Hyo Seok Ji

Enhancement of the Thermoelectric Figure-of-Merit in a Wide Temperature Range in In4Se3–xCl0.03 Bulk Crystals

IO N Because of the increasing awareness of renewable energy issues, much attention has been devoted to thermoelectric energy harvesting technology. The dimensionless thermoelectric fi gureof-merit is defi ned by ZT = S 2 σ T/ κ , where S , σ , T , and κ are the Seebeck coeffi cient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. Regarding a high ZT , it has long been sought both by lowering the thermal conductivity by exploiting the phonon-glass and electron-crystal concept [ 1–5 ] and by enhancing the power factor S 2 σ by utilizing the quantum confi nement effect [ 6 , 7 ] in low-dimensional nanostructured materials. Recently, we proposed that the charge density wave represents a new direction for high thermoelectric performance in bulk crystalline materials. [ 8 , 9 ] Low-dimensional electronic transport with strong electron–phonon coupling breaks the translational symmetry of the lattice because of the energy instability in a high-symmetry crystalline lattice. [ 10 ] The lattice dimerization along the electronic transport plane lowers the lattice thermal conductivity as a result of lowering of the phonon energy. Through quasi one-dimensional lattice distortion (Peierls distortion) in In 4 Se 3– x bulk single crystals, we achieved a high ZT of 1.48 at 705 K. [ 9 ] However, two challenges remain for practical applications. Firstly, the reported ZT could be increased further if we could increase the carrier concentration of the In 4 Se 3– x crystals because it is far from the carrier concentration of a heavily doped semiconductor (on the order of 10 19 cm − 3 ) that is generally considered to be optimal for thermoelectric materials. Secondly, ZT decreases signifi cantly as the temperature decreases, which limits the operational temperature range to within 350 –430 ° C. [ 9 ] Here, we report a signifi cant increase in ZT (maximum ZT ( ZT max ) = 1.53) over a wide temperature range in chlorine-doped In 4 Se 3– x Cl 0.03

Orbital selective Fermi surface shifts and mechanism of high Tc superconductivity in correlated AFeAs (A=Li,Na)

Based on the dynamical mean field theory and angle resolved photoemission spectroscopy, we have investigated the mechanism of high T(c) superconductivity in stoichiometric LiFeAs. The calculated spectrum is in excellent agreement with the measured angle resolved photoemission spectroscopy. The Fermi surface (FS) nesting, which is predicted in the conventional density functional theory method, is suppressed due to the orbital-dependent correlation effect within the dynamical mean field theory method. We have shown that such marginal breakdown of the FS nesting is an essential condition to the spin-fluctuation mediated superconductivity, while the good FS nesting in NaFeAs induces a spin density wave ground state. Our results indicate that a fully charge self-consistent description of the correlation effect is crucial in the description of the FS nesting-driven instabilities.

Descriptor-based crystal structure prediction of magnetic transition metals: Orbital-spin occupancy rule

Prediction of structural phase of transition metal composites is highly required because the electronic and magnetic properties are deeply related to the crystal structures. The d-orbital occupancy has been suggested as a simple descriptor to predict the structural phase of transition metal composition in nonmagnetic ground state. In this work, we suggest new rule, orbital-spin occupancy rule with new descriptor nd-σd (σd is spin moment.) to predict stable crystal structure, which should be generally applied to nonmagnetic as well as magnetic system. Using first-principles calculation, we show that all 3d, 4d, and 5d transition metals follow this rule. Also, we confirm that structural phase can be controlled by changing nd-σd with pressure and electron doping. We suggest that orbital-spin occupancy rule should be widely applied to the prediction of various transition metal composites.