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We report neutron scattering measurements of cooperative spin excitations in antiferromagnetically ordered BaFe2As2, the parent phase of an iron pnictide superconductor. The data extend up to ~100meV 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 0.17eV. The large characteristic spin fluctuation energy suggests that magnetism could play a role in the formation of the superconducting state.
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The evolution of electronic (spin and charge) excitations upon carrier doping is an extremely important issue in superconducting layered cuprates and the knowledge of its asymmetry between electron- and hole-dopings is still fragmentary. Here we combine x-ray and neutron inelastic scattering measurements to track the doping dependence of both spin and charge excitations in electron-doped materials. Copper L3 resonant inelastic x-ray scattering spectra show that magnetic excitations shift to higher energy upon doping. Their dispersion becomes steeper near the magnetic zone center and deeply mix with charge excitations, indicating that electrons acquire a highly itinerant character in the doped metallic state. Moreover, above the magnetic excitations, an additional dispersing feature is observed near the {Gamma}-point, and we ascribe it to particle-hole charge excitations. These properties are in stark contrast with the more localized spin-excitations (paramagnons) recently observed in hole-doped compounds even at high doping-levels.
We have measured the spin susceptibility of the underdoped high temperature superconductor, YBa2Cu4O8 by Gd^{3+} electron spin resonance in single crystals and aligned powders at several magnetic fields between 3 and 15.4 T. At low temperatures and high fields, the spin susceptibility of the CuO2 planes is enhanced slightly in the $Bparallel c$ orientation with respect to the $Bperp c$ orientation. The enhancement in an applied field of 15.4 T ($approx 0.15 H_{c2}$) at 16 K (0.2 $T_c$) is approximately 10 percent of the susceptibility measured at $T_c$. Such a small magnitude suggests that the second critical field of superconductivity, $H_{c2}approx 100 T$, would not suppress the pseudogap. This work demonstrates the potential of high field ESR in single crystals for studying high $T_c$ superconductors.
We performed inelastic neutron experiments on underdoped La_2-xSr_xCuO_4(x=0.10, T_c=28.6K) using a time-of-flight neutron scattering technique. Four incommensurate peaks on the two-dimensional reciprocal plane disperse inwards toward an antiferromagnetic zone center as the energy increases. These peaks merge into a single peak at an energy E_cross around w=40+-3meV. Beyond E_cross, the peak starts to broaden and ``hourglass-like excitations are observed. The E_cross in the underdoped sample is smaller than that reported for the optimally doped La_1.84Sr_0.16CuO_4. The reduction of the E_cross is explained by the doping-independent slope of the downward dispersion below the E_cross combined with the smaller incommensurability in the underdoped sample. In the energy spectrum of chi(w), we observed a similar peak-dip-hump structure in the energy region of 10~45meV to that reported for the optimally doped sample. We discuss the relation between the hourglass-shaped dispersion and the peak-dip-hump energy spectrum.
Magnetic interactions are generally believed to play a key role in mediating electron pairing for superconductivity in iron arsenides; yet their character is only partially understood. Experimentally, the antiferromagnetic (AF) transition is always preceded by or coincident with a tetragonal to orthorhombic structural distortion. Although it has been suggested that this lattice distortion is driven by an electronic nematic phase, where a spontaneously generated electronic liquid crystal state breaks the C4 rotational symmetry of the paramagnetic state, experimental evidence for electronic anisotropy has been either in the low-temperature orthorhombic phase or the tetragonal phase under uniaxial pressure that breaks this symmetry. Here we use inelastic neutron scattering to demonstrate the presence of a large in-plane spin anisotropy above TN in the unstressed tetragonal phase of BaFe2As2. In the low-temperature orthorhombic phase, we find highly anisotropic spin waves with a large damping along the AF a-axis direction. On warming the system to the paramagnetic tetragonal phase, the low-energy spin waves evolve into quasi-elastic excitations, while the anisotropic spin excitations near the zone boundary persist. These results strongly suggest that the spin nematicity we find in the tetragonal phase of BaFe2As2 is the source of the electronic and orbital anisotropy observed above TN by other probes, and has profound consequences for the physics of these materials.
Neutron diffraction studies of Ba(Fe[1-x]Co[x])2As2 reveal that commensurate antiferromagnetic order gives way to incommensurate magnetic order for Co compositions between 0.056 < x < 0.06. The incommensurability has the form of a small transverse splitting (0, +-e, 0) from the nominal commensurate antiferromagnetic propagation vector Q[AFM] = (1, 0, 1) (in orthorhombic notation) where e = 0.02-0.03 and is composition dependent. The results are consistent with the formation of a spin-density wave driven by Fermi surface nesting of electron and hole pockets and confirm the itinerant nature of magnetism in the iron arsenide superconductors.
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