Dr. Hisao Nakamura

My research is developing simulation methods of molecular and material sciences based on first-principles theory. Our methods were applied to molecular electronics and various nano-scale devices like thermoelectric device and nonvolatile memory.

Computational chemistry of excited states

Many important chemical reactions include non-adiabatic processes, where standard Born-Oppenheimer approximation breaks down. We developed the direct method to calculate diabatic wavefunctions instead of adiabatic wavefunctions. The resulting potential energy surfaces are smooth and the coupling terms, which correspond to electric transition probability by electron-ion energy conversion, are scalar. The method is very powerful tool to perform simulation of chemical reaction dynamics. My computational method was implemented to GAMMES, which is one of the most standard quantum chemistry program packages.

Nano-scale electronic device materials

Device scale goes to sub 10 nm scale by “More than Moore” and electric transport is quantum transport process. We develop first principles method to evaluate electric and thermal transport properties including electron-phonon interactions based on nonequilibrium Green’s function (NEGF) theory. My recent studies are in the fields of molecular electronics and nonvolatile memory materials like HfOx-ReRAM and interfacial phase change memory (iPCM). Some of ReRAM devices are now in the market, however, the fundamental physics of SET/RESET operation of sub 10 nm ReRAM device is almost unknown. Recently, we succeed in showing details of transport mechanism. Theoretical simulation of iPCM is another scope. The iPCM is superlattice by ordered stacking of topological insulator and normal insulator blocks. With understanding structural phase change mechanism, topological phase transition of “embedded” 2D states is also a challenging topic.

First-principles Simulation of Thermoelectric Materials

Efficiency of thermoelectric energy conversion is estimated by figure of merit, ZT. In order to increase ZT, the following strategies are possible (1) increasing Seebeck coefficient, (2) increasing electric conductance, (3) suppression of thermal conductance. However, directions of material design for (1) and (2) often conflict. I extended our NEGF simulation program to calculate thermoelectric properties including phonon thermal conductance as well as improvement of the standard Boltzmann equation approach. As one of our recent achievements, we found that organometallic molecular film can be potentially a good thermoelectric material by controlling quantum interference effects. Applications to metal-organic hybrid materials or nano composite materials are now in the scope.