Tunable goniopolarity of graphenelike boron layers in metal diborides

Abstract

The orientation-dependent conduction polarity, e.g., $p$-type conduction along one direction and $n$-type conduction along another direction, namely ``goniopolarity,'' observed in highly anisotropic materials brought us an alternative concept for novel electronic devices. But goniopolarity has hardly been utilized due to the rarity of goniopolar materials. Here, we associate goniopolarity with the saddle points in electronic band structure and the hyperbolic Fermi surface of materials and highlight the potential of layered metal diboride ($M{\mathrm{B}}_{2}$) materials in achieving this interesting phenomenon using first-principles calculations. We demonstrate that the electron-doped ${\mathrm{MgB}}_{2}$, ${\mathrm{CaB}}_{2}$, and ${\mathrm{SrB}}_{2}$ can exhibit remarkable orientation-dependent conduction polarity with opposite Seebeck coefficients which are detectable in verifying goniopolarity. We attribute the goniopolarity with the ${p}_{x,y}$ orbitals of the boron layers of these materials which constitute anisotropic electronic states along in-plane and out of plane directions. Moreover, their anisotropic Seebeck coefficients can be efficiently enhanced by applying tensile strain. This work offers a promising strategy for design of goniopolar materials and regulation of goniopolarity.