arXiv subjects

Naoki Morishita

Publications and source records attributed to Naoki Morishita.

5 recordsLinked to original sources

arXiv2025

Modulated Dirac bands and integer hopping ratios in a honeycomb lattice of phenalenyl-tessellation molecules

A family of nanographene molecules called phenalenyl-tessellation molecules (PTMs) exhibits two types of zero modes: a $\sqrt{3} \times \sqrt{3}$ type that spreads over the entire molecule and a vacancy-localized type. A periodic system of PTMs is expected to have low-energy bands that strongly reflect the properties of the zero modes of PTMs as effective atoms. In this study, we show that the low-energy Dirac bands in a class of honeycomb PTMs (H-PTM) can be represented by an effective honeycomb model which is determined only by the connections between neighboring effective atoms.The hopping parameters of H-PTM in each direction take positive integer ratios according to the connection order between two PTMs.By structurally designing each PTM, we can change the connection order of the PTMs and hence modulate the energy gap and the Fermi velocity of the Dirac band of the H-PTM. Moreover, we confirm that Dirac bands coexist with vacancy-localized zero modes in the H-PTM with vacancies.The result indicates that the nanographene structure arranging PTMs as effective atoms extends material design freedom that effectively generates a modulated Dirac electron system with coexisting localized electron spins for graphene-based electronic and quantum devices.

cond-mat.mes-hall↗

arXiv2025

Designing a polymerized phenalenyl tessellation molecule to realize a super-honeycomb antiferromagnetic S = 3/2 spin system

In a multiply hydrogenated polymer of phenalenyl tessellation molecules (PTMs), spatially overlapping zero modes appear, and three spin-aligned electron spins per PTM are generated through direct exchange interactions in the strongly correlated electron system. This interaction was used to design a two-dimensional (2D) $S = 3/2$ Heisenberg spin system on a honeycomb lattice. Simulations of the electronic structure using density functional theory with the Wannierization method revealed an array of nonbonding molecular orbitals (zero modes) in the hydrogenated nanographene structure. Our analysis of the onsite interaction strength indicated that each zero mode was half-filled with a spin-active electron owing to electron correlation effects. The low-energy subspace of the resulting zero mode-tight-binding model suggests the formation of a 2D antiferromagnetic $S = 3/2$ Heisenberg system with an entangled quantum spin ground state.

cond-mat.mtrl-sci↗

arXiv2021

Zero-energy modes in super-chiral nanographene networks of phenalenyl-tessellation molecules

We have derived a general rule for the appearance of zero-energy modes in super-chiral defective nanographene. This so-called "super-zero-sum rule" defines the appearance of zero modes in a new class of materials, which we call polymerized phenalenyl-tessellation molecules (poly-PTMs). Through theoretical modeling of the electronic states in these molecular forms, we provide concrete solutions for achieving the quantum-spin systems needed in quantum-information devices. The two-dimensional graph of electronic $π$-orbitals in the poly-PTM possesses a number of localized zero modes equivalent to that of vacancies in PTMs. In addition to the modes confined to each PTM, another type of zero mode may appear according to the super-zero-sum rule supported by super-chirality. Since the magnetic interactions among quantum spins in the zero modes are determined by how they appear (which is governed by the super-zero-sum rule), our rule is indispensable for designing quantum-information devices using electron zero modes in poly-aromatic hydrocarbons and defective graphene with vacancies.

cond-mat.mes-hall↗

arXiv2016

Theoretical Analysis on Pseudo-Degenerate Zero-Energy Modes in Vacancy-Centered Hexagonal Armchair Nanographene

Deriving mathematical expressions of two zero modes for a $π$-band tight-binding model, we identify a class of bipartite graphs having the same number of subgraph sites, where each graph represents one of the quasi-hexagonal nanographene molecule with a center vacancy (VANG). Indeed, in a VANG molecule, C$_{60}$H$_{24}$, showing stability in a density-functional simulation at the highest occupied level, there appear two pseudo-degenerate zero modes, a vacancy-centered quasi-localized zero mode, and extending zero mode with a $\sqrt{3} \times \sqrt{3}$ structure. Since there is a finite energy gap between these two zero-energy modes and the other modes, low-lying states composed of quasi-degenerate zero modes appear as magnetic multiplets. Thus, the unique magnetic characteristics derived in our theory are expected to hold for synthesized VANG molecules in reality.

cond-mat.mes-hall↗

arXiv2014

Magnetic correlation effects by the topological zero mode in a hydrogenated graphene vacancy $V_{111}$

Electron correlation effects caused by the topological zero mode of a hydrogenated graphene vacancy, $V_{111}$, with three adsorbed hydrogen atoms is discussed theoretically. A Kondo model is derived from the multi-reference representation of the density functional theory, where exchange scattering processes between the zero mode and low-energy modes in the Dirac cones are estimated. Even when the Dirac cone is slightly off from the charge neutral point, a finite on-site correlation energy, $U_0$, for the zero mode of an isolated $V_{111}$ allows the half-filling of the localized level giving a spin $s=1/2$. The anti-ferromagnetic Kondo screening mediated by higher order scattering processes becomes dominant in the dilute limit of the vacancies. Our estimation of relevant two body interactions certifies appearance of the Kondo effect at low temperatures.

cond-mat.mes-hall↗