Frontiers in Synthetic Moiré Quantum Matter Proceedings of a Workshop (2022) / Chapter Skim
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Pages 38-49
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From page 38... ...
Thin films are close cousins of the van der Waals systems being discussed at the workshop, he noted, and they had been around for a very long time before a few developments in the 1950s and 1960s led to a revolution in the development of molecular beam epitaxy (MBE)
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From page 39... ...
There have also been new deposition methods and new characterization tools developed over time. Thinking about the lessons from this history that might apply to the new work in van der Waals materials, Checkelsky said that the most important thing for these thin films was the substrate and the ability to find different substrates and understand what the different substates would mean for the film grown on them.
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From page 40... ...
An important issue is the types of technical challenges that will be encoun tered in trying to fabricate devices using materials beyond what has already been tried -- graphene, hexagonal boron nitride (hBN) , and transition metal dichalco genides?
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From page 41... ...
One is working with the systems that already exist and really understanding the phase diagrams -- for example, where a chiral spin liquid is found. The second is working with design principles to decide where to search in the huge phase space -- a space that is far too large to search through systematically either experimentally or with theory.
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From page 42... ...
Finally, Ashvin Vishwanath of Harvard University offered his thoughts. What his experience has shown to him, he said, is how much condensed matter physics is one subject despite the enormous phase space that it explores.
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From page 43... ...
An entire suite of tools was build up over decades to work in solid state physics, he noted, but when researchers started working with optical lattices, while they were inspired by some of the probes that had been developed for the earlier area, "at the end of the day you do something completely different." Researchers developed and used tools that were more natural at that scale, thus gaining some new tools while also losing some earlier ones that had been developed for solid state physics. "So what are the analogous tools we will need to develop at the moiré scale?
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From page 44... ...
She answered that at present there seems more to be gained from making simple models than doing database searches. "I think the most fruitful thing to start out," she said, "might be to identify a few classes of simple low-energy physics, like Dirac cones, quadratic band touching, and maybe quadratic insulators and then identify a few low-energy k∙p models and then see which of those puts you in the right place.
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From page 45... ...
Indeed, he added, even in magic-angle twisted bilayer graphene, people have suggested it may be possible to establish particular chemical links in a moiré structure to, for instance, stabilize an angle. Given all these possibilities, he suggested that researcher might want to explore twisted structures made with metal-organic frameworks.
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From page 46... ...
So it would be valuable to have a machine learning approach to ab initio calculations or perhaps some other approach to making the calculations more tractable. "Tricks or tools to getting low-energy physics out of ab initio calculations of these huge moiré cells might be a theoretical advance worth pursuing." Vishwanath added that it is common today to use an effective low-energy model to calculate twisted band structures, and it has been found that by and large calculations with the low-energy model agree reasonably well with ab initio calcula tions, so people will often rely just on the low-energy model.
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From page 47... ...
In contrast to molecular beam epitaxy, where layers are laid down in a vacuum chamber that essentially eliminates any contaminants, the single layers of graphene created by exfoliation are products of "lousy conditions, so it seems like dirt must play some role in these materials, he said, and this should be taken into account. Not only may this dirt be responsible for keeping layers from slipping past each other, it may also cause a deterioration in various properties of the system, such as mobility, although the electronic structure may not be so greatly affected.
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From page 48... ...
Cano added that tunable correlation strength is unique to this platform because usually when researchers are looking for spin liquid candidates, they are working with a phase diagram where each point represents an individual material that has been synthesized and tested, and adding points to the phase diagram means synthesizing and measuring new sample, while the tunable correlation strength of twisted bilayer graphene makes it possible to explore a large phase diagram within one material. "I think that's one of the things that makes this platform really unique for exploring fundamental physics," she said.
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From page 49... ...
There are many such types of quantum sensing and quantum computing applications that one could imagine, he said. A different sort of applications could use hexagonal boron nitride, which is the thinnest of all out-of-plane ferroelectric materials.
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