Atomically-flat,chemically-stable,superconducting epitaxial thin film of iron-based superconductor,cobalt-doped BaFe2As2

The epitaxial growth of Fe-based superconductors such as Co-doped SrFe₂As₂ (SrFe₂As₂:Co) was reported recently, but has still insufficient properties for a device application because they have rough surfaces and are decomposed by reactions with water vapor in an ambient atmosphere. This paper reports that epitaxial films of Co-doped BaFe₂As₂ grown at 700 ^°C show the onset superconducting transition temperature of ～20 K. The transition is sharper than those observed on the SrFe₂As₂:Co films, which would originate from their improved crystallinity. These films also have atomically-flat surfaces with step-and-terrace structures and exhibit chemical stability against exposure to water vapor.     

Inelastic neutron scattering from H2 on vapour deposited CO2

Inelastic neutron scattering spectra at 17 meV and 68 meV incident energies of molecular hydrogen adsorbed on vapour deposited substrates of pure CO₂, 90:10 and 50:50 CO₂:Kr are reported. The ortho-para transition is shifted from 14.7 meV in the free H₂-molecule to 9.4 meV in a presumably commensurate ortho-H₂ monolayer on the CO₂ surface. The quadrupole-quadrupole interaction of ortho-H₂ molecules with the CO₂ substrate results in a strongly anisotropic potential. In addition to rotations the dynamics of this layer comprise a local Einstein mode and phonons in resonance with the substrate, giving rise to intense multiphonon transitions. Quasielastic scattering on warmer samples is assigned to a liquidlike adsorption layer, in which the H₂ rotations are strongly perturbed. Received: 15 October 1996 / Revised: 25 August 1997 / Accepted: 24 October 1997     

Existence, character, and origin of surface-related bands in the high temperature iron pnictide superconductor BaFe(2-x)Co(x)As2

Low energy electron diffraction (LEED) experiments, LEED simulations, and finite slab density functional calculations are combined to study the cleavage surface of Co doped BaFe(2-x)Co(x)As2 (x = 0.1,0.17). We demonstrate that the energy dependence of the LEED data can only be understood from a terminating 1/2 Ba layer accompanied by distortions of the underlying As-Fe2-As block. As a result, surface-related Fe 3d states are present in the electronic structure, which we identify in angle resolved photoemission spectroscopy (ARPES) experiments. The close proximity of the surface-related states to the bulk bands inevitably leads to broadening of the ARPES signals, which excludes the use of the BaFe(2-x)Co(x)As2 system for accurate determination of self-energies using ARPES.     

Inelastic neutron scattering from superconducting rings

For the first time the differential cross section for the inelastic magnetic neutron scattering by superconducting rings is derived taking account of the interaction of the neutron magnetic moment with the magnetic field generated by the superconducting current. Calculations of the scattering cross section are carried out for cold neutrons and thin film rings from type-II superconductors with the magnetic fields not exceeding the first critical field.     

Inelastic neutron scattering study of antiferromagnetic CoO

In order to study the energy levels of the magnetic excitations in antiferromagnetic CoO, we have measured the cross-section for inelastic n-scattering of CoO powder at various scattering angles θ and energy transfer from 15 to 70 meV. For large θ purely nuclear scattering is observed, while for small θ the cross-section is predominantly due to magnetic scattering with some nuclear contributions. The cross-section shows several peaks, which have not yet been observed.By comparison of the results of pure CoO and of Co_0.8Ni_0.2O two peaks in the cross-section can be assigned to single ion-like transitions in the ground state multiplet of Co²⁺ in the antiferromagnetic phase. The parameters of a simple molecular field model are discussed.     

Inelastic neutron scattering from cerium under pressure

Inelastic neutron scattering from Ce metal at 300 K was studied both below and above the first order γ-α phase transition, using a triple axis spectrometer. It was found that (a) there is no indication of any residual magnetic scattering in the collapsed α phase, and (b) the energy width of the paramagnetic scattering in the γ-phase increases with pressure. We conclude that J_sf increases with pressure, as has been predicted byCoqblin.     

Inelastic neutron scattering to very high pressures

Progress in high-pressure and neutron scattering methods has recently allowed measurements of phonon dispersion curves of simple solids at high pressures to 10 GPa. In this technique single crystals of 10–25 mm³ volume are compressed by the Paris-Edinburgh cell and the phonon frequencies are measured on high-flux triple axis spectrometers. Detailed studies of the lattice dynamics of low-compressible systems are feasible, including measurements of mode Grüneisen parameters, elastic constants, and precursor effects of phase transitions. We describe the experimental set-up and illustrate its potential by results on semiconductors (Ge and GaSb) and metals (Fe and Zn) obtained at the LLB (Saclay) and ILL (Grenoble) reactor sources. This revised version was published online in July 2006 with corrections to the Cover Date.     

Electronic correlations and unconventional spectral weight transfer in the high-temperature pnictide BaFe(2-x)Co(x)As(2) superconductor using infrared spectroscopy

We report an infrared optical study of the pnictide high-temperature superconductor BaFe(1.84)Co(0.16)As(2) and its parent compound BaFe(2)As(2). We demonstrate that electronic correlations are moderately strong and do not change across the spin-density wave transition or with doping. By examining the energy scale and direction of spectral weight transfer, we argue that Hund's coupling J is the primary mechanism that gives rise to correlations.     

Josephson vortices and intrinsic Josephson junctions in the layered iron-based superconductor Ca10(Pt3As8)((Fe0.9Pt0.1)2As2)5

Modulated electronic state due to the layered crystal structures brings about moderate anisotropy of superconductivity in the iron-based superconductors and thus Abrikosov vortices are expected in the mixed state. However, based on the angular and temperature dependent transport measurements in iron-based superconductor Ca$_{10}$(Pt$_3$As$_8$)((Fe$_{0.9}$Pt$_{0.1}$)$_2$As$_2$)$_5$ with $T_{\rm c} \simeq 12$ K, we find clear evidences of a crossover from Abrikosov vortices to Josephson vortices at a crossover temperature $T^{\star} \simeq 7 $ K, when the applied magnetic field is parallel to the superconducting FeAs layers, i.e., the angle between the magnetic field and the FeAs layers $\theta = 0^\circ$. This crossover to Josephson vortices is demonstrated by an abnormal decrease (increase) of the critical current (flux-flow resistance) below $T^{\star}$, in contrast to the increase (decrease) of the critical current (flux-flow resistance) above $T^{\star}$ expected for Abrikosov vortices. Furthermore, when $\theta$ is larger than $0.5^\circ$, the flux-flow resistance and critical current have no anomalous behaviors across $T^{\star}$. These anomalous behaviors can be understood in terms of the distinct transition from the well-pinned Abrikosov vortices to the weakly-pinned Josephson vortices upon cooling, when the coherent length perpendicular to the FeAs layers $\xi_\bot$ becomes shorter than half of the interlayer distance $d/2$. These experimental findings indicate the existence of intrinsic Josephson junctions below $T^{\star}$ and thus quasi-two-dimensional superconductivity in Ca$_{10}$(Pt$_3$As$_8$)((Fe$_{0.9}$Pt$_{0.1}$)$_2$As$_2$)$_5$, similar to those in the cuprate superconductors.     

Inelastic neutron scattering and lattice dynamics of minerals

We review current research on minerals using inelastic neutron scattering and lattice dynamics calculations. Inelastic neutron scattering studies in combination with first principles and atomistic calculations provide a detailed understanding of the phonon dispersion relations, density of states and their manifestations in various thermodynamic properties. The role of theoretical lattice dynamics calculations in the planning, interpretation and analysis of neutron experiments are discussed. These studies provide important insights in understanding various anomalous behaviour including pressure-induced amorphization, phonon and elastic instabilities, prediction of novel high pressure phase transitions, high pressure-temperature melting, etc.      

Inelastic electron scattering from 31P

Form factors for the electroexcitation of the $\text{3}\text{2}$⁺ (1.27 MeV), $\text{5}\text{2}$⁺ (2.23 MeV), and $\text{3}\text{2}$⁺ (3.51 MeV) states in ³¹P have been measured for momentum transfers from 0.36 to 0.80 fm^?1 at the NBS electron scattering facility. In addition, form factors for the 2.23 MeV state in the momentum transfer range 0.74 to 1.78 fm^?1 have been extracted from data obtained in a previous experiment. Using the DUELS distorted-wave code the B(E2, ω) ↓ were found to be 6.0 ± 0.9, 6.9 ± 0.3, and 2.7 ± 0.3 W.u. for the 1.27, 2.23, and 3.51 MeV states, respectively. The form factors for these states, calculated using wave functions derived in an intermediate-coupling vibrational calculation, are compared with the data.     

Vortex imaging in Co-doped BaFe2As2

We review superconducting vortex imaging in Co-doped BaFe₂As₂ by Bitter decoration and small-angle neutron scattering (SANS). At all measured fields a highly disordered vortex configuration is observed, which is attributed to strong pinning. Further support of this conclusion comes from the absence of a Meissner rim in decoration images obtained close to the sample edge. The evolution of the SANS scattering vector with increasing applied field indicates vortex lattice domains of (distorted) hexagonal symmetry. This is consistent with the decoration images which show small, six fold coordinated ordered vortex domains. The SANS scattered intensity is found to decrease rapidly with increasing field, exceeding the rate expected from estimates of the upper critical field. This is consistent with the large degree of vortex “lattice” disorder.     

Non-magnetic impurity effect on the optimally carrier doped superconductor BaFe1.87Co0.13As2 prepared at ambient pressure

We report properties of the Zn-substituted polycrystalline superconductors BaFe_1.87?xZn_xCo_0.13As₂ (x = 0, 0.04, 0.06 and 0.08) prepared at ambient pressure condition. Electrical conductivity and magnetization measurements revealed that the superconductivity is suppressed by at most 2% substitution of Zn, which shows typical behavior of an anisotropically gaped superconductivity rather than an isotropically gaped superconductivity. With increasing the amount of Zn, weak change in superconductivity was observed. It is likely impossible to obtain BaFe_1.87?xZn_xCo_0.13As₂ crystals with x > 0.04 at ambient pressure condition. On the basis of the structural and the physical properties of the present system, we discuss a possible mechanism of the superconducting pairing.     

Inelastic neutron scattering and lattice dynamics of ZrO2, Y2O3 and ThSiO4

Zirconia (ZrO₂), yttria (Y₂O₃) and thorite (ThSiO₄) are ceramic materials used for a wide range of industrial applications. The dynamical properties of these materials are of interest as they exhibit numerous interesting phase transitions at high temperature and pressure. Using a combination of inelastic neutron scattering and theoretical lattice dynamics we have studied the phonon spectra and thermodynamic properties of these compounds. The experimental data validate the theoretical model, while the model enables microscopic interpretations of the observed data. The calculated thermodynamic properties are in good agreement with the experimental data.      

Inelastic neutron scattering study of spin waves in MnO

The dispersion relation E(q) for spin waves in MnO has been measured at 4.2°K by neutron inelastic scattering. The isotropic exchange integrals as well as the anisotropy constants have been determined by fitting the Hamiltonian to the data. It is found that the exchange striction plays main role in the anomaly in the magnetic interactions in MnO and the biquadratic exchange interaction j₁(S₁·S₂)² is almost absent (j₁/J₁ ? 0.002).