2,791 research outputs found

    Impurity states in antiferromagnetic Iron Arsenides

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    We explore theoretically impurity states in the antiferromagnetic spin-density wave state of the iron arsenide. Two types of impurity models are employed: one has only the intraband scattering while the other has both the intraband and interband scattering with the equal strength. Interestingly, the impurity bound state is revealed around the impurity site in the energy gap for both models. However, the impurity state is doubly degenerate with respect to spin for the first case; while the single impurity state is observed in either the spin-up or spin-down channel for the second one. The impurity-induced variations of the local density of states are also examined.Comment: 4 pages, 2 figure

    Effect of anisotropy on the ground-state magnetic ordering of the spin-one quantum J1XXZJ_{1}^{XXZ}--J2XXZJ_{2}^{XXZ} model on the square lattice

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    We study the zero-temperature phase diagram of the J1XXZJ_{1}^{XXZ}--J2XXZJ_{2}^{XXZ} Heisenberg model for spin-1 particles on an infinite square lattice interacting via nearest-neighbour (J11J_1 \equiv 1) and next-nearest-neighbour (J2>0J_2 > 0) bonds. Both bonds have the same XXZXXZ-type anisotropy in spin space. The effects on the quasiclassical N\'{e}el-ordered and collinear stripe-ordered states of varying the anisotropy parameter Δ\Delta is investigated using the coupled cluster method carried out to high orders. By contrast with the spin-1/2 case studied previously, we predict no intermediate disordered phase between the N\'{e}el and collinear stripe phases, for any value of the frustration J2/J1J_2/J_1, for either the zz-aligned (Δ>1\Delta > 1) or xyxy-planar-aligned (0Δ<10 \leq \Delta < 1) states. The quantum phase transition is determined to be first-order for all values of J2/J1J_2/J_1 and Δ\Delta. The position of the phase boundary J2c(Δ)J_{2}^{c}(\Delta) is determined accurately. It is observed to deviate most from its classical position J2c=1/2J_2^c = {1/2} (for all values of Δ>0\Delta > 0) at the Heisenberg isotropic point (Δ=1\Delta = 1), where J2c(1)=0.55±0.01J_{2}^{c}(1) = 0.55 \pm 0.01. By contrast, at the XY isotropic point (Δ=0\Delta = 0), we find J2c(0)=0.50±0.01J_{2}^{c}(0) = 0.50 \pm 0.01. In the Ising limit (Δ\Delta \to \infty) J2c0.5J_2^c \to 0.5 as expected.Comment: 20 pages, 5 figure

    Doping evolution of itinerant magnetic excitations in Fe-based oxypnictides

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    Employing the four-band tight-binding model we study theoretically the doping dependence of the spin response in the normal state of novel Fe-based pnictide superconductors. We show that the commensurate spin density wave (SDW) transition that arises due to interband scattering between the hole α\alpha-pockets and the electron β\beta-pockets disappears already at the doping concentration x0.04x \approx 0.04 reflecting the evolution of the Fermi surfaces. Correspondingly, with further increase of the doping the antiferromagnetic fluctuations are suppressed for x>0.1x > 0.1 and the Imχ(QAFM,ω)\chi({\bf Q_{AFM}},\omega) becomes nearly temperature independent. At the same time, we observe that the uniform susceptibility deviates from the Pauli-like behavior and is increasing with increasing temperature reflecting the activation processes for the α\alpha-Fermi surfaces up to temperatures of about T=800K. With increase of the doping the absolute value of the uniform susceptibility lowers and its temperature dependence changes. In particular, it is a constant at low temperatures and then decreases with increasing temperature. We discuss our results in a context of recent experimental data.Comment: 5 pages, accepted in EP

    Effect of the tetrahedral distortion on the electronic properties of iron-pnictides

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    We study the dependence of the electronic structure of iron pnictides on the angle formed by the arsenic-iron bonds. Within a Slater-Koster tight binding model which captures the correct symmetry properties of the bands, we show that the density of states and the band structure are sensitive to the distortion of the tetrahedral environment of the iron atoms. This sensitivity is extremely strong in a two-orbital (d_xz, d_yz) model due to the formation of a flat band around the Fermi level. Inclusion of the d_xy orbital destroys the flat band while keeping a considerable angle dependence in the band structure.Comment: 5 pages, including 5 figures. Fig. 5 replaced. Minor changes in the tex

    Raman spectra in iron-based quaternary CeO1x_{1-x}Fx_xFeAs and LaO1x_{1-x}Fx_xFeAs

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    Raman spectra have been measured on iron-based quaternary CeO1x_{1-x}Fx_xFeAs and LaO1x_{1-x}Fx_xFeAs with varying fluorine doping at room temperatures. A group analysis has been made to clarify the optical modes. Based on the first principle calculations, the observed phonon modes can be assigned accordingly. In LaO1x_{1-x}Fx_xFeAs, the Eg_g and A1g_{1g} modes related to the vibrations of La, are suppressed with increasing F doping. However F doping only has a small effect on the Eg_g and A1g_{1g} modes of Fe and As. The Raman modes of La and As are absent in rare-earth substituted CeO1x_{1-x}Fx_xFeAs, and the Eg_g mode of oxygen, corresponding to the in-plane vibration of oxygen, moves to around 450 cm1^{-1} and shows a very sharp peak. Electronic scattering background is low and electron-phonon coupling is not evident for the observed phonon modes. Three features are found above 500 cm1^{-1}, which may be associated with multi-phonon process. Nevertheless it is also possible that they are related to magnetic fluctuations or interband transitions of d orbitals considering their energies.Comment: 12 pages, 3 figures, 2 table

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

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    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 electron-antineutrino disappearance at Daya Bay

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    The Daya Bay Reactor Neutrino Experiment has measured a non-zero value for the neutrino mixing angle θ13\theta_{13} with a significance of 5.2 standard deviations. Antineutrinos from six 2.9 GWth_{\rm th} reactors were detected in six antineutrino detectors deployed in two near (flux-weighted baseline 470 m and 576 m) and one far (1648 m) underground experimental halls. With a 43,000 ton-GW_{\rm th}-day livetime exposure in 55 days, 10416 (80376) electron antineutrino candidates were detected at the far hall (near halls). The ratio of the observed to expected number of antineutrinos at the far hall is R=0.940±0.011(stat)±0.004(syst)R=0.940\pm 0.011({\rm stat}) \pm 0.004({\rm syst}). A rate-only analysis finds sin22θ13=0.092±0.016(stat)±0.005(syst)\sin^22\theta_{13}=0.092\pm 0.016({\rm stat})\pm0.005({\rm syst}) in a three-neutrino framework.Comment: 5 figures. Version to appear in Phys. Rev. Let

    Nuclear magnetic relaxation and superfluid density in Fe-pnictide superconductors: An anisotropic \pm s-wave scenario

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    We discuss the nuclear magnetic relaxation rate and the superfluid density with the use of the effective five-band model by Kuroki et al. [Phys. Rev. Lett. 101, 087004 (2008)] in Fe-based superconductors. We show that a fully-gapped anisotropic \pm s-wave superconductivity consistently explains experimental observations. In our phenomenological model, the gaps are assumed to be anisotropic on the electron-like \beta Fermi surfaces around the M point, where the maximum of the anisotropic gap is about four times larger than the minimum.Comment: 10 pages, 8 figures; Submitted versio
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