草部 浩一

We investigated the stability of the ferromagnetism of CrAs and CrSb in the zinc-blende structure against the lattice distortion, systematically. A calculation within the generalized gradient approximation using a full potential linearized augmented plane wave method was performed. We compared the ferromagnetic state and the antiferromagnetic state assuming tetragonal distortion with the lattice constants a and c changing independently and determined the spin polarization ratio in the ferromagnetic phase. The result shows that complete spin polarization (half-metallic ferromagnetism) remains stable even in the presence of large tetragonal distortion. On the other hand, our calculation shows that two monolayers of CrAs is enough to produce a half-metallic state in the CrAs/GaAs multilayer. Thus, the present result suggests that the half-metallic nature persists in various atomic-scale superlattices made of distorted CrAs or CrSb.

We investigate magnetic properties of the nanoscale ferrimagnetic ring [Mn(hfac)(2)NITPh](6) that is suggested to have a tendency of frustration. The spin densities of the candidate models are discussed. The spin density of the model with frustration shows characteristic behaviour different from those of the other models. Except for the quantum fluctuation which is a little bigger for the frustrated model, the difference in overall behaviour of the spin density is explained by the spin alignment in corresponding classical spin systems. The spin alignment is determined by competition of the Zeeman energy and the Mn-radical exchange interaction, when a canted spin alignment of Mn spins from the axis of the external magnetic field appears in the magnetization process for the other models, while it is absent for our frustrated model.

We have searched for a new highly spin-polarized ferromagnet which has a higher spin moment than that of known half-metallic transition metal pnictides with the zinc-blende structure by first-principles calculations. To generate the high spin moment we focus on Gd compounds. Our calculation shows that a (GdN)(1)/(CrAs)(1) structure is a ferromagnetic material. The total magnetic moment of this ferromagnet is over 9.9 mu(B) per chemical formula.

Electron transport is investigated theoretically for an electron which is coupled to a localized spin system. The dc electrical conductivity of spin-fermion models in one dimension is studied by the direct numerical estimation of the Kubo formula. The dependence of the conductivity on temperature and external magnetic field is investigated. When the localized spins are antiferromagnetically coupled on a chain, the conductivity shows a monotonic increase with magnetic field, which is ordinary magnetoresistance. However, in a system where the localized spins form an antiferromagnetically coupled two-leg ladder, the conductivity shows an anomalous non-monotonic field dependence. Analysis of the spin configuration of the localized spin system reveals that quantum fluctuation determines the conductivity through a dimerized-rung spin configuration, and the formation of local triplets on the ladder rungs causes the decrease in conductivity.

Based on our theoretical prediction of magnetic nanographite, we examined magnetism in various graphitic structures in nanometer scale using the first-principles calculation. When di-hydrogenated carbon atoms are created at a zigzag edge, high-spin ground states are found in carbon nanorings, which are short zigzag nanotubes, as well as graphene structures. The maximum total spin is proportional to the length of graphene ribbon or the diameter of the nanoring. The magnetism is interpreted as a realization of flat-band ferromagnetism known in the Hubbard model on bipartite lattices. (C) 2003 Elsevier B. V. All rights reserved.

Newly proposed aromatic molecules and graphene fragments are shown to have the high-spin ground state by the first-principles electronic structure calculations. Our strategy to predict magnetic carbon materials is based on our previous conclusion that mono-hydrogenated, di-hydrogenated or mono-fluorinated zigzag edges of honeycomb networks are magnetic. Structural optimization as well as determination of the electronic states was performed for various nanographite ribbons and high-spin molecules, e.g. 1,8,9-di-hydro-anthracene, C19H14 and C14F13. For hydrogenated molecules and ribbons, the total spin S determined by the LSDA calculation coincides with the value expected from a counting rule for the total spin on a bipartite network. However, S depends on structures of fluorinated nanographite. (C) 2003 Elsevier Ltd. All rights reserved.

A constructive method to find flat bands is demonstrated for a model describing tight-binding electrons on the hexagonal lattice in which each atomic site has three orbitals. The system may represent sigma-electrons of an sp(2)-hybridized material but we assume transfer integrals between orbitals to be free parameters for general discussion. Finding highly degenerate eigenfunctions of each flat band is identified as finding a localized eigenfunction, which is determined as a basis function of an irreducible representation of, for example, a point group C-6nu. This approach yields not only flat bands on all over the k-space but also some partial flat bands only on a partial k-space. In the former case, if the flat band locates at the bottom (or the top) of a band structure, it is shown that Mielke's condition holds, allowing occurrence of the flat band ferromagnetism.

A numerical study on the Josephson current through a quantum dot has been done by means of the quantum Monte Carlo method. We consider a dot which contacts to two gapless superconductors with d-wave pairing or p-wave pairing to enhance the possible Kondo effect at low temperatures. The junction becomes a pi junction caused by the Coulomb blockade in a temperature range depending on strength of the repulsion U on the dot. Our result on temperature dependence of the current shows that the current through the pi junction is suppressed by decreasing the temperature, when the zero energy states enhance the Kondo effect. (C) 2003 Elsevier Science B.V. All rights reserved.

Hydrogenated nanographite can display spontaneous magnetism. Recently, we proposed that hydrogenation of nanographite is able to induce finite magnetization. We have performed theoretical investigation of a graphene ribbon, in which each carbon is bonded to two hydrogen atoms at one edge and to a single hydrogen atom at another edge. Application of the local-spin-density approximation to the calculation of the electronic band structure of the ribbon shows the appearance of a spin-polarized flat band at the Fermi energy. Producing different numbers of monohydrogenated carbons and dihydrogenated carbons can create magnetic moments in nanographite.

Hydrogenated nanographite can display spontaneous magnetism. Recently, we proposed that hydrogenation of nanographite is able to induce finite magnetization. We have performed theoretical investigation of a graphene ribbon, in which each carbon is bonded to two hydrogen atoms at one edge and to a single hydrogen atom at another edge. Application of the local-spin-density approximation to the calculation of the electronic band structure of the ribbon shows the appearance of a spin-polarized flat band at the Fermi energy. Producing different numbers of monohydrogenated carbons and dihydrogenated carbons can create magnetic moments in nanographite.

Hydrogenated nanographite can display spontaneous magnetism. Recently, we proposed that hydrogenation of nanographite is able to induce finite magnetization. We have performed theoretical investigation of a graphene ribbon, in which each carbon is bonded to two hydrogen atoms at one edge and to a single hydrogen atom at another edge. Application of the local-spin-density approximation to the calculation of the electronic band structure of the ribbon shows the appearance of a spin-polarized flat band at the Fermi energy. Producing different numbers of monohydrogenated carbons and dihydrogenated carbons can create magnetic moments in nanographite.

We have performed a quantum Monte-Carlo calculation for the DC Josephson current through a quantum dot. Electron Green's functions dressed by a short range repulsion U on the dot were obtained by the Hirsch-Fye algorithm. Temperature dependence of the Josephson current was determined for a junction made of a single dot connected with two d-wave superconductors. Our calculation reproduced a double-peaked spectral function and change in the sign of the current when the Coulomb blockade occurred at low temperatures. Indication of resonant tunneling was seen in the current, when the chemical potential on the dot was shifted by about U/2 from the band center.

We studied phase stability, structural properties, and electronic band structures of graphite fluorides CnF (n=1,2,...,6) using density-functional theory with local density approximation. The calculation shows that CnF with sp(3) bonding has a direct gap, when n=1 or 2. Even though CnF with n>2 has not been identified experimentally, our calculation predicts that they become an indirect gap semiconductor.

The ground-state properties of the two-band Hubbard model (TBHM) with semimetallic-band structures are studied by using numerical and analytical methods. Based on the density matrix renormalization group calculation in one dimension (1D), we find that the partially ferromagnetic state is realized by doping the compensated semimetal in the intermediate-coupling regime. We present an exact proof of partial ferromagnetism by using the Perron-Frobenius theorem in a 1D strongly correlated electron model derived from TBHM. © 2002 Elsevier Science B.V. All rights reserved.

We have studied theoretically the Josephson effect through a quantum dot array connected with two superconducting electrodes. The dot array has been studied by two models: (1) a Hubbard chain with boundary pair-potentials and (2) an Anderson impurity model coupled to superconducting leads. The first model shows a parity effect. Namely, the system becomes a pi-junction for odd N-e or a 0-junction for even N-e, where N-e is the average number of electrons in the array. By shifting the on-site potential at the dot array, N-e increases one by one for finite U. The maximum Josephson current shows peaks of resonant tunneling determined by both the parity effect and the Hubbard gap. In order to consider the Josephson effect at finite temperature, we have generalized the quantum Monte Carlo method by Hirsch and Fye and have applied it to the second model with a quantum dot, which shows occurrence of the Coulomb blockade, (C) 2002 Elsevier Science B.V. All rights reserved.

Magnetism of nanographite is examined by consideration based on known rigorous theorems for the Hubbard model. For a bearded nanographite ribbon with the zigzag edge on one side and Klein's decorated edge on the other side, the surface magnetization of O(mu(B)) per a carbon atom is exactly shown to appear for the pi-electron system described by the Hubbard model. The surface magnetism appears due to existence of the edge states, which are degenerate localized states specific to these edges of nanographite. The same magnetic mechanism holds for network structures called the hypergraphite, when edges are prepared so that completely degenerate edge states appear.

By the first-principles calculation, we show that graphite fluorides have a potential for application as optical devices with near ultraviolet light emission.

DC Josephson current in a superconductor/a quantum dot/superconductor junction is studied by a quantum Monte Carlo calculation. Dressed temperature Green’s functions are evaluated by using the Hirsch-Fye algorithm and the current is obtained when the bias voltage is zero. Our result reproduces that a π-junction is realized when the Coulomb blockade occurs at finite temperatures. © 2002 Elsevier Science Ltd. All rights reserved.

To consider interaction effects in a directed random system, we have studied a directed random XXZ model numerically. The exact diagonalizaton method is utilized to obtain distribution of complex eigenvalues. Starting from a strong non-Hermiticity limit with J(z) = 0, where the system becomes a non-interacting one-way model, inclusion of antiferromagnetic J(z) makes eigenvalues distributed on many concentric circles grouped by the number of neighboring anti-parallel spin configurations in an Ising limit. In this transition, an Ising gap opens as a result of continual recombination of spectral flows into smaller circles in the complex plane.

By introducing a set of auxiliary equations representing a many-body system, we have derived an extension of the Kohn-Sham scheme for the density functional theory. These equations consist of a Kohn-Sham-type equation determining single-particle orbitals and an eigen-value equation for an effective many-body problem. A variational method similar to the Kohn-Sham technique was utilized to derive effective interactions as well as effective potentials without artificial substitution of a Hubbard-type interact-ion and a mean-field correction in the energy functional. The second equation is described by an effective many-body Hamiltonian with both 2-body interactions and mean-field terms. Rigorous formulation of the extended Kohn-Sham equation is also given in accordance with the Hadjisavvas-Theophilou formulation. Our formulation can be interpreted as a way to define models of the strongly correlated electron systems, e.g. the Hubbard model.

Temperature dependence of the Josephson current through a one-dimensional (1D) doped Mott's insulator is investigated by theoretically analyzing a Josephson junction with a 1D electron system (IDES) in the vicinity of the charge gapped phase. Formation of the zero-energy states (ZES) at interfaces between 1DES and anisotropic superconductors affects the Josephson current in the metallic phase. However, approaching the Mott insulating phase, effects of ZES become less prominent. (C) 2001 Elsevier Science B.V. All rights reserved.

We discuss a possibility of new three-dimensional all-sp(2) phases of carbon. The most interesting property of them is appearance of special surface states, i. e. the edge states. One of characters of edge states is large degeneracy at the Fermi level. Thus, these structures are expected to be metallic carbon-materials. In this paper, we show pi -band structure and band structure based on the first principle calculation within local density approximation(LDA).

We review fundamental issues arising in the exact solution of the one-dimensional Hubbard model. We perform a careful analysis of the Lieb-Wu equations, paying particular attention to the so-called 'string solutions'. Two kinds of string solutions occur: Lambda strings, related to spin degrees of freedom and k-Lambda strings, describing spinless bound states of electrons. Whereas Lambda strings were thoroughly studied in the literature, less is known about k-Lambda strings. We carry out a thorough analytical and numerical analysis of k-Lambda strings. We further review two different approaches to the thermodynamics of the Hubbard model, the Yang-Yang approach and the quantum transfer matrix approach, respectively. The Yang-Yang approach is based on strings, the quantum transfer matrix approach is not. We compare the results of both methods and show that they agree. Finally, we obtain the dispersion curves of all elementary excitations at zero magnetic field for the less than half-filled band by considering the zero-temperature limit of the Yang-Yang approach. (C) 2000 Elsevier Science B.V. All rights reserved.

We present a theory for the Josephson effect in an unconventional superconductor/Luttinger liquid/unconventional superconductor (s/LL/s) junction where the Josephson current is carried by components injected perpendicular to the interface. We apply the bosonization method by Haldane and Maslov to the cases with arbitrary barrier height of the insulator formed between the superconductor and the LL. When superconductors on both sides have triplet symmetries, the Josephson current is enhanced at low temperatures due to the zero-energy states formed near the interface independent of the strength of electron-electron interaction and the transparency of the junction.

We have shown a theoretical method to construct structures called "hyper-graphite which can be regarded as a generalization of the graphite structure. Based on the method, we propose a possible 3-dimensional network, which may be realized as a pi-electron system of hydro-carbon. Naturally, localized edge states emerge at the Fermi level similar to the zigzag-edged graphite sheet.

We present a theory for the Josephson effect in various unconventional super- conductor / Luttinger liquid / unconventional superconductor junctions, applying the bosonization method by Haldane and Maslov. Depending on the parity of superconductors and appearance of the zero-energy state, the temperature dependence of the maximum Josephson current (J(c)) shows different properties. The interaction effect on the Josephson current appears in the renormarized velocity in our formulation, but essential temperature dependence are the same as that in non-interacting cases.

We present a theory for the Josephson effect in an unconventional superconductor/one-dimensional electron gas/unconventional superconductor (s/o/s) junction, where the Josephson current is carried by components injected perpendicular to the interface. When superconductors on both sides have triplet symmetries, the Josephson current is enhanced at low temperature due to the zero-energy states formed near the interface. Measuring Josephson current in this s/o/s junction, we can identify parity of the superconductor. [S0163-1829(99)10333-3].

We study electronic structures of an extended three-dimensional graphite network and its slab performing of the tight binding model. The one of networks we study is bipartite and three-coordinated, and also found in some materials like α-ThSi2. The electronic structures of this networks have characters which are peculiar to 2D graphite. One of these characters is that these networks are a semi metal and the other is that these networks which are cut in proper direction have a surface localized states (Edge State), which originate from the topological feature of the network as well as the zigzag edge in 2D graphite. We call these networks which are made by concept extending graphite network for higher dimensions 'the Hyper Graphite'.

We have studied the magnetic properties of a certain class of Hubbard models proposed as models for quantum atomic wires with flat single-particle bands. These Hubbard models belong to a class of models to which Mielke-Tasaki theorem for flat-band ferromagnetism can be applied. We also found that the Mielke-Tasaki theorem is applicable in networks of quantum atomic wires. The present findings will be useful for producing flat-band ferromagnetism in quantum atomic wires and two dimensional networks.

Electronic states for two possible high-pressure phases (with the ThSi2 structure and with the AlB2 structure) of CaSi2 are determined by means of a local density approximation band-structure calculation. We confirm that these polymorphs can be regarded as doped sp(2) networks of Si. However, a simple picture in terms of K-orbitals is not adequate for explaining the conduction bands, because (i) an s-like band, which is anti-bonding in sp2 connections but bonding between segments of the network, appears and (ii) there is pi-d hybridization between Si and Ca.

Eder and Ohta have found a violation of the Luttinger rule in the spectral function for the t-t'-J model, which was interpreted as a possible breakdown of the Tomonaga-Luttinger (TL) description in models where electrons can pass each other. Here we have computed the spin correlation along with the spectral function for the one-dimensional t-t' Hubbard model and two-leg Hubbard ladder. By varying the Hubbard U we have identified that such a phenomenon is in fact a spinless-fermion-like behavior of holes moving in a spiral spin configuration that has a spin correlation wavelength equal to the system size. [S0163-1829(98)50942-3].

We present results of first-principles band calculations as well as structural optimization of the orthorhombic KC60 polymer. Our band dispersion is significantly anisotropic, and different from previous band calculations. The character of the conduction band is explained in terms of linear combinations of the molecular orbitals of I-h C-60 and a perturbation coming from the polymerization. [S0163-1829(98)05944-X].

We examined the magnetic property of newly extended Delta-chain Hubbard models which may be relevant to quantum atomic wires formed on solid surfaces. We rigorously show that the extended models exhibit so-called flat-band ferromagnetism, based on the Mielke-Tasaki theorem. Moreover, we numerically find that the ferromagnetic state is stable against some deviations from completely flat band, by using the exact diagonalization method. The present result will be also useful as a guiding principle possibly display the flat-band ferromagnetism. [S0163-1829(98)09139-5].

We discuss the ground state and some excited states of the half-filled Hubbard model defined on an open chain with L sites, where only one of the boundary sites has a different value of chemical potential. We consider the case when the boundary site has a negative chemical potential -p and the Hubbard coupling U is positive. By an analytic method we show that when p is larger than the transfer integral some of the ground-state solutions of the Bethe ansatz equations become complex-valued. It follows that there is a 'surface phase transition' at some critical value pc when p &lt pc all the charge excitations have the gap for the half-filled band, while there exists a massless charge mode when p &gt pc.

We have investigated the electronic states of several BN/C-2 (111) ultrathin superlattices, which are the most energetically favorable among atomically-mixed heterodiamond structures, using first-principles calculations. The resulting band structures and charge densities indicate variations of the band-edge states depending on the slacking orientation of B and N. Inversion of BN orientation in alternating BN layers leads to two-dimensionally localized band-edge states, although superlattices with common BN orientation show spread states throughout the entire crystal.

Constant-pressure first-principles molecular dynamics method is applied to study of the graphite-to-diamond transformations under pressure and to theoretical prediction of structural properties of BC2N with diamond-like structures. The transition states of the transformations to cubic and hexagonal diamonds are investigated to clarify the difference of the mechanisms between them. It is shown that cubic diamond is formed with less activation energy than that for hexagonal diamond, provided that collective slide of graphite planes is not prohibited. As to BC2N, equilibrium lattice parameters and bulk moduli are calculated for some hypothetical arrangements of atoms. Among them, an arrangement which contains more C-C and B-N bonds turns out to be the most stable and the hardest, in spite of breaking the Grimm-Sommerfeld rule.

We study the magnetic field effect on nanographite ribbons based on the tight binding model with Peierls phase. It is found that the edge shape of graphite ribbons gives significant difference in the π electronic structure under magnetic field. We exhibit the ribbon width dependence of the total magnetization and the edge effect on the local distribution of the magnetization based on the armchair and zigzag edge-shaped ribbons. © 1998 Elsevier Science B.V. All rights reserved.

We propose probable synthesis paths using high pressure condition for well-crystallized energetically-favored structures of BC2N heterodiamonds, and further show their expected mechanical and electronic properties, from a viewpoint of material design by first-principles calculations. The results indicate that BN/C2 (111) superlattice is the most stable among atomically-mixed heterodiamond structures, and that compression of a graphitic superlattice at low temperature will be a promising way for the well-crystallized sample. The bulk modulus is found to be comparable to that of diamond. It is also found that the band-edge states of the superlattices dramatically change depending on the stacking orientation of boron and nitrogen. [first-principles calculation, high pressure synthesis, superlattice, bulk modulus, electronic states.]. © 1998, The Japan Society of High Pressure Science and Technology. All rights reserved.

Stability and structural transformation under high pressure are investigated for polymorphs of calcium di-silicide (CaSi2) theoretically. Using LDA calculation we can show that sp2-bonded structures are stabilized at high pressure against popular trigonal structure with sp3-bonded corrugated Si-planes. Our result suggests that even A1B2 structure with flat honeycomb Si-planes, which has not been found for CaSi2 experimentally, could be obtained under high pressure at low temperature. Characteristics of structural transformation of CaSi2 is discussed in comparison with the graphite-diamond transformation in which sp3 bonding is selected at high pressure. [CaSi2, structural transformation, LDA calculation, layered material, high pressure synthesis]. © 1998, The Japan Society of High Pressure Science and Technology. All rights reserved.

The extended spectral flow at the normal-superconducting transition is investigated for the one-dimensional Hubbard model. Recombination of spectral flow lines at U = +/-0; which causes halving of the extended Aharonov-Bohm period for U < 0 found in a recent paper [K. Kusakabe and H. Aoki: J. Phys. Sec. Jpn. 65 (1996) 2772], is exactly analyzed using the Bethe ansatz solution. Adiabatic motion of charge rapidities which are real for U > 0 (normal Luttinger liquid) or complex for U < 0 (superconducting Luther-Emery liquid) dominates sudden response of the spectral Row at the transition point. The extended period is obtained not only against the charge flux but also against the spin Aux for both U > 0 and U < 0.

The 'extended Aharonav-Bohm (AB) period' recently proposed by [K. Kusabahe and H. Aoki: J. Phys. Sec, Jpn. 85 (1996) 2772] is extensively studied numerically for finite size systems of strongly correlated electrons. While the extended AB period is the system length times the flux quantum for noninteracting systems; we have found the existence of the boundary across which the period is halved or another boundary into an et-en shorter period on the phase diagram for these models, If me compare this result with the phase diagram predicted From the Tomonaga-Luttinger theory, devised for low-energy physics, the halved period (or shorter periods) has a one-to-one correspondence to the existence of the pairing (phase separation or metal-insulator transition) in these models, We have also found for the t-J model that the extended AB period does not change across the integrable-nonintegrable boundary despite the totally different level statistics.

A probable heterodiamond BC2N structure that can be obtained from compression of graphitic BC2N at low temperature is proposed using first-principles calculations. The structure consists of a rhombohedral atomic arrangement and has a large bulk modulus comparable to that of diamond as well as a wide band gap. We also found that the structure can be synthesized from a superlattice with alternate stacking of graphite and hexagonal BN monolayers. Transformations from such layered superlattices open a possibility for control of the properties in synthesized materials.

A probable heterodiamond (Formula presented)N structure that can be obtained from compression of graphitic (Formula presented)N at low temperature is proposed using first-principles calculations. The structure consists of a rhombohedral atomic arrangement and has a large bulk modulus comparable to that of diamond as well as a wide band gap. We also found that the structure can be synthesized from a superlattice with alternate stacking of graphite and hexagonal BN monolayers. Transformations from such layered superlattices open a possibility for control of the properties in synthesized materials. © 1997 The American Physical Society.