We study the existence of normalized ground states for the 3D dipolar Bose-Einstein condensate equation with attractive three-body interactions: \begin{align}\label{1} -Δu+βu+λ_1|u|^2 u+λ_2 (K*|u|^2)u-|u|^4u=0.\tag{DBEC} \end{align} When
λ2=0 or
u is radial, (\ref{1}) reduces to the cubic-quintic NLS \begin{align}\label{2} -Δu+βu+λ_1|u|^2 u-|u|^4u=0\tag{CQNLS}, \end{align} which has been recently studied by Soave in [31]. In particular, it was shown that for any
λ1<0 and
c>0, (\ref{2}) possesses a radially symmetric ground state solution with mass
c and for
λ1≥0, (\ref{2}) has no non-trivial solution. We show that by adding a dipole-dipole interaction to (\ref{2}), the geometric nature of (\ref{2}) changes dramatically and techniques as the ones from [31] cannot be used anymore to obtain similar results. More precisely, due to the axisymmetric nature of the dipole-dipole interaction potential, the energy corresponding to (\ref{1}) is not stable under symmetric rearrangements, hence conventional arguments based on the radial symmetry of solutions are inapplicable. We will overcome this difficulty by appealing to subtle variational and perturbative methods and prove the following: (i) If the pair
(λ1,λ2) is unstable and
λ1<0, then for any
c>0, (\ref{1}) has a ground state solution with mass
c. (ii) If the pair
(λ1,λ2) is unstable and
λ1≥0, then there exists some
c∗=c∗(λ1,λ2)≥0 such that for all
c>c∗, (\ref{1}) has a ground state solution with mass
c. Moreover, any non-trivial solution of (\ref{1}) in this case must be non-radial. (iii) If the pair
(λ1,λ2) is \textit{stable}, then (\ref{1}) has no non-trivial solutions.
We study analytically the ground-state stability of a Bose–Einstein condensate (BEC) confined in an harmonic trap with repulsive or attractive zero-range interaction by minimizing the energy functional of the system. In the case of repulsive interaction the BEC mean radius grows by increasing the number of bosons, instead in the case of attractive interaction the BEC mean radius decreases by increasing the number of bosons: to zero if the system is one-dimensional and to a constant minimum radius, with a maximum number of bosons, if the system is three-dimensional.
The partially attractive character of the dipole-dipole interaction leads to phonon instability in dipolar Bose-Einstein condensates, which is followed by collapse in 3D geometries. We show that in 2D, the nature of the post-instability dynamics is fundamentally different, due to the stabilization of 2D solitons. As a result, a transient gas of attractive solitons is formed, and collapse may be avoided. In the presence of an harmonic trap, the post-instability dynamics is characterized by a transient pattern formation followed by the creation of stable 2D solitons. This dynamics should be observable in ongoing experiments, allowing for the creation of stable 2D solitons for the first time ever in quantum gases.
We study the rotational properties of a dipolar Bose-Einstein condensate confined in a quasi-two-dimensional anisotropic trap for an arbitrary orientation of the dipoles with respect to their plane of motion. Within the mean-field approximation, we find that the lowest-energy state of the system depends strongly on the relative strength between the dipolar and the contact interactions, as well as on the size and the orientation of the dipoles and the size and the orientation of the deformation of the trapping potential.
We study the nonlinear Schrödinger equation arising in dipolar Bose--Einstein condensate in the unstable regime. Two cases are studied: the first when the system is free, the second when gradually a trapping potential is added. In both cases we first focus on the existence and stability/instability properties of standing waves. Our approach leads to the search of critical points of a constrained functional which is unbounded from below on the constraint. In the free case, by showing that the constrained functional has a so-called mountain pass geometry, we prove the existence of standing states with least energy, the ground states, and show that any ground state is orbitally unstable. Moreover, when the system is free, we show that small data in the energy space scatter in all regimes, stable and unstable. In the second case, if the trapping potential is small, we prove that two different kinds of standing waves appear: one corresponds to a topological local minimizer of the constrained energy functional and consists in ground states, and the other is again of mountain pass type but now corresponds to excited states. We also prove that any ground state is a topological local minimizer. Despite the problem being mass supercritical and the functional being unbounded from below, the standing waves associated to the set of ground states turn out to be orbitally stable. Actually, from the physical point of view, the introduction of the trapping potential stabilizes the system that is initially unstable. Related to this we observe that it also creates a gap in the ground state energy level of the system. In addition when the trapping potential is active the presence of standing waves with arbitrary small norm does not permit small data scattering. Eventually some asymptotic results are also given.