A `checkerboard' electronic crystal state in lightly hole-doped Ca_{2-x}Na_{x}CuO_{2}Cl_{2}

TL;DR

This study uncovers a checkerboard-like electronic crystal state in lightly hole-doped Ca_{2-x}Na_{x}CuO_{2}Cl_{2}, revealing a potential hidden electronic order within the pseudogap regime of copper oxides.

Contribution

It provides direct imaging evidence of a crystalline electronic state with checkerboard order in a lightly doped cuprate, suggesting a new form of hidden electronic order.

Findings

Observation of a V-shaped energy gap at low energies.

Detection of spatial checkerboard modulations in electronic states.

Evidence supporting a crystalline electronic order in the pseudogap regime.

Abstract

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, ofunknown 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: Ca_{2-x}Na_{x}CuO_{2}Cl_{2}. 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 CuO_{2}-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 Ca_{2-x}Na_{x}CuO_{2}Cl_{2}.