4,208 research outputs found

    Persistence of Topological Order and Formation of Quantum Well States in Topological Insulators Bi2(Se,Te)3 under Ambient Conditions

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    The topological insulators represent a unique state of matter where the bulk is insulating with an energy gap while the surface is metallic with a Dirac cone protected by the time reversal symmetry. These characteristics provide a venue to explore novel quantum phenomena in fundamental physics and show potential applications in spintronics and quantum computing. One critical issue directly related with the applications as well as the fundamental studies is how the topological surface state will behave under ambient conditions (1 atmosphere air and room temperature). In this paper, we report high resolution angle-resolved photoemission measurements on the surface state of the prototypical topological insulators, Bi2Se3, Bi2Te3 and Bi2(Se0.4Te2.6), upon exposing to ambient conditions. We find that the topological order persists even when the surface is exposed to air at room temperature. However, the surface state is strongly modified after such an exposure. Particularly, we have observed the formation of two-dimensional quantum well states near the surface of the topological insulators after the exposure which depends sensitively on the original composition, x, in Bi2(Se3-xTex). These rich information are crucial in utilizing the surface state and in probing its physical properties under ambient conditions.Comment: 15 Pages, 4 Figure

    Measurement of the branching fractions of psi(2S) -> 3(pi+pi-) and J/psi -> 2(pi+pi-)

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    Using data samples collected at sqrt(s) = 3.686GeV and 3.650GeV by the BESII detector at the BEPC, the branching fraction of psi(2S) -> 3(pi+pi-) is measured to be [4.83 +- 0.38(stat) +- 0.69(syst)] x 10^-4, and the relative branching fraction of J/psi -> 2(pi+pi-) to that of J/psi -> mu+mu- is measured to be [5.86 +- 0.19(stat) +- 0.39(syst)]% via psi(2S) -> (pi+pi-)J/psi, J/psi -> 2(pi+pi-). The electromagnetic form factor of 3(pi+pi-) is determined to be 0.21 +- 0.02 and 0.20 +- 0.01 at sqrt(s) = 3.686GeV and 3.650GeV, respectively.Comment: 17pages, 7 figures, submitted to Phys. Rev.

    First Measurement of the Branching Fraction of the Decay psi(2S) --> tau tau

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    The branching fraction of the psi(2S) decay into tau pair has been measured for the first time using the BES detector at the Beijing Electron-Positron Collider. The result is Bττ=(2.71±0.43±0.55)×103B_{\tau\tau}=(2.71\pm 0.43 \pm 0.55) \times 10^{-3}, where the first error is statistical and the second is systematic. This value, along with those for the branching fractions into e+e- and mu+mu of this resonance, satisfy well the relation predicted by the sequential lepton hypothesis. Combining all these values with the leptonic width of the resonance the total width of the psi(2S) is determined to be (252±37)(252 \pm 37) keV.Comment: 9 pages, 2 figure

    Measurements of the Cross Section for e+e- -> hadrons at Center-of-Mass Energies from 2 to 5 GeV

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    We report values of R=σ(e+ehadrons)/σ(e+eμ+μ)R = \sigma(e^+e^-\to {hadrons})/\sigma(e^+e^-\to\mu^+\mu^-) for 85 center-of-mass energies between 2 and 5 GeV measured with the upgraded Beijing Spectrometer at the Beijing Electron-Positron Collider.Comment: 5 pages, 3 figure

    Measurements of J/psi decays into phi pi^0, phi eta, and phi eta^prime

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    Based on 5.8x10^7 J/psi events detected in BESII, the branching fractions of J/psi--> phi eta and phi eta^prime are measured for different eta and eta^prime decay modes. The results are significantly higher than previous measurements. An upper limit on B(J/psi--> phi pi^0) is also obtained.Comment: 9 pages, 10 figure

    Measurement of the chi_{c2} Polarization in psi(2S) to gamma chi_{c2}

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    The polarization of the chi_{c2} produced in psi(2S) decays into gamma chi_{c2} is measured using a sample of 14*10^6 psi(2S) events collected by BESII at the BEPC. A fit to the chi_{c2} production and decay angular distributions in psi(2S) to gamma chi_{c2}, chi_{c2} to pi pi and KK yields values x=A_1/A_0=2.08+/-0.44 and y=A_2/A_0=3.03 +/-0.66, with a correlation rho=0.92 between them, where A_{0,1,2} are the chi_{c2} helicity amplitudes. The measurement agrees with a pure E1 transition, and M2 and E3 contributions do not differ significantly from zero.Comment: 6 pages, 4 figures, 1 tabl

    Observation of p pbar pi^0 and p pbar eta in psi' decays

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    The processes psi'-->p pbar pi^0 and psi'-->p pbar eta are studied using a sample of 14 million psi' decays collected with the Beijing Spectrometer at the Beijing Electron-Positron Collider. The branching fraction of psi'-->p pbar pi^0 is measured with improved precision as (13.2\pm 1.0\pm 1.5)\times 10^{-5}, and psi'-->p pbar eta is observed for the first time with a branching fraction of (5.8\pm 1.1\pm 0.7)\times 10^{-5}, where the first errors are statistical and the second ones are systematic.Comment: 15 pages, 8 figures and 3 table

    Measurement of \psip Radiative Decays

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    Using 14 million psi(2S) events accumulated at the BESII detector, we report first measurements of branching fractions or upper limits for psi(2S) decays into gamma ppbar, gamma 2(pi^+pi^-), gamma K_s K^-pi^++c.c., gamma K^+ K^- pi^+pi^-, gamma K^{*0} K^- pi^+ +c.c., gamma K^{*0}\bar K^{*0}, gamma pi^+pi^- p pbar, gamma 2(K^+K^-), gamma 3(pi^+pi^-), and gamma 2(pi^+pi^-)K^+K^- with the invariant mass of hadrons below 2.9GeV/c^2. We also report branching fractions of psi(2S) decays into 2(pi^+pi^-) pi^0, omega pi^+pi^-, omega f_2(1270), b_1^\pm pi^\mp, and pi^0 2(pi^+pi^-) K^+K^-.Comment: 5 pages, 4 figure

    Partial Wave Analysis of χc0π+πK+K\chi_{c0}\to\pi^+\pi^-K^+K^-

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    A partial wave analysis of χc0π+πK+K\chi_{c0}\to\pi^+\pi^-K^+K^- in ψ(2S)γχc0\psi(2S)\to\gamma\chi_{c0} decay is presented using a sample of 14 million ψ(2S)\psi(2S) events accumulated by the BES II detector. The data are fitted to the sum of relativistic covariant tensor amplitudes for intermediate resonant decay modes. From the fit, significant contributions to χc0\chi_{c0} decays from the channels f0(980)f0(980)f_0(980)f_0(980), f0(980)f0(2200)f_0(980)f_0(2200), f0(1370)f0(1710)f_0(1370)f_0(1710), K(892)0Kˉ(892)0K^*(892)^0\bar K^*(892)^0, K0(1430)Kˉ0(1430)K^*_0(1430)\bar K^*_0(1430), K0(1430)Kˉ2(1430)+c.c.K^*_0(1430)\bar K^*_2(1430) + c.c., and K1(1270)KK_1(1270)K are found. Flavor-SU(3)-violating K1(1270)K1(1400)K_1(1270)-K_1(1400) asymmetry is observed. Values obtained for the masses and widths of the resonances f0(1710)f_0(1710), f0(2200)f_0(2200), f0(1370)f_0(1370), and K0(1430)K^*_0(1430) are presented.Comment: 16 pages, 9 figures, and 4 table

    Measurements of J/ψJ/\psi and ψ(2S)\psi(2S) decays into ΛΛˉπ0\Lambda \bar{\Lambda}\pi^0 and ΛΛˉη\Lambda \bar{\Lambda}\eta

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    Using 58 million J/ψJ/\psi and 14 million ψ(2S)\psi(2S) events collected by the BESII detector at the BEPC, branching fractions or upper limits for the decays J/ψJ/\psi and ψ(2S)ΛΛˉπ0\psi(2S) \to \Lambda \bar{\Lambda}\pi^0 and ΛΛˉη\Lambda \bar{\Lambda}\eta are measured. For the isospin violating decays, the upper limits are determined to be B(J/ψΛΛˉπ0)<6.4×105{\cal B}(J/\psi \to \Lambda \bar{\Lambda}\pi^0)<6.4\times 10^{-5} and B(ψ(2S)ΛΛˉπ0)<4.9×105{\cal B}(\psi(2S) \to \Lambda \bar{\Lambda}\pi^0)<4.9\times 10^{-5} at the 90% confidence level. The isospin conserving process J/ψΛΛˉηJ/\psi \to \Lambda \bar{\Lambda}\eta is observed for the first time, and its branching fraction is measured to be B(J/ψΛΛˉη)=(2.62±0.60±0.44)×104{\cal B}(J/\psi \to \Lambda \bar{\Lambda}\eta)=(2.62\pm 0.60\pm 0.44)\times 10^{-4}, where the first error is statistical and the second one is systematic. No ΛΛˉη\Lambda \bar{\Lambda}\eta signal is observed in ψ(2S)\psi(2S) decays, and B(ψ(2S)ΛΛˉη)<1.2×104{\cal B}(\psi(2S) \to \Lambda \bar{\Lambda}\eta)<1.2\times 10^{-4} is set at the 90% confidence level. Branching fractions of J/ψJ/\psi decays into Σ+πbarΛ\Sigma^+ \pi^- bar{\Lambda} and Σˉπ+Λ\bar{\Sigma}^- \pi^+ \Lambda are also reported, and the sum of these branching fractions is determined to be B(J/ψΣ+πΛˉ+c.c.)=(1.52±0.08±0.16)×103{\cal B}(J/\psi \to \Sigma^+\pi^- \bar{\Lambda} + c.c.)=(1.52\pm 0.08\pm 0.16)\times 10^{-3}.Comment: 7 pages, 10 figures. Phys.Rev.D comments considere
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