Superfluidity in sympathetic cooling with atomic Bose-Einstein condensates

The dynamical structure of an atomic Bose-Einstein condensate limits the efficiency of the condensate in cooling slow impurity atoms. To illustrate the point, we show that an impurity atom moving in a homogeneous zero-temperature condensate is not scattered incoherently if its velocity is lower than the condensate sound velocity c, limiting cooling to velocities v greater than or equal to c. This striking effect is an expression of superfluidity and provides a direct means to detect the fundamental property of superfluidity in atomic condensates. Furthermore, we show that the fermionic lithium isotope, Li-6, is a reasonable candidate for sympathetic cooling by a Na-23 condensate.