Resonant Control Of Elastic Scattering Between²³Na⁴⁰K Molecules And ⁴⁰K Atoms

Feshbach resonance is a significant quantum effect in the scattering process.Feshbach resonance is useful for studying ultracold quantum gases.As an important physical process in the ultracold collision,Feshbach resonances between molecules and atoms are very complex.This dissertation presents the experimental research conducted during my doctoral study on the elastic collision of ultracold ²³Na⁴⁰K ground state molecules and ⁴⁰K atoms in the vicinity of the atom-molecule Feshbach resonances.The study consists of four parts.The first part involved an extensive investigation of the Feshbach resonance between ²³Na⁴⁰K molecules and ⁴⁰K atoms.Experimentally,we performed measurements in 25 collision channels in the range of 16G to 120G and observed 84 new atommolecule Feshbach resonances.Through theoretical analysis of these resonances,we determined the quantum numbers of the bound states and obtained an important conclusion that these resonances are caused by long-range bound states.This means that the resonant bound states may have similar characteristics to the incoming scattering states.The long-range nature of the resonant states allows the understanding of these resonances and provides the possibility of controlling molecule-atom interactions through Feshbach resonance.Similar resonances may occur in other systems of alkali-metal atom+diatomic molecule.The second part demonstrates the association of triatomic molecules near the Feshbach resonance through the radio-frequency association method experimentally.We also extracted the binding energy of the triatomic molecules near the resonance via the radio-frequency spectra.This work opens up an avenue toward the preparation of ultracold triatomic molecules and contributes to the understanding of the complicated Feshbach resonances between atom and molecule.The third part focused on the study of elastic collision between atoms and molecules near the Feshbach resonance.Studying the elastic scattering process between molecules and atoms near the Feshbach resonance is challenging due to serious inelastic loss,and it is uncertain whether the Feshbach resonance can be used to control the elastic scattering cross-section between molecules and atoms.To address this issue,we used resonant light to heat the atom cloud,driving the thermalization process of molecules and atoms while reducing the density of atoms and inhibiting inelastic loss.We extracted the elastic scattering cross-section from cross-species thermalization measurements.By studying the thermalization process caused by the elastic collision between atoms and molecules,we obtained the Fano profile of the elastic scattering cross-section changing with the magnetic field,which is the hallmark of Feshbach resonance.The key parameters that characterize the s-wave Feshbach resonance,including the resonance position,resonance width,and background scattering length,are also determined from the measured elastic scattering cross-sections.Lastly,a broad Feshbach resonance in the ²³Na⁴⁰K-⁴⁰K mixture was observed in both the elastic collision and inelastic collision.By characterizing the resonance through the measurements of the elastic scattering cross-sections and the loss rate coefficients,we found that the resonance had a large background scattering length and resonance width.Experimentally,we further investigated the elastic collision process of molecules and atoms at the magnetic field away from the resonance center and found that due to the high elastic collision rate,the center-of-mass oscillation of molecules and atoms in the optical dipole trap exhibited hydrodynamic properties.This work indicates that it is possible to use Feshbach resonance to achieve strongly interacting molecular-atomic fermion mixtures in the future.