  First-principles calculations reveal half metallicity in zigzag boron nitride
(BN) nanoribbons (ZBNNRs). When the B edge, but not the N edge, of the ZBNNR is
passivated, despite being a pure $sp$-electron system, the ribbon shows a giant
spin splitting. The electrons at the Fermi level are 100% spin polarized with a
half-metal gap of 0.38 eV and its conductivity is dominated by metallic
single-spin states. The two states across at the Dirac point have different
molecular origins, which signals a switch of carrier velocity. The ZBNNR should
be a good potential candidate for widegap spintronics.
