arXiv subjects

Hideo Aoki

Publications and source records attributed to Hideo Aoki.

At least 73 records · Page 4Linked to original sources

arXiv2012

Investigation on synthesis and physical properties of metal doped picene solids

We report electronic structure and physical properties of metal doped picene as well as selective synthesis of the phase exhibiting 18 K superconducting transition. First, Raman scattering is used to characterize the number of electrons transferred from the dopants to picene molecules. The charge transfer leads to a softening of Raman scattering peaks, which enables us to determine the number of transferred electrons. From this we have identified that three electrons are transferred to each picene molecule in the superconducting doped-picene solids. Second, we report the pressure dependence of Tc in 7 and 18 K phases of K3picene. The 7 K phase shows a negative pressure-dependence, while the 18 K phase exhibits a positive pressure-dependence which cannot be understood with a simple phonon mechanism of BCS superconductivity. Third, we report a new synthesis method for superconducting K3picene by a solution process with monomethylamine, CH3NH2. This method enables one to prepare selectively the K3picene sample exhibiting 18 K superconducting transition. The discovery of suitable way for preparing K3picene with Tc = 18 K may facilitate clarification of the mechanism of superconductivity.

cond-mat.supr-con↗

arXiv2012

First-principles band structure and FLEX approach to the pressure effect on $T_c$ of the cuprate superconductors

High-temperature cuprate superconductors have been known to exhibit significant pressure effects. In order to fathom the origin of why and how Tc is affected by pressure, we have recently studied the pressure effects on Tc adoptig a model that contains two cupper d-orbitals derived from first-principles band calculations, where the dz2 orbital is considere on top of the usually considered dx2-y2 orbital. In that paper, we have identified two origins for the Tc enhancement under hydrostatic pressure: (i) while at ambient pressure the smaller the hybridization of other orbital components the higher the Tc, an application of pressure acts to reduce the multiorbital mxing on the Fermi surface, which we call the orbital distillation effects, and (ii) the increase of the band width with pressure also contributes to the enhancement. In the present paper, we further elabolrate the two points. As for point (i), while the reduction of the apical oxygen height under pressure tends to increase the dz2 mixture, hence to lower Tc, here we show that this effect is strongly reduced in bi-layer materials due to the pyramidal coordination of oxygen atoms. As for point (ii), we show that the enhancement of Tc due to the increase in the band width is caused by the effect that the many-body renormalization arising from the self-energy is reduced.

cond-mat.supr-con↗

arXiv2012

Multiorbital analysis of the effects of uniaxial and hydrostatic pressure on $T_c$ in the single-layered cuprate superconductors

The origin of uniaxial and hydrostatic pressure effects on $T_c$ in the single-layered cuprate superconductors is theoretically explored. A two-orbital model, derived from first principles and analyzed with the fluctuation exchange approximation gives axial-dependent pressure coefficients, $\partial T_c/\partial P_a>0$, $\partial T_c/\partial P_c<0$, with a hydrostatic response $\partial T_c/\partial P>0$ for both La214 and Hg1201 cuprates, in qualitative agreement with experiments. Physically, this is shown to come from a unified picture in which higher $T_c$ is achieved with an "orbital distillation", namely, the less the $d_{x^2-y^2}$ main band is hybridized with the $d_{z^2}$ and $4s$ orbitals higher the $T_c$. Some implications for obtaining higher $T_c$ materials are discussed.

cond-mat.supr-con↗

arXiv2012

Faraday rotation in bilayer and trilayer graphene in the quantum Hall regime

Optical Hall conductivity, as directly related to Faraday rotation, is theoretically studied for bilayer and trilayer graphene. In bilayer graphene, the trigonal warping of the band dispersion greatly affects the resonance structures in Faraday rotation not only in the low-energy region where small Dirac cones emerge, but also in the higher-energy parabolic bands as a sequence of satellite resonances. In ABA-stacked trilayer, the resonance spectrum is a superposition of effective monolayer and bilayer contributions with band gaps, while ABC trilayer exhibits a distinct spectrum peculiar to the cubic-dispersed bands with a strong trigonal warping, where the signals associated with low-energy Dirac cones should be directly observable owing to a large Lifshitz transition energy ($\sim 10 meV$).

cond-mat.mes-hall↗

arXiv2012

Accessing surface Brillouin zone and band structure of picene single crystals

We have experimentally revealed the band structure and the surface Brillouin zone of insulating picene single crystals (SCs), the mother organic system for a recently discovered aromatic superconductor, with ultraviolet photoelectron spectroscopy (UPS) and low-energy electron diffraction with laser for photoconduction. A hole effective mass of 2.24 m_0 and the hole mobility mu_h >= 9.0 cm^2/Vs (298 K) were deduced in Gamma-Y direction. We have further shown that some picene SCs did not show charging during UPS even without the laser, which indicates that pristine UPS works for high-quality organic SCs.

cond-mat.mtrl-sci↗

arXiv2012

Chiral Symmetry and Many-Body Effect in Multilayer Graphene

Influence of the chiral symmetry on the many-body problem in multilayer graphene in magnetic fields is investigated. For a spinless electron model on the honeycomb lattice the many-body ground state is shown to be a doubly-degenerate chiral condensate irrespective of the number of layers. The energy spectrum calculated numerically with the exact diagonalization method reveals for ABC-stacked multilayer graphenes that the many-body gap decreases monotonically with the number of layers.

cond-mat.mes-hall↗

arXiv2012

Magneto-orbital effect without spin-orbit interactions --- noncentrosymmetric zeolite-templated carbon structure

A peculiar manifestation of orbital angular momentum is proposed for a zeolite-templated carbon system, C36H9. The structure, being a network of nanoflakes in the shape of a "pinwheel", lacks inversion symmetry. While the unit cell is large, the electronic structure obtained with a first-principles density functional theory and captured as an effective tight-binding model in terms of maximally-localized Wannier functions, exhibits an unusual feature that the valence band top comes from two chiral states having orbital magnetic momenta of $\pm 1$. The noncentrosymmetric lattice structure then makes the band dispersion asymmetric, as reminiscent of, but totally different from, spin-orbit systems. The unusual feature is predicted to imply a current-induced orbital magnetism when holes are doped.

cond-mat.mtrl-sci↗

arXiv2012

Spin Hall effect in iron-based superconductors: A Dirac-point effect

We have theoretically explored the intrinsic spin Hall effect (SHE) in the iron-based superconductor family with a variety of materials. The study is motivated by an observation that, in addition to an appreciable spin-orbit coupling in the Fe 3d states, a character of the band structure in which Dirac cones appear below the Fermi energy may play a crucial role in producing a large SHE. Our investigation does indeed predict a substantially large spin Hall conductivity in the heavily hole-doped regime such as KFe$_2$As$_2$. The magnitude of the SHE has turned out to be comparable with that for Pt despite a relatively small spin-orbit coupling, which we identify to come from a huge contribution from the gap opening induced by the spin-orbit coupling at the Dirac point, which can become close to the Fermi energy for the heavy hole doping.

cond-mat.str-el↗

arXiv2012

Chiral condensate with topological degeneracy in graphene and its manifestation in edge states

Role of chiral symmetry in many-body states of graphene in strong magnetic fields is theoretically studied with the honeycomb lattice model. For a spin-split Landau level where the leading electron-electron interaction is the nearest-neighbor repulsion, a chiral condensate is shown to be, within the subspace of n = 0 Landau level, an exact many-body ground state with a finite gap, for which calculation of Chern numbers reveals that the ground state is a Hall insulator with a topological degeneracy of two. The topological nature of the ground state is shown to manifest itself as a Kekuléan bond order along armchair edges, while the pattern melts in the bulk due to quantum fluctuations. The whole story can be regarded as a realization of the bulk-edge correspondence peculiar to the chiral symmetry.

cond-mat.mes-hall↗

arXiv2012

Two parameter flow of σ_{xx}(ω) - σ_{xy}(ω) for the graphene quantum Hall system in ac regime

Flow diagram of $(σ_{xx}, σ_{xy})$ in finite-frequency ($ω$) regime is numerically studied for graphene quantum Hall effect (QHE) system. The ac flow diagrams turn out to show qualitatively similar behavior as the dc flow diagrams, which can be understood that the dynamical length scale determined by the frequency poses a relevant cutoff for the renormalization flow. Then the two parameter flow is discussed in terms of the dynamical scaling theory. We also discuss the larger-$ω$ regime which exhibits classical flows driven by the raw frequency $ω$.

cond-mat.mes-hall↗

arXiv2012

Repulsion-to-attraction transition in correlated electron systems triggered by a monocycle pulse

We study the time evolution of the Hubbard model driven by a half-cycle or monocycle pulsed electric field F(t) using the nonequilibrium dynamical mean-field theory. We find that for properly chosen pulse shapes the electron-electron interaction can be effectively and permanently switched from repulsive to attractive if there is no energy dissipation. The physics behind the interaction conversion is a nonadiabatic shift $δ$ of the population in momentum space. When $δ\simπ$, the shifted population relaxes to a negative-temperature state, which leads to the interaction switching. Due to electron correlation effects $δ$ deviates from the dynamical phase $ϕ=\int dt F(t)$, which enables the seemingly counterintuitive repulsion-to-attraction transition by a monocycle pulse with $ϕ=0$.

cond-mat.str-el↗

arXiv2012

Temperature dependent magnetotransport around $ν$= 1/2 in ZnO heterostructures

The sequence of prominent fractional quantum Hall states up to $ν$=5/11 around $ν$=1/2 in a high mobility two-dimensional electron system confined at oxide heterointerface (ZnO) is analyzed in terms of the composite fermion model. The temperature dependence of $\Rxx$ oscillations around $ν$=1/2 yields an estimation of the composite fermion effective mass, which increases linearly with the magnetic field. This mass is of similar value to an enhanced electron effective mass, which in itself arises from strong electron interaction. The energy gaps of fractional states and the temperature dependence of $\Rxx$ at $ν$=1/2 point to large residual interactions between composite fermions.

cond-mat.mes-hall↗

arXiv2012

A perspective of superconductivity as multiband phenomena: Cuprate, iron and aromatic systems

A theoretical overview of the classes of superconductors encompassing (a) high-Tc cuprate, (b) iron-based and (c) aromatic superconductors is given. Emphasis is put on the multiband natures of all the three classes, where the differences in the multiorbits and their manifestations in the electronic structures and pairing are clarified. From these, future directions and prospects are discussed.

cond-mat.supr-con↗

arXiv2012

Generalization of Chiral Symmetry for Tilted Dirac Cones

The notion of chiral symmetry for the conventional Dirac cone is generalized to include the tilted Dirac cones, where the generalized chiral operator turns out to be non-hermitian. It is shown that the generalized chiral symmetry generically protects the zero modes (n=0 Landau level) of the Dirac cone even when tilted. The present generalized symmetry is equivalent to the condition that the Dirac Hamiltonian is elliptic as a differential operator, which provides an explicit relevance to the index theorem.

cond-mat.mes-hall↗

arXiv2011

Topologically protected Landau levels in bilayer graphene in finite electric fields

The zero-energy Landau level of bilayer graphene is shown to be anomalously sharp (delta-function like) against bond disorder as long as the disorder is correlated over a few lattice constants.The robustness of the zero-mode anomaly can be attributed to the preserved chiral symmetry. Unexpectedly, even when we apply a finite potential difference (i.e., an electric field) between the top and the bottom layers, the valley-split $n=0$ Landau levels remain anomalously sharp although they are now shifted away from the zero energy, while the $n=1$ Landau levels exhibit the usual behavior.

cond-mat.mes-hall↗

arXiv2011

First-principles structural optimization and electronic structure of the superconductor picene for various potassium doping levels

We theoretically explore the crystal structures of K$_x$picene, for which a new aromatic superconductivity has recently been discovered for $x=3$, by systematically performing first-principles full structural optimization covering the concentration range $x=1$-4. The crystal symmetry (space group) of the pristine picene is shown to be preserved in all the optimized structures despite significant deformations of each picene molecule and vast rearrangements of herringbone array of molecules. For K$_x$picene ($x=1$-4) optimization indicates that (i) multiple structures exist in some cases, and (ii) dopants can enter not only in the interlayer region between the stack of herringbone structures, but also in the intralayer region. In the electronic structure obtained with the local density approximation for the optimized structures, the dopants are shown to affect the electronic properties not only through the rearrangement and distortion of molecules, but also molecule-metal atom hybridization. In other words, the rigid-band approximation is invalidated for multifold reasons. As a consequence the resultant Fermi surface exhibits a variety of multiband structures which take diverse topology for K$_1$picene and K$_3$picene.

cond-mat.supr-con↗

arXiv2011

Dynamical band flipping in fermionic lattice systems: An ac-field-driven change of the interaction from repulsive to attractive

We show theoretically that the sudden application of an appropriate ac field to correlated lattice fermions flips the band structure and effectively switches the interaction from repulsive to attractive. The nonadiabatically driven system is characterized by a negative temperature with a population inversion. We numerically demonstrate the converted interaction in an ac-driven Hubbard model with the nonequilibrium dynamical mean-field theory solved by the continuous-time quantum Monte Carlo method. Based on this, we propose an efficient ramp-up protocol for ac fields that can suppress heating, which leads to an effectively attractive Hubbard model with a temperature below the superconducting transition temperature of the equilibrium system.

cond-mat.quant-gas↗

arXiv2011

Electronic structure of solid coronene: differences and commonalities to picene

We have obtained the first-principles electronic structure of solid coronene, which has been recently discovered to exhibit superconductivity with potassium doping. Since coronene, along with picene, the first aromatic superconductor, now provide a class of superconductors as solids of aromatic compounds, here we compare the two cases in examining the electronic structures. In the undoped coronene crystal, where the molecules are arranged in a herringbone structure with two molecules in a unit cell, the conduction band above an insulating gap is found to comprise four bands, which basically originate from the lowest two unoccupied molecular orbitals (doubly-degenerate, reflecting the high symmetry of the molecular shape) in an isolated molecule but the bands are entangled as in solid picene. The Fermi surface for a candidate of the structure of K$_x$coronene with $x=3$, for which superconductivity is found, comprises multiple sheets, as in doped picene but exhibiting a larger anisotropy with different topology.

cond-mat.supr-con↗