Jisook Hong

Organic–inorganic hybrid perovskites ABI3 (A = CH3NH3, NH2CHNH2; B = Sn, Pb) as potential thermoelectric materials: a density functional evaluation

To assess the feasibility of the organic–inorganic perovskite iodides ABI3 (A = CH3NH3, NH2CHNH2; B = Sn, Pb; X = I) for thermoelectric applications, we estimated their figures of merit (ZTs) as well as that of Bi2Te3, which is optimized for temperatures around 300 K, as a function of chemical potential on the basis of density functional theory calculations. Our analysis employed the tetragonal structures (P4mm) of (CH3NH3)PbI3 and (CH3NH3)SnI3, the trigonal (P3m1) structure of (NH2CHNH2)PbI3, and the orthorhombic (Amm2) structure of (NH2CHNH2)SnI3 to examine their thermoelectric properties around room temperature. Our work reveals that the ZTs of electron-doped ABI3 perovskites can be as large as that of hole-doped Bi2Te3 whereas those of hole-doped ABI3 are rather smaller so that, in thermoelectric performance, electron-doped perovskites ABI3 can be as good as hole-doped Bi2Te3.

Quantitative analysis on electric dipole energy in Rashba band splitting.

We report on quantitative comparison between the electric dipole energy and the Rashba band splitting in model systems of Bi and Sb triangular monolayers under a perpendicular electric field. We used both first-principles and tight binding calculations on p-orbitals with spin-orbit coupling. First-principles calculation shows Rashba band splitting in both systems. It also shows asymmetric charge distributions in the Rashba split bands which are induced by the orbital angular momentum. We calculated the electric dipole energies from coupling of the asymmetric charge distribution and external electric field, and compared it to the Rashba splitting. Remarkably, the total split energy is found to come mostly from the difference in the electric dipole energy for both Bi and Sb systems. A perturbative approach for long wave length limit starting from tight binding calculation also supports that the Rashba band splitting originates mostly from the electric dipole energy difference in the strong atomic spin-orbit coupling regime.

Magnetic structure of (C5H12N)CuBr3: origin of the uniform Heisenberg chain behavior and the magnetic anisotropy of the Cu2+ (S = 1/2) ions

The magnetic properties and electric polarization of the organic/inorganic hybrid system (C5H12N)CuBr3 (C5H12N = piperidinium) were examined on the basis of density functional theory calculations. The spin exchanges of (C5H12N)CuBr3 evaluated by energy-mapping analysis show that its uniform Heisenberg antiferromagnetic chain behavior is not caused by the CuBr3 chains made up of edge-sharing CuBr5 square pyramids, but by the two-leg spin ladders resulting from interchain interactions. The magnetic anisotropy of the Cu2+ ions in (C5H12N)CuBr3 originates largely from the Br− ligands rather than the Cu2+ ions. The electric polarization of (C5H12N)CuBr3 arises from the absence of inversion symmetry in the crystal structure, and is weakly affected by the magnetic structure.