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-

The electrical properties of polyaniline changes by orders of magnitude upon exposure to analytes such as acids or bases, making it a useful material for detection of these analytes in the gas phase. The objectives of this lab are to synthesize different diameter polyaniline nanofibers and compare them as sensor materials. In this experiment polyaniline nanofibers are synthesized using a two-phase interfacial polymerization method that yields nanofibers with relatively narrow diameter distributions centered around 30, 50, and 120 nm. The sensors are then fabricated by drop-casting aqueous dispersions of nanofibers onto electrode arrays to form films and measuring their change in resistance upon exposure to acids or bases. The sensor response is dependent on the surface area, diameter, and porosity of the nanofiber films. The larger diameter nanofibers have slower response times because of the difficulty for gas to diffuse through more material. The advantages to this lab include simplicity and low cost, making it suitable for both high school and college students, particularly in departments with modest means.

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...

Composite aerogels (with varying concentrations of silica and poly-dimethylsiloxane) were developed and their acoustic absorption coefficient as a function of composition and average pores size have been measured. The polydimethylsiloxane modified the ceramic structure of the silica aerogels, decreasing the material's rigidity while maintaining the high porosity of the aerogel structure. The composite aerogels were found to exhibit different modes of acoustic absorption than that of typical porous absorbers such as fiberglass. At some frequencies, the composite aerogels had 40% higher absorption than that of commercial fiberglass. Physical data show that these materials have a large surface area (> 400 m2/g) and varying pore sizes (d ~ 5 -20 nm).