arXiv subjects

Shigeki Onoda

Publications and source records attributed to Shigeki Onoda.

51 records · Page 3Linked to original sources

arXiv2004

Ginzburg-Landau Theory and Classical Critical Phenomena of Mott Transition

Theory of classical critical phenomena of Mott transition is developed for the dimensionality $d \le \infty$. Reconsidering a cluster dynamical mean-field theory (DMFT), Ginzburg-Landau free energy is derived in terms of hybridization function for a cluster-impurity model. Its expansion around a cluster DMFT solution reduces to a $ϕ^4$ model. Inherent thermal Mott transition without spontaneous symmetry breaking is described by a scalar field reflecting the charge degrees of freedom. In the space of local Coulomb repulsion, chemical potential and temperature, a first-order transition surface terminates at a critical end curve. The criticality belongs to the Ising universality as a liquid-gas transition. Various quantities including double occupancy, electron filling and entropy show diverging responses at the criticality and discontinuities at the first-order transition. Particularly, electron effective mass shows a critical divergence in $2\le d\le 4$. Only at a certain curve on the surface, a filling-control transition and its related singularities disappear. We discuss detailed critical behaviors, effects of interplay with other critical fluctuations, and relevant experimental results.

cond-mat.str-el↗

arXiv2004

Two-dimensional charge order in layered 2-1-4 perovskite oxides

Monte Carlo simulations are performed on the three-dimensional (3D) Ising model with the 2-1-4 layered perovskite structure as a minimal model for checkerboard charge ordering phenomena in layered perovskite oxides. Due to the interlayer frustration, only 2D long-range order emerges with a finite correlation length along the c axis. Critical exponents of the transition change continuously as a function of the interlayer coupling constant. The interlayer long-range Coulomb interaction decays exponentially and is negligible even between the second-neighbor layers. Instead, monoclinic distortion of a tetragonal unit cell lifts the macroscopic degeneracy to induce a 3D charge ordering. The dimensionality of the charge order in La$_{0.5}$Sr$_{1.5}$MnO$_4$ is discussed from this viewpoint.

cond-mat.stat-mech↗

arXiv2003

Mott Transition vs Multicritical Phenomenon of Superconductivity and Antiferromagnetism -- Application to $κ$-(BEDT-TTF)$_2$X --

Interplay between the Mott transition and the multicritical phenomenon of d-wave superconductivity (SC) and antiferromagnetism (AF) is studied theoretically. We describe the Mott transition, which is analogous to a liquid-gas phase transition, in terms of an Ising-type order parameter $η$. We reveal possible mean-field phase diagrams produced by this interplay. Renormalization group analysis up to one-loop order gives flows of coupling constants, which in most cases lead to fluctuation-induced first-order phase transitions even when the SO(5) symmetry exists betwen the SC and AF. Behaviors of various physical quantities around the Mott critical point are predicted. Experiments in $κ$-(BEDT-TTF)$_2$X are discussed from this viewpoint.

cond-mat.str-el↗

arXiv2003

Finite-Temperature Mott Transition in the Two-Dimensional Hubbard Model

Mott transitions are studied in the two-dimensional Hubbard model by a non-perturbative theory of correlator projection that systematically includes spatial correlations into the dynamical mean-field approximation. Introducing a nonzero second-neighbor transfer, a first-order Mott transition appears at finite temperatures and ends at a critical point or curve.

cond-mat.str-el↗

arXiv2002

Mott Transitions in the 2D Half-Filled Hubbard Model: Correlator Projection Method with Projective Dynamical Mean-Field Approximation

The 2D half-filled Hubbard model is studied by a nonperturbative analytic theory of correlator projection. The original dynamical mean-field approximation (DMFA) is reproduced at the first-order projection and then improved by systematic inclusion of spatial correlations at higher orders. A geometrical frustration induces a first-order Mott transition surface with a finite-temperature critical end curve. Growth of antiferromagnetic correlations gives single-particle spectra strongly modified from DMFA with shadow bands and flat dispersions observed in high-T_c cuprates.

cond-mat.str-el↗

arXiv2002

Spin Chirality Fluctuation and Anomalous Hall Effect in Itinerant Ferromagnets

The anomalous Hall effect due to the spin chirality order and fluctuation is studied theoretically in a Kondo lattice model without the relativistic spin-orbit interaction. Even without the correlations of the localized spins, $σ_{xy}$ can emerge depending on the lattice structure and the spin anisotropy. We reveal the condition for this chirality-fluctuation driven mechanism for $σ_{xy}$. Our semiquantitative estimates for a pyrochlore oxide Nd$_2$Mo$_2$O$_7$ give a finite $σ_{xy} \sim 10 \Ohm^{-1} \cm^{-1}$ together with a high resistivity $ρ_{xx} \sim 10^{-4}-10^{-3} \Ohm \cm$, in agreement with experiments.

cond-mat.str-el↗

arXiv2001

Operator Projection Theory for Electron Differentiation in Underdoped Cuprate Superconductors

Metals approaching the Mott insulator generate a new hierarchy in the electronic structure accompanied by a momentum dependent electron differentiation, beyond the Mott-Hubbard, Brinkman-Rice and Slater pictures of the Mott transition. To consider such nonlinear phenomenon, we develop an analytic nonperturbative theory based on operator projections combined with a self-consistent treatment of the low-energy excitations. This reproduces the formation of the Hubbard bands, Mott gap, spin fluctuations, mass divergence, diverging charge compressibility, and strongly renormalized flat and damped dispersion similar to angle-resolved photoemission data in high-T_c cuprates. Main structures in electronic spectra show a remarkable similarity to numerical results.

cond-mat.str-el↗

arXiv2001

Filling-Control Metal-Insulator Transition in the Hubbard Model Studied by the Operator Projection Method

Thermodynamic and dynamical properties of filling-control metal-insulator transition (MIT) in the Hubbard model are studied by the operator projection method, especially in two dimensions. This is a non-perturbative analytic approach to many-body systems. The present theory incorporates the Mott-Hubbard, Brinkman-Rice and Slater pictures of the MIT into a unified framework, together with reproducing low-energy narrow band arising from spin-charge fluctuations. At half filling, single-particle spectra $A(ω, k)$ show formation of two Hubbard bands and their antiferromagnetic shadows separated by a Mott gap in the plane of energy $ω$ and momentum $k$ with lowering temperatures. These four bands produce splitting to two low-energy narrow bands and two SDW-like bands in the dispersion. Near half filling, the low-energy narrow band persists at low temperatures. This narrow band has a particularly weak dispersion and large weights around $(π, 0)$ and $(0, π)$ momenta. The velocity of these low-energy excitations is shown to vanish towards the MIT, indicating the mass divergence as in the Brinkman-Rice picture, but most prominently around $(π,0)$ and $(0,π)$ with strong momentum dependence. This reflects the suppression of the coherence near the MIT. Main structures in $A(ω, k)$ show remarkable similarities to quantum Monte-Carlo results in two dimensions as well as in the one-dimensional Hubbard model. The charge compressibility appears to diverge with decreasing doping concentration in both one and two dimensions in agreement with the exact and quantum Monte-Carlo results. We also discuss implications of the flat dispersion formed near the Fermi level to the observations in high-$\Tc$ cuprate superconductors.

cond-mat.str-el↗

arXiv2001

Operator projection method applied to the single-particle Green's function in the Hubbard model

A new non-perturbative framework for many-body correlated systems is formulated by extending the operator projection method (OPM). This method offers a systematic expansion which enables us to project into the low-energy structure after extracting the higher-energy hierarchy. This method also opens a way to systematically take into account the effects of collective excitations. The Mott-Hubbard metal-insulator transition in the Hubbard model is studied by means of this projection beyond the second order by taking into account magnetic and charge fluctuations in the presence of the high-energy Mott-Hubbard structure. At half filling, the Mott-Hubbard gap is correctly eproduced between the separated two bands. Near half filling, a strongly renormalized low-energy single-particle excitations coexisting with the Mott-Hubbard bands are shown to appear. Signifcance of momentum-dependent self-energy in the results is stressed.

cond-mat.str-el↗

arXiv2000

Pseudogap and Kinetic Pairing Under Critical Differentiation of Electrons in Cuprate Superconductors

Superconducting mechanism of cuprates is discussed in the light of the proximity of the Mott insulator. The proximity accompanied by suppression of coherence takes place in an inhomogeneous way in the momentum space in finite-dimensional systems. Studies on instabilities of metals consisted of such differentiated electrons in the momentum space are reviewed from a general point of view. A typical example of the differentiation is found in the flattening of the quasiparticle dispersion discovered around momenta $(π,0)$ and $(0,π)$ on 2D square lattices. This flattening even controls the criticality of the metal-insulator transition. Such differentiation and suppressed coherence subsequently cause an instability to the superconducting state in the second order of the strong coupling expansion. The d-wave pairing interaction is generated from such local but kinetic processes in the absence of disturbance from the coherent single-particle excitations. The superconducting mechanism emerges from a direct kinetic origin which is conceptually different from the pairing mechanism mediated by bosonic excitations as in magnetic, excitonic, and BCS mechanisms. Pseudogap phenomena widely observed in the underdoped cuprates are then naturally understood from the mode-mode coupling of d-wave superconducting (dSC) fluctuations repulsively coupled with antiferromagnetic (AFM) ones. When we assume the existence of a strong d-wave channel repulsively competing with AFM fluctuations under the formation of flat and damped single-particle dispersion, we reproduce basic properties of the pseudogap seen in the magnetic resonance, neutron scattering, angle resolved photoemission and tunneling measurements in the cuprates.

cond-mat.str-el↗

arXiv2000

Pseudogap and Superconducting Fluctuations in High-Tc Cuprates

We analyze pseudogap phenomena widely observed in the underdoped cuprates. We assume the existence of a strong d-wave pairing force competing with antiferromagnetic(AFM) fluctuations and the formation of flat and damped dispersion around the $(π,0)$ and $(0,π)$ region as two important elements caused by the proximity from the Mott insulator. Using the mode-mode coupling theory for the d-wave superconducting(dSC) and AFM fluctuations, we reproduce basic properties of the pseudogap seen in the magnetic resonance, neutron scattering, angle resolved photoemission and tunneling measurements in the cuprates. Then minimal requirements to understand the pseudogap phenomena are clarified as the above two elements. A strong competition of the pairing with the antiferromagnetic fluctuations suppresses the transition temperature thereby generates the pseudogap in the underdoped region while the weakness of the AFM fluctuations leads to the absence of the pseudogap at the optimal doping concentration.}

cond-mat.supr-con↗

arXiv2000

Theory of Electron Differentiation, Flat Dispersion and Pseudogap Phenomena

Aspects of electron critical differentiation are clarified in the proximity of the Mott insulator. The flattening of the quasiparticle dispersion appears around momenta $(π,0)$ and $(0,π)$ on square lattices and determines the criticality of the metal-insulator transition with the suppressed coherence in that momentum region of quasiparticles. Such coherence suppression at the same time causes an instability to the superconducting state if a proper incoherent process is retained. The d-wave pairing interaction is generated from such retained processes without disturbance from the coherent single-particle excitations. Pseudogap phenomena widely observed in the underdoped cuprates are then naturally understood from the mode-mode coupling of d-wave superconducting(dSC) fluctuations with antiferromagnetic ones. When we assume the existence of a strong d-wave pairing force repulsively competing with antiferromagnetic(AFM) fluctuations under the formation of flat and damped single-particle dispersion, we reproduce basic properties of the pseudogap seen in the magnetic resonance, neutron scattering, angle resolved photoemission and tunneling measurements in the cuprates.

cond-mat.supr-con↗

arXiv2000

Magnetic properties of the Hubbard model on three-dimensional lattices: fluctuation-exchange and two-particle self-consistent studies

The relation between three-dimensional lattice structure and magnetism in correlated electron systems is explored for face centered cubic (FCC), body centered cubic (BCC), and simple cubic (SC) lattices. In particular, we question which lattice structure has the strongest tendency toward the ferromagnetism or antiferromagnetism. We employ the Hubbard model to calculate the spin susceptibility and the single-particle spectrum with the fluctuation-exchange (FLEX) and the two-particle self-consistent (TPSC) approximations in the weak coupling regime. We have shown that (i) ferromagnetic spin fluctuations become dominant when the Fermi level lies around a sharp peak (divergence) in the density of states (D(E)) near the bottom of the band, which occurs for FCC with/without next nearest neighbor hoppings (t') or BCC with an appropriate value of t'. Among the cases studied, the ferromagnetic fluctuation is found to be the strongest for FCC with a finite t'. (ii) When the peak in D(E) resides around the band center as in bipartite SC or BCC, antiferromagnetic fluctuations become dominant when the band is close to the half-filling, with the fluctuation being much stronger in BCC.

cond-mat↗

arXiv1999

Single-Particle Pseudogap in Two-Dimensional Electron Systems

We investigate pseudogap phenomena in the 2D electron system. Based on the mode-mode coupling theory of antiferromagnetic (AFM) and $d_{x^2-y^2}$-wave superconducting ($d$SC) fluctuations, single-particle dynamics is analyzed. For the parameter values of underdoped cuprates, pseudogap structure grows in the single-particle spectral weight $A(k,ω)$ around the wave vector $(π,0)$ and $(0,π)$ below the pseudo-spin-gap temperature $\TPG$ signaled by the reduction of dynamical spin correlations in qualitative agreement with the experimental data. The calculated results for the overdoped cuprates also reproduce the absence of the pseudogap in the experiments. We also discuss limitations of our weak-coupling approach.

cond-mat.str-el↗

arXiv1999

d_{x^2-y^2}-Wave Pairing Fluctuations and Pseudo Spin Gap in Two-Dimensional Electron Systems

Pseudogap phenomena of high-T_c cuprates are examined. In terms of AFM (antiferromagnetic) and dSC (d_{x^2-y^2}-wave superconducting) auxiliary fields introduced to integrate out the fermions, the effective action for 2D electron systems with AFM and dSC fluctuations is considered. By the self-consistent renormalization (SCR), the NMR relaxation rate T_1^{-1}, the spin correlation length ξ_σand the pairing correlation length ξ_d are calculated. From this calculation, a mechanism of the pseudogap formation emerges as the region of dominant d-wave short-range order (SRO) over AFM-SRO. When damping for the AFM fluctuation strongly depends on the dSC correlation length through the formation of precursor singlets around (π,0) and (0,π) points in the momentum space, the pseudogap appears in a region of the normal state characterized by decreasing 1/T_1T and increasing AFM correlation length with decrease in temperature. This reproduces a characteristic feature of the pseudogap phenomena in many underdoped cuprates. When the damping becomes insensitive to the dSC correlation length, the pseudogap region shrinks as in the overdoped cuprates.

cond-mat.str-el↗