An unexpected finding by RIKEN researchers shows that hidden stripes in a crystal may be able to shed some light on the puzzling behaviour of electrons in some quantum systems, including high-temperature superconductors.
Most materials have very weak interactions between electrons. But in materials with high electron interactions, physicists frequently notice intriguing features. In these materials, the electrons frequently behave as particles when they are all together, creating “quasiparticles.”
Butler and colleagues looked at a crystal that contained a layer of nickel atoms organised in a chessboard-like square lattice. Despite having a tiny mass on their own, individual electrons in this crystal appeared as massless quasiparticles.
The scientists attempted to use a scanning tunnelling microscope to investigate this peculiar behavior, but this proved difficult. A roomful of machinery that generates extremely low temperatures and pressures similar to those at the surface of the moon surrounds the walnut-sized microscope, which is housed inside a vacuum chamber.
“To examine the pristine surface of these crystals, we try to cleave off a small flake, much as geologists do,” says Butler. “But we have to do this inside the vacuum, and these crystals are so brittle they are prone to explode into dust.”
They finally succeeded after multiple failed efforts and utilised the microscope to scan the flake using a tiny needle—similar to a record player with a voltage across it. They were able to investigate many aspects by altering the voltage.
The group verified that the arrangement of the nickel atoms resembled a checkerboard. The electrons, however, had violated this pattern and were now arranged in stripes, which surprised them. (Fig. 1). When system interactions cause the electrons to exhibit less symmetry than the underlying material, this is referred to as nematicity.
Butler likens the discovery to standing by a pond and throwing in a pebble. “You’d expect to see circular ripples, so if you saw ripples appearing in parallel lines, you would know something weird is going on,” he says. “It demands an explanation.”
These tests will allow scientists to compare several hypotheses for the behaviour of quantum systems with numerous particle interactions, including high-temperature superconductors. These new findings, for instance, are consistent with theories put out by the study’s co-authors from Nagoya University in Japan utilising a “density-wave” framework.
Butler claims that even with supercomputers, it is challenging to forecast the behaviour of many interacting electrons. But at least we can watch them in action through a microscope.
