We report on a diluted magnetic semiconductor based on the Sb 2 Te 3 tetradymite structure doped with very low concentrations of vanadium ͑1-3 at. %͒. The anomalous transport behavior and robust magnetic hysteresis loops observed in magnetotransport and magnetic measurements are experimental manifestations of the ferromagnetic state in these materials. The p-d exchange between holes and vanadium 3d spins is estimated from the behavior of the magnetoresistance. A Curie temperature of at least 22 K is observed for Sb 1.97 V 0.03 Te 3 . This discovery offers possibilities for exploring magnetic properties of other tetradymite structure semiconductors doped with a wide range of 3d transition metals.

The temperature dependence of the resistivity and the Hall effect in the range 0.3-300 K, and the Shubnikov-de Haas effect have been investigated in Sn-doped p-(Bi 1Àx Sb x ) 2 Te 3 ð0 x 1:0Þ single crystals. Doping of (Bi 1Àx Sb x ) 2 Te 3 with tin showed that Sn exhibits acceptor properties in all crystals. The anomalous temperature and magnetic field behavior of the Hall coefficient was explained quantitatively by a model, which involves the complicated two-valence band structure of p-(Bi 1Àx Sb x ) 2 Te 3 . The quantization of the Hall resistivity r H in the form of plateaus in the dependence of r H on the magnetic field B is observed. The minima of the transverse magnetoresistivity r correspond to the start of plateaus. The oscillation of r H is due to the presence of a carrier reservoir. An impurity resonant band with a high density of states in Sb 2 Te 3 or the second lower valence band with a higher hole effective mass in (Bi 1Àx Sb x ) 2 Te 3 serves as the reservoir. The valence band structure of (Bi 1Àx Sb x ) 2 Te 3 is also discussed.

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The Hall effect and the Shubnikov-de Haas (SdH) effect have been investigated in magnetic fields up to 54 T in p-(Bi 1−x Sb x ) 2 Te 3 (0 x 1.0) Sn doped single crystals. Doping of (Bi 1−x Sb x ) 2 Te 3 with tin has shown that Sn exhibits acceptor properties in all crystals. We discuss the valence band structure of (Bi 1−x Sb x ) 2 Te 3 with the upper valence band (light hole band (LHB)), the lower valence band (heavy hole band (HHB)) and Sn-induced impurity band (IB). The Hall resistivity ρ H as a function of magnetic field shows quantization in the form of plateaus. The calculated Landau levels of the LHB with the best-fit parameters are in agreement with the experiment. The oscillation of ρ H is due to the presence of the carrier reservoir. The impurity resonant band with a high density of states or the HHB with a higher hole effective mass serve as the reservoir.

Single crystals of Hf-doped Sb2Te3 (CW= 0-2.4~10'~cm") were prepared from the elements of Sb, Hf, and Te of the 5N purity by a modified Bridgman method. The obtained crystals were characterized by the measurements of the reflectance R in the plasma resonance frequency region. Values of the plasma resonance frequency, op, were determined by fitting the reflectance spectra using the relations for the real and imaginary parts of the complex dielectric function, following from the Drude-Zener theory. From the observed changes in the value w,' it was concluded that the doping of Hfatoms into the crystal structure of Sb2Te3 results in a decrease in the concentration of free current carriers -holes. This conclusion was confirmed by the measurement of temperature dependencies of the Hall coefficient RH, electrical conductivity 0. and the Seebeck coefficient a. within the temperature range of 100-400K. Moreover, temperature dependences of the power factor a , 'and mobility g-RH.o of free carriers are discussed.