Activation conductivity and superconducting state in solid solutions (PbzSn1-z)0.8In0.2Te.

Abstract

We have studied temperature (1.5 K < T < 300 K) and magnetic field H < 5 T dependences of the electrical resistivity in bulk polycrystalline samples of semiconductor solid solutions (PbzSn1-z)0.8In0.2Te with various Pb content 0.1 ≤ z ≤ 0.9. The transition to the superconducting (SC) state was observed in samples with z ≤ 0.5 at helium temperatures with the critical transition temperature increasing up to Tc = 4.1 K with an increase of the Pb content up to z = 0.5. The bulk superconductivity in solid solutions with z ≥ 0.6 was not detected at temperatures T > 1.5 K. An exponential increase of the resistivity was observed in samples (PbzSn1-z)0.8In0.2Te with z ≥ 0.5 in the temperature range 40 K < T < 120 K. Experimental data was interpreted considering a shift of the energy position of the indium impurity band EIn in the complex valence band spectrum of (PbzSn1-z)0.8In0.2Te with changing lead content z. The exponential increase of the resistivity with decreasing temperature in studied solid solutions is observed when an energy barrier Ea appears between valence band states and quasilocal impurity states of In. The energy barrier increases with an increase of the Pb content in compounds with 0.5 ≤ z ≤ 0.8 and reaches maximum value Ea = 9.7 meV in the solid solution with z = 0.8. The SC state at helium temperatures was observed in (PbzSn1-z)0.8In0.2Te at z ≤ 0.4 when there is no energy barrier between zone and impurity states (i.e. the indium impurity band is located within the valence Σ-band), or at z = 0.5 when the activation energy Еа = 0.7 meV is less than the superconducting gap ΔS. Possible reasons for a decrease in the resistivity at T < 4.2 K in non-SC samples (PbzSn1-z)0.8In0.2Te with z ≥ 0.6 are also discussed.