arXiv subjects

S. Maekawa

Publications and source records attributed to S. Maekawa.

At least 127 records · Page 7Linked to original sources

arXiv2000

Orbital Structure and Magnetic Ordering in Layered Manganites: Universal Correlation and Its Mechanism

Correlation between orbital structure and magnetic ordering in bilayered manganites is examined. A level separation between the $3d_{3z^2-r^2}$ and $3d_{x^2-y^2}$ orbitals in a Mn ion is calculated in the ionic model for a large number of the compounds. It is found that the relative stability of the orbitals dominates the magnetic transition temperatures as well as the magnetic structures. A mechanism of the correlation between orbital and magnetism is investigated based on the theoretical model with the two $e_g$ orbitals under strong electron correlation.

cond-mat.str-el↗

arXiv1999

Spin Liquid State around a Doped Hole in Insulating Cuprates

The numerically exact diagonalization study on small clusters of the t-J model with the second- and third- neighbor hopping terms shows that a novel spin liquid state is realized around a doped hole with momentum k=(pi,0) and energy \sim 2J compared with that with (pi/2,pi/2) in insulating cuprates, where the spin and charge degrees of freedom are approximately decoupled. Our finding implies that the excitations in the insulating cuprates are mapped onto the the d-wave resonating valence bond state.

cond-mat.str-el↗

arXiv1999

Momentum Dependence of Resonant Inelastic X-Ray Scattering Spectrum in Insulating Cuprates

The resonant inelastic x-ray scattering spectrum in insulating cuprates is examined by using the exact diagonalization technique on small clusters in the two-dimensional Hubbard model with second and third neighbor hopping terms. When the incident photon energy is tuned near the Cu K absorption edges, we find that the features of the unoccupied upper Hubbard band can be extracted from the spectrum through an anisotropic momentum dependence. They provide an opportunity for the understanding of the different behavior of hole- and electron-doped superconductors.

cond-mat.str-el↗

arXiv1999

Phase Transition in Perovskite Manganites with Orbital Degree of Freedom

Roles of orbital degree of freedom of Mn ions in phase transition as a function of temperature and hole concentration in perovskite manganites are studied. It is shown that the orbital order-disorder transition is of the first order in the wide region of hole concentration and the N$\rm \acute{e}$el temperature for the anisotropic spin ordering, such as the layer-type antiferromagnetic one, is lower than the orbital ordering temperature due to the anisotropy in the orbital space. The calculated results of the temperature dependence of the spin and orbital order parameters explain a variety of the experiments observed in manganites.

cond-mat.str-el↗

arXiv1999

Interchain interactions and magnetic properties of Li2CuO2

An effective Hamiltonian is constructed for an insulating cuprate with edge-sharing chains Li2CuO2.The Hamiltonian contains the nearest and next-nearest neighboring intrachain and zigzag-type interchain interactions.The values of the interactions are obtained from the analysis of the magnetic susceptibility, and this system is found to be described as coupled frustrated chains.We calculate the dynamical spin correlation function S(q,ω) by using the exact diagonalization method, and show that the spectra of S(q,ω) are characterized by the zigzag-type interchain interactions. The results of the recent inelastic neutron scattering experiment are discussed in the light of the calculated spectra.

cond-mat↗

arXiv1999

Effect of Stripes on Electronic States in Underdoped La_{2-x}Sr_xCuO_4

We investigate the electronic states of underdoped La_{2-x}Sr_xCuO_4 (LSCO) by using a microscopic model, i.e., t-t'-t''-J model, containing vertical charge stripes. The numerically exact diagonalization calculation on small clusters shows the consistent explanation of the physical properties in the angle-resolved photoemission, neutron magnetic scattering and optical conductivity experiments such as the antiphase domain and quasi-one-dimensional charge transport. The pair correlation function of the d-channel is suppressed by the stripes. These results demonstrate a crucial role of the stripes in LSCO

cond-mat.str-el↗

arXiv1999

Spin-Imbalance and Magnetoresistance in Ferromagnet/Superconductor/Ferromagnet Double Tunnel Junctions

We theoretically study the spin-dependent transport in a ferromagnet/super- conductor/ferromagnet double tunnel junction. The tunneling current in the antiferromagnetic alignment of the magnetizations gives rise to a spin imbalance in the superconductor. The resulting nonequilibrium spin density strongly suppresses the superconductivity with increase of bias voltage and destroys it at a critical voltage Vc. The results provide a new method not only for measuring the spin polarization of ferromagnets but also for controlling superconductivity and tunnel magnetoresistance (TMR) by applying the bias voltage.

cond-mat↗

arXiv1999

Effect of Coulomb blockade on STM current through a granular film

The electron transport through an array of tunnel junctions consisting of an STM tip and a granular film is studied both theoretically and experimentally. When the tunnel resistance between the tip and a granule on the surface is much larger than those between granules, a bottleneck of the tunneling current is created in the array. It is shown that the period of the Coulomb staircase(CS) is given by the capacitance at the bottleneck. Our STM experiments on Co-Al-O granular films show the CS with a single period at room temperature. This provides a new possibility for single-electron-spin-electronic devices at room temperature.

cond-mat.mes-hall↗

arXiv1999

Stripe Stability in the Extended t-J Model on Planes and Four-Leg Ladders

The tendencies to phase-separation and stripe formation of the t-J model on planes and four-leg ladders have been here reexamined including hole hopping terms t', t'' beyond nearest-neighbor sites. The motivation for this study is the growing evidence that such terms are needed for a quantitative description of the cuprates. Using a variety of computational techniques it is concluded that the stripe tendencies considerably weaken when experimentally realistic t'<0, t''>0 for hole-doped cuprates are considered. However, a small t'>0 actually enhances the stripe formation.

cond-mat.str-el↗

arXiv1999

Orbital Degree of Freedom and Phase Separation in Ferromagnetic Manganites at Finite Temperatures

The spin and orbital phase diagram for perovskite manganites are investigated as a function of temperature and hole concentration. The superexchange and double exchange interactions dominate the ferromagnetic phases in the low and high concentration regions of doped holes, respectively.The two interactions favor different orbital states each other. Between the phases, two interactions compete with each other and the phase separation appears in the wide range of temperature and hole concentration. The anisotropy of the orbital space causes discontinuous changes of the orbital state and promotes the phase separation. The relation between the phase separation and the stripe- and sheet-type charge segregation is discussed.

cond-mat.str-el↗

arXiv1999

Difference of optical conductivity between one- and two-dimensional doped nickelates

We study the optical conductivity in doped nickelates, and find the dramatic difference of the spectrum in the gap ($ω$$\alt$4 eV) between one- (1D) and two-dimensional (2D) nickelates. The difference is shown to be caused by the dependence of hopping integral on dimensionality. The theoretical results explain consistently the experimental data in 1D and 2D nickelates, Y$_{2-x}$Ca$_x$BaNiO$_5$ and La$_{2-x}$Sr$_x$NiO$_4$, respectively. The relation between the spectrum in the X-ray aborption experiments and the optical conductivity in La$_{2-x}$Sr$_x$NiO$_4$ is discussed.

cond-mat.str-el↗

arXiv1999

Finite Temperature Effects in One-dimensional Mott-Hubbard Insulator: Angle-Resolved Photoemission Study of Na_{0.96}V_{2}O_{5}

We have made an angle-resolved photoemission study of a one-dimensional (1D) Mott-Hubbard insulator Na_{0.96}V_{2}O_{5} and found that the spectra of the V 3d lower Hubbard band are strongly dependent on the temperature. We have calculated the one-particle spectral function of the one-dimensional t-J model at finite temperatures by exact diagonalization and compared them with the experimental results. Good overall agreement is obtained between experiment and theory. The strong finite temperature effects are discussed in terms of the existence of the ``Fermi surface'' of the spinon band.

cond-mat.str-el↗

arXiv1999

Transition between two ferromagnetic states driven by orbital ordering in La_{0.88}Sr_{0.12}MnO_3

A lightly doped perovskite mangantite La_{0.88}Sr_{0.12}MnO_3 exhibits a phase transition at T_{OO}=145 K from a ferromagnetic metal (T_C=172 K) to a novel ferromagnetic insulator.We identify that the key parameter in the transition is the orbital degree of freedom in e_g electrons. By utilizing the resonant x-ray scattering technique, orbital ordering is directly detected below T_{OO}, in spite of a significant diminution of the cooperative Jahn-Teller distortion. The new experimental features are well described by a theory treating the orbital degree of freedom under strong electron correlation. The present experimental and theoretical studies uncover a crucial role of the orbital degree in the metal-insulator transition in lightly doped manganites.

cond-mat.str-el↗

arXiv1998

Field Induced Transition from Metal to Insulator in the CMR Manganites

The gigantic reduction of the electric resistivity under the applied magnetic field, CMR effect, is now widely accepted to appear in the vicinity of the insulator to metal transition of the perovskite manganites. Recently, we have discovered the first order transition from ferromagnetic metal to insulator in $\rm La_{0.88}Sr_{0.12}MnO_3$ of the CMR manganite. This phase transition induces the tremendous increase of the resistivity under the external magnetic field just near above the phase transition temperature. We report here fairly detailed results from the systematic experiments including neutron and synchrotron X-ray scattering studies.

cond-mat.str-el↗

arXiv1998

Two Ferromagnetic States in Magnetoresistive Manganites - First Order Transition Driven by Orbitals -

A systematic study of the electronic structure in perovskite manganites is presented. The effective Hamiltonian is derived by taking into account the degeneracy of $e_g$ orbitals and strong electron correlation in Mn ions. The spin and orbital orderings are examined as functions of carrier concentration in the mean-field approximation applied to the effective Hamiltonian. We obtain the first order phase transition between ferromagnetic metallic and ferromagnetic insulating states in the lightly doped region. The transition is accompanied with the orbital order-disorder one which is directly observed in the anomalous X-ray scattering experiments. The present investigation shows a novel role of the orbital degree of freedom on metal-insulator transition in manganites.

cond-mat.str-el↗

arXiv1998

Effect of magnetic frustration on single-hole spectral function in the t-t'-t''-J model

We examine the effect of the magnetic frustration J' on the single-hole spectral function in the t-t'-t''-J model. At zero temperature, the exact diagonalization (ED) and the self-consistent Born approximation (SCBA) methods are used. We find that the frustration suppresses the quasiparticle (QP) weight at small momentum k, whereas the QP peak at k=(pi/2,pi/2) remains sharp. We also show the temperature dependence of the single-hole spectral function by using the ED method. It is found that the lineshapes at (pi/2,0) and (pi/2,pi/2) show different temperature dependence. These findings are consistent with the angle-resolved photoemission data on Sr2CuO2Cl2, and indicate the importance of the magnetic frustration on the electronic states of the insulating cuprates.

cond-mat.str-el↗

arXiv1998

Electronic and magnetic states in doped LaCoO_3

The electronic and magnetic states in doped perovskite cobaltites, (La, Sr)CoO_3, are studied in the numerically exact diagonalization method on Co_2O_{11} clusters. For realistic parameter values, it is shown that a high spin state and an intermediate spin state coexist in one-hole doped clusters due to strong p-d mixing. The magnetic states in the doped cobaltites obtained in the calculation explain various experimental results.

cond-mat.str-el↗

arXiv1997

Insulator-Metal Transition in One Dimension Induced by Long-Range Electronic Interactions

The effects of a long range electronic potential on a one dimensional commensurate Charge Density Wave (CDW) state are investigated. Using numerical techniques it is shown that a transition to a metallic ground state is reached as the range of the electron-electron repulsion increases. In this metallic state, the optical conductivity exhibits a large Drude weight. Possible interpretations of our results are discussed.

cond-mat.str-el↗