  The paper presents a metamaterial for ballistic electrons, which consists of
a quantum barrier formed in a semiconductor with negative effective electron
mass. This barrier is the analogue of a metamaterial for electromagnetic waves
in media with negative electrical permittivity and magnetic permeability.
Besides applications similar to those of optical metamaterials, a nanosized
slab of a metamaterial for ballistic electrons, sandwiched between quantum
wells of positive effective mass materials, reveals unexpected conduction
properties, e.g. single or multiple room temperature negative differential
conductance regions at very low voltages and with considerable peak-to-valley
ratios, while the traversal time of ballistic electrons can be tuned to larger
or smaller values than in the absence of the metamaterial slab. Thus, slow and
fast electrons, analogous to slow and fast light, occur in metamaterials for
ballistic electrons.
