 The link between exact quantization and gauge invariance was subsequently found by Robert Laughlin, who connected the quantized conductivity to the quantized charge transport in Thouless charge pump.Most integer quantum Hall experiments are now performed on gallium arsenide heterostructures, although many other semiconductor materials can be used.In 2007, the integer quantum Hall effect was reported in graphene at temperatures as high as room temperature, and in the magnesium zinc oxide ZnO–Mg"x"Zn1−"x"O.