  Scanning tunneling microscopy and transport measurements have been performed
to investigate the electronic structure and its temperature dependence in
heavily Sr and Na codoped PbTe, which is recognized as one of the most
promising thermoelectric materials. Our main findings are as follows: (i) Below
T=4.5 K, all carriers are distributed in the first valence band at the L point
(L band), which forms tube-shaped Fermi surfaces with concave curvature. With
Sr and Na doping, the dispersion of the L band changes, and the band gap
increases from 200 meV to 300 meV. (ii) At T=4.5 K, the Fermi energy is located
~100 meV below the edge of the L band for the Sr/Na codoped PbTe. The second
valence band at the Sigma point (Sigma band) is lower than the L band by 150
meV, which is significantly smaller than that of pristine PbTe (200 meV). The
decrease in the band offset, leading to band convergence, provides a desirable
condition for thermoelectric materials.(iii) With increasing temperature, the
carrier distribution to the Sigma band starts at T=100 K and we estimate that
about 50 percent of the total carriers are redistributed in the Sigma band at
T=300 K.Our work demonstrates that scanning tunneling microscopy and angular
dependent magnetoresistance measurements are particularly powerful tools to
determine the electronic structure and carrier distribution. We believe that
they will provide a bird's eye view of the doping strategy towards realizing
high-efficiency thermoelectric materials.
