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Superconductivity enters a new dimension

04 May 2021 Isabelle Dumé
Photo of Carolina Parra in the laboratory with two colleagues
As a postdoc at Stanford University, Carolina Parra (centre) carried out the experiments that led to the visualization of these intriguing results. (Courtesy: Carolina Parra)

Two-dimensional “puddles” of electrons that form inside a three-dimensional superconductor could be a way for some superconductors to reorganize themselves before undergoing an abrupt phase transition into an insulating state. The phenomenon, dubbed “inter-dimensional superconductivity” by the researchers who discovered it, might make it easier to fabricate 2D materials for electronics applications.

Superconductors are materials that, when cooled to below their superconducting transition temperature, Tc, can conduct electricity without any resistance. In the Bardeen-Cooper-Schrieffer (BCS) theory of conventional superconductivity, this occurs when electrons overcome their mutual repulsion and form so-called Cooper pairs that travel unimpeded through the material as a supercurrent.

The first superconductors to be discovered (beginning with solid mercury in 1911) had transition temperatures only a few Kelvin above absolute zero, meaning that expensive liquid helium coolant was required to keep them in the superconducting phase. Beginning in the late 1980s, however, a new class of “high-temperature” superconductors with Tat liquid nitrogen rather than liquid helium temperatures began to emerge. These materials were not metals but insulators made of copper oxides, or cuprates.

“The other high-temperature superconductor”

In the new work, researchers led by Hari Manoharan of Stanford University and the Stanford Institute for Materials and Energy Sciences (SIMES) at the US Department of Energy‘s SLAC National Accelerator Laboratory studied a somewhat similar material: a bismuthate known as BPBO that has the chemical formula BaPb1-xBixO3. Manoharan explains that unlike the cuprate high-temperature (d-wave) superconductors, BPBO is believed to be a conventional (s-wave) superconductor that behaves according to BCS theory. “BPBO does, however, have higher Tthan other conventional superconductors,” he tells Physics World. “In fact, it is sometimes called ‘the other high-temperature superconductor’.”

During experiments aimed at pinning down the temperature at which BPBO becomes an insulator – a point known as the superconducting-insulator transition (SIT) – the researchers observed that the electrons in the material behaved as if they were confined to ultrathin 2D layers or stripes. This was unexpected behaviour, since BPBO is a 3D superconductor in which electrons can move in any direction.

Further investigations with a scanning tunnelling microscope (STM), which can directly image individual atoms in the top few atomic layers of a material, revealed that the stripes were domains (or “puddles”) that formed in the material at its SIT. These domains were separated by distances short enough to allow the electrons in them to interact and coherently couple (pair up).

Emergent behaviour

The observation closely matches the predictions of so-called “emergent electronic granularity” theory, which is specific to 2D materials. This theory was first articulated by Nandini Trivedi of Ohio State University, US, and it describes what happens when superconducting domains on the scale of the material’s coherence length (one of the characteristic parameters for describing superconductors) are embedded in an insulating matrix and coupled via a phenomenon known as Josephson tunnelling.

Usually, the stronger the superconductor, the greater the energy required to break the bonds between its electron pairs. This bond-breaking energy is known as the “energy gap” and is related to the material’s coherence length. Trivedi and her colleagues predicted, however, that in certain disordered types of superconductor, like BPBO, the opposite would be true: the system would form emergent domains in which superconductivity was strong, but the pairs could be broken with much less energy than expected.

Collective reorganization

Trivedi says that it is “quite thrilling” to see her predictions being confirmed by the STM measurements from the Stanford group. These observations suggest that the electrons in a 3D superconductor collectively reorganize themselves into a 2D granular state before the material ultimately transforms to an insulator.

The results, which are detailed in PNAS, could have implications for crafting 2D materials, says team member Caroline Parra, who now heads the Nanobiomaterials Laboratory at the Universidad Técnica Federico Santa María, Valparaíso, Chile. “Most methods for fabricating 2D materials rely on growing films a few atomic layers thick or creating a sharp interface between two materials and confining a 2D state in these materials. The new finding offers an additional way to reach these 2D superconducting states.”

The only tricky part, she explains, would be to make sure that the composition of the superconducting material was just right.

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