Fermi surface instability in topological materials
The quasi-two-dimensional bismuth layer-like AMnBi2 (A = alkaline as well as rare earth atom) lately advanced as an arena for the investigation of low-energy quasiparticle excitations in topological materials. The A = Sr or Ca compositions have attracted special attention because anisotropic Dirac cones may be realized. This latter property can be exploited for making new electronic devices with electrons propagating differently from one direction to the other. In a broader context, the title compound also provides an opportunity to study low-dimensional magnetism and its putative relationship to the electronic properties, a central topic in condensed matter. In this context, a transition at temperatures of about Ts ~ 50–100 K in the dc resistivity is very intriguing and it is possibly related to a spin-reorientation, for instance due to a spin-canting.
Our work describes novel results of reflectivity measurements from the far-infrared up to the ultraviolet that probe the optical response as a function of temperature. This gives access to the optical conductivity which captures the relevant energy scales shaping the electronic structure. We discover a depletion of spectral weight, for energy scales up to 0.2 eV, in the real part of the optical conductivity at T < Ts (Fig. 1.13), which signals the partial gapping of the Fermi surface and seems to directly affect the electronic properties at the Dirac cones. This may reveal the inclination towards a Fermi surface instability in topological materials, possibly related to a substantial reconstruction of the electronic structure upon the onset of the spin-canting at Ts.