Efimov Effect in the Dirac Semi-metals.

Efimov effect refers to quantum states with discrete scaling symmetry and a universal scaling factor, and has attracted considerable interests from nuclear to atomic physics communities. In a Dirac semi-metal, when an electron interacts with a static impurity though a Coulomb interaction, the same scaling of the kinetic and interaction energies also gives rise to such a Efimov effect. However, even when the Fermi energy exactly lies at the Dirac point, the vacuum polarization of electron-hole pair fluctuation can still screen the Coulomb interaction, which leads to derivation from this scaling symmetry and eventually breakdown of the Efimov effect. This distortion of the Efimov bound state energy due to vacuum polarization is a relativistic electron analogy of the Lamb shift for the hydrogen atom. Motivated by recent experimental observations in two- and three-dimensional Dirac semi-metals, in this paper we investigate this many-body correction to the Efimov effect, and answer the question that under what condition a good number of Efimov-like bound states can still be observed in these condensed matter experiments.