arXiv subjects
Publications and source records attributed to J. L. Roberts.
We study the evolution of ultracold plasmas by measuring the electron temperature. Shortly after plasma formation, competition between heating and cooling mechanisms drives the electron temperature to a value within a narrow range regardless of the initial energy imparted to the electrons. In agreement with theory predictions, plasmas exhibit values of the Coulomb coupling parameter $Γ$ less than 1.
We report extentions and corrections to the measurement of the Feshbach resonance in 85Rb cold atom collisions reported earlier [J. L. Roberts et al., Phys. Rev. Lett. 81, 5109 (1998)]. In addition to a better determination of the position of the resonance peak (154.9(4) gauss) and its width (11.0(4) gauss), improvements in our techniques now allow the measurement of the absolute size of the elastic scattering rate. This provides a new measure of the s-wave scasttering length as a function of magnetic field near the Feshbach resonance and contrains the Rb-Rb interaction potential.
The point of instability of a Bose-Einstein Condensate (BEC) due to attractive interactions was studied. Stable 85-Rb BECs were created and then caused to collapse by slowly changing the atom-atom interaction from repulsive to attractive using a Feshbach resonance. At a critical value, an abrupt transition was observed in which atoms were ejected from the condensate. By measuring the onset of this transition as a funstion of number and attractive interactions strength, we determined the stability condition to be N|a|/(aho)=0.461(12)(54), slightly lower than the predicted value of 0.574.
Bose-Einstein condensation has been achieved in a magnetically trapped sample of 85Rb atoms. Long-lived condensates of up to 10^4 atoms have been produced by using a magnetic-field-induced Feshbach resonance to reverse the sign of the scattering length. This system provides many unique opportunities for the study of condensate physics. The variation of the scattering length near the resonance has been used to magnetically tune the condensate self-interaction energy over a very wide range. This range extended from very strong repulsive to large attractive self-interactions. When the interactions were switched from repulsive to attractive, the condensate shrank to below our resolution limit, and after ~5 ms emitted a burst of high-energy atoms.
Inelastic collision rates for ultracold $^{85}$Rb atoms in the F=2 m$_{f}$=-2 state have been measured as a function of magnetic field. Dramatic change in the vicinity of a Feshbach resonance at 155 G was observed. Similar to the elastic rate, the inelastic rates show a high peak and a deep trough. Both two- and three-body processes are important, and individual contributions have been determined and compared with theory. This work has made it possible to create an $^{85}$Rb Bose-Einstein condensate with a highly adjustable scattering length.
A magnetic field dependent Feshbach resonance has been observed in the elastic scattering collision rate between atoms in the F = 2, M = -2 state of 85 Rb. Changing the magnetic field by several Gauss caused the collision rate to vary by a factor of 10,000, and the sign of the scattering length could be reserved. The resonance peak is at 155.2(4) G and its width is 11.6(5) G. From these results we extract much improved values for the three quantities that characterize the interaction porential: the van der Waals coefficient C6, the singlet scattering length, and the triplet scattering length.