arXiv subjects

Koichi Kusakabe

Publications and source records attributed to Koichi Kusakabe.

50 records · Page 3Linked to original sources

arXiv2008

Two-Site shift Product Wave Function Renormalization Group Method Applied to Quantum Systems

We report a way of wave function estimation for the density matrix renormalization group (DMRG) method applied to quantum systems, which has 2-site modulation, when the system size extension is necessary in both the finite and the infinite algorithms. The estimation is performed by renormalization group (RG) transformation applied to the ground state wave function, which is represented as the matrix product. This RG scheme is known as the product wave function renormalization group (PWFRG) method. In order to treat the 2-site modulation, the operation of the RG transformation is shifted by amount of 2 lattice sites. It turns out that this 2-site shift algorithm provides better wave function estimation in the thermodynamic limit, compared with the previously known PWFRG algorithm.

quant-ph↗

arXiv2007

A self-consistent first-principles calculation scheme for correlated electron systems

A self-consistent calculation scheme for correlated electron systems is created based on the density-functional theory (DFT). Our scheme is a multi-reference DFT (MR-DFT) calculation in which the electron charge density is reproduced by an auxiliary interacting Fermion system. A short-range Hubbard-type interaction is introduced by a rigorous manner with a residual term for the exchange-correlation energy. The Hubbard term is determined uniquely by referencing the density fluctuation at a selected localized orbital. This strategy to obtain an extension of the Kohn-Sham scheme provides a self-consistent electronic structure calculation for the materials design. Introducing an approximation for the residual exchange-correlation energy functional, we have the LDA+U energy functional. Practical self-consistent calculations are exemplified by simulations of Hydrogen systems, i.e. a molecule and a periodic one-dimensional array, which is a proof of existence of the interaction strength U as a continuous function of the local fluctuation and structural parameters of the system.

cond-mat.str-el↗

arXiv2006

New Ferromagnetic Nitrides CaN and SrN and their synthesis process

We introduce new type of ferromagnets, CaN and SrN, which were designed using first-principles calculations. These are half-metallic ferromagnets and they have magnetic moments of 1 $μ_{\rm B}$ per chemical formula unit. Out of the typical structures of binary compounds, the rock-salt structure is the most stable form for both CaN and SrN. The majority of the magnetic moment of these compound originates from the N sites since the $p$ states of N are spin-polarized. Their formation energies were calculated and the results show that it should be feasible to synthesize these materials. The structural stability of CaN was confirmed by performing first-principles molecular dynamics simulations. We propose a synthesis process for CaN basd on the first-principles.

cond-mat.mtrl-sci↗

arXiv2006

Edge state on hydrogen-terminated graphite edges investigated by scanning tunneling microscopy

The edge states that emerge at hydrogen-terminated zigzag edges embedded in dominant armchair edges of graphite are carefully investigated by ultrahigh-vacuum scanning tunneling microscopy (STM) measurements. The edge states at the zigzag edges have different spatial distributions dependent on the $α$- or $β$-site edge carbon atoms. In the case that the defects consist of a short zigzag (or a short Klein) edge, the edge state is present also near the defects. The amplitude of the edge state distributing around the defects in an armchair edge often has a prominent hump in a direction determined by detailed local atomic structure of the edge. The tight binding calculation based on the atomic arrangements observed by STM reproduces the observed spatial distributions of the local density of states.

cond-mat.mtrl-sci↗

arXiv2005

A uniqueness theorem in a density-matrix functional theory

Uniqueness of effective interaction defined in an extension of the Kohn-Sham theory is proved, if the model with a non-degenerate ground state exists and to reproduce a correlation function as well as the single-particle density of an electron system. The two-body interaction term is regarded as a Hubbard-type short range interaction term. The interaction strength is a functional of an element of a two-body reduced density matrix of the electron system. The uniqueness theorem gives a basic principle for effective description of electron systems.

cond-mat.str-el↗

arXiv2005

MCSCF-DFT based on an interacting reference system

We present the MCSCF version of density functional theory. Two sets of equations, which correspond to the CI and orbital relaxation respectively, are derived. An important feature is that the correlation potential of DFT for CI wavefunction and that for orbitals are different to each other. These relations ensure that a density yielded by a effective MCSCF solution also satisfies the Euler equation of DFT

physics.chem-ph↗

arXiv2005

Observation of zigzag and armchair edges of graphite using scanning tunneling microscopy and spectroscopy

The presence of structure-dependent edge states of graphite is revealed by both ambient- and ultra-highvacuum- (UHV) scanning tunneling microscopy (STM) / scanning tunneling spectroscopy (STS) observations. On a hydrogenated zigzag (armchair) edge, bright spots are (are not) observed together with (SQRT(3) by SQRT(3))R30 superlattice near the Fermi level (V_S = −30 mV for a peak of the local density of states (LDOS)) under UHV, demonstrating that a zigzag edge is responsible for the edge states, although there is no appreciable difference between as-prepared zigzag and armchair edges in air. Even in hydrogenated armchair edge, however, bright spots are observed at defect points, at which partial zigzag edges are created in the armchair edge.

cond-mat.mtrl-sci↗

arXiv2004

Zinc-blende CaP, CaAs and CaSb as half-metals: A new route to magnetism in calcium compounds

Existence of ferromagnetism in bulk calcium compounds is discovered theoretically. First-principles calculations of calcium phosphide, calcium arsenide and calcium antimonide in the zinc-blende structure have been performed to show the half-metallic ground state in each optimized stable structure. Magnetism comes from spin-polarization of electrons in $p$-orbitals of P, As or Sb and $d$-orbitals of calcium atoms. The half-metallicity is analogous to the half-metallic zinc-blende compounds, {\it e.g.} CrAs or CrSb, but the predicted compounds become ferromagnetic without transition metals. In (In$_{1-x}$Ca$_x$)Sb, the magnetism remains to be stable in a range of the doping rate ($x>0.8$).

cond-mat.mtrl-sci↗

arXiv2002

Magnetic 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 appearance of a spin-polarized flat band at the Fermi energy. Producing different numbers of mono-hydrogenated carbons and di-hydrogenated carbons can create magnetic moments in nanographite.

cond-mat.mtrl-sci↗

arXiv2001

A Rigorous Extension of the Kohn-Sham Equation for Strongly Correlated Electron Systems

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 interaction 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

cond-mat.mtrl-sci↗

arXiv1999

Theory of the Josephson effect in superconductor / one-dimensional electron gas / superconductor junction

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.

cond-mat.supr-con↗

arXiv1998

Spectral function of the spiral spin state in the trestle and ladder Hubbard model

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 length of the system size.

cond-mat↗

arXiv1998

A gapless charge mode induced by the boundary states in the half-filled Hubbard open-chain

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 p_c; when p p_c.

cond-mat.stat-mech↗

arXiv1997

Extended Aharonov-Bohm period analysis of strongly correlated electron systems

The `extended Aharonov-Bohm (AB) period' recently proposed by Kusakabe and Aoki [J. Phys. Soc. Jpn (65), 2772 (1996)] 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 even shorter period on the phase diagram for these models. If we 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.

cond-mat↗