Experimental Research Of ⁴¹K-⁶Li Bose-Fermi Quantum Degenerate Mixture

This thesis aims at describing our 41K-6Li ultracold atomic gas mixture system and several experimental researches on it.At the beginning,we firstly give some related background knowledge,including cooling and trapping of atoms,Feshbach resonance and Boson/Fermion properties,and so on,to help readers establish a preliminary cognition on neutral atom manipulations.Next,we systematically introduce our 41K-6Li mixture machine and key experimental techniques and results.Besides the conventional MOT,we develop two advanced sub-doppler cooling methods,UV MOT for 6Li and gray molasses for 41K,and the laser cooled temperatures of both atoms are reduced to about 50 ?K.After delivering the cold mixture to the science chamber with magnetic transfer,we apply the sympathetic cooling both in optical-plugged magnetic trap and disk-like optical dipole trap.In this way,we produce the 41K-6Li mass-imbalanced Bose-Fermi superfluid mixture for the first time.We then study the vortex lattice and collective excitation in this spectacular sys-tem.By laser stirring,we simultaneously create vortex lattices in both species.From the measurement of vortex formation and decay,we observe the significant influence of inter-species interaction.Collective excitation is an important tool to gain insight of trapped Bose and Fermi cloud.We take the explorations at both axial and radial di-rection for the first time.When the interaction goes through BEC-BCS crossover,the oscillation frequency shift display several unexpected features:Boson's frequency shift on axial and radial directions are opposite;There is a resonant-like behaviour at BCS side on both directions and both species.Considering the density alternation induced by large interspecies interaction,we propose an improved mean field model,however only a part of the data can be quantitatively described.We accidentally find the very broad d-wave shape resonance around 17 G on 41K|1>.The loss spectroscopy displays an obvious triplet structure,and varies seriously with temperature,as the theory predicts.Through applying resonant oscillatory mag-netic field,we measure the binding energy spectroscopy of all the three resonance split.The results are in perfect agreement with the predictions from Multichannel Quantum Defect Theory(MQDT).After the field is ramped across the resonance point,the cloud will have a long lifetime breathing mode oscillation.By analysing the oscillation ampli-tude,we confirm the existence of d-wave molecules.Besides,with the help of MQDT calculation,we take precise measurements on 41K broad d-wave resonances and 41K-6Li s-and p-wave resonances.With molecular spectroscopy technique,we precisely measure the closed-channel fraction Z of 6Li Feshbach molecule from unitary point to deep BCS region.There is only one experiment work before this,in which the data analysis is too simple and mea-surement accuracy is limited due to the small atom number and not good molecular laser frequency lock.Two channel model tells us that Z depends not only on the interaction strength,but also on the system Fermi temperature(TF).We firstly improve the scheme of molecular laser lock and a stability of less than 1 MHz is acquired.We verify the linear dependence between Z and(?).As the interaction goes to deep BCS region,the discrepancy between measurement and theoretical predictions becomes larger and larger.Finally,we observe the real-time order-parameter dynamics across a superfluid phase transition in a strongly interacting Fermi gas near unitarity.Performing a quench in the confining potential for the trapped atomic ensemble,we observe spontaneous vor-tex generation,that goes beyond the description of conventional Kibble-Zurek mech-anism,which we attribute to the intermediate pseudo-gap region of strongly attractive fermions.At longer-timescale,we find universal decay dynamics of vortices following a power law.The vortex-decay time shows distinct behaviors in the weak and strong pairing regions,owing to the interplay of diffusion and vortex annihilation effects.This observation makes the superfluid formation time in the fermionic coarsening dynamics largely tunable in the system with Feshbach resonance techniques.