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G. Xu

Publications and source records attributed to G. Xu.

At least 55 records · Page 3Linked to original sources

Two successive field-induced spin-flop transitions in single-crystalline CaCo$_{2}$As$_{2}$

CaCo$_{2}$As$_{2}$, a ThCr$_{2}$Si$_{2}$-structure compound, undergoes an antiferromagnetic transition at \emph{T$_{N}$}=76K with the magnetic moments being aligned parallel to the \emph{c} axis. Electronic transport measurement reveals that the coupling between conducting carriers and magnetic order in CaCo$_{2}$As$_{2}$ is much weaker comparing to the parent compounds of iron pnictide. Applying magnetic field along \emph{c} axis induces two successive spin-flop transitions in its magnetic state. The magnetization saturation behaviors with \emph{\textbf{H}$\parallel$c} and \emph{\textbf{H}$\parallel$ab} at 10K indicate that the antiferromagnetic coupling along \emph{c} direction is very weak. The interlayer antiferromagntic coupling constant \emph{J$_{c}$} is estimated to be about 2 meV.

cond-mat.str-el↗

Effects of Ru Substitution on Dimensionality and Electron Correlations in Ba(Fe_{1-x}Ru_x)_2As_2

We report a systematic angle-resolved photoemission spectroscopy study on Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ for a wide range of Ru concentrations (0.15 $\leq$ \emph{x} $\leq$ 0.74). We observed a crossover from two-dimension to three-dimension for some of the hole-like Fermi surfaces with Ru substitution and a large reduction in the mass renormalization close to optimal doping. These results suggest that isovalent Ru substitution has remarkable effects on the low-energy electron excitations, which are important for the evolution of superconductivity and antiferromagnetism in this system.

cond-mat.supr-con↗

Observation of the rare eta->e+e-e+e- decay with the KLOE experiment

We report the first observation of the rare eta->e+e-e+e- decay based on 1.7 fb^{-1} collected by the KLOE experiment at the DAFNE phi-factory. The selection of the e+e-e+e- final state is fully inclusive of radiation. We have identified 362 +- 29 events resulting in a branching ratio of (2.4 +- 0.2_stat+bckg +- 0.1_syst) x 10^{-5}.

hep-ex↗

Measurement of $Γ(η\to π^+π^-γ)/Γ(η\to π^+π^-π^0)$ with KLOE experiment

We report the measurement of the ratio $Γ(η\to π^+π^-γ)/Γ(η\to π^+π^-π^0)$ analyzing a large sample of $ϕ\to ηγ$ decays recorded with the KLOE experiment at the DA$Φ$NE $e^+ e^-$ collider, corresponding to an integrated luminosity of 558 pb$^{-1}$. The $η\to π^+π^-γ$ process is supposed to proceed both via a resonant contribution, mediated by the $ρ$ meson, and a non resonant direct term, connected to the box anomaly. The presence of the direct term affects the partial width value. Our result $R_η=Γ(η\to π^+ π^- γ)/Γ(η\to π^+ π^- π^0)= 0.1838\pm 0.0005_{stat} \pm 0.0030_{syst}$ is in agreement with a recent CLEO measurement, which differs by more 3 $σ$ from the average of previous results.

hep-ex↗

Infrared spectrum and its implications for the electronic structure of the semiconducting iron selenide K$_{0.83}$Fe$_{1.53}$Se$_2$

We report an infrared spectroscopy study on K$_{0.83}$Fe$_{1.53}$Se$_2$, a semiconducting parent compound of the new iron-selenide system. The major spectral features are found to be distinctly different from all other Fe-based superconducting systems. Our measurement revealed two peculiar spectral structures: a double peak structure between 4000-6000 cm$^{-1}$ and abundant phonon modes much more than those expected for a 122 structure. We elaborate that those features could be naturally explained from the blocked antiferromagnetism due to the presence of Fe vacancy ordering as determined by recent neutron diffraction experiments. The double peaks reflect the coexistence of ferromagnetic and antiferromagnetic couplings between the neighboring Fe sites.

cond-mat.supr-con↗

Absence of holelike Fermi surface in superconducting K0.8Fe1.7Se2 revealed by ARPES

We have performed an angle-resolved photoemission spectroscopy study of the new iron-based superconductor K0.8Fe1.7Se2 (Tc ~ 30 K). Clear band dispersion is observed with the overall bandwidth renormalized by a factor of 2.5 compared to our local density approximation calculations, indicating relatively strong correlation effects. Only an electronlike band crosses the Fermi energy, forming a nearly circular Fermi surface (FS) at M (π, 0). The holelike band at Γ sinks ~ 90 meV below the Fermi energy, with an indirect band gap of 30 meV to the bottom of the electronlike band. The observed FS topology in this superconductor favors (π, π) inter-FS scattering between the electronlike FSs at the M points, in sharp contrast with other iron-based superconductors which favor (π, 0) inter-FS scattering between holelike and electronlike FSs.

cond-mat.supr-con↗

Three dimensionality of band structure and a large residual quasiparticle population in Ba$_{0.67}$K$_{0.33}$Fe$_2$As$_2$ as revealed by the c-axis polarized optical measurement

We report on a c-axis polarized optical measurement on a Ba$_{0.67}$K$_{0.33}$Fe$_2$As$_2$ single crystal. We find that the c-axis optical response is significantly different from that of high-T$_c$ cuprates. The experiments reveal an anisotropic three-dimensional optical response with the absence of the Josephson plasma edge in R($ω$) in the superconducting state. Furthermore, different from the ab-plane optical response, a large residual quasiparticle population down to $T\sim\frac{1}{5}T_c$ was observed in the c-axis polarized reflectance measurement. We elaborate that there exist nodes for the superconducting gap in regions of the 3D Fermi surface that contribute dominantly to the c-axis optical conductivity.

cond-mat.supr-con↗

Fermiology of Sr4V2O6Fe2As2: Quasi-Nested Fe vs Mott-Insulating V Orbitals

We have performed an angle-resolved photoemission spectroscopy study of a new iron-based superconductor Sr4V2O6Fe2As2. While V 3d orbitals are found to be in a Mott insulator state and show an incoherent peak at ~ 1 eV below the Fermi level, the dispersive Fe 3d bands form several hole- and electron-like Fermi surfaces (FSs), some of which are quasi-nested by the (pi, 0) wave vector. This differs from the local density approximation (LDA) calculations, which predict non-nested FSs for this material. However, LDA+U with a large effective Hubbard energy U on V 3d electrons can reproduce the experimental observation reasonably well. The observed fermiology in superconducting Sr4V2O6Fe2As2 strongly supports that (pi, 0) interband scattering between quasi-nested FSs is indispensable to superconductivity in pnictides.

cond-mat.supr-con↗

Electron-Hole Asymmetry in Superconductivity of Pnictides Originated from the Observed Rigid Chemical Potential Shift

We have performed a systematic photoemission study of the chemical potential shift as a function of carrier doping in a pnictide system based on BaFe$_2$As$_2$. The experimentally determined chemical potential shift is consistent with the prediction of a rigid band shift picture by the renormalized first-principle band calculations. This leads to an electron-hole asymmetry (EHA) in the Fermi surface (FS) nesting condition due to different effective masses for different FS sheets, which can be calculated from the Lindhard function of susceptibility. This built-in EHA from the band structure, which matches well with observed asymmetric superconducting domes in the phase diagram, strongly supports FS near-nesting driven superconductivity in the iron pnictides.

cond-mat.supr-con↗

Measurement of the $η\to 3π^{0}$ slope parameter $α$ with the KLOE detector

We present a measurement of the slope parameter $α$ for the $η\to 3π^{0}$ decay, with the KLOE experiment at the DA$Φ$NE $ϕ$-factory, based on a background free sample of $\sim$ 17 millions $η$ mesons produced in $ϕ$ radiative decays. By fitting the event density in the Dalitz plot we determine $α= -0.0301 \pm 0.0035\,stat\;_{-0.0035}^{+0.0022}\,syst\,$. The result is in agreement with recent measurements from hadro- and photo-production experiments.

hep-ex↗

Investigation of the spin-glass regime between the antiferromagnetic and superconducting phases in Fe$_{1+y}$Se$_x$Te$_{1-x}$

Using bulk magnetization along with elastic and inelastic neutron scattering techniques, we have investigated the phase diagram of Fe$_{1+y}$Se$_{x}$Te$_{1-x}$ and the nature of magnetic correlations in three nonsuperconducting samples of Fe$_{1.01}$Se$_{0.1}$Te$_{0.9}$, Fe$_{1.01}$Se$_{0.15}$Te$_{0.85}$ and Fe$_{1.02}$Se$_{0.3}$Te$_{0.7}$. A cusp and hysteresis in the temperature dependence of the magnetization for the $x=0.15$ and 0.3 samples indicates spin-glass (SG) ordering below $T_{\rm sg} = 23$K. Neutron scattering measurements indicate that the spin-glass behavior is associated with short-range spin density wave (SDW) ordering characterized by a static component and a low-energy dynamic component with a characteristic incommensurate wave vector of ${\bf Q}_m = (0.46, 0, 0.50)$ and an anisotropy gap of $\sim$ 2.5 meV. Our high ${\bf Q}$-resolution data also show that the systems undergo a glassy structural distortion that coincides with the short-range SDW order.

cond-mat.supr-con↗

A global fit to determine the pseudoscalar mixing angle and the gluonium content of the eta' meson

We update the values of the eta-eta' mixing angle and of the eta' gluonium content by fitting our measurement R_phi = BR(phi to eta' gamma)/ BR(phi to eta gamma) together with several vector meson radiative decays to pseudoscalars (V to P gamma), pseudoscalar mesons radiative decays to vectors (P to V gamma) and the eta' to gamma gamma, pi^0 to gamma gamma widths. From the fit we extract a gluonium fraction of Z^2_G = 0.12 +- 0.04, the pseudoscalar mixing angle psi_P = (40.4 +- 0.6) degree and the phi-omega mixing angle psi_V = (3.32 +- 0.09) degree. Z^2_G and psi_P are fairly consistent with those previously published. We also evaluate the impact on the eta' gluonium content determination of future experimental improvements of the eta' branching ratios and decay width.

hep-ph↗

Search for the decay phi -> K0 K0bar gamma with the KLOE experiment

We have searched for the decay phi -> K0 K0bar gamma, by detecting K_s pairs plus a photon and with the K_s-mesons decaying to pi^+ pi^-, in a sample of about 1.5x 10^9 phi-decays collected by the KLOE experiment at DAFNE. The reaction proceeds through the intermediate states f_0(980) gamma, a_0(980) gamma. We find five events with 3.2 events expected from background processes. We obtain the upper limit: BR (phi -> K0 K0bar gamma) < 1.9x10^-8 at 90% C.L. .

hep-ex↗

Test of a LYSO matrix with an electron beam

The angular coverage extension of the KLOE-2 electromagnetic calorimeter, from a polar angle of 20$^{\circ}$ down to $8^{\circ}$, will increase the multiphoton detection capability of the experiment enhancing the search reach for rare kaon, $η$ and $η'$ prompt decay channels. The basic layout of the calorimeter extension consists of two small barrels of LYSO crystals readout with APD photosensors aiming to achieve a timing resolution between 300 and 500 ps for 20 MeV photons. The first test of a (5.5$\times6\times$13) cm$^3$ prototype for such a detector was carried out in april 2009 at the Beam Test Facility of Laboratori Nazionali di Frascati of INFN with an electron beam from 100 to 500 MeV. In the selected energy range, we measured a light yield of 500$÷$800 p.e./Mev, an energy resolution which can be parametrized as $0.05 \oplus 0.01/({\rm E/GeV}) \oplus 0.015/\sqrt{\rm{E/GeV}}$, a position resolution of 2.8 mm and a timing resolution of 200$÷$300 ps.

physics.ins-det↗

Gutzwiller density functional calculations of the electronic structure of FeAs-based superconductors: Evidence for a three-dimensional Fermi surface

The electronic structures of FeAs-compounds strongly depend on the Fe-As bonding, which can not be described successfully by the local density approximation (LDA). Treating the multi-orbital fluctuations from $ab$-$initio$ by LDA+Gutzwiller method, we are now able to predict the correct Fe-As bond-length, and find that Fe-As bonding-strength is 30% weaker, which will explain the observed "soft phonon". The bands are narrowed by a factor of 2, and the $d_{3z^2-r^2}$ orbital is pushed up to cross the Fermi level, forming 3-dimensional Fermi surfaces, which suppress the anisotropy and the ($π,π$) nesting. The inter-orbital Hund's coupling $J$ rather than $U$ plays crucial roles to obtain these results.

cond-mat.supr-con↗

Origin of the structural phase transition at 130 K in BaNi$_2$As$_2$: a combined study of optical spectroscopy and band structure calculations

BaNi$_2$As$_2$ exhibits a first order structural transition at 130 K. Understanding this structural transition is a crucial step towards understanding the electronic properties of the material. We present a combined optical spectroscopy and band structure calculation study on the compound across the transition. The study reveals that BaNi$_2$As$_2$ is a good metal with a rather high plasma frequency. The phase transition leads to a small reduction of conducting carriers. We identify that this reduction is caused by the removal of several small Fermi surface sheets contributed dominantly from the As-As bonding and Ni-As antibonding states.

cond-mat.supr-con↗

Measurement of the branching ratio and search for a CP violating asymmetry in the eta -> pi+pi-e+e-(gamma) decay at KLOE

We have studied the eta->pi+pi-e+e-(gamma) decay using about 1.7 fb^-1 collected by the KLOE experiment at the DAFNE phi-factory. This corresponds to about 72 millions eta mesons produced in phi radiative decays. We have measured the branching ratio, inclusive of radiative effects, with 4% accuracy: BR(eta->pi+pi-e+e-(gamma)) = (26.8 +/- 0.9_Stat. +/- 0.7_Syst.) x 10^-5. We have obtained the first measurement of the CP-odd pipi-ee decay planes angular asymmetry, A_phi = (-0.6 +/- 2.5_Stat. +/- 1.8_Syst.) x 10^-2.

hep-ex↗

Study of the a_0(980) meson via the radiative decay phi->eta pi^0 gamma with the KLOE detector

We have studied the phi->a_0(980) gamma process with the KLOE detector at the Frascati phi-factory DAPhNE by detecting the phi->eta pi^0 gamma decays in the final states with eta->gamma gamma and eta->pi^+ pi^- pi^0. We have measured the branching ratios for both final states: Br(phi->eta pi^0 gamma)=(7.01 +/- 0.10 +/- 0.20)x10^-5 and (7.12 +/- 0.13 +/- 0.22)x10^-5 respectively. We have also extracted the a_0(980) mass and its couplings to eta pi^0, K^+ K^-, and to the phi meson from the fit of the eta pi^0 invariant mass distributions using different phenomenological models.

hep-ex↗