Unusual Shubnikov–de Haas oscillations in BiTeCl

We report measurements of Shubnikov--de Haas oscillations in single crystals of BiTeCl at magnetic fields up to 31 T and at temperatures as low as 0.4 K. Two oscillation frequencies were resolved at the lowest temperatures, F₁=65±4 T and F₂=156±5 T. We also measured the infrared optical reflectance [R(ω)] and Hall effect; we propose that the two frequencies correspond respectively to the inner and outer Fermi sheets of the Rashba spin-split bulk conduction band. The bulk carrier concentration was nₑ≈1×10¹⁹0.28em0excm^-3 and the effective masses m₁*=0.20m₀ for the inner and m₂*=0.27m₀ for the outer sheet. Surprisingly, despite its low effective mass, we found that the amplitude of F₂ is very rapidly suppressed with increasing temperature, being almost undetectable above T≈4 K.