Recent experiments published in Nature challenge our understanding of electron behavior in quantum materials. Researchers have used stacked layers of tungsten ditelluride to observe electrons behave in two dimensions as though they were in one dimension. This has led to the creation of what the researchers call a new electronic state.
Sanfeng Wu (assistant professor of physics at Princeton University) said, "This is really an entirely new horizon." He was also the senior author. "We were able to create a new electronic phase with this experiment--basically, a new type of metallic state."
The current understanding of interactions electrons within metals can be described using a theory that is compatible with both two- and three-dimensional systems. However, it breaks down when we try to describe the interaction of electrons in one dimension.
Wu stated that this theory "describes the majority of the metals we know." It states that electrons within metals, even though they are strongly interconnected, should behave as free electrons except that their values may differ in certain characteristic quantities such as the mass or magnetic moment.
This "Fermi fluid theory" is not applicable to one-dimensional systems. Instead, it gives way to "the Luttinger theory," which describes the interaction of electrons.
Wu stated that "Luttinger liquid theory is a good starting point for understanding the interactions electrons in one dimension." "Electrons in one-dimensional lattices are so closely correlated that they don't act like free electrons in a sense."
L.D. Landau, the Nobel Prize winner for physics, first proposed the Fermi liquid theory. Landau. Before being widely accepted by physicists, Luttinger's theory underwent a lengthy process of refinement. Shinichiro Tomonaga, a Japanese Nobel Prize winner, proposed a theoretical model in 1950s. It was later independently developed by J.M. Luttinger later in 1963.
Luttinger's solution was not satisfactory. In 1965, Princeton mathematician Elliott Lieb became the Eugene Higgins Professor Emeritus of Physics. He eventually found the correct solution. F. Duncan Haldane (Princeton's Sherman Fairchild University Professor) of Physics was another Nobel Prize winner and physicist who used the model to study the interaction effects of one dimensional metals. Haldane was the first to use the term "Luttinger fluids" and provided the basis for modern theory of Luttinger Liquids, which is a general description of one-dimensional metals.
These two theories, the Fermi liquid theory (or the Luttinger theory) have been key to understanding the behavior of electrons within condensed matter Physics.
There have been indications that electron interactions are more complicated than the simple classification. In the 1990s, Philip Anderson, another Nobel Prize winner, and Princeton physicist suggested that there could be "exotic" cases where the behavior of electrons within two-dimensional systems could, in rare instances, also match the predictions of Luttinger fluid theory. Anderson asked if electrons in two-dimensional systems could be explained by Fermi liquid theory. However, Anderson thought that these electrons might behave counterintuitively as a Luttinger fluid, just as they would in a one-dimensional system.
This was only a hypothetical. Wu stated that there were no experiments that could have been connected to these unusual cases.
Wu and his team found that electrons in a two-dimensional structure were able to suddenly behave like predicted by the Luttinger liquid theory when they are cooled to very low temperatures. They were acting as correlated electrons in one-dimensional states.
Researchers used a semimetal called tungsten telluride (WTe2) to carry out their experiment. Semimetals are compounds that have intermediate properties, which place them between metals or insulators. Princeton researchers Leslie Schoop (assistant professor of Chemistry) and Robert Cava (Russell Wellman Moore Professor in Chemistry), created the finest quality tungsten ditelluride crystals. Wu's team created single atomic layers from this material and then stacked two of them vertically for the study.
Pengjie Zhang, co-first author and postdoctoral research assistant, said that they stacked monolayers made of tungsten ditelluride on top one another. This created a rectangular lattice known as a moire, which is similar to a French textile design.
The original intention of the team was to examine how the twist angle affected other types quantum phenomena in thetungsten ditelluride. They were astonished at what they discovered.
Wang stated, "At first we were confused at the results." It turned out to be correct.
Researchers observed that electrons began to cluster strongly in a linear array, rather than acting naturally. This was indicative of electrons in one-dimensional systems.
Wu stated, "What we have is a two-dimensional metallic phase that is not described in the Fermi liquid theory." "For the first-time, we have discovered a completely new electronic state of matter in two dimensions as described by the Luttinger liquid theorem."
Guo Yu, coauthor of the paper, is a graduate student at electrical and computer engineering. He described the material's properties as being remarkably switchable. They can be either uniform in all directions (isotropic), or they can vary strongly in physical properties when measured from different directions (anisotropic).
Yu stated, "What's unique about our twisted bilayer-tungsten ditelluride is that unlike most monolayer materials and the moire superlattices, our sample is highly anisotropic. This is crucial for hosting the one-dimensional Physics."
Wu warned that although a new metallic phase may sound appealing, it is still preliminary research. He said that further research is needed before such applications are possible.
Wu remains optimistic about the future. He said, "This could help open up new windows to look at novel quantum phase of matter." This research will yield many new discoveries over the next few years.