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Nobuo Furukawa

Publications and source records attributed to Nobuo Furukawa.

At least 73 records · Page 4Linked to original sources
arXiv2001

Non-equilibrium Relaxation Study of Ferromagnetic Transition in Double-Exchange Systems

Ferromagnetic transition in double-exchange systems is studied by non-equilibrium relaxation technique combined with Monte Carlo calculations. Critical temperature and critical exponents are estimated from relaxation of the magnetic moment. The results are consistent with the previous Monte Carlo results in thermal equilibrium. The exponents estimated by these independent techniques suggest that the universality class of this transition is the same as that of short-range interaction models but is different from the mean-field one.

cond-mat.str-el↗
arXiv2001

Antiferromagnetism of the Hubbard Model on a Layered Honeycomb Lattice - Is MgB$_2$ a Nearly-Antiferromagnetic Metal? -

As a model for the B 2pz bands of MgB$_2$, a half-filled Hubbard model on a honeycomb lattice is studied, and a possibility of an antiferromagnetism (AF) at the ground state is examined by Monte Carlo calculations as well as a random phase approximation method. On a single-layer honeycomb lattice, a paramagnetic to AF phase transition occurs at a finite value of the repulsion energy $U_c/t \simeq 3.6$, due to the loss of the density of states at the fermi level. On a layered honeycomb lattice, however, inter-layer hopping term creates a perfect nesting of the fermi surface, and leads the AF with $U_c=0$. From these facts, we discuss that MgB$_2$ is possibly a nearly-AF metal, where the AF of the 2pz bands is destroyed by carrier doping.

cond-mat.str-el↗
arXiv2001

Monte Carlo Algorithm for the Double Exchange Model Optimized for Parallel Computations

A new algorithm for Monte Carlo calculation of the double exchange model is studied. The algorithm is commonly applicable to wide classes of strongly correlated electron systems which involve itinerant electrons coupled with thermodynamically fluctuating fields. Using moment expansions of the density of states with Chebyshev polynomials, the algorithm provides an efficient calculation on large size clusters, especially on parallel computers. Benchmark calculations are performed on Beowulf-type cluster systems with over 100 CPUs in parallel.

cond-mat.str-el↗
arXiv2001

Critical Phenomena of Ferromagnetic Transition in Double-Exchange Systems

Critical phenomena of ferromagnetic transition at finite temperatures are studied in double-exchange systems. In order to investigate strong interplay between charge and spin degrees of freedom, Monte Carlo technique is applied to include fluctuations in a controlled and unbiased manner. By using finite-size scaling analysis, critical exponents and transition temperature are estimated for a model with Ising spin symmetry in two dimensions. The obtained exponents are far distinct from the mean-field values, but consistent with those of spin models with short-range exchange interactions. The universality class of this transition belongs to that of short-range interaction with the same spin symmetry. We also discuss the case for three dimensions. The results are compared with experimental results in perovskite manganites which show colossal magnetoresistance.

cond-mat.str-el↗
arXiv2000

Critical Temperature of Ferromagnetic Transition in Three-Dimensional Double-Exchange Models

Ferromagnetic transition in three-dimensional double-exchange models is studied by the Monte Carlo method. Critical temperature $T_{\rm c}$ is precisely determined by finite-size scaling analysis. Strong spin fluctuations in this itinerant system significantly reduce $T_{\rm c}$ from mean-field estimates. By choosing appropriate parameters, obtained values of $T_{\rm c}$ quantitatively agree with experiments for the ferromagnetic metal regime of (La,Sr)MnO$_{3}$, which is a typical perovskite manganite showing colossal magnetoresistance. This indicates that the double-exchange mechanism alone is sufficient to explain $T_{\rm c}$ in this material. Critical exponents are also discussed.

cond-mat.str-el↗
arXiv2000

A Monte Carlo Method for Fermion Systems Coupled with Classical Degrees of Freedom

A new Monte Carlo method is proposed for fermion systems interacting with classical degrees of freedom. To obtain a weight for each Monte Carlo sample with a fixed configuration of classical variables, the moment expansion of the density of states by Chebyshev polynomials is applied instead of the direct diagonalization of the fermion Hamiltonian. This reduces a cpu time to scale as $O(N_{\rm dim}^{2} \log N_{\rm dim})$ compared to $O(N_{\rm dim}^{3})$ for the diagonalization in the conventional technique; $N_{\rm dim}$ is the dimension of the Hamiltonian. Another advantage of this method is that parallel computation with high efficiency is possible. These significantly save total cpu times of Monte Carlo calculations because the calculation of a Monte Carlo weight is the bottleneck part. The method is applied to the double-exchange model as an example. The benchmark results show that it is possible to make a systematic investigation using a system-size scaling even in three dimensions within a realistic cpu timescale.

cond-mat.str-el↗
arXiv2000

Unconventional one-magnon scattering resistivity in half metals

Low-temperature resistivity of half-metals is investigated. To date it has been discussed that the one-magnon scattering process in half-metals is irrelevant for low-temperature resistivity, due to the fully spin-polarized electronic structure at the ground state. If one takes into account the non-rigid-band behavior of the minority band due to spin fluctuations at finite temperatures, however, the unconventional one-magnon scattering process is shown to be most relevant and gives T^3 dependence in resistivity. This behavior may be used as a crucial test in the search for half-metallic materials which are potentially important for applications. Comparison with resistivity data of La_1-x Sr_x MnO_3 as candidates for half-metals shows good agreement.

cond-mat.str-el↗
arXiv1999

Magnon Broadening Effects in Double Layered Manganite La_1.2 Sr_1.8 Mn_2 O_7

Magnon linewidth of La_1.2 Sr_1.8 Mn_2 O_7 near the Brillouin zone boundary is investigated from both theoretical and experimental points of view. Abrupt magnon broadening is ascribed to a strong magnon-phonon coupling. Magnon broadening observed in cubic perovskite manganites is also discussed.

cond-mat.str-el↗
arXiv1999

Magnon Broadening Effect by Magnon-Phonon Interaction in Colossal Magnetoresistance Manganites

In order to study the magnetic excitation behaviors in colossal magnetoresistance manganites, a magnon-phonon interacting system is investigated. Sudden broadening of magnon linewidth is obtained when a magnon branch crosses over an optical phonon branch. Onset of the broadening is approximately determined by the magnon density of states. Anomalous magnon damping at the brillouine zone boundary observed in low Curie temperature manganites is explained.

cond-mat.str-el↗
arXiv1998

Truncation of a 2-dimensional Fermi surface due to quasiparticle gap formation at the saddle points

We study a two-dimensional Fermi liquid with a Fermi surface containing the saddle points $(π,0)$ and $(0,π)$. Including Cooper and Peierls channel contributions leads to a one-loop renormalization group flow to strong coupling for short range repulsive interactions. In a certain parameter range the characteristics of the fixed point, opening of a spin and charge gap and dominant pairing correlations are similar to those of a 2-leg ladder at half-filling. An increase of the electron density we argue leads to a truncation of the Fermi surface with only 4 disconnected arcs remaining.

cond-mat.str-el↗
arXiv1998

Scaling relations in charge and spin excitations for (La,Sr)MnO3

Scaling relations in the charge and spin excitations of (La,Sr)MnO3 are studied from both theoretical and experimental points of view. In the ferromagnetic metal phase, we investigate optical conductivity and neutron inelastic scattering, and compare with a theoretical calculation based on the dynamical mean-field theory of the double-exchange hamiltonian. Spin and charge dynamics of (La,Sr)MnO3 exhibit typical behaviors of half metals. In these manganite compounds with high Curie temperature, various behaviors in spin and charge properties are explained by the double-exchange hamiltonian alone. Magnetoresistance of these compounds as well as other compounds with lower Curie temperature are also discussed.

cond-mat.str-el↗
arXiv1997

Scaling relation of spin wave lifetime in double-exchange systems

Spin excitation spectrum of CMR manganites are studied from both theoretical and experimental sides. Scaling relations for the spin wave lifetime in the electronic model with double-exchange interaction is observed, which is consistent with results for neutron inelastic scattering experiments for (La,Y)$_{0.8}$Sr$_{0.2}$MnO$_3$. Roles of other interactions and degrees of freedom such as polaron effects are also discussed.

cond-mat.str-el↗
arXiv1997

Anomalous Shift of Chemical Potential in the Double-Exchange Systems

Double-exchange system is investigated by the dynamical mean-field theory. We show that the chemical potential shifts as a function of temperature and magnetization, which is anomalously large. We also discuss the influences of dynamic Jahn-Teller effect to the shift of the chemical potential. Measurement of the shift of the chemical potential casts a constraint to theoretical approaches for the magnetoresistance phenomena in ($R$,$A$)MnO$_3$ such as double-exchange effects and dynamic Jahn-Teller effects. We also propose a method to measure the shift of $μ$.

cond-mat.str-el↗
arXiv1996

Critical Exponents of the Metal-Insulator Transition in the Two-Dimensional Hubbard Model

We study the filling-controlled metal-insulator transition in the two-dimensional Hubbard model near half-filling with the use of zero temperature quantum Monte Carlo methods. In the metallic phase, the compressibility behaves as $κ\propto |μ- μ_c|^{-0.58\pm0.08}$ where $μ_c$ is the critical chemical potential. In the insulating phase, the localization length follows $ξ_l \propto |μ- μ_c|^{-ν_l}$ with $ν_l = 0.26 \pm 0.05$. Under the assumption of hyperscaling, the compressibility data leads to a correlation length exponent $ν_κ= 0.21 \pm 0.04$. Our results show that the exponents $ν_κ$ and $ν_l$ agree within statistical uncertainty. This confirms the assumption of hyperscaling with correlation length exponent $ν= 1/4$ and dynamical exponent $z = 4$. In contrast the metal-insulator transition in the generic band insulators in all dimensions as well as in the one-dimensional Hubbard model satisfy the hyperscaling assumption with exponents $ν= 1/2$ and $z = 2$.

cond-mat↗
arXiv1996

Impurity Effect on Spin Ladder System

Effects of nonmagnetic impurity doping in a spin ladder system with a spin gap are investigated by the exact diagonalization as well as by the variational Monte Carlo calculations. Substantial changes in macroscopic properties such as enhancements in spin correlations and magnetic susceptibilities are observed in the low impurity concentration region, which are caused by the increase of low-energy states. These results suggest that small but finite amount of nonmagnetic impurity doping relevantly causes the reduction or the vanishment of the spin gap. This qualitatively explains the experimental result of Zn-doped SrCu$_{2}$O$_{3}$ where small doping induces gapless nature. We propose a possible scenario for this drastic change as a quantum phase transition in a spin gapped ladder system due to spinon doping effects.

cond-mat↗
arXiv1996

Magnetic and Transport Properties of (La,Sr)MnO$_3$

Magnetic and transport properties of the perovskite-type $3d$ transition-metal oxide (La,Sr)MnO$_3$ are theoretically studied using the double-exchange model in infinite dimension. Magnetoresistance properties as well as the magnetic transition temperatures are in good agreement with the experimental data.

cond-mat↗
arXiv1996

Spin Excitation Spectrum of La$_{1-x}A_x$MnO$_3$

As an effective model to describe perovskite-type manganates (La,$A$)MnO$_3$, the double-exchange model on a cubic lattice is investigated. Spin excitation spectrum of the model in the ground state is studied using the spin wave approximation. Spin wave dispersion relation observed in the inelastic neutron scattering experiment of La$_{0.7}$Pb$_{0.3}$MnO$_3$ is reproduced. Effective values for the electron bandwidth as well as Hund's coupling is estimated from the data.

cond-mat↗