What limits the conductivity in a metal? 60 years after the Bloch-Wilson theory of solids, research is still uncovering surprises. The key quantity is the electron mobility, which measures the current carried per electron. In the conventional picture, the mobility at 4 Kelvin is limited only by impurity scattering (Panel A). Progress in purification over the decades has resulted in mobilities as high as 30 million cm2/Vs in the 2D electron gas (2DEG) in GaAs-AlGaAs quantum wells, 90 million cm2/Vs in ultrapure bismuth, and 1 million cm2/Vs in suspended graphene (Panel C).
Research on topological phases of matter sheds new light on this question. In topological insulators (e.g. Bi2Se3), the quantum properties at the surface can enhance the mobility by “protecting” the electrons against back-scattering events, which reverse their momentum (Panel A). Recently, strong interest has focused on a new class of materials called Dirac semimetals, which are 3D analogs of graphene (Fig. B). A group at Princeton reports  mobilities in Cd3As2 (9 million cm2/Vs) comparable to those seen in GaAs-AlGaAs and Bi, even though the Cd3As2 crystals show significant disorder. Although the mechanism that shields the electrons from back scattering in Cd3As2 is still not understood, the experiments show that it is rapidly suppressed when a magnetic field is applied. The research may lead to future applications in ultralow-dissipation electronics.
1. T. Liang et al., “Ultrahigh
mobility and giant magnetoresistance in a Dirac semimetal, Cd3As2,”
Nov. 24 2014, doi:
Research supported by U. S.
National Science Foundation and the Army Research Office.
Figure (A) Back scattering of electrons degrades mobility. (B) Schematic of the dispersion of the bulk Dirac states in Cd3As2. (C) Comparison of the mobilities (at 4 K) in Cd3As2, as the 2DEG in GaAs/A1GaAs quantum wells, in pure bismuth and graphene.