Ultrasonic Wave and the Analysis of Excitation Energy in Alkali Metals: Clogston-Chandrasekhar Limit

Ultracold alkali atoms refer to atoms such as lithium, sodium, and potassium. By applying a magnetic field, we can tune how the atoms attract or repel each other. We investigate the excitation energy of alkali atoms when the Clogston–Chandrasekhar limit is reached, leading to possible normal–superfluid phase separation. This separation occurs when the system is spin imbalanced. Spin in lithium-6 refers to its hyperfine states. Among the possible phase-separated states, we consider the case where an unpolarized superfluid component coexists with a partially polarized normal component. Unlike a conventional Fermi gas, the excitation energy in this system depends on several parameters that can be tuned by an external magnetic field. Then, using the second-order perturbation approach, the excitation energy is analyzed when the system is subjected to a weak ultrasonic wave. We assume that the frequency of the ultrasonic wave is lower than the breaking energy of each pair Using these results, we show that when the ultrasonic wave is applied, the energy absorption increases with increasing the average chemical potential. However, this is not due to the creation of new quasiparticles or quasiholes, since analysis of the dependence of the excitation energy on average chemical potential shows otherwise. Thermal quasiparticles already present in the system are responsible for the enhanced energy absorption as the average chemical potential increases.