A 'checkerboard' electronic crystal state in lightly hole-doped Ca 2- x Na x CuO 2 Cl 2

• Published in: Nature Vol. 430; no. 7003; pp. 1001 - 1005
• Main Authors: Hanaguri, T, Davis, J. C, Lupien, C, Kohsaka, Y, Lee, D.-H, Azuma, M, Takano, M, Takagi, H
• Format: Journal Article
• Language: English
• Published: 26.08.2004
• ISSN: 0028-0836
• DOI: 10.1038/nature02861

The phase diagram of hole-doped copper oxides shows four different electronic phases existing at zero temperature. Familiar among these are the Mott insulator, high-transition-temperature superconductor and metallic phases. A fourth phase, of unknown identity, occurs at light doping along the zero-temperature bound of the 'pseudogap' regime. This regime is rich in peculiar electronic phenomena, prompting numerous proposals that it contains some form of hidden electronic order. Here we present low-temperature electronic structure imaging studies of a lightly hole-doped copper oxide: Ca2-xNaxCuO2Cl2. Tunnelling spectroscopy (at energies |E| > 100 meV) reveals electron extraction probabilities greatly exceeding those for injection, as anticipated for a doped Mott insulator. However, for |E| < 100 meV, the spectrum exhibits a V-shaped energy gap centred on E = 0. States within this gap undergo intense spatial modulations, with the spatial correlations of a four CuO2-unit-cell square 'checkerboard', independent of energy. Intricate atomic-scale electronic structure variations also exist within the checkerboard. These data are consistent with an unanticipated crystalline electronic state, possibly the hidden electronic order, existing in the zero-temperature pseudogap regime of Ca2-xNaxCuO2Cl2.