Temperature-induced modification of the Dirac cone in the tetradymite topological insulator Bi2Te2Se

The thermal excitation of electrons to higher, unoccupied states leads in certain cases to the paradox situation that the chemical potential needs to be shifted to lower energies. Here, a manipulation of Dirac fermions through the temperature dependence of the chemical potential is analyzed while maintaining sufficient insulating character in the bulk. The appearance of a bulk conduction band and consequently, a remarkable energy shift of the Dirac point is observed in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{2}\mathrm{Se}$ at low temperatures (20 K), compared to the high temperatures (200--300 K), as revealed by hard x-ray photoelectron spectroscopy as well as momentum-resolved photoelectron microscopy. The temperature-induced shift of the Dirac cone and the appearance of bulk bands are related to the paradoxical shift of the chemical potential. The experiments are completely reversible, i.e., repeated cooling and heating of the sample up to room temperature recovers the original spectra. Such bulk related energy shifts must be considered not only in photoelectron spectroscopy but also in analysis of other measurable physical quantities and while designing devices for applications.