A technique to exaggerate with graphene beyond the Van Hove singularity

For over a decade, theoretical physicists have predicted that graphene’s van Hove singularity could be associated with several exotic phases of matter, the most notable of which is chiral superconductivity.

A van Hove singularity is essentially a non-smooth point in the density of states (DOS) of a crystalline solid. When graphene reaches or is close to this specific energy level, a flat band develops in its electronic structure that can occupy an exceptionally large number of electrons. This leads to strong multi-body interactions that promote or allow the existence of exotic states of matter.

Until now, it has been very difficult to determine the exact degree to which the available energy levels of graphene must be filled with electrons (ie, “doped”) for the individual phases to stabilize has been very difficult to determine using model calculations. Identifying or designing techniques that can be used to doping graphene up to or beyond the Van Hove singularity could ultimately lead to interesting observations related to exotic phases of matter, which could in turn pave the way for the development of a new technology based on the graphene.

Researchers at the Max Planck Institute for Solid State Research in Stuttgart, Germany, recently devised an approach to excessive graphene beyond the van Hove singularity. Their method, presented in an article published in Physical Review Letters, combines two different techniques, namely the intercalation of ytterbium and the adsorption of potassium.

“An experimentally tunable electron density near the van Hove singularity would be highly desirable,” Philipp Rosenzweig, one of the researchers who led the study, told Phys.org. “Previous experiments have shown that graphene can actually be stabilized (‘locked’) at the van Hove level and that charge carriers can subsequently be removed from this locking scenario. The question we have asked ourselves, however, is that we can also transfer more electrons to the graphene layer, overcome the van Hove block and overcome the singularity? Apart from pure proof of principle, this would open up an unexplored playing field of interrelated phases with exciting promise. “

Doping graphene at the van Hove singularity is a challenging task in itself, as it requires the transfer of over 100 trillion (10¹⁴) electrons per cm² on the graphene layer. Doping of graphene can be achieved by depositing other atomic species on top of it, which donate some of their electrons to it.

An alternative method of doping graphene, known as intercalation, involves sandwiching dopants between graphene and its supporting substrate. Over the past decade, this technique has proved very useful for fine-tuning the electronic properties of the material.

Typically, even when the deposition and intercalation approaches are combined, it is difficult to increase the graphene vector density to an arbitrary value. This is mainly due to the fact that the charge transfer will eventually become saturated, preventing it from being doped beyond a certain level.

“Recently, we have found that the intercalation of some rare earth elements, due to their enormous doping efficiency, is already enough to fix graphene to its van Hove singularity,” Rosenzweig said. “In that case, the graphene surface still remains free to occupy additional dopants. Starting from the van Hove scenario of graphene intercalated with ytterbium, by depositing potassium atoms on top, we were thus able to increase the vector density by another factor. of 1.5, going well beyond the singularity level. “

In their experiments, the researchers used ytterbium intercalation and potassium adsorption methods. This approach allowed them to dop a graphene layer placed on a semiconductor silicon carbide (SiC) substrate beyond the van Hove singularity, achieving a charge carrier density of 5.5 x 10.¹⁴ centimeter^-2.

“You can compare the strategy we used with a situation in everyday life where a bulky object has to be carried up the stairs to the top floor (in our case, beyond van Hove’s singularity),” Rosenzweig explained. “This could only become possible by simultaneously pushing from below (ie, intercalation of ytterbium) and pulling from above (ie, adsorption of potassium).”

The study conducted by Rosenzweig and his colleagues shows that it is indeed possible to drug graphene beyond its van Hove singularity in an experimental setting. The researchers examined their graphene system using a technique called angle-resolved photoelectron spectroscopy, in tests performed at the BESSY II synchrotron, Helmholtz-Zentrum in Berlin. This method allows direct visualization of the energy band structure of graphene and its evolution through doping.

“The feasibility of doping excess was previously far from clear, as the system is first locked down at the singularity level that occupies a huge number of charge carriers,” Rosenzweig said. “In practice, by pushing graphene doping to new heights, our study also opens up a new and unexplored landscape in the phase diagram of this two-dimensional material prototype. As such, we hope our work will help strengthen the search for fundamental states. correlates in monolayer graphene that would certainly be of interest in various subfields of physics “.

In the future, the results gathered by Rosenzweig and his colleagues could open exciting new possibilities for studying the exotic states of matter in graphene that is doped beyond its van Hove singularity. Furthermore, this recent study could improve the current understanding of the strong non-local interactions of many bodies in van Hove-doped graphene that have been shown to have significant deformation effects on its energy levels. Researchers have shown that such effects are still present in the hyper-doped regimen and that they become more and more as graphene approaches the van Hove singularity. The data collected could therefore also inspire the development of new theoretical models that go beyond the conventional Fermi theory of liquids.

“Now that we can regularly adjust the level of doping in experiments around the van Hove level, we are looking for any of the various exotic phases predicted by the theory,” Rosenzweig concluded. “Aiming for the stars, realizing unconventional superconductivity in a monolayer of epitaxial graphene would obviously be a revolutionary discovery that could one day lead to technological applications. In any case, exciting times are ahead for doped van-Hove graphene.”

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