On Drude metallicity of 2- <i>Pmmn</i> borophene and possible emergence of bad metal

The metallic character of 2-Pmmn borophene is discussed within the minimal model Hamiltonian, which is derived from the ab initio band structure. Working on Boltzmann’s transport equation, which is solved within the relaxation time approximation, charge and heat transport with and without a magnetic field are determined with the aim of describing transport in a sample with random impurities and vibration of lattice atoms. By examining various observables, it is demonstrated that 2-Pmmn borophene represents a paradigmatic example of anisotropic Drude metal with signatures that remain robust in spite of impurity perturbation. Deviation from the ideal Drude picture into a bad metal, however, becomes increasingly apparent whenever the incoherence that arises from scattering between carrier and lattice vibration enhances. Their signatures particularly appear in the resistivity and optical conductivity, marked by the emergence of axis-selective metal, i.e., the coexistence of metal and bad metal, and ensued by non-metal characteristic. It is then argued that being a good metal, a supported borophene in the impurity dominated regime could find its use in the optical setup experiment as a photon reflector.