arXiv subjects

Shota Ono

Publications and source records attributed to Shota Ono.

At least 19 recordsLinked to original sources

arXiv2026

Beyond geometrical screening in predicting two-dimensional materials

This perspective overviews the family of two-dimensional (2D) materials, which have attracted significant attention due to their properties and potential applications, and discusses how novel 2D materials including van der Waals (vdW) and non-vdW 2D materials have been predicted so far. A few thousand 2D materials have been predicted to be exfoliable or dynamically/thermodynamically stable, whereas a few hundred 2D materials have been synthesized so far, highlighting a gap between the theoretical prediction and experiments. This perspective introduces the recent developments in predicting the synthesis of non-vdW 2D materials.

cond-mat.mtrl-sci↗

arXiv2026

Site preference of chalcogen atoms in 1T$^\prime$ $MX_{2(1-x)}Y_{2x}$ ($M=$ Mo and W; $X, Y=$ S, Se, and Te)

The insulator-metal transition, accompanying the structural phase transition from 2H to 1T$^\prime$ structure, has been reported in two-dimensional W-S-Te and W-Se-Te systems. It is also reported that Te atoms tend to occupy a specific site of the 1T$^\prime$ structure. Here, we study the site preference of chalcogen atoms in $MX_{2(1-x)}Y_{2x}$ ($M=$ Mo and W; $X, Y=$ S, Se, and Te; $0\le x \le 1$) using first-principles approach. We demonstrate that the site preference of chalcogen atoms explains the universal correlation between the formation energy and the Peierls-like distortion amplitude in the 1T$^\prime$ phase. The impact of the site preference on the linear elastic properties is strong, whereas its impact is weak in the non-linear regime. This establishes the structure-property relationships in $MX_{2(1-x)}Y_{2x}$ systems.

cond-mat.mtrl-sci↗

arXiv2025

Hidden layered structures from carbon-analog metastability in metal dichalcogenides

Carbon exhibits both a layered ground state structure that produces two-dimensional (2D) nanosheets and a non-layered diamond structure created under high pressure conditions. Motivated by this metastability relationship, we revisit the ground state structure of metal dichalcogenides that are known to have non-layered pyrite-type structure. Ultrathin films of pyrite-type ZnSe$_2$ spontaneously transform into a layered phase. This phase is identified as a ground state, and the monolayer exhibits strong elastic anisotropy and a semiconducting bandgap larger than that of the pyrite phase by a factor of two. We demonstrate that a two-valued but directional potential energy surface exists along a Bain-like distortion path, hiding the layered ground state. This work implies that many 2D materials are hidden in non-layered materials and connects 2D materials science with surface and high-pressure science.

cond-mat.mtrl-sci↗

arXiv2025

A Framework for Identifying Non-van der Waals 2D Materials

Two-dimensional (2D) materials are categorized into van der Waals (vdW) and non-vdW types. However, no relevant descriptors have been proposed for identifying the latter. Here, we identify the non-vdW 2D materials by calculating the thickness-dependence of total energy of thin films truncated from surfaces. The non-vdW 2D materials exhibit a deviation from the law of exfoliation energy inverse to the number of layers in the monolayer limit. This framework is applied to explore single- and multi-component systems, which predicts the synthesizability of several non-vdW 2D materials including silicene and goldene that are overlooked in the dimensional analysis of the parent crystals and also predicts that a Janus structure exists in nature but is hidden in 3D crystals.

cond-mat.mtrl-sci↗

arXiv2025

Composition-dependent ultrafast luminescence in Cu-Ni alloys: Combined experimental and ab initio study

Properties of Cu-Ni solid solutions have long been studied in physical and materials sciences. Yet, their many-body properties have not been well understood. Here, we investigate ultrafast luminescence in near infrared region for Cu$_{1-x}$Ni$_x$ alloys. The luminescence intensity was the highest in Cu and decreased dramatically by adding Ni, approaching close to the value for pure Ni at $x=0.45$. This composition dependence was well reproduced by calculations assuming two body scattering of the energetic electrons. The luminescent decay rate was not straightforward, i.e., it decreased first by adding Ni up to $x=0.17$ and then started to increase approaching twice the initial value at $x=0.45$. This behavior was in good agreement with ab initio calculations of electron-phonon (e-ph) coupling strength. This work provides a new perspective on the electron relaxation dynamics in solid solutions systems.

cond-mat.mtrl-sci↗

arXiv2024

Breakdown of continuum elasticity due to electronic effects in gold nanotubes

A recent experiment reports a creation of goldene, which is two-dimensional gold with hexagonal structure. By rolling up the goldene, gold nanotubes (GNT) should exist, but their structural and electronic properties are not understood well. Based on first-principles calculations, we demonstrate a breakdown of inverse square law, wherein the curvature energy stored in a GNT decreases with the inverse square of the GNT radius. This is due to the enhanced curvature energy in specific GNTs having nearly flat bands around the Fermi level. We show that the electron states on the flat band of GNT reflect those on the Fermi surface of goldene by using the Bloch and geometric boundary conditions, and that in-plane character of the latter states enhances the curvature energy.

cond-mat.mtrl-sci↗

arXiv2024

Fluorite-type materials in the monolayer limit

The 2H, 1T, and their distorted structures are known as prototype structures of $AB_2$ monolayers. Here, we study a puckered structure that is truncated from the (110) surface of fluorite-type materials. 53 fluorite-type materials are investigated based on first-principles approach. The formation energy calculations indicate that seven systems form the puckered structure in the monolayer limit, while other systems form either 1T, 2H, or distorted 1T structures. The puckered structures of PbF$_2$, PRh$_2$, and Ga$_2$Au exhibit negative Poisson's ratio (NPR) in the out-of-plane direction. An analytical model for the NPR is derived. The surface energy calculations predict the appearance of NPR.

cond-mat.mtrl-sci↗

arXiv2024

Bain distortion of noble metal thin films that exhibit fcc, bct, and reoriented fcc structures

A recent experiment has reported that body-centered cubic (bcc)-structured Ag is realized by bending face-centered cubic (fcc)-structured Ag nanowires [S. Sun {\it et al}., Phys. Rev. Lett. {\bf 128}, 015701 (2022)]. However, the bcc phase has been observed only near the Ag surface. Here, we explore how the bcc phase is stabilized near the surface by compressions. Our first-principles calculations for noble metals, Cu, Ag, and Au, indicate that body-centered tetragonal (bct) rather than bcc structure is preferred due to the surface effect. The bct-fcc boundary treated as a fixed boundary condition is necessary to thermodynamically stabilize the bct phase of Ag nanowire. The correlation between crystal structure and electron density-of-states is also discussed for three noble metals.

cond-mat.mtrl-sci↗

arXiv2023

Structural Properties of Two-Dimensional Strontium Titanate: A First-Principles Investigation

Motivated by the experimental synthesis of two-dimensional (2D) perovskite materials, we study the stability of 2D SrTiO$_3$ from first principles. We find that the TiO$_6$ octahedral rotations emerge in 2D SrTiO$_3$ with a rotation angle twice that in the 3D bulk. The rotation angle decreases significantly with the film thickness, reflecting the strong interlayer coupling that is absent in the conventional 2D materials. Using the molecular dynamics simulations, the cubic-like phase is found to appear above 1000 K that is much higher than the transition temperature of 3D SrTiO$_3$.

cond-mat.mtrl-sci↗

arXiv2022

Small atoms fall into bulk from non-close-packed surfaces?

Surface rippling has been observed when atoms of $X$ and $A$ are mixed on the $A$ substrate surface. The rippling amplitude has been estimated using hard sphere models. We present a gedanken experiment predicting a penetration of small atoms into bulk through the (100) surface. To understand how the electronic effects alter this picture, we investigate the surface rippling of $X/A(100)$ from first-principles, assuming $X=$ H to Bi except for noble gases and $A=$ Cu, Ag, and Au. We show that the small atoms (such as H, C, N, O and F) attract electrons from the substrate due to the large electronegativity, which prevent them from passing through the void in the (100) surface. The behaviors of small atoms are further explored by studying lateral displacements of the top layer in the $A$ substrate and a formation of the $X$ dimer above, below, and across the top layer. The present work provides an example to understand when atoms are not hard spheres.

cond-mat.mtrl-sci↗

arXiv2022

Two-dimensional ionic crystals: The cases of IA-VII alkali halides and IA-IB CsAu

The alkali halides, known as ionic crystals, have the NaCl-type or CsCl-type structure as the ground state. We study the structural, vibrational, and electronic properties of two-dimensional (2D) ionic crystals from first-principles. Two potential structures that are hexagonal and tetragonal are investigated as structural templates. Through phonon dispersion calculations, 8 and 16 out of 20 alkali halides in the hexagonal and tetragonal structures are dynamically stable, respectively. The electron energy gaps range from 6.8 eV for LiF to 3.9 eV for RbI and CsI in the tetragonal structure within the generalized gradient approximation. By considering the Madelung energy and the core-core repulsion, we propose a hard sphere model that accounts for the nearest-neighbor bond length and the cohesive energy of 2D alkali halides. The 2D CsAu in the tetragonal structure is also predicted to be stable as an ionic crystal including only metallic elements, showing a band gap of 2.6 eV that is higher than that of the 3D counterparts.

cond-mat.mtrl-sci↗

arXiv2022

Finding the stable structures of 2D hexagonal materials with Bayesian optimization: Beyond the structural relationship with 3D crystals in weakly-bonded binary systems

The graphene-graphite relationship in structural geometry is a basic principle to predict novel two-dimensional (2D) materials. Here, we demonstrate that this is not the case in binary metallic systems. We use the Bayesian optimization framework combined with the density-functional theory approach to determine the stable configuration of atomic species on a hexagonal plane. We show that the optimized structure of 2D Cu-Au exhibits the hexagonal lattice of a hexagonal ring of Cu atoms containing one Au atom, where the number of the Cu atoms is larger than that of the Au atoms in the unit cell, which is difficult to speculate from the atomic distribution of CuAu in the L1$_0$ structure. We also show that 2D Cu-$X$ with $X=$ Be, Zn, and Pd have hexagonal or elongated rings containing different atoms in the unit cell. Based on the binary Lennard-Jones model, we propose that such structures can appear for weakly-bonded systems located in between the phase-separated and strongly-bonded systems with the interatomic interaction energy between different species.

cond-mat.mtrl-sci↗

arXiv2021

Mapping the metastability of Lennard-Jones clusters by the maximum vibrational frequency

We study the structure-stability relationship of the Lennard-Jones (LJ) clusters from a point of view of vibrations. By assuming the size up to $N=1610$, we demonstrate that the $N$-dependence of the maximum vibrational frequency reflects the geometry of the core (the interior of cluster) that will determine the overall geometry of the cluster. This allows us to identify the formation of non-icosahedral structures for $N\le 150$, the vacancy formation at the core for $N\ge 752$, and the transition from icosahedral to decahedral structures at $N = 1034$. We apply the maximum frequency analysis to classify metastable clusters for $19\le N \le 39$, where transformation pathways between different structures are visualized, and the energy barrier height is estimated simultaneously.

cond-mat.mtrl-sci↗

arXiv2021

Stability of B2 compounds: Role of the $M$ point phonons

Although many binary compounds have the B2 (CsCl-type) structure in the thermodynamic phase diagram, an origin of the structural stability is not understood well. Here, we focus on 416 compounds in the B2 structure extracted from the Materials Project, and study the dynamical stability of those compounds from first principles. We demonstrate that the B2 phase stability lies in whether the lowest frequency phonon at the $M$ point in the Brillouin zone is endowed with a positive frequency. We show that the interatomic interactions up to the fourth nearest neighbor atoms are necessary for stabilizing such phonon modes, which should determine the minimum cutoff radius for constructing the interatomic potentials of binary compounds with guaranteed accuracy.

cond-mat.mtrl-sci↗

arXiv2021

Hypothetical FrAu: An outlier in the B2 compounds

The ordered alloys of alkali metals (Rb and Cs) and gold (Au) have the B2 (CsCl-type) structure and show a semiconducting property, irrespective to the metallic constituents. Francium (Fr) is classed as an alkali metal and is expected to form the B2 structure with Au. However, it is difficult to synthesize such a compound experimentally due to a half-life of a few ten minutes in the Fr atom. In this paper, by using the first-principles method, we study the structural and electronic properties of FrAu in the B2 structure. The FrAu has a relatively large lattice constant and a relatively small bulk modulus among 310 B2 compounds. The profiles of the electron and phonon band structures of the FrAu are quite similar to those of the CsAu. We predict that the FrAu is included to one of the ionic compounds as well as the RbAu and CsAu.

cond-mat.mtrl-sci↗

arXiv2021

Magic numbers for vibrational frequency of charged particles on a sphere

Finding minimum energy distribution of $N$ charges on a sphere is known as the Thomson problem. Here, we study the vibrational properties of the $N$ charges in the lowest energy state within the harmonic approximation for $10\le N\le 200$ and for selected sizes up to $N=372$. The maximum frequency $ω_{\rm max}$ increases with $N^{3/4}$, which is rationalized by studying the lattice dynamics of a two-dimensional triangular lattice. The $N$-dependence of $ω_{\rm max}$ identifies magic numbers of $N=12, 32, 72, 132, 192, 212, 272, 282$, and 372, reflecting both a strong degeneracy of one-particle energies and an icosahedral structure that the $N$ charges form. $N=122$ is not identified as a magic number for $ω_{\rm max}$ because the former condition is not satisfied. The magic number concept can hold even when an average of high frequencies is considered. The maximum frequency mode at the magic numbers has no anomalously large oscillation amplitude (i.e., not a defect mode).

cond-mat.mes-hall↗

arXiv2021

Comprehensive search for buckled honeycomb binary compounds based on noble metals (Cu, Ag, and Au)

Honeycomb structure has been frequently observed in two-dimensional (2D) materials. CuAu in the buckled honeycomb (BHC) structure has been synthesized recently, which is the first case of 2D intermetallic compounds. Here, the dynamical stability of 2D $AX$ in the BHC structure, where $A=$ Cu, Ag, and Au and $X$ is a metallic element in the periodic table, is systematically studied by calculating phonon dispersions from first-principles. Among 135 $AX$, more than 50 $AX$ are identified to be dynamically stable. In addition, (i) a relationship between the dynamical stability and the formation energy, (ii) a correlation of dynamical stability between different constituents $A$, (iii) a trend of lattice parameters, and (iv) electronic and magnetic properties are discussed. Furthermore, a stable phase of B11-type AuZr is predicted based on both the result (ii) and the stability relationship between 2D and three-dimensional structures. The present findings stimulate future studies exploring physics and chemistry of 2D intermetallic compounds.

cond-mat.mtrl-sci↗

arXiv2021

Metastability relationship between two- and three-dimensional crystal structures: A case study of the Cu-based compounds

Some of the three-dimensional (3D) crystal structures are constructed by stacking two-dimensional (2D) layers. It remains unclear whether this geometric concept is related to the stability of ordered compounds and whether this can be used to computational materials design. Here, using first principles calculations, we investigate the dynamical stability of copper-based compounds Cu$X$ (a metallic element $X$) in the B$_h$ and L1$_1$ structures constructed from the buckled honeycomb (BHC) structure and in the B2 and L1$_0$ structures constructed from the buckled square (BSQ) structure. We demonstrate that (i) if Cu$X$ in the BHC structure is dynamically stable, those in the B$_h$ and L1$_1$ structures are also stable. Although the interrelationship of the metastability between the BSQ and the 3D structures (B2 and L1$_0$) is not clear, we find that (ii) if Cu$X$ in the B2 (L1$_0$) structure is dynamically stable, that in the L1$_0$ (B2) is unstable, analogous to the metastability relationship between the bcc and the fcc structures in elemental metals. The total energy curves for Cu$X$ along the tetragonal and trigonal paths are also investigated.

cond-mat.mtrl-sci↗