A new type of low temperature conductivity in InSb doped with Mn

We investigated unusual low temperature transport properties of InSb single crystals doped with manganese to concentrations in the range of 1017<NMn<2*1017cm-3 which includes the critical concentration for metal-insulator transition Ncr= 2*1017cm-3. It was found that in the temperature range T=10÷1.5K the resisitivity had an unusual behavior described by an exponential quadratic function ρ ∼ exp(Δ1/kT)2 where Δ1 varied from Δ1=0.3meV to Δ1=0. Coexistence of electrons and holes at temperatures around the metal-insulator transition in p-InSb(Mn) crystals gives the ground to explain the unusual temperature - resistivity dependence in the framework of excitonic insulator phase model (L. V. Keldysh and Yu. V. Kopaev, Sov. Phys. Solid State 6, 2219 (1965)).