Research (English)

Excited states

High-Lying Triplet Excitons of Thermally Activated Delayed Fluorescence Molecules: Y. Noguchi and O. Sugino, J. Phys. Chem. Phys. C 121, 20687-20695 (2017).

Thermally activated delayed fluorescence (TADF) has attracted attention for its extreme efficiency for emitting light. The key is in the efficient transition of the spin triplet exciton to the spin singlet one. For the ultrahigh efficiency, those spin states need to be nearly degenerated in energy. We performed first-principles GW+BSE simulation of candidate TADF molecules to show how this condition is satisfied. This is a first step towards designing the TADF material.

Molecular size insensitivity of optical gap of [n] cycloparaphenylenes (n= 3-16): Y. Noguchi and O. Sugino, J. Chem. Phys. 146 (2017) 144304.

Cycloparaphenylene (CPP) is a recently synthesized shortest carbon nanotube. This paper investigates the excited states using the many-body Green’s function method. With increasing size of the tube-diameter, the electronic wave function is spatially more extended and is more clearly characterized by the envelope function. The geometry of CPP also has a size dependence, but these size-dependences happen to cancel with each other. This explains the size-independence of the photoabsorption spectrum observed experimentally.

Quantitative characterization of exciton from GW+Bethe-Salpeter calculation D. Hirose, Y. Noguchi, and O. Sugino: J. Chem. Phys. 146 (2017) 044303.

Excitons are classified into the Frenkel and Wannier-Mott types, or into the local, charge-transfer type, and Rydberg type. The classification has been done only approximately. In this context, we have developed a classification method using the exciton wave function provided by the many-body Green’s function method. This classification method enables us to relate the atomic configuration to the excited state property, and thus will be useful in finding a material for an organic luminescence or photovoltaic material through an extensive data-based research.

GW Γ + Bethe-Salpeter equation approach for photoabsorption spectra: Importance of self-consistent GW Γ calculations in small atomic systems: R. Kuwahara, Y. Noguchi and K. Ohno, Phys. Rev. B 94 (2016) 121116.

This is done in collaboration with Ohno group in Yokohama National Universiy. We have been trying to level up the many-body Green’s function method.

Interfaces

Emergence of Negative Capacitance in Multidomain Ferroelectric-Paraelectric Nanocapacitors at Finite Bias: S. Kasamatsu, S. Watanabe, C. S. Hwang and S. Han, Adv. Mater. 28 (2016) 335.

This work shows that the negative capacitance can be realized at a thin film consisting of a ferroelectric-paraelectric interface. The negative capacitance corresponds, in the free-energy curve of the Landau theory, to the region of negative curvature. The region is unstable in the bulk but can be metastable at an interface when driving force for domain formation is relatively weak. Our density functional simulation shows this can happen. This work was done in collaboration with Watanabe group in the University of Tokyo and others.

Reverse Stability of Oxyluciferin Isomers in Aqueous Solutions: Y. Noguchi, M. Hiyama, M. Shiga, O. Sugino and H. Akiyama, Journal of Physical Chemisty B 120 (2016) 8776-8783.

Biomaterials are interfaced with solution. The interfaces are crucially important. This work investigates the interface of an oxyluciferin anion, a bioluminescence molecule of firefly, to show that the previous understanding based on the continuum solvation models is too approximate. The solvation occurs differently depending on the charge distribution within the oxyluciferin, and thus explicit structure of water molecules plays an important role in determining the stability in solution. This work was done in collaboration within ISSP.

Electron correlation

Four-body correlation embedded in antisymmetrized geminal power wave function: A. Kawasaki and O. Sugino, The Journal of Chemical Physics 145 (2016) 244110.

This proposes a method to calculate a strongly correlate region enbedded in a material. The formulation is based on the Antisymmetrized Geminal Powers (AGP), which is a mean-field approximation for the electron pair; AGP is a natural extension of Hartree-Fock theory, which is a mean-field approximation for one electron. AGP is then multiplied by a correlation factor representing their correlation to show that a single-impurity Anderson model can be accurately simulated. This work is an example to show that the method of low-rank tensor decomposition is a key to advance the method of calculating strongly-correlated electron systems.

Phase transition and phase diagram

First-principles description of van der Waals bonded spin-polarized systems using the vdW-DF+U method: Application to solid oxygen at low pressure, S. Kasamatsu, T. Kato, and O. Sugino, Physical Review B95 (2017)  235120.

Oxygen molecules exist in a gas phase at ambient temperature and pressure, but is crystallized at low temperature and/or high pressure. Oxygen molecule has a spin-triplet ground state and is interacting with each other magnetically, thereby the stable phase depends on the applied magnetic field. The distance between the oxygen molecules is determined via delicate balance between van der Waals and magnetic interactions. This fact has hampered computational determination of the structure. In this context, we have used vdW-DF + U approach, which is a technique of the density functional theory, to show that the structure can be determined theoretically for the alpha phase, which is stable at a low temperature and low pressure region.With this, we predict a stable structure at a high pressure region, in consistent with experiment.