R. G. Herb Condensed Matter Seminars

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Events During the Week of February 5th through February 12th, 2023

Monday, February 6th, 2023

No events scheduled

Tuesday, February 7th, 2023

Building Tools for the Quantum Many-Body Problem: The Case of Fractional Chern Insulators
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Daniel Parker , Harvard
Abstract: The intermediate coupling regime of the quantum many-body problem --- where kinetic and potential energy compete on the same order --- is notoriously difficult. Analytical approaches there are generically uncontrolled, and numerical tools are often insufficient. However, experiments on the growing array of two-dimensional materials routinely probe this regime. To understand such experiments, a new intermediate coupling toolkit must be developed. This talk will focus on recent experiments in magic-angle graphene that detected fractional Chern insulators. Fractional Chern insulators (FCIs) realize the remarkable physics of the fractional quantum Hall effect in crystalline systems with Chern bands, casting off the typical requirements of Landau levels and strong magnetic fields. Understanding these intermediate-coupling experiments required an "all of the above" approach, combining close experimental collaborations with new theoretical and numerical tools. I will start by briefly overviewing the experimental phenomenology: in small magnetic fields, FCIs compete with charge-density waves due to the significant band dispersion. Next I will introduce ``vortexable bands", a class of beyond-Landau level systems with exact FCI ground states. Magic-angle graphene is vortexable in a limit, making the realistic system almost ideal for FCIs. Finally, I will use the new numerical technique of ``MPO compression" to compute the phase diagram of FCIs in magic-angle graphene, predicting where experiments should look to find FCIs in zero external magnetic field. Future work will study pairs of Chern bands related by time-reversal symmetry that, instead of FCIs, naturally host superconductors.
Host: Victor Brar
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Wednesday, February 8th, 2023

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Thursday, February 9th, 2023

Constructive approaches to frustrated magnetism: Moiré and Measurements
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: Zhu-Xi Luo, Harvard University
Abstract: Frustrated magnetism arises when spins interact through competing exchange interactions which cannot be simultaneously satisfied. When the frustrations are strong enough, exotic states can emerge such as long-range entangled spin liquids. Unfortunately, solid state materials are complicated and frustrations are hard to control: To this date, quantum spin liquids are still challenging to be realized in experiments. Naturally, researchers seek more manageable experimental systems, in the hope of engineering frustrated magnetism constructively. I will discuss my recent works in two types of such manageable systems: moire heterostructures in van der Waals materials where many tuning knobs are available; and monitored quantum circuits where designer gates and measurements are exploited as new sources of frustrations.
Host: Victor Barr
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Friday, February 10th, 2023

No events scheduled