Biomedical subjects

Naoto Nagaosa

Publications and source records attributed to Naoto Nagaosa.

At least 19 recordsLinked to original sources

PubMed2006

Intrinsic versus extrinsic anomalous Hall effect in ferromagnets.

A unified theory of the anomalous Hall effect (AHE) is presented for multiband ferromagnetic metals with dilute impurities. In the clean limit, the AHE is mostly due to extrinsic skew scattering. When the Fermi level is located around anticrossing of band dispersions split by spin-orbit interaction, the intrinsic AHE to be calculated ab initio is resonantly enhanced by its nonperturbative nature, revealing the extrinsic-to-intrinsic crossover which occurs when the relaxation rate is comparable to the spin-orbit coupling.

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PubMed2006

Electron localization or delocalization in incommensurate helical magnets.

The electronic states in incommensurate helical magnets are studied theoretically from the viewpoint of the localization or delocalization. It is found that in the multiband system with a relativistic spin-orbit interaction, the electronic wave functions show both an extended and localized nature along the helical axis depending on the orbital, helical wave number, and the direction of the plane on which spins rotate. The possible realization of this localization is discussed.

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PubMed2006

Dynamical diffraction theory for wave packet propagation in deformed crystals.

We develop a theory for the trajectory of an x ray in the presence of a crystal deformation. A set of equations of motion for an x-ray wave packet including the dynamical diffraction is derived, taking into account the Berry phase as a correction to geometrical optics. The trajectory of the wave packet has a shift of the center position due to a crystal deformation. Remarkably, in the vicinity of the Bragg condition, the shift is enhanced by a factor omega/deltaomega (omega: frequency of an x ray, deltaomega: gap frequency induced by the Bragg reflection). Comparison with the conventional dynamical diffraction theory is also made.

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PubMed2006

Dynamics of localized spins coupled to the conduction electrons with charge and spin currents.

The dynamics of the localized spins coupled to the conduction electrons is studied theoretically in the wide range of magnitudes of the charge and spin currents including the regime which has never been explored but is now possible in terms of the pure spin-current injection methods, e.g., the spin Hall effect and spin battery. The equations of motion for the two-spin system are investigated in detail, and its phase diagram of the dynamics is presented. It is found that the dynamics depends sensitively upon the relative magnitudes of the charge and spin currents; i.e., it shows steady state, periodic motion, and even chaotic behavior. The extension to the multispin system and its implications including a possible "spin-current detector" are also discussed.

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PubMed2005

Interfaces of correlated electron systems: proposed mechanism for colossal electroresistance.

Mott's metal-insulator transition at an interface due to band bending is studied by the density matrix renormalization group approach. We show that the result can be recovered by a simple modification of the conventional Poisson's equation approach used in semiconductor heterojunctions. A novel mechanism of colossal electroresistance is proposed, which incorporates the hysteretic behavior of the transition in higher dimensions.

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PubMed2005

Optical magnetoelectric effect in multiferroic materials: evidence for a Lorentz force acting on a ray of light.

We theoretically propose that the optical analog of a Lorentz force acting on a ray of light is realized in multiferroic materials such as GaFeO3 showing the magnetoelectric effect. The toroidal moment T --> = sigma(j)r(j) x S(j) plays the role of a "vector potential," while its rotation corresponds to a "magnetic field" for photons. Hence, the light is subject to the Lorentz force when propagating through the domain wall region of the ferromagnetic or ferroelectric order. A realistic estimate on the magnitude of this effect is given.

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PubMed2005

Spin current and accumulation generated by the spin Hall insulator.

Spin current and accumulation generated by the electric field in a spin Hall insulator (SHI) are investigated theoretically in terms of the Keldysh formalism. In contrast to the quantum Hall system, there are no massless edge modes in general. The spin current is generated near the contacts to the electrodes by the hybridization between the metallic states and the conduction/valence bands of the SHI, but is truncated by the sink and source of the spin. However, one can produce the spin current flowing out to the conductors, which is attached to the SHI, and also the spin accumulation there, due to the leakage charge current which breaks the time-reversal symmetry.

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PubMed2005

Spin current and magnetoelectric effect in noncollinear magnets.

A new mechanism of the magnetoelectric effect based on the spin supercurrent is theoretically presented in terms of a microscopic electronic model for noncollinear magnets. The electric polarization P(ij) produced between the two magnetic moments S(i) and S(j) is given by P proportional e(ij) X (S(i) X S(j)) with e(ij) being the unit vector connecting the sites i and j. Applications to the spiral spin structure and the gauge theoretical interpretation are discussed.

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PubMed2005

Ferroelectricity near room temperature in co-crystals of nonpolar organic molecules.

The research on ferroelectric materials-mostly inorganic compounds or organic polymers-is increasingly motivated by both basic scientific concerns and the potential for practical applications in electronics and optics. Ferroelectricity in organic solids would be important for the development of all-organic electronic and photonic devices. The conventional approach to making organic ferroelectrics is based on the use of polar molecules. Here we report that through supramolecular assembly of nonpolar conjugated molecules, a remarkable ferroelectric response can be obtained in co-crystals of low-molecular-weight organic compounds. Co-crystals of phenazine and chloranilic acid reveal large spontaneous polarization and sizable room-temperature dielectric constants exceeding 100. The present findings provide an approach to making potentially useful organic ferroelectric materials.

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PubMed2004

Quantum versus Jahn-Teller orbital physics in YVO3 and LaVO3.

We argue that the large Jahn-Teller (JT) distortions in YVO3 and LaVO3 should suppress the quantum orbital fluctuation. The unusual magnetic properties can be well explained based on local density approximation + Hubbard U calculations using experimental structures, in terms of the JT orbital. The observed splitting of the spin-wave dispersions for YVO3 in a C-type antiferromagnetic state is attributed to the inequivalent VO2 layers in the crystal structure, instead of the "orbital-Peierls state." Alternative stacking of ab-plane exchange couplings produces the c-axis spin-wave splitting; thus, the spin system is highly three dimensional rather than quasi-one-dimensional. Similar splitting is also predicted for LaVO3, although it is weak.

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PubMed2004

Topological nature of polarization and charge pumping in ferroelectrics.

Electric polarization in insulators is represented by the transferred charge through a shift of the Bloch wave functions induced by an adiabatic change of external parameters Q-->. It is found that this covalent/quantum contribution is determined nonlocally by the topological structure in the Q--> space. The condition for the charge pumping for a cyclic change of Q--> is also obtained. Applications of this picture to various organic ferroelectrics and BaTiO3 are discussed.

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PubMed2004

Spin-Hall insulator.

Recent theories predict dissipationless spin current induced by an electric field in doped semiconductors. Nevertheless, the charge current is still dissipative in these systems. In this work, we theoretically predict the dissipationless spin-Hall effect, without any accompanying charge current, in some classes of band insulators, including zero-gap semiconductors such as HgTe and narrow-gap semiconductors such as PbTe. This effect is similar to the quantum-Hall effect in that all the states below the gap contribute and there occurs no dissipation. However, the spin-Hall conductance is not quantized even in two dimensions. This is the first example of a nontrivial topological structure in a band insulator without any magnetic field.

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PubMed2004

Hall effect of light.

We derive the semiclassical equation of motion for the wave packet of light taking into account the Berry curvature in momentum-space. This equation naturally describes the interplay between orbital and spin angular momenta, i.e., the conservation of the total angular momentum of light. This leads to the shift of wave-packet motion perpendicular to the gradient of the dielectric constant, i.e., the polarization-dependent Hall effect of light. An enhancement of this effect in photonic crystals is also proposed.

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PubMed2004

Two-dimensional charge order in layered 2-1-4 perovskite oxides.

Monte Carlo simulations are performed on the three-dimensional (3D) Ising model with the 2-1-4 layered perovskite structure as a minimal model for checkerboard charge ordering phenomena in layered perovskite oxides. Because of the interlayer frustration, only 2D long-range order emerges with a finite correlation length along the c axis. Critical exponents of the transition change continuously as a function of the interlayer coupling constant. The interlayer long-range Coulomb interaction decays exponentially and is negligible even between the second-neighbor layers. Instead, monoclinic distortion of a tetragonal unit cell lifts the macroscopic degeneracy to induce a 3D charge ordering. The dimensionality of the charge order in La0.5Sr1.5MnO4 is discussed from this viewpoint.

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PubMed2003

Competing orders and disorder-induced insulator to metal transition in manganites.

Effects of disorder on the two competing phases, i.e., the ferromagnetic metal and the commensurate charge/lattice ordered insulator, are studied by Monte Carlo simulation. The disorder suppresses the charge/lattice ordering more strongly than the ferromagnetic order, driving the commensurate insulator to the ferromagnetic metal near the phase boundary in the pure case. Above the ferromagnetic transition temperature, on the contrary, the disorder makes the system more insulating, which might cause an enhanced colossal magnetoresistance as observed in the half-doped or Cr-substituted manganites. No indication of the percolation or the cluster formation is found, and there remains the charge/lattice fluctuations instead which are enhanced toward the transition temperature.

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PubMed2003

The anomalous Hall effect and magnetic monopoles in momentum space.

Efforts to find the magnetic monopole in real space have been made in cosmic rays and in particle accelerators, but there has not yet been any firm evidence for its existence because of its very heavy mass, approximately 10(16) giga-electron volts. We show that the magnetic monopole can appear in the crystal momentum space of solids in the accessible low-energy region (approximately 0.1 to 1 electron volts) in the context of the anomalous Hall effect. We report experimental results together with first-principles calculations on the ferromagnetic crystal SrRuO3 that provide evidence for the magnetic monopole in the crystal momentum space.

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PubMed2003

Dissipationless quantum spin current at room temperature.

Although microscopic laws of physics are invariant under the reversal of the arrow of time, the transport of energy and information in most devices is an irreversible process. It is this irreversibility that leads to intrinsic dissipations in electronic devices and limits the possibility of quantum computation. We theoretically predict that the electric field can induce a substantial amount of dissipationless quantum spin current at room temperature, in hole-doped semiconductors such as Si, Ge, and GaAs. On the basis of a generalization of the quantum Hall effect, the predicted effect leads to efficient spin injection without the need for metallic ferromagnets. Principles found here could enable quantum spintronic devices with integrated information processing and storage units, operating with low power consumption and performing reversible quantum computation.

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PubMed2003

Quantized anomalous Hall effect in two-dimensional ferromagnets: quantum Hall effect in metals.

We study the effect of disorder on the anomalous Hall effect (AHE) in two-dimensional ferromagnets. The topological nature of the AHE leads to the integer quantum Hall effect from a metal, i.e., the quantization of sigma(xy) induced by the localization except for the few extended states carrying Chern numbers. Extensive numerical study on a model reveals that Pruisken's two-parameter scaling theory holds even when the system has no gap with the overlapping multibands and without the uniform magnetic field. Therefore, the condition for the quantized AHE is given only by the Hall conductivity sigma(xy) without the quantum correction, i.e., /sigma(xy)/>e(2)/(2h).

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