arXiv subjects

Koichi Kusakabe

Publications and source records attributed to Koichi Kusakabe.

At least 19 recordsLinked to original sources

arXiv2025

Spin current generation driven by skyrmion dynamics under magnetic anisotropy and polarized microwaves

We have investigated the spin-current pumped by the skyrmion-host material with the lack of inversion symmetry through the microwave resonance process. The effects of magnetic anisotropy and polarized microwaves are examined by micromagnetic simulations. Our results reveal two distinct skyrmion phases, designated as SkX type-I and II, which emerge at low ($K_z<0.1$ meV) and high ($K_z>0.1$ meV) magnetic anisotropy constants, respectively, having different characteristics of spin excitations. The SkX type-I exhibits spin dynamics where the resonant frequency of the breathing mode is lying in between the clockwise and counterclockwise gyration modes of Bloch-type skyrmion at a very low anisotropy, and is crossing over the counterclockwise mode at $K_z \sim 0.04$ meV. Meanwhile, the SkX type-II exhibits distinct spin excitations in which the clockwise mode is notably absent, while the counterclockwise modes exist at both low and high resonant frequencies. This suggests that the magnetic anisotropy plays an essential role in the spin dynamics. Furthermore, the resulting spin excitations induce spin currents with exotic features under the polarized microwaves. The spin currents induced, for instance, by low-lying in-plane excitations are strongly enhanced under the left-handed circularly polarized microwaves, but quenched by the right-handed circularly polarized microwaves regardless of the sign of the Dzyaloshinskii-Moriya interaction. These results may pave the way for understanding the non-trivial interplay between magnetic anisotropy and polarized microwaves in the generation of spin currents by a resonant process.

cond-mat.mtrl-sci↗

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

Exploration of the potential energy surface for the conformational interconversion of the amyloid $β$ peptide at the fibril end

The formation of amyloid fibrils comprising amyloid $β$ (A$β$) peptides is associated with the pathology of Alzheimer's disease. In this study, we theoretically investigated the A$β$ structure at the fibril end using the density functional theory calculation. Several twisted conformations were identified as local minima in which a part of the peptide chain bends upward while the rest remains bound to the lower A$β$ monomer. Fibril-to-twisted conformational transition exhibited endothermic behavior, with endothermic energy increasing as more backbone hydrogen bonds were broken. In addition, the loss of van der Waals interaction from the hydrophobic sidechain contributed to endothermicity. The nudged elastic band method was applied to analyze the potential energy surface connecting the fibril and twisted conformations. Comparison of the activation barriers between different twisted conformations revealed that certain twisted conformations returned relatively easily to the fibril conformation, whereas others encountered a higher activation barrier and reverted less readily. Detailed structural analysis revealed that the twisted conformation's propensity to return originates from the local steric hindrance imposed by the sidechain near the torsional axis.

physics.bio-ph↗

arXiv2025

Synthesis, crystal and electronic structures, and second harmonic generation of La 4Ge 3S12

The crystal structure of La 4 Ge 3S 12 has been known to be noncentrosymmetric for almost four decades. This characteristic inversion symmetry breaking suggests the presence of nonlinear optical properties. Yet only recently have nonlinear optical phenomena such as second harmonic generation (SHG) been reported in this material. In this study, we synthesized La4Ge3S12 using the direct reaction method and characterized the composition, crystal structure, and electronic structure using electron probe microanalysis, powder and single-crystal X-ray diffraction, and X-ray photoelectron spectroscopy. The experimentally measured electronic structure is in line with that obtained using first-principles calculations. In addition, we observed the nonlinear optical properties of La4Ge3S12 in response to an ultrashort infrared pulsed laser. We found that the intensity of the SHG depends quadratically on the intensity of the incident light, mirroring the intrinsic nature of nonlinear optics.

cond-mat.mtrl-sci↗

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↗

arXiv2024

Impact of Monoatomic Vacancies in 2D Materials on the Performance of Magnetic Tunnel Junction Devices: Insights from Configurations and Interface Interactions

We investigate the impact of monoatomic vacancies in 2D materials on the performance of magnetic tunnel junction (MTJ) devices using first-principles calculations within Density Functional Theory (DFT). Specifically, we analyze the influence on hexagonal boron nitride (hBN) with various layer configurations, uncovering distinct transmission probability patterns. Transmission calculations were conducted using the Landauer-Büttiker formula employing the Non-Equilibrium Green's Function (NEGF) method. In the Ni/hBN(V$_B$)-hBN/Ni system, a significant reduction in transmission probability was observed compared to non-vacancy configurations. However, when two hBN vacancies were considered, creating the Ni/hBN(V$_B$)-hBN(V$_B$)/Ni MTJ system, a new transmission channel mediated by vacancy localized states emerged. The introduction of a monoatomic boron vacancy in the middle hBN layer of the Ni/3hBN/Ni system revealed nuanced effects on the transmission probability, highlighting alterations in the spin minority and majority channels. Additionally, we explore the monoatomic vacancy in the graphene layer in the Ni/hBN-Gr-hBN/Ni MTJ, uncovering a unique transmission channel influenced by the proximity effect. Our findings suggest that the creation of monoatomic vacancies on the insulator barrier of 2D materials induces distinctive characteristics shaped by the interaction between the surface state of the electrode and the localized state of the monoatomic vacancy layer in the MTJ system.

cond-mat.mtrl-sci↗

arXiv2023

High Magnetoresistance Ratio on hBN Boron-Vacancy/Graphene Magnetic Tunnel Junction

We presents a new strategy to create a van der Waals-based magnetic tunnel junction (MTJ) that consists of a three-atom layer thickness of graphene (Gr) sandwiched with hexagonal boron nitride (hBN) by introducing a monoatomic Boron vacancy in both hBN layers. The magnetic properties and electronic structure of the system were investigated using density functional theory (DFT), while the transmission probability of the MTJ was investigated using the Landauer-Büttiker formalism within the non-equilibrium Green function method. The Stoner gap was found to be created between the spin-majority channel and the spin-minority channel on LDOS of the hBN monoatomic boron-vacancy (V$_B$) near the vicinity of Fermi energy, creating a possible control of the spin valve by considering two different magnetic allignment of hBN(V$_B$) layers, anti-parallel and parallel configuration. The results of the transmission probability calculation showed a high electron transmission in the parallel configuration of the hBN(V$_B$) layers and a low transmission when the antiparallel configuration was considered. A high TMR ratio of approximately 400% was observed when comparing the antiparallel and parallel configuration of hBN(V$_B$) layers in the hBN (V$_B$)/Gr/hBN(V$_B$), giving the highest TMR for the thinnest MTJ system.

cond-mat.mes-hall↗

arXiv2023

Possible bi-stable structures of pyrene-butanoic-acid-linked protein molecules adsorbed on graphene: Theoretical study

We theoretically analyze possible multiple conformations of protein molecules immobilized by 1-pyrenebutanoic-acid-succinimidyl-ester (PASE) linkers on graphene. The activation barrier between two bi-stable conformations exhibited by PASE is confirmed to be based on the steric hindrance effect between a hydrogen on the pyrene group and a hydrogen on the alkyl group of this molecule. Even after the protein is supplemented, this steric hindrance effect remains if the local structure of the linker consisting of an alkyl group and a pyrene group is maintained. Therefore, it is likely that the kinetic behavior of a protein immobilized with a single PASE linker exhibits an activation barrier-type energy surface between the bi-stable conformations on graphene. We discuss the expected protein sensors when this type of energy surface appears and provide a guideline for improving the sensitivity, especially as an oscillator-type biosensor.

physics.chem-ph↗

arXiv2022

Theoretical Analysis on the Stability of 1-Pyrenebutanoic Acid Succinimidyl Ester Adsorbed on Graphene

The adsorbed structure of 1-pyrenebutanoic acid succinimidyl ester (PASE) on graphene was investigated based on density functional theory. We found two locally stable structures: a straight structure with the chainlike part of butanoic acid succinimidyl ester (BSE) lying down and a bent structure with the BSE part directed away from graphene, keeping the pyrene (Py) part adsorbed on graphene. Then, to elucidate the adsorption mechanism, we separately estimated the contributions of the Py and BSE parts to the entire PASE adsorption, and the adsorption effect of the BSE part was found to be secondary in comparison to the contribution of the Py. Next, the mobility of the BSE part at room temperature was confirmed by the activation energy barrier between straight and bent structures. To take account of the external environment, we considered the presence of amino acids and the hydration effect by a three-dimensional reference interaction site model. The contributions of glycine molecules and the solvent environment to stabilizing the bent PASE structure relative to the straight PASE structure were found. Therefore, the effect of the external environment around PASE is of importance when the standing-up process of the BSE part from graphene is considered.

cond-mat.mtrl-sci↗

arXiv2022

Discovery of nanographene for hydrogen storage solving low reversibility issues

We found good reversibility of hydrogen uptake-release in vacancy-centered hexagonal armchair nanographene (VANG) based on density functional theory calculation. VANG has a triply hydrogenated vacancy (V$_{111}$) at the center and acts as a self-catalytic property to reduce an activation barrier of hydrogen uptake-release. We found remarkable features in an almost equal value of the activation energy barrier of 1.19 eV for hydrogen uptake and 1.25 eV for hydrogen release on V$_{111}$ of VANG. The dehydrogenation showed slightly exothermic and the hydrogenation became slightly endothermic, suggesting the efficiency of hydrogen uptake-release. In high hydrogen coverage, the quintuply hydrogenated vacancy (V$_{221}$) is formed with some hydrogenated located in the in-plane and armchair edges. This structure produces an exothermic hydrogen release from the in-plane with an energy barrier of not more than 2 eV. This finding potentially addresses the low reversibility issues in the organic chemical hydrides as hydrogen storage materials.

cond-mat.mtrl-sci↗

arXiv2021

Ultimate In-plane Magnetoresistance Ratio of Graphene by Controlling the Gapped Dirac Cone through Pseudospin

$\require{mediawiki-texvc}$ A theoretical study is presented on the in-plane conductance of graphene that is partially sandwiched by Ni(111) slabs with a finite size and atom-scale width of $\approx12.08 Å$. In the sandwiched part, the gapped Dirac cone of graphene can be controlled via pseudospin by changing the magnetic alignment of the Ni(111) slabs. When the magnetic moments of the upper and lower Ni(111) slabs have antiparallel and parallel configurations, the bandgap at the Dirac cone is open and closed, respectively. The transmission probability calculation for the in-plane conductance of the system indicated that the antiparallel configuration would result in nearly zero conductance of $E-E_F=0.2$ eV. In the parallel configuration, the transmission probability calculation indicated that the system would have a profile similar to that of pristine graphene. A comparison of the transmission probabilities of the antiparallel and parallel configurations indicated that a high magnetoresistance of $1450\%$ could be achieved. An ultimate magnetoresistance can be expected if the Ni(111) slab widths are increased to the nanometer scale.

cond-mat.mes-hall↗

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↗

arXiv2020

Material Optimization of Potential High-$T_{\text{c}}$ Superconducting Single-layer Cuprates

We investigated the material parameters of several single-layer cuprates, including those with fluorinated buffer layers, with the aim of identifying possible high-temperature superconductors. To evaluate the material parameters, we use the Wannierization techniques and the constrained random phase approximation. The obtained single-band Hubbard models are studied using the fluctuation-exchange approximation. Comparison among several cuprates reveals unknown high-$T_{\text{c}}$ superconductors. In, Ga, Al, and Cd compounds in particular show the potential to exhibit higher-$T_{\text{c}}$ superconductivity than Hg1201.

cond-mat.supr-con↗

arXiv2020

Effect of on-site Coulomb repulsion on ferromagnetic fluctuations in heavily over-doped cuprates

We theoretically study ferromagnetic (FM) fluctuations that are experimentally observed in the heavily overdoped region of cuprate superconductors. To explore the origin of FM fluctuations, we evaluate the spin susceptibilities of a single-band Hubbard model within the fluctuation exchange approximation. Model parameters are derived using the Wannierization technique and the constrained random phase approximation method based on the maximally localized Wannier functions. The constrained random phase approximation calculations reveal that the on-site Coulomb interaction decreases with an increase in hole doping. By taking this reduction of the on-site Coulomb interaction into account, the emergence of FM fluctuations in heavily overdoped cuprates can be explained.

cond-mat.supr-con↗

arXiv2019

Interplanar stiffness in defect-free monocrystalline graphite

The interplanar bond strength in graphite has been identified to be very low owing to the contribution of the van der Waals interaction. However, in this study, we use microscopic picosecond ultrasound to demonstrate that the elastic constant, $C_{33}$, along the $c$ axis of defect-free monocrystalline graphite exceeds 45 GPa, which is higher than reported values by 20\%. Existing theories fail to reproduce this strongly correlated interplanar system, and our results, thus, indicate the necessity for improvement. Since the LDA+U+RPA method, including both random phase approximation correlation and short-range correlation in $p$ Wannier orbitals, shows better agreement with the observation than LDA or even than ACFDT-RPA, the experimental results indicate non-negligible electron correlation effects with respect to both the short-range and long-range interactions.

cond-mat.mtrl-sci↗

arXiv2019

Spin-Current Control by Induced Electric-Polarization Reversal in Ni/hBN/Ni: A Cross-Correlation Material

We undertook an ab-initio study of hexagonal boron nitride (hBN) sandwiched between Ni(111) layers to examine the interface of this material structure. We considered Ni(111)/hBN/Ni(111) with a slab with three Ni atomic layers to determine the exact atom arrangement at the interface. The density functional theory calculations for 36 stacking arrangements, which are doubled with respect to the magnetic alignment of slabs in an anti-parallel configuration (APC) and parallel configuration (PC), revealed that the number of formed weak chemical bonds, in the pd-hybridization between the N and Ni atoms, is decisive. A maximum of two pd-hybridization bonds stabilized the structure, with APC proving to be the most favorable magnetic alignment, in line with the results of previous experimental studies. In the lowest energy state, an induced magnetic moment at an N site appears when N is moved closer to one of the Ni atoms. Interestingly, the moment direction is switched by the position of the N layer in the resulting bi-stable state with electrical polarization when APC is chosen. The transmission probability calculation of Ni/hBN/Ni having the determined interface structure at the center of the junction exhibits a spin-filtering effect where the spin-polarized current is controlled by the electric field when a field-induced reversal of the polarization is realized.

cond-mat.mes-hall↗

arXiv2018

Tunable induced magnetic moment and in-plane conductance of graphene in Ni/graphene/Ni nano-spin-valve-like structure: a first principles study

This study theoretically investigated the magnetic properties and electronic structure of a graphene-based nano-spin-valve-like structure. Magnetic nickel layers on both sides of the graphene were considered. A spin-polarized generalized-gradient approximation determined the electronic states. In an energetically stable stacking arrangement of graphene and two nickel layers, the anti-parallel spin configuration of the underlayer and overlayer magnetic moments had the lowest energy, which is in agreement with previous experimental studies. The spin density mapping and obtained band-structure results show that when the upper and lower Ni(111) slabs have an anti-parallel (parallel) magnetic-moment configuration, the carbon atoms of sublattices A and B will have an antiferromagnetic (ferromagnetic) spin configuration. A band gap at the Dirac cone was open when the alignment had an anti-parallel configuration and closed when the alignment had a parallel configuration. Therefore, the in-plane conductance of the graphene layer depends on the magnetic alignment of the two nickel slabs when the Fermi level is adjusted at the Dirac point. Both the magnetic properties and electronic structures of the Ni/graphene/Ni nanostructure cause the system to be a new prospective spintronic device showing controllable in-plane magnetoresistance.

cond-mat.mes-hall↗

arXiv2018

Superconductivity arising from layer-differentiation in multi-layer cuprates

In order to theoretically identify the factors governing superconductivity in multi-layer cuprates, a three-layer Hubbard model is studied with the two-particle self-consistent (TPSC) approach so as to incorporate electron correlations. The linearized Eliashberg equation is then solved for the gap function in a matrix form to resolve the role of outer CuO$_2$ planes (OPs) and inner plane (IP). We show that OPs dominate IP in the $d_{x^{2}-y^{2}}$-wave superconductivity, while IP dominates in the antiferromagnetism. This comes from an electron correlation effect in that the correlation makes the doping rates different between OPs and IP (i.e., a self-doping effect), which occurs in intermediate and strong correlation regimes. Namely, the antiferromagnetic fluctuations in IP are stronger due to a stronger electron correlation, which simultaneously reduces the quasiparticle density of states in IP with a suppressed $d_{x^{2}-y^{2}}$-wave superconductivity. Intriguingly, while the off-diagonal (inter-layer) elements in the gap function matrix are tiny, {\it inter-layer pair scattering} processes are in fact at work in enhancing the superconducting transition temperature $T_{\text{c}}$ through the inter-layer Green's functions. This actually causes the trilayer system to have higher $T_{\text{c}}$ than the single-layer in a weak- and intermediate-coupling regimes. This picture holds for a range of the on-site Hubbard repulsion $U$ that contains those estimated for the cuprates. The present result is qualitatively consistent with nuclear magnetic resonance experiments in multi-layer cuprates superconductors.

cond-mat.supr-con↗