arXiv subjects

Hideo Aoki

Publications and source records attributed to Hideo Aoki.

At least 145 records · Page 8Linked to original sources

arXiv2003

Integer quantum Hall effect and Hofstadter's butterfly spectra in three-dimensional metals in external periodic modulations

We propose that Hofstadter's butterfly accompanied by quantum Hall effect that is similar to those predicted to occur in 3D tight-binding systems by Koshino {\it et al.} [Phys. Rev. Lett. {\bf 86}, 1062 (2001)] can be realized in an entirely different system -- 3D metals applied with weak external periodic modulations (e.g., acoustic waves). Namely, an effect of two periodic potentials interferes with Landau's quantization due to an applied magnetic field $\Vec{B}$, resulting generally in fractal energy gaps as a function of the tilting angle of $\Vec{B}$, for which the accompanying quantized Hall tensors are computed. The phenomenon arises from the fact that, while the present system has a different physical origin for the butterfly from the 3D tight-binding systems, the mathematical forms are remarkably equivalent.

cond-mat.mes-hall↗

arXiv2003

Interaction and dimensionality in the quantum Hall physics

While the composite fermion picture is so effective as to describe the excitation spectra including the spin wave for Laughlin's quantum liquid, ``how heavy and how strongly-interacting" remains a formidable question for the composite fermions, to which this article first addresses. The effective mass (purely interaction originated) defined from the excitation spectrum and obtained for various even- as well as odd-fractions exhibits a curious, step-like filling dependence basically determined by the number of flux quanta attached to each fermion, where the non-monotonic behaviour indicates a strong effect of gauge-field fluctuations. The excitation spectrum fits a Fermi liquid, but again a large effect of inter-composite fermion interaction appears as anomalous Landau's parameters. We have then moved on to see how the introduction of three-dimensionality (where the shape of the Fermi surface becomes relevant) affects the interacting electron system, and propose the magnetic-field induced SDW in three-dimensional systems. This should be a good candidate, in entirely realistic magnetic fields, for the integer QHE recently predicted by Koshino et al to occur in 3D on the fractal energy spectrum similar to Hofstadter's. The mechanism for the field-induced phase is an effect of interaction in Landau's quantisation on incompletely-nested (i.e., multiply-connected) Fermi surfaces, so the interplay of many-body physics and the magnetic quantisation on various Fermi surfaces may provide an interesting future avenue for 3D systems.

cond-mat.mes-hall↗

arXiv2003

Flat-band ferromagnetism proposed for an organic polymer crystal

Motivated from the flat-band ferromagnetism conceived theoretically for a single chain of five membered rings (polyaminotriazole) by Arita {\it et al.}, [Phys. Rev. Lett. {\bf 88}, 127202 (2002)], we have studied whether the magnetism can indeed occur as a bulk, i.e., in a three-dimensional crystal of the polymer, by means of the spin density functional calculation. We find that the intra-chain ferromagnetism is robust against crystallization as far as the flat band is made half-filled. We have further investigated the actual crystal doped with various compounds, where HF$_2$ is shown to put the system close to the bulk ferromagnetism while stronger anions such as BF$_4$ or PF$_6$ should be promising.

cond-mat.str-el↗

arXiv2003

Flat-Band Ferromagnetism in Organic Polymers Designed by a Computer Simulation

By coupling a first-principles, spin-density functional calculation with an exact diagonalization study of the Hubbard model, we have searched over various functional groups for the best case for the flat-band ferromagnetism proposed by R. Arita et al. [Phys. Rev. Lett. {\bf 88}, 127202 (2002)] in organic polymers of five-membered rings. The original proposal (poly-aminotriazole) has turned out to be the best case among the materials examined, where the reason why this is so is identified here. We have also found that the ferromagnetism, originally proposed for the half-filled flat band, is stable even when the band filling is varied away from the half-filling. All these make the ferromagnetism proposed here more experimentally inviting.

cond-mat.mtrl-sci↗

arXiv2003

Electronic properties of metal induced gap states at insulator/metal interfaces -- dependence on the alkali halide and the possibility of excitonic mechanism of superconductivity

Motivated from the experimental observation of metal induced gap states (MIGS) at insulator/metal interfaces by Kiguchi {\it et al.} [Phys. Rev. Lett. {\bf 90}, 196803 (2003)], we have theoretically investigated the electronic properties of MIGS at interfaces between various alkali halides and a metal represented by a jellium with the first-principles density functional method. We have found that, on top of the usual evanescent state, MIGS generally have a long tail on halogen sites with a $p_z$-like character, whose penetration depth ($λ$) is as large as half the lattice constant of bulk alkali halides. This implies that $λ$, while little dependent on the carrier density in the jellium, is dominated by the lattice constant (hence by energy gap) of the alkali halide, where $λ_{\rm LiF} < λ_{\rm LiCl} < λ_{\rm LiI}$. We also propose a possibility of the MIGS working favorably for the exciton-mediated superconductivity.

cond-mat.mtrl-sci↗

arXiv2003

Superconductivity from the repulsive electron interaction -- from 1D to 3D

An overview is given on how superconductivity with anisotropic pairing can be realised from repulsive electron-electron interaction. (i) We start from the physics in one dimension, where the Tomonaga-Luttinger theory predicts that, while there is no superconducting phase for the repulsive case for a single chain, the phase does exists in ladders with the number of legs equal to or greater than two, as shown both by analytically (renormalisation) and numerically (quantum Monte Carlo). (ii) We then show how this pairing has a natural extension to the two-dimensional case, where anisotropic (usually d) pairing superconductivity arises mediated by spin fluctuations (usually antiferromagnetic), as shown both by analytically (renormalisation) and numerically (quantum Monte Carlo). (iii) We finally discuss how the superconductivity from the electron repulsion can be "optimised" (i.e., how $T_C$ can be raised) in 2D and 3D, where we propose that the anisotropic pairing is much favoured in systems having {\it disconnected Fermi surfaces} where $T_C$ can be almost an order of magnitude higher.

cond-mat.str-el↗

arXiv2003

Breakdown of a Mott insulator -- non-adiabatic tunneling mechanism

Time-dependent nonequilibrium properties of a strongly correlated electron system driven by large electric fields is obtained by means of solving the time-dependent Schrödinger equation for the many-body wave function numerically in one dimension. While the insulator-to-metal transition depends on the electric field and the interaction, the metallization is found to be described in terms of a universal Landau-Zener quantum tunneling among the many-body levels. These processes induces current oscillation for small systems, while give rise to finite resistivity through dissipation for larger systems/on longer time scales.

cond-mat.str-el↗

arXiv2003

Electronic properties of alkali-metal loaded zeolites -- a "supercrystal" Mott insulator

First-principles band calculations are performed for the first time for an open-structured zeolite (LTA) with guest atoms (potassium) introduced in their cages. A surprisingly simple band structure emerges, which indicates that this system may be regarded as a "supercrystal", where each cluster of guest atoms with diameter $\sim$10Åacts as a "superatom" with well-defined $s$- and $p$-like orbitals, which in turn form the bands around the Fermi energy. The calculated Coulomb and exchange energies for these states turn out to be in the strongly-correlated regime. With the dynamical mean-field theory we show the system should be on the Mott-insulator side, and, on a magnetic phase diagram for degenerate-orbital systems, around the ferromagnetic regime, in accord with experimental results. We envisage this class of systems can provide a new avenue for materials design.

cond-mat.str-el↗

arXiv2003

Possibility of superconductivity in the repulsive Hubbard model on the Shastry-Sutherland lattice

Possibility of superconductivity from electron repulsion in the Shastry-Sutherland lattice, which has a spin gap at half filling, is explored with the repulsive Hubbard model in the fluctuation-exchange approximation. We find that, while superconductivity is not favored around the half-filling, superconductivity is favored around the quarter-filling. Our results suggest that the Fermi surface nesting is more important than the spin dimerization for superconductivity.

cond-mat.supr-con↗

arXiv2003

Metal-Induced Gap States at Well Defined Alkali-Halide/Metal Interfaces

In order to search for states specific to insulator/metal interfaces, we have studied epitaxially grown interfaces with element-selective near edge X-ray absorption fine structure (NEXAFS). An extra peak is observed below the bulk edge onset for LiCl films on Cu and Ag substrates. The nature of chemical bonds as probed by X-ray photoemission spectroscopy and Auger electron spectroscopy remains unchanged, so we regard this as evidence for metal-induced gap states(MIGS) formed by the proximity to a metal, rather than local bonds at the interface. The dependence on the film thickness shows that the MIGS are as thin as one monolayer. An ab initio electronic structure calculation supports the existence of the MIGS that are strongly localized at the interface.

cond-mat.mtrl-sci↗

arXiv2002

Hybridization-induced superconductivity from the electron repulsion on a tetramer lattice having a disconnected Fermi surface

Plaquette lattices with each unit cell containing multiple atoms are good candidates for disconnected Fermi surfaces, which are shown by Kuroki and Arita to be favorable for spin-flucutation mediated superconductivity from electron repulsion. Here we find an interesting example in a tetramer lattice where the structure within each unit cell dominates the nodal structure of the gap function. We trace its reason to the way in which a Cooper pair is formed across the hybridized molecular orbitals, where we still end up with a T_c much higher than usual.

cond-mat.supr-con↗

arXiv2002

Magnetic-field induced triplet superconductivity in the Hubbard model on a triangular lattice

We propose theoretically that a magnetic field can realize spin-triplet superconductivity in repulsively interacting electron systems having strong ferromagnetic spin fluctuations. We confirm the general idea for the low-density Hubbard model on a triangular lattice, whose Fermi surface consists of disconnected pieces, by calculating the pairing susceptibility in a moderate magnetic field with the quantum Monte-Carlo method combined with the dynamical cluster approximation.

cond-mat.str-el↗

arXiv2002

Superconductivity induced by inter-band nesting in the three-dimensional honeycomb lattice

In order to study whether the inter-band nesting can favor superconductivity arising from electron-electron repulsion in a three-dimensional system, we have looked at the repulsive Hubbard model on a stack of honeycomb (i.e., non-Bravais) lattices with the FLEX method, partly motivated by the superconductivity observed in MgB2. By systematically changing the shape of Fermi surface with varied band filling n and the third-direction hopping, we have found that the pair scattering across the two-bands is indeed found to give rise to gap functions that change sign across the bands and behave as an s- or d-wave within each band. This implies (a) the electron repulsion can assist gapful pairing when a phonon-mechanism pairing exists, and (b) the electron repulsion alone, when strong enough, can give rise to a d-wave-like pairing, which should be, for a group-theoretic reason, a time-reversal broken d+id with point nodes in the gap.

cond-mat.supr-con↗

arXiv2002

Wrapping Current versus Bulk Integer Quantum Hall Effect in Three Dimensions

Surface electron currents are studied for the integer quantum Hall effect in three dimensions(3D) proposed previously by Kohmoto et al. and by Koshino et al. We predict the current wraps the facets of the sample with its intensity and direction dictated by 3D Chern numbers which are just the quantized Hall conductivities in 3D ($σ_{xy}, σ_{zx}$), so a natural connection exists between the surface and bulk currents just as in 2D. An experiment to detect the 3D integer quantum Hall effect through the wrapping current is proposed.

cond-mat.mes-hall↗

arXiv2001

Image-potential band-gap narrowing at a metal/semiconductor interface

GW approximation is used to systematically revisit the image-potential band-gap narrowing at metal/semiconductor interfaces proposed by Inkson in the 1970's. Here we have questioned how the narrowing as calculated from quasi-particle energy spectra for the jellium/Si interface depends on $r_s$ of the jellium. The gap narrowing is found to only weakly depend on $r_s$ (i.e., narrowing $\simeq 0.3$ eV even for a large $r_s = 6)$. Hence we can turn to smaller polarizability in the semiconductor side as an important factor in looking for larger narrowing.

cond-mat.mtrl-sci↗

arXiv2001

Gate-induced band ferromagnetism in an organic polymer

We propose that a chain of five-membered rings (polyaminotriazole) should be ferromagnetic with an appropriate doping that is envisaged to be feasible with an FET structure. The ferromagnetism is confirmed by a spin density functional calculation, which also shows that ferromagnetism survives the Peierls instability. We explain the magnetism in terms of Mielke and Tasaki's flat-band ferromagnetism with the Hubbard model. This opens a new possibility of band ferromagnetism in purely organic polymers.

cond-mat.str-el↗

arXiv2001

Effective mass staircase and the Fermi liquid parameters for the fractional quantum Hall composite fermions

Effective mass of the composite fermion in the fractional quantum Hall system, which is of purely interaction originated, is shown, from a numerical study, to exhibit a curious nonmonotonic behavior with a staircase correlated with the number (=2,4,...) of attached flux quanta. This is surprising since the usual composite-fermion picture predicts a smooth behavior. On top of that, significant interactions are shown to exist between composite fermions, where the excitation spectrum is accurately reproduced in terms of Landau's Fermi liquid picture with negative (i.e., Hund's type) orbital and spin exchange interactions.

cond-mat.mes-hall↗

arXiv2001

A crib-shaped triplet pairing gap function for an orthogonal pair of quasi-one dimensional Fermi surfaces in Sr$_2$RuO$_4$

The competition between spin-triplet and singlet pairings is studied theoretically for the tight-binding $α$-$β$ bands in Sr$_2$RuO$_4$, which arise from two sets of quasi-one dimensional Fermi surfaces. Using multiband FLEX approximation, where we incorporate an anisotropy in the spin fluctuations as suggested from experiments, we show that (i) the triplet can dominate over the singlet (which turns out to be extended s), and (ii) the triplet gap function optimized in the Eliashberg equation has an unusual, very non-sinusoidal form, whose time-reversal-broken combination exhibits a crib-shaped amplitude with dips.

cond-mat.str-el↗