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N Nagaosa

Publications and source records attributed to N Nagaosa.

At least 19 recordsLinked to original sources
PubMed2005

One-dimensional orbital excitations in vanadium oxides.

The d electron orbital is a hidden but important degree of freedom controlling novel properties of transition-metal oxides. A one-dimensional orbital system is especially intriguing due to its enhanced quantum fluctuation. We present a combined experimental and theoretical study on the Raman scattering spectra in perovskite oxides NdVO(3) and LaVO(3) to prove that the quasi-one-dimensional orbital chain described by fermionic pseudospinons bears orbital excitations exchanging occupied orbital states on the neighboring sites, termed a two-orbiton in analogy with two-magnon.

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PubMed2005

Orbital nature of ferromagnetic magnons in manganites.

Magnon excitation in a ferromagnetic state of Sm(0.55)Sr(0.45)MnO(3) located on the verge of the metal-insulator transition has been studied in terms of the neutron scattering experiment. The anomalous magnon dispersion with the zone-boundary softening is well described by the Heisenberg model with extended exchange coupling constants J(s). In particular the fourth neighbor coupling J(4) is as large as 0.6 times the nearest neighbor one J(1). Theoretical analysis based on the local density approximation + Hubbard U band calculation reveals that this one-dimensional exchange path is due to the (3z(2)-r(2))-type orbital correlation, in sharp contrast to previous proposals.

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PubMed2004

Metallic ferromagnet with square-lattice CoO2 sheets.

A single-crystalline film of Sr2CoO4 with square-lattice CoO2 sheets (K2NiF4-type structure) was synthesized, and its electronic properties were characterized. The compound exhibited both ferromagnetic and metallic behaviors, with a fairly high Curie temperature T(C) of approximately 250 K, in contrast to the superconductivity recently found in a triangular-lattice CoO2-sheet compound. The film's large magnetoresistance with current perpendicular to the CoO2 plane showed field-hysteretic behavior analogous to tunneling magnetoresistance. The electronic structure of Sr2CoO4 was also investigated to characterize the quasi-two-dimensional ferromagnetic metal state in terms of optical spectroscopy and first-principles calculation.

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PubMed2004

Coupling of the B1g phonon to the antinodal electronic states of Bi2Sr2Ca0.92Y0.08Cu2O8+delta.

Angle-resolved photoemission spectroscopy on optimally doped Bi(2)Sr(2)Ca(0.92)Y(0.08)Cu(2)O(8+delta) uncovers a coupling of the electronic bands to a 40 meV mode in an extended k-space region away from the nodal direction, leading to a new interpretation of the strong renormalization of the electronic structure seen in Bi2212. Phenomenological agreements with neutron and Raman experiments suggest that this mode is the B(1g) oxygen bond-buckling phonon. A theoretical calculation based on this assignment reproduces the electronic renormalization seen in the data.

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PubMed2004

Anisotropic electron-phonon interaction in the cuprates.

We explore manifestations of electron-phonon coupling on the electron spectral function for two phonon modes in the cuprates exhibiting strong renormalizations with temperature and doping. Applying simple symmetry considerations and kinematic constraints, we find that the out-of-plane, out-of-phase O buckling mode (B(1g)) involves small momentum transfers and couples strongly to electronic states near the antinode while the in-plane Cu-O breathing modes involve large momentum transfers and couples strongly to nodal electronic states. Band renormalization effects are found to be strongest in the superconducting state near the antinode, in full agreement with angle-resolved photoemission spectroscopy data.

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PubMed2004

Electron-phonon coupling and a polaron in the t-J model: from the weak to the strong coupling regime.

We present numeric results for ground state and angle resolved photoemission spectra (ARPES) for a single hole in the t-J model coupled to optical phonons. The systematic-error-free diagrammatic Monte Carlo method is employed where the Feynman graphs for the Matsubara Green function in imaginary time are summed up completely with respect to phonon variables, while magnetic variables are subjected to the noncrossing approximation. We obtain that at electron-phonon coupling constants relevant for high T(c) cuprates the polaron undergoes a self-trapping crossover to the strong-coupling limit and theoretical ARPES demonstrate features observed in experiment: A broad peak in the bottom of the spectra has momentum dependence which coincides with that of a hole in the pure t-J model.

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PubMed2004

Commensurate-incommensurate crossover of charge stripe in La2-xSrxNiO4 (x approximately 1/3).

The temperature (T) dependence of the charge-stripe order in La2-xSrxNiO4 has been investigated in the vicinity of x approximately 1/3 by synchrotron radiation x-ray diffraction measurements. With decreasing T, a prominent commensurate-incommensurate (C-IC) crossover is observed in the x<1/3 region, while for the x>1/3 region the IC order is dominant over the whole T range. Such a C-IC crossover is interpreted as the entropy-driven self-doping of the charge stripes, and its x dependence indicates the clear electron-hole asymmetry with the x=1/3 compound as the Mott insulator.

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PubMed2003

Optical conductivity of the Fröhlich polaron.

We present accurate results for optical conductivity of the three dimensional Fröhlich polaron in all coupling regimes. The systematic-error free diagrammatic quantum Monte Carlo method is employed where the Feynman graphs for the momentum-momentum correlation function in imaginary time are summed up. The real-frequency optical conductivity is obtained by the analytic continuation with stochastic optimization. We compare numerical data with available perturbative and nonperturbative approaches to the optical conductivity and show that the picture of sharp resonances due to relaxed excited states in the strong-coupling regime is "washed out" by large broadening of these states. As a result, the spectrum contains only a single-maximum broad peak with peculiar shape and a shoulder.

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PubMed2001

Quasidegenerate self-trapping in one-dimensional charge transfer exciton.

The self-trapping by the nondiagonal particle-phonon interaction between two quasidegenerate energy levels of the excitonic system is studied. We propose this is realized in the charge-transfer exciton, where the directions of the polarization give the quasidegeneracy. It is shown that this mechanism, unlike the conventional diagonal one, allows a coexistence and resonance of the free and self-trapped states even in one-dimensional systems and a quantitative theory for the optical properties (light absorption and time-resolved luminescence) of the resonating states is presented. This theory gives a consistent resolution for the long-standing puzzles in quasi-one-dimensional compound A-PMDA.

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PubMed2001

Spin chirality, Berry phase, and anomalous Hall effect in a frustrated ferromagnet.

An electron hopping on non-coplanar spin sites with spin chirality obtains a complex phase factor (Berry phase) in its quantum mechanical amplitude that acts as an internal magnetic field, and is predicted to manifest itself in the Hall effect when it is not cancelled. The present combined work of transport measurement, neutron scattering, and theoretical calculation provides evidence that the gigantic anomalous Hall effect observed in Nd2Mo2O7, a pyrochlore ferromagnet with geometrically frustrated lattice structure, is mostly due to the spin chirality and the associated Berry phase originating from the Mo spin tilting.

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PubMed2000

Orbital physics in transition-metal oxides

An electron in a solid, that is, bound to or nearly localized on the specific atomic site, has three attributes: charge, spin, and orbital. The orbital represents the shape of the electron cloud in solid. In transition-metal oxides with anisotropic-shaped d-orbital electrons, the Coulomb interaction between the electrons (strong electron correlation effect) is of importance for understanding their metal-insulator transitions and properties such as high-temperature superconductivity and colossal magnetoresistance. The orbital degree of freedom occasionally plays an important role in these phenomena, and its correlation and/or order-disorder transition causes a variety of phenomena through strong coupling with charge, spin, and lattice dynamics. An overview is given here on this "orbital physics," which will be a key concept for the science and technology of correlated electrons.

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