Excitation spectrum and effective mass of the even-fraction quantum Hall liquid

To probe the nature of the even-fraction quantum Hair system, we have inves tigated the low-lying excitation spectrum by exact diagonalization for fini te systems. We have found (i) a striking one-to-one correspondence (i.e., a shell structure) between the spectrum and those for free (composite) fermi ons, (ii) a surprisingly straight scaling plot for the excitation energy th at gives a zero gap (metal) in the thermodynamic limit, (iii) the effective mass evaluated from the scaling becoming heavier for nu = 1/2, 1/4, 1/6, b ut (iv) some deviations from the single-mode or the Hartree-Fock composite fermion approximation.