Chromium Doping of the Topological Insulator Bi$_{\mathrm{1.5}}$Sb$_{\mathrm{0.5}}$Te$_{\mathrm{1.7}}$Se$_{\mathrm{1.3}}$

A major materials science challenge is to minimize bulk conductivity in topological insulators so that topological surface state physics can be cleanly accessed. One solution to this problem has been the development of quaternary bismuth chalcogenides Bi$_{\mathrm{2-x}}$Sb$_{\mathrm{x}}$Te$_{\mathrm{3-y}}$Se$_{\mathrm{y}}$ (BSTS) which can be tuned to place the mid-gap Fermi level near the Dirac point associated with the topological surface state. This is ideal for accessing interesting topological physics including the exotic magnetoelectric effects associated with breaking time reversal symmetry. With this goal in mind, we have grown Cr-doped crystals of Bi$_{\mathrm{1.5}}$Sb$_{\mathrm{0.5}}$Te$_{\mathrm{1.7}}$Se$_{\mathrm{1.3}}$ to assess the impact of magnetic dopants on the electronic and magnetic properties of this material. For 4 percent Cr doping we find electronic structure modifications measured by angle-resolved ultraviolet photoelectron spectroscopy and observe magnetic ordering below 50 K in bulk magnetometry. Higher doping levels show evidence of phase segregation.