LMU researcher Dmitri Efetov: Exotic physics in the second dimension

A material that exists only in the second dimension? For Professor Dmitri Efetov, what might sound like science fiction is simply routine research: “Graphene is a material that is no more than an atom thick,” the solid state physicist explains. “It’s as thin as you get: existent on a plane, but virtually non-existent in terms of height – an extremely flat nanomaterial.” Nor is this the only futuristic property of graphene, a modification of the chemical element carbon: In a layer of graphene, the electrons become massless. And if you put two layers of it in relation to each other at a ‘magic’ angle, the result is a plethora of exotic ‘quantum phases’. The graphene becomes superconductive, magnetic and topological, for example. “Graphene really does possess all the qualities that are currently of interest to modern solid state physics,” Efetov says.

The German-Russian scientist took the Chair of Experimental Solid State Physics at LMU in August 2021. Graphene research has been a constant theme throughout his career: Born in Moscow, Efetov grew up in Stuttgart and Bochum, studied physics at establishments such as the ETH in Zurich, and was already conducting research into graphene while studying for his doctorate at Columbia University, New York. Back then, this remarkable and astonishingly simple-to-make material had only just been investigated closely for the first time, leading to a Nobel prize for scientists in England.

Superconductivity is the goal

“’Relativistic electrons’ are one of many features that make graphene special,” says Efetov. “They don’t behave like normal electrons, but more like light particles in that they are massless.” In the course of his doctoral research (supported by a Faculty PhD fellowship from Columbia University), he too attempted to make graphene superconductive – a state that, back then, appeared virtually impossible. “Superconductivity is a material state in which electric current can flow with no loss of energy. Since conventional conductors give off heat – i.e. energy – a great deal of energy gets lost in traditional power lines, most of which could be avoided by using superconductors.” Efetov adds that superconductors are central elements in modern quantum technologies such as quantum computers, which consist of complex superconductive networks: “One of research’s major objectives with graphene is to develop new kinds of superconductive states that exhibit completely new properties.” During his doctoral research, however, the scientist managed this feat only after bringing graphene into contact with an existing and familiar superconductor – whose properties were carried over to the new material.

As a postdoctoral researcher at the Massachusetts Institute of Technology (MIT), Efetov applied himself as of 2014 to developing quantum detectors that detect individual light particles. Even here, his graphene research continued in the background. Answering a call to the Institut de Ciències Fotòniques (ICFO) in Barcelona in 2017, he then led a group conducting research into these detectors. Just at this time, academics elsewhere succeeded in making graphene itself superconductive: Efetov’s team became the third group in the world to reproduce this achievement. “The method used to finally make graphene intrinsically superconductive is as ground-breaking as it is unique. It works if you take two layers of graphene and set them at an angle of 1.1 degrees to each other,” the physicist explains. “That’s what we call the magic angle, because the superconductivity is lost at even a 1.0 or 1.2-degree angle.”

The researcher is aware that this state has not yet been fully understood. “But we are assuming that it is similar to high-temperature superconductivity.” Many practical aspects remain to be resolved along the path to superconductivity that can be applied at room temperature, he admits. “But this discovery is a kind of milestone. The opportunities it opens up in physics are utterly new and decidedly exotic.” Efetov’s team has since made further discoveries relating to graphene and today ranks as “one of the key groups in this field”, as he puts it.

Lessons worth learning

His working group’s move to LMU should be completed in summer 2022. Right now, the cleanroom for the Chair of Experimental Solid State Physics is being refined and improved for graphene research. Efetov’s focus is an “excellent” fit with the goals of locally based quantum research groups who are working on correlated states and superconductors. “Especially in quantum information research, Munich – with all its research institutes – is extremely well represented,” he notes. “LMU had always been on my radar.” Efetov points out that initiatives such as the Munich Quantum Valley and the Munich Center for Quantum Science and Technology adopt a highly interdisciplinary approach: “Solid state physicists like me interact here with researchers into quantum information, laser physics and quantum optics.” Within the broad-based discipline of ‘quantum technologies’, he himself tends to work more on basic research. “I myself don’t build quantum computers, even though my research means that I could contribute components to this end – rather like a supplier to automotive manufacturers.” There is another thing that Efetov appreciates about Munich: “The students here are really very good. That is a tremendous asset for a professor, because it lets you recruit good people.” Having recently become a father for the first time, Efetov values the benefits of his teaching activities: “It forces you to rework your own research material and explain it to the students. You learn a lot yourself in the process.”

He continues to concentrate his research on the potential properties of graphene as a superconductor. “Graphene happens to be ‘topological’ as well, with currents flowing at its edges, not in the middle. It is also magnetic and, by no means least, what is known as a ‘correlated isolator’. That is a reference to a very exotic many-particle state that is likewise the object of intensive research at present,” Efetov explains. “To sum up: Graphene is rich in complicated ‘quantum phases’.” Even here at LMU’s Chair of Experimental Solid State Physics, it looks like the angled, two-dimensional, magic material will keep him busy for a long time to come.