  The HgTe quantum well (QW) is a well-characterized two-dimensional
topological insulator (2D-TI). Its band gap is relatively small (typically on
the order of 10 meV), which restricts the observation of purely topological
conductance to low temperatures. Here, we utilize the strain-dependence of the
band structure of HgTe QWs to address this limitation. We use
$\text{CdTe}-\text{Cd}_{0.5}\text{Zn}_{0.5}\text{Te}$ strained-layer
superlattices on GaAs as virtual substrates with adjustable lattice constant to
control the strain of the QW. We present magneto-transport measurements, which
demonstrate a transition from a semi- metallic to a 2D-TI regime in wide QWs,
when the strain is changed from tensile to compressive. Most notably, we
demonstrate a much enhanced energy gap of 55 meV in heavily compressively
strained QWs. This value exceeds the highest possible gap on common II-VI
substrates by a factor of 2-3, and extends the regime where the topological
conductance prevails to much higher temperatures.

Materials Science