Observation of the Castaing instability in a trapped ultracold Bose gas

Ultracold trapped atomic systems are ideal model systems to investigate physical phenomena in the quantum regime. In this work we studied instabilities in spin dynamics of a sample of nondegenerate trapped $^{87}$Rb gas. This is the first experimental investigation of the so-called Castaing instability in such systems. The Castaing instability is an instability in the spin dynamics of spin-polarized systems as a result of introducing sharp spin gradients in their spin profile. We used an optical technique via the ac Stark effect to initialize arbitrary spin profiles in a sample of evaporatively cooled nondegenerate $^{87}$Rb gas. The experimental results manifest evidence for the presence of the Castaing instability in both transverse and longitudinal spin channels. The results agree reasonably well with theoretical studies and results of a numerical solution to the spin transport equation. Some imperfection in the longitudinal spin gradient due to the spin-state preparation technique makes the signature of the instability less clear in the transverse channel. We propose another preparation technique to overcome this shortcoming in future experiments.