  In two-dimensional Dirac semimetals, massless Dirac fermions can form a
series of Efimov-like quasibound states in an attractive Coulomb potential. In
an applied magnetic field, these quasibound states can become bound states
around the Dirac point, due to the energy gap between two successive Landau
levels. Through the calculation of the energy spectrum directly, we show that
there exists a crossover from the Landau level to the bound state as the
magnetic field increases. Because of the decreasing magnetic length, the deep
quasibound states are pushed up to the Dirac point relatively and transformed
into bound states gradually. Furthermore, the magnetic positions of the
emerging bound states in the crossovers obey a geometric scaling law, which can
be regarded as an analogy of the radial law in the Efimov effect. We extend our
analysis to massless three-component fermions, which also demonstrate this
phenomenon. Our results show the universality of fermions with linear
dispersion in two dimension and pave a way for the future experimental
exploration.
