Phonons in single crystals of PrFeAsO_{1-y} are investigated using high-resolution inelastic x-ray scattering and ab initio pseudopotential calculations. Extensive measurements of several samples at temperatures spanning the magnetic ordering tempera
ture and the superconducting transition temperature show that there are some changes in phonon spectra with temperature and/or doping. We compare our measurements with several ab initio pseudopotential models (nonmagnetic tetragonal, oxygen-deficient O_{7/8} supercell, magnetic orthorhombic, and magnetic tetragonal) and find that the experimentally observed changes are much smaller than the differences between the experimental data and the calculations. Agreement is improved if magnetism is included in the calculations via the local spin density approximation, as the Fe atomic motions parallel to the ferromagnetic ordering direction are softened. However, the antiferromagnetically polarized modes remain hard, and in disagreement with the experimental data. In fact, given the increasing evidence for anisotropy in the iron pnictide materials, the phonon response is surprisingly isotropic. We consider several modifications of the ab initio calculations to improve the agreement with the experimental data. Improved agreement is found by setting the matrix to zero (clipping the bond) between nearest-neighbor antiferromagnetically aligned Fe atoms in the magnetic calculation, or by softening only the in-plane nearest-neighbor Fe-As force constant in the nonmagnetic calculation. We discuss these results in the context of other measurements, especially of phonons, for several FeAs systems. Fluctuating magnetism may be a partial explanation for the failure of the calculations, but seems incomplete in the face of the similarity of the measured phonon response in all the systems investigated here including those known to have static magnetism.
In order to consistently explain controversial experimental results on superconducting states observed by different probes in typical iron-based superconductors, we construct a realistic multi-band $pm s$-wave pairing model by combining the quasiclas
sical formalism with the first-principles calculation. The model successfully resolves the controversies in contrast to the fact that simplified models such as two-band $pm s$-wave one fail to do. A key in the model is the existence of relatively small gaps which leads to material-dependent peculiarities.
Electrostatic carrier doping using a field-effect-transistor structure is an intriguing approach to explore electronic phases by critical control of carrier concentration. We demonstrate the reversible control of the insulator-metal transition (IMT)
in a two dimensional (2D) electron gas at the interface of insulating SrTiO$_3$ single crystals. Superconductivity was observed in a limited number of devices doped far beyond the IMT, which may imply the presence of 2D metal-superconductor transition. This realization of a two-dimensional metallic state on the most widely-used perovskite oxide is the best manifestation of the potential of oxide electronics.
A puzzle in the iron-based superconductor LaFeAsO_{1-x}F_x is that the magnetic moment obtained by first-principle electronic structure calculations is unexpectedly much larger than the experimentally observed one. For example, the calculated value i
s ~ 2.0 mu_B in the mother compound, while it is ~ 0.3 mu_B in experiments. We find that the puzzle is solved within the framework LDA + U by expanding the U value into a slightly negative range. We show U dependence of the obtained magnetic moment in both the undoped x=0.0 and doped x = 0.125. These results reveal that the magnetic moment is drastically reduced when entering to the slightly negative range of U. Moreover, the negative U well explains other measurement data, e.g., lattice constants and electronic DOS at the Fermi level. We discuss possible origins of the negative U in these compounds.
