Fig. 7-6

Schematic views of electron pair wave functions, and a phase diagram of a theoretical model on a honeycomb lattice (a) Various kinds of electron pair wave-functions are illustrated. The BCS theory envisions spin-singlet s-wave pairing mediated by the electron-lattice interaction. In high-Tc cuprates, spin-singlet d-wave pairing is realized in order to avoid the effects of Coulomb repulsion. For the case of spin triplet pairing, there occurs a p-wave state with odd-parity under space inversion. (b) Phase diagram obtained by analyzing a theoretical model on a honeycomb lattice. An unphysical region is shown in gray. In the ferromagnetic state, a superconducting phase with odd-parity triplet pairing occurs. The unit cell of the honeycomb lattice contains two atomic sites, 1 and 2. As denoted by a cross, an inversion center is located just midway between the two sites. In such a case, local triplets due to the Hund's rule interaction can form bonding and anti-bonding states inside the unit cell, leading to even and odd-parity triplet pairing states, respectively.

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