Masao Nakao

Half-metallic antiferromagnetism in LaOCr0.5Fe0.5As as a probe of single spin superconductivity

The normal ground state of LaOFeAs is an antiferromagnet (AFM) with magnetically stripe-ordered FeAs planes. By using a full-potential augmented plane wave method within density functional theory, we find LaOCr0.5Fe0.5As to be a quasi-2D half-metallic (HM) AFM, where two component magnetic ions in a unit cell have antialigned local moments that cancel exactly because of the integer filling of an insulating channel. Both striped and checkerboard structures in CrFeAs2 planes show stable HM-AFM behaviors in the ground state.

Bonding nature and wave function around the Fermi level of MgB2-related compounds

Abstract We present first-principles molecular-orbital calculations for the 39 K superconductor MgB 2 and related diborides using the discrete variational (DV)-Xα method. The electronic structure of a slab cluster embedded in the Madelung potential is determined self-consistently in terms of charge transfer between Mg and B. In contrast with the simplest ionic picture Mg 2+ B − 2 , the negative charge of B is estimated to be 0.39. Our results, which include total and partial density of states, overlap population in the neighboring B–B and Mg–B bonds and wave functions around the Fermi energy E F consisting of B 2p x , y orbitals in the B planes, indicate the incomplete filling of the σ orbitals in MgB 2 . We discuss possible ways to achieve higher transition temperatures in non-periodic systems such as surfaces, interfaces and artificially layered hetero-structures based on this approach.

Cluster Approach to Tetrahedrally Bonded Half‐Metallic Antiferromagnets

We present first‐principles molecular‐orbital calculations of electronic structures of chalcopyrite‐derived model clusters to search for half‐metallic (HM) antiferromagnetic (AFM) materials that are nonmagnetic metals with 100% spin polarization of the conduction electrons, and are within the class of tetrahedrally coordinated ternary systems compatible with ordinary III–V and II–VI semiconductors. By replacing Cu and/or Fe in the chalcopyrite CuFeS2 with 3d transition‐metal ions, we find two HM AFM results for CrFeS2 and VCoS2, where two constituent magnetic ions have antialigned local moments that cancel exactly by virtue of the integer filling of one spin channel in stoichiometric half‐metals. This work provides a new class of viable candidates for HM AFM materials, in addition to the Heusler alloys and ordered double perovskites, though none of them have been synthesized yet.