Computation of Stationary States for Rotating Bose-Einstein Condensates using Spectral Methods

Abstract: The Bose-Einstein condensate is a phase of matter that arises when cooling gases of bosons to extremely low temperatures. When studying these condensates one may use the Gross-Pitaevskii equation, which is a non-linear variant of the Schrödinger equation. An interesting phenomenon that arises when rotating a Bose-Einstein condensate is the appearance of vortices. We implement a semi-implicit Euler scheme using spectral methods proposed in [1] to numerically calculate the ground state of a rotating Bose-Einstein condensate. We start with implementing a simpler iterative fixed-point method to solve the Euler scheme but show that this method fails to converge for large rotations. Because of this we implement multiple Krylov subspace solvers that in fact do converge for large rotations and show that the Preconditioned Conjugate Gradient method has better performance than the BiConjugate Gradient Stabilized method. After the implementation we briefly look at the performance of the method and improve it with simple tricks that do not compromise the accuracy or robustness and which reduce the computation time slightly. Lastly we look at the formation of vortices in 2-dimensional and 3-dimensional Bose-Einstein condensates. We show that the number of vortices increases exponentially for increasing angular velocity in 2D until the condensate breaks apart, but in 3D we ultimately find that the required computation time and RAM storage is too large to be able to analyze the vortices in a similar way on our personal computers.