Origin of the checkerboard phases in KxC60 fullerides and high Tc cuprates revealed by STM

 

Yayu Wang

Department of Physics, Tsinghua University

 

One of the main themes in condensed matter research is to unravel the mechanisms of various phases and phase transitions in novel quantum materials. Scanning Tunneling Microscopy (STM) has been a powerful technique in this aspect owing to its ability to map out the atomic scale real space structure and electronic structure simultaneously. In this talk I will discuss two distinct systems: monolayers of KxC60 fullerides and the cuprate high Tc superconductors, where a checkerboard-like pattern has been observed. In KxC60, we demonstrate that the metal-insulator transition from K3C60 to K4C60 is caused by intra-molecular Jahn-Teller effect, and the accompanying checkerboard structure is due to inter-molecular electron hopping. In the cuprates, our results suggest that the checkerboard most likely originate from the charge-density-wave (CDW) formation in the anti-nodal region of the Fermi surface. The possible implications to the pseudogap phase and superconductivity will be discussed. These results illustrate the intricate interplay between the electronic, orientational, and vibronic (lattice) degrees of freedom in novel low dimensional materials.