Giant negative piezoresistance effect in copper-doped germanium

We have observed a stress-induced decrease of over ten orders of magnitude in the low-temperature electrical resistivity of copper- doped germanium single crystals. The application of large uniaxial stresses in a direction leas to a change in the copper ground- state wavefunction from the highly localized (1s){sup 3} to the much more extended (1s){sup 2}(2s){sup 1} configuration. We attribute the decrease in the resistivity to impurity band conduction by the 2s - holes of the high pressure configuration.