arXiv subjects

Hideo Aoki

Publications and source records attributed to Hideo Aoki.

176 records · Page 10Linked to original sources

arXiv1997

A consistent description of the pairing symmetry in hole and electron doped cuprates within the two dimensional Hubbard model

Quantum Monte Carlo is used to calculate various pairing correlations of the 2D Hubbard model possessing band features experimentally observed in the cuprates. In the hole-doped case, where the Fermi level lies close to the van Hove singularities around $(0,π)$, the d$_{x^2-y^2}$ pairing correlation is selectively enhanced, while in the electron-doped case, where the singularities are far below the Fermi level and the Fermi surface runs through $(\pm π/2,\pm π/2)$, both d$_{x^2-y^2}$ and d$_{xy}$ correlations are enhanced with the latter having a $\sqrt{2}\times \sqrt{2}$ structure. The two pairing symmetries can mix to result in a nodeless gap.

cond-mat.str-el↗

arXiv1997

Enhancement of the d_{x^2-y^2} pairing correlation in the two-dimensional Hubbard model: a quantum Monte Carlo study

Quantum Monte Carlo is used to investigate the possibility of d_{x^2-y^2} superconductivity in the two-dimensional repulsive Hubbard model. A small energy scale relevant to possible pairing requires a care (i.e., sufficiently small level separation between the $k$ points $(δk,π-δk')$ and $(π-δk'',δk''')$ with small $δk$'s) to detect enhanced correlations in finite-size studies, as motivated from a previous study on Hubbard ladders. Our calculation indeed detects long-tailed enhancements in the d_{x^2-y^2} pairing correlation when the system is near, but not exactly at, half-filling.

cond-mat.str-el↗

arXiv1997

Pairing correlation in the two- and three-leg Hubbard ladders --- Renormalization and quantum Monte Carlo studies

In order to shed light whether the `even-odd conjecture' (even numbers of legs will superconduct accompanied by a spin gap while odd ones do not) for correlated electrons in ladder systems, the pairing correlation is studied for the Hubbard model on a two- and three-leg ladders. We have employed both the weak-coupling renormalization group and the quantum Monte Carlo (QMC) method for strong interactions. For the two-leg Hubbard ladder, a systematic QMC (with a controlled level spacings) has detected an enhanced pairing correlation, which is consistent with the weak-coupling prediction. We also calculate the correlation functions in the three-leg Hubbard ladder and show that the weak-coupling study predicts the dominant superconductivity, which refutes the naive even-odd conjecture. A crucial point is a spin gap for only some of the multiple spin modes is enough to make the ladder superconduct with a pairing symmetry (d-like here) compatible with the gapped mode. A QMC study for the three-leg ladder endorses the enhanced pairing correlation.

cond-mat.str-el↗

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↗

arXiv1997

Numerical study of a superconductor-insulator transition in a half-filled Hubbard chain with distant transfers

The ground state of a one-dimensional Hubbard model having the next-nearest neighbor hopping (t') as well as the nearest-neighbor one (t) is numerically investigated at half-filling. A quantum Monte Carlo result shows a slowly decaying pairing correlation for a sizeable interaction strength $(U \leq 2t)$, while the system is shown to become insulating for yet larger $U>U_C\sim 3t$ from a direct evaluation of the charge gap with the density-matrix renormalization group method. The results are consistent with Fabrizio's recent weak-coupling theory which suggests a transition from a superconductor into an insulator at a finite U.

cond-mat.str-el↗

arXiv1996

Superconductivity in the three-leg Hubbard ladder: a Quantum Monte Carlo study

Quantum Monte Carlo method is used to look into the superconductivity in the three-leg Hubbard ladder. The enhanced correlation for the pairing across the central and edge chains, which has been predicted in the weak-coupling renormalization as an effect of coexistence of gapful and gapless spin modes, is here shown to persist for intermediate interaction strengths.

cond-mat.supr-con↗

arXiv1996

Quantum Monte Carlo study of the pairing correlation in the Hubbard ladder

An extensive Quantum Monte Carlo calculation is performed for the two-leg Hubbard ladder model to clarify whether the singlet pairing correlation decays slowly, which is predicted from the weak-coupling theory but controversial from numerical studies. Our result suggests that the discreteness of energy levels in finite systems affects the correlation enormously, where the enhanced pairing correlation is indeed detected if we make the energy levels of the bonding and anti-bonding bands lie close to each other at the Fermi level to mimic the thermodynamic limit.

cond-mat.str-el↗

arXiv1996

Correlation functions in the three-chain Hubbard ladder

In order to check whether odd-numbered Tomonaga-Luttinger ladders are dominated by antiferromagnetic correlations associated with gapless spin excitations, correlation functions of the doped three-chain Hubbard model are obtained with the bosonization at the renormalization-group fixed point. The correlation of the singlet superconducting pairing across the central and edge chains is found to be dominant, reflecting two gapful spin modes, while the intra-edge spin density wave correlation, reflecting the gapless mode, is only subdominant. This implies that, when there are multiple spin modes, a dominant superconductivity can arise from the presence of {\it some} spin gap(s) despite the coexistence of power-law correlated spins.

cond-mat↗

arXiv1996

Hofstadter butterflies for flat bands

Hofstadter's diagram, or the energy spectrum against the magnetic field in tight-binding systems, is obtained for the models having flat (dispersionless) one-electron band(s) that have originally been proposed for itinerant spin ferromagnetism. Magnetic fields preserve those flat bands that arise from a topological reason, while dispersions emerge in a singular manner for the flat bands arising from interference, implying an anomalous orbital magnetism.

cond-mat↗

arXiv1996

Multifractality of the quantum Hall wave functions in higher Landau levels

To probe the universality class of the quantum Hall system at the metal-insulator critical point, the multifractality of the wave function $ψ$ is studied for higher Landau levels, $N=1,2$, for various range $(σ)$ of random potential. We have found that, while the multifractal spectrum $f(α)$ (and consequently the fractal dimension) does vary with $N$, the parabolic form for $f(α)$ indicative of a log-normal distribution of $ψ$ persists in higher Landau levels. If we relate the multifractality with the scaling of localization via the conformal theory, an asymptotic recovery of the single-parameter scaling with increasing $σ$ is seen, in agreement with Huckestein's irrelevant scaling field argument.

cond-mat↗

arXiv1996

Detection of pairing correlation in the two-dimensional Hubbard model

Quantum Monte Carlo method is used to re-examine superconductivity in the single-band Hubbard model in two dimensions. Instead of the conventional pairing, we consider a `correlated pairing', $\langle \tilde{c}_{i\uparrow} \tilde{c}_{i'\downarrow} %\tilde{c}_{j'\downarrow}^\dagger \tilde{c}_{j \uparrow}^\dagger \rangle$ with $\tilde{c}_{iσ} \equiv c_{iσ}(1-n_{i-σ})$, which is inferred from the $t$-$J$ model, the strong-coupling limit of the Hubbard model. The pairing in the $d$-wave channel is found to possess both a divergence like $1/T$ in the pairing susceptibility and a growth of the ground-state pairing correlation with sample size, indicating an off-diagonal long-range order near (but not exactly at) half-filling.

cond-mat↗

arXiv1996

Magic Numbers and Optical Absorption Spectrum in Vertically Coupled Quantum Dots in the Fractional Quantum Hall Regime

Exact diagonalization is used to study the quantum states of vertically coupled quantum dots in strong magnetic fields. We find a new sequence of angular momentum magic numbers which are a consequence of the electron correlation in the double dot. The new sequence occurs at low angular momenta and changes into the single dot sequence at a critical angular momentum determined by the strength of the inter-dot electron tunneling. We also propose that the magic numbers can be investigated experimentally in vertically coupled dots. Because of the generalized Kohn theorem, the far-infrared optical absorption spectrum of a single dot is unaffected by correlation but the theorem does not hold for two vertically coupled dots which have different confining potentials. We show that the absorption energy of the double dot should exhibit discontinuities at the magnetic fields where the total angular momentum changes from one magic number to another.

cond-mat↗

arXiv1995

Quantum Monte Carlo evidence for superconductivity in the three-band Hubbard model in two dimensions

A possibility of the electronic origin of the high-temperature superconductivity in cuprates is probed with the quantum Monte Carlo method by revisiting the three-band Hubbard model comprising Cu$3d_{x^2-y^2}$ and O$2p_σ$ orbitals. The $d_{x^2-y^2}$ pairing correlation is found to turn into an increasing function of the repulsion $U_d$ within the $d$ orbitals or the $d$-$p$ level off-set $Δ\varepsilon$, where the correlation grows with the system size. % and is long-ranged as also seen from a real-space analysis. We have detected this in both the charge-transfer and Mott-Hubbard regimes upon entering the strong-correlation region ($U_d$ or $Δ\varepsilon >$ bare band width).

cond-mat↗

arXiv1995

Generation of spin-polarized currents in Zeeman-split Tomonaga-Luttinger models

In a magnetic field an interacting electron gas in one dimension may be described as a Tomonaga-Luttinger model comprising two components with different Fermi velocities due to the Zeeman splitting. This destroys the spin-charge separation, and even the quantities such as the density-density correlation involve spin and charge critical exponents (K). Specifically, the ratio of the up-spin and down-spin conductivities in a dirty system diverges at low temperatures like an inverse power of the temperature, $T^{-(K_{\uparrow}-K_{\downarrow})}$, resulting in a spin-polarized current. In finite, clean systems the conductance becomes different for up- and down-spins as another manifestation of the electron-electron interaction.

cond-mat↗