Here is a list of our recent publications (published after December 1998).

Stripes in Manganites |

T. Hotta, E. Dagotto, H. Koizumi, and Y. Takada |

Int. J. Mod. Phys. B 14, 3494-3499 (2000) |

Topological aspects of the stripe structure in manganese
oxides are discussed in terms of the "winding number" w associated with
the Berry-phase connection for e_{g }orbitals acquired by the parallel
transport through the periodic array of Jahn-Teller centers. For La_{1-x}Ca_{x}MnO_{3}, w is shown to characterize both the three-dimensional spin-charge-orbital
structures in the antiferromagnetic phase for x > 1/2 and the charge-orbital
stripes in the two-dimensional ferromagnetic phase for x < 1/2. |

Self-Energy Revision Operator Theory for the Many-Body Problem: Application to Dynamic Properties of the Electron Gas |

Yasutami Takada |

Int. J. Mod. Phys. B 15, 2595-2610 (2001) (Plenary
Talk at SCES-Y2K) |

An approximation scheme is proposed for implementing the algorithm to obtain the exact self-energy in the many-body problem [Phys. Rev. B {\bf 52}, 12708 (1995)]. This scheme relates the self-energy revision operator ${\cal F}$, the key quantity in the algorithm, with $f_{xc}(q,\omega)$ the frequency-dependent exchange-correlation kernel appearing in the time-dependent density functional theory. We illustrate this scheme by applying it to the calculation of dynamic properties of the electron gas at metallic densities. |

Concluding Remarks on CMR and Related Problems |

Y. Takada and T. Hotta |

Int. J. Mod. Phys. B 15, 4267-4270 (2001) (Summary
Talk at SCES-Y2K) |

We review the talks on the colossal mangetoresistance (CMR) and its related subjects addressed at the Conference, together with a very brief explanation of our recent work. We also suggest some problems left in the future. |

Comment
on "Charge-Orbital Stripe Structure in La_{1-x}Ca_{x}MnO_{3}
(x=1/2,2/3)" |

T. Hotta, E. Dagotto, H. Koizumi, and Y. Takada |

Phys. Rev. Lett. 86, 2478 (2001) |

Comments on the paper by T. Mutou and H. Kontani [Phys.
Rev. Lett. 83, 3685 (1999)]. |

Structural phase transition at high temperatures in solid molecular hydrogen and deuterium |

T. Cui, Y. Takada, Q. Cui, Y. Ma, and G. Zou |

Phys. Rev. B 64, 024108: 1-7 (2001) |

We study the effect of temperature up to 1000K on the structure of dense molecular para-hydrogen (p-H$_{2}$) and {ortho-deuterium (o-D$_{2}$), using the path-integral Monte Carlo method. We find a structural phase transition from orientationally disordered hexagonal close packed (hcp) to an orthorhombic structure of Cmca symmetry before melting. The transition is basically induced by thermal fluctuations, but quantum fluctuations of protons (deuterons) are important in determining the transition temperature through effectively hardening the intermolecular interaction. We estimate the phase line between hcp and Cmca phases as well as the melting line of the Cmca solid. |

Polaron in the Jahn-Teller system and its superconductivity |

Y. Takada |

Physica C 364-365, 71-73 (2001) (Plenary Talk
at New3SC-3) |

We approach superconductivity in a crystal composed of the $E \otimes e$ Jahn-Teller centers from the weak-coupling region. We find that the vertex correction is less important in this system than in the usual electron-phonon coupled systems due to the presence of rotational symmetry in the pseudospin space representing twofold degeneracy. This ineffectiveness of the vertex correction results in the expansion of the applicable range of the conventional Migdal-Eliashberg theory. |

Inclusion of Vertex Corrections in the Self-Consistent Calculation of Quasiparticles in Metals |

Y. Takada |

Phys. Rev. Lett. 87, 226402: 1-4 (2001) |

Based on the Bethe-Salpeter equation and the Ward identity derived from it, we provide a scheme for constructing the vertex function in the self-consistent iteration loop to determine the electron self-energy. The scheme is implemented in the homogeneous electron gas at the sodium density. |

Geometric Phase Current in Solids:Derivation in a Path-Integral Approach |

H. Koizumi and Y. Takada |

Phys. Rev. B 65, 153104: 1-4 (2002) |

We consider a path-integral representation for the propagator
of a Bloch electron with band crossings. The geometric phase arising from
the crossings is included through the modification of the matrix element
for the unitary transformation from Bloch to Wannier bases. We adopt the
semiclassical approximation to the path integral to obtain a fundamental
expression for the electron velocity, based on which we derive an anomalous
current, named geometric phase current, of Bloch electrons induced by the
geometric phase connection in k-space. |

Dynamical Structure Factor of the Homogeneous Electron Liquid: Its Accurate Shape and the Interpretation of Experiments on Aluminum |

Y. Takada and H. Yasuhara |

Phys. Rev. Lett. 89, 216402: 1-4 (2002) |

Based on a highly self-consistent theory maintaining
the exact functional relations between the self-energy and the vertex part,
we evaluate the dynamical structure factor S(q,w) of the electron
liquid. We find striking deviations from S(q,w) in the RPA for |q|
larger than the Fermi wave number; besides a broad peak in the one-pair
excitation region as seen in the RPA, a clear shoulder appears along a
steepened slope at low w due to electron-hole multiple scattering and a
flattened structure follows due to inseparable interference between one-
and multi-pair excitations. Our result agrees with experiments on Al on
the whole. The remaining discrepancy is ascribed to the band-structure-induced
effect. |

Possibility of a Metallic Phase in the CDW-SDW Crossover Region in the One-Dimensional Hubbard-Holstein Model at Half Filling |

Y. Takada and A. Chatterjee |

Phys. Rev. B 67, 081102 (R) : 1-4 (2003) |

The Hubbard-Holstein model is studied at half-filling in one dimension using a variational method based on the variable-displacement Lang-Firsov canonical transformation and the exact solution to the Hubbard model due to Lieb and Wu. It is usually believed that the system undergoes a direct insulator-to-insulator transition from charge-density wave (CDW) to spin-density wave (SDW) with the increase of the on-site Coulomb repulsion $U$ for a given strength of the electron-phonon coupling. Here we show indications that, at least in the antiadiabatic region, an intervening metallic state may exist in the crossover region of the CDW-SDW transition. |

Quantum Fluctuations in a Four-Body Coulomb System and Breakdown of the Adiabatic Approximation |

Y. Takada and T. Cui |

J. Phys. Soc. Jpn. 67, 081102 (2003) |

Accuracy of the Born-Oppenheimer adiabatic approximation
to the ground state of a hydrogenlike molecule (M^{+}M^{+}m^{-}m^{-}^{)}
is examined in comparison with the exact results obtained by diffusion
Monte Carlo simulations, fully incorporating quantum fluctuations. For
the mass ratio m/M<0.1, the relative error in the ground-state
energy is found to be less than 1%. We discuss the change in nature of
the binding mechanism of this system with the increase of m/M from
the usual chemical bonding at m/M << 1 to the one in which
nonadiabatic effects such as the retardation of electron response to proton
motion play a crucial role. |

The Hubbard-Holstein Model with Anharmonic Phonons in One Dimension |

A. Chatterjee and Y. Takada |

J. Phys. Soc. Jpn. 67, 081102 (2004) |

Effects of cubic and quartic anharmonic phonons on the
polaronic properties are investigated in the one-dimensional Hubbard-Holstein
model at half filling, using the variable-displacement Lang-Firsov canonical
transformation and the exact solution of Lieb and Wu. Although the quartic
anharmonicity always reduces the effect of the Coulomb repulsion U,
the cubic one mainly responsible for the thermal expansion of lattice brings
about the effect of strong asymmetric nature in the sign of the electron-phonon
coupling constant g; it enhances the effect of U for positive
g, while it suppresses for negative g.
As a result, the overall features of the anharmonic system are basically
similar to those in the harmonic one for positive
g, but even in
the range of the realistic magnitudes for those anharmonicities, they may
become qualitatively so different for negative g as to provide a first-order
phase transition. |

We no longer publish our preprints in the form of usual ISSP Reports in order to save our paper resources. Instead, we frquently revise this page to upload our new preprints. Those who wish to obtain our upload information through e-mail are kindly requested to email me.