At low temperatures, a material can either be insulating or electrically conducting. The difference is that in an insulator, if you add an extra electron, it will get trapped. That is why no current flows. In a conductor, extra electrons will immediately flow away to restore the equilibrium. The more conductive the material is, the faster this will happen.

Conductor or insulator?

This is why the research group led by Leiden physicist Milan Allan and involving UvA physicists Mark Golden and Yingkai Huang, were so surprised when they discovered charge trapping in the best possible conductor - a superconductor that has zero resistance.

How can charge trapping, a telltale sign of an insulator, be seen in a superconductor? The group found that the key to this apparent paradox is that the material is built up of two-atom thick layers of charge-trapping insulator in between the metallic layers that cause the whole material to enter a state of no resistance – an effect occurring below a temperature of 85 degrees above absolute zero.

Club sandwich

Together with Leiden theoretical physicist Jan Zaanen, examining single crystals that were grown by the UvA researchers, the group found that this ‘club-sandwich’ picture could unravel a long-standing mystery about the very anisotropic electrical transport in a family of materials called cuprates, which lose their resistance at relatively high temperatures, and are therefore called high-temperature superconductors.

The discovery was made possible by a tour-de-force experimental effort. Leiden PhD students Koen Bastiaans and Tjerk Benschop, together with postdoc Doohee Cho, spent two years building a new type of tunneling microscope. On top of measuring the average signal, it determines the temporal fluctuations of the signal —usually called the noise. These fluctuations indicated the trapping of electrons in the insulating layers. The microscope registers noise at the atomic scale, which was vital for the discovery. Bastiaans: ‘The noise centers only appear in very few, localized spots, as if some atoms are noisier than others. Our microscope helps us to understand not only these materials, but also the noisy impurity spots themselves.’

Reference

Charge trapping and super-Poissonian noise centres in a cuprate superconductor, K. M. Bastiaans, D. Cho, T. Benschop, I. Battisti, Y. Huang, M.S. Golden, Q. Dong, Y. Jin, J. Zaanen and M. P. Allan, Nature Physics (2018).