Abstract

Does a hidden ordered electronic state exist in the underdoped pseudogap regime of the cuprate phase diagram? This question has been the focus of intensive research worldwide. But after more than a decade of searches, no long range ordered state has ever been detected there.

Our spectroscopic imaging STM studies of Ca2-xNaxCuO 2Cl2 in that regime do reveal a 4a0X4a0 checkerboard pattern of density-of-state modulations which persists into the low doped superconducting region. But the actual charge density modulation in the checkerboard pattern is negligible.

So, to examine the doped-hole density in Ca2-xNaxCuO 2Cl2 directly, we have recently developed new Mottness imaging techniques . We find atomic-scale variations in hole-density (primarily on the oxygen sites) which have no long range order. Instead it looks like a hole glass with short range 4a0X4a0 correlations. Embedded in this hole glass are more ordered nano-regions of dimensions ~4a0X16a0. It seems plausible that the checkerboard density-of-state modulations are due to scattering of quasiparticles by the Coulomb potential of this 4a0X4a0 correlated hole glass.

I will discuss the relationship of these observations to results from other probes and argue that the electronic state in lightly-doped cuprates has not been hidden, merely overlooked. I will also discuss the likelihood that this type of hole glass is, in fact, the native state of lightly doped cuprates (when dopant disorder is present).