The story of Jacutingaite: how a wonder material went from the mine to theory, crystal growth and experiments

by Nicola Nosengo, NCCR MARVEL

What does “success” mean for a project like NCCR MARVEL, that for 10 years has brought together experts from leading Swiss institutions to develop new computational tools for the study of novel materials? For sure, one key goal of the project has always been to prove that theory, computations and experiments can work hand in hand, resulting in something that is more than the sum of each part.

That’s exactly what happened, and more than once, in the years-long story of jacutingaite, a very exotic material that has kept both experimentalists and theorists busy for years, collaborating and taking turns in leading new discoveries.

The story started in 2008, when a strange tiny inclusion was noticed in minerals collected from a mine in the Brazilian state of Minas Geiras. A group of Czech mineralogists who got hold of the sample were surprised to find platinum in it, which is rare and unexpected. They extracted the sample, crashed it, solved the crystal structure, and wrote a paper in The Canadian Mineralogists reporting that it was an unexpected ternary compound containing platinum, with the chemical formula Pt₂HgSe₃. The material was called jacutingaite, from the local name of the mine where the first discovery had happened.

Not much happened for the following decade, apart from the fact that the material was added to one of the several databases that collect crystal structures of known compounds. From there, it entered the 2D materials database, created by MARVEL scientists and including materials that – according to calculations – can potentially be exfoliated from their 3D counterpart. But the potential to be turned into a two-dimensional layer would prove to be only one of the intriguing properties of this material.

It was around that time that jacutingaite got on the radar of Enrico Giannini, an expert in quantum materials and crystal growth at the University of Geneva, where he leads the crystal growth facility. “In 2017 I was organizing the meeting of Swiss society for crystallography”, he recalls, “and I invited Nicola Marzari to introduce to our community how theorists were using crystallographic descriptions to create tools that could predict physical properties”. Marzari could not attend, but sent Marco Gibertini, at the time a member of his lab and now a professor in Modena and Reggio Emilia. “He gave a great talk, after which he told me that Antimo Marrazzo, at the time a PhD student in Marzari’s group at EPFL and now assistant professor at SISSA in Italy, had been going through the 2D database looking for topological materials, and he had found an incredibly exciting compound, but could not tell me much yet”.

Six months later, Marrazzo, Gibertini and Marzari were sure that they had calculated something reliable: monolayers of jacutingaite appeared to be a rare example of a quantum spin Hall insulator (QSHI), a class of so-called two-dimensional topological materials where the boundary acts as a conductor while the interior is an insulator —electrons, in other words, can only move along the edge of the material. In addition, electrons with opposite spin directions propagate towards opposite directions along the edge, an effect called spin-momentum locking. Even more remarkably,  first-principle simulations showed that jacutingaite was the first ever material embodying the Kane-Mele physics with an energy scale such that could be potentially be observed at room temperature – but why was that so important?

This finding is related to the 2016 Nobel prize in Physics, that was assigned to Thouless, Haldane and Kosterlitz “for theoretical discoveries of topological phase transitions and topological phases of matter". In 1988, Haldane developed a theoretical model that was the first prototype of what we now call topological insulators: for the first time, topological matter was conceived to exist as an intrinsic property of materials, without external perturbations such as strong applied magnetic fields. Kane and Mele in 2005 showed that the magnetic field is not even necessary: graphene, which is made of a single honeycomb layer of carbon atoms, is a topological insulator, once relativistic effects are taken into account. However, relativistic effects are weak for light elements such as carbon, and the effect in graphene is way too small to be observed.

More than ten years after the breakthrough by Kane and Mele, Marrazzo and the EPFL team showed that jacutingaite was the first ever material embodying strong Kane-Mele physics, with an energy scale that was several orders of magnitude larger than in graphene and that could actually be observed. In addition, monolayer jacutingaite exhibits other interesting effects: for instance, the robust topological phase could be switched on and off with relatively low electric fields due to a unique strong interplay between relativistic effects, crystal-symmetry breaking, and dielectric response. Those calculations would end up in a publication in Physical Review Letters and in Marrazzo’s PhD thesis Electronic Structure and Topology of Novel Materials. But, asked theorists, could Giannini grow crystals of it to confirm the prediction?

It was a challenging task. Jacutingaite includes both mercury, that is highly volatile and tends to evaporate when heated, and platinum, that requires very high temperatures to react with other elements. “And yet, the fact that it was found as a mineral told us that it can be stable" says Giannini. "So we asked, how can nature make it? And the answer had to be ‘by applying pressure”.

Giannini and his group used their multi-anvil cubic press in Geneva and began looking for the right combination of pressure and temperature to grow the jacutingaite crystal. “We found it around 15,000 times atmospheric pressure and 950 degrees Celsius”, explains Giannini. “The choice of the temperature is critical. You have a narrow window, because there is a secondary phase that has basically the same structure but without mercury and is stable in the same pressure range. Add more heat, and mercury will disappear from the crystal”.

What Giannini and his team managed to grow in Geneva was the bulk material, from which two-dimensional flakes could be isolated, but not the coveted 2D monolayer. “Since then, various groups have tried chemical or mechanical techniques for exfoliation, but nothing has worked. To this day, the isolated monolayer does not exist.”

Still, experiments were conducted on the 3D bulk material and on two-dimensional exfoliated flakes, using transport, electrical properties and tunnelling, angle-resolved photoemission. Not only were all data in excellent agreement with predictions, says Giannini, but they also pointed theorists towards new things.

“The experiments found things that theorists had not predicted, in particular a dual topology. The material was showing both a 2-dimensional topology creating protected edge states, and 3-dimensional topology producing surface states, that became evident in photoemission experiments”. That result went back to MARVEL, and Marrazzo, Gibertini and Marzari extended the model to also explain this dual topology, resulting in a joint publication of an experimental and a theory paper. “This back and forth between theorists and experimentalists was very exciting, and does not happen often” says Giannini.

Things have kept moving since 2020. At least a couple of groups in Geneva are continuing experiments on jacutingaite, using optics techniques (in the case of Alexei Kuzmenko’s group) – or tunnelling microscopy (in Christoph Renner’s lab), and the results may be published soon. Experimental effort is also needed to replicate the observation of a topological edge state reported by a 2020 study by a Hungarian group that used scanning tunnelling microscopy. Also on the theory side, several groups around the world have been working on jacutingaite following the prediction by the EPFL team and the first experiments done on Giannini’s samples in Geneva, and interesting theoretical results about this material continue to appear regularly in the literature.

Jacutingaite has also become a template for a whole family of materials. In 2020, a group of Brazilian theorists predicted that nine compounds must exist with the same crystal structure, replacing palladium or nickel for platinum, sulphur or telliurium for selenium, cadmium and zinc for mercury.  For all these theoretical structures the team calculated stability and predicted which phases may exist as well as their topological properties, in particular the energy gap that protects the topological state. What came out from these studies is that one of these compounds – the one with platinum, zinc and tellurium (Pt₂ZnTe₃) is predicted to have a higher topological gap than classic jacutingaite. “If it exists, it is even more interesting than jacutingaite” says Giannini, who’s been trying to synthesize this material since then but without success. “The problem is the simultaneous presence of zinc and tellurium, that tend to create very stable binary compounds.  We are trying various processing routes to achieve this goal”.

Still, experimentalists and theorists keep surprising each other with this family of materials. “There is a yet unexplored possibility to play around with chemistry and find the same structure to exist with different compositions, thus providing theorists with new input for further calculations of physical properties”, Giannini explains.

The story that started in that Brazilian mine 16 years ago may still hold surprises. “And we have to be proud of the fact that this story is entirely Swiss Made”,  says Giannini. “Everything was done between Lausanne and Geneva, where we have all the competences and equipment to go from theory to crystal growth to experiments, and back”.