

Vibronic pairing mechanism for La,2-xSrxCuO4. The role of local structural distortions

“…Yet large isotope constants seem inherent to strong-coupling theories. The experimental value is about 0.4 (Faltens et al 1987). …”