Abstract

High-temperature superconductivity in the cuprates arises from carrier doping into Mott insulators. Whereas the ground states of cuprates are phenomenologically conventional, the mysterious pseudogap excitation prevents us from fully understanding the electronic evolution. Although dozens of proposals to unify such complex states have been made, consensus is yet to be built. Therefore, detecting emergence of broken symmetry associated with the pseudogap state, from Mott insulating state upon carrier doping, is of fundamental importance to discern those proposals.

Scanning tunneling spectroscopy provides unique capability for this purpose. With this technique, we found bond-centered unidirectional electronic domains, which break both translational and rotational symmetries [1]. Later, these broken symmetries were turned out to be a characteristic at energies of the inhomogeneous pseudogap excitations [2]. Here we apply these mapping techniques to insulating Ca2-xNaxCuO2Cl2 (x = 0.06, 0.08) to elucidate evolution of broken symmetries with hole doping. We find that the broken symmetry patterns in the superconducting samples appear in virtually identical form in the insulating samples as well. However, density of such patterns is smaller in the insulating samples, and areas preserving the symmetry of crystalline lattice are found in between. Moreover, spectra in the former patterns are V-shaped pseudogap as observed in the underdoped superconducting samples, while those in the latter areas are U-shaped insulating ones. These results indicate that hole doping into the Mott insulating state causes the local broken symmetries with the pseudogap.

[1] Y. Kohsaka et al., Science 315, 1380 (2007). [2] Y. Kohsaka et al., Nature 454, 1072 (2008).