Peter Abbamonte (bottom) and student researcher Andrivo Rusydi.
Researchers from the US, the Netherlands, Japan, and Canada have found evidence of a hole crystal in a layered copper oxide material, Sr₁₄Cu₂₄O₄₁ or SCO [Abbamonte et al., Nature (2004) 431, 1078]. “A hole crystal is a very unusual phenomenon,” explains Peter Abbamonte of Brookhaven National Laboratory and SUNY Stony Brook. “Its existence is a direct result of the correlations between holes, which are believed to produce superconductivity in other cuprates.”
The structure of SCO consists of two different types of copper oxide sheets, described as chain and ladder layers because of their atomic arrangements. These sheets are stacked alternately and separated by Sr atoms. It is an intrinsically hole-doped material and, when alloyed with Ca and subjected to a pressure of 3 GPa, superconducts below 12 K. Without Ca, its electron transport properties indicate a carrier density that is modulated in real space.
Resonant X-ray scattering can distinguish between the chain and ladder layers in SCO. The researchers observed a reflection that indicates a standing wave in the hole density along the ladders without any lattice distortion. This can be interpreted as a crystallized state of holes in the ladder.
The hole crystal is of interest because proximity to a charge-ordered state is believed to be a general property of high-temperature superconductors. Other superconductors, for example, show ‘stripes’ of condensed holes in between regions of antiferromagnetism. The hole crystal could be another example. “We believe the hole crystal and stripes may be linked,” says Abbamonte. “Specifically, the hole crystal in SCO may be a ‘low-dimensional’ precursor to stripes, meaning it exists only along the copper oxide ladders, rather than in an entire copper oxide plane. Clearly, more research needs to be done to study these phases and their possible link to superconductivity,” he says.
As well as examining how the hole crystal is related to stripes in other cuprates, Abbamonte and coworkers plan to investigate how varying the chemical composition of SCO affects the hole crystal.