| This thesis reports on the experimental research on physical properties of a unitary Fermi gas near superfluid phase transition during my doctoral study.On the basis of a new apparatus for preparing and manipulating quantum degenerate mixtures of lithium and dysprosium atoms,we have produced homogeneous unitary Fermi degenerate gases composed of 6Li atoms.With the improvement of Bragg spectroscopy and momentum-resolved radio frequency spectroscopy,we have made a further exploration on thermodynamic properties,sound waves,linear transport,spectral function,and pseudogap.In comparison with the harmonic trapping potential,the three-dimensional box potential has the advantage of producing quantum degenerate gases with uniform density,which facilitates the interpretation of experimental results and the test of theoretical models.In the establishment of the new laboratory,we integrated and developed many advanced methods of cooling atoms and finally achieved the state-of-the-art technique of preparing Fermi degenerate gases with uniform density.Moreover,we are able to prepare a unitary Fermi superfluid with Fermi energy of about hx50 kHz in which h is Planck constant,temperature precision better than 0.01 times of Fermi temperature,and density non-uniformity less than 2%,all of which are among the top in the world.Bragg spectroscopy plays an important part in the research on ultracold atoms.For quantum degenerate gases,the conventional Bragg spectroscopy focused on the total momentum imparted to the many-body system by Bragg lasers.The experimental results are not only broadened by the finite duration of Bragg lasers but also proportional to the wave number of the Bragg potential,which is not conducive to the study of longwavelength perturbation and responses.We have improved the Bragg spectroscopy so that the measurement of density waves of unitary Fermi superfluids directly gives the density-density response function.The basis is the linearly dissipative two-fluid model,from which we extracted not only speed but also attenuation rate of first and second sound.Thermodynamic functions such as energy and entropy were obtained,while the superfluid fraction is more accurate than the previous experimental result.Furthermore,we obtained transport coefficients such as shear viscosity,bulk viscosity and thermal conductivity independently.We found that the bulk viscosity of a unitary Fermi superfluid is almost zero,while both shear viscosity and thermal conductivity are close to the quantum limit nh and nhkB/m respectively,where n is the atomic number density,m is the atomic mass,h is the reduced Planck constant,and kB is the Boltzmann constant.We also found that the critical region of the superfluid phase transition of a unitary Fermi superfluid is about 100 times larger than that of superfluid helium,which provides an excellent foundation for further study of critical transport in the future.The momentum-resolved radio frequency spectroscopy can reveal the microscopic properties of quantum degenerate gases,especially the pairing between atoms.However,the three lowest Zeeman sublevels of the ground state of a 6Li atom all have very wide Feshbach resonances overlapping with each other.No matter which two states are selected to prepare unitary Fermi degenerate gases and the third state is used as the final state of the radio frequency transition,the final-state effect cannot be completely avoided.Moreover,the momentum distribution cannot be measured either.To overcome this obstacle,we chose a new Zeeman sublevel as the final state of the radio frequency transition,at the cost of requiring magnetic field to be extremely stable.We combined magnetostatic shielding,feedforward and current filtering to improve the stability of the magnetic field to the extent that experiments can be carried out successfully.With all these efforts,we measured radio frequency spectra of homogeneous unitary Fermi gases near the superfluid phase transition and succeeded in extracting momentum-resolved radio frequency spectra,which can be used to analyze the spectral function and pseudogap. |
