They have published their findings in Nature Physics.
The university writes:
A new direction that has recently emerged in graphene research is to try to modify graphene’s electronic properties by combining it with other similar materials in multilayered stacks. This creates an additional landscape for electrons moving through graphene and, therefore, its electronic properties can change strongly.
The University of Manchester scientists have used capacitance measurements to probe these changes. They found that in combination with a magnetic field this creates numerous replicas of the original graphene spectrum. This phenomenon is known as the Hofstadter butterfly but it is the first time that well developed replica spectra have been observed.
The researchers found a wealth of unexpected physics in this new system. For example, the Hofstadter butterflies turned out to be strongly contorted, very different from the theoretical predictions. This happens because electrons feel not only the landscape but also each other, which modifies the butterfly.
The paper, 'Hierarchy of Hofstadter states and replica quantum Hall ferromagnetism in graphene superlattices' by G. L. Yu, R. V. Gorbachev, J. S. Tu, A. V. Kretinin, Y. Cao, R. Jalil, F.Withers, L. A. Ponomarenko, B. A. Piot, M. Potemski, D. C. Elias, X. Chen, K.Watanabe, T. Taniguchi, I. V. Grigorieva, K. S. Novoselov, V. I. Fal’ko, A. K. Geim and A. Mishchenko, is available on request from the University Press Office.