Pressure Effect on the superconducting properties of LaO_{1-x}F_{x}FeAs(x=0.11) superconductor

Diamagnetic susceptibility measurements under high hydrostatic pressure (up to 1.03 GPa) were carried out on the newly discovered Fe-based superconductor LaO_{1-x}F_{x}FeAs(x=0.11). The transition temperature T_c, defined as the point at the maximum slope of superconducting transition, was enhanced almost linearly by hydrostatic pressure, yielding a dT_c/dP of about 1.2 K/GPa. Differential diamagnetic susceptibility curves indicate that the underlying superconducting state is complicated. It is suggested that pressure plays an important role on pushing low T_c superconducting phase toward the main (optimal) superconducting phase.Comment: 7 pages, 4 figure

Nernst effect of the new iron-based superconductor LaO1−x_{1-x}Fx_{x}FeAs

We report the first Nernst effect measurement on the new iron-based superconductor LaO$_{1-x}$F$_{x}$FeAs $(x=0.1)$. In the normal state, the Nernst signal is negative and very small. Below $T_{c}$ a large positive peak caused by vortex motion is observed. The flux flowing regime is quite large compared to conventional type-II superconductors. However, a clear deviation of the Nernst signal from normal state background and an anomalous depression of off-diagonal thermoelectric current in the normal state between $T_{c}$ and 50 K are observed. We propose that this anomaly in the normal state Nernst effect could correlate with the SDW fluctuations.Comment: 8 pages, 4 figures; Latex file changed, references adde

Renormalized mean-field theory of the neutron scattering in cuprate superconductors

The magnetic excitation spectrum of the t-t'-J-model is studied in mean-field theory and compared to inelastic neutron-scattering (INS) experiments on YBCO and BSCCO superconductors. Within the slave-particle formulation the dynamical spin response is calculated from a renormalized Fermi liquid with an effective interaction ~J in the magnetic particle--hole channel. We obtain the so-called 41meV resonance at wave vector (pi,pi) as a collective spin-1 excitation in the d-wave superconducting state. It appears sharp (undamped), if the underlying Fermi surface is hole-like with a sufficient next-nearest-neighbor hopping t'<0. The double-layer structure of YBCO or BSCCO is not important for the resonance to form. The resonance energy \omega_{res} and spectral weight at optimal doping come out comparable to experiment. The observed qualitative behavior of \omega_{res} with hole filling is reproduced in the underdoped as well as overdoped regime. A second, much broader peak becomes visible in the magnetic excitation spectrum if the 2D wave-vector is integrated over. It is caused by excitations across the maximum gap, and in contrast to the resonance its energy is almost independent of doping. At energies above or below \omega_{res} the commensurate resonance splits into incommensurate peaks, located off (pi,pi). Below \omega_{res} the intensity pattern is of `parallel' type and the dispersion relation of incommensurate peaks has a negative curvature. This is in accordance with recent INS experiments on YBCO.Comment: 17pp including 14 figure

Measurement of proton electromagnetic form factors in e+e−→ppˉe^+e^- \to p\bar{p} in the energy region 2.00-3.08 GeV

The process of $e^+e^- \rightarrow p\bar{p}$ is studied at 22 center-of-mass energy points ($\sqrt{s}$) from 2.00 to 3.08 GeV, exploiting 688.5~pb$^{-1}$ of data collected with the BESIII detector operating at the BEPCII collider. The Born cross section~($\sigma_{p\bar{p}}$) of $e^+e^- \rightarrow p\bar{p}$ is measured with the energy-scan technique and it is found to be consistent with previously published data, but with much improved accuracy. In addition, the electromagnetic form-factor ratio ($|G_{E}/G_{M}|$) and the value of the effective ($|G_{\rm{eff}}|$), electric ($|G_E|$) and magnetic ($|G_M|$) form factors are measured by studying the helicity angle of the proton at 16 center-of-mass energy points. $|G_{E}/G_{M}|$ and $|G_M|$ are determined with high accuracy, providing uncertainties comparable to data in the space-like region, and $|G_E|$ is measured for the first time. We reach unprecedented accuracy, and precision results in the time-like region provide information to improve our understanding of the proton inner structure and to test theoretical models which depend on non-perturbative Quantum Chromodynamics

A Generic Two-band Model for Unconventional Superconductivity and Spin-Density-Wave Order in Electron and Hole Doped Iron-Based Superconductors

Based on experimental data on the newly synthesized iron-based superconductors and the relevant band structure calculations, we propose a minimal two-band BCS-type Hamiltonian with the interband Hubbard interaction included. We illustrate that this two-band model is able to capture the essential features of unconventional superconductivity and spin density wave (SDW) ordering in this family of materials. It is found that bound electron-hole pairs can be condensed to reveal the SDW ordering for zero and very small doping, while the superconducting ordering emerges at small finite doping, whose pairing symmetry is qualitatively analyzed to be of nodal d-wave. The derived analytical formulas not only give out a nearly symmetric phase diagram for electron and hole doping, but also is likely able to account for existing main experimental results. Moreover, we also derive two important relations for a general two-band model and elaborate how to apply them to determine the band width ratio and the effective interband coupling strength from experimental data.Comment: 6 pages, 4 figures, refs. added, typos correcte

Study of J/ψ→ppˉJ/\psi\to p\bar{p} and J/ψ→nnˉJ/\psi\to n\bar{n}

The decays $J/\psi\to p\bar{p}$ and $J/\psi\to n\bar{n}$ have been investigated with a sample of 225.2 million $J/\psi$ events collected with the BESIII detector at the BEPCII $e^+e^-$ collider. The branching fractions are determined to be $\mathcal{B}(J/\psi\to p\bar{p})=(2.112\pm0.004\pm0.031)\times10^{-3}$ and $\mathcal{B}(J/\psi\to n\bar{n})=(2.07\pm0.01\pm0.17)\times10^{-3}$. Distributions of the angle $\theta$ between the proton or anti-neutron and the beam direction are well described by the form $1+\alpha\cos^2\theta$, and we find $\alpha=0.595\pm0.012\pm0.015$ for $J/\psi\to p\bar{p}$ and $\alpha=0.50\pm0.04\pm0.21$ for $J/\psi\to n\bar{n}$. Our branching-fraction results suggest a large phase angle between the strong and electromagnetic amplitudes describing the $J/\psi\to N\bar{N}$ decay.Comment: 16 pages, 13 figures, the 2nd version, submitted to PR

Search for Baryonic Decays of \psi(3770) and \psi(4040)

By analyzing data samples of 2.9 fb^{-1} collected at \sqrt s=3.773 GeV, 482 pb^{-1} collected at \sqrt s=4.009 GeV and 67 pb^{-1} collected at \sqrt s=3.542, 3.554, 3.561, 3.600 and 3.650 GeV with the BESIII detector at the BEPCII storage ring, we search for \psi(3770) and \psi(4040) decay to baryonic final states, including \Lambda\bar\Lambda\pi^+\pi^-, \Lambda \bar\Lambda\pi^0, \Lambda\bar\Lambda\eta, \Sigma^+ \bar\Sigma^-, \Sigma^0 \bar\Sigma^0, \Xi^-\bar\Xi^+ and \Xi^0\bar\Xi^0 decays. None are observed, and upper limits are set at the 90% confidence level.Comment: 9 pages, 3 figure

Pressure-induced magnetic transition and volume collapse in FeAs superconductors: An orbital-selective Mott scenario

Motivated by pressure experiments on FeAs-122 superconductors, we propose a scenario based on local-moment physics to explain the simultaneous disappearance of magnetism, reduction of the unit cell volume, and decrease in resistivity. In this scenario, the low-pressure magnetic phase derives from Fe moments, which become screened in the paramagnetic high-pressure phase. The quantum phase transition can be described as an orbital-selective Mott transition, which is rendered first order by coupling to the lattice, in analogy to a Kondo volume collapse. Spin-fluctuation driven superconductivity competes with antiferromagnetism and may be stabilized at low temperatures in the high-pressure phase. The ideas are illustrated by a suitable mean-field analysis of an Anderson lattice model.Comment: 9 pages, 3 figs; (v2) robustness of OS Mott transition vs. fragility of superconductivity discussed, final version to be publishe

Observation of χc1\chi_{c1} decays into vector meson pairs ϕϕ\phi\phi, ωω\omega\omega, and ωϕ\omega\phi

Decays of $\chi_{c1}$ to vector meson pairs $\phi\phi$, $\omega\omega$ and $\omega\phi$ are observed for the first time using $(106\pm4)\times 10^6$ \psip events accumulated at the BESIII detector at the BEPCII $e^+e^-$ collider. The branching fractions are measured to be $(4.4\pm 0.3\pm 0.5)\times 10^{-4}$, $(6.0\pm 0.3\pm 0.7)\times 10^{-4}$, and $(2.2\pm 0.6\pm 0.2)\times 10^{-5}$, for $\chi_{c1}\to \phi\phi$, $\omega\omega$, and $\omega\phi$, respectively. The observation of $\chi_{c1}$ decays into a pair of vector mesons $\phi\phi$, $\omega\omega$ and $\omega\phi$ indicates that the hadron helicity selection rule is significantly violated in $\chi_{cJ}$ decays. In addition, the measurement of $\chi_{cJ}\to \omega\phi$ gives the rate of doubly OZI-suppressed decay. Branching fractions for $\chi_{c0}$ and $\chi_{c2}$ decays into other vector meson pairs are also measured with improved precision.Comment: 4 pages, 2 figure

First observation of the M1 transition ψ(3686)→γηc(2S)\psi(3686)\to \gamma\eta_c(2S)

Using a sample of 106 million \psi(3686) events collected with the BESIII detector at the BEPCII storage ring, we have made the first measurement of the M1 transition between the radially excited charmonium S-wave spin-triplet and the radially excited S-wave spin-singlet states: \psi(3686)\to\gamma\eta_c(2S). Analyses of the processes \psi(2S)\to \gamma\eta_c(2S) with \eta_c(2S)\to \K_S^0 K\pi and K^+K^-\pi^0 gave an \eta_c(2S) signal with a statistical significance of greater than 10 standard deviations under a wide range of assumptions about the signal and background properties. The data are used to obtain measurements of the \eta_c(2S) mass (M(\eta_c(2S))=3637.6\pm 2.9_\mathrm{stat}\pm 1.6_\mathrm{sys} MeV/c^2), width (\Gamma(\eta_c(2S))=16.9\pm 6.4_\mathrm{stat}\pm 4.8_\mathrm{sys} MeV), and the product branching fraction (\BR(\psi(3686)\to \gamma\eta_c(2S))\times \BR(\eta_c(2S)\to K\bar K\pi) = (1.30\pm 0.20_\mathrm{stat}\pm 0.30_\mathrm{sys})\times 10^{-5}). Combining our result with a BaBar measurement of \BR(\eta_c(2S)\to K\bar K \pi), we find the branching fraction of the M1 transition to be \BR(\psi(3686)\to\gamma\eta_c(2S)) = (6.8\pm 1.1_\mathrm{stat}\pm 4.5_\mathrm{sys})\times 10^{-4}.Comment: 7 pages, 1 figure, 1 tabl