Spin Susceptibility Of Asymmetric Fermi Gas

The special physical phenomena that the Fermi system exhibits at low temperature are the hot issues in the field of physics.In recent years,the continuous improvement and advancement of experimental techniques have experimentally realized molecular Bose-Einstein condensation and Fermion superfluidity.When the Fermi system is in the external magnetic field,the external magnetic field will affect the spin direction of the particles and makes the system show a certain magnetism.In order to reveal the magnetic phenomena and related mechanisms of quantum systems,the physics of the Fermi system with asymmetric spin orientation is a very important physical problem.In this paper,the Fermi gas with asymmetrical spin component is taken as the research object to investigate the behavior of the spin susceptibility under extreme conditions.The thesis is divided into two parts.In the first part,we mainly study the spin susceptibility of free Fermi gas.Using the finite temperature field and linear response theory,the spin susceptibility of the fermion system is obtained.Then the spin susceptibility with the Sommerfeld expansion method at low temperature was developed.At the same time,the spin susceptibility was expanded at the high temperature Boltzmann region.The spin susceptibility is consistent with the Pauli paramagnetic result at the equilibrium limit of spin components.For the fermion system with unbalanced spin component,we investigate in detail the effect of spin polarization on the spin susceptibility.The results show that the spin susceptibility decreases with increasing spin polarization.When the spin polarization tends to 1,the spin susceptibility approaches zero.We further study the effect of temperature on the spin susceptibility.The results reveal that the spin susceptibility decreases when the temperature increases.In the second part,we study the important effect of the interaction on the spin susceptibility.For interacting quantum systems,there are usually two methods of calculating spin susceptibility in the literature,i.e.,through the calculation of the incompletely polarized tensor and the fully polarized tensor.We use the linear response method to further compare the difference between the spin susceptibility obtained respectively from the incompletely polarized tensor and the fully polarized one.The calculated results show that for the strong interaction Fermi systems,the result given by the incompletely polarized tensor tends to be negative and the value obtained by the completely polarized tensor better reflects the actual physics.This research can play a guiding role in understanding the spin susceptibility of ultracold Fermi gas.