254 research outputs found

    Relativistic J-matrix method

    Get PDF
    The relativistic version of the J-matrix method for a scattering problem on the potential vanishing faster than the Coulomb one is formulated. As in the non-relativistic case it leads to a finite algebraic eigenvalue problem. The derived expression for the tangent of phase shift is simply related to the non-relativistic case formula and gives the latter as a limit case. It is due to the fact that the used basis set satisfies the ``kinetic balance condition''.Comment: 21 pages, RevTeX, accepted for publication in Phys. Rev.

    Universal magnetic and structural behaviors in the iron arsenides

    Full text link
    Commonalities among the order parameters of the ubiquitous antiferromagnetism present in the parent compounds of the iron arsenide high temperature superconductors are explored. Additionally, comparison is made between the well established two-dimensional Heisenberg-Ising magnet, K2_2NiF4_4 and iron arsenide systems residing at a critical point whose structural and magnetic phase transitions coincide. In particular, analysis is presented regarding two distinct classes of phase transition behavior reflected in the development of antiferromagnetic and structural order in the three main classes of iron arsenide superconductors. Two distinct universality classes are mirrored in their magnetic phase transitions which empirically are determined by the proximity of the coupled structural and magnetic phase transitions in these materials.Comment: 6 pages, 4 figure

    NMR evidence for Friedel-like oscillations in the CuO chains of ortho-II YBa2_2Cu3_3O6.5_{6.5}

    Full text link
    Nuclear magnetic resonance (NMR) measurements of CuO chains of detwinned Ortho-II YBa2_2Cu3_3O6.5_{6.5} (YBCO6.5) single crystals reveal unusual and remarkable properties. The chain Cu resonance broadens significantly, but gradually, on cooling from room temperature. The lineshape and its temperature dependence are substantially different from that of a conventional spin/charge density wave (S/CDW) phase transition. Instead, the line broadening is attributed to small amplitude static spin and charge density oscillations with spatially varying amplitudes connected with the ends of the finite length chains. The influence of this CuO chain phenomenon is also clearly manifested in the plane Cu NMR.Comment: 4 pages, 3 figures, refereed articl

    Electron doping evolution of the magnetic excitations in BaFe2-xNixAs2

    Full text link
    We use inelastic neutron scattering (INS) spectroscopy to study the magnetic excitations spectra throughout the Brioullion zone in electron-doped iron pnictide superconductors BaFe2x_{2-x}Nix_{x}As2_{2} with x=0.096,0.15,0.18x=0.096,0.15,0.18. While the x=0.096x=0.096 sample is near optimal superconductivity with Tc=20T_c=20 K and has coexisting static incommensurate magnetic order, the x=0.15,0.18x=0.15,0.18 samples are electron-overdoped with reduced TcT_c of 14 K and 8 K, respectively, and have no static antiferromagnetic (AF) order. In previous INS work on undoped (x=0x=0) and electron optimally doped (x=0.1x=0.1) samples, the effect of electron-doping was found to modify spin waves in the parent compound BaFe2_2As2_2 below \sim100 meV and induce a neutron spin resonance at the commensurate AF ordering wave vector that couples with superconductivity. While the new data collected on the x=0.096x=0.096 sample confirms the overall features of the earlier work, our careful temperature dependent study of the resonance reveals that the resonance suddenly changes its QQ-width below TcT_c similar to that of the optimally hole-doped iron pnictides Ba0.67_{0.67}K0.33_{0.33}Fe2_2As2_2. In addition, we establish the dispersion of the resonance and find it to change from commensurate to transversely incommensurate with increasing energy. Upon further electron-doping to overdoped iron pnictides with x=0.15x=0.15 and 0.18, the resonance becomes weaker and transversely incommensurate at all energies, while spin excitations above \sim100 meV are still not much affected. Our absolute spin excitation intensity measurements throughout the Brillouin zone for x=0.096,0.15,0.18x=0.096,0.15,0.18 confirm the notion that the low-energy spin excitation coupling with itinerant electron is important for superconductivity in these materials, even though the high-energy spin excitations are weakly doping dependent.Comment: 16 pages, 16 figure

    Antiferromagnetic Critical Fluctuations in BaFe2_2As2_2

    Full text link
    Magnetic correlations near the magneto-structural phase transition in the bilayer iron pnictide parent compound, BaFe2_2As2_2, are measured. In close proximity to the antiferromagnetic phase transition in BaFe2_2As2_2, a crossover to three dimensional critical behavior is anticipated and has been preliminarily observed. Here we report complementary measurements of two-dimensional magnetic fluctuations over a broad temperature range about TN_N. The potential role of two-dimensional critical fluctuations in the magnetic phase behavior of BaFe2_2As2_2 and their evolution near the anticipated crossover to three dimensional critical behavior and long-range order are discussed.Comment: 6 pages, 4 figures; Accepted for publication in Physical Review

    Magnetic structure of the antiferromagnetic half-Heusler compound NdBiPt

    Full text link
    We present results of single crystal neutron diffraction experiments on the rare-earth, half-Heusler antiferromagnet (AFM) NdBiPt. This compound exhibits an AFM phase transition at TN=2.18T_{\mathrm N}=2.18~K with an ordered moment of 1.78(9)1.78(9)~μB\mu_{\mathrm B} per Nd atom. The magnetic moments are aligned along the [001][001]-direction, arranged in a type-I AFM structure with ferromagnetic planes, alternating antiferromagnetically along a propagation vector τ\tau of (100)(100). The RRBiPt (RR= Ce-Lu) family of materials has been proposed as candidates of a new family of antiferromagnetic topological insulators (AFTI) with magnetic space group that corresponds to a type-II AFM structure where ferromagnetic sheets are stacked along the space diagonal. The resolved structure makes it unlikely, that NdBiPt qualifies as an AFTI.Comment: As resubmitted to PRB, corrected typos and changed symbols in Fig.

    Magnetic properties of LaFe1-xCrxO3 and Fe2-2xCr2xO3 mixed oxides

    Get PDF
    Mixed oxides with formula LaFe1-xCrxO3 and Fe2-2xCr2xO3, where 0≤x≤1, are studied. The samples have been prepared using solid state reaction technique in air. The X-ray diffraction spectra indicated that the samples crystallize in a corundum phase with space group ( R3c ) for Fe2-2xCr2xO3 and in the perovskite structure for LaFe1-xCrxO3. Many techniques have been used to explore the magnetic properties of the systems. High field, ZFC and FC magnetization vs. temperature, d.c. susceptibility and Mössbauer spectroscopy were carried out. High temperature magnetic susceptibility measurements and high field magnetic magnetization (H ≤ 20 T) show that the behavior of the susceptibility and the magnetization are complex. Mössbauer spectra of the solid solutions have been measured at 4.2 K and in the temperature range 77 K to 300 K. The shapes of spectra are unusual, showing strong relaxation phenomena in a wide temperature range as recently observed for many frustrated systems. The results are discussed by establishing the existence of various magnetic structures, inducing intermediate magnetic phases between the antiferromagnetic and the paramagnetic states. Preliminary magnetic phase diagrams of the systems have been established.Mixed oxides with formula LaFe1-xCrxO3 and Fe2-2xCr2xO3, where 0≤x≤1, are studied. The samples have been prepared using solid state reaction technique in air. The X-ray diffraction spectra indicated that the samples crystallize in a corundum phase with space group ( R3c ) for Fe2-2xCr2xO3 and in the perovskite structure for LaFe1-xCrxO3. Many techniques have been used to explore the magnetic properties of the systems. High field, ZFC and FC magnetization vs. temperature, d.c. susceptibility and Mössbauer spectroscopy were carried out. High temperature magnetic susceptibility measurements and high field magnetic magnetization (H ≤ 20 T) show that the behavior of the susceptibility and the magnetization are complex. Mössbauer spectra of the solid solutions have been measured at 4.2 K and in the temperature range 77 K to 300 K. The shapes of spectra are unusual, showing strong relaxation phenomena in a wide temperature range as recently observed for many frustrated systems. The results are discussed by establishing the existence of various magnetic structures, inducing intermediate magnetic phases between the antiferromagnetic and the paramagnetic states. Preliminary magnetic phase diagrams of the systems have been established

    J-matrix method of scattering in any L2 basis

    Full text link
    The restriction imposed on the J-matrix method of using specific L2 bases is lifted without compromising any of the advantages that it offers. This opens the door to a wider range of application of the method to physical problems beyond the restrictive SO(2,1) dynamical symmetry. The numerical scheme developed to achieve this objective projects the J-matrix formalism in terms of the eigenvalues of a finite Hamiltonian matrix and its submatrices in any convenient L2 basis. Numerical stability and convergence of the original analytic J-matrix method is still maintained in the proposed scheme, which can be applied to multi-channel nonrelativistic as well as relativistic scattering problems

    The nature of the magnetic and structural phase transitions in BaFe2_{2}As2_{2}

    Full text link
    We present the results of an investigation of both the magnetic and structural phase transitions in a high quality single crystalline sample of the undoped, iron pnictide compound BaFe2_2As2_2. Both phase transitions are characterized via neutron diffraction measurements which reveal simultaneous, continuous magnetic and structural orderings with no evidence of hysteresis, consistent with a single second order phase transition. The onset of long-range antiferromagnetic order can be described by a simple power law dependence ϕ(T)2(1TTN)2β\phi(T)^2\propto(1-\frac{T}{T_N})^{2\beta} with β=0.103±0.018\beta=0.103\pm0.018; a value near the β=0.125\beta=0.125 expected for a two-dimensional Ising system. Biquadratic coupling between the structural and magnetic order parameters is also inferred along with evidence of three-dimensional critical scattering in this system.Comment: New figure and discussion added. Length: 11 pages, 7 figure
    corecore