The lines in the crystal hint at the behavior of electrons in a quantum system
(Nanowerk News) Hidden lines in crystals can help scientists understand the mysterious behavior of electrons in certain quantum systems, including high-temperature superconductors, according to an unexpected discovery by physicist RIKEN (Proceedings of the National Academy of Sciences, “Correlation-driven electronic nematicity in Dirac semimetal BaNiS2“).
Electrons in most materials interact with each other very weakly. But physicists often observe interesting properties in materials where electrons interact strongly with each other. In these materials, the electrons often collectively behave as particles, giving rise to ‘quasiparticles’.
“Crystals can be thought of as alternative universes with different physical laws that allow different fundamental particles to live there,” said Christopher Butler of the RIKEN Center for Emergent Matter Science.
Butler and colleagues studied crystals in which layers of nickel atoms were arranged in a square lattice, like a chessboard. Individual electrons have little mass, but within these crystals, they appear as massless, pseudo-particles.
The team set out to examine this strange effect using a scanning tunneling microscope, but this proved challenging. A walnut-sized microscope is housed in a vacuum, surrounded by a chamber full of equipment that creates low temperatures and extremely low pressures comparable to those on the surface of the Moon.
“To examine the pristine surface of these crystals, we try to split small pieces of debris, as geologists do,” said Butler. “But we have to do this in a vacuum, and these crystals are so fragile that they easily explode into dust.”
After many attempts, they succeeded and used a microscope to scan the flakes with a tiny needle—like a record player—with voltage across it. Varying the voltage allowed them to investigate different features.
The team confirmed the nickel atoms were arranged in a checkerboard-like arrangement. But to their surprise, the electrons had broken this pattern and were streaked instead (Fig. 1). This is called nematicity — where interactions within the system make the electrons less symmetrical than the matter beneath them.
Butler likened the discovery to standing by a pond and throwing pebbles. “You’d see circular ripples, so if you saw ripples appear in parallel lines, you’d know something strange was going on,” he said. “That demands an explanation.”
Such experiments will help physicists test various proposed theories for the behavior of quantum systems with many particle interactions, such as high-temperature superconductors. These new results, for example, match predictions made using the ‘wave density’ framework proposed by study co-authors at Nagoya University in Japan.
The behavior of many interacting electrons is difficult to predict even with supercomputers,” said Butler. “But at least we can observe what they do under a microscope.”