Anomalous temperature dependence of the effective mass inn-type PbTe

We have measured the cyclotron resonance in n-type PbTe in the temperature range between 12 and 340 K at wavelengths of 9.6 and 16.9 μm in the Faraday configuration (B∥k∥[111]) in ultrahigh magnetic fields up to 150 T. Samples of single-crystal n-type PbTe are grown on the ${\mathrm{BaF}}_2$(111) surface by the hot-wall technique. The effective masses measured around 20 K are explained well by the six-band model. However, the temperature dependence of the effective masses in the temperature range between 20 and 300 K is at least 2.6 times larger than the calculation taking into account the temperature dependence of the fundamental energy-band gap. The large discrepancy suggests that the conventional theory, which assumes the temperature-independent momentum matrix elements, should be reexamined, or may expose the applicable limit of k⋅p theory at finite temperatures.