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Visit to UIUC
Thanks so much to everyone at UIUC for a wonderful visit, particularly my hosts, Taylor Hughes and Philip Phillips. It was a tremendous honor to receive this year’s McMillan award. Bill was an incredible physicist and apparently quite the character. You can read more about Bill here.
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Publication: Deterministic fabrication of graphene hexagonal boron nitride moiré superlattices
I am excited to announce that Rupini Kamat and my paper has been accepted to PNAS! When trying to form a moiré between different materials, one previously relied on aligning straight edges of exfoliated materials as a proxy for the crystallographic axes. However, for graphene and hexagonal boron nitride, these straight edges can represent either zig-zag or arm-chair termination in roughly equal proportion. This results in a 50-50 chance of improper alignment. Rupini and I set out to remove this ambiguity by directly measuring the various crystallographic axes in the problem. Check it out here: https://www.pnas.org/doi/10.1073/pnas.2410993121.
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Publication: Torsional force microscopy of van der Waals moirés and atomic lattices
Congrats to Mihir et al. on the acceptance of his paper to PNAS. Mihir and others have developed a really nice and reliable technique for imaging moire and atomic lattices. It is quite easy to get up and running on any Bruker Icon: ( https://www.pnas.org/doi/abs/10.1073/pnas.2314083121).
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Publication: Unusual magnetotransport in twisted bilayer graphene, now with strain!
Congrats to Xiaoyu and Joe on the acceptance of their paper to PNAS. They really have a beautiful understanding of just about every detail in the device! Check it out here: https://doi.org/10.1073/pnas.2307151120.
When incorporating uniaxial heterostrain into the continuum model, the degeneracy of the Dirac cones and van Hove points are lifted. This leads to three Lifshitz transitions: (1) a transition from two to one Fermi pockets which we observe in quantum oscillations; (2) a transition to open orbits where we observe non-saturating magnetoresistance; and (3) a transition to hole-like pockets.
This model also predicts that the principal axes of transport are now density-dependent! They will rotate as a function of filling for a given uniaxial strain amount and direction.
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Temporarily moving to Boston
For the next 6 months, I will be visiting MIT, primarily collaborating with Pablo Jarillo-Herero. If you are in the area and would like to catch up, send me an email!
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March Meeting
It was great to catch up with a lot of people I have not seen in a long time and meet a bunch of new people at March meeting. If you made it to my site to check out the slides of my talk, you can find them here. Feel free to reach out with any questions!
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Publication: Unusual magnetotransport in twisted bilayer graphene from strain-induced open Fermi surfaces
Here is the latest paper to come out of the strain project I have been lucky to be a part of with Joe Finney and (now) Xiaoyu Wang. Uniaxial heterostrain can lead to a large range of densities where the Fermi surface becomes open, providing a very plausible explanation for some of the effects we previously observed, like non-saturating magnetoresistance. You can check it out here.
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Publication: Stacks on stacks on stacks
I recently served as a referee for two exciting papers focusing on multilayer twisted graphene stacks (1, 2). These papers found physics reminiscent but slightly different than magic-angle twisted bilayer graphene in four- and five-layer graphene stacks with an alternating twist angle between adjacent layers. After reviewing, I was invited to write a News and Views article on the two papers. You can check it out here.
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Publication: Super-geometric electron focusing on the hexagonal Fermi surface of PdCoO2
Congrats to Maja Bachmann on the acceptance of her latest paper to Nature Physics. Check it out here: (https://rdcu.be/cNbEN). We hopefully will publish one more paper on this topic.
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Publication: Unusual magnetotransport in twisted bilayer graphene
Congrats to Joe Finney (spxtr) on the acceptance of his paper to PNAS. Check it out here: (https://www.pnas.org/doi/10.1073/pnas.2118482119).
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Publication: Tunable ferromagnetism at non-integer filling of a moiré superlattice
Our preprint on magnetism in ABC-trilayer graphene aligned with hBN occuring at non-integer filling of the lattice has been published in Nano letters! You can check it out here: (https://pubs.acs.org/doi/10.1021/acs.nanolett.1c03699). You can check out my previous post on the preprint of this paper for a brief summary.
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Seminar Series: Practical lab skills
I recently started a seminar series on practical skills for the lab. The goal is for the course to help new students get a jump start in familiarizing themselves with everything they may come across in the lab. Check out all the content here.
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News: Sandia LabNews article
I was featured in a recent Sandia LabNews article along with other Sandia fellows (including fellow Stanford PhD Bette Webster). Check it out here.
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Publication: Evidence of orbital ferromagnetism in twisted bilayer graphene aligned to hexagonal boron nitride
Our manuscript was accepted for publication in Nanoletters! Check it out here.
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Preprint: Unusual magnetotransport in twisted bilayer graphene
A new preprint I was involved in just came out (https://arxiv.org/abs/2105.01870). Here we study a twisted bilayer device with an angle (1.38 degrees) quite far away from the magic angle (1.05 degrees). In this device, we do not observe correlated insulating states. However, we do observe superconductivity and extremely strange behavior in magnetotransport. We observe large magnetoresistance over a large range in density near half filling. We also see that the gaps between Landau levels split and bend in field/density. Interestingly, it seems that this can be entirely modeled using a simple tight-binding model of Hofstadter’s butterfly on an entirely different lattice (square) if one includes anisotropic hopping term, indicating that they are likely a manifestation of some more generic class of phenomena.
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Preprint: Directional ballistic transport in two-dimensional metal PdCoO2
A new preprint I was involved in came just came out (https://arxiv.org/abs/2012.10075). In this work, we have been looking at ballistic transport in an extremely clean material: PdCoO2. We can see all sorts of strange transport signatures from the system being both ballistic and having a hexagonal Fermi surface. The electrons will travel mostly along the facets of the Fermi surface in the limit of zero field, leading to anisotropy in the transport. We studied this by studying the transport through very long bars that are oriented at a specific angle relative to the crystal axis (and hence the Fermi surface).
Code for modeling ballistic transport in a mesoscopic device can be found at https://github.com/dgglab/ballistic_montecarlo
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Preprint: Evidence of orbital ferromagnetism in twisted bilayer graphene aligned to hexagonal boron nitride
Check out our latest preprint (arXiv:2102.04039), where we perform hyestersis loops on the magnetic state found in twisted bilayer graphene at various tilt angles. We find that at above some threshold tilt angle, the magnetic state responds to a threshold out-of-plane magnetic field indicating that the magnetism is highly anistropic. This tells us that the orbital magnetic moment is likely dominant, as it is the most obvious route for a high degree of magnetic anisotropy. As the tilt angle becomes very small (field is mostly in the plane of the sample), the story becomes much more complicated and we no longer see this clear response.
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Preprint: Tunable ferromagnetism at non-integer filling of a moiré superlattice
Check out our latest preprint (https://arxiv.org/abs/2012.10075). Up to now, all of the moiré systems that have displayed correlated behavior have done so at an integer number of carriers per moiré unit cell. It is somewhat natural to expect this given that there may be a large exchange energy favoring the system to polarize the avaliable degrees of freedom near integer filling. In this preprint, we discuss some of our recent findings of evidence of magnetism away from integer filling of the lattice. It seems that there are also some similar rumblings of evidence of correlations at non-integer fillings in other moiré systems that may also be coming out soon!
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Press release: Emergent ferromagnetism near three-quarters filling in twisted bilayer graphene
Here are two press releases from Stanford regarding our recent publication: Emergent ferromagnetism near three-quarters filling in twisted bilayer graphene.
Thanks to Stanford libraries and the Stanford Digital Repository. They host a number of great resources and helped us post all of the data for our publication.
Stanford physicists discover new quantum trick for graphene: magnetism
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Press release: A Graphene Superconductor That Plays More Than One Tune - Berkeley Lab News
Press release from LBL News regarding our recent publication: Tunable correlated Chern insulator and ferromagnetism in a moiré superlattice
https://newscenter.lbl.gov/2020/03/04/2d-material-gets-a-new-gig/
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Press release: A Graphene Superconductor That Plays More Than One Tune - Berkeley Lab News
Press release from LBL News regarding our recent publication: Signatures of tunable superconductivity in a trilayer graphene moiré superlattice
https://newscenter.lbl.gov/2019/07/17/a-graphene-superconductor-that-plays-more-than-one-tune/
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New correlated phenomena in magic-angle twisted bilayer graphene/s
This recent article from the Journal Club for Condensed Matter Physics mentions our recent preprint arXiv:1901.03520, a [great paper by Matt Yankowitz et al] (http://science.sciencemag.org/content/early/2019/01/25/science.aav1910), and an incredibly interesting talk Dmitri Efetov gave as part of the moiré flat bands program hosted by KITP. I was lucky enough to attend this program and it is definitely worth checking out.
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