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Khaliullin, G.*; Mori, Michiyasu; Toyama, Takami; Maekawa, Sadamichi
no journal, ,
Sakai, Toru; Nakano, Hiroki*
no journal, ,
Magnetization process of the S=1/2 isotropic Heisenberg antiferromagnet on the Kagome lattice is studied. Data from numerical-diagonalization method up to 39-spin systems, are reexamined from the viewpoint of the derivative of the magnetization with respect to the magnetic field. We find that the behavior of the derivative around the 1/3 height of the magnetization saturation is quite different from the cases of typical magnetization plateaux. The magnetization process of the Kagome-lattice antiferromagnet reveals a new phenomena, which we call the "magnetization ramp". We also compare it with the 1/3 magnetization plateau of the triangular antiferromagnet.
Adachi, Hiroto; Uchida, Kenichi*; Saito, Eiji; Oe, Junichiro; Takahashi, Saburo; Maekawa, Sadamichi
no journal, ,
Ota, Yukihiro; Machida, Masahiko; Koyama, Tomio*
no journal, ,
We theoretically study the macroscopic quantum tunneling (MQT) in a hetero Josephson junction formed by a conventional single-gap superconductor and a multi-gap one such as 
and iron-based superconductors. We clarify the quantum dynamics of the phase differences and construct a theory for the MQT in the multi-gap Josephson junctions. The dynamics of the phase differences are strongly affected by the Josephson-Leggett mode, which is the out-of-phase oscillation mode of the phase differences. The escape rate is drastically enhanced when the frequency of the Josephson-Leggett mode is less than the Josephson-plasma frequency.
Matsuo, Mari; Okamoto, Satoshi*; Koshibae, Wataru*; Mori, Michiyasu; Maekawa, Sadamichi
no journal, ,
Gu, B.; Ziman, T.*; Guo, G.-Y.*; Nagaosa, Naoto*; Maekawa, Sadamichi
no journal, ,
Machida, Masahiko; Nakamura, Hiroki
no journal, ,
Since the discovery of the iron-based superconductor, a large discrepancy between experimental observations and first-principles calculations in the magnetic moment of the antiferromagnetic state of the parent compounds has been intensively debated. The observed moment values are about 3 to 5 times smaller than those of the calculation although there is a variety of the difference depending on the materials. Very recently, an interesting calculation data fully reproducing the observed low moment has been suggested by F. Cricchio et al., (Phys. Rev. B 81 (2010) 140403) who performed first-principles calculations using a LDA+U scheme. In this study, we suggest that the new state is a possible candidate to well explain the other data, e.g., strong anisotropy in spatial patterns measured by STM and magnetic excitations found by neutron scattering. Furthermore, we compare the result with other theoretical works reproducing similar low moment in terms of orbital ordering.
Nakamura, Hiroki; Machida, Masahiko
no journal, ,
Since the discovery of the iron-based superconductor, LaFeAsO
F
whose Tc reached 26 K, various types of iron-based superconductors have been fabricated to attain higher 
. Recently, it is reported that Tc of an iron-based superconductor LaFeAsO
is enhanced to 35 K by doping hydrogen. This result implies that atoms of light elements penetrate into the crystal of iron-based superconductors and transform their structures into more useful ones for superconductivity. In this talk, we investigate how the light elements are doped in the iron-based superconductors by using the first-principles density functional theory. Furthermore, we evaluate the effects of doping on the crystal structures and electronic states and explore the origin of the enhancements.
Yamamoto, Atsushi; Yamada, Susumu; Machida, Masahiko
no journal, ,
We present single-particle excitation spectra of ultracold fermions in one-dimensional(1D) optical lattices by using dynamical density-matrix renormalization group method. In an analysis of weakly-interaction regimes, we find that the spectrum structure changes from a typical Hubbard band as obtained from periodical 1D lattice to band branching as increasing the trap potential, and finally, we observe clear discrete bound-state levels. On the other hand, in case of strongly-interacting regimes, we confirm the multiple flat bound-state levels lying above 1D Tomonaga-Luttinger (TL) liquid spectrum on a central Mott-plateau phase surrounded by metallic regions. Furthermore, we also investigate spectral changes as a metallic state partially emerges at the center region and find one-dimensional TL spectrum breakdown with an emergence of a new dispersive band due to the central metal portion.