Yoshiyuki Kawazoe - Academia.edu (original) (raw)

Papers by Yoshiyuki Kawazoe

Research paper thumbnail of Design of wide-gap fluoride heterostructures for deep ultraviolet optical devices

Journal of Applied Physics, 2004

The design of fluoride-based optical devices for deep ultraviolet applications is discussed. Vari... more The design of fluoride-based optical devices for deep ultraviolet applications is discussed. Variations in the band-gap energy and band structure with respect to composition are investigated for Li ͑1−x͒ K x Ba ͑1−y͒ Mg y F 3 perovskites. The band-gap energy, lattice constant, and band structure of perovskitelike fluorides are estimated based on ab initio calculations within the local-density approximation. The lattice-matched double heterostructure with direct band-gap compounds (Li ͑1−x͒ K x Ba ͑1−y͒ Mg y F 3 on either LiBaF 3 or KMgF 3 substrates) is promising for fabrication.

Research paper thumbnail of Adsorption and electronic structure of single C60F18 molecule on Si(111)-7×7 surface

Chemical Physics Letters, 2009

... The addition of F atoms makes four nearest neighbors around each such carbon atom and the loc... more ... The addition of F atoms makes four nearest neighbors around each such carbon atom and the local bonding character changes from predominantly sp 2 to sp 3 type, as well as the C–C bonds elongate in the range of 1.5–1.6 Å. ...

Research paper thumbnail of Experimental and theoretical investigations on the epitaxial growth of 4,4(') stilbenedicarboxylic acid molecules on Au(111)

The Journal of Chemical Physics

We report on scanning tunneling microscope observations of the epitaxial growth of 4,4(') sti... more We report on scanning tunneling microscope observations of the epitaxial growth of 4,4(') stilbenedicarboxylic acid (SDA) molecules on Au(111), with the coverage ranges from submonolayer to one monolayer. The surface assembly evolves from one-dimensional molecular chains to striped islands and finally monolayer films. For two-dimensional assembly, the head-to-tail hydrogen bonding is found to dominate the molecule-molecule interactions. Each linking region of the SDA molecular chains consists of two hydrogen bonds. This is confirmed by our first-principles calculations where the hydrogen bond length, hydrogen bond energy, and SDA-Au interaction energy are deduced theoretically. Moreover, the configurations with interchain hydrogen bonds are energetically unstable. The roles of the herringbone reconstruction of Au(111) and the compression effect of a complete film on the formation of molecular ribbons are discussed.

Research paper thumbnail of Strain-Induced Spin Crossover in Phthalocyanine-Based Organometallic Sheets

The journal of physical chemistry letters, 2012

Motivated by the recent success in synthesizing two-dimensional Fe-phthalocyanine (poly-FePc) por... more Motivated by the recent success in synthesizing two-dimensional Fe-phthalocyanine (poly-FePc) porous sheets, we studied strain-induced spin crossover in poly-TMPc (TM = Mn, Fe, Co, and Ni) systems by using first-principle calculations based on density functional theory. A small amount of biaxial tensile strain is found to not only significantly enhance the magnetic moment of the central TM atoms by 2 μB when the strain reaches a critical value, but also the systems undergo low-spin (LS) to high-spin (HS) transition. These systems, however, show different response to strain, namely, poly-FePc sheet becomes ferromagnetic (FM) while poly-MnPc and poly-NiPc sheets become antiferromagnetic (AFM). Poly-CoPc, on the other hand, remains AFM. These predicted results can be observed in suspended poly-TMPc sheets by using scanning tunneling microscope (STM) tips to manipulate strain.

Research paper thumbnail of Evolution of the dizincocene metal–metal bond in the series Zn2L2 where L=H, CH3, F, η5-C5H5

Journal of Molecular Structure: THEOCHEM, 2006

Ab initio plane-wave-based density functional calculations for the molecules Zn 2 L 2 with the li... more Ab initio plane-wave-based density functional calculations for the molecules Zn 2 L 2 with the ligands L = H, CH 3 , F, g 5 -C 5 H 5 (cyclopentadienyl Cp) show how the ZnZn and ZnL bonds evolve to dimetallocene from simple H to the electronically and spatially extended Cp ligand. The metal-metal bond distances (pm) decrease in the order: 240.2 (H), 239 (CH 3 ), 230.4 (F), 230 (Cp) compared to distances: 266 (Zn metal), 246 (Zn 2 2+ ), 230.5 (crystalline Zn 2 [C 5 (CH 3 ) 5 ] 2 ). Analysis is supported by calculation of the total charge and partial charge density distributions and the electron localization function (ELF). In the dizinc region, the ELF consists of a separate ring (torus) and mid-bond axis basin. For molecules with the ligands H, CH 3 , and F, the mid-bond basin attractor appears on the bond axis at higher ELF followed at lower values by the ring. For Cp, the reverse is true. In the Cp compound, the bonding from ring to metal atom involves both sigma r-contributions with s-, p-, and d-components and p-contributions with p-and d-components. In the Cp compound, the ELF shows some differentiation in the CC bonding within the C 5 ring towards three single C-C and two double C@C bonds.

Research paper thumbnail of Quantum approach for magnetic multilayers at finite temperatures

Physical Review B, 1998

The Green's function technique has been employed to examine the finite-temperature properties of ... more The Green's function technique has been employed to examine the finite-temperature properties of a magnetic multilayer both in aligned and in canted spin configurations. A local coordinates system is introduced to describe complicated spin configurations which are determined by minimizing the free energy of the magnetic multilayer. Thermal averages of S m z m in each layer are selected to be independent variables, and the necessarily self-consistent equations are obtained successfully based on the random phase approximation. The temperature dependences of the coercivity, the hysteresis, and the magnetic multivalued recording have been discussed, and an application of the present approach to the canted spin state is given in a double-film system.

Research paper thumbnail of Ab initio GW quasiparticle energies of small sodium clusters by an all-electron mixed-basis approach

Physical Review B, 2001

A state-of-the-art GW calculation is carried out for small sodium clusters, Na 2 , Na 4 , Na 6 , ... more A state-of-the-art GW calculation is carried out for small sodium clusters, Na 2 , Na 4 , Na 6 , and Na 8 . The quasiparticle energies are evaluated by employing an ab initio GW code based on an all-electron mixed-basis approach, which uses both plane waves and atomic orbitals ...

Research paper thumbnail of Single adatom dynamics at monatomic steps of free-standing few-layer reduced graphene

Scientific Reports, 2014

Steps and their associated adatoms extensively exist and play prominent roles in affecting surfac... more Steps and their associated adatoms extensively exist and play prominent roles in affecting surface properties of materials. Such impacts should be especially pronounced in two-dimensional, atomically-thin membranes like graphene. However, how single adatom behaves at monatomic steps of few-layer graphene is still illusive. Here, we report dynamics of individual adatom at monatomic steps of free-standing few-layer reduced graphene under the electron beam radiations, and demonstrate the prevalent existence of monatomic steps even down to unexpectedly ultrasmall lateral size of a circular diameter of ,5 Å . Single adatom prefers to stay at the edges of the atomic steps of few-layer reduced graphene and evolve with the steps. Moreover, we also find that how the single adatom behaves at atomic step edges can be remarkably influenced by the type of adatoms and step edges. Such single adatoms at monatomic steps and ultrasmall atomic steps open up a new window for surface physics and chemistry for graphene-based as well as other two-dimensional materials.

Research paper thumbnail of A new carbon allotrope with six-fold helical chains in all-sp2 bonding networks

Scientific reports, 2014

Using a recently developed approach to constructing covalent network structures from linear carby... more Using a recently developed approach to constructing covalent network structures from linear carbyne, we identify by ab initio calculations a new carbon allotrope in R3m (D(3d)(5) symmetry that comprises six-fold helical chains with alternating sp(2)-type single and double bonds along the chains that are connected via zigzag benzene rings. This 6-fold carbene is characterized as a three-dimensional three-connected chiral crystalline modification of graphite. Phonon and electronic band calculations indicate that this new structure is dynamically stable and is a semiconductor with a band gap of 0.47 eV, in contrast to the semimetallic nature of graphite. Simulated x-ray diffraction patterns of the 6-fold carbene provide an excellent match to the previously unexplained distinct diffraction peak of a new carbon allotrope found in recent detonation experiments. These results establish a new carbon phase and offer insights into its outstanding structural and electronic properties.

Research paper thumbnail of Quantum theory of the coercive force and the capping effect for magnetic multilayers

Physical review. B, Condensed matter, 1996

A quantum theory of coercivity is established for magnetic systems. The concept of a metastable s... more A quantum theory of coercivity is established for magnetic systems. The concept of a metastable state is adopted and an order parameter that is actually the gap of the magnon excitations is used to tell the stability of the metastable state and to determine the coercive force of the system. Quantum theory is applied to study the capping effects in a magnetic double-film structure. A model Hamiltonian is presented for such a system, in which there are several magnetic layers with perpendicular easy-axis anisotropy capped by a few layers with in-plane easy-axis anisotropy. The Hamiltonian is diagonalized under the harmonic approximation and the magnon excitation gap is determined. Numerical calculations are carried out and the experimental results about the coercive force in the double-film structure are explained qualitatively. Finally, we show that there are two types of phase transition in the capping effect. The results show that the capping effect can be significantly enhanced if one selects the anisotropy and the number of capping layers appropriately.

Research paper thumbnail of Insertion of Be Atoms in C{sub {bold 60}} Fullerene Cages: Be{bold @} C{sub {bold 60}}

Physical Review Letters, 1996

Radioactive endohedral ⁷Be@Cââ can be detected using radiochemical and radiochromatographic tec... more Radioactive endohedral ⁷Be@Cââ can be detected using radiochemical and radiochromatographic techniques in the final solvent. Such a ⁷Be atom can penetrate into the Cââ cage to produce ⁷Be@Cââ by a recoil process of the nuclear reactions. An {ital ab} {ital initio} molecular dynamics simulation was carried out to demonstrate that a direct insertion process is really possible. Both the experimental

Research paper thumbnail of Penta-graphene: A new carbon allotrope

Proceedings of the National Academy of Sciences, 2015

A 2D metastable carbon allotrope, penta-graphene, composed entirely of carbon pentagons and resem... more A 2D metastable carbon allotrope, penta-graphene, composed entirely of carbon pentagons and resembling the Cairo pentagonal tiling, is proposed. State-of-the-art theoretical calculations confirm that the new carbon polymorph is not only dynamically and mechanically stable, but also can withstand temperatures as high as 1000 K. Due to its unique atomic configuration, penta-graphene has an unusual negative Poisson's ratio and ultrahigh ideal strength that can even outperform graphene. Furthermore, unlike graphene that needs to be functionalized for opening a band gap, penta-graphene possesses an intrinsic quasi-direct band gap as large as 3.25 eV, close to that of ZnO and GaN. Equally important, penta-graphene can be exfoliated from T12-carbon. When rolled up, it can form pentagon-based nanotubes which are semiconducting, regardless of their chirality. When stacked in different patterns, stable 3D twin structures of T12-carbon are generated with band gaps even larger than that of T12-carbon. The versatility of penta-graphene and its derivatives are expected to have broad applications in nanoelectronics and nanomechanics. carbon allotrope | carbon pentagon | stability | negative Poisson's ratio | electronic structure C arbon is one of the most versatile elements in the periodic table and forms a large number of allotropes ranging from the well-known graphite, diamond, C 60 fullerene (1), nanotube (2), and graphene (3) to the newly discovered carbon nanocone (4), nanochain (5), graphdiyne (6), as well as 3D metallic structures (7, 8). The successful synthesis of graphene (3) has triggered considerable interest in exploring novel carbon-based nanomaterials. A wealth of 2D carbon allotropes beyond graphene has since been studied (see SI Appendix, for details). Although some of these polymorphs such as graphdiyne (6) are metastable compared with graphene, they have been successfully synthesized. Moreover, some 2D carbon allotropes are predicted to exhibit remarkable properties that even outperform graphene, such as anisotropic Dirac cones (9), inherent ferromagnetism (10), high catalytic activity (6), and potential superconductivity related to the high density of states at the Fermi level (11). These results demonstrate that many of the novel properties of carbon allotropes are intimately related to the topological arrangement of carbon atoms and highlight the importance of structure-property relationships (12).

Research paper thumbnail of Highly controlled acetylene accommodation in a metal-organic microporous material

Nature, 2005

Metal-organic microporous materials (MOMs) have attracted wide scientific attention owing to thei... more Metal-organic microporous materials (MOMs) have attracted wide scientific attention owing to their unusual structure and properties, as well as commercial interest due to their potential applications in storage, separation and heterogeneous catalysis. One of the advantages of MOMs compared to other microporous materials, such as activated carbons, is their ability to exhibit a variety of pore surface properties such as

Research paper thumbnail of Ab Initio Molecular Dynamics Simulations for Collision between C60- and Alkali-Metal Ions: A Possibility of Li@C60

Physical Review Letters, 1996

Collisions between C 2 60 and alkali-metal ion are investigated by an ab initio molecular dynamic... more Collisions between C 2 60 and alkali-metal ion are investigated by an ab initio molecular dynamics simulation at 1000 K. When Li 1 with kinetic energy ϳ5 eV hits the center of a six-membered ring of C 2 60 , Li@C 60 is created. If either the kinetic energy is lower or the collision occurs off center, the Li 1 ion stays outside, and C 60 is deformed by the shock. In some cases, several bonds of C 60 break after the bombardment; this may make the insertion of alkali-metal ion easier. [S0031-9007(96)00138-X] PACS numbers: 61.46. + w

Research paper thumbnail of Insertion of Xe and Kr Atoms into C60 and C70 Fullerenes and the Formation of Dimers

Physical Review Letters, 1998

The radioactive endohedral fullerenes 127 Xe@ C 60, 127 Xe@ C 70, 79 Kr@ C 60, and 79 Kr@ C 70, a... more The radioactive endohedral fullerenes 127 Xe@ C 60, 127 Xe@ C 70, 79 Kr@ C 60, and 79 Kr@ C 70, and their dimers, are detected using radiochemical and radiochromatographic techniques in the final solution of samples in which Xe or Kr has penetrated into C 60 or C ...

Research paper thumbnail of Resonant Tunneling in an Aharonov-Bohm Ring with a Quantum Dot

Physical Review Letters, 1998

We investigate the resonant tunneling in an Aharonov-Bohm ring with a quantum dot embedded in one... more We investigate the resonant tunneling in an Aharonov-Bohm ring with a quantum dot embedded in one of its arms. A complete transmission mechanism, considering both the resonance of the dot and the interference effect, is presented. As a consequence, the experimental results, the phase features for conductance peaks, are well explained by a one-dimensional noninteracting model. New features concerning temperature effects are predicted. [S0031-9007(98)

Research paper thumbnail of Nonlinear Coherent Transport Through Doped Nanotube Junctions

Physical Review Letters, 1999

The electronic and phase-coherent transport properties of doped carbon nanotube junctions are stu... more The electronic and phase-coherent transport properties of doped carbon nanotube junctions are studied. It is shown that there are regions of negative differential resistance in the I-V characteristic of small radius metallic tubes, which are not seen for semiconducting tubes. Their origin, as discussed, is different from that of traditional Esaki diodes and resonant tunneling structures. Semiconducting tube characteristics, however, show a region of zero current for nonzero voltages which is asymmetric with respect to the applied bias if there is different doping on each side of the junction.

Research paper thumbnail of Molecular Dynamics Simulation for the Formation of Magic-Number Clusters with a Lennard-Jones Potential

Physical Review Letters, 1996

The formation of clusters from a gas state of particles interacting with a Lennard-Jones potentia... more The formation of clusters from a gas state of particles interacting with a Lennard-Jones potential was simulated by a large-scale molecular dynamics calculation with a new temperature-control scheme. Decreasing the translational temperature of atoms and clusters followed by a long-term evaporation produced several peaks in a cluster-size distribution. The peak positions obtained correspond to sizes 13, 19, 23, and 26,

Research paper thumbnail of Fast molecular-dynamics simulation for ferroelectric thin-film capacitors using a first-principles effective Hamiltonian

Physical Review B, 2008

A newly developed fast molecular dynamics method is applied to BaTiO 3 ferroelectric thin-film ca... more A newly developed fast molecular dynamics method is applied to BaTiO 3 ferroelectric thin-film capacitors with short-circuited electrodes or under applied voltage. The molecular dynamics simulations based on a first-principles effective Hamiltonian clarify that dead layers ͑or passive layers͒ between ferroelectrics and electrodes markedly affect the properties of capacitors, and predict that the system is unable to hop between a uniformly polarized ferroelectric structure and a striped ferroelectric domain structure at low temperatures. Simulations of hysteresis loops of thin-film capacitors are also performed, and their dependence on film thickness, epitaxial constraints, and electrodes are discussed.

Research paper thumbnail of Fast-ionic conductivity of Li^{+} in LiBH_{4}

Physical Review B, 2011

High Li+ conductivity in the high-temperature (hexagonal) phase of LiBH4 is revealed through a fi... more High Li+ conductivity in the high-temperature (hexagonal) phase of LiBH4 is revealed through a first-principles molecular dynamics simulation of 1200 atoms using periodic boundary conditions. The high ionic conductivity originates from the generation of a Li+ metastable state located at an interstitial site surrounded by three Li+ ions and three BH4- ions in the a-b plane. A defect is created

Research paper thumbnail of Design of wide-gap fluoride heterostructures for deep ultraviolet optical devices

Journal of Applied Physics, 2004

The design of fluoride-based optical devices for deep ultraviolet applications is discussed. Vari... more The design of fluoride-based optical devices for deep ultraviolet applications is discussed. Variations in the band-gap energy and band structure with respect to composition are investigated for Li ͑1−x͒ K x Ba ͑1−y͒ Mg y F 3 perovskites. The band-gap energy, lattice constant, and band structure of perovskitelike fluorides are estimated based on ab initio calculations within the local-density approximation. The lattice-matched double heterostructure with direct band-gap compounds (Li ͑1−x͒ K x Ba ͑1−y͒ Mg y F 3 on either LiBaF 3 or KMgF 3 substrates) is promising for fabrication.

Research paper thumbnail of Adsorption and electronic structure of single C60F18 molecule on Si(111)-7×7 surface

Chemical Physics Letters, 2009

... The addition of F atoms makes four nearest neighbors around each such carbon atom and the loc... more ... The addition of F atoms makes four nearest neighbors around each such carbon atom and the local bonding character changes from predominantly sp 2 to sp 3 type, as well as the C–C bonds elongate in the range of 1.5–1.6 Å. ...

Research paper thumbnail of Experimental and theoretical investigations on the epitaxial growth of 4,4(') stilbenedicarboxylic acid molecules on Au(111)

The Journal of Chemical Physics

We report on scanning tunneling microscope observations of the epitaxial growth of 4,4(') sti... more We report on scanning tunneling microscope observations of the epitaxial growth of 4,4(') stilbenedicarboxylic acid (SDA) molecules on Au(111), with the coverage ranges from submonolayer to one monolayer. The surface assembly evolves from one-dimensional molecular chains to striped islands and finally monolayer films. For two-dimensional assembly, the head-to-tail hydrogen bonding is found to dominate the molecule-molecule interactions. Each linking region of the SDA molecular chains consists of two hydrogen bonds. This is confirmed by our first-principles calculations where the hydrogen bond length, hydrogen bond energy, and SDA-Au interaction energy are deduced theoretically. Moreover, the configurations with interchain hydrogen bonds are energetically unstable. The roles of the herringbone reconstruction of Au(111) and the compression effect of a complete film on the formation of molecular ribbons are discussed.

Research paper thumbnail of Strain-Induced Spin Crossover in Phthalocyanine-Based Organometallic Sheets

The journal of physical chemistry letters, 2012

Motivated by the recent success in synthesizing two-dimensional Fe-phthalocyanine (poly-FePc) por... more Motivated by the recent success in synthesizing two-dimensional Fe-phthalocyanine (poly-FePc) porous sheets, we studied strain-induced spin crossover in poly-TMPc (TM = Mn, Fe, Co, and Ni) systems by using first-principle calculations based on density functional theory. A small amount of biaxial tensile strain is found to not only significantly enhance the magnetic moment of the central TM atoms by 2 μB when the strain reaches a critical value, but also the systems undergo low-spin (LS) to high-spin (HS) transition. These systems, however, show different response to strain, namely, poly-FePc sheet becomes ferromagnetic (FM) while poly-MnPc and poly-NiPc sheets become antiferromagnetic (AFM). Poly-CoPc, on the other hand, remains AFM. These predicted results can be observed in suspended poly-TMPc sheets by using scanning tunneling microscope (STM) tips to manipulate strain.

Research paper thumbnail of Evolution of the dizincocene metal–metal bond in the series Zn2L2 where L=H, CH3, F, η5-C5H5

Journal of Molecular Structure: THEOCHEM, 2006

Ab initio plane-wave-based density functional calculations for the molecules Zn 2 L 2 with the li... more Ab initio plane-wave-based density functional calculations for the molecules Zn 2 L 2 with the ligands L = H, CH 3 , F, g 5 -C 5 H 5 (cyclopentadienyl Cp) show how the ZnZn and ZnL bonds evolve to dimetallocene from simple H to the electronically and spatially extended Cp ligand. The metal-metal bond distances (pm) decrease in the order: 240.2 (H), 239 (CH 3 ), 230.4 (F), 230 (Cp) compared to distances: 266 (Zn metal), 246 (Zn 2 2+ ), 230.5 (crystalline Zn 2 [C 5 (CH 3 ) 5 ] 2 ). Analysis is supported by calculation of the total charge and partial charge density distributions and the electron localization function (ELF). In the dizinc region, the ELF consists of a separate ring (torus) and mid-bond axis basin. For molecules with the ligands H, CH 3 , and F, the mid-bond basin attractor appears on the bond axis at higher ELF followed at lower values by the ring. For Cp, the reverse is true. In the Cp compound, the bonding from ring to metal atom involves both sigma r-contributions with s-, p-, and d-components and p-contributions with p-and d-components. In the Cp compound, the ELF shows some differentiation in the CC bonding within the C 5 ring towards three single C-C and two double C@C bonds.

Research paper thumbnail of Quantum approach for magnetic multilayers at finite temperatures

Physical Review B, 1998

The Green's function technique has been employed to examine the finite-temperature properties of ... more The Green's function technique has been employed to examine the finite-temperature properties of a magnetic multilayer both in aligned and in canted spin configurations. A local coordinates system is introduced to describe complicated spin configurations which are determined by minimizing the free energy of the magnetic multilayer. Thermal averages of S m z m in each layer are selected to be independent variables, and the necessarily self-consistent equations are obtained successfully based on the random phase approximation. The temperature dependences of the coercivity, the hysteresis, and the magnetic multivalued recording have been discussed, and an application of the present approach to the canted spin state is given in a double-film system.

Research paper thumbnail of Ab initio GW quasiparticle energies of small sodium clusters by an all-electron mixed-basis approach

Physical Review B, 2001

A state-of-the-art GW calculation is carried out for small sodium clusters, Na 2 , Na 4 , Na 6 , ... more A state-of-the-art GW calculation is carried out for small sodium clusters, Na 2 , Na 4 , Na 6 , and Na 8 . The quasiparticle energies are evaluated by employing an ab initio GW code based on an all-electron mixed-basis approach, which uses both plane waves and atomic orbitals ...

Research paper thumbnail of Single adatom dynamics at monatomic steps of free-standing few-layer reduced graphene

Scientific Reports, 2014

Steps and their associated adatoms extensively exist and play prominent roles in affecting surfac... more Steps and their associated adatoms extensively exist and play prominent roles in affecting surface properties of materials. Such impacts should be especially pronounced in two-dimensional, atomically-thin membranes like graphene. However, how single adatom behaves at monatomic steps of few-layer graphene is still illusive. Here, we report dynamics of individual adatom at monatomic steps of free-standing few-layer reduced graphene under the electron beam radiations, and demonstrate the prevalent existence of monatomic steps even down to unexpectedly ultrasmall lateral size of a circular diameter of ,5 Å . Single adatom prefers to stay at the edges of the atomic steps of few-layer reduced graphene and evolve with the steps. Moreover, we also find that how the single adatom behaves at atomic step edges can be remarkably influenced by the type of adatoms and step edges. Such single adatoms at monatomic steps and ultrasmall atomic steps open up a new window for surface physics and chemistry for graphene-based as well as other two-dimensional materials.

Research paper thumbnail of A new carbon allotrope with six-fold helical chains in all-sp2 bonding networks

Scientific reports, 2014

Using a recently developed approach to constructing covalent network structures from linear carby... more Using a recently developed approach to constructing covalent network structures from linear carbyne, we identify by ab initio calculations a new carbon allotrope in R3m (D(3d)(5) symmetry that comprises six-fold helical chains with alternating sp(2)-type single and double bonds along the chains that are connected via zigzag benzene rings. This 6-fold carbene is characterized as a three-dimensional three-connected chiral crystalline modification of graphite. Phonon and electronic band calculations indicate that this new structure is dynamically stable and is a semiconductor with a band gap of 0.47 eV, in contrast to the semimetallic nature of graphite. Simulated x-ray diffraction patterns of the 6-fold carbene provide an excellent match to the previously unexplained distinct diffraction peak of a new carbon allotrope found in recent detonation experiments. These results establish a new carbon phase and offer insights into its outstanding structural and electronic properties.

Research paper thumbnail of Quantum theory of the coercive force and the capping effect for magnetic multilayers

Physical review. B, Condensed matter, 1996

A quantum theory of coercivity is established for magnetic systems. The concept of a metastable s... more A quantum theory of coercivity is established for magnetic systems. The concept of a metastable state is adopted and an order parameter that is actually the gap of the magnon excitations is used to tell the stability of the metastable state and to determine the coercive force of the system. Quantum theory is applied to study the capping effects in a magnetic double-film structure. A model Hamiltonian is presented for such a system, in which there are several magnetic layers with perpendicular easy-axis anisotropy capped by a few layers with in-plane easy-axis anisotropy. The Hamiltonian is diagonalized under the harmonic approximation and the magnon excitation gap is determined. Numerical calculations are carried out and the experimental results about the coercive force in the double-film structure are explained qualitatively. Finally, we show that there are two types of phase transition in the capping effect. The results show that the capping effect can be significantly enhanced if one selects the anisotropy and the number of capping layers appropriately.

Research paper thumbnail of Insertion of Be Atoms in C{sub {bold 60}} Fullerene Cages: Be{bold @} C{sub {bold 60}}

Physical Review Letters, 1996

Radioactive endohedral ⁷Be@Cââ can be detected using radiochemical and radiochromatographic tec... more Radioactive endohedral ⁷Be@Cââ can be detected using radiochemical and radiochromatographic techniques in the final solvent. Such a ⁷Be atom can penetrate into the Cââ cage to produce ⁷Be@Cââ by a recoil process of the nuclear reactions. An {ital ab} {ital initio} molecular dynamics simulation was carried out to demonstrate that a direct insertion process is really possible. Both the experimental

Research paper thumbnail of Penta-graphene: A new carbon allotrope

Proceedings of the National Academy of Sciences, 2015

A 2D metastable carbon allotrope, penta-graphene, composed entirely of carbon pentagons and resem... more A 2D metastable carbon allotrope, penta-graphene, composed entirely of carbon pentagons and resembling the Cairo pentagonal tiling, is proposed. State-of-the-art theoretical calculations confirm that the new carbon polymorph is not only dynamically and mechanically stable, but also can withstand temperatures as high as 1000 K. Due to its unique atomic configuration, penta-graphene has an unusual negative Poisson's ratio and ultrahigh ideal strength that can even outperform graphene. Furthermore, unlike graphene that needs to be functionalized for opening a band gap, penta-graphene possesses an intrinsic quasi-direct band gap as large as 3.25 eV, close to that of ZnO and GaN. Equally important, penta-graphene can be exfoliated from T12-carbon. When rolled up, it can form pentagon-based nanotubes which are semiconducting, regardless of their chirality. When stacked in different patterns, stable 3D twin structures of T12-carbon are generated with band gaps even larger than that of T12-carbon. The versatility of penta-graphene and its derivatives are expected to have broad applications in nanoelectronics and nanomechanics. carbon allotrope | carbon pentagon | stability | negative Poisson's ratio | electronic structure C arbon is one of the most versatile elements in the periodic table and forms a large number of allotropes ranging from the well-known graphite, diamond, C 60 fullerene (1), nanotube (2), and graphene (3) to the newly discovered carbon nanocone (4), nanochain (5), graphdiyne (6), as well as 3D metallic structures (7, 8). The successful synthesis of graphene (3) has triggered considerable interest in exploring novel carbon-based nanomaterials. A wealth of 2D carbon allotropes beyond graphene has since been studied (see SI Appendix, for details). Although some of these polymorphs such as graphdiyne (6) are metastable compared with graphene, they have been successfully synthesized. Moreover, some 2D carbon allotropes are predicted to exhibit remarkable properties that even outperform graphene, such as anisotropic Dirac cones (9), inherent ferromagnetism (10), high catalytic activity (6), and potential superconductivity related to the high density of states at the Fermi level (11). These results demonstrate that many of the novel properties of carbon allotropes are intimately related to the topological arrangement of carbon atoms and highlight the importance of structure-property relationships (12).

Research paper thumbnail of Highly controlled acetylene accommodation in a metal-organic microporous material

Nature, 2005

Metal-organic microporous materials (MOMs) have attracted wide scientific attention owing to thei... more Metal-organic microporous materials (MOMs) have attracted wide scientific attention owing to their unusual structure and properties, as well as commercial interest due to their potential applications in storage, separation and heterogeneous catalysis. One of the advantages of MOMs compared to other microporous materials, such as activated carbons, is their ability to exhibit a variety of pore surface properties such as

Research paper thumbnail of Ab Initio Molecular Dynamics Simulations for Collision between C60- and Alkali-Metal Ions: A Possibility of Li@C60

Physical Review Letters, 1996

Collisions between C 2 60 and alkali-metal ion are investigated by an ab initio molecular dynamic... more Collisions between C 2 60 and alkali-metal ion are investigated by an ab initio molecular dynamics simulation at 1000 K. When Li 1 with kinetic energy ϳ5 eV hits the center of a six-membered ring of C 2 60 , Li@C 60 is created. If either the kinetic energy is lower or the collision occurs off center, the Li 1 ion stays outside, and C 60 is deformed by the shock. In some cases, several bonds of C 60 break after the bombardment; this may make the insertion of alkali-metal ion easier. [S0031-9007(96)00138-X] PACS numbers: 61.46. + w

Research paper thumbnail of Insertion of Xe and Kr Atoms into C60 and C70 Fullerenes and the Formation of Dimers

Physical Review Letters, 1998

The radioactive endohedral fullerenes 127 Xe@ C 60, 127 Xe@ C 70, 79 Kr@ C 60, and 79 Kr@ C 70, a... more The radioactive endohedral fullerenes 127 Xe@ C 60, 127 Xe@ C 70, 79 Kr@ C 60, and 79 Kr@ C 70, and their dimers, are detected using radiochemical and radiochromatographic techniques in the final solution of samples in which Xe or Kr has penetrated into C 60 or C ...

Research paper thumbnail of Resonant Tunneling in an Aharonov-Bohm Ring with a Quantum Dot

Physical Review Letters, 1998

We investigate the resonant tunneling in an Aharonov-Bohm ring with a quantum dot embedded in one... more We investigate the resonant tunneling in an Aharonov-Bohm ring with a quantum dot embedded in one of its arms. A complete transmission mechanism, considering both the resonance of the dot and the interference effect, is presented. As a consequence, the experimental results, the phase features for conductance peaks, are well explained by a one-dimensional noninteracting model. New features concerning temperature effects are predicted. [S0031-9007(98)

Research paper thumbnail of Nonlinear Coherent Transport Through Doped Nanotube Junctions

Physical Review Letters, 1999

The electronic and phase-coherent transport properties of doped carbon nanotube junctions are stu... more The electronic and phase-coherent transport properties of doped carbon nanotube junctions are studied. It is shown that there are regions of negative differential resistance in the I-V characteristic of small radius metallic tubes, which are not seen for semiconducting tubes. Their origin, as discussed, is different from that of traditional Esaki diodes and resonant tunneling structures. Semiconducting tube characteristics, however, show a region of zero current for nonzero voltages which is asymmetric with respect to the applied bias if there is different doping on each side of the junction.

Research paper thumbnail of Molecular Dynamics Simulation for the Formation of Magic-Number Clusters with a Lennard-Jones Potential

Physical Review Letters, 1996

The formation of clusters from a gas state of particles interacting with a Lennard-Jones potentia... more The formation of clusters from a gas state of particles interacting with a Lennard-Jones potential was simulated by a large-scale molecular dynamics calculation with a new temperature-control scheme. Decreasing the translational temperature of atoms and clusters followed by a long-term evaporation produced several peaks in a cluster-size distribution. The peak positions obtained correspond to sizes 13, 19, 23, and 26,

Research paper thumbnail of Fast molecular-dynamics simulation for ferroelectric thin-film capacitors using a first-principles effective Hamiltonian

Physical Review B, 2008

A newly developed fast molecular dynamics method is applied to BaTiO 3 ferroelectric thin-film ca... more A newly developed fast molecular dynamics method is applied to BaTiO 3 ferroelectric thin-film capacitors with short-circuited electrodes or under applied voltage. The molecular dynamics simulations based on a first-principles effective Hamiltonian clarify that dead layers ͑or passive layers͒ between ferroelectrics and electrodes markedly affect the properties of capacitors, and predict that the system is unable to hop between a uniformly polarized ferroelectric structure and a striped ferroelectric domain structure at low temperatures. Simulations of hysteresis loops of thin-film capacitors are also performed, and their dependence on film thickness, epitaxial constraints, and electrodes are discussed.

Research paper thumbnail of Fast-ionic conductivity of Li^{+} in LiBH_{4}

Physical Review B, 2011

High Li+ conductivity in the high-temperature (hexagonal) phase of LiBH4 is revealed through a fi... more High Li+ conductivity in the high-temperature (hexagonal) phase of LiBH4 is revealed through a first-principles molecular dynamics simulation of 1200 atoms using periodic boundary conditions. The high ionic conductivity originates from the generation of a Li+ metastable state located at an interstitial site surrounded by three Li+ ions and three BH4- ions in the a-b plane. A defect is created