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تسجيل مستخدم جديدCorrelated Insulators in Twisted Bilayer Graphene
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Experiments on graphene bilayers, where the top layer is rotated with respect to the one below, have displayed insulating behavior when the moire bands are partially filled. We calculate the charge distributions in these phases, and estimate the excitation gaps.
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We introduce and analyze a model that sheds light on the interplay between correlated insulating states, superconductivity, and flavor-symmetry breaking in magic angle twisted bilayer graphene. Using a variational mean-field theory, we determine the
normal-state phase diagram of our model as a function of the band filling. The model features robust insulators at even integer fillings, occasional weaker insulators at odd integer fillings, and a pattern of flavor-symmetry breaking at non-integer fillings. Adding a phonon-mediated inter-valley retarded attractive interaction, we obtain strong-coupling superconducting domes, whose structure is in qualitative agreement with experiments. Our model elucidates how the intricate form of the interactions and the particle-hole asymmetry of the electronic structure determine the phase diagram. It also explains how subtle differences between devices may lead to the different behaviors observed experimentally. A similar model can be applied with minor modifications to other moir{e} systems, such as twisted trilayer graphene.
Even as the understanding of the mechanism behind correlated insulating states in magic-angle twisted bilayer graphene converges towards various kinds of spontaneous symmetry breaking, the metallic normal state above the insulating transition tempera
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Magic-angle twisted bilayer graphene (MATBG) exhibits a range of correlated phenomena that originate from strong electron-electron interactions. These interactions make the Fermi surface highly susceptible to reconstruction when $ pm 1, pm 2, pm 3$ e
lectrons occupy each moir e unit cell and lead to the formation of correlated insulating, superconducting and ferromagnetic phases. While some phases have been shown to carry a non-zero Chern number, the local microscopic properties and topological character of many other phases remain elusive. Here we introduce a set of novel techniques hinging on scanning tunneling microscopy (STM) to map out topological phases in MATBG that emerge in finite magnetic field. By following the evolution of the local density of states (LDOS) at the Fermi level with electrostatic doping and magnetic field, we visualize a local Landau fan diagram that enables us to directly assign Chern numbers to all observed phases. We uncover the existence of six topological phases emanating from integer fillings in finite fields and whose origin relates to a cascade of symmetry-breaking transitions driven by correlations. The spatially resolved and electron-density-tuned LDOS maps further reveal that these topological phases can form only in a small range of twist angles around the magic-angle value. Both the microscopic origin and extreme sensitivity to twist angle differentiate these topological phases from the Landau levels observed near charge neutrality. Moreover, we observe that even the charge-neutrality Landau spectrum taken at low fields is considerably modified by interactions and exhibits an unexpected splitting between zero Landau levels that can be as large as ${sim },3-5$ meV. Our results show how strong electronic interactions affect the band structure of MATBG and lead to the formation of correlation-enabled topological phases.
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res. We propose that the superconducting and correlated insulating orders are connected by Wess-Zumino-Witten terms, so that defects of one order contain quanta of another order and skyrmion fluctuations of the correlated insulator are a mechanism for superconductivity. We present a comprehensive listing of plausible low-temperature orders, and the parent flavor symmetry breaking orders. The previously characterized topological nature of the band structure of twisted bilayer graphene plays an important role in this analysis.
When bilayer graphene is rotationally faulted to an angle $thetaapprox 1.1^circ$, theory predicts the formation of a flat electronic band and correlated insulating, superconducting, and ferromagnetic states have all been observed at partial band fill
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