Landau level spectrum and magneto-optical conductivity in tilted Weyl semimetal

We present a systematic investigation of the magnetoresponses of the Weyl points (WPs) with a topological charge of n=2,3, and 4, and with both linear and quadratic energy tilt. The linear tilt always tends to squeeze the Landau levels (LLs) of both conduction and valence bands of all the WPs, and eventually leads to LL collapse in the type-II phase. However, the quadratic energy tilt has more complex influences on the LLs of the unconventional WPs. For charge-n (n=2,4) WP, the influence of the quadratic tilt on the LLs of conduction and valence bands are opposite, i.e., if the LLs of conduction (valence) bands are squeezed, then that of the valence (conduction) bands are broadened and the squeezed LL spectrum will be collapsed in the type-III phase. But, the LL collapse generally cannot be found in the type-III charge-3 WP. Moreover, for charge-n (n=2,3) WP, the quadratic tilt breaks the degeneracy of the chiral LLs regardless of the direction of the magnetic field, leading to additional optical transitions and magneto-optical conductivity peaks at low frequencies. Interestingly, the four chiral LLs in charge-4 WP are always not degenerate. Hence there inevitably exist magneto-optical conductivity peaks at low frequencies for charge-4 WP. Since the density of state of the LL spectrum is very large, one can expect that the low-frequency magneto-optical responses in unconventional WPs would be significant and may be used for developing efficient terahertz photodetectors.