[ { "id": "authors:7btgy-p9d44", "collection": "authors", "collection_id": "7btgy-p9d44", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20220816-183023896", "type": "monograph", "title": "Andreev reflection spectroscopy in strongly paired superconductors", "author": [ { "family_name": "Lewandowski", "given_name": "Cyprian", "orcid": "0000-0002-6944-9805", "clpid": "Lewandowski-Cyprian" }, { "family_name": "Lantagne-Hurtubise", "given_name": "\u00c9tienne", "orcid": "0000-0003-0417-6452", "clpid": "Lantagne-Hurtubise-\u00c9tienne" }, { "family_name": "Thomson", "given_name": "Alex", "orcid": "0000-0002-9938-5048", "clpid": "Thomson-Alex" }, { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" }, { "family_name": "Alicea", "given_name": "Jason", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" } ], "abstract": "Motivated by recent experiments on low-carrier-density superconductors, including twisted multilayer graphene, we study signatures of the BCS to BEC evolution in Andreev reflection spectroscopy. We establish that in a standard quantum point contact geometry, Andreev reflection in a BEC superconductor is unable to mediate a zero-bias conductance beyond e\u00b2/h per lead channel. This bound is shown to result from a duality that links the sub-gap conductance of BCS and BEC superconductors. We then demonstrate that sharp signatures of BEC superconductivity, including perfect Andreev reflection, can be recovered by tunneling through a suitably designed potential well. We propose various tunneling spectroscopy setups to experimentally probe this recovery.", "doi": "10.48550/arXiv.2207.09494", "publisher": "arXiv", "publication_date": "2022-07-19" }, { "id": "authors:06ybb-rx004", "collection": "authors", "collection_id": "06ybb-rx004", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20220524-180258498", "type": "monograph", "title": "Hierarchy of Symmetry Breaking Correlated Phases in Twisted Bilayer Graphene", "author": [ { "family_name": "Polski", "given_name": "Robert", "orcid": "0000-0003-0887-8099", "clpid": "Polski-Robert-M" }, { "family_name": "Zhang", "given_name": "Yiran", "orcid": "0000-0002-8477-0074", "clpid": "Zhang-Yiran" }, { "family_name": "Peng", "given_name": "Yang", "orcid": "0000-0002-8868-2928", "clpid": "Peng-Yang" }, { "family_name": "Arora", "given_name": "Harpreet Singh", "orcid": "0000-0002-7674-735X", "clpid": "Arora-Harpreet-Singh" }, { "family_name": "Choi", "given_name": "Youngjoon", "orcid": "0000-0001-9783-5992", "clpid": "Choi-Youngjoon" }, { "family_name": "Kim", "given_name": "Hyunjin", "orcid": "0000-0001-9886-0487", "clpid": "Kim-Hyunjin" }, { "family_name": "Watanabe", "given_name": "Kenji", "orcid": "0000-0003-3701-8119", "clpid": "Watanabe-Kenji" }, { "family_name": "Taniguchi", "given_name": "Takashi", "orcid": "0000-0002-1467-3105", "clpid": "Taniguchi-Takashi" }, { "family_name": "Refael", "given_name": "Gil", "clpid": "Refael-G" }, { "family_name": "von Oppen", "given_name": "Felix", "orcid": "0000-0002-2537-7256", "clpid": "von-Oppen-Felix" }, { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" } ], "abstract": "Twisted bilayer graphene (TBG) near the magic twist angle of \u223c1.1\u00b0 exhibits a rich phase diagram. However, the interplay between different phases and their dependence on twist angle is still elusive. Here, we explore the stability of various TBG phases and demonstrate that superconductivity near filling of two electrons per moir\u00e9 unit cell alongside Fermi surface reconstructions, as well as entropy-driven high-temperature phase transitions and linear-in-T resistance occur over a range of twist angles which extends far beyond those exhibiting correlated insulating phases. In the vicinity of the magic angle, we also find a metallic phase that displays a hysteretic anomalous Hall effect and incipient Chern insulating behaviour. Such a metallic phase can be rationalized in terms of the interplay between interaction-driven deformations of TBG bands leading to Berry curvature redistribution and Fermi surface reconstruction. Our results provide an extensive perspective on the hierarchy of correlated phases in TBG as classified by their robustness against deviations from the magic angle or, equivalently, their electronic interaction requirements.", "doi": "10.48550/arXiv.2205.05225", "publisher": "arXiv", "publication_date": "2022-05-11" }, { "id": "authors:qqvh4-vrf77", "collection": "authors", "collection_id": "qqvh4-vrf77", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20220524-180301852", "type": "monograph", "title": "Spin-Orbit Enhanced Superconductivity in Bernal Bilayer Graphene", "author": [ { "family_name": "Zhang", "given_name": "Yiran", "orcid": "0000-0002-8477-0074", "clpid": "Zhang-Yiran" }, { "family_name": "Polski", "given_name": "Robert", "orcid": "0000-0003-0887-8099", "clpid": "Polski-Robert-M" }, { "family_name": "Thomson", "given_name": "Alex", "orcid": "0000-0002-9938-5048", "clpid": "Thomson-Alex" }, { "family_name": "Lantagne-Hurtubise", "given_name": "\u00c9tienne", "orcid": "0000-0003-0417-6452", "clpid": "Lantagne-Hurtubise-\u00c9tienne" }, { "family_name": "Lewandowski", "given_name": "Cyprian", "orcid": "0000-0002-6944-9805", "clpid": "Lewandowski-Cyprian" }, { "family_name": "Zhou", "given_name": "Haoxin", "orcid": "0000-0003-1235-0035", "clpid": "Zhou-Haoxin" }, { "family_name": "Watanabe", "given_name": "Kenji", "orcid": "0000-0003-3701-8119", "clpid": "Watanabe-Kenji" }, { "family_name": "Taniguchi", "given_name": "Takashi", "orcid": "0000-0002-1467-3105", "clpid": "Taniguchi-Takashi" }, { "family_name": "Alicea", "given_name": "Jason", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" } ], "abstract": "In the presence of a large perpendicular electric field, Bernal-stacked bilayer graphene (BLG) features several broken-symmetry metallic phases as well as magnetic-field-induced superconductivity. The superconducting state is quite fragile, however, appearing only in a narrow window of density and with a maximum critical temperature T\ua700 \u2248 30~mK. Here, we show that placing monolayer tungsten diselenide (WSe\u2082) on BLG promotes Cooper pairing to an extraordinary degree: superconductivity appears at zero magnetic field, exhibits an order of magnitude enhancement in T\ua700, and occurs over a density range that is wider by a factor of eight. By mapping quantum oscillations in BLG-WSe\u2082 as a function of electric field and doping, we establish that superconductivity emerges throughout a region whose normal state is polarized, with two out of four spin-valley flavours predominantly populated. In-plane magnetic field measurements further reveal a striking dependence of the critical field on doping, with the Chandrasekhar-Clogston (Pauli) limit roughly obeyed on one end of the superconducting dome yet sharply violated on the other. Moreover, the superconductivity arises only for perpendicular electric fields that push BLG hole wavefunctions towards WSe\u2082 -- suggesting that proximity-induced (Ising) spin-orbit coupling plays a key role in enhancing the pairing. Our results pave the way for engineering robust, highly tunable, and ultra-clean graphene-based superconductors.", "doi": "10.48550/arXiv.2205.05087", "publisher": "arXiv", "publication_date": "2022-05-10" }, { "id": "authors:m8brt-1ce36", "collection": "authors", "collection_id": "m8brt-1ce36", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20220524-180254587", "type": "monograph", "title": "Unsupervised learning of two-component nematicity from STM data on magic angle bilayer graphene", "author": [ { "family_name": "Taranto", "given_name": "William", "clpid": "Taranto-William" }, { "family_name": "Lederer", "given_name": "Samuel", "orcid": "0000-0002-7443-3859", "clpid": "Lederer-Samuel" }, { "family_name": "Choi", "given_name": "Youngjoon", "clpid": "Choi-Youngjoon" }, { "family_name": "Izmailov", "given_name": "Pavel", "clpid": "Izmailov-Pavel" }, { "family_name": "Wilson", "given_name": "Andrew Gordon", "orcid": "0000-0002-2011-3315", "clpid": "Wilson-Andrew-Gordon" }, { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" }, { "family_name": "Kim", "given_name": "Eun-Ah", "clpid": "Kum-Eun-Ah" } ], "abstract": "Moir\u00e9 materials such as magic angle twisted bilayer graphene (MATBG) exhibit remarkable phenomenology, but present significant challenges for certain experimental methods, particularly scanning probes such as scanning tunneling microscopy (STM). Typical STM studies that can image tens of thousands of atomic unit cells can image roughly ten moir\u00e9 cells, making data analysis statistically fraught. Here, we propose a method to mitigate this problem by aggregating STM conductance data from several bias voltages, and then using the unsupervised machine learning method of gaussian mixture model clustering to draw maximal insight from the resulting dataset. We apply this method, using as input coarse-grained bond variables respecting the point group symmetry, to investigate nematic ordering tendencies in MATBG for both charge neutral and hole-doped samples. For the charge-neutral dataset, the clustering reveals the surprising coexistence of multiple types of nematicity that are unrelated by symmetry, and therefore generically nondegenerate. By contrast, the clustering in the hole doped data is consistent with long range order of a single type. Beyond its value in analyzing nematicity in MATBG, our method has the potential to enhance understanding of symmetry breaking and its spatial variation in a variety of moir\u00e9 materials.", "doi": "10.48550/arXiv.2203.04449", "publisher": "arXiv", "publication_date": "2022-03-08" }, { "id": "authors:n990w-e2f28", "collection": "authors", "collection_id": "n990w-e2f28", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20220113-234609742", "type": "monograph", "title": "Ascendance of Superconductivity in Magic-Angle Graphene Multilayers", "author": [ { "family_name": "Zhang", "given_name": "Yiran", "orcid": "0000-0002-8477-0074", "clpid": "Zhang-Yiran" }, { "family_name": "Polski", "given_name": "Robert", "orcid": "0000-0003-0887-8099", "clpid": "Polski-Robert-M" }, { "family_name": "Lewandowski", "given_name": "Cyprian", "orcid": "0000-0002-6944-9805", "clpid": "Lewandowski-Cyprian" }, { "family_name": "Thomson", "given_name": "Alex", "orcid": "0000-0002-9938-5048", "clpid": "Thomson-Alex" }, { "family_name": "Peng", "given_name": "Yang", "orcid": "0000-0002-8868-2928", "clpid": "Peng-Yang" }, { "family_name": "Choi", "given_name": "Youngjoon", "clpid": "Choi-Youngjoon" }, { "family_name": "Kim", "given_name": "Hyunjin", "orcid": "0000-0001-9886-0487", "clpid": "Kim-Hyunjin" }, { "family_name": "Watanabe", "given_name": "Kenji", "orcid": "0000-0003-3701-8119", "clpid": "Watanabe-Kenji" }, { "family_name": "Taniguchi", "given_name": "Takashi", "orcid": "0000-0002-1467-3105", "clpid": "Taniguchi-Takashi" }, { "family_name": "Alicea", "given_name": "Jason", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "von Oppen", "given_name": "Felix", "orcid": "0000-0002-2537-7256", "clpid": "von-Oppen-Felix" }, { "family_name": "Refael", "given_name": "Gil", "clpid": "Refael-G" }, { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" } ], "abstract": "Graphene moire superlattices have emerged as a platform hosting and abundance of correlated insulating, topological, and superconducting phases. While the origins of strong correlations and non-trivial topology are shown to be directly linked to flat moire bands, the nature and mechanism of superconductivity remain enigmatic. In particular, only alternating twisted stacking geometries of bilayer and trilayer graphene are found to exhibit robust superconductivity manifesting as zero resistance and Fraunhofer interference patterns. Here we demonstrate that magic-angle twisted tri-, quadri-, and pentalayers placed on monolayer tungsten diselenide exhibit flavour polarization and superconductivity. We also observe insulating states in the trilayer and quadrilayer arising at finite electric displacement fields, despite the presence of dispersive bands introduced by additional graphene layers. Moreover, the three multilayer geometries allow us to identify universal features in the family of graphene moire structures arising from the intricate relations between superconducting states, symmetry-breaking transitions, and van Hove singularities. Remarkably, as the number of layers increases, superconductivity emerges over a dramatically enhanced filling-factor range. In particular, in twisted pentalayers, superconductivity extends well beyond the filling of four electrons per moire unit cell, demonstrating the non-trivial role of the additional bands. Our results highlight the importance of the interplay between flat and dispersive bands in extending superconducting regions in graphene moire superlattices and open new frontiers for developing graphene-based superconductors.", "doi": "10.48550/arXiv.2112.09270", "publisher": "arXiv", "publication_date": "2021-12-17" }, { "id": "authors:sz595-h0961", "collection": "authors", "collection_id": "sz595-h0961", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20220113-182215445", "type": "monograph", "title": "Spectroscopic Signatures of Strong Correlations and Unconventional Superconductivity in Twisted Trilayer Graphene", "author": [ { "family_name": "Kim", "given_name": "Hyunjin", "orcid": "0000-0001-9886-0487", "clpid": "Kim-Hyunjin" }, { "family_name": "Choi", "given_name": "Youngjoon", "clpid": "Choi-Youngjoon" }, { "family_name": "Lewandowski", "given_name": "Cyprian", "orcid": "0000-0002-6944-9805", "clpid": "Lewandowski-Cyprian" }, { "family_name": "Thomson", "given_name": "Alex", "orcid": "0000-0002-9938-5048", "clpid": "Thomson-Alex" }, { "family_name": "Zhang", "given_name": "Yiran", "orcid": "0000-0002-8477-0074", "clpid": "Zhang-Yiran" }, { "family_name": "Polski", "given_name": "Robert", "orcid": "0000-0003-0887-8099", "clpid": "Polski-Robert-M" }, { "family_name": "Watanabe", "given_name": "Kenji", "orcid": "0000-0003-3701-8119", "clpid": "Watanabe-Kenji" }, { "family_name": "Taniguchi", "given_name": "Takashi", "orcid": "0000-0002-1467-3105", "clpid": "Taniguchi-Takashi" }, { "family_name": "Alicea", "given_name": "Jason", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" } ], "abstract": "Magic-angle twisted trilayer graphene (MATTG) has emerged as a novel moir\u00e9 material that exhibits both strong electronic correlations and unconventional superconductivity. However, spectroscopic studies of its electronic properties are lacking, and the nature of superconductivity and the corresponding order parameter in this system remain elusive. Here we perform high-resolution scanning tunneling microscopy and spectroscopy of MATTG and reveal extensive regions of atomic reconstruction that favor mirror-symmetric stacking. In these regions, we observe a cascade of symmetry-breaking electronic transitions and doping-dependent band structure deformations similar to those realized in magic-angle bilayers, as expected theoretically given the commonality of flat bands. More strikingly, in a density window spanning two to three holes per moire unit cell, spectroscopic signatures of superconductivity are manifest as pronounced dips in the tunneling conductance at the Fermi level accompanied by coherence peaks that become gradually suppressed at elevated temperatures and magnetic fields. The observed evolution of the conductance with doping is consistent with a gate-tunable transition from a gapped to a nodal superconductor, which we show theoretically is compatible with a sharp transition from a Bardeen-Cooper-Schrieffer (BCS) to a Bose-Einstein condensation (BEC) superconductor with a nodal order parameter. Within this doping window, we also detect peak-dip-hump structures suggesting that superconductivity is driven by strong coupling to bosonic modes of MATTG. Our results pave the way for further understanding of superconductivity and correlated states in graphene-based moir\u00e9 structures beyond twisted bilayers, where unconventional superconductivity and nodal pairing were reported.", "doi": "10.48550/arXiv.2109.12127", "publisher": "arXiv", "publication_date": "2021-09-24" } ]