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Naoto Nagaosa

Publications and source records attributed to Naoto Nagaosa.

At least 325 records · Page 18Linked to original sources
arXiv2002

Anisotropic Optic Conductivities due to Spin and Orbital Orderings in LaVO3 and YVO3: First-Principles Studies

The anisotropy of low energy (0$\sim$5eV) optical excitations in strongly correlated transition-metal oxides is closely related to the spin and orbital orderings. The recent successes of LDA+$U$ method in describing the magnetic and electronic structures enable us to calculate the optical conductivity from first-principles. The LaVO$_3$ and YVO$_3$, both of which have $3d^2$ configuration and have various spin and orbital ordered phases at low temperature, show distinct anisotropy in the optical spectra. The effects of spin and orbital ordering on the anisotropy are studied in detail based on our first-principles calculations. The experimental spectra of both compounds at low temperature phases can be qualitatively explained with our calculations, while the studies for the intermediate temperature phase of YVO$_3$ suggest the substantial persistence of the low temperature phase at elevated temperature.

cond-mat.str-el↗
arXiv2002

Theory of Excitonic States in CaB6

We study the excitonic states in CaB6 in terms of the Ginzburg-Landau theory. By minimizing the free energy and by comparing with experimental results, we identify two possible ground states with exciton condensation. They both break time-reversal and inversion symmetries. This leads to various magnetic and optical properties. As for magnetic properties, it is expected to be an antiferromagnet, and its spin structure is predicted. It will exhibit the magnetoelectric effect, and observed novel ferromagnetism in doped samples and in thin-film and powder samples can arise from this effect. Interesting optical phenomena such as the nonreciprocal optical effect and the second harmonic generation are predicted. Their measurement for CaB6 will clarify whether exciton condensation occurs or not and which of the two states is realized.

cond-mat.str-el↗
arXiv2002

Spin Chirality Fluctuation and Anomalous Hall Effect in Itinerant Ferromagnets

The anomalous Hall effect due to the spin chirality order and fluctuation is studied theoretically in a Kondo lattice model without the relativistic spin-orbit interaction. Even without the correlations of the localized spins, $σ_{xy}$ can emerge depending on the lattice structure and the spin anisotropy. We reveal the condition for this chirality-fluctuation driven mechanism for $σ_{xy}$. Our semiquantitative estimates for a pyrochlore oxide Nd$_2$Mo$_2$O$_7$ give a finite $σ_{xy} \sim 10 \Ohm^{-1} \cm^{-1}$ together with a high resistivity $ρ_{xx} \sim 10^{-4}-10^{-3} \Ohm \cm$, in agreement with experiments.

cond-mat.str-el↗
arXiv2002

Theory for Slightly Doped Antiferromagnetic Mott Insulators

Trial wavefunctions, constructed explicitly from the unique 2-dimensional Mott insulating state with antiferromagnetic order, are proposed to describe the low-energy states of a Mott insulator slightly doped with holes or electrons. With the state behaving like charged quasi-particles with well-defined momenta, a rigid band is observed. These states have much less pairing correlations than previously studied ones. Small Fermi patches obtained are consistent with recent experiments on high Tc cuprates doped lightly with holes or electrons. States showing the incoherent and spin-bag behaviors are also discussed. Using these wavefunctions, a number of results obtained by exact calculations are reproduced.

cond-mat.str-el↗
arXiv2002

Colossal magnetoresistance in manganites as a multicritical phenomenon

The colossal magnetoresistance in manganites AMnO_3 is studied from the viewpoint of multicritical phenomena. To understand the complicated interplay of various phases, we study the Ginzburg-Landau theory in terms of both the mean-field approximation and the renormalization-group analysis to compare with the observed phase diagram. Several novel features, such as the first-order ferromagnetic transition, and the dip in the transition temperature near the multicritical point, can be understood as driven by enhanced fluctuations near the multicritical point. Furthermore, we obtain a universal scaling relation for the H/M-M^2 plot (Arrott plot), which fits rather well with the experimental data, providing the further evidence for the enhanced fluctuation.

cond-mat.str-el↗
arXiv2002

Particle-hole symmetry and transport properties of the flux state in underdoped cuprates

Transport properties are studied for the flux state with the gauge flux $ϕ$ per plaquett, which may model the underdoped cuprates, with the emphasis on the particle-hole and parity/chiral symmetries.This model is reduced to the Dirac fermions in (2+1)D with a mass gap introduced by the antiferromagnetic (AF) long range order and/or the stripe formation. Without the mass gap, the Hall constant $R_H$ and the thermopower $S$ obey two-parameter scaling laws, and show the strong temperature dependence due to the recovery of the particle-hole symmetry at high temperature. The $x$-dependences of $σ_{xx} (\propto \sqrt{x})$ and $σ_{xy}$ (independent of $x$) are in a sharp contradiction with the experiments. (Here $x$ is the hole concentration.) Therefore there is no signature of the particle-hole symmetry or the massless Dirac fermions in the underdoped cuprates even above the Neel temperature $T_N$. With the mass gap introduced by the AF order, there occurs the parity anomaly for each of the Dirac fermions. However the contributions from different valleys and spins cancel with each other to result in no spontaneous Hall effect even if the time-reversal symmetry is broken with $ϕ\ne π$. The effects of the stripes are also studied. The diagonal and vertical (horizontal) stripes have quite different influence on the transport properties. The suppression of $R_H$ occurs at low temperature only when (i) both the AF order and the vertical (horizontal) stripe coexist, and (ii) the average over the in-plane direction is taken. Discussions on the recent experiments are given from the viewpoint of these theoretical results.

cond-mat.supr-con↗
arXiv2002

Jahn-Teller effect and Electron correlation in manganites

Jahn-Teller (JT) effect both in the absence and presence of the strong Coulomb correlation is studied theoretically focusing on the reduction $ΔK$ of the kinetic energy gain which is directly related to the spin wave stiffness. Without the Coulomb interaction, the perturbative analysis gives $ΔK / (g^2/MΩ^2) \cong 0.05-0.13$ depending on the electron number [$g$: electron-phonon(el-ph) coupling constant, $M$: mass of the oxygen atom, $Ω$: frequency of the phonon]. Although there occurs many channels of the JT el-ph interaction in the multiband system, the final results of $ΔK$ roughly scales with the density of states at the Fermi energy. In the limit of strong electron correlation, the magnitude of the orbital polarization saturate and the relevant degrees of freedom are the direction (phase) of it. An effective action is derived for the phase variable including the effect of the JT interaction. In this limit, JT interaction is {\it{enhanced}} compared with the non-interacting case, and $ΔK$ is given by the lattice relaxation energy $E_{L}$ for the localized electrons, although the electrons remains itinerant. Discussion on experiments are given based on these theoretical results.

cond-mat.str-el↗
arXiv2002

Theory of Ferromagnetism in Ca1-xLaxB6

Novel ferromagnetism in Ca$_{1-x}$La$_{x}$B$_6$ is studied in terms of the Ginzburg-Landau theory for excitonic order parameters, taking into account symmetry of the wavefunctions. We found that the minima of the free energy break both inversion and time-reversal symmetries, while the product of these two remains preserved. This explains various novelties of the ferromagnetism and predicts a number of magnetic properties, including the magnetoelectric effect, which can be tested experimentally.

cond-mat.str-el↗
arXiv2001

Topological Nature of Anomalous Hall Effect in Ferromagnet

The anomalous Hall effect in two-dimensional ferromagnets is discussed to be the physical realization of the parity anomaly in (2+1)D, and the band crossing points behave as the topological singularity in the Brillouin zone. This appears as the sharp peaks and the sign changes of the transverse conductance $σ_{xy}$ as a function of the Fermi energy and/or the magnetization. The relevance to the experiments including the three dimensional systems is also discussed.

cond-mat.str-el↗
arXiv2000

Multicritical Phenomena of Superconductivity and Antiferromagnetism in Organic Conductor $κ$-(BEDT-TTF)$_2$X

We study theoretically the multicritical phenomena of the superconductivity (SC) and antiferromagnetism (AF) in organic conductors $κ$-BEDT salts. The phase diagram and the experimental data on the NMR relaxation rate $1/T_1$ is analysed in terms of the renormalization group method. The bicritical phenomenon observed experimentally indicates the rotational symmetry, i.e., SO(5) symmetry, of the SC and the AF. The critical exponent $x$ for the divergence of $1/T_1$ is well explained by $ x = ν(z - 1 - η)$ with the dynamical exponent $z = 3/2$ for the AF region while $z = ϕ/ν\sim 1.84 $ at the bicritical point. These results strongly suggest that the origin of the SC is in common with that of the AF and that its symmetry is d-wave.

cond-mat.supr-con↗
arXiv2000

Complex orbital state in manganites

The $e_g$-orbital states with complex coefficients of the linear combination of $x^2-y^2$ and $3z^2-r^2$ are studied for the ferromagnetic state in doped manganites. Especially the focus is put on the competition among uniform complex, staggered complex, and real orbital states. As the hole-doping $x$ increases, the real, the canted complex, and the staggered complex orbital states appears successively. Uniform complex state analoguous to Nagaoka ferromagnet does not appear. These complex states can be expressed as a resonating state among the planer orbitals as the orbital liquid, accompanied by no Jahn-Teller distortion.

cond-mat.str-el↗
arXiv2000

Impurity Effects on Quantum Depinning of Commensurate Charge Density Waves

We investigate quantum depinning of the one-dimensional (1D) commensurate charge-density wave (CDW) in the presence of one impurity theoretically. Quantum tunneling rate below but close to the threshold field is calculated at absolute zero temperature by use of the phase Hamiltonian within the WKB approximation. We show that the impurity can induce localized fluctuation and enhance the quantum depinning. The electric field dependence of the tunneling rate in the presence of the impurity is different from that in its absence.

cond-mat.dis-nn↗
arXiv1999

Spin Anisotropy and Quantum Hall Effect in the Kagome Lattice - Chiral Spin State based on a Ferromagnet-

A ferromagnet with spin anisotropies on the 2D Kagome lattice is theoretically studied. This is a typical example of the flat-band ferromagnet. The Berry phase induced by the tilting of the spins opens the band gap and quantized Hall conductance σ_{xy}=\pm e^2/h is realized without external magnetic field. This is the most realistic chiral spin state based on the ferromagnetism. We also discuss the implication of our results to anomalous Hall effect observed in the metallic pyrochlore ferromagnets R_2Mo_2O_7(R=Nd, Sm, Gd).

cond-mat.str-el↗
arXiv1999

Features of renormalization induced by interaction in 1D transport

One-dimensional interacting electrons in a quantum wire connected to reservoirs are studied theoretically. The difference in the Tomonaga-Luttinger interaction constants between the wire (g) and reservoirs $(g_{\infty})$ produces the cross-correlation between the right- and left-going chiral components of the charge density wave field. The low energy asymptotics of this field correlator, which is determined by (g) and $(g_{\infty})$, specifies renormalization of physical quantities. We have found that charge of the carriers in the shot noise is determined by $g_\infty$ (no renormalization for the Fermi liquid reservoirs) at any energy, meanwhile the factor g renormalizing the charge and spin susceptibilities emerges in the threshold structures at some rational fillings.

cond-mat.str-el↗
arXiv1999

Impurity pinning in transport through 1D Mott-Hubbard and spin gap insulators

A low energy crossover (see cond-mat/9711167) induced by Fermi liquid reservoirs in transport through a 1D Mott-Hubbard insulator of finite length $L$ is examined in the presence of impurity pinning. Under the assumption that the Hubbard gap 2M is large enough: $M > T_L \equiv v_c/L$ ($v_c$: charge velocity in the wire) and the impurity backscattering rate $Γ_1 \ll T_L$, the conductance vs. voltage/temperature displays a zero-energy resonance. Transport through a spin gapped 1D system is also described availing of duality between the backscattered current of this system and the direct current of the Mott-Hubbard insulator.

cond-mat↗
arXiv1999

Orbital Polarization and Fluctuation in Manganese Oxides

In a form which include the mother compound, we studied the extended Hubbard-type model with the orbital degeneracy for any doping concentration. Meanfield phase diagram and the spin wave excitation in RPA are calculated. It turned out that a large orbital polarization is essential to reproduce the magnetic phases depending on the doping concentration. Base on this result, we discuss the spin canting, the spin wave dispersion, and spin wave stiffness from the standing point of the large orbital polarization. We also discuss the orbital fluctuation which turned out to be important in the ferromagnetic metallic region where the CMR is observed. Spin wave softening near the zone boundary is also discussed in this context.

cond-mat.str-el↗
arXiv1999

Theory of spin wave excitation in manganites

The role of the orbital degrees of freedom is studied theoretically for the spin dynamics of $R_{1-x}A_x$MnO$_3$. Based on the meanfield solution, an RPA calculation has been done and it is found that the $d_{x^2-y^2}$-type orbital is essential for the double-exchange (DE) interactions, i.e., the DE is basically two-dimensional interaction. Based on this results compared with experiments, we propose that the orbital wavefunction is $d_{x^2-y^2}$-type locally even in the metallic ferromagnetic state, which fluctuate quantum mechanically. Well agreement of the estimation with experiments suggest that the Jahn-Teller phonon has less importance on the spin dynamics.

cond-mat.str-el↗