#### PC6-2

##### Simulation of Collective Motion of Vortex Using Molecular +Field Dynamics
###### *Jun Yamanaka1, Masaru Kato1
• Department of Physics and Electronics, Osaka Prefecture University

For applications of superconductors it is important to control vortices. However, dynamics of vortices in a dirty super conductor under an external current is still not clean. For example, Nishida et al. showed vortices move as bundles separated by glide plane of dislocation[1]. In order to clarify vortex dynamics ,we have developed “Molecular + Field Dynamics method.”

In a usual molecular dynamics (MD) method, an equation of motion for the i-th vortex is given by
ηdri/dt=fdi+fpi+fvi+ffi
where η is viscosity coefficient, ri is position of the vortex, fdi is a driving force by an external current, fpi is a pinning force due to an impurity potential, fvi is a repulsive force from other vortices, and ffi is a force due to thermal fluctuations.

In this method, we can only consider a few thousands vortices because of computation time of repulsive interactions between all points of vortices.

In order to treat more than ten thousands vortices, we consider a current field that consists of an external current and current around vortices.

The driving force from the external current and repulsive force from other vortices can be calculated locally as a current force fCi. Then the equation of motion becomes
ηdri/dt=fCi+fpi+ffi
The current field is obtained using the finite element method.In the simulations, a square superconductor was assumed, and the length of a piece was set to 5λ0 where λ0 is magnetic field penetration length.

Figure1 shows a vortex distribution from a simulation of 20000 vortices. Colors of vortices show direction of velocities of vortices.Using this method, we will show collective motions of vortices.

[1]N. Nishida, K. Hirata, H. Takeda and H. Takeya, Physica C 470(2010)S795-S796

Keywords: Molecular dynamics method, Vortex, Finite Element method