Andrew Boothroyd

Abstract:

We report neutron inelastic-scattering measurements on the stoichiometric iron-based superconductor LiFeAs. We find evidence for (i) magnetic scattering consistent with strong antiferromagnetic fluctuations and (ii) an increase in intensity in the superconducting state at low energies, similar to the resonant magnetic excitation observed in other iron-based superconductors. The results do not support a recent theoretical prediction of spin-triplet p-wave superconductivity in LiFeAs but instead suggest that the mechanism of superconductivity is similar to that in the other iron-based superconductors. © 2011 American Physical Society.

Abstract:

We report inelastic neutron-scattering measurements of the magnetic excitations in SrFe2As2, the parent of a family of iron-based superconductors. The data extend throughout the Brillouin zone and up to energies of ~260 meV. The spectrum calculated from a J1-J 2 model does not accurately describe our data, and we show that some of the qualitative features that the model fails to describe are readily explained by calculations from a five-band itinerant mean-field model. In particular, the high-energy part of the spectra recorded above TN do not differ significantly from those at low temperature, which is explained by the itinerant model and which has implications for theories of electronic nematic and orbital ordering. © 2011 American Physical Society.

Abstract:

We used ultrafast resonant soft x-ray diffraction to probe the picosecond dynamics of spin and orbital order in La(0.5)Sr(1.5)MnO(4) after photoexcitation with a femtosecond pulse of 1.5 eV radiation. Complete melting of antiferromagnetic spin order is evidenced by the disappearance of a (1/4,1/4,1/2) diffraction peak. On the other hand, the (1/4,1/4,0) diffraction peak, reflecting orbital order, is only partially reduced. We interpret the results as evidence of destabilization in the short-range exchange pattern with no significant relaxation of the long-range Jahn-Teller distortions. Cluster calculations are used to analyze different possible magnetically ordered states in the long-lived metastable phase. Nonthermal coupling between light and magnetism emerges as a primary aspect of photoinduced phase transitions in manganites.