arXiv subjects

S. Maekawa

Publications and source records attributed to S. Maekawa.

At least 91 records · Page 5Linked to original sources

arXiv2004

Mott gap excitations in twin-free YBa2Cu3O7-d (Tc = 93 K) studied by RIXS

Mott gap excitations in the high-Tc superconductor of the optimal doped YBa2Cu3O7-d (Tc = 93 K) have been studied by the resonant inelastic x-ray scattering method. Anisotropic spectra in the ab-plane are observed in a twin-free crystal. The excitation from the one-dimensional CuO chain is enhanced at 2 eV near the zone boundary of the b* direction, while the excitation from the CuO2 plane is broad at 1.5-4 eV and almost independent of the momentum transfer. Theoretical calculation based on the one-dimensional and two-dimensional Hubbard model reproduces the observed spectra by taking the different parameters of the on-site Coulomb energy. The fact of the Mott gap of the CuO chain site is much smaller than that of CuO2 plane site is observed for the first time.

cond-mat.str-el↗

arXiv2004

Excitonic effect on optical response in one-dimensional two-band Hubbard model

Motivated by the gigantic nonlinear optical response in the halogen-bridged Ni-compounds, the underlying electronic states of the compounds are examined in the one-dimensional two-band Hubbard model, by studying the current-current correlation function and the charge density in the ground state. The dynamical density matrix renormalization group method is employed. We find that the low-energy peak of the correlation function consists of a single Lorentzian component for a parameter range appropriate to the compounds. This is due to an excitonic state induced by the intersite Coulomb repulsion between holes on the metal and halogen ions. This is consistent with the optical absorption spectra of the compounds. We suggest that the localization of holes on the metal ions in the ground state brings about the formation of the excitonic state.

cond-mat.str-el↗

arXiv2004

Temperature and Dimensionality Dependences of Optical Absorption Spectra in Mott Insulators

We investigate the temperature dependence of optical absorption spectra of one-dimensional (1D) and two-dimensional (2D) Mott insulators by using an effective model in the strong-coupling limit of a half-filed Hubbard model. In the numerically exact diagonalization calculations on finite-size clusters, we find that in 1D the energy position of the absorption edge is almost independent of temperature, while in 2D the edge position shifts to lower energy with increasing temperature. The different temperature dependence between 1D and 2D is attributed to the difference of the coupling of the charge and spin degrees of freedom. The implications of the results on experiments are discussed in terms of the dimensionality dependence.

cond-mat.str-el↗

arXiv2004

Gate-controlled spin-splitting in quantum dots with ferromagnetic leads in the Kondo regime

The effect of a gate voltage ($V_g$) on the spin-splitting of an electronic level in a quantum dot (QD) attached to ferromagnetic leads is studied in the Kondo regime using a generalized numerical renormalization group technique. We find that the $V_g$-dependence of the QD level spin-splitting strongly depends on the shape of the density of states (DOS). For one class of DOS shapes there is nearly no $V_g$-dependence, for another, $V_g$ can be used to control the magnitude and sign of the spin-splitting, which can be interpreted as a local exchange magnetic field. We find that the spin-splitting acquires a new type of logarithmic divergence. We give an analytical explanation for our numerical results and explain how they arise due to spin-dependent charge fluctuations.

cond-mat.mtrl-sci↗

arXiv2004

Dynamical density matrix renormalization group study of photoexcited states in one-dimensional Mott insulators

One-dimensional Mott insulators exhibit giant nonlinear optical response. Based on the dynamical density matrix renormalization group method, photoexcited states and optical response in the insulators are studied as functions of the on-site and the nearest neighbor Coulomb interactions, $U$ and $V$, respectively. We find that the lowest optically-allowed and forbidden excited states, which have odd and even parities, respectively, are degenerate for $V/t\ltsim 2$ with $t$ being the hopping integral of an electron between nearest neighbor sites. For $V/t\gtsim 2$, the bound states with even and odd parities occur and are not degenerate. The nature of the degeneracy and its effect on the optical response are examined.

cond-mat.str-el↗

arXiv2004

Resonant inelastic x-ray scattering study of hole-doped manganites La1-xSrxMnO3 (x=0.2 and 0.4)

Electronic excitations near the Fermi energy in the hole doped manganese oxides (La1-xSrxMnO3, x=0.2 and 0.4) have been elucidated by using the resonant inelastic x-ray scattering (RIXS) method. A doping effect in the strongly correlated electron systems has been observed for the first time. The scattering spectra show that a salient peak appears in low energies indicating the persistence of the Mott gap. At the same time, the energy gap is partly filled by doping holes and the energy of the spectral weight shifts toward lower energies. The excitation spectra show little change in the momentum space as is in undoped LaMnO3, but the scattering intensities in the low energy excitations of x=0.2 are anisotropic as well as temperature dependent, which indicates a reminiscence of the orbital nature.

cond-mat.str-el↗

arXiv2004

Current-driven domain wall motion in thin ferromagnetic wires

The coupling between a current and a Bloch wall is examined in the half-metal limit of the double exchange model. The conduction electrons transfer angular momentum to the Bloch wall with 100% efficiency in the absence of pinning. The wall is displaced without distortion with velocity proportional to the current. In the presence of a pinning potential either the angular momentum is destroyed by the perpendicular component of the anisotropy field or is converted to coherent magnons. A moving wall has its velocity reduced by pinning. The expression for the velocity agrees surprisingly well with experiment.

cond-mat.str-el↗

arXiv2004

Bi-layer splitting in overdoped high $T_{c}$ cuprates

Recent angle-resolved photoemission data for overdoped Bi2212 are explained. Of the peak-dip-hump structure, the peak corresponds the $\vec q =0$ component of a hole condensate which appears at $T_c$. The fluctuating part of this same condensate produces the hump. The bilayer splitting is large enough to produce a bonding hole and an electron antibonding quasiparticle Fermi surface. Smaller bilayer splittings observed in some experiments reflect the interaction of the peak structure with quasiparticle states near, but not at, the Fermi surface.

cond-mat.supr-con↗

arXiv2004

Kondo effect in quantum dots coupled to ferromagnetic leads

We study the Kondo effect in a quantum dot which is coupled to ferromagnetic leads and analyse its properties as a function of the spin polarization of the leads. Based on a scaling approach we predict that for parallel alignment of the magnetizations in the leads the strong-coupling limit of the Kondo effect is reached at a finite value of the magnetic field. Using an equation-of-motion technique we study nonlinear transport through the dot. For parallel alignment the zero-bias anomaly may be split even in the absence of an external magnetic field. For antiparallel spin alignment and symmetric coupling, the peak is split only in the presence of a magnetic field, but shows a characteristic asymmetry in amplitude and position.

cond-mat.mes-hall↗

arXiv2004

Crossed Andreev reflection in a d-wave superconductor with two quantum point contacts

We theoretically study the crossed Andreev reflection in a hybrid nanostructure which comprises a d-wave superconductor and two normal-metal quantum wires. When the superconductor of the (110) oriented surface is in contact with the wires parallel and placed close to each other, the Andreev bound state is formed by the crossed Andreev reflection. When the contact barrier potential is sufficiently large, two sharp peaks appear in the conductance well below the gap structure, which originate from the bonding and antibonding Andreev bound states. We propose that these Andreev bound states form a two-level quantum system (qubit).

cond-mat.mes-hall↗

arXiv2003

Mott Gap Excitations and Resonant Inelastic X-Ray Scattering in Doped Cuprates

Predictions are made for the momentum- and carrier-dependent degradation of the Mott gap upon doping in high-Tc cuprates as would be observed in Cu K-edge resonant inelastic x-ray scattering (RIXS). The two-dimensional Hubbard model with second- and third-nearest-neighbor hopping terms has been studied by numerical exact diagonalization. Special emphasis is placed on the particle-hole asymmetry of the Mott gap excitations. We argue that the Mott gap excitations observed by RIXS are significantly influenced by the interaction between charge carriers and antiferromagnetic correlations.

cond-mat.str-el↗

arXiv2003

Friedel oscillations in a two-band Hubbard model for CuO chains

Friedel oscillations induced by open boundary conditions in a two-band Hubbard model for CuO chains are numerically studied. We find that for physically realistic parameters and close to quarter filling, these oscillations have a 2k_F modulation according with experimental results on YBa_2Cu_3O_{7-delta}. In addition, we predict that, for the same parameters, as hole doping is reduced from quarter filling to half filling, Friedel oscillations would acquire a 4k_F modulation, typical of a strongly correlated electrons regime. The 4k_F modulation dominates also in the electron doped region. The range of parameters varied is very broad, and hence the results reported could apply to other cuprates and other strongly correlated compounds with quasi-one dimensional structures. On a more theoretical side, we stress the fact that the copper and oxygen subsystems should be described by two different Luttinger liquid exponents.

cond-mat.str-el↗

arXiv2003

Spin Dependent Transport in Magnetic Nanostructures

Recent progress in physics on spin dependent transport in magnetic nanostructures is reviewed. Special attention is paid on the spin accumulation and spin current caused by spin injection into non-magnetic metals and semiconductors and superconductors. A variety of phenomena induced in nano-superconductor/ferromagnet devices are proposed, examining the spin-charge separation in superconductors.

cond-mat.mes-hall↗

arXiv2003

Kagomé in triangular lattice: electronic state of CoO_2 layer with hexagonal structure

The electronic state in layered cobalt oxides with hexagonal structure is examined. We find that the electronic structure reflects the nature of the Kagomé lattice hidden in the CoO_2 layer which consists of stacked triangular lattices of oxygen ions and of cobalt ions. A fundamental model for the electron system is proposed and the mechanism of the unique transport and magnetic properties of the cobalt oxides are discussed in the light of the model.

cond-mat.str-el↗

arXiv2003

Thermodynamic and thermoelectric properties of high-temperature cuprate superconductors in the stripe phase

We examine the thermodynamic and thermoelectric properties in the stripe phase of high-Tc cuprates, by using the finite-temperature Lanczos technique for the t-J model with a potential that stabilizes vertical charge stripes. When the stripe potential is turned on, the entropy is suppressed as a consequence of the formation of one-dimensional charge stripes accompanied by an enhancement of antiferromagnetic spin correlation in the spin domains. The stripe formation leads also to weak temperature dependence of the chemical potential, leading to the suppression of the thermoelectric power. The suppression of the entropy and thermoelectric power is consistent with experimental data in the stripe phase of La_{1.6-x}Nd_{0.4}Sr_xCuO_4.

cond-mat.str-el↗

arXiv2003

Doping dependence of chemical potential and entropy in hole- and electron-doped high-Tc cuprates

We examine the thermodynamic properties of the hole- and electron-doped cuprates by using the t-t'-t''-J model. We find that the chemical potential shows different doping dependence between the hole and electron dopings. Recent experimental data of the chemical potential shift are reproduced except for lightly underdoped region in the hole doping where stripe and/or charge inhomogeneity are expected to be important. The entropy is also calculated as a function of the carrier concentration. It is found that the entropy of the electron-doped system is smaller than that of the hole-doped systems. This is related to strong antiferromagnetic short-range correlation that survives in the electron-doped system.

cond-mat.str-el↗

arXiv2002

Spin Injection and Detection in Magnetic Nanostructures

We study theoretically the spin transport in a nonmagnetic metal connected to ferromagnetic injector and detector electrodes. We derive a general expression for the spin accumulation signal which covers from the metallic to the tunneling regime. This enables us to discuss recent controversy on spin injection and detection experiments. Extending the result to a superconducting device, we find that the spin accumulation signal is strongly enhanced by opening of the superconducting gap since a gapped superconductor is a low carrier system for spin transport but not for charge. The enhancement is also expected in semiconductor devices.

cond-mat.mes-hall↗

arXiv2002

Resonant Two-Magnon Raman Scattering and Photoexcited States in Two-Dimensional Mott Insulators

We investigate the resonant two-magnon Raman scattering in two-dimensional (2D) Mott insulators by using a half-filled 2D Hubbard model in the strong coupling limit. By performing numerical diagonalization calculations for small clusters, we find that the Raman intensity is enhanced when the incoming photon energy is not near the optical absorption edge but well above it, being consistent with experimental data. The absence of resonance near the gap edge is associated with the presence of background spins, while photoexcited states for resonance are found to be characterized by the charge degree of freedom. The resonance mechanism is different from those proposed previously.

cond-mat.str-el↗