Typically, electrons move through an electronic device like marbles rolling down a hill. A team of researchers were curious if it might be possible to alter a material ever so slightly to initiate the exotic quantum states locked within electrons. The results of their study are available in the June issue of the journal Nature Communications.
“In our work, we discovered that in ultra-thin materials electrons confined to one-dimensional channels can split into separate waves of charge and spin, moving independently of each other,” said Antonio Rossi, a researcher at the Italian Institute of Technology and co-first author on the study. “It’s as if a marble rolled down a wire and suddenly turned into two ripples traveling at different speeds, one electric and one magnetic.”
Engineering a thin film with exotic properties
Rossi joined his colleague John Thomas, senior scientific engineering associate at the Advanced Light Source (ALS), in engineering a thin film composed of a tungsten disulfide (WS2) matrix where one tungsten atom is sandwiched between two sulfur atoms. The WS2 film is grown on top of a single layer of carbon atoms (graphene), which sits on a silicon carbide wafer. The research team bombarded the three-atom-high construction with argon gas to insert one-dimensional defects in the thin film. These defects offered a pathway to explore a whole new world of electron behavior.
“When you shrink things down to the atomic scale, just a few atoms wide, electrons start behaving in strange and surprising ways,” said Rossi. “What’s especially exciting is that we found a way to engineer this behavior by stacking two materials in just the right way.”
The team performed measurements at the Molecular Foundry at Lawrence Berkeley National Laboratory (Berkeley Lab) using ultra-stable scanning tunneling microscopy and spectroscopy, a technique that illustrates the distribution of electrons on the surface at the atomic level within individual defects. They paired this analysis with quantum materials growth and electronic structure (MAESTRO) at Beamline 7.0.2 at the ALS at Berkeley Lab. This technique uses a high-flux, tunable photon source and high resolution electron energy analyzer to directly measure how electrons move through the material and how their energies are arranged. This cross-correlative endeavor provides atomic to nanoscopic level information about the origin of Tomonaga–Luttinger liquid (TLL) behavior.
Defying conventional behavior, electrons in TLLs form new collective entities, called quasiparticles, that have completely different properties. TLLs allow for the separation of an electron’s charge and spin, which travel independently and in fractional amounts. As a result, materials that are normally insulators can suddenly become superconductors or exhibit entirely new types of behavior. In addition, TLLs can propagate charge 10 to 100 times faster compared to conventional electronics, opening a unique solution to low-power electronics.
Materials with new commercial applications
In this study, the team found line defects in atomically thin materials that can host exotic quantum states. In particular, they found that direct contact with graphene enabled the formation of the TLL state. A small separation between the active thin film layer and the graphene was enough to suppress this behavior. This result shows that quantum states in low-dimensional systems can be highly sensitive to the material’s structure as well as its immediate environment.
While two-dimensional, material-based devices are still evolving at the research level, this insight opens up new opportunities to “turn on” or “turn off” correlated electronic states simply by choosing the right supporting material, which is both conceptually powerful and practically useful for material design.
“When you turn on a light in a room, press a button on a controller, or use a phone, you’re controlling the flow of electrons through wires or components,” said Thomas. “If you can better control where electrons go and, perhaps, how fast they move, one can design the next generation of devices.”
The research findings could contribute to the development of ultra-compact and energy-efficient technologies, such as quantum information platforms, low-power logic elements, or novel interconnects in integrated circuits.