{"topic":"moire-superconductivity","sort":"top","limit":25,"offset":0,"count":25,"papers":[{"id":"arxiv:1803.02342","kind":"article","title":"Unconventional superconductivity in magic-angle graphene superlattices","authors":["Yuan Cao","Valla Fatemi","Shiang Fang","Kenji Watanabe","Takashi Taniguchi","Efthimios Kaxiras","Pablo Jarillo‐Herrero"],"author_count":7,"year":2018,"published":"2018-03-05","venue":"Nature","cited_by":8329,"doi":"10.1038/nature26160","arxiv_id":"1803.02342","nct_id":null,"source_url":"https://doi.org/10.1038/nature26160","archive_url":"https://space-kitty.com/papers/arxiv:1803.02342","updated":"2018-05-20","pmid":"29512651","pmcid":null,"openalex_id":"W2793369479","pdf_url":"https://www.nature.com/articles/nature26160.pdf","abstract":"The understanding of strongly-correlated materials, and in particular unconventional superconductors, has puzzled physicists for decades. Such difficulties have stimulated new research paradigms, such as ultra-cold atom lattices for simulating quantum materials. Here we report on the realization of intrinsic unconventional superconductivity in a 2D superlattice created by stacking two graphene sheets with a small twist angle. For angles near $1.1^\\circ$, the first `magic' angle, twisted bilayer graphene (TBG) exhibits ultra-flat bands near charge neutrality, which lead to correlated insulating states at half-filling. Upon electrostatic doping away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature $T_c$ up to 1.7 K. The temperature-density phase diagram shows similarities with that of the cuprates, including superconducting domes. Moreover, quantum oscillations indicate small Fermi surfaces near the correlated insulating phase, in analogy with under-doped cuprates. The relative high $T_c$, given such small Fermi surface (corresponding to a record-low 2D carrier density of $10^{11} \\textrm{cm}^{-2}$ , renders TBG among the strongest coupling superconductors, in a regime close to the BCS-BEC crossover. These novel results establish TBG as the first purely carbon-based 2D superconductor and as a highly tunable platform to investigate strongly-correlated phenomena, which could lead to insights into the physics of high-$T_c$ superconductors and quantum spin liquids.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["Physics of Superconductivity and Magnetism"],"details":null,"provenance":"openalex+arxiv","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:1802.00553","kind":"article","title":"Correlated insulator behaviour at half-filling in magic-angle graphene superlattices","authors":["Yuan Cao","Valla Fatemi","Ahmet Demir","Shiang Fang","Spencer Tomarken","Jason Y. Luo","Javier Sanchez-Yamagishi","Kenji Watanabe","Takashi Taniguchi","Efthimios Kaxiras","R. C. Ashoori","Pablo Jarillo‐Herrero"],"author_count":12,"year":2018,"published":"2018-03-05","venue":"Nature","cited_by":4884,"doi":"10.1038/nature26154","arxiv_id":"1802.00553","nct_id":null,"source_url":"https://doi.org/10.1038/nature26154","archive_url":"https://space-kitty.com/papers/arxiv:1802.00553","updated":"2018-02-02","pmid":"29512654","pmcid":null,"openalex_id":"W2787584289","pdf_url":"https://arxiv.org/pdf/1802.00553","abstract":"Van der Waals (vdW) heterostructures are an emergent class of metamaterials comprised of vertically stacked two-dimensional (2D) building blocks, which provide us with a vast tool set to engineer their properties on top of the already rich tunability of 2D materials. One of the knobs, the twist angle between different layers, plays a crucial role in the ultimate electronic properties of a vdW heterostructure and does not have a direct analog in other systems such as MBE-grown semiconductor heterostructures. For small twist angles, the moiré pattern produced by the lattice misorientation creates a long-range modulation. So far, the study of the effect of twist angles in vdW heterostructures has been mostly concentrated in graphene/hexagonal boron nitride (h-BN) twisted structures, which exhibit relatively weak interlayer interaction due to the presence of a large bandgap in h-BN. Here we show that when two graphene sheets are twisted by an angle close to the theoretically predicted 'magic angle', the resulting flat band structure near charge neutrality gives rise to a strongly-correlated electronic system. These flat bands exhibit half-filling insulating phases at zero magnetic field, which we show to be a Mott-like insulator arising from electrons localized in the moiré superlattice. These unique properties of magic-angle twisted bilayer graphene (TwBLG) open up a new playground for exotic many-body quantum phases in a 2D platform made of pure carbon and without magnetic field. The easy accessibility of the flat bands, the electrical tunability, and the bandwidth tunability though twist angle may pave the way towards more exotic correlated systems, such as unconventional superconductors or quantum spin liquids.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["Graphene research and applications"],"details":null,"provenance":"openalex+arxiv","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:1903.06513","kind":"article","title":"Superconductors, orbital magnets and correlated states in magic-angle bilayer graphene","authors":["Xiaobo Lu","Petr Stepanov","Wei Yang","Ming Xie","Mohammed Ali Aamir","Ipsita Das","C. Urgell","Kenji Watanabe","Takashi Taniguchi","Guangyu Zhang","Adrian Bachtold","Allan H. MacDonald","Dmitri K. Efetov"],"author_count":13,"year":2019,"published":"2019-10-30","venue":"Nature","cited_by":1597,"doi":"10.1038/s41586-019-1695-0","arxiv_id":"1903.06513","nct_id":null,"source_url":"https://doi.org/10.1038/s41586-019-1695-0","archive_url":"https://space-kitty.com/papers/arxiv:1903.06513","updated":"2019-04-10","pmid":"31666722","pmcid":null,"openalex_id":"W2982376788","pdf_url":"https://arxiv.org/pdf/1903.06513","abstract":"Superconductivity often occurs close to broken-symmetry parent states and is especially common in doped magnetic insulators. When twisted close to a magic relative orientation angle near 1 degree, bilayer graphene has flat moire superlattice minibands that have emerged as a rich and highly tunable source of strong correlation physics, notably the appearance of superconductivity close to interaction-induced insulating states. Here we report on the fabrication of bilayer graphene devices with exceptionally uniform twist angles. We show that the reduction in twist angle disorder reveals insulating states at all integer occupancies of the four-fold spin/valley degenerate flat conduction and valence bands, i.e. at moire band filling factors nu = 0, +(-) 1, +(-) 2, +(-) 3, and superconductivity below critical temperatures as high as 3 K close to - 2 filling. We also observe three new superconducting domes at much lower temperatures close to the nu = 0 and nu = +(-) 1 insulating states. Interestingly, at nu = +(-) 1 we find states with non-zero Chern numbers. For nu = - 1 the insulating state exhibits a sharp hysteretic resistance enhancement when a perpendicular magnetic field above 3.6 tesla is applied, consistent with a field driven phase transition. Our study shows that symmetry-broken states, interaction driven insulators, and superconducting domes are common across the entire moire flat bands, including near charge neutrality.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["Graphene research and applications"],"details":null,"provenance":"openalex+arxiv","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:2012.01434","kind":"article","title":"Tunable strongly coupled superconductivity in magic-angle twisted trilayer graphene","authors":["Jeong Min Park","Yuan Cao","Kenji Watanabe","Takashi Taniguchi","Pablo Jarillo‐Herrero"],"author_count":5,"year":2021,"published":"2021-02-01","venue":"Nature","cited_by":857,"doi":"10.1038/s41586-021-03192-0","arxiv_id":"2012.01434","nct_id":null,"source_url":"https://doi.org/10.1038/s41586-021-03192-0","archive_url":"https://space-kitty.com/papers/arxiv:2012.01434","updated":"2020-12-02","pmid":"33526935","pmcid":null,"openalex_id":"W3127466677","pdf_url":"https://arxiv.org/pdf/2012.01434","abstract":"Moiré superlattices have recently emerged as a novel platform where correlated physics and superconductivity can be studied with unprecedented tunability. Although correlated effects have been observed in several other moiré systems, magic-angle twisted bilayer graphene (MATBG) remains the only one where robust superconductivity has been reproducibly measured. Here we realize a new moiré superconductor, mirror symmetric magic-angle twisted trilayer graphene (MATTG) with dramatically richer tunability in electronic structure and superconducting properties. Hall effect and quantum oscillations measurements as a function of density and electric field allow us to determine the system's tunable phase boundaries in the normal state. Zero magnetic field resistivity measurements then reveal that the existence of superconductivity is intimately connected to the broken symmetry phase emerging from two carriers per moiré unit cell. Strikingly, we find that the superconducting phase gets suppressed and bounded at the van Hove singularities (vHs) partially surrounding the broken-symmetry phase, which is difficult to reconcile with weak-coupling BCS theory. Moreover, the extensive in situ tunability of our system allows us to achieve the ultra-strong coupling regime, characterized by a Ginzburg-Landau coherence length reaching the average inter-particle distance and very large $T_\\mathrm{BKT}/T_{F}$ ratios in excess of 0.1, where $T_\\mathrm{BKT}$ and $T_F$ are the Berezinskii-Kosterlitz-Thouless transition and Fermi temperatures, respectively. These observations suggest that MATTG can be electrically tuned close to the two-dimensional BCS-BEC crossover. Our results establish a new generation of tunable moiré superconductors with the potential to revolutionize our fundamental understanding and the applications of strong coupling superconductivity.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["Physics of Superconductivity and Magnetism"],"details":null,"provenance":"openalex+arxiv","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:2008.08129","kind":"review","title":"Graphene bilayers with a twist","authors":["Eva Y. Andrei","Allan H. MacDonald"],"author_count":2,"year":2020,"published":"2020-11-18","venue":"Nature Materials","cited_by":913,"doi":"10.1038/s41563-020-00840-0","arxiv_id":"2008.08129","nct_id":null,"source_url":"https://doi.org/10.1038/s41563-020-00840-0","archive_url":"https://space-kitty.com/papers/arxiv:2008.08129","updated":"2020-08-18","pmid":"33208935","pmcid":null,"openalex_id":"W3062278829","pdf_url":"https://arxiv.org/pdf/2008.08129","abstract":"Near a magic twist angle, bilayer graphene transforms from a weakly correlated Fermi liquid to a strongly correlated two-dimensional electron system with properties that are extraordinarily sensitive to carrier density and to controllable environmental factors such as the proximity of nearby gates and twist-angle variation. Among other phenomena magic-angle twisted bilayer graphene hosts superconductivity, interaction induced insulating states, magnetism, electronic nematicity, linear-in-T low-temperature resistivity, and quantized anomalous Hall states. We highlight some key research results in this field, point to important questions that remain open, and comment on the place of magic angle twisted bilayer graphene in the strongly correlated quantum matter world.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["Graphene research and applications"],"details":null,"provenance":"openalex+arxiv","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:1812.08776","kind":"article","title":"Maximized electron interactions at the magic angle in twisted bilayer graphene","authors":["Alexander Kerelsky","L. J. McGilly","Dante Marvin Kennes","Lede Xian","Matthew Yankowitz","Shao-Wen Chen","Kenji Watanabe","Takashi Taniguchi","James Hone","Cory R. Dean","Ángel Rubio","Abhay N. Pasupathy"],"author_count":12,"year":2019,"published":"2019-07-31","venue":"Nature","cited_by":936,"doi":"10.1038/s41586-019-1431-9","arxiv_id":"1812.08776","nct_id":null,"source_url":"https://doi.org/10.1038/s41586-019-1431-9","archive_url":"https://space-kitty.com/papers/arxiv:1812.08776","updated":"2018-12-26","pmid":"31367030","pmcid":null,"openalex_id":"W2904994892","pdf_url":"https://arxiv.org/pdf/1812.08776","abstract":"The electronic properties of heterostructures of atomically-thin van der Waals (vdW) crystals can be modified substantially by Moiré superlattice potentials arising from an interlayer twist between crystals. Moiré-tuning of the band structure has led to the recent discovery of superconductivity and correlated insulating phases in twisted bilayer graphene (TBLG) near the so-called \"magic angle\" of $\\sim$1.1°, with a phase diagram reminiscent of high T$_c$ superconductors. However, lack of detailed understanding of the electronic spectrum and the atomic-scale influence of the Moiré pattern has so far precluded a coherent theoretical understanding of the correlated states. Here, we directly map the atomic-scale structural and electronic properties of TBLG near the magic angle using scanning tunneling microscopy and spectroscopy (STM/STS). We observe two distinct van Hove singularities (vHs) in the LDOS which decrease in separation monotonically through 1.1° with the bandwidth (t) of each vHs minimized near the magic angle. When doped near half Moiré band filling, the conduction vHs shifts to the Fermi level and an additional correlation-induced gap splits the vHs with a maximum size of 7.5 meV. We also find that three-fold (C$_3$) rotational symmetry of the LDOS is broken in doped TBLG with a maximum symmetry breaking observed for states near the Fermi level, suggestive of nematic electronic interactions. The main features of our doping and angle dependent spectroscopy are captured by a tight-binding model with on-site (U) and nearest neighbor Coulomb interactions. We find that the ratio U/t is of order unity, indicating that electron correlations are significant in magic angle TBLG. Rather than a simple maximization of the DOS, superconductivity arises in TBLG at angles where the ratio U/t is largest, suggesting a pairing mechanism based on electron-electron interactions.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["Graphene research and applications"],"details":null,"provenance":"openalex+arxiv","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:1808.05250","kind":"article","title":"Origin of Magic Angles in Twisted Bilayer Graphene","authors":["Grigory Tarnopolsky","Alex J. Kruchkov","Ashvin Vishwanath"],"author_count":3,"year":2018,"published":"2018-08-15","venue":"Phys. Rev. Lett. 122, 106405 (2019)","cited_by":791,"doi":"10.1103/physrevlett.122.106405","arxiv_id":"1808.05250","nct_id":null,"source_url":"https://arxiv.org/abs/1808.05250","archive_url":"https://space-kitty.com/papers/arxiv:1808.05250","updated":"2018-08-15","pmid":"30932657","pmcid":null,"openalex_id":"W2887373946","pdf_url":"https://arxiv.org/pdf/1808.05250","abstract":"Twisted Bilayer graphene (TBG) is known to feature isolated and relatively flat bands near charge neutrality, when tuned to special magic angles. However, different criteria for the magic angle such as the vanishing of Dirac speed, minimal bandwidth or maximal band gap to higher bands typically give different results. Here we study a modified continuum model for TBG which has an infinite sequence of magic angles $θ$ at which, we simultaneously find that (i) the Dirac speed vanishes (ii) absolutely flat bands appear at neutrality and (iii) bandgaps to the excited bands are maximized. When parameterized in terms of $α\\sim 1/θ, $ they recur with the simple periodicity of $Δα\\simeq 3/2$, which, beyond the first magic angle, differs from earlier calculations. Further, in this model we prove that the vanishing of the Dirac velocity ensures the exact flatness of the band and show that the flat band wave functions are related to doubly-periodic functions composed of ratios of theta functions. Also, using perturbation theory up to $α^8$ we capture important features of the first magic angle $θ\\approx1.09^{\\circ}$ ($α\\approx 0.586$), which precisely explains the numerical results. Finally, based on our model we discuss the prospects for observing the second magic angle in TBG.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["cond-mat.str-el","cond-mat.mes-hall"],"details":null,"provenance":"arxiv+openalex","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:1903.08596","kind":"article","title":"Tunable correlated states and spin-polarized phases in twisted bilayer–bilayer graphene","authors":["Yuan Cao","Daniel Rodan‐Legrain","Oriol Rubies-Bigordà","Jeong Min Park","Kenji Watanabe","Takashi Taniguchi","Pablo Jarillo‐Herrero"],"author_count":7,"year":2020,"published":"2020-05-06","venue":"Nature","cited_by":674,"doi":"10.1038/s41586-020-2260-6","arxiv_id":"1903.08596","nct_id":null,"source_url":"https://doi.org/10.1038/s41586-020-2260-6","archive_url":"https://space-kitty.com/papers/arxiv:1903.08596","updated":"2019-03-25","pmid":"32499644","pmcid":null,"openalex_id":"W2924236731","pdf_url":"https://arxiv.org/pdf/1903.08596","abstract":"The recent discovery of correlated insulator states and superconductivity in magic-angle twisted bilayer graphene has paved the way to the experimental investigation of electronic correlations in tunable flat band systems realized in twisted van der Waals heterostructures. This novel twist angle degree of freedom and control should be generalizable to other 2D systems, which may exhibit similar correlated physics behavior while at the same time enabling new techniques to tune and control the strength of electron-electron interactions. Here, we report on a new highly tunable correlated system based on small-angle twisted bilayer-bilayer graphene (TBBG), consisting of two rotated sheets of Bernal-stacked bilayer graphene. We find that TBBG exhibits a rich phase diagram, with tunable correlated insulators states that are highly sensitive to both twist angle and to the application of an electric displacement field, the latter reflecting the inherent polarizability of Bernal-stacked bilayer graphene. We find correlated insulator states that can be switched on and off by the displacement field at all integer electron fillings of the moiré unit cell. The response of these correlated states to magnetic fields points towards evidence of electrically switchable magnetism. Moreover, the strong dependence of the resistance at low temperature and near the correlated insulator states indicates possible proximity to a superconducting phase. Furthermore, in the regime of lower twist angles, TBBG shows multiple sets of flat bands near charge neutrality, resulting in numerous correlated states corresponding to half-filling of each of these flat bands. Our results pave the way to the exploration of novel twist-angle and electric-field controlled correlated phases of matter in novel multi-flat band twisted superlattices.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["Graphene research and applications"],"details":null,"provenance":"openalex+arxiv","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:1901.04621","kind":"article","title":"Signatures of tunable superconductivity in a trilayer graphene moiré superlattice","authors":["Guorui Chen","Aaron L. Sharpe","Patrick Gallagher","Ilan T. Rosen","Eli J. Fox","Lili Jiang","Bosai Lyu","Hongyuan Li","Kenji Watanabe","Takashi Taniguchi","Jeil Jung","Zhiwen Shi","David Goldhaber‐Gordon","Yuanbo Zhang","Feng Wang"],"author_count":15,"year":2019,"published":"2019-07-17","venue":"Nature","cited_by":668,"doi":"10.1038/s41586-019-1393-y","arxiv_id":"1901.04621","nct_id":null,"source_url":"https://doi.org/10.1038/s41586-019-1393-y","archive_url":"https://space-kitty.com/papers/arxiv:1901.04621","updated":"2019-01-15","pmid":"31316203","pmcid":null,"openalex_id":"W2909737023","pdf_url":"https://arxiv.org/pdf/1901.04621","abstract":"Understanding the mechanism of high temperature (high Tc) superconductivity is a central problem in condensed matter physics. It is often speculated that high Tc superconductivity arises from a doped Mott insulator as described by the Hubbard model. An exact solution of the Hubbard model, however, is extremely challenging due to the strong electron-electron correlation. Therefore, it is highly desirable to experimentally study a model Hubbard system in which the unconventional superconductivity can be continuously tuned by varying the Hubbard parameters. Here we report signatures of tunable superconductivity in ABC-trilayer graphene (TLG) / boron nitride (hBN) moiré superlattice. Unlike \"magic angle\" twisted bilayer graphene, theoretical calculations show that under a vertical displacement field the ABC-TLG/hBN heterostructure features an isolated flat valence miniband associated with a Hubbard model on a triangular superlattice. Upon applying such a displacement field we find experimentally that the ABC-TLG/hBN superlattice displays Mott insulating states below 20 Kelvin at 1/4 and 1/2 fillings, corresponding to 1 and 2 holes per unit cell, respectively. Upon further cooling, signatures of superconducting domes emerge below 1 kelvin for the electron- and hole-doped sides of the 1/4 filling Mott state. The electronic behavior in the TLG/hBN superlattice is expected to depend sensitively on the interplay between the electron-electron interaction and the miniband bandwidth, which can be tuned continuously with the displacement field D. By simply varying the D field, we demonstrate transitions from the candidate superconductor to Mott insulator and metallic phases. Our study shows that TLG/hBN heterostructures offer an attractive model system to explore rich correlated behavior emerging in the tunable triangular Hubbard model.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["Physics of Superconductivity and Magnetism"],"details":null,"provenance":"openalex+arxiv","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:1906.09274","kind":"article","title":"Spectroscopic signatures of many-body correlations in magic angle twisted bilayer graphene","authors":["Yonglong Xie","Biao Lian","Berthold Jäck","Xiaomeng Liu","Cheng-Li Chiu","Kenji Watanabe","Takashi Taniguchi","B. Andrei Bernevig","Ali Yazdani"],"author_count":9,"year":2019,"published":"2019-06-21","venue":"Nature 572, 101 (2019)","cited_by":665,"doi":"10.1038/s41586-019-1422-x","arxiv_id":"1906.09274","nct_id":null,"source_url":"https://arxiv.org/abs/1906.09274","archive_url":"https://space-kitty.com/papers/arxiv:1906.09274","updated":"2019-06-21","pmid":"31367031","pmcid":null,"openalex_id":"W2965205629","pdf_url":"https://arxiv.org/pdf/1906.09274","abstract":"The discovery of superconducting and insulating states in magic angle twisted bilayer graphene (MATBG) has ignited considerable interest in understanding the nature of electronic interactions in this chemically pristine material system. The phenomenological similarity of the MATBG transport properties as a function of doping with those of the high-Tc cuprates and other unconventional superconductors suggests the possibility that MATBG may be a highly interacting system. However, there have not been any direct experimental evidence for strong many-body correlations in MATBG. Here we provide such evidence from using high-resolution spectroscopic measurements, as a function of carrier density, with a scanning tunneling microscopy (STM). We find MATBG to display unusual spectroscopic characteristics that can be attributed to electron-electron interactions over a wide range of doping, including when superconductivity emerges in this system. We show that our measurements cannot be explained with a mean-field approach for modeling electron-electron interaction in MATBG. The breakdown of a mean-field approach for understanding the properties of other correlated superconductors, such as cuprates, has long inspired the study of highly correlated Hubbard model. We show that a phenomenological extended Hubbard model cluster calculation, motivated by the nearly localized nature of the relevant electronic states of MATBG produces spectroscopic features similar to those we observe experimentally. Our findings demonstrate the critical role of many-body correlations in understanding the properties of MATBG.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["cond-mat.mes-hall","cond-mat.str-el","cond-mat.supr-con"],"details":null,"provenance":"arxiv+openalex","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:1903.08130","kind":"article","title":"Tunable spin-polarized correlated states in twisted double bilayer graphene","authors":["Xiaomeng Liu","Zeyu Hao","Eslam Khalaf","Jong Yeon Lee","Yuval Ronen","Hyobin Yoo","Danial Haei Najafabadi","Kenji Watanabe","Takashi Taniguchi","Ashvin Vishwanath","Philip Kim"],"author_count":11,"year":2020,"published":"2020-07-08","venue":"Nature","cited_by":618,"doi":"10.1038/s41586-020-2458-7","arxiv_id":"1903.08130","nct_id":null,"source_url":"https://doi.org/10.1038/s41586-020-2458-7","archive_url":"https://space-kitty.com/papers/arxiv:1903.08130","updated":"2019-03-25","pmid":"32641816","pmcid":null,"openalex_id":"W2922318702","pdf_url":"https://arxiv.org/pdf/1903.08130","abstract":"Ferromagnetism and superconductivity typically compete with each other since the internal magnetic field generated in a magnet suppresses the formation of spin-singlet Cooper pairs in conventional superconductors. Only a handful of ferromagnetic superconductors are known in heavy fermion systems, where many-body electron interactions promoted by the narrow energy bands play a key role in stabilizing these emergent states. Recently, interaction-driven superconductivity and ferromagnetism have been demonstrated as separate phenomena in different density regimes of flat bands enabled by graphene moire superlattices. Combining superconductivity and magnetism in a single ground state may lead to more exotic quantum phases. Here, employing van der Waals heterostructures of twisted double bilayer graphene (TDBG), we realize a flat electron band that is tunable by perpendicular electric fields. Similar to the magic angle twisted bilayer graphene, TDBG exhibits energy gaps at the half and quarter filled flat bands, indicating the emergence of correlated insulating states. We find that the gaps of these insulating states increase with in-plane magnetic field, suggesting a ferromagnetic order. Upon doping the ferromagnetic half-filled insulator, superconductivity emerges with a critical temperature controlled by both density and electric fields. We observe that the in-plane magnetic field enhances the superconductivity in the low field regime, suggesting spin-polarized electron pairing. Spin-polarized superconducting states discovered in TDBG provide a new route to engineering interaction-driven topological superconductivity.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["Graphene research and applications"],"details":null,"provenance":"openalex+arxiv","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:2012.02773","kind":"article","title":"Electric field–tunable superconductivity in alternating-twist magic-angle trilayer graphene","authors":["Zeyu Hao","A. M. Zimmerman","Patrick Ledwith","Eslam Khalaf","Danial Haie Najafabadi","Kenji Watanabe","Takashi Taniguchi","Ashvin Vishwanath","Philip Kim"],"author_count":9,"year":2021,"published":"2021-02-04","venue":"Science","cited_by":518,"doi":"10.1126/science.abg0399","arxiv_id":"2012.02773","nct_id":null,"source_url":"https://doi.org/10.1126/science.abg0399","archive_url":"https://space-kitty.com/papers/arxiv:2012.02773","updated":"2020-12-07","pmid":"33542148","pmcid":null,"openalex_id":"W3113282607","pdf_url":"https://arxiv.org/pdf/2012.02773","abstract":"We construct a van der Waals heterostructure consisting of three graphene layers stacked with alternating twisting angles $\\pmθ$. At the average twist angle $θ\\sim 1.56^{\\circ}$, a theoretically predicted magic angle for the formation of flat electron bands, narrow conduction and valence moiré bands appear together with a linearly dispersing Dirac band. Upon doping the half-filled moiré valence band with holes, or the half-filled moiré conduction band with electrons, displacement field tunable superconductivity emerges, reaching a maximum critical temperature of 2.1 K at optimal doping and displacement field. By tuning the doping level and displacement fields, we find that superconducting regimes occur in conjunction with flavour polarization of moiré bands bounded by a van Hove singularity (vHS) at high displacement fields. This experimental observation is found to be inconsistent with a weak coupling description, suggesting that the observed moiré superconductivity has an unconventional nature.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["Graphene research and applications"],"details":null,"provenance":"openalex+arxiv","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:1901.03710","kind":"article","title":"Strange Metal in Magic-Angle Graphene with near Planckian Dissipation","authors":["Yuan Cao","Debanjan Chowdhury","Daniel Rodan‐Legrain","Oriol Rubies-Bigordà","Kenji Watanabe","Takashi Taniguchi","T. Senthil","Pablo Jarillo‐Herrero"],"author_count":8,"year":2020,"published":"2020-02-18","venue":"Physical Review Letters","cited_by":528,"doi":"10.1103/physrevlett.124.076801","arxiv_id":"1901.03710","nct_id":null,"source_url":"https://doi.org/10.1103/physrevlett.124.076801","archive_url":"https://space-kitty.com/papers/arxiv:1901.03710","updated":"2019-01-11","pmid":"32142336","pmcid":null,"openalex_id":"W2910872414","pdf_url":"https://arxiv.org/pdf/1901.03710","abstract":"Recent experiments on magic-angle twisted bilayer graphene have discovered correlated insulating behavior and superconductivity at a fractional filling of an isolated narrow band. In this paper we show that magic-angle bilayer graphene exhibits another hallmark of strongly correlated systems --- a broad regime of $T-$linear resistivity above a small, density dependent, crossover temperature--- for a range of fillings near the correlated insulator. We also extract a transport \"scattering rate\", which satisfies a near Planckian form that is universally related to the ratio of $(k_BT/\\hbar)$. Our results establish magic-angle bilayer graphene as a highly tunable platform to investigate strange metal behavior, which could shed light on this mysterious ubiquitous phase of correlated matter.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["Graphene research and applications"],"details":null,"provenance":"openalex+arxiv","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:1911.09198","kind":"article","title":"Untying the insulating and superconducting orders in magic-angle graphene","authors":["Petr Stepanov","Ipsita Das","Xiaobo Lu","Ali Fahimniya","Kenji Watanabe","Takashi Taniguchi","Frank H. L. Koppens","Johannes Lischner","Leonid S Levitov","Dmitri K. Efetov"],"author_count":10,"year":2020,"published":"2020-07-06","venue":"Nature","cited_by":511,"doi":"10.1038/s41586-020-2459-6","arxiv_id":"1911.09198","nct_id":null,"source_url":"https://doi.org/10.1038/s41586-020-2459-6","archive_url":"https://space-kitty.com/papers/arxiv:1911.09198","updated":"2020-12-24","pmid":"32632215","pmcid":null,"openalex_id":"W3041558139","pdf_url":"https://arxiv.org/pdf/1911.09198","abstract":"The coexistence of superconducting and correlated insulating states in magic-angle twisted bilayer graphene prompts fascinating questions about the relationship of these orders. Independent control of the microscopic mechanisms governing these phases could help uncover their individual roles and shed light on their intricate interplay. Here we report on direct tuning of electronic interactions in this system by changing its separation from a metallic screening layer. We observe quenching of correlated insula-tors in devices with screening layer separations that are smaller than a typical Wannier orbital size of 15nm, and with the twist angles slightly deviating from the magic value 1.10 plus(minus) 0.05 degrees. Upon extinction of the insulating orders, the vacated phase space is taken over by superconducting domes that feature critical temperatures comparable to those in the devices with strong insulators. In addition, we find that insulators at half-filling can reappear in small out-of-plane magnetic fields of 0.4 T, giving rise to quantized Hall states with a Chern number of 2. Our study suggests reexamination of the often-assumed mother-child relation between the insulating and superconducting phases in moire graphene, and illustrates a new approach to directly probe microscopic mechanisms of superconductivity in strongly-correlated systems.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["Graphene research and applications"],"details":null,"provenance":"openalex+arxiv","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:2003.11072","kind":"article","title":"Tuning electron correlation in magic-angle twisted bilayer graphene using Coulomb screening","authors":["Xiaoxue Liu","Zhi Wang","K. Watanabe","T. Taniguchi","Oskar Vafek","J. I. A. Li"],"author_count":6,"year":2020,"published":"2020-03-24","venue":"Science","cited_by":274,"doi":"10.1126/science.abb8754","arxiv_id":"2003.11072","nct_id":null,"source_url":"https://arxiv.org/abs/2003.11072","archive_url":"https://space-kitty.com/papers/arxiv:2003.11072","updated":"2020-07-08","pmid":"33737488","pmcid":null,"openalex_id":"W3013345219","pdf_url":"https://arxiv.org/pdf/2003.11072","abstract":"The ability to control the strength of interaction is essential for studying quantum phenomena emerging from a system of correlated fermions. For example, the isotope effect illustrates the effect of electron-phonon coupling on superconductivity, providing an important experimental support for the BCS theory. In this work, we report a new device geometry where the magic-angle twisted bilayer graphene (tBLG) is placed in close proximity to a Bernal bilayer graphene (BLG) separated by a 3 nm thick barrier. Using charge screening from the Bernal bilayer, the strength of electron-electron Coulomb interaction within the twisted bilayer can be continuously tuned. Transport measurements show that tuning Coulomb screening has opposite effect on the insulating and superconducting states: as Coulomb interaction is weakened by screening, the insulating states become less robust, whereas the stability of superconductivity is enhanced. Out results demonstrate the ability to directly probe the role of Coulomb interaction in magic-angle twisted bilayer graphene. Most importantly, the effect of Coulomb screening points toward electron-phonon coupling as the dominant mechanism for Cooper pair formation, and therefore superconductivity, in magic-angle twisted bilayer graphene.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["cond-mat.mes-hall","cond-mat.str-el","cond-mat.supr-con"],"details":null,"provenance":"arxiv+openalex","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:1911.13302","kind":"article","title":"Independent superconductors and correlated insulators in twisted bilayer graphene","authors":["Yu Saito","Jingyuan Ge","Kenji Watanabe","Takashi Taniguchi","Andrea F. Young"],"author_count":5,"year":2020,"published":"2020-06-01","venue":"Nature Physics","cited_by":462,"doi":"10.1038/s41567-020-0928-3","arxiv_id":"1911.13302","nct_id":null,"source_url":"https://doi.org/10.1038/s41567-020-0928-3","archive_url":"https://space-kitty.com/papers/arxiv:1911.13302","updated":"2019-12-03","pmid":null,"pmcid":null,"openalex_id":"W2991405141","pdf_url":"https://arxiv.org/pdf/1911.13302","abstract":"When bilayer graphene is rotationally faulted to an angle $θ\\approx 1.1^\\circ$, theory predicts the formation of a flat electronic band and correlated insulating, superconducting, and ferromagnetic states have all been observed at partial band filling. The proximity of superconductivity to correlated insulators has suggested a close relationship between these states, reminiscent of the cuprates where superconductivity arises by doping a Mott insulator. Here, we show that superconductivity can appear without correlated insulating states. While both superconductivity and correlated insulating behavior are strongest near the flat band condition, superconductivity survives to larger detuning of the angle. Our observations are consistent with a \"competing phases\" picture, in which insulators and superconductivity arise from disparate mechanisms.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["Graphene research and applications"],"details":null,"provenance":"openalex+arxiv","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:1805.08735","kind":"article","title":"Theory of Phonon-Mediated Superconductivity in Twisted Bilayer Graphene","authors":["Fengcheng Wu","Allan H. MacDonald","Ivar Martin"],"author_count":3,"year":2018,"published":"2018-12-17","venue":"Physical Review Letters","cited_by":477,"doi":"10.1103/physrevlett.121.257001","arxiv_id":"1805.08735","nct_id":null,"source_url":"https://doi.org/10.1103/physrevlett.121.257001","archive_url":"https://space-kitty.com/papers/arxiv:1805.08735","updated":"2018-12-17","pmid":"30608789","pmcid":null,"openalex_id":"W2804628806","pdf_url":"https://arxiv.org/pdf/1805.08735","abstract":"We present a theory of phonon-mediated superconductivity in near magic angle twisted bilayer graphene. Using a microscopic model for phonon coupling to moiré band electrons, we find that phonons generate attractive interactions in both $s$ and $d$ wave pairing channels and that the attraction is strong enough to explain the experimental superconducting transition temperatures. Before including Coulomb repulsion, the $s$-wave channel is more favorable; however, on-site Coulomb repulsion can suppress $s$-wave pairing relative to $d$-wave. The pair amplitude varies spatially with the moiré period, and is identical in the two layers in the $s$-wave channel but phase shifted by $π$ in the $d$-wave channel. We discuss experiments that can distinguish the two pairing states.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["Physics of Superconductivity and Magnetism"],"details":null,"provenance":"openalex+arxiv","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:1908.04595","kind":"article","title":"Mapping the twist-angle disorder and Landau levels in magic-angle graphene","authors":["Aviram Uri","Sameer Grover","Yuan Cao","J. A. Crosse","K. Bagani","Daniel Rodan‐Legrain","Yuri Myasoedov","Kenji Watanabe","Takashi Taniguchi","Pilkyung Moon","Mikito Koshino","Pablo Jarillo‐Herrero","Eli Zeldov"],"author_count":13,"year":2020,"published":"2020-05-06","venue":"Nature","cited_by":414,"doi":"10.1038/s41586-020-2255-3","arxiv_id":"1908.04595","nct_id":null,"source_url":"https://doi.org/10.1038/s41586-020-2255-3","archive_url":"https://space-kitty.com/papers/arxiv:1908.04595","updated":"2019-08-13","pmid":"32376964","pmcid":null,"openalex_id":"W2967788151","pdf_url":"https://arxiv.org/pdf/1908.04595","abstract":"The emergence of flat electronic bands and of the recently discovered strongly correlated and superconducting phases in twisted bilayer graphene crucially depends on the interlayer twist angle upon approaching the magic angle $θ_M \\approx 1.1°$. Although advanced fabrication methods allow alignment of graphene layers with global twist angle control of about 0.1$°$, little information is currently available on the distribution of the local twist angles in actual magic angle twisted bilayer graphene (MATBG) transport devices. Here we map the local $θ$ variations in hBN encapsulated devices with relative precision better than 0.002$°$ and spatial resolution of a few moir$é$ periods. Utilizing a scanning nanoSQUID-on-tip, we attain tomographic imaging of the Landau levels in the quantum Hall state in MATBG, which provides a highly sensitive probe of the charge disorder and of the local band structure determined by the local $θ$. We find that even state-of-the-art devices, exhibiting high-quality global MATBG features including superconductivity, display significant variations in the local $θ$ with a span close to 0.1$°$. Devices may even have substantial areas where no local MATBG behavior is detected, yet still display global MATBG characteristics in transport, highlighting the importance of percolation physics. The derived $θ$ maps reveal substantial gradients and a network of jumps. We show that the twist angle gradients generate large unscreened electric fields that drastically change the quantum Hall state by forming edge states in the bulk of the sample, and may also significantly affect the phase diagram of correlated and superconducting states. The findings call for exploration of band structure engineering utilizing twist-angle gradients and gate-tunable built-in planar electric fields for novel correlated phenomena and applications.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["Graphene research and applications"],"details":null,"provenance":"openalex+arxiv","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:1808.02482","kind":"article","title":"Faithful tight-binding models and fragile topology of magic-angle bilayer graphene","authors":["Hoi Chun Po","Liujun Zou","T. Senthil","Ashvin Vishwanath"],"author_count":4,"year":2019,"published":"2019-05-29","venue":"Physical review. B./Physical review. B","cited_by":394,"doi":"10.1103/physrevb.99.195455","arxiv_id":"1808.02482","nct_id":null,"source_url":"https://doi.org/10.1103/physrevb.99.195455","archive_url":"https://space-kitty.com/papers/arxiv:1808.02482","updated":"2019-06-03","pmid":null,"pmcid":null,"openalex_id":"W2886074776","pdf_url":"https://arxiv.org/pdf/1808.02482","abstract":"Correlated insulators and superconductivity have been observed in \"magic-angle\" twisted bilayer graphene, when the nearly flat bands close to neutrality are partially filled. While a momentum-space continuum model accurately describes these flat bands, interaction effects are more conveniently incorporated in tight-binding models. We have previously shown that no fully symmetric tight-binding model can be minimal, in the sense of capturing just the flat bands, so extended models are unavoidable. Here, we introduce a family of tight-binding models that capture the flat bands while simultaneously retaining all symmetries. In particular, we construct three concrete models with five, six, or ten bands per valley and per spin. These models are also faithful, in that the additional degrees of freedom represent energy bands further away from neutrality, and they serve as optimal starting points for a controlled study of interaction effects. Furthermore, our construction demonstrates the \"fragile topology\" of the nearly flat bands; i.e., the obstruction to constructing exponentially localized Wannier functions can be resolved when a particular set of trivial bands is added to the model.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["Topological Materials and Phenomena"],"details":null,"provenance":"openalex+arxiv","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:2109.13944","kind":"article","title":"Evidence for unconventional superconductivity in twisted bilayer graphene","authors":["Myungchul Oh","Kevin P. Nuckolls","Dillon Wong","Ryan L. Lee","Xiaomeng Liu","Kenji Watanabe","Takashi Taniguchi","Ali Yazdani"],"author_count":8,"year":2021,"published":"2021-10-20","venue":"Nature","cited_by":363,"doi":"10.1038/s41586-021-04121-x","arxiv_id":"2109.13944","nct_id":null,"source_url":"https://doi.org/10.1038/s41586-021-04121-x","archive_url":"https://space-kitty.com/papers/arxiv:2109.13944","updated":"2021-12-20","pmid":"34670267","pmcid":null,"openalex_id":"W3204940466","pdf_url":"https://arxiv.org/pdf/2109.13944","abstract":"The emergence of superconductivity with doping from correlated insulators in magic-angle twisted bilayer graphene (MATBG) has raised the intriguing possibility that its pairing mechanism is distinct from that of conventional superconductors, as described by the Bardeen-Cooper-Schrieffer (BCS) theory. While there is now ample evidence for strong electronic correlations in MATBG, recent studies have claimed that unlike correlated insulators, superconductivity persists even when these interactions are partially screened. This suggests that the pairing in MATBG might be conventional in nature, a consequence of the large density of states of its nearly flat bands, perhaps phonon-mediated as in BCS superconductors. Here we combine tunneling and Andreev reflection spectroscopy with the scanning tunneling microscope (STM) to observe several key experimental signatures for unconventional superconductivity in MATBG. We show that the tunneling spectra below the transition temperature $T_c$ are inconsistent with that of a conventional s-wave superconductor, but rather resemble that of a nodal superconductor with an anisotropic pairing mechanism. We observe a large discrepancy between the tunneling energy gap $Δ_T$, which far exceeds the mean-field BCS ratio (with $2Δ_T/k_BT_c \\sim 25$) and the energy gap $Δ_{AR}$ extracted from Andreev reflection spectroscopy ($2Δ_{AR}/k_BT_c \\sim 6$). The tunneling gap persists even when superconductivity is suppressed, indicating its emergence from a pseudogap phase, with a suppressed density of states at the Fermi level. Moreover, the pseudogap state and superconductivity are both absent when MATBG is aligned with the hexagonal boron nitride (hBN) underneath. These findings and other observations reported here provide a preponderance of evidence for a non-BCS mechanism for superconductivity in MATBG.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["Physics of Superconductivity and Magnetism"],"details":null,"provenance":"openalex+arxiv","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:1911.02045","kind":"article","title":"Ground State and Hidden Symmetry of Magic-Angle Graphene at Even Integer Filling","authors":["Nick Bultinck","Eslam Khalaf","Shang Liu","Shubhayu Chatterjee","Ashvin Vishwanath","Michael P. Zaletel"],"author_count":6,"year":2020,"published":"2020-08-12","venue":"Physical Review X","cited_by":372,"doi":"10.1103/physrevx.10.031034","arxiv_id":"1911.02045","nct_id":null,"source_url":"https://doi.org/10.1103/physrevx.10.031034","archive_url":"https://space-kitty.com/papers/arxiv:1911.02045","updated":"2020-06-19","pmid":null,"pmcid":null,"openalex_id":"W2985847189","pdf_url":"http://link.aps.org/pdf/10.1103/PhysRevX.10.031034","abstract":"In magic angle twisted bilayer graphene, electron-electron interactions play a central role resulting in correlated insulating states at certain integer fillings. Identifying the nature of these insulators is a central question and potentially linked to the relatively high temperature superconductivity observed in the same devices. Here we address this question using a combination of analytical strong-coupling arguments and a comprehensive Hartree-Fock numerical calculation which includes the effect of remote bands. The ground state we obtain at charge neutrality is an unusual ordered state which we call the Kramers intervalley-coherent (K-IVC) insulator. In its simplest form, the K-IVC exhibits a pattern of alternating circulating currents which triples the graphene unit cell leading to an \"orbital magnetization density wave\". Although translation and time reversal symmetry are broken, a combined `Kramers' time reversal symmetry is preserved. Our analytic arguments are built on first identifying an approximate ${\\rm U}(4) \\times {\\rm U}(4)$ symmetry, resulting from the remarkable properties of the tBG band structure, which helps select a low energy manifold of states, which are further split to favor the K-IVC. This low energy manifold is also found in the Hartree-Fock numerical calculation. We show that symmetry lowering perturbations can stabilize other insulators and the semi-metallic state, and discuss the ground state at half filling and a comparison with experiments.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":"cc-by","is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["Graphene research and applications"],"details":null,"provenance":"openalex+arxiv","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:1807.04382","kind":"article","title":"Twisted Bilayer Graphene: A Phonon-Driven Superconductor","authors":["Biao Lian","Zhijun Wang","Bogdan Andrei Bernevig"],"author_count":3,"year":2019,"published":"2019-06-28","venue":"Physical Review Letters","cited_by":374,"doi":"10.1103/physrevlett.122.257002","arxiv_id":"1807.04382","nct_id":null,"source_url":"https://doi.org/10.1103/physrevlett.122.257002","archive_url":"https://space-kitty.com/papers/arxiv:1807.04382","updated":"2019-01-15","pmid":"31347876","pmcid":null,"openalex_id":"W2823732984","pdf_url":"https://arxiv.org/pdf/1807.04382","abstract":"We study the electron-phonon coupling in twisted bilayer graphene (TBG), which was recently experimentally observed to exhibit superconductivity around the magic twist angle $θ\\approx 1.05^\\circ$. We show that phonon-mediated electron electron attraction at the magic angle is strong enough to induce a conventional intervalley pairing between graphene valleys $K$ and $K'$ with a superconducting critical temperature $T_c\\sim1K$, in agreement with the experiment. We predict that superconductivity can also be observed in TBG at many other angles $θ$ and higher electron densities in higher Moiré bands, which may also explain the possible granular superconductivity of highly oriented pyrolytic graphite. We support our conclusions by \\emph{ab initio} calculations.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["Graphene research and applications"],"details":null,"provenance":"openalex+arxiv","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:2011.00011","kind":"article","title":"Gate-Defined Josephson Junctions in Magic-Angle Twisted Bilayer Graphene","authors":["Folkert K. de Vries","Elias Portoles","Giulia Zheng","Takashi Taniguchi","Kenji Watanabe","Thomas Ihn","Klaus Ensslin","Peter Rickhaus"],"author_count":8,"year":2020,"published":"2020-10-30","venue":"Nature Nanotechnology, volume 16, pages 760-763 (2021)","cited_by":108,"doi":"10.1038/s41565-021-00896-2","arxiv_id":"2011.00011","nct_id":null,"source_url":"https://arxiv.org/abs/2011.00011","archive_url":"https://space-kitty.com/papers/arxiv:2011.00011","updated":"2020-11-03","pmid":"33941917","pmcid":null,"openalex_id":"W3096660331","pdf_url":"https://arxiv.org/pdf/2011.00011","abstract":"In the past two years, magic-angle twisted bilayer graphene has emerged as a uniquely versatile experimental platform that combines metallic, superconducting, magnetic and insulating phases in a single crystal. In particular the ability to tune the superconducting state with a gate voltage opened up intriguing prospects for novel device functionality. Here we present the first demonstration of a device based on the interplay between two distinct phases in adjustable regions of a single magic-angle twisted bilayer graphene crystal. We electrostatically define the superconducting and insulating regions of a Josephson junction and observe tunable DC and AC Josephson effects. We show that superconductivity is induced in different electronic bands and describe the junction behaviour in terms of these bands, taking in consideration interface effects as well. Shapiro steps, a hallmark of the AC Josephson effect and therefore the formation of a Josephson junction, are observed. This work is an initial step towards devices where separate gate-defined correlated states are connected in single-crystal nanostructures. We envision applications in superconducting electronics and quantum information technology as well as in studies exploring the nature of the superconducting state in magic-angle twisted bilayer graphene.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["cond-mat.mes-hall","cond-mat.str-el","cond-mat.supr-con"],"details":null,"provenance":"arxiv+openalex","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"arxiv:2002.03003","kind":"article","title":"Superconductivity in metallic twisted bilayer graphene stabilized by WSe2","authors":["Harpreet Singh Arora","Robert Polski","Yiran Zhang","Alex Thomson","Youngjoon Choi","Hyunjin Kim","Zhong Lin","Ilham Zaky Wilson","Xiaodong Xu","Jiun‐Haw Chu","Kenji Watanabe","Takashi Taniguchi","Jason Alicea","Stevan Nadj-Perge"],"author_count":14,"year":2020,"published":"2020-07-15","venue":"Nature","cited_by":346,"doi":"10.1038/s41586-020-2473-8","arxiv_id":"2002.03003","nct_id":null,"source_url":"https://doi.org/10.1038/s41586-020-2473-8","archive_url":"https://space-kitty.com/papers/arxiv:2002.03003","updated":"2020-02-07","pmid":"32669697","pmcid":null,"openalex_id":"W3043226974","pdf_url":"https://arxiv.org/pdf/2002.03003","abstract":"Magic-angle twisted bilayer graphene (TBG), with rotational misalignment close to 1.1$^\\circ$, features isolated flat electronic bands that host a rich phase diagram of correlated insulating, superconducting, ferromagnetic, and topological phases. The origins of the correlated insulators and superconductivity, and the interplay between them, are particularly elusive. Both states have been previously observed only for angles within $\\pm0.1^\\circ$ from the magic-angle value and occur in adjacent or overlapping electron density ranges; nevertheless, it is still unclear how the two states are related. Beyond the twist angle and strain, the dependence of the TBG phase diagram on the alignment and thickness of insulating hexagonal boron nitride (hBN) used to encapsulate the graphene sheets indicates the importance of the microscopic dielectric environment. Here we show that adding an insulating tungsten-diselenide (WSe$_2$) monolayer between hBN and TBG stabilizes superconductivity at twist angles much smaller than the established magic-angle value. For the smallest angle of $θ$ = 0.79$^\\circ$, we still observe clear superconducting signatures, despite the complete absence of the correlated insulating states and vanishing gaps between the dispersive and flat bands. These observations demonstrate that, even though electron correlations may be important, superconductivity in TBG can exist even when TBG exhibits metallic behaviour across the whole range of electron density. Finite-magnetic-field measurements further reveal breaking of the four-fold spin-valley symmetry in the system, consistent with large spin-orbit coupling induced in TBG via proximity to WSe$_2$. Our results highlight the importance of symmetry breaking effects in stabilizing electronic states in TBG and open new avenues for engineering quantum phases in moiré systems.","abstract_license":"CC0-1.0 (arXiv descriptive metadata)","license":null,"is_open_access":true,"retracted":false,"peer_reviewed":null,"categories":["Graphene research and applications"],"details":null,"provenance":"openalex+arxiv","attribution":"Abstract: arXiv descriptive metadata (CC0). Thank you to arXiv for use of its open access interoperability. This service was not reviewed or approved by, nor does it necessarily express or reflect the policies or opinions of, arXiv."},{"id":"openalex:W3163903440","kind":"preprint","title":"Nematicity and competing orders in superconducting magic-angle graphene","authors":["Yuan Cao","Daniel Rodan‐Legrain","Jeong Min Park","Noah F. Q. Yuan","Kenji Watanabe","Takashi Taniguchi","Rafael M. Fernandes","Liang Fu","Pablo Jarillo‐Herrero"],"author_count":9,"year":2021,"published":"2021-01-01","venue":"DSpace@MIT (Massachusetts Institute of Technology)","cited_by":335,"doi":null,"arxiv_id":null,"nct_id":null,"source_url":"https://hdl.handle.net/1721.1/141864","archive_url":"https://space-kitty.com/papers/openalex:W3163903440","updated":null,"pmid":null,"pmcid":null,"openalex_id":"W3163903440","pdf_url":null,"abstract":null,"abstract_license":null,"license":"cc-by-nc-sa","is_open_access":true,"retracted":false,"peer_reviewed":false,"categories":["Physics of Superconductivity and Magnetism"],"details":null,"provenance":"openalex","attribution":"Metadata and citation counts: OpenAlex (CC0)."}],"next":"https://space-kitty.com/api/v1/topics/moire-superconductivity/papers?sort=top&limit=25&offset=25"}