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Tomoyuki Yamamoto

Publications and source records attributed to Tomoyuki Yamamoto.

13 recordsLinked to original sources
arXiv2023

Electronic, Optical and Elastic Properties of Perovskite CsSnI$_{3-x}$Cl$_x$ (x = 0, 1, 2, 3): Using First Principles Study

The structural, electronic, optical and elastic properties of perovskites of the $α$-, $β$- and $γ$-phase family CsSnI$_{3-x}$Cl$_x$ (x = 0, 1, 2, 3) are analyzed by ab initio methods using GGA, SCAN and HSE06. It is found that the lattice constants of CsSnI$_{3-x}$Cl$_x$ decrease with decreasing anion size from I to Cl, and when passing from CsSnI$_3$ to CsSnCl$_3$, the band gap width decreases slightly at the beginning and then increases. The optical characteristics were evaluated and it was found that the $α$-, $β$-phase of CsSnI$_{3-x}$Cl$_x$ at $x=1$ has a wide absorption range and high absorption coefficients compared to other Cl concentrations. The elastic constants of $α$-CsSnI$_{3-x}$Cl$_x$ perovskite have been calculated and their bulk moduli, shear moduli, sound velocities and Debye temperatures under pressure in the range from 0 to 30 GPa have been considered, with the external pressure induction increasing in steps of 5 GPa. These materials are found to be mechanically stable perovskites according to Born stability criteria and low bulk moduli. Thus, we predict that perovskites of the CsSnI$_{3-x}$Cl$_x$ family are good candidates for optical materials used in solar cells.

cond-mat.mtrl-sci↗
arXiv2023

The Effect of Structural Phase Changes on Fermi Level Shifts and Optoelectronic Properties of Lead-Free CsSnI3 Perovskites

The work carried out first-principles calculations within the framework of density functional theory to study the structural stability of the CsSnI3 compound and the influence of phase transitions on their electronic and optical properties. Using the GGA and SCAN functionals, the relaxed structures of the CsSnI3 phases were obtained and their geometric characteristics were assessed. Using the Phonopy code based on VASP, calculations of phonon and thermodynamic properties were performed, and the temperatures of phase transitions of CsSnI3 were determined. Electronic properties and Fermi level shifts as a result of phase transformations of CsSnI3 were assessed using the HSE06 functional and machine learning prediction. The values of the complex dielectric constant and the refractive index of all phases of the CsSnI3 were determined.

cond-mat.mtrl-sci↗
arXiv2023

Construction of topological representation of geometric patterns using Cantor self-similar set

Universal representation of geometric patterns of disordered matters is investigated with the aid of general topology. By utilizing the result obtained in the previous study (S. Ohmori, et.al., Phys. Scr. 94, 105213 (2019)) that any patterns can be represented by a specific topological space, a construction of topological representation of patterns using Cantor set is shown. The obtained topological representations are then demonstrated by the contractions that characterize the self-similarity of Cantor set. For some practical geometric patterns, e.g., network, dendritic, and clusterized patterns, their topological representations are focused on.

math-ph↗
arXiv2018

Universal topological representation of geometric patterns

We discuss here geometric structures of condensed matters by means of a fundamental topological method. Any geometric pattern can be universally represented by a decomposition space of a topological space consisting of the infinite product space of 0 and 1, in which a partition with a specific topological structure determines a character of each geometric structure.

math-ph↗
arXiv2017

Topological characterization of network structures of aggregates of atoms

Two distinct structures of aggregates of atoms connected by anisotropic bonds with a network configuration are discussed from the viewpoint of a point set topology. A specific topological space connects the two types of topological structures based on the equivalence classes, which show the main difference of the structures between the aggregates of atoms with or without a clusterized structure.

math-ph↗
arXiv2014

A product space of {0, 1} and an abstract polycrystal

A partition of a $Λ(CardΛ\succ \aleph_0)$-product space of $\{0,1\}$ defines an abstract polycrystal composed of abstract singlecrystals a decomposition space (space of equivalence classes) of each of which is self-similar.

math.GN↗
arXiv2014

On a difference in topological nature between Cantor middle-third set and Sierpiński carpet

Both Cantor middle-third set and Sierpiński carpet are self-similar, perfect, compact metric spaces. In spite of the similarity of the mathematical procedure of construction, there exists between them a fundamental difference in topological nature, and this difference affects the methods of construction of an interesting non-trivial quotient space of them. The totally disconnectedness (or, more generally, zero-dimensional) enables Cantor middle-third set to have a non-trivial quotient space which is self-similar. On the other hand, concerning Sierpiński carpet, because of the connectedness of its structure, no non-trivial quotient space which is self-similar can be constructed by such an elegant procedure as that for Cantor middle-third set. Various topologically significant nature specific to Cantor middle-third set owe mainly to the totally disconnectedness of the set.

math.GN↗
arXiv2014

A dendrite generated from {0,1}^Λ, CardΛ\succ \aleph

The existence of a decomposition space with a dendritic structure of a topological space $(\{0,1\}^Λ,τ_{0}^Λ)$ is discussed. Here, $Λ$ is any set with the cardinal number $\succ \aleph , \{0,1\}^{Λ}=\{φ:Λ\rightarrow \{0,1\}\}, τ_0$ is the discrete topology for $\{0,1\}$ and the topology $τ_0^{Λ}$ for $\{0,1\}^Λ$ is the topology with the base $β=\{ ~;~G_{λ_1}\in τ_0,\dots,G_{λ_n}\in τ_0, \{λ_1,\dots,λ_n\}\subset Λ,n\in {\bf N}\}$ where the notation $ $ concerning the subset $E_{λ_i}, i=1,\dots,n$ of $\{0,1\}$ denotes the set $\{φ:Λ\rightarrow \{0,1\}~;~φ(λ_1)\in E_{λ_1},\dots,φ(λ_n)\in E_{λ_n}, φ(λ)\in \{0,1\}, λ\in Λ-\{λ_1,\dots,λ_n\}\}$.

math.GN↗
arXiv2011

On the Existence of a Self-Similar Coarse Graining of a Self-Similar Space

A topological space homeomorphic to a self-similar space is demonstrated to be self-similar. There exists a self-similar space $S$ whose coarse graining is homeomorphic to $S$. The coarse graining of $S$ is, therefore, self-similar again. In the same way, the coarse graining of the self-similar coarse graining of $S$ is, furthermore, self-similar. These situations succeed endlessly. Such a self-similar $S$ is generated actually from an intense quadratic dynamics.

math-ph↗
arXiv2006

Awaking of ferromagnetism in GaMnN through control of Mn valence

Room temperature ferromagnetism of GaMnN thin films is awaked by a mild hydrogenation treatment of samples synthesized by molecular beam epitaxy. Local environment of Mn atoms is monitored by Mn-L2,3 near edge x-ray absorption fine structure (NEXAFS) technique. Doped Mn ions are present at substitutional sites of Ga both before and after the hydrogenation. No secondary phase can be detected. Major valency of Mn changes from +3 to +2 by the hydrogenation. The present result supports the model that the ferromagnetism occurs when Mn2+ and Mn3+ are coexistent and holes in the mid- gap Mn band mediate the magnetic coupling.

cond-mat.mtrl-sci↗
arXiv2005

Intrinsic mechanism for high temperature ferromagnetism in GaMnN

Considerable efforts have been devoted recently to synthesize diluted magnetic semiconductors having ferromagnetic properties at room temperature because of their technological impacts for spintronic devices. In 2001 successful growth of GaMnN films showing room temperature ferromagnetism and p-type conductivity has been reported. The estimated Curie temperature was 940K at 5.7% of Mn, which is highest among diluted magnetic semiconductors ever been reported. However, the electronic mechanism behind the ferromagnetic behaviour has still been controversial. Here we show experimental evidence using the ferromagnetic samples that Mn atoms are substitutionally dissolved into the GaN lattice and they exhibit mixed valences of +2 (majority) and +3 (minority). The p-type carrier density decreases significantly at very low temperatures. At the same time, magnetization dramatically decreases. The results imply that the ferromagnetic coupling between Mn atoms is mediated by holes in the mid-gap Mn band.

cond-mat.mtrl-sci↗