{"686158":{"#nid":"686158","#data":{"type":"event","title":"CMP\/AMO Seminar - Dr. Yahui Zhang, Johns Hopkins University - RVB theory of superconductivity in magic angle twisted bilayer graphene","body":[{"value":"\u003Cp\u003E\u003Cstrong\u003ESpeaker:\u003C\/strong\u003E Dr. Yahui \u0026nbsp;Zhang - Johns Hopkins University\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003ETitle:\u003C\/strong\u003E RVB theory of superconductivity in magic angle twisted bilayer graphene\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EAbstract:\u003C\/strong\u003E TBG superconductor remains mysterious after 7 years. \u0026nbsp;In the first March meeting talk by Pablo, the physics was proposed to be similar \u0026nbsp;to doped Mott insulator in high Tc cuprates. \u0026nbsp;However, theoretical efforts were largely distracted to the topological aspects and symmetry breaking orders based on momentum space Hartree Fock in the last a few years. \u0026nbsp; Only recently the relevance of Mott physics has been finally recognized. \u0026nbsp; In this talk, I will return to the most obvious theory of superconductor based on doping a n=2 Mott insualtor. \u0026nbsp; We use the so-called topological heavy fermion model with f orbital on AA site and itinerant c bands. However, the physics is essentially captured in a f-orbital-only Hubbard model on triangular lattice, so band topology \u0026nbsp;and the c band does not play any significant role other than renormalizing the dispersion. \u0026nbsp; \u0026nbsp; In our theory, there is already preformed pairing of the two localized electrons in the parent n=2 Mott insualtor, which is from \u0026nbsp;an on-site spin interaction mediated by optical phonon. \u0026nbsp; \u0026nbsp;Then at doping n=2-x, we find an unconventional metallic state with small hole pockets (A_{FS}=-x\/4) \u0026nbsp;on top of these paired singlets. \u0026nbsp; At lower temperature, a slave boson condenses, and \u0026nbsp;the local pairing of neutral moments (or spinons) induce a secondary \u0026nbsp;pairing for the mobile carriers and opens a smaller superconducting gap on the small Fermi surface. \u0026nbsp; The supercnoductor naturally has two gaps: \u0026nbsp;a pseudogap from pairing of spinons, and a smaller superconductor gap. \u0026nbsp; Above Tc, superconductor gap is killed, but the pseudogap persists. \u0026nbsp; This theory provides, to our knowledge, the first unified explanation of the experimental observation of the pseudogap and two-gap superconductor in TBG.\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003E\u003Cimg src=\u0022https:\/\/krieger.jhu.edu\/physics\/wp-content\/uploads\/sites\/11\/2021\/12\/citations.jpg\u0022 alt=\u0022Yahui Zhang\u0022\u003E\u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EShort \u0026nbsp;Bio:\u003C\/strong\u003E Yahui Zhang is an assistant professor at Johns Hopkins University. He got his PhD from MIT in 2019, mentored by T. Senthil. \u0026nbsp;He did his postdoc with Ashvin Vishwanath and Subir Sachdev at Harvard. \u0026nbsp;He is working on theory of strongly correlated electron systems, including morie system, cuprate, bilayer nickelate.\u003C\/p\u003E","summary":"","format":"limited_html"}],"field_subtitle":"","field_summary":[{"value":"\u003Cp\u003ETBG superconductor remains mysterious after 7 years. \u0026nbsp;In the first March meeting talk by Pablo, the physics was proposed to be similar \u0026nbsp;to doped Mott insulator in high Tc cuprates. \u0026nbsp;However, theoretical efforts were largely distracted to the topological aspects and symmetry breaking orders based on momentum space Hartree Fock in the last a few years. \u0026nbsp; Only recently the relevance of Mott physics has been finally recognized. \u0026nbsp; In this talk, I will return to the most obvious theory of superconductor based on doping a n=2 Mott insualtor. \u0026nbsp; We use the so-called topological heavy fermion model with f orbital on AA site and itinerant c bands. However, the physics is essentially captured in a f-orbital-only Hubbard model on triangular lattice, so band topology \u0026nbsp;and the c band does not play any significant role other than renormalizing the dispersion. \u0026nbsp; \u0026nbsp; In our theory, there is already preformed pairing of the two localized electrons in the parent n=2 Mott insualtor, which is from \u0026nbsp;an on-site spin interaction mediated by optical phonon. \u0026nbsp; \u0026nbsp;Then at doping n=2-x, we find an unconventional metallic state with small hole pockets (A_{FS}=-x\/4) \u0026nbsp;on top of these paired singlets. \u0026nbsp; At lower temperature, a slave boson condenses, and \u0026nbsp;the local pairing of neutral moments (or spinons) induce a secondary \u0026nbsp;pairing for the mobile carriers and opens a smaller superconducting gap on the small Fermi surface. \u0026nbsp; The supercnoductor naturally has two gaps: \u0026nbsp;a pseudogap from pairing of spinons, and a smaller superconductor gap. \u0026nbsp; Above Tc, superconductor gap is killed, but the pseudogap persists. \u0026nbsp; This theory provides, to our knowledge, the first unified explanation of the experimental observation of the pseudogap and two-gap superconductor in TBG.\u003C\/p\u003E","format":"limited_html"}],"field_summary_sentence":[{"value":"Dr. Yahui Zhang, JHU - RVB theory of superconductivity in magic angle twisted bilayer graphene"}],"uid":"36625","created_gmt":"2025-11-03 23:58:16","changed_gmt":"2025-11-04 00:05:19","author":"crichardson76","boilerplate_text":"","field_publication":"","field_article_url":"","field_event_time":{"event_time_start":"2025-11-19T14:00:00-05:00","event_time_end":"2025-11-19T15:00:00-05:00","event_time_end_last":"2025-11-19T15:00:00-05:00","gmt_time_start":"2025-11-19 19:00:00","gmt_time_end":"2025-11-19 20:00:00","gmt_time_end_last":"2025-11-19 20:00:00","rrule":null,"timezone":"America\/New_York"},"location":"Howey Building - Room N110","extras":[],"hg_media":{"678524":{"id":"678524","type":"image","title":"Dr. Yahui Zhang","body":null,"created":"1762214600","gmt_created":"2025-11-04 00:03:20","changed":"1762214600","gmt_changed":"2025-11-04 00:03:20","alt":"Dr. Yahui Zhang","file":{"fid":"262573","name":"Dr.-Yahui-Zhang.jpg","image_path":"\/sites\/default\/files\/2025\/11\/03\/Dr.-Yahui-Zhang.jpg","image_full_path":"http:\/\/hg.gatech.edu\/\/sites\/default\/files\/2025\/11\/03\/Dr.-Yahui-Zhang.jpg","mime":"image\/jpeg","size":11278,"path_740":"http:\/\/hg.gatech.edu\/sites\/default\/files\/styles\/740xx_scale\/public\/2025\/11\/03\/Dr.-Yahui-Zhang.jpg?itok=PptgdVof"}}},"media_ids":["678524"],"groups":[{"id":"126011","name":"School of Physics"}],"categories":[],"keywords":[],"core_research_areas":[],"news_room_topics":[],"event_categories":[{"id":"1789","name":"Conference\/Symposium"},{"id":"1795","name":"Seminar\/Lecture\/Colloquium"}],"invited_audience":[{"id":"78761","name":"Faculty\/Staff"},{"id":"177814","name":"Postdoc"},{"id":"174045","name":"Graduate students"},{"id":"78751","name":"Undergraduate students"}],"affiliations":[],"classification":[],"areas_of_expertise":[],"news_and_recent_appearances":[],"phone":[],"contact":[],"email":[],"slides":[],"orientation":[],"userdata":""}}}