arXiv subjects

Koichi Kusakabe

Publications and source records attributed to Koichi Kusakabe.

At least 37 records · Page 2Linked to original sources

arXiv2018

Material-dependent screening of Coulomb interaction in single-layer cuprates

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

cond-mat.supr-con↗

arXiv2017

An enhancement mechanism of two-particle exchange interactions in single- and multi-layer cuprate superconductors

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

cond-mat.supr-con↗

arXiv2017

Self-doping effect arising from electron correlations in multi-layer cuprates

A self-doping effect between outer and inner CuO$_2$ planes (OPs and IPs) in multi-layer cuprate superconductors is studied. When one considers a three-layer tight-binding model of the Hg-based three-layer cuprate derived from the first principle calculations, the electron concentration gets to be large in the OP compared to IP. This is inconsistent with the experimental fact that more hole carriers tend to be introduced into the OP than IP.We investigate a three-layer Hubbard model with the two-particle self-consistent approach for multi-layer systems to incorporate electron correlations. We observe that the double occupancy (antiferromagnetic instability) in the IP decreases (increases) more than the OP, and also reveal that more electrons tend to be introduced into the IP than OP to obtain the energy gain from the on-site Hubbard interaction. These results are consistent with the experimental facts, and this electron distribution between the OP and IP can be interpreted as a self-doping effect arising from strong electron correlations.

cond-mat.supr-con↗

arXiv2016

Theoretical Analysis on Pseudo-Degenerate Zero-Energy Modes in Vacancy-Centered Hexagonal Armchair Nanographene

Deriving mathematical expressions of two zero modes for a $π$-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 quasi-hexagonal nanographene molecule with a center vacancy (VANG). Indeed, in a VANG molecule, C$_{60}$H$_{24}$, showing stability in a density-functional simulation at the highest occupied level, there appear two pseudo-degenerate zero modes, a vacancy-centered quasi-localized zero mode, and extending zero mode with a $\sqrt{3} \times \sqrt{3}$ structure. Since there is a finite energy gap between these two zero-energy modes and the other modes, low-lying states composed of quasi-degenerate zero modes appear as magnetic multiplets. Thus, the unique magnetic characteristics derived in our theory are expected to hold for synthesized VANG molecules in reality.

cond-mat.mes-hall↗

arXiv2014

Magnetic correlation effects by the topological zero mode in a hydrogenated graphene vacancy $V_{111}$

Electron correlation effects caused by the topological zero mode of a hydrogenated graphene vacancy, $V_{111}$, with three adsorbed hydrogen atoms is discussed theoretically. A Kondo model is derived from the multi-reference representation of the density functional theory, where exchange scattering processes between the zero mode and low-energy modes in the Dirac cones are estimated. Even when the Dirac cone is slightly off from the charge neutral point, a finite on-site correlation energy, $U_0$, for the zero mode of an isolated $V_{111}$ allows the half-filling of the localized level giving a spin $s=1/2$. The anti-ferromagnetic Kondo screening mediated by higher order scattering processes becomes dominant in the dilute limit of the vacancies. Our estimation of relevant two body interactions certifies appearance of the Kondo effect at low temperatures.

cond-mat.mes-hall↗

arXiv2014

Orbital mixture effect on the Fermi surface-$T_c$ correlation in the cuprate superconductors --- bilayer vs single layer

By constructing $d_{x^2-y^2}-d_{z^2}$ two-orbital models from first principles, we have obtained a systematic correlation between the Fermi surface warping and the evaluated $T_c$ for various bilayer as well as single-layer cuprates. This reveals that smaller mixture of the $d_{z^2}$ orbital component on the Fermi surface leads to both of larger Fermi surface warping and higher $T_c$. The theoretical correlation strikingly resembles a systematic plot for the experimentally observed $T_c$ against the Fermi surface warping due to Pavarini {\it et al.} [Phys. Rev. Lett. {\bf 87}, 047003 (2001)], and the present result unambiguously indicates that the $d_{z^2}$ mixture is a key factor that determines $T_c$ in the cuprates.

cond-mat.supr-con↗

arXiv2014

Direct imaging of monovacancy-hydrogen complexes in single graphitic layer

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 ultra-high 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 sigma-dangling bond is passivated with one hydrogen atom shows a well-defined signal from the non-bonding pi-state which penetrates into the bulk with a (\sqrt 3 \times \sqrt 3)R30^ \circ periodicity. However, a single atomic vacancy with full hydrogen termination of sigma-dangling bonds and additional hydrogen passivation of the extended pi-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.

cond-mat.mtrl-sci↗

arXiv2013

Material Optimization for Fermi Surface Shape Control of Tl-based Cuprate Superconductors

To show an optimization method of element substitution for cuprate superconductors, we investigate Fermi surface shape of TlR2A2Cu3O9 with R=La, Y, and A=Li, Na, K, Rb, Cs. We adopt the generalized gradient approximation in the density-functional theory (DFT-GGA) for the study of over-doped phases of these unknown cuprates. The electronic structures of crystals optimized by DFT-GGA show systematic element dependence in a Fermi surface shape controlling parameter, r, of Cu dx2-y2 bands, where nearly absent dz2 component at the Fermi level and smaller r keeping t1 suggest enhancement of the superconducting transition temperature within the spin-fluctuation mechanism. For TlYRb2Cu3O9, smaller r by a reduction factor larger than 10% compared to a reference system of TlBa2Ca2Cu3O9 (TBCCO) appears in the outer CuO2 plane, but with 12 % reduction in t1. For TlR2Li2Cu3O9, (R=Y, La), smaller r by a factor larger than 1% appears keeping t1 as large as TBCCO in the inner plane. Our method may be used to predict an optimized material structure referencing known cuprate superconductors.

cond-mat.supr-con↗

arXiv2011

Determination of Boundary Scattering, Intermagnon Scattering, and the Haldane Gap in Heisenberg Chains

Low-lying magnon dispersion in a S=1 Heisenberg antiferromagnetic (AF) chain is analyzed using the non-Abelian DMRG method. The scattering length $a_{\rm b}$ of the boundary coupling and the inter-magnon scattering length $a$ are determined. The scattering length $a_{\rm b}$ 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 $ξ$.

cond-mat.str-el↗

arXiv2011

Hyperbolic Deformation Applied to S = 1 Spin Chains - Scaling Relation in Excitation Energy -

We investigate excitation energies of hyperbolically deformed S = 1 spin chains, which are specified by the local energy scale f_j^{~} = \cosh j λ, where j is the lattice index and λis the deformation parameter. The elementary excitation is well described by a quasiparticle hopping model, which is also expressed in the form of hyperbolic deformation. It is possible to estimate the excitation gap Δin the uniform limit λ\rightarrow 0, by means of a finite size scaling with respect to the system size N and the deformation parameter λ.

cond-mat.str-el↗

arXiv2010

Theorems on ground-state phase transitions in Kohn-Sham models given by the Coulomb density functional

Some theorems on derivatives of the Coulomb density functional with respect to the coupling constant $λ$ are given. Consider an electron density $n_{GS}({\bf r})$ given by a ground state. A model Fermion system with the reduced coupling constant, $λ<1$, is defined to reproduce $n_{GS}({\bf r})$ and the ground state energy. Fixing the charge density, possible phase transitions as level crossings detected in a value of the reduced density functional happen only at discrete points along the $λ$ axis. If the density is $v$-representable also for $λ<1$, accumulation of phase transition points is forbidden when $λ\rightarrow 1$. Relevance of the theorems for the multi-reference density functional theory is discussed.

cond-mat.stat-mech↗

arXiv2010

A Theoretical Simulation of Deformed Carbon Nanotubes with Adsorbed Metal Atoms: Enhanced Reactivity by Deformation

First-principles simulations were performed to investigate reaction of carbon nanotubes with adsorbed metal atoms. Mechanical modification of their structures enhances chemical reactivity of carbon nanotubes. Adsorption of a tungsten, tantalum, or niobium atom on a (5, 0) nanotube with a Stone-Wales defect was shown to have characteristically strong chemisorption. Bond-breaking in the carbon-carbon network and formation of a local metal-carbon complex were observed during the simulation. Adsorption of W, Ta, Ni or Mo on a twisted (5, 0) nanotube showed a preferred breaking of several bonds, even creating an opening in the wall. The enhanced chemical reactivity of deformed nanotubes is characterized by formation of a metal-carbon complex. Applications of the reaction are suggested.

cond-mat.mtrl-sci↗

arXiv2010

Scaling Relation for Excitation Energy Under Hyperbolic Deformation

We introduce a one-parameter deformation for one-dimensional (1D) quantum lattice models, the hyperbolic deformation, where the scale of the local energy is proportional to cosh lambda j at the j-th site. Corresponding to a 2D classical system, the deformation does not strongly modify the ground state. In this situation, the effective Hamiltonian of the quantum system shows that the quasi particle is weakly bounded around the center of the system. By analyzing this binding effect, we derive scaling relations for the mean-square width of confinement, the energy correction with respect to the excitation gap Δ, and the deformation parameter $λ$. This finite-size scaling allows us to investigate excitation gap of 1D non-deformed bulk quantum systems.

cond-mat.other↗

arXiv2009

Pair-Hopping Mechanism for Layered Superconductors

We propose a possible charge fluctuation effect expected in layered superconducting materials. In the multireference density functional theory, relevant fluctuation channels for the Josephson coupling between superconducting layers include the interlayer pair hopping derived from the Coulomb repulsion. When interlayer single-electron tunneling processes are irrelevant in the Kohn-Sham electronic band structure calculation, the two-body effective interactions stabilize a superconducting phase. This state is also regarded as a valence-bond solid in a bulk electronic state. The hidden order parameters coexist with the superconducting order parameter when the charging effect of a layer is comparable to the pair hopping. Relevant materials structures favorable for the pair-hopping mechanism are discussed.

cond-mat.supr-con↗

arXiv2009

Gap Estimation by means of Hyperbolic Deformation

We present a way of numerical gap estimation applicable for one-dimensional infinite uniform quantum systems. Using the density matrix renormalization group method for a non-uniform Hamiltonian, which has deformed interaction strength of $j$-th bond proportional to $\cosh λj$, the uniform Hamiltonian is analyzed as a limit of $λ\to 0$. As a consequence of the deformation, an excited quasi-particle is weakly bounded around the center of the system, and kept away from the system boundary. Therefore, insensitivity of an estimated excitation gap of the deformed system to the boundary allows us to have the bulk excitation gap $Δ(λ)$, and shift in $Δ(λ)$ from $Δ(0)$ is nearly linear in $λ$ when $λ\ll 1$. Efficiency of this estimation is demonstrated through application to the S=1 antiferromagnetic Heisenberg chain. Combining the above estimation and another one obtained from the technique of convergence acceleration for finite-size gaps estimated by numerical diagonalizations, we conclude that the Haldane gap is in $[0.41047905,~0.41047931]$.

cond-mat.stat-mech↗

arXiv2008

Finite-size scaling of string order parameters characterizing the Haldane phase

We have developed a numerical procedure to clarify the critical behavior near a quantum phase transition by analyzing a multi-point correlation function characterizing the ground state. This work presents a successful application of this procedure to the string order parameter of the S = 1 XXZ chain with uniaxial single-ion anisotropy. The finite-size string correlation function is estimated by the density matrix renormalization group method. We focus on the gradient of the inversedsystem- size dependence of the correlation function on a logarithmic plot. This quantity shows that the finite-size scaling sensitively changes at the critical point. The behavior of the gradient with increasing system size is divergent, stable at a finite value, or rapidly decreases to zero when the system is in the disordered phase, at the critical point, or in the ordered phase, respectively. The analysis of the finite-size string correlation functions allows precise determination of the boundary of the Haldane phase and estimation of the critical exponent of the correlation length. Our estimates of the transition point and the critical exponents, which are determined only by the ground-state quantities, are consistent with results obtained from the analysis of the energy-level structure. Our analysis requires only the correlation functions of several finite sizes under the same condition as a candidate for the long-range order. The quantity is treated in the same manner irrespective of the kind of elements which destroy the order concerned. This work will assist in the development of a method to directly observe quantum phase transitions.

cond-mat.str-el↗

arXiv2008

Generation of Nano-Catalyst Particles by Spinodal Nano-Decomposition in Perovskite

A new mechanism of nano-catalyst generation based on the spinodal nano-decomposition in self-regenerating perovskite catalysts for automotive-emissions control is proposed. To demonstrate existence of the spinodal nano-decomposition in real perovskite catalysts, we performed first-principles calculations to evaluate the free energy of La(Fe$_{1-x}$Pd$_x$)O$_3$ and La(Fe$_{1-x}$Rh$_x$)O$_3$. The result indicates appearance of a spinodal region in the phase diagram of each material. Formation of nano-catalyst particles in the perovskite host matrix is crucial for the self-regeneration of perovskite catalyst. Based on the spinodal nano-decomposition model, possible materials are designed for new three-way catalyst with no contents of precious metal.

cond-mat.mtrl-sci↗

arXiv2008

General Rule and Materials Design of Negative Effective U System for High-T_c Superconductivity

Based on the microscopic mechanisms of (1) charge-excitation-induced negative effective U in s^1 or d^9 electronic configurations, and (2) exchange-correlation-induced negative effective U in d^4 or d^6 electronic configurations, we propose a general rule and materials design of negative effective U system in itinerant (ionic and metallic) system for the realization of high-T_c superconductors. We design a T_c-enhancing layer (or clusters) of charge-excitation-induced negative effective $U$ connecting the superconducting layers for the realistic systems.

cond-mat.supr-con↗