Switching superconductivity on and off in materials

Materials World magazine
29 Oct 2019

Researchers have proved that an old scientific model for understanding material behaviour could, through simulation, enable superconductivity to be turned on and off again. Idha Valeur reports.

Superconductivity of certain materials can be changed based on a mathematical model, researchers at Stanford University, USA, and the Department of Energy’s SLAC National Accelerator Laboratory, found.

Based on the Hubbard model, which characterises the transition between conducting and isolating systems, the team ran simulations that revealed how it could be possible for superconductivity in copper-based materials, often called cuprates, to be activated. Results were published in the paper, Superconductivity in the doped Hubbard model and its interplay with next-nearest hopping t’, in Science in September 2019.

‘The big thing you want to know is how to make superconductors operate at higher temperatures and how to make superconductivity more robust. It’s about finding the knobs you can turn to tip the balance in your favour,’ Institute for Materials and Energy Sciences (SIMES) Director and co-author, Thomas Devereaux, said in a university release.

SIMES Staff Scientist and co-author, Hong-Chen Jiang, said this development has been a long time coming, as scientists previously believed the Hubbard model could also apply to copper-based, high-temperature superconductors, but that evidence was missing. ‘Because there are a number of competing states in these materials, we have to rely on unbiased simulations to answer these questions, but the computational problems are very difficult, so progress has been slow,’ Jiang said.

Using an algorithm called density matrix renormalisation group (DMRG) to simulate the old model, the team found that how an electron hopped on the constructed lattice could determine the relationship of charge stripes and superconductivity. If an electron hopped to its closest neighbour, there was no superconductivity, but the charge stripes became more powerful. Superconducting-state only appeared when the electrons hopped diagonally and the charge stripes weakened.

‘Our results indicate that a route toward stable, long-range superconducting order may lie in mechanisms that perturb the balance between various intertwined correlations,’ the paper stated. ‘Most likely, t’ alone is not solely responsible for depopulating filled charge stripes in real materials, as other factors such as longer-range hoppings, other orbital contributions, and dynamical lattice effects may also destabilise insulating charge stripes. Answering these open questions may lead to a better understanding of the robust superconductivity observed in cuprates’.