Influence of Magnetic Dopants on the Metal-Insulator Transition in Semiconductors

InSb:Mn and InSb:Ge reveal differences in their resistivity near the metal-insulator transition although both are acceptors of comparable depth. InSb:Ge shows the commonly observed behavior whereas InSb:Mn exhibits a strong enhancement of the resistivity below 10 K and pronounced negative magnetoresistance effects at 1.6 K. Both effects increase by applying hydrostatic pressure. The different behavior arises from the differences in the filling of the $3d$ shell, half filled $3d^5$ for Mn with a total spin of $S=5/2$ and entirely filled $3d^{10}$ for Ge with total angular momentum of $J=0$. The exchange interaction between the hole spin of the Mn acceptor and the $S=5/2$ spin of its $3d^5$ shell is the dominant correlation effect leading to the formation of an antiferromagnetic alignment of the Mn $3d^5$ spins along the percolation path which inhibits hopping of holes between neighboring Mn sites.

Figure 1

Arrhenius plot of the resistivity of InSb:Mn and InSb:Ge crystals for various hydrostatic pressures (ambient, 5.2 kbar both InSb:Mn and InSb:Ge, 12.2 kbar and 11.1 kbar for InSb:Mn and InSb:Ge, respectively). InSb:Mn shows a dramatic increase of the activation energy in contrast to InSb:Ge.

Figure 2

Magnetoresistance in (a) $p$-InSb:Mn and (b) $p$-InSb:Ge as a function of magnetic field shown for various pressures at $T=1.6\mathrm{K}$.

Figure 3

Schematic illustration of hopping between impurity (hole) levels along the percolation chain in (a) InSb:Ge (b) InSb:Mn at zero magnetic field and ambient pressure (c) InSb:Mn under the influence of hydrostatic pressure (d) InSb:Mn with applied magnetic field. For simplicity, we assume a hole spin $s=1/2$ in this illustration.