Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session L67: Spectrocopic Measurements of Magic Angle Twisted Bilayer GrapheneInvited
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Sponsoring Units: DCMP Chair: Ali Yazdani, Princeton Univ Room: Four Seasons 2-3 |
Wednesday, March 4, 2020 8:00AM - 8:36AM |
L67.00001: Electronic Correlations and Superconductivity in Twisted Bilayer Graphene near Magic Angle Invited Speaker: Stevan Nadj-Perge Twisted bilayer graphene (TBG) with a rotational misalignment (twist) angle close to the magic value of 1.1°, features isolated flat electronic bands. These bands form a strongly correlated electronic system that exhibits a range of quantum phases, including superconductivity, ferromagnetism, and correlated insulating states. In this talk, I will present the latest results from our lab on magic-angle graphene obtained using scanning tunneling microscopy/spectroscopy (STM/STS) and transport measurements [1,2]. The local spectroscopy data shows that the flat bands get highly deformed when aligned with the Fermi level. Near half-filling of the bands, we observe the development of gaps originating from correlated insulating states, while near charge neutrality, we find a regime featuring an enhanced flat band splitting that can be described within a model predicting a strong tendency towards nematic ordering. In the second part of the talk, I will discuss transport measurements using novel stack geometry showing the existence of superconductivity in the absence of insulating states. Our findings provide a basis for a microscopic understanding of correlated quantum phases in twisted bilayer graphene. |
Wednesday, March 4, 2020 8:36AM - 9:12AM |
L67.00002: Flat Bands and Electronic Correlations in Graphene with and without a Twist Invited Speaker: Alexander Kerelsky Van der Waals bilayers with an interlayer twist are an excellent platform towards achieving gate-tunable electronic flat bands. A number of emergent phases driven by electron-electron interactions have been observed when tuning the Fermi level through these flat bands. I will present atomically-resolved doping-dependent scanning tunneling spectroscopy studies of two graphene-based systems with emergent correlated phases due to flat bands – magic angle twisted bilayer graphene (tBG) and ABCA graphene, a simple system with a flat band without a moire potential. Our spectroscopy shows that magic angle tBG features two flat bands separated by 40-55 meV. Correlated states emerge when tuning the Fermi level through an individual flat band with minimized bandwidth suggesting the importance of maximized electronic correlations at the magic angle. We measure the correlated insulator gap at half filling and show evidence of nematicity near half-filling in magic angle tBG. I will then show that twisted double bilayer graphene (tDBG) features a fundamentally different moire pattern than tBG, hosting rhombohedral (ABCA) stacking sites. At tiny tDBG angles this creates micron-scale ABCA graphene domains, a robust platform to stabilize the ABCA graphene. Our spectroscopy reveals that ABCA graphene hosts a flat band of 3-4 meV half-width (versus 9-10 meV in magic angle tBG). The unprecedentedly narrow electronic band enhances electronic correlations inducing a correlated gap at charge neutrality making ABCA graphene a model graphene-based correlated system with no moire superlattice potential or integer fillings. Finally, I will show that ABCA graphene hosts topological surface helical edge states which can be turned on and off with gate voltage. |
Wednesday, March 4, 2020 9:12AM - 9:48AM |
L67.00003: Charge order and broken rotational symmetry in magic-angle twisted bilayer graphene Invited Speaker: Jinhai Mao Bilayer graphene can be modified by rotating (twisting) one layer with respect to the other. The interlayer twist gives rise to a moiré superlattice that affects the electronic motion and alters the band structure. Near a ‘magic angle’ of twist, where the emergence of a flat band causes the charge carriers to slow down, correlated electronic phases including Mott-like insulators and superconductors were recently discovered by using electronic transport. These measurements revealed an intriguing similarity between magic-angle twisted bilayer graphene and high-temperature superconductors, which spurred intensive research into the underlying physical mechanism. Essential clues to this puzzle, such as the symmetry and spatial distribution of the spectral function, can be accessed through scanning tunnelling spectroscopy. Here we use scanning tunnelling microscopy and spectroscopy to visualize the local density of states and charge distribution in magic-angle twisted bilayer graphene. Doping the sample to partially fill the flat band, we observe a pseudogap phase accompanied by a global stripe charge order that breaks the rotational symmetry of the moiré superlattice. Both the pseudogap and the stripe charge order disappear when the band is either empty or full. The close resemblance to similar observations in high-temperature superconductors provides new evidence of a deeper link underlying the phenomenology of these systems. |
Wednesday, March 4, 2020 9:48AM - 10:24AM |
L67.00004: Spectroscopic signatures of many-body correlations in magic-angle twisted bilayer graphene Invited Speaker: Yonglong Xie The discovery of superconducting and insulating states in magic-angle twisted bilayer graphene (MATBG)1,2 has ignited considerable interest in understanding the nature of electronic interactions in this chemically pristine material. The transport properties of MATBG as a function of doping are similar to those of high-transition-temperature copper oxides and other unconventional superconductors1-3, which suggests that MATBG may be a highly interacting system. However, there is no direct experimental evidence of strong many-body correlations in MATBG. In this talk, I will present the unusual spectroscopic characteristics found on MATBG, obtained using a scanning tunneling microscope, and describe how these correlated features allow us to establish a more concrete connection between MATBG and high-Tc cuprates beyond the phenomenological resemblance of their transport phase diagrams4. |
Wednesday, March 4, 2020 10:24AM - 11:00AM |
L67.00005: Spectroscopy and Correlation Effect at Magic Angle Invited Speaker: Biao Lian The twisted bilayer graphene (TBG) near the magic angle develops topological flat electron bands at low energies, and exhibits both superconducting and correlated insulating phases at low temperatures. The STM spectroscopy measurements of magic angle TBG shows that the differential tunneling conductances dI/dV at both low and relatively high temperatures have a strong dependence on the doping density in the flat bands. We show that the Hartree Fock mean field theory with spontaneous spin/valley polarization and strain effect fails to capture the spectroscopy at the magic angle. Instead, we show that charge correlation effects beyond the mean field theory are significant for explaining the spectroscopy data, and can be demonstrated in the exact diagonalization of a phenomenological Hubbard model. We also discuss the possible effects of phonon-mediated electron interaction. Lastly, we talk about the effects of the TBG band topology on the Hofstadter butterfly and the interacting phases. |
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