TY - JOUR AB - Partially filled Landau levels host competing electronic orders. For example, electron solids may prevail close to integer filling of the Landau levels before giving way to fractional quantum Hall liquids at higher carrier density1,2. Here, we report the observation of an electron solid with non-collinear spin texture in monolayer graphene, consistent with solidification of skyrmions3—topological spin textures characterized by quantized electrical charge4,5. We probe the spin texture of the solids using a modified Corbino geometry that allows ferromagnetic magnons to be launched and detected6,7. We find that magnon transport is highly efficient when one Landau level is filled (ν=1), consistent with quantum Hall ferromagnetic spin polarization. However, even minimal doping immediately quenches the magnon signal while leaving the vanishing low-temperature charge conductivity unchanged. Our results can be understood by the formation of a solid of charged skyrmions near ν=1, whose non-collinear spin texture leads to rapid magnon decay. Data near fractional fillings show evidence of several fractional skyrmion solids, suggesting that graphene hosts a highly tunable landscape of coupled spin and charge orders. AU - Zhou, H. AU - Polshyn, Hryhoriy AU - Taniguchi, T. AU - Watanabe, K. AU - Young, A. F. ID - 10620 IS - 2 JF - Nature Physics KW - General Physics and Astronomy SN - 1745-2473 TI - Solids of quantum Hall skyrmions in graphene VL - 16 ER - TY - JOUR AB - Since the discovery of correlated insulators and superconductivity in magic-angle twisted bilayer graphene (tBLG) ([1, 2], JCCM April 2018), theorists have been excitedly pursuing the alluring mix of band topology, symmetry breaking, Mott insulators and superconductivity at play, as well as the potential relation (if any) to high-Tc physics. Now a new stream of experimental work is arriving which further enriches the story. To briefly recap Episodes 1 and 2 (JCCM April and November 2018), when two graphene layers are stacked with a small rotational mismatch θ, the resulting long-wavelength moire pattern leads to a superlattice potential which reconstructs the low energy band structure. When θ approaches the “magic-angle” θM ∼ 1 ◦, the band structure features eight nearly-flat bands which fill when the electron number per moire unit cell, n/n0, lies between −4 < n/n0 < 4. The bands can be counted as 8 = 2 × 2 × 2: for each spin (2×) and valley (2×) characteristic of monolayergraphene, tBLG has has 2× flat bands which cross at mini-Dirac points. AU - Yankowitz, Mathew AU - Chen, Shaowen AU - Polshyn, Hryhoriy AU - Watanabe, K. AU - Taniguchi, T. AU - Graf, David AU - Young, Andrea F. AU - Dean, Cory R. AU - Sharpe, Aaron L. AU - Fox, E.J. AU - Barnard, A.W. AU - Finney, Joe ID - 10664 JF - Journal Club for Condensed Matter Physics TI - New correlated phenomena in magic-angle twisted bilayer graphene/s VL - 03 ER - TY - JOUR AB - The quantum anomalous Hall (QAH) effect combines topology and magnetism to produce precisely quantized Hall resistance at zero magnetic field. We report the observation of a QAH effect in twisted bilayer graphene aligned to hexagonal boron nitride. The effect is driven by intrinsic strong interactions, which polarize the electrons into a single spin- and valley-resolved moiré miniband with Chern number C = 1. In contrast to magnetically doped systems, the measured transport energy gap is larger than the Curie temperature for magnetic ordering, and quantization to within 0.1% of the von Klitzing constant persists to temperatures of several kelvin at zero magnetic field. Electrical currents as small as 1 nanoampere controllably switch the magnetic order between states of opposite polarization, forming an electrically rewritable magnetic memory. AU - Serlin, M. AU - Tschirhart, C. L. AU - Polshyn, Hryhoriy AU - Zhang, Y. AU - Zhu, J. AU - Watanabe, K. AU - Taniguchi, T. AU - Balents, L. AU - Young, A. F. ID - 10619 IS - 6480 JF - Science KW - multidisciplinary SN - 0036-8075 TI - Intrinsic quantized anomalous Hall effect in a moiré heterostructure VL - 367 ER - TY - CONF AB - Twisted bilayer graphene (tBLG) near the flat band condition is a versatile new platform for the study of correlated physics in 2D. Resistive states have been observed at several commensurate fillings of the flat miniband, along with superconducting states near half filling. To better understand the electronic structure of this system, we study electronic transport of graphite gated superconducting tBLG devices in the normal regime. At high magnetic fields, we observe full lifting of the spin and valley degeneracy. The transitions in the splitting of this four-fold degeneracy as a function of carrier density indicate Landau level (LL) crossings, which tilted field measurements show occur between LLs with different valley polarization. Similar LL structure measured in two devices, one with twist angle θ=1.08° at ambient pressure and one at θ=1.27° and 1.33GPa, suggests that the dimensionless combination of twist angle and interlayer coupling controls the relevant details of the band structure. In addition, we find that the temperature dependence of the resistance at B=0 shows linear growth at several hundred Ohm/K in a broad range of temperatures. We discuss the implications for modeling the scattering processes in this system. AU - Polshyn, Hryhoriy AU - Zhang, Yuxuan AU - Yankowitz, Matthew AU - Chen, Shaowen AU - Taniguchi, Takashi AU - Watanabe, Kenji AU - Graf, David E. AU - Dean, Cory R. AU - Young, Andrea ID - 10724 IS - 2 SN - 0003-0503 T2 - APS March Meeting 2019 TI - Normal state transport in superconducting twisted bilayer graphene VL - 64 ER - TY - CONF AB - Bilayer graphene, rotationally faulted to ~1.1 degree misalignment, has recently been shown to host superconducting and resistive states associated with the formation of a flat electronic band. While numerous theories exist for the origins of both states, direct validation of these theories remains an outstanding experimental problem. Here, we focus on the resistive states occurring at commensurate filling (1/2, 1/4, and 3/4) of the two lowest superlattice bands. We test theoretical proposals that these states arise due to broken spin—and/or valley—symmetry by performing direct magnetic imaging with nanoscale SQUID-on-tip microscopy. This technique provides single-spin resolved magnetometry on sub-100nm length scales. I will present imaging data from our 4.2K nSOT microscope on graphite-gated twisted bilayers near the flat band condition and discuss the implications for the physics of the commensurate resistive states. AU - Serlin, Marec AU - Tschirhart, Charles AU - Polshyn, Hryhoriy AU - Zhu, Jiacheng AU - Huber, Martin E. AU - Young, Andrea ID - 10722 IS - 2 SN - 0003-0503 T2 - APS March Meeting 2019 TI - Direct Imaging of magnetic structure in twisted bilayer graphene with scanning nanoSQUID-On-Tip microscopy VL - 64 ER -