Group Seminar via Zoom: Quantum droplet in a mixture of Na and Rb Bose-Einstein condensates

July 20, 2021

Zhichao Guo, CUHK
Group Seminar via video conference (Zoom)
Tuesday, July 20th, 9:00 am (MEZ)

Dealing with the unique situation of partial lock downs worldwide and home office solutions at our Institute due to the current spreading of the Covid 19 virus, we are now holding our group seminars and journal clubs via video conference.This procedure enables us to continue our research, enhance discussions and exchange important information.


According to the mean-field description, an atomic Bose-Einstein condensate (BEC) will collapse when the inter-particle interaction is attractive. However, the mixture of two BECs in the mean-field collapse region can be stabilized
by the beyond mean-field Lee-Huang-Yang(LHY) correction in the format of self-bound quantum droplets. This liquefaction mechanism is universal and has been confirmed not only in the double-BEC samples but also in the dipolar condensates. In this seminar, I will present our progress in studying the heteronuclear quantum droplet with a double BEC of Na and Rb. Thanks to an interspecies Feshbach resonance(FR) at 347 G, we can arbitrarily set Na-Rb scattering length without changing intraspecies interactions. When setting the sample to the weak MF collapse region, we observe the self-bound behaviour as the signature of the droplet, during the time of flight expansion upon releasing the mixture from the optical trap. We measured the liquid-gas phase diagram; however, a discrepancy of about 300 mG was found between measurement and numerical simulation. After careful compensation of magnet field gradient and calibration of atomic number, the gap is still there. So, we questioned the FR parameters of Na-Rb, which is measured previously by 3B-loss of atoms and by association method. In the second part of this seminar, I will focus on the calibration of the 347 G Na-Rb FR by the dissociation method. We measure the binding energies of weakly bound Feshbach molecules by dissociating them with magnetic field modulation. We use the binding energies to refine the singlet and triplet potential energy curves, using coupled-channel calculations. Then, the resulting potentials offer us a high-precision mapping between magnetic field and scattering length. Back to the droplet experiment, we match the experiment and theory well now. Finally, we investigate the LHY correction in a gaseous sample with near-zero MF energy. By measuring its expansion velocity, we obtain the release energy as a function of interaction strength. With a simplified variational calculation, we try to explain the abnormal expansion for the gaseous sample after vaporized from the droplet phase.

If you would like to join our group seminars via Zoom, please contact us for more information.

Go to Editor View