How Two Physicists Unlocked the Secrets of Two Dimensions

Condensed matter physics is the study of matter and the diverse forms of matter that arise when atoms and electrons are assembled in different ways. It's the most active field of contemporary physics and has yielded some of the biggest breakthroughs of the past century. But as rapidly as technology has advanced, scientists have only scratched the surface of the electron's potential, harnessing just a few of its behaviors. Now, for the first time ever, a married couple of physicists at Cornell have figured out a way to create artificial atoms in the lab, opening the door to a new era of research. In this field of physics called condensed matter physics, it's basically about the properties of behavior of trillions of trillions of electrons. I'm just curious, you know, about something I don't understand. Both Jie and Fai, they're pioneers. Fai and Jie live more or less completely devoted to physics. We are good at different things, I think. That actually makes us a good team. Electrons are the animating actors of the material world, yet their behavior remains elusive. They can tunnel, orbit, spin, and behave in other mysterious ways, making them notoriously difficult to study, especially in large groups. To understand and manipulate the dynamics of many electrons together, Jie Shan and Fai Mak have had to get creative in their lab at Cornell. We try to deal with electrons in a more controllable way and try to explore the quantum nature of electrons more. After a decade of patient and steady work, Shan and Mak have developed an experimental setup that is as surprising as it is groundbreaking. So the problem is so complicated that you just really want to capture this essential of interacting electrons, and they are jiggling around randomly. So you want to look at this problem in its simplest way. One way to simplify experimental conditions is to use what are called 2D materials, sheets of metal a few atoms thick. 2D materials are an excellent platform to control and also understand the behavior of electrons. Two-dimensional materials are great because we are not two-dimensional, we are three-dimensional. So that is just really convenient because you can look at it from the top and you can push on it with electric fields or you can put magnetic fields on it and then you can look and see what happens. A popular choice of 2D material is graphene, the building block of graphite and the thinnest material known to scientists. As an ideal conductor, electricity passes through graphene with minimal resistance, but its conductivity is difficult to control. Once an electrical current starts flowing, scientists can't turn it off. In search of more control, Shan and Mak turn to alternative 2D materials known as semiconductors. You can control the number of electrons in those systems really precisely. I mean you don't want a current, you can turn off the current and then you can make a transistor out of this. So what we actually find is basically a semiconductor that is just a few atoms thick. This platform they've been working on based on To construct their tiny playground to study the atomic world, the researchers developed a method to isolate sheets of semiconducting metals known as TMDs, short for transition metal decalcogenides, each one just a few atoms thick. They meticulously stack the metal flakes on top of each other and the slight structural differences create a repeating pattern known as a moiré superlattice. This idea of moiré superlattice really opens a new kind of avenue. In a series of landmark papers, the researchers have used their tightly controlled stack of semiconductor sheets to explore various exotic quantum phenomena. This material that they've made just sort of shows all of this textbook physics plus surprises all in one system. Perhaps the most significant breakthrough of all came when the researchers demonstrated the ability to simulate the behavior of artificial atoms. By harnessing an electromagnetic attraction between the overlaid sheets in their stack, the researchers can create simulated atoms 100 times the size of real atoms, allowing for far greater experimental control. And then by turning a knob, the researchers can adjust the voltage, probe the system with this laser, and change the properties of their artificial atoms in a new form of scientific alchemy. When we change the number of electrons in each atom, it's like changing one element to another element, right? If you put one electron into the atom, you create like an artificial hydrogen atom. And if you put two electrons into the atom, you create an artificial helium atom. If you put three electrons, you create an artificial lithium atom. By just controlling the voltage, we are controlling the filling of the electron in each atom, one, two, three. And every time we do it, we change the chemistry. So in this case, it's really a game changer because now we can actually sort of design chemistry or design artificial material just by stacking this material and applying voltages to just how to tame the electrons. Shan and Mack have used their Moray system to coax electrons into realizing a series of unusual states of matter, including Mott insulators, Wigner crystals, and Chern insulators, with potential applications to quantum computing, energy storage, and other next-generation technologies. The sort of beautiful successes have in some ways come from realizing a really elegant physical system. It's that compromise between having a lot of experimental control, understanding how it relates to something that you understand well theoretically, but having enough tunable parameters where you can still be surprised. Insights gained from a tiny stack of semiconductor flakes may one day reveal secrets that will power a future era of superconductors, a holy grail of technology. The Moray materials themselves may not have high enough critical temperature to be useful superconductor in any near future, but the principles that we can learn from them may actually help us to understand the principles and therefore design superconductors that have higher transition temperature or critical temperatures in the future. It would be great to be able to transmit electricity for long distances without any dissipation. That would, you know, that would have a huge technological impact on a very basic part of our energy infrastructure. This is an amazing moment, you know, in my life and my students' life. Many phenomena have been observed, but I think many more are coming, so I think this is just the beginning of it.