Topological and transport properties of Dirac fermions in an antiferromagnetic metallic phase of iron-based superconductors.

We investigate Dirac fermions in the antiferromagnetic metallic state of iron-based superconductors. Deriving an effective Hamiltonian for Dirac fermions, we reveal that there exist two Dirac cones carrying the same chirality, contrary to graphene, compensated by a Fermi surface with a quadratic energy dispersion as a consequence of a nontrivial topological property inherent in the band structure. We also find that the presence of the Dirac fermions gives the difference of sign-change temperatures between the Hall coefficient and the thermopower.

Pressure-induced magnetic transition in manganite (La0.75Ca0.25MnO3).

Low temperature Mn K-edge x-ray magnetic circular dichroism and x-ray diffraction measurements were carried out to investigate the stability of the ferromagnetic ground state in manganite La0.75Ca0.25MnO3 under nearly uniform compression using diamond anvil cells.

Phonon anomalies due to collective stripe modes in high T(c) cuprates.

Phonon anomalies observed in various high T(c) cuprates by neutron experiments are analyzed theoretically in terms of the stripe concept. The phonon self-energy correction is evaluated by taking into account the charge collective modes of stripes, giving rise to dispersion gap, or kink and shadow phonon modes at twice the wave number of spin stripe. These features coincide precisely with observations.