Magic-angle twisted graphene systems, including bilayer (MATBG) and trilayer (MATTG) structures, constitute a highly tunable platform for exploring strongly correlated electronic phenomena and unconventional superconductivity. Despite extensive studies, the local electronic structure, symmetry-breaking transitions, and their interplay with superconductivity remain elusive. In this thesis, we employ high-resolution scanning tunneling microscopy and spectroscopy, to investigate the evolution, and hierarchy of correlated phases in twisted multilayer graphene as functions of doping, temperature, magnetic field, and twist angle.
\r\n\r\nIn twisted bilayer graphene, we map the evolution of flat electronic bands and detect filling-dependent band flattening, which drives cascades of symmetry-breaking transitions and the emergence of correlated gaps. Correlated gaps that occur at high magnetic fields are identified as Chern insulators, driven by interaction induced degeneracy breaking. In twisted trilayer graphene, we identify a sequence of correlated gaps at the Fermi level, including a robust outer gap associated with intervalley coherence and a more fragile inner gap linked to superconductivity. Atomic-scale reconstruction reveals Kekul\u00e9 reconstruction indicative of inter-valley coherence, which coexists with moir\u00e9-scale translation symmetry breaking.
\r\n\r\nOur results demonstrate that superconductivity in twisted multilayer graphene emerges from a hierarchy of correlated states, starting from cascade physics, to formation of Kondo resonance, flavor symmetry breaking to superconductivity. Our findings provide an insightful microscopic framework that is relevant to many moir\\'e systems and offer guiding principles for engineering correlated and topological states in designer quantum materials.
", "doi": "10.7907/2rk8-2q20", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:16642", "collection": "thesis", "collection_id": "16642", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08152024-222635476", "primary_object_url": { "basename": "Yiran_Zhang_2024_thesis.pdf", "content": "final", "filesize": 106668206, "license": "other", "mime_type": "application/pdf", "url": "/16642/1/Yiran_Zhang_2024_thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Spin-Orbit Enhanced Superconductivity in Graphene Heterostructures", "author": [ { "family_name": "Zhang", "given_name": "Yiran", "orcid": "0000-0002-8477-0074", "clpid": "Zhang-Yiran" } ], "thesis_advisor": [ { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" } ], "thesis_committee": [ { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Falson", "given_name": "Joseph", "orcid": "0000-0003-3183-9864", "clpid": "Falson-Joseph" }, { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Flat electronic bands in moire and crystalline graphene multilayers showcase emergent correlated phenomena including correlated insulators, superconductivity, topological orders, etc. This thesis focuses on the electrical transport characterization of superconductivity in moire and crystalline graphene, with the proximity of a layer of tungsten diselenide (WSe\u2082) that induces spin-orbit coupling (SOC). The interplay between spontaneous symmetry-breaking and explicit spin-orbit interactions emerges various unconventional superconducting pairing.
\r\n\r\nIn the case of moire graphene multilayers, superconductivity in twisted bilayer graphene persists much far away from the magic angle at which electronic correlations dominate. At the lowest twist angle 0.79\u00b0, superconductivity appears despite the absence of any insulating states. By changing the moire twist angle, the ratio between Coulomb interactions and kinetic energy is reduced, and we thus established a hierarchy of various symmetry-breaking orders. Importantly, superconductivity is tightly related to the half-filling symmetry-breaking reconstructions. We further generalize the twisted moire graphene to trilayer, quadrilayer and pentalayer cases. Characterizations around their respective magic angle show that superconductivity is more prominent in filling phase space when the number of layers is increased.
\r\n\r\nWe then investigated the effect of SOC on correlated phases in crystalline Bernal-stacked bilayer graphene. Surprisingly, placing monolayer WSe\u2082 on bilayer graphene promotes Cooper pairing to an extraordinary degree: field-induced superconductivity is stabilized at zero magnetic field, exhibits an order of magnitude enhancement in critical temperature and occurs over a density range that is wider by a factor of eight. The superconductivity descends from a broken-symmetry parent state with two out of the four spin-valley flavors being predominantly populated. Moreover, the superconductivity arises only for perpendicular electric fields that push hole wavefunctions toward WSe\u2082, indicating that proximity-induced Ising spin-orbit coupling plays a key role in stabilizing the pairing.
\r\n\r\nThe last part of the thesis focuses on a new degree of freedom: interfacial twisting between graphene and WSe\u2082. We experimentally demonstrate the \"moireless\" tuning of superconductivity in Bernal bilayer graphene proximitized by WSe\u2082. The precise alignment between the two materials systematically controls the strength of the induced Ising SOC, profoundly altering the phase diagram. As Ising SOC is increased, superconductivity onsets at a higher displacement field and features a higher critical temperature, reaching up to 0.5K. Within the main superconducting dome and in the strong Ising SOC limit, we find an unusual phase transition characterized by a nematic redistribution of holes among trigonally warped Fermi pockets and enhanced resilience to in-plane magnetic fields. Moreover, we identify two additional superconducting regions, one of which descends from an inter-valley coherent normal state and exhibits a Pauli-limit violation ratio exceeding 40, among the highest for all known superconductors.
", "doi": "10.7907/nfyx-3565", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:15081", "collection": "thesis", "collection_id": "15081", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:01052023-230400021", "type": "thesis", "title": "Electronic Correlations and Topology in Graphene Moir\u00e9 Multilayers and InAs/GaSb-Derivative Systems", "author": [ { "family_name": "Polski", "given_name": "Robert Michael", "orcid": "0000-0003-0887-8099", "clpid": "Polski-Robert-Michael" } ], "thesis_advisor": [ { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" } ], "thesis_committee": [ { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Falson", "given_name": "Joseph", "orcid": "0000-0003-3183-9864", "clpid": "Falson-Joseph" }, { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Twisted bilayer graphene (TBG) near the magic angle exhibits a wide variety of correlated and topological phases such as superconductivity, correlated insulators, and orbital ferromagnetism. We show using electrical transport measurements that adding a layer of tungsten diselenide in proximity to twisted bilayer graphene stabilizes superconductivity to twist angles significantly below the magic angle despite the disappearance of correlated insulators and insulators at full moir\u00e9 filling. These findings--along with our report of a relationship between superconductivity and symmetry breaking Fermi surface reconstruction--suggest constraints on theories of the origin of superconductivity in TBG. In the context of this TBG-tungsten diselenide system, we study how the correlated phases evolve over a wide twist angle range and classify them into a hierarchy based on where they occur relative to the magic angle (or where bands have been maximally flattened). While effects such as orbital ferromagnetism near one electron per moir\u00e9 unit cell and gapped correlated insulators only exist in close proximity to the magic angle, superconductivity and high-temperature cascade transitions survive in a wider twist angle range.
\r\n\r\nWe also analyze the structures of twisted trilayer, quadrilayer, and pentalayer graphene (and all proximitized to tungsten diselenide) near their respective theoretical magic angles, revealing robust electron- and hole-side superconductivity in each heterostructure. We additionally find previously unreported insulating states in twisted trilayer and quadrilayer graphene along with an enlarged filling range of superconductivity in pentalayer. Our studies on twisted graphene multilayers beyond two layers allow us to generalize the correlated physics found in TBG and consider the role of the additional bands introduced.
\r\n\r\nIn the last part of this thesis, we measure the two-dimensional topological insulator candidate system InAs/GaSb with added stoichiometric impurities. Previous studies in pure InAs/GaSb structures have revealed low bulk resistivity and edge states that arise from trivial effects which can be easily mistaken for topological effects. Due, in part, to the strain effects of Indium impurities added to GaSb, our results show high bulk resistivity. We also, due to the wide gate-tunability in our devices, are able to measure the expected spin-orbit-split valence band structure. Our development of highly tunable InAs/GaSb-derivative structures paves the way for another look at two-dimensional topological insulator behavior in these systems and for their integration into superconducting devices.
", "doi": "10.7907/yhws-0f08", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:14372", "collection": "thesis", "collection_id": "14372", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09242021-222116257", "primary_object_url": { "basename": "Youngjoon_Choi_PhD_Thesis_Final.pdf", "content": "final", "filesize": 49417580, "license": "other", "mime_type": "application/pdf", "url": "/14372/1/Youngjoon_Choi_PhD_Thesis_Final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "A Spectroscopic Study of Electronic Correlations in Twisted Bilayer Graphene by Scanning Tunneling Microscopy", "author": [ { "family_name": "Choi", "given_name": "Youngjoon", "orcid": "0000-0001-9783-5992", "clpid": "Choi-Youngjoon" } ], "thesis_advisor": [ { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" } ], "thesis_committee": [ { "family_name": "Refael", "given_name": "Gil", "orcid": "0009-0007-4566-8441", "clpid": "Refael-G" }, { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Lee", "given_name": "Patrick A.", "orcid": "0000-0001-7809-8157", "clpid": "Lee-Patrick-A" }, { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Twisted bilayer graphene around the magic angle has shown variety of correlated phases such as superconductivity, correlated insulators, and magnetism due to its flat band structure. The unconventional nature of the superconductivity and its pos- sible relation to high temperature superconductors have sparked a lot of theoretical and experimental efforts to understand the properties of the magic angle twisted bilayer graphene. While electrical transport measurements revealed the interesting phases, spectroscopic understanding is strongly needed to connect the phases with theoretical calculations. We present the spectroscopic studies of gate-tunable magic angle twisted bilayer graphene using scanning tunneling microscopy. We report that the band structure is significantly modified even at charge neutrality due to exchange interaction. We apply a perpendicular magnetic field and develop a novel method that enables scanning tunneling microscopy to reveal Landau fan diagrams. We discover topologically non-trivial states appearing at finite magnetic field, and from spectroscopy we are able to identify the mechanism. Finally, we verify inter- action driven band flattening experimentally in twisted bilayer graphene, which is responsible for creating strong correlations.
", "doi": "10.7907/ajgk-7246", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:13781", "collection": "thesis", "collection_id": "13781", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06052020-175219708", "primary_object_url": { "basename": "PhD_Thesis_Harpreet_Arora_Final.pdf", "content": "final", "filesize": 43517279, "license": "other", "mime_type": "application/pdf", "url": "/13781/14/PhD_Thesis_Harpreet_Arora_Final.pdf", "version": "v9.0.0" }, "type": "thesis", "title": "Superconductivity in Graphene Hetero-Structures: From Fundamental Physics to Functional Devices", "author": [ { "family_name": "Arora", "given_name": "Harpreet Singh", "orcid": "0000-0002-7674-735X", "clpid": "Arora-Harpreet-Singh" } ], "thesis_advisor": [ { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" } ], "thesis_committee": [ { "family_name": "Roukes", "given_name": "Michael Lee", "orcid": "0000-0002-2916-6026", "clpid": "Roukes-M-L" }, { "family_name": "Rosenbaum", "given_name": "Thomas F.", "orcid": "0009-0008-6152-666X", "clpid": "Rosenbaum-T-F" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "While graphene has been dubbed as a \"wonder material\" because of its amazing characteristics, such as the ability to conduct electricity better than copper and being two hundred times stronger than steel, until recently, the key quantum phenomenon of superconductivity was missing from the list of properties exhibited by graphene. In 2018, an astonishing discovery showed that by placing two sheets of graphene on top of each other in a structure known as Twisted Bilayer Graphene, it is possible to realize superconductivity when the rotation angle between the sheets is close to the \"Magic Angle\" value of 1.1\u00b0. More surprisingly, superconductivity in the initial reports was observed in close proximity to insulating states - resembling the phase diagram of High Tc superconductors. This sparked a fierce debate about its origin and its possible relation to High Tc superconductors. In this thesis, we show that by carefully engineering the dielectric environment of TBG, it is possible to stabilize superconductivity in non-magic angle TBG devices without the presence of any insulating states. This discovery imposes severe constraints on the origin of superconductivity in TBG. We also report, for the first time, the successful induction of spin-orbit coupling in TBG and discuss its implications.
\r\n\r\nSuperconductivity can also be induced into graphene via coupling to conventional superconductors, and the strength of the induced supercurrent depends strongly on temperature. We employ this thermal dependence by integrating graphene into superconducting circuits that serves two purposes a) to investigate graphene's thermal behavior at milliKelvin temperatures and b) to utilize its extremely low heat capacity in making functional devices that have the potential to achieve ultra-high thermal sensitivity.
", "doi": "10.7907/nc05-gr15", "publication_date": "2020", "thesis_type": "phd", "thesis_year": "2020" } ]