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We investigate single-particle properties of a mass-imbalanced Fermi gas inthe BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossoverregion. In the presence of mass imbalance, we point out that the ordinary$T$-matrix approximation, which has been extensively used to clarify variousBCS-BEC crossover physics in the mass-balanced case, unphysically gives adouble-valued solution in terms of the superfluid phase transition temperature$T_{\rm c}$ in the crossover region. To overcome this serious problem, weinclude higher order strong-coupling corrections beyond the $T$-matrix level.Using this extended $T$-matrix theory, we calculate single-particle excitationsin the normal state above $T_{\rm c}$. The so-called pseudogap phenomenaoriginating from pairing fluctuations are shown to be different between thelight mass component and heavy mass component, which becomes more remarkable athigher temperatures. Since Fermi condensates with hetero-Cooper pairs haverecently been discussed in various fields, such as exciton (polariton)condensates, as well as color superconductivity, our results would be usefulfor the further development of Fermi superfluid physics, beyond theconventional superfluid state with homo-Cooper pairs.

Excitation properties and effects of mass imbalance in the BCS-BEC crossover regime of an ultracold Fermi gas