| Ultra-cold atomic gas has the characteristics of controllable interaction,clean,and rich freedom degrees.It is an ideal system for studying the quantum behavior of matter.In the field of ultra-cold atomic physics,the research on ultra-cold Fermi gas has also developed vigorously with the continuous progress of experimental technology.Especially in recent years,a series of novel quantum phenomena have been discovered,including superfluid phase transition of Cooper pair,scale invarance during expansion,matter wave solitons and so on.These phenomena exist in different systems and involve some basic physics problems in the intersection of condensed matter physics,particle physics,and atomic and molecular physics.By using Feshbach resonance technology,people can easily adjust the interactions of ultra-cold Fermi gases,which provides a way to study the properties of strong interaction Fermi gases,especially the atoms in the BCS-BEC crossover.The 6Li atom is a perfect system for studying the strong correlation effect,because the magnetic field that regulates the interaction of the 6Li atom has a large range of 300 Gauss.In addition,6Li is a fermi gas,with a small three-body loss rate and long lifetime.In the strong interaction regime,the system is stable,which provides a good experimental condition for studying the strong interaction systems.This dissertation mainly introduces the production of degenerate Fermi gases of 6Li atoms and studies the anisotropic expansion of the strong interaction Fermi gas,the three-body recombination loss process,and the formation process of ultra-cold molecular BEC and the physical properties of atoms in the BEC-BCS regime.Firstly,we design and build up of the experimental platform for the study of degenerate Fermi gas,including vacuum system,laser system,magnetic field system,imaging system,control system and data manipulation.The vacuum in the science chamber reaches 3×10-9 Pa,and the lifetime of atoms in a single optical dipole trap is more than 25 s.The laser system includes a resonant laser part of 671 nm and a dipole laser part of 1064 nm.The magnetic field system includes MOT magnetic field,compensation magnetic field,Zeeman decelerating magnetic field and Feshbach magnetic field.The vertical imaging system is composed by two lenses with a resolution of about 8 ?m.The control system was realized by writing programs with Labview software to control the Pxie6738 and Pxie7858R boards of NI Company.At the same time,Labview software was also used to write the data manipulation programs,which was mainly to convert the ACSII code of the image acquired by the CCD into common constants such as the size and number of atoms.Secondly,a magneto-optical trap of 6Li atoms was realized,loaded 1×109 atoms in 20 s.After compressing the magneto-optical trap,the number of atoms was 5×108,and the temperature was 500 ?K.In order to improve the loading efficiency of atoms into optical dipole trap,sub-Doppler cooling of 6Li atoms was carried out,after then the temperature of the atoms was reduced to 57 ?K,the remaining atom number was 3×108,and the phase space density reached 6.2×10-5.The number of atoms loaded in the crossed optical dipole trap has been increased by nearly four times.Thirdly,the Feshbach magnetic field control circuit was designed,the current in the Feshbach coil was controlled by MOSFET whose resistance will change with the different voltage on the G?S terminal.By changing the reference voltage of the PID circuit the magnetic field can be scanned from 0 Gauss to 1000 Gauss.During the scanning process,an analog signal was added to the external control port of the large current source to control the power supply voltage output,which can guarantee the MOSFET works below the rated power.At the same time,a high-field detection laser was prepared by frequency offset locking technology.By the calculation of Zeeman splitting with the magnetic field we got relation between the reference voltage of the PID input(the product of the current in the coil and the sampling resistance)and the magnetic field.Nearly 1.2×106 atoms were loaded into the crossed optical dipole trap.When the power of dipole trap is reduced to P=5.8 mW,the total number of atoms in the two components was 7.1 × 104 and the temperature is T/TF=0.11.Fourth,the scattering length was adjusted by controlling the magnetic field.The anisotropic expansion was studied in different scattering lengths and sign;the relationship between the three-body recombination process and the atomic temperature and magnetic field were studied;we got the molecular Bose-Einstein condensate either by directly evaporation or scanning the magnetic field and observed the progress of BEC-BCS crossover. |
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