Optical conductivity of tilted higher pseudospin Dirac-Weyl cones

Abstract

We investigate the finite-frequency optical response of systems described at low energies by Dirac-Weyl Hamiltonians with higher pseudospin $\mathcal{S}$ values. In particular, we examine the situation where a tilting term is applied in the Hamiltonian, which results in tilting of the Dirac electronic band structure. We calculate and discuss the optical conductivity for the cases $\mathcal{S}=1$, $3/2$, and $2$, in both two and three dimensions in order to demonstrate the expected signatures in the optical response. We examine both undertilted (type I) and overtilted (type II) as well as the critically-tilted case (type III). Along with the well-known case of $\mathcal{S} =1/2$, a pattern emerges for any $\mathcal{S}$. We note that in situations with multiple nested cones, such as happens for $\mathcal{S}>1$, the possibility of having one cone being type I while the other is type II allows for more rich variations in the optical signature, which we will label as type IV behavior. We also comment on the presence of optical sum rules in the presence of tilting. Finally, we discuss tilting in the $\alpha$-T$_3$ model in two dimensions, which is a hybrid of the $\mathcal{S}=1/2$ (honeycomb lattice) and $\mathcal{S}=1$ (dice or T$_3$ lattice) model with a variable Berry's phase. We contrast this model's conductivity with that of $\mathcal{S}=3/2$ and $\mathcal{S}=2$ as the resultant optical response has some similarities, although there are clear distinguishing features between the these cases.