Raymond Osborn

Unconventional superconductivity in Ba0.6K0.4Fe2As2 from inelastic neutron scattering

A new family of superconductors containing layers of iron arsenide has attracted considerable interest because of their high transition temperatures (Tc), some of which are >50 K, and because of similarities with the high-Tc copper oxide superconductors. In both the iron arsenides and the copper oxides, superconductivity arises when an antiferromagnetically ordered phase has been suppressed by chemical doping. A universal feature of the copper oxide superconductors is the existence of a resonant magnetic excitation, localized in both energy and wavevector, within the superconducting phase. This resonance, which has also been observed in several heavy-fermion superconductors, is predicted to occur when the sign of the superconducting energy gap takes opposite values on different parts of the Fermi surface, an unusual gap symmetry which implies that the electron pairing interaction is repulsive at short range. Angle-resolved photoelectron spectroscopy shows no evidence of gap anisotropy in the iron arsenides, but such measurements are insensitive to the phase of the gap on separate parts of the Fermi surface. Here we report inelastic neutron scattering observations of a magnetic resonance below Tc in Ba0.6K0.4Fe2As2, a phase-sensitive measurement demonstrating that the superconducting energy gap has unconventional symmetry in the iron arsenide superconductors.

Two-dimensional resonant magnetic excitation in BaFe1.84Co0.16As2.

Inelastic neutron scattering measurements on single crystals of superconducting BaFe1.84Co0.16As2 reveal a magnetic excitation located at wave vectors (1/2 1/2 L) in tetragonal notation. On cooling below T_{C}, a clear resonance peak is observed at this wave vector with an energy of 8.6(0.5) meV, corresponding to 4.5(0.3) k_{B}T_{C}. This is in good agreement with the canonical value of 5 k_{B}T_{C} observed in the cuprates. The spectrum shows strong dispersion in the tetragonal plane but very weak dispersion along the c axis, indicating that the magnetic fluctuations are two dimensional in nature. This is in sharp contrast to the anisotropic three dimensional spin excitations seen in the undoped parent compounds.

Phonon density of states in MgB2

We report inelastic neutron scattering measurements of the phonon density of states in Mg 11B2, which has a superconducting transition at 39.2 K. The acoustic phonons extend in energy to 36 meV, and there are highly dispersive optic branches peaking at 54, 78, 89, and 97 meV. A simple Born-von Kàrmàn model reproduces the mode energies, and provides an estimate of the electron-phonon coupling of lambda approximately 0.9. Furthermore, the estimated boron and magnesium contributions to the isotope effect are in qualitative agreement with experiment. The data confirm that a conventional phonon mechanism, with moderately strong electron-phonon coupling, can explain the observed superconductivity.

Nanomagnetic droplets and implications to orbital ordering in La1-xSrxCoO3

Inelastic cold-neutron scattering on LaCoO3 provided evidence for a distinct low energy excitation at 0.6 meV coincident with the thermally induced magnetic transition. Coexisting strong ferromagnetic (FM) and weaker antiferromagnetic correlations that are dynamic follow the activation to the excited state, identified as the intermediate S = 1 spin triplet. This is indicative of dynamical orbital ordering favoring the observed magnetic interactions. With hole doping as in La(1-x)Sr(x)CoO3 , the FM correlations between Co spins become static and isotropically distributed due to the formation of FM droplets. The correlation length and condensation temperature of these droplets increase rapidly with metallicity due to the double exchange mechanism.

Magnetically driven suppression of nematic order in an iron-based superconductor

A theory of superconductivity in the iron-based materials requires an understanding of the phase diagram of the normal state. In these compounds, superconductivity emerges when stripe spin density wave (SDW) order is suppressed by doping, pressure or atomic disorder. This magnetic order is often pre-empted by nematic order, whose origin is yet to be resolved. One scenario is that nematic order is driven by orbital ordering of the iron 3d electrons that triggers stripe SDW order. Another is that magnetic interactions produce a spin-nematic phase, which then induces orbital order. Here we report the observation by neutron powder diffraction of an additional fourfold-symmetric phase in Ba1-xNaxFe2As2 close to the suppression of SDW order, which is consistent with the predictions of magnetically driven models of nematic order.

The NeXus data format

A description is presented of the NeXus data format for X-ray and neutron scattering and muon spectroscopy.

Emergence of coherence in the charge-density wave state of 2H-NbSe2

A charge-density wave (CDW) state has a broken symmetry described by a complex order parameter with an amplitude and a phase. The conventional view, based on clean, weak-coupling systems, is that a finite amplitude and long-range phase coherence set in simultaneously at the CDW transition temperature Tcdw. Here we investigate, using photoemission, X-ray scattering and scanning tunnelling microscopy, the canonical CDW compound 2H-NbSe2 intercalated with Mn and Co, and show that the conventional view is untenable. We find that, either at high temperature or at large intercalation, CDW order becomes short-ranged with a well-defined amplitude, which has impacts on the electronic dispersion, giving rise to an energy gap. The phase transition at Tcdw marks the onset of long-range order with global phase coherence, leading to sharp electronic excitations. Our observations emphasize the importance of phase fluctuations in strongly coupled CDW systems and provide insights into the significance of phase incoherence in ‘pseudogap’ states.

NeXus: A common format for the exchange of neutron and synchroton data

Abstract NeXus is a data format for the exchange of neutron and synchrotron scattering data between facilities and user institutions. It has been developed by an international team of scientists and computer programmers from neutron and X-ray facilities around the world. The NeXus format uses the hierarchical data format (HDF) which is portable, binary, extensible and self-describing. The NeXus format defines the structure and contents of these HDF files in order to facilitate the visualization and analysis of neutron and X-ray data. In addition, an application program interface (API) is being produced in order to simplify the reading and writing of NeXus files. The details of the format are available at 〈http://www.neutron.anl.gov/NeXus/〉.

The magnetic state of Pr in PrBa2Cu3O7

Abstract We present the results of a detailed theoretical analysis of neutron inelastic scattering measurements of the crystal-field excitations in PrBa 2 Cu 3 O 7 . Using a scaling procedure we estimate the crystal-field parameters for Pr 3+ from those of Ho 3+ determined from neutron spectra of HoBa 2 Cu 3 O 7 . With the scaled parameters as a starting point and guide we refine a model which is able to account for the main features of the crystal-field spectra of PrBa 2 Cu 3 O 7 , including the position of a peak in the neutron spectrum near 280 meV, presented here for the first time, arising from 3 H 4 to 3 H 5 intermultiplet transitions. The analysis leads us to propose the sequence Γ 4 -Γ 2 -Γ 1 for the symmetries of the levels within the crystal-field-split quasi-triplet ground state, and we show that this level scheme is consistent with the observed magnetic anisotropy. We predict the excitation spectrum of Pr 4+ by scaling the Pr 3+ parameters, and use it to estimate an upper limit of 10% for the concentration of stable Pr 4+ ions on the yttrium site. The prevalence of Pr 3+ is consistent with a magnetic interaction, rather than hole-filling, as the major factor responsible for the absence of superconductivity in PrBa 2 Cu 3 O 7 .

Electron-Phonon Coupling and the Soft Phonon Mode in TiSe2

We report high-resolution inelastic x-ray measurements of the soft phonon mode in the charge-density-wave compound TiSe(2). We observe a complete softening of a transverse optic phonon at the L point, i.e., q=(0.5, 0, 0.5), at T≈T(CDW). Detailed ab initio calculations for the electronic and lattice dynamical properties of TiSe(2) are in quantitative agreement with experimental frequencies for the soft phonon mode. The observed broad range of renormalized phonon frequencies, (0.3, 0, 0.5)≤q≤(0.5, 0, 0.5), is directly related to a broad peak in the electronic susceptibility stabilizing the charge-density-wave ordered state. Our analysis demonstrates that a conventional electron-phonon coupling mechanism can explain a structural instability and the charge-density-wave order in TiSe(2) although other mechanisms might further boost the transition temperature.