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    BES Recent Results and Future Plans

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    We report the preliminary R values for all the 85 energy points scanned in the energy region of 2-5 GeV with the upgraded Beijing Spectrometer (BESII) at Beijing Electron Positron Collider (BEPC). Preliminary results from the J/psi data collected with both BESI and BESII are presented. Measurements of the branching fraction of the psi(2S) decays and the psi(2S) resonance parameters are reported. The future plans, i.e. significantly upgrade the machine and detector are also discussed.Comment: Talk given at APPAC2000, 6 pages, 8 figure

    Evidence for very strong electron-phonon coupling in YBa_{2}Cu_{3}O_{6}

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    From the observed oxygen-isotope shift of the mid-infrared two-magnon absorption peak of YBa2_{2}Cu3_{3}O6_{6}, we evaluate the oxygen-isotope effect on the in-plane antiferromagnetic exchange energy JJ. The exchange energy JJ in YBa2_{2}Cu3_{3}O6_{6} is found to decrease by about 0.9% upon replacing 16^{16}O by 18^{18}O, which is slightly larger than that (0.6%) in La2_{2}CuO4_{4}. From the oxygen-isotope effects, we determine the lower limit of the polaron binding energy, which is about 1.7 eV for YBa2_{2}Cu3_{3}O6_{6} and 1.5 eV for La2_{2}CuO4_{4}, in quantitative agreement with angle-resolved photoemission data, optical conductivity data, and the parameter-free theoretical estimate. The large polaron binding energies in the insulating parent compounds suggest that electron-phonon coupling should also be strong in doped superconducting cuprates and may play an essential role in high-temperature superconductivity.Comment: 4 pages, 1 figur

    Oxygen-isotope effect on the in-plane penetration depth in underdoped Y_{1-x}Pr_xBa_2Cu_3O_{7-delta} as revealed by muon-spin rotation

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    The oxygen-isotope (^16O/^18O) effect (OIE) on the in-plane penetration depth λab(0)\lambda_{ab} (0) in underdoped Y_{1-x}Pr_xBa_2Cu_3O_{7-delta} was studied by muon-spin rotation. A pronounced OIE on λab−2(0)\lambda_{ab}^{-2}(0) was observed with a relative isotope shift of Δλab−2/λab−2\Delta\lambda^{-2}_{ab}/\lambda^{-2}_{ab}=-5(2)% for x =0.3 and -9(2)% for x=0.4. It arises mainly from the oxygen-mass dependence of the in-plane effective mass mab∗m_{ab}^{\ast}. The OIE exponents of T_{c} and of λab−2(0)\lambda_{ab}^{-2}(0) exhibit a relation that appears to be generic for cuprate superconductors.Comment: 4 pages, 4 eps figures, RevTex
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