arXiv subjects

Hideo Aoki

Publications and source records attributed to Hideo Aoki.

At least 91 records · Page 5Linked to original sources

arXiv2011

Collective modes in multiband superfluids and superconductors: Multiple dynamical classes

One important way to characterize the states having a gauge symmetry spontaneously broken over multibands should be to look at their collective excitation modes. We find that a three-band system has multiple Leggett modes with significantly different masses, which can be classified into different dynamical classes according to whether multiple inter-band Josephson currents add or cancel. This provides a way to dynamically characterize multiband superconductivity while the pairing symmetry is a static property.

cond-mat.supr-con↗

arXiv2011

Generalized Chiral Symmetry and Stability of Zero Modes for Tilted Dirac Cones

While it has been well-known that the chirality is an important symmetry for Dirac-fermion systems that gives rise to the zero-mode Landau level in graphene, here we explore whether this notion can be extended to tilted Dirac cones as encountered in organic metals. We have found that there exists a "generalized chiral symmetry" that encompasses the tilted Dirac cones, where a generalized chiral operator $γ$, satisfying $γ^{\dagger} H + Hγ=0$ for the Hamiltonian $H$, protects the zero mode. We can use this to show that the $n=0$ Landau level is delta-function-like (with no broadening) by extending the Aharonov-Casher argument. We have numerically confirmed that a lattice model that possesses the generalized chirality has an anomalously sharp Landau level for spatially correlated randomness.

cond-mat.mes-hall↗

arXiv2010

Impurity effect in multi-orbital, sign-reversing s-wave superconductors

We study the impurity effects on the transition temperature Tc with use of the T-matrix approximation. We propose a way to visualize the multi-orbital effect by introducing the hybridization function characterizing the multi-orbital effect for the impurity scattering. Characterizing function does not depend on the superconducting pairing symmetry, since this function is defined by the eigenvectors in normal states. The result indicates that an impurity-robust superconductivity does not necessarily imply a sign- preserving pairing. Visualizing the hybridization effect in the effective five-band model for LaFeAsO, we show that the impurity effect on Tc is relatively weaker than that in single-band models.

cond-mat.supr-con↗

arXiv2010

Non-equilibrium superconductivity in a correlated electron system studied with the Keldysh+FLEX approach

Non-equilibrium phase transitions are studied theoretically for the two-dimensional Hubbard model subject to bias voltages from the electrodes coupled to the system. By combining the fluctuation exchange approximation with the Keldysh method for non-equilibrium, we have studied the properties of the non-equilibrium Fermi liquid phase and determined the phase diagram with transition to non-equilibrium magnetic and superconducting phases.

cond-mat.str-el↗

arXiv2010

Half-integer contributions to the quantum Hall conductivity from single Dirac cones

While the quantum Hall effect in graphene has been regarded as a realization of the anomaly associated with the massless Dirac particle carrying half the usual topological integer, this is hidden due to the doubling of the Dirac cones. In order to confirm the half-integer contribution from each Dirac cone, here we theoretically consider a lattice model in which the relative energy between the two Dirac points is systematically shifted. With an explicit calculation of the topological (Chern) number, we have demonstrated that each Dirac cone does indeed contribute to the Hall conductivity as the half odd integer series (... -3/2, -1/2, 1/2, 3/2, ...) when the Fermi energy traverses the (shifted sets of) Landau levels. The picture is also endorsed, via the bulk-edge correspondence, from the edge mode spectrum for the present model.

cond-mat.mes-hall↗

arXiv2010

Spin-density functional study of the organic polymer dimethylaminopyrrole: A realization of the organic periodic Anderson model

While the periodic Anderson model (PAM) has been recognized as a good model for various heavy f-electron systems, here we design a purely organic polymer whose low-energy physics can be captured by PAM. By means of the spin density functional calculation, we show that polymer of dimethylaminopyrrole is a candidate, where its ground state can indeed be magnetic depending on the doping. We discuss the factors favoring ferromagnetic ground state.

cond-mat.mtrl-sci↗

arXiv2010

Nonequilibrium magnetic and superconducting phases in the two-dimensional Hubbard model coupled to electrodes

A theory is presented for a nonequilibrium phase transition in the two-dimensional Hubbard model coupled to electrodes. Nonequilibrium magnetic and superconducting phase diagram is determined by the Keldysh method, where the electron correlation is treated in the fluctuation exchange approximation. The nonequilibrium distribution function in the presence of electron correlation is evoked to capture a general feature in the phase diagram.

cond-mat.str-el↗

arXiv2010

Anomalous criticality in the quantum Hall transition at $n=0$ Landau level of graphene with chiral-symmetric disorders

We investigate numerically whether the chiral symmetry is the sole factor dominating the criticality of the quantum Hall transitions in disordered graphene. When the disorder respects the chiral symmetry, the plateau-to-plateau transition at the $n=0$ Landau level is shown to become anomalous (i.e., step-function-like). Surprisingly, however, the anomaly is robust against the inclusion of the uniform next-nearest neighbor hopping, which degrades the chiral symmetry of the lattice models. We have also shown that the ac (optical) Hall conductivity exhibits a robust plateau structure when the disorder respects the chiral symmetry.

cond-mat.mes-hall↗

arXiv2010

Two-Orbital Model Explains the Higher Transition Temperature of the Single-Layer Hg-Cuprate Superconductor Compared to That of the La-Cuprate Superconductor

In order to explore the reason why the single-layered cuprates, La$_{2-x}$(Sr/Ba)$_x$CuO$_4$ ($T_c\simeq$ 40K) and HgBa$_2$CuO$_{4+δ}$ ($T_c\simeq$ 90K), have such a significant difference in $T_c$, we study a two-orbital model that incorporates the $d_{z^2}$ orbital on top of the $d_{x^2-y^2}$ orbital. It is found, with the fluctuation exchange approximation, that the $d_{z^2}$ orbital contribution to the Fermi surface, which is stronger in the La system, works against d-wave superconductivity, thereby dominating over the effect of the Fermi surface shape. The result resolves the long-standing contradiction between the theoretical results on Hubbard-type models and the experimental material dependence of $T_c$ in the cuprates.

cond-mat.supr-con↗

arXiv2010

All Optical Measurement Proposed for the Photovoltaic Hall Effect

We propose an all optical way to measure the recently proposed "photovoltaic Hall effect", i.e., a DC Hall effect induced by a circularly polarized light in the absence of static magnetic fields. For this, we have calculated the Faraday rotation angle induced by the photovoltaic Hall effect with the Kubo formula extended for photovoltaic optical response in the presence of strong AC electric fields treated with the Floquet formalism. We also point out the possibility of observing the effect in three-dimensional graphite, and more generally in multi-band systems such as materials described by the dp-model.

cond-mat.mes-hall↗

arXiv2010

Optical Hall Effect in the Integer Quantum Hall Regime

Optical Hall conductivity $σ_{xy}(ω)$ is measured from the Faraday rotation for a GaAs/AlGaAs heterojunction quantum Hall system in the terahertz frequency regime. The Faraday rotation angle ($\sim$ fine structure constant $\sim$ mrad) is found to significantly deviate from the Drude-like behavior to exhibit a plateau-like structure around the Landau-level filling $ν=2$. The result, which fits with the behavior expected from the carrier localization effect in the ac regime, indicates that the plateau structure, although not quantized, still exists in the terahertz regime.

cond-mat.mes-hall↗

arXiv2010

Dynamical scaling analysis of the optical Hall conductivity in the quantum Hall regime

Dynamical scaling analysis is theoretically performed for the ac (optical) Hall conductivity $σ_{xy}(\varepsilon_F,ω)$ as a function of Fermi energy $\varepsilon_F$ and frequency $ω$ for the two-dimensional electron gas and for graphene. In both systems, results based on exact diagonalization show that $σ_{xy}(\varepsilon_F,ω)$ displays a well-defined dynamical scaling, for which the dynamical critical exponent as well as the localization exponent are fitted and plugged in. A crossover from the dc-like bahavior to the ac regime is identified. The dynamical scaling analysis has enabled us to quantify the plateau in the ac Hall conductivity previously obtained, and to predict that the plateaux structure in ac is robust enough to be observed in the THz regime.

cond-mat.mes-hall↗

arXiv2010

Tight-binding photonic bands in metallophotonic waveguide networks and flat bands in kagome lattices

We propose that we can realize "tight-binding photonic bands" in metallophotonic waveguide networks, where the photonic bound states localized around the crossings of a network form a tight-binding band. The formation of bound states at the crossings is distinct from the conventional bound states at defects or virtual bound states in photonic crystals, but comes from a photonic counterpart of the zero-point states in wave mechanics. Model calculations show that the low-lying photon dispersions are indeed described accurately by the tight-binding model. To exemplify how we can exploit the tight-binding analogy for a {\it designing} of photonic bands, we propose a "flat photonic band" in the kagome network, in which the photonic flat band is shown to arise with group velocities that can be as small as 1/1000 times the velocity of light in vacuum.

cond-mat.mtrl-sci↗

arXiv2009

Dielectric Breakdown in a Mott Insulator: Many-body Schwinger-Landau-Zener Mechanism studied with a Generalized Bethe Ansatz

The nonadiabatic quantum tunneling picture, which may be called the many-body Schwinger-Landau-Zener mechanism, for the dielectric breakdown of Mott insulators in strong electric fields is studied in the one-dimensional Hubbard model. The tunneling probability is calculated by a metod due to Dykhne-Davis-Pechukas with an analytical continuation of the Bethe-ansatz solution for excited states to a non-Hermitian case. A remarkable agreement with the time-dependent density matrix renormalization group result is obtained.

cond-mat.stat-mech↗

arXiv2009

First-Principles Electronic Structure of Solid Picene

To explore the electronic structure of the first aromatic superconductor, potassium-doped solid picene which has been recently discovered by Mitsuhashi et al with the transition temperatures $T_c=7 - 20$ K, we have obtained a first-principles electronic structure of solid picene as a first step toward the elucidation of the mechanism of the superconductivity. The undoped crystal is found to have four conduction bands, which are characterized in terms of the maximally localized Wannier orbitals. We have revealed how the band structure reflects the stacked arrangement of molecular orbitals for both undoped and doped (K$_3$picene) cases, where the bands are not rigid. The Fermi surface for K$_3$picene is a curious composite of a warped two-dimensional surface and a three-dimensional one.

cond-mat.supr-con↗

arXiv2009

Photovoltaic Berry curvature in the honeycomb lattice

Photovoltaic Hall effect -- the Hall effect induced by intense, circularly-polarized light in the absence of static magnetic fields -- has been proposed in Phys. Rev. B 79, 081406R (2009) for graphene where a massless Dirac dispersion is realized. The photovoltaic Berry curvature (a nonequilibrium extension of the standard Berry curvature) is the key quantity to understand this effect, which appears in the Kubo formula extended to Hall transport in the presence of strong AC field backgrounds. Here we elaborate the properties of the photovoltaic curvature such as the frequency and field strength dependence in the honeycomb lattice.

cond-mat.mes-hall↗

arXiv2009

Optical Hall conductivity in 2DEG and graphene QHE systems

We have revealed from a numerical study that the Hall plateaus are retained in the optical Hall conductivity $σ_{xy}(ω)$ in the ac ($\sim$ THz) regime in both of the ordinary two-dimensional electron gas and graphene in the quantum Hall regime, although the plateau height in ac deviates from integer multiples of $e^2/h$. The effect remains unexpectedly robust against a significant strength of disorder, which we attribute to an effect of localization. We predict the ac Hall plateaus are observable through the Faraday rotation with the rotation angle characterized by the fine-structure constant $α$. In this paper we clarify the relationship between plateau structures and the disorder strength by performing numerical calculation.

cond-mat.mes-hall↗

arXiv2009

Quantum Hall plateau transition in graphene with spatially correlated random hopping

We investigate how the criticality of the quantum Hall plateau transition in disordered graphene differs from those in the ordinary quantum Hall systems, based on the honeycomb lattice with ripples modeled as random hoppings. The criticality of the graphene-specific n=0 Landau level is found to change dramatically to an anomalous, almost exact fixed point as soon as we make the random hopping spatially correlated over a few bond lengths. We attribute this to the preserved chiral symmetry and suppressed scattering between K and K' points in the Brillouin zone. The results suggest that a fixed point for random Dirac fermions with chiral symmetry can be realized in free-standing, clean graphene with ripples.

cond-mat.mes-hall↗