R. I. Bewley

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.

Topological Magnon Bands in a Kagome Lattice Ferromagnet

There is great interest in finding materials possessing quasiparticles with topological properties. Such materials may have novel excitations that exist on their boundaries which are protected against disorder. We report experimental evidence that magnons in an insulating kagome ferromagnet can have a topological band structure. Our neutron scattering measurements further reveal that one of the bands is flat due to the unique geometry of the kagome lattice. Spin wave calculations show that the measured band structure follows from a simple Heisenberg Hamiltonian with a Dzyaloshinkii-Moriya interaction. This serves as the first realization of an effectively two-dimensional topological magnon insulator--a new class of magnetic material that should display both a magnon Hall effect and protected chiral edge modes.

Effect of fermi surface nesting on resonant spin excitations in Ba 1-xKxFe2As2

We report inelastic neutron scattering measurements of the resonant spin excitations in Ba(1-x)K(x)Fe(2)As(2) over a broad range of electron band filling. The fall in the superconducting transition temperature with hole doping coincides with the magnetic excitations splitting into two incommensurate peaks because of the growing mismatch in the hole and electron Fermi surface volumes, as confirmed by a tight-binding model with s(±)-symmetry pairing. The reduction in Fermi surface nesting is accompanied by a collapse of the resonance binding energy and its spectral weight, caused by the weakening of electron-electron correlations.

Two-Dimensional Spin Density Wave State in LaFeAsO

The parent compound of the Fe oxypnictide superconductor, LaFeAsO, has been studied by pulsed neutron powder inelastic scattering measurement. The inelastic scattering intensity along the Q -axis at T =140 K exhibits a prominent asymmetric peak ascribed to the (π,π, l ) magnetic rod, suggesting the two-dimensionality of the spin density wave above the magnetic transition temperature. It persists even in the tetragonal phase up to room temperature, which is well above the magnetic transition temperature, corresponding to two antiferromagnetic sublattices or domains with a strong effective antiferromagnetic interaction J 2 between next-nearest-neighbour Fe magnetic moments in a single Fe square lattice.

High-energy spin excitations in BaFe 2 As 2 observed by inelastic neutron scattering

We report neutron-scattering measurements of cooperative spin excitations in antiferromagnetically ordered

A high energy inelastic neutron scattering investigation of the Gd-Fe exchange coupling in Gd2Fe17Dx (x = 0, 3 and 5)

{\text{BaFe}}_{2}{\text{As}}_{2}

Antiferromagnetic Spin Ice Correlations at (1/2,1/2,1/2) in the Ground State of the Pyrochlore Magnet Tb

, the parent phase of an iron pnictide superconductor. The data extend up to

_2

\ensuremath{\sim}100\text{ }\text{meV}

Ti

and show that the spin excitation spectrum is sharp and highly dispersive. By fitting the spectrum to a linear spin-wave model we estimate the magnon bandwidth to be in the region of 0.17 eV. The large characteristic spin-fluctuation energy suggests that magnetism could play a role in the formation of the superconducting state.

_2

The structural and magnetic properties of the Gd2Fe17Dx compounds (x = 0, 3, and 5) have been investigated by means of x-ray powder diffraction, high energy inelastic neutron scattering and magnetic measurements. The Gd2Fe17Dx compounds crystallize in the R-3m space group with the Th2Zn17-like structure. The increase of the lattice parameters with D content reveals a two-step filling of the interstitial sites with deuterium first filling the octahedral 9e sites for x = 3 and then partially filling the tetrahedral 18g sites for x = 5. The evolution of the Jex exchange coupling versus the deuterium (D) content is discussed. Although it is well known that the permanent magnet properties of D-doped samples are considerably better than those of the pure compound Gd2Fe17 there is a reduction in the Gd-Fe exchange field. Finally, the influence of deuterium insertion on the exchange interaction between the Gd and Fe sublattice is compared to that of nitrogen (N) or carbon (C). The C and N atoms are found to be more efficient in reducing the intersublattice coupling.