Biomedical subjects

Hiroki Tutu

Publications and source records attributed to Hiroki Tutu.

3 recordsLinked to original sources

PubMed2004

Magnetic walls in the anisotropic XY-spin system in an oscillating magnetic field.

Wall structures associated with dynamic phase transitions in the anisotropic XY -spin system in a temporally oscillating magnetic field h cos (Omegat) in a one-dimensional system are analyzed by using the time-dependent Ginzburg-Landau model. It is numerically confirmed that there exist two types of magnetic walls, i.e., the Néel and Bloch walls, and is found that the transition between the two walls can occur for changing h or Omega . The phase diagram for the stable regions of each wall is obtained by both numerical and analytical methods. Furthermore, the critical behavior of the modulus of the Bloch wall around the Néel-Bloch transition point is studied, and it is found that the transition can be either continuous or discontinuous with respect to h, depending on Omega .

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PubMed2003

Ordering dynamics of one-dimensional Bloch wall system and domain size distribution function.

The dynamics of domain size distribution in the ordering process for a one-dimensional classical anisotropic XY model is studied with a reduced equation of motion for the assembly of domain sizes. The system possesses two types of the domain wall structures, the Néel or Bloch walls, depending on the strength of magnetization anisotropy. In the Néel wall situation the neighboring walls interact with one another in only an attractive way. On the other hand, in the Bloch one, these walls interact in either an attractive or a repulsive way depending on their chiralities. For the Bloch wall situation, we found that the domain size distribution is characterized by solitonlike translational motion with a function form h(y-y(t)) and a characteristic domain size y(t) for the domain size y. This is in contrast to that in the Néel wall situation, which can be described as a scaling-type distribution function g[y/l(t)]/l(t), as was obtained by Nagai and Kawasaki, with a certain scaling length l(t). We discuss why such a solitonlike motion appears instead of the scaling-type distribution function, show a proof for the absence of the scaling-type distribution, a qualitative estimation for the distribution function in the Bloch wall situation, and an analysis for the realization probability of a specified twistness.

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PubMed2002

Dynamic phase transitions in the anisotropic XY spin system in an oscillating magnetic field.

The Ginzburg-Landau model for the anisotropic XY spin system in an oscillating magnetic field below the critical temperature T(c), psi;(r,t)=(T(c)-T)psi-/psi/(2)psi+gammapsi(*)+ nabla (2)psi+h cos(Omegat) is both theoretically and numerically studied. Here psi is the complex order parameter and gamma stands for the real anisotropy parameter. It is numerically shown that the spatially uniform system shows various characteristic oscillations (dynamical phases), depending on the amplitude h and the frequency Omega of the external field. As the control parameter, either h or Omega, is changed, there exist dynamical phase transitions (DPT), separating them. By making use of the mode expansion analysis, we obtain the phase diagrams, which turn out to be in a qualitative agreement with the numerically obtained ones. By carrying out the Landau expansion, the reduced equations of motion near the DPT are derived. Furthermore, taking into account the spatial variation of order parameters, we will derive the analytic expressions for domain walls, which are represented by the Néel and Bloch type walls, depending on the difference of the coexistence of phases.

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