Lee Chang

Double-layer Bose-Einstein condensates with a large number of vortices

In this paper we systematically study the double-layer vortex lattice model, which is proposed to illustrate the interplay between the physics of a fast rotating Bose-Einstein condensate and the macroscopic quantum tunneling. The phase diagram of the system is obtained. We find that under certain conditions the system will exhibit a phase transition which is a consequence of the competition between interlayer coherent hopping and the interlayer density-density interaction. In one phase the vortices in one layer coincide with those in the other layer. In another phase two sets of vortex lattices are staggered, and as a result the quantum tunneling between two layers is suppressed. To obtain the phase diagram we use the quantum Hall mean field and Thomas-Fermi mean field theories. Two different criteria for the transition taking place are obtained, which reveals some fundamental differences between these two mean-field states. The sliding mode excitation is also discussed.

Skyrmion excitation in a two-dimensional spinor Bose-Einstein condensate

We study the properties of coreless vortices (skyrmions) in spinor Bose-Einstein condensates. We find that this excitation is always energetically unstable, it always decays to an uniform spin texture. We obtain the skyrmion energy as a function of its size and position, a key quantity in understanding the decay process. We also point out that the decay rate of a skyrmion with high winding number will be slower. The interaction between skyrmions and other excitation modes are also discussed.

Magnetization quantum tunneling at excited levels for a biaxial spin system in an arbitrarily directed magnetic field

The quantum tunneling of the magnetization vector between excited levels are studied theoretically in single-domain ferromagnetic nanoparticles with biaxial crystal symmetry placed in an external magnetic field at an arbitarily directed angle in the ZX plane. The temperature dependences of the tunneling frequency and the decay rate are clearly shown for each case.

Spin tunnelling of trigonal and hexagonal ferromagnets in an arbitrarily directed magnetic field

The quantum tunneling of the magnetization vector are studied theoretically in single-domain ferromagnetic nanoparticles placed in an external magnetic field at an arbitrarily directed angle in the

Quantum nucleation in ferromagnets with tetragonal and hexagonal symmetries

ZX

Resonant quantum tunnelling and coherence of the Néel vector for different crystal symmetries

plane. We consider the magnetocrystalline anisotropy with trigonal and hexagonal crystal symmetry, respectively. By applying the instanton technique in the spin-coherent-state path-integral representation, we calculate the tunnel splittings, the tunneling rates and the crossover temperatures in the low barrier limit for different angle ranges of the external magnetic field (

Macroscopic magnetization tunneling and coherence in antiferromagnetic particles

\theta_{H}=\pi/2

Ferromagnetic Instanton Interference in Magnetic Fields — Detailed Calculation of Tunneling-Rate

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Topological phase interference induced by a magnetic field along hard anisotropy axis in nanospin systems with different crystal symmetries

\pi/2\ll\theta_{H}\ll\pi

Resonant quantum coherence of the Néel vector in a magnetic field applied along the medium axis

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