Koichi Kusakabe

Abstract Topological insulators (TI) hold significant potential for various electronic and optoelectronic devices that rely on the Dirac surface state (DSS), including spintronic and thermoelectric devices, as well as terahertz detectors. The behavior of electrons within the DSS plays a pivotal role in the performance of such devices. It is expected that DSS appear on a surface of three dimensional(3D) TI by mechanical exfoliation. However, it is not always the case that the surface terminating atomic configuration and corresponding band structures are homogeneous. In order to investigate the impact of surface terminating atomic configurations on electron dynamics, we meticulously examined the electron dynamics at the exfoliated surface of a crystalline 3D TI (Bi$$_2$$Se$$_3$$) with time, space, and energy resolutions. Based on our comprehensive band structure calculations, we found that on one of the Se-terminated surfaces, DSS is located within the bulk band gap, with no other surface states manifesting within this region. On this particular surface, photoexcited electrons within the conduction band effectively relax towards DSS and tend to linger at the Dirac point for extended periods of time. It is worth emphasizing that these distinct characteristics of DSS are exclusively observed on this particular surface.

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.

Enhanced spin control in graphene/hBN MTJ: boron vacancy tuning yields high TMR ratio of 400%, paving the way for ultra-thin spin valves.

Abstract Martensitic crystal structures are usually obtained by rapid thermal quenching of certain alloys, which induces stress and subsequent shear deformation. Here, we demonstrate that it is also possible to intentionally excite a suitable transverse acoustic phonon mode to induce a local shear deformation. We irradiate the surface of a partially stabilized zirconia plate with intense terahertz pulses and verify martensitic transformation from the tetragonal to the monoclinic phases by Raman spectroscopy and the observed destructive spallation of the zirconia microcrystals. We calculate the phonon modes in tetragonal zirconia and determine the decay channel that triggers the transformation. The phonon mode required for the martensitic transformation can be excited via the Klemens process. Since terahertz pulses can induce a specific local shear deformation beyond thermal equilibrium, they can be used to elucidate phase transformation mechanisms with approaches based on nonlinear phononics.

A spin-topological electronic valve was discovered in a Ni/hBN–graphene–hBN/Ni magnetic junction to control the in-plane conductance of graphene. By manipulating the mass-gapped Dirac cone (MGDC) of graphene’s topology using the magnetic proximity effect, the spin-topological electronic valve was made possible. The first-principles investigation was conducted to show how the mechanism of graphene’s MGDC is controlled. Twelve stacking configurations for the anti-parallel configuration (APC) and parallel configuration (PC) of the magnetic alignment of Ni slabs were calculated using spin-polarized density functional theory. Three groups can be made based on the relative total energy of the 12 stacking configurations, which corresponds to a van der Waals interaction between hBN and graphene. Each group exhibits distinctive features of graphene’s MGDC. The configuration of the Ni(111) surface state’s interaction with graphene as an evanescent wave significantly impacts how the MGDC behaves. By utilizing the special properties of graphene’s MGDC, which depend on the stacking configuration, a controllable MGDC using mechanical motion was proposed by suggesting a device that can translate the top and bottom Ni(111)/hBN slabs. By changing the stacking configuration from Group I to II and II to III, three different in-plane conductances of graphene were observed, corresponding to three non-volatile memory states. This device provides insight into MJs having three or more non-volatile memory states that cannot be found in conventional MJs.

This work presents an ab initio study of a few-layer hexagonal boron nitride (hBN) and hBN–graphene heterostructure sandwiched between Ni(111) layers.

In this study, we present a theoretical study on the in-plane conductance of graphene partially sandwiched between Ni(111) nanostructures with a width of ∼12.08 Å.

<title>Abstract</title> A resource state for measurement-based quantum computation is proposed using a material design of <italic>S</italic> = 1 antiferromagnetic spin chains. Specifying hydrogen adsorption positions on polymerized phenalenyl-tessellation molecules gives rise to formation of graphene zero modes that produce local <italic>S</italic> = 1 spins or <italic>S</italic> = 1/2 spins in the required order through exchange interactions. When the <italic>S</italic> = 1 antiferromagnetic Heisenberg models serve as quantum-computation resources, hydrogen adatoms inducing zero modes can also work as local electron-spin probes in nuclear spin spectroscopy, which could be used for controlling and measuring local spins.

© 2020 American Physical Society. 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, C33, along the c axis of defect-free monocrystalline graphite exceeds 45 GPa, which is higher than reported values by 20%. Based on the experimental finding, we find that the LDA+U+RPA method, including both random phase approximation correlation and short-range correlation in 2p Wannier orbitals, can be a promising solution. The agreement of thus calculated stiffness with the observation indicates non-negligible electron correlation effects with respect to both the short-range and long-range interactions.

The rule of phenalenyl unit tiling specifies a class of polyaromatic hydrocarbon molecules. These nanographene molecules, which we call phenalenyl-tessellation molecules (PTMs), show a particular electronic property owing to their topology. We derive a relationship between the number of vacancies n and the difference N-A - N-B between the numbers of atomic A-sites N-A and atomic B-sites N-B for a PTM as N-A - N-B = n - 1. In addition, because there is always one B-site zero mode, the number of A-site zero modes is given by n for a PTM. The resulting ground state with a total spin of vertical bar n - 1 vertical bar/2 becomes a non-trivial entangled state for n > 0.

To explore material dependence of cuprate superconductors, we evaluate<br /> effective Coulomb interactions for Hg1201 and Tl1201, where Tl1201 having a<br /> nearly half value of T_c of Hg1201 even at the optimal oxygen concentration.<br /> Although structures are similar for these superconductors, there is an apparent<br /> difference in the occupied levels below EF. The characteristic difference in<br /> the band structure is correlated with oxygen contents in the buffer layer. By<br /> using constrained Random Phase Approximation, effective screened Coulomb<br /> interactions are estimated for HgBa_2CuO_4 and TlBa_2CuO_5. The results shows<br /> that the value of screened on-site Coulomb interaction in Hg1201 is nearly<br /> twice bigger than that in Tl1201. In addition, The eigenvalues of the<br /> linearized Eliashberg equation of single-band Hubbard model within FLEX can<br /> show apparent difference in T_c. When we assume that the twice big screened<br /> on-site Coulomb for Hg, the material dependent T_c might be explained.

A self-doping effect between outer and inner CuO2 planes (OPs and IPs) in multilayer cuprate superconductors is studied. When one considers a three-layer tight-binding model of the Hg-based three-layer cuprate derived from first-principles calculations, the electron concentration becomes larger in the OPs than in the IP. This is inconsistent with the experimental finding that more hole carriers tend to be introduced into the OPs than into the IP. We investigate a three-layer Hubbard model with the two-particle self-consistent approach for multilayer systems to incorporate electron correlations. We observe that the double occupancy (antiferromagnetic instability) in the IP decreases (increases) more than that in the OPs, and also reveal that more electrons tend to be introduced into the IP than into the OPs to obtain an energy gain from the on-site Hubbard interaction. These results are consistent with the experimental findings, and this electron distribution between the OPs and IP can be interpreted as a self-doping effect arising from strong electron correlations.

A self-doping effect between outer and inner CuO2 planes (OPs and IPs) in multilayer cuprate superconductors is studied. When one considers a three-layer tight-binding model of the Hg-based three-layer cuprate derived from first-principles calculations, the electron concentration becomes larger in the OPs than in the IP. This is inconsistent with the experimental finding that more hole carriers tend to be introduced into the OPs than into the IP. We investigate a three-layer Hubbard model with the two-particle self-consistent approach for multilayer systems to incorporate electron correlations. We observe that the double occupancy (antiferromagnetic instability) in the IP decreases (increases) more than that in the OPs, and also reveal that more electrons tend to be introduced into the IP than into the OPs to obtain an energy gain from the on-site Hubbard interaction. These results are consistent with the experimental findings, and this electron distribution between the OPs and IP can be interpreted as a self-doping effect arising from strong electron correlations.

To explore material dependence of layered cuprate superconductors, we examine<br /> effective two-particle interactions for Hg1201 and Tl1201, where Tl1201 having<br /> a nearly half value of Tc of Hg1201 even at the optimal oxygen concentration.<br /> Although the 3dx_2-y_2 band, the Fermi surface, and its Wannier-orbitals are<br /> similar for these superconductors, there is an apparent difference in the<br /> unoccupied levels above EF. Based on a multi-reference<br /> density-functional-theory formulation, effective two-particle exchange<br /> interactions are estimated to derive enhancement in intra-layer exchange<br /> interactions for HgBa2CuO4, while it is weakened in TlBaLaCuO5 and furthermore<br /> it is weak in TlBa2CuO5. The characteristic difference in the band structure is<br /> correlated with oxygen contents in the buffer layer. We also comment on the<br /> similar feature in triple-layered compounds. Our spin-fluctuation enhancement<br /> mechanism in an electron-correlation regime is consistent with the experimental<br /> fact.

We provide a theoretical proof of the protected degeneracy in zero modes found in a set of molecular structures called the vacancy-centered hexagonal armchair nanographene (VANG). Recently, two of the authors (G.K.S, K.K.), T. Enoki, and I. Maruyama have proposed that the molecular nanographene is an ideal material for hydrogen storage applications. In our proof of the tight-binding description of the pi-electron system, we utilize the mirror reflection symmetry preserved in the VANG structure as well as the chiral symmetry. Provided that the symmetric operations are maintained, even if the molecular structure is deformed, curious zero modes, the quasi-localized zero mode and the extending zero mode, are degenerate with each other and create the singlet ground state.

We propose rules to determine the charge-Transfer rates (CTR) from hydrogen to graphene in a wide range of coverage. Based on first-principle calculations, the inherent correlation between CTR and the number of hydrogen atoms (coverage) is detected. CTR show roughly linear behavior from 0.22e for a dilute limit to 0.15e for a high-coverage limit. Furthermore, CTR from hydrogen to graphene depend on factors such as hydrogen coverage, local surface morphology typically given by the convex and concave regions of graphene surfaces and the location/arrangement of the hydrogen atoms. Our simulation of the charge transfer of hydrogenated graphene sheets provides a method for quantitatively controlling CTR, which may also be used to identify the surface morphology of hydrogen-Adsorbed graphene.

Deriving mathematical expressions for two zero modes for a pi-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 vacancy-centered hexagonal armchair nanographene (VANG) molecules. Indeed, in the VANG molecule C60H24, which shows stability in a density functional theory simulation, two pseudodegenerate zero modes, a vacancy-centered quasilocalized zero mode and an extended zero mode with a root 3 x root 3 structure, appear at the highest occupied level. Since there is a finite energy gap between these two zero-energy modes and the other modes, low-lying states composed of pseudodegenerate zero modes appear as magnetic multiplets. Thus, the unique magnetic characteristics derived using our theory are expected to hold for synthesized VANG molecules in reality.

Based on density functional theory, we investigate the interaction between a hydrogen molecule and a hydrogenated vacancy V11in graphene surface. V11is graphene mono-vacancy with two hydrogen atoms adsorbed at the edge of vacancy. The hydrogen molecule physisorbed on deformed V11is shown to dissociate producing a known stable vacancy V211, in which two carbon atoms are mono-hydrogenated and another is di-hydrogenated at the edge of the vacancy. We observe that an energy barrier, which is a little above 0.5 eV, exists along all reaction pathways from V11+H2 to V211. There is a gradual change in the charge-transfer rate from a reacting hydrogen atom to the graphene surface, which reaches a common value of 0.19e per a hydrogen atom in the product. We also observed the energy barrier for a migrated hydrogen atom from the vacancy site to in-plane graphene site is around 2 eV. Since the reaction energy of H2 on V11is as large as 2.5 eV, the migration motion may be easily induced after the dissociative adsorption of H2 on this defective graphene effectively enhancing dissociative adsorption of hydrogen molecules.

The geometry and chemistry of graphene nanostructures significantly affects their electronic properties. Despite a large number of experimental and theoretical studies dealing with the geometrical shape-dependent electronic properties of graphene nanostructures, experimental characterisation of their chemistry is clearly lacking. This is mostly due to the difficulties in preparing chemically-modified graphene nanostructures in a controlled manner and in identifying the exact chemistry of the graphene nanostructure on the atomic scale. Herein, we present scanning probe microscopic and first-principles characterisation of graphene nanostructures with different edge geometries and chemistry. Using the results of atomic scale electronic characterisation and theoretical simulation, we discuss the role of the edge geometry and chemistry on the electronic properties of graphene nanostructures with hydrogenated and oxidised linear edges at graphene boundaries and the internal edges of graphene vacancy defects. Atomic-scale details of the chemical composition have a strong impact on the electronic properties of graphene nanostructures, i.e.; the presence or absence of non-bonding π states and the degree of resonance stability.

Understanding how foreign chemical species bond to atomic vacancies in graphene layers can advance our ability to tailor the electronic and magnetic properties of defective graphenic materials. Here, we use ultrahigh-vacuum scanning tunneling microscopy (UHV-STM) and density functional theory to identify the precise structure of hydrogenated single atomic vacancies in a topmost graphene layer of graphite and establish a connection between the details of hydrogen passivation and the electronic properties of a single atomic vacancy. Monovacancy-hydrogen complexes are prepared by sputtering of the graphite surface layer with low-energy ions and then exposing it briefly to an atomic hydrogen environment. High-resolution experimental UHV-STM imaging allows us to determine unambiguously the positions of single missing atoms in the defective graphene lattice and, in combination with the ab initio calculations, provides detailed information about the distribution of low-energy electronic states on the periphery of the monovacancy-hydrogen complexes. We found that a single atomic vacancy where each σ dangling bond is passivated with one hydrogen atom shows a well-defined signal from the nonbonding π state, which penetrates into the bulk with a 3×3R30 periodicity. However, a single atomic vacancy with full hydrogen termination of σ dangling bonds and additional hydrogen passivation of the extended π state at one of the vacancy's monohydrogenated carbon atoms is characterized by complete quenching of low-energy localized states. In addition, we discuss the migration of hydrogen atoms at the periphery of the monovacancy-hydrogen complexes, which dramatically change the vacancy's low-energy electronic properties, as observed in our low-bias, high-resolution STM imaging. © 2014 American Physical Society.

Diamond is the stiffest known material. Here we report that nanopolycrystal diamond synthesized by direct-conversion method from graphite is stiffer than natural and synthesized monocrystal diamonds. This observation departs from the usual thinking that nanocrystalline materials are softer than their monocrystals because of a large volume fraction of soft grain-boundary region. The direct conversion causes the nondiffusional phase transformation to cubic diamond, producing many twins inside diamond grains. We give an ab initio-calculation twinned model that confirms the stiffening. We find that shorter interplane bonds along [111] are significantly strengthened near the twinned region, from which the superstiff structure originates. Our discovery provides a novel step forward in the search for superstiff materials. © 2013 Macmillan Publishers Limited. All rights reserved.

The relation between electronic structure and hydrogen content at the edges of nanosized holes (nanoholes) in graphite single layer is studied by means of atomically resolved scanning tunneling microscopy and first-principles calculations. Repeatable production of nanoholes with predominantly zigzag hydrogenated edges was realized by bombardment of the top graphene layer of graphite with low-energy (100 eV) Ar+ ions and its further treatment (etching) in an atomic hydrogen environment. Two main types of nanohole zigzag edge, with a striking contrast to each other, are identified: (i) monohydrogenated zigzag edge supporting edge-localized π state on its boundary; (ii) zigzag edge with repeating two monohydrogenated sites and one dihydrogenated site, without features of the edge-localized state and characterized by a prominent standing wave pattern. Absence of the localized state at a general (chiral) hydrogenated graphitic edge consisting of a mixture of zigzag and armchair fragments is also discussed. © 2013 American Physical Society.

Criterion for occurrence of the high-temperature superconductivity by the pair-hopping mechanism in layered materials is given. Based on the multi-reference density functional theory, vanishing inter-layer single particle hopping processes around the Fermi level and appearance of high-energy unoccupied orbitals mediating the Coulomb-repulsion driven pair scattering among the layers in the ordinal band-structure calculation certify appearance of the super-pair-hopping processes as the off-diagonal process of the quantum density-density fluctuation. The criterion is certified for alpha-K0.25TiNCl in the band structure calculation using the generalized gradient approximation. (C) 2011 Elsevier Ltd. All rights reserved.

Based on the density functional theory (DFT), a theoretical determination method is applied to find structural and electronic properties of potassium-doped picene. In a unit cell of K(2)picene observed experimentally, the atomic positions and the electronic state are determined within a generalized-gradient approximation of DFT using the plane-wave expansion method. Some metastable structures of K2picene are found in the optimization simulations. Notable properties of a structure with AB stacking, which is the lowest in the energy than others, are 1) appearance of electron doped molecular bands with the band width of 0.1 similar to 0.4 eV, 2) gap formation with a gap of 0.04 similar to 0.1 eV at the Fermi energy, and 3) narrow conduction bands with the band width of similar to 0.1 eV. In all meta-stable structures determined in this study, these characteristics are commonly found. Assuming further electron doping in the conduction bands of K2picene, electron-correlation effects may appear as the pair-hopping processes between doped narrow bands and empty (or filled) broader high-energy bands to stabilize superconductivity by a kind of the Suhl-Kondo mechanism.

Low-lying magnon dispersion in a S=1 Heisenberg antiferromagnetic (AF) chain is analyzed using the non-Abelian density-matrix-renormalization-group (DMRG) method. The scattering length ab of the boundary coupling and the intermagnon scattering length a are determined. The scattering length a b is found to exhibit a characteristic diverging behavior at the crossover point. In contrast, the Haldane gap Δ, the magnon velocity v, and a remain constant at the crossover. Our method allowed estimation of the gap of the S=2 AF chain to be Δ=0.0891623(9) using a chain length longer than the correlation length ξ. © 2011 American Physical Society.

Low-lying magnon dispersion in a S = 1 Heisenberg antiferromagnetic (AF) chain is analyzed using the non-Abelian density-matrix-renormalization-group (DMRG) method. The scattering length a(b) of the boundary coupling and the intermagnon scattering length a are determined. The scattering length a(b) is found to exhibit a characteristic diverging behavior at the crossover point. In contrast, the Haldane gap Delta, the magnon velocity v, and a remain constant at the crossover. Our method allowed estimation of the gap of the S = 2 AF chain to be Delta = 0.0891623(9) using a chain length longer than the correlation length xi.

Low-lying magnon dispersion in a S = 1 Heisenberg antiferromagnetic (AF) chain is analyzed using the non-Abelian density-matrix-renormalization-group (DMRG) method. The scattering length a(b) of the boundary coupling and the intermagnon scattering length a are determined. The scattering length a(b) is found to exhibit a characteristic diverging behavior at the crossover point. In contrast, the Haldane gap Delta, the magnon velocity v, and a remain constant at the crossover. Our method allowed estimation of the gap of the S = 2 AF chain to be Delta = 0.0891623(9) using a chain length longer than the correlation length xi.

Using the density functional calculations for a designed material structure, we obtained a modified graphene structure on a SrO(111) oxygen surface as a self-consistent stable solution. The structure is easily realized by placing graphene on the substrate and is stable with a reaction energy of 4.6eV per a surface unit cell. All surface oxygen possess carbon-oxygen covalent bondings. Determined electronic density of states shows appearance of two shallow acceptor bands of embedded edge states above the filled SrO bands. These in-gap states have non-bonding character because an effective AB-bipartite lattice of the p network is formed in the oxidized graphene. When the lower band is partially filled, each localized orbital can acts as a quantum dot with a spin 1/2. Possible density of the dots can reach 2x10(14)[cm(-2)], which amounts to 200 terabits per 1 cm(2). We show a clue that a magnetic solution is obtained, when the upper flat band is half-filled, indicating effective realization of flat-band ferromagnetism. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim