Magnetic properties and pseudogap phenomenon in an ultracold Fermi gas with population imbalance

We discuss the magnetic properties of an ultracold Fermi gas with population imbalance. In the unpolarized case, the photoemisson spectroscopy have observed a gap-like (pseudogap) structure in the normal state above the superfluid phase transition temperature, such an anomalous structure has not been detected in the highly-polarized regime. In this talk, we discuss how the poseudogap phenomenon is affected by the polarization of the system. Within the framework of an extendend $T$-matrix theory, we calculate the polarization dependence of DOS to show that the pseudogap gradually disappeas with increasing the polarization rate. In a highly-polarized regime, the system is simply described as a gas of long-lived quasiparticles. We also show that the calculated polarization as a function of an effective ``magnetic'' field $h=(\mu_\uparrow - \mu_\downarrow)/2$ agrees well with the experimental data [where $\mu_\sigma$ is the chemical potential of atoms with pseudospin $\sigma(=\uparrow, \downarrow)$].