京都大学 / 新制・課程博士 / 博士(工学) / 甲第24904号 / 工博第5184号 / 新制||工||1990(附属図書館) / 京都大学大学院工学研究科分子工学専攻 / (主査)教授 関 修平, 准教授 須田 理行, 教授 生越 友樹, 教授 水落 憲和 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Over the past two decades, chirality induced spin selectivity (CISS) has emerged as its own field of research. As a phenomenon, it reveals how the shape of molecules interplay with electron dynamics, showcasing how a helical molecule acts as a filter for electron spins, and thus, provides valuable insights into the correlation between molecular structure and electron properties. The interpretation is that the influence of the helical molecular structure, along with spin-orbit interaction and non-equilibrium conditions causes the charge-carrying electrons to become spin-polarized. Consequently, the structure of these molecules effectively filters electrons based on their spin direction. In this project, we investigate the time evolution of the probability densities for each spin as well as the spin polarization in a simplified model of the helical molecule under non-equilibrium. The problem is treated both analytically for a simpler case and numerically for a more general one. In the more general case, the solutions are divided into two parts, namely 1) The unperturbed system, to determine the properties of the system, and 2) The perturbed system, to investigate consequences of different types of perturbations. Two types of sources of perturbation were considered, namely metals, both magnetic and non-magnetic, and external electric fields, where these are assumed both constant and time-dependent, such as harmonic or decaying. The results from the analytical and numerical models turned out to give essentially the same results, however a larger molecule than the very smallest was required to display the expected symmetry of spin polarization between helicities. The properties of the unperturbed system were consistent withprevious research, and the perturbations had a varying effect, with the magnetic metals producing distinct changes to the spin polarization while the electric field mostly shifted the total charge.

The physical origin of the so-called chirality-induced spin selectivity (CISS) effect has puzzled experimental and theoretical researchers over the past few years. Early experiments were interpreted in terms of unconventional spin–orbit interactions mediated by the helical geometry. However, more recent experimental studies have clearly revealed that electronic exchange interactions also play a key role in the magnetic response of chiral molecules in singlet states. In this investigation, we use spin-polarized closed-shell density functional theory calculations to address the influence of exchange contributions to the interaction between helical molecules as well as of helical molecules with magnetized substrates. We show that exchange effects result in differences in the interaction properties with magnetized surfaces, shedding light into the possible origin of two recent important experimental results: enantiomer separation and magnetic exchange force microscopy with AFM tips functionalized with helical peptides.