The unexpected pleasures of a theoretical physicist

Prof. Efrat Shimshoni from the Department of Physics at BIU joined BINA in 2015. She develops theories and makes predictions, searching for clues in an attempt to understand physical phenomena. Here, Shimshoni offers a sneak peek into those rare transcendent events in the career of a theoretical physicist whose ultimate goal is to advance the quest for finding new quantum phases of matter.

Prof. Efrat Shimshoni attained her undergraduate degree at the Hebrew University of Jerusalem, and her MSc and PhD degrees at the Weizmann Institute. Following a postdoc at the University of Illinois at Urbana-Champaign, she received an academic appointment at the University of Haifa, and later at Bar Ilan, where she is presently a Professor at the Physics Department.

Since early in her career, Shimshoni has studied the effects of quantum phenomena on the physical properties of materials. She focuses on how interactions among electrons affect the formation of novel phases of matter. “One of our main objectives is to find materials that a slight turn of a dial can significantly change their conductance properties”, she explained. “We would like to be able to predict their behaviour in advance”.

Theoretical physicists use mathematical tools and fundamental concepts to extract insights on natural phenomena. They typically work together with experimental physicists who perform lab research and measure how different conditions affect certain materials. Then they compare the results to theoretical models. Shimshoni reveals that an exciting moment in her line of work is when something unexpected occurs. Something as minor as a puzzling lab measurement would prompt her to look for a scientific explanation that could lead potentially to a surprising discovery.

Huge breakthrough on the tip of a pencil

Graphene is a honeycomb-shaped crystal of carbon atoms which form a membrane – a single atom thick. These ultra-thin layers stacked on top of each other are the building blocks of graphite − the material pencils are made of. In 2010 Andre Geim and Konstantin Novoselov were awarded jointly the Nobel Prize in Physics for their ground-breaking achievement separating graphite into individual layers, through an ingenious use of scotch-tape. This allowed them to measure its conductance properties and observe phenomena in a lab, which until then could be done only in particle accelerators. Carbon, the basis of all known life on earth, had surprised scientists once again.

"It had been known for a long time that electrical conductivity is the result of movement of electrons – particles with an electric charge and a mass", said Shimshoni. "Interestingly, however, the electrons in graphene behave as ‘massless’ particles. Their laws of motion can be described using the Dirac equation, a theoretical framework combining the principles of Einstein’s theory of special relativity with quantum mechanics. Accordingly, under the influence of electromagnetic fields, electrons in graphene react very differently than electrons in ordinary conductors. They behave more like light rays, though their velocity is much lower than the speed of light", she explained.

“Like many theorists, I was intrigued by the potential implications of this discovery, particularly by the graphene’s peculiar electrical conductivity observed in strong magnetic fields, when it switched seamlessly from nearly perfect to completely insulating”, Shimshoni said.

A new phase is born

“In a recent study, we found that in the presence of a magnetic and an electric field, graphene exhibits a broad variety of phases, or states of matter. These phases have radically different conductivity levels, depending on how the electrons are arranged in the crystal. We found that tuning the strength of either of the fields induces a transition from one phase to another− a similar process to the solid-liquid-gas transition when changing a material’s temperature or pressure", stated Shimshoni.

Past research on graphene in the presence of a magnetic and an electric field, had demonstrated that it acts almost invariably as an insulator, with two distinct insulating phases: One in which the electrons' spin forms a magnetically ordered pattern, and the other one in which they become electrically polarized. Both phases have similarly low conductivity, showing a very narrow ‘phase transition’ region between phases.

However, as Shimshoni points out, new experimental evidence showed that under slightly altered conditions a conductive region emerges in a finite range of electric and magnetic fields (Fig. 1). This led her and her colleagues to wonder: Had they discovered a new phase, undetected in previous studies?

“Using a theoretical model,  we were able to prove the existence of such a phase, characterized by the uncertainty of both its electrons’ spin orientation, and their position on the crystal’s atoms”, stated Shimshoni. “This can be illustrated by Schreodinger's paradox, in which a cat can be both alive and dead simultaneously. The new phase (marked as "Novel Metal" in Fig. 1) is electrically conducting, in agreement with the experiment", she added.

According to Shimshoni, such sensitivity to minute changes in external conditions is among the hallmarks of phase transitions. Subtle manipulations manifested as dramatic changes in conductivity open up exciting new possibilities for potential applications, such as highly accurate sensors, read/write memory devices, and many others.