Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session B20: Bilayer & Trilayer Graphene Electronic Phenomena |
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Sponsoring Units: DCMP Chair: Yiran Zhang, California Institute of Technology Room: Room 212 |
Monday, March 6, 2023 11:30AM - 11:42AM |
B20.00001: Studying the broken symmetry states of Bernal-stacked bilayer graphene in a magnetic field by scanning tunneling microscope Yuwen Hu, Yen-Chen Tsui, Minhao He, Umut Kamber, Kenji Watanabe, Takashi Taniguchi, Ali Yazdani Bernal-stacked bilayer graphene forms eight-fold degenerate lowest Landau levels with valley, spin and orbital degree of freedoms under a magnetic field. The Landau level degeneracy can be lifted by electron-electron interactions and leads to a rich set of symmetry-breaking ground states. We use a dilution refrigerator scanning tunneling microscope (STM) to study the ground states of a back-gated bilayer graphene sample under high magnetic field. With a combination of gate-dependent spectroscopy measurement and real-space imaging of the wavefunctions, we study the symmetry-breaking ground states of the lowest Landau level at each integer fillings. Our study reveals how the electron-electron interactions and external field tune the ground state of integer quantum Hall states in bilayer graphene. This research also paves the way for studying fractional quantum Hall states in bilayer graphene with STM. |
Monday, March 6, 2023 11:42AM - 11:54AM |
B20.00002: Scanning tunneling microscopy study of robust fractional quantum Hall states in Bernal-stacked bilayer graphene Yen-Chen Tsui, Yuwen Hu, Minhao He, Umut Kamber, Kenji Watanabe, Takashi Taniguchi, Ali Yazdani Fractional quantum Hall states (FQHs) emerge when a 2D electron system is subjected to a strong magnetic field. We performed millikelvin scanning tunneling microscopy (STM) study of the FQHs in Bernal-stacked bilayer graphene. A robust sequence of odd-denominator FQHs are observed, and their charge excitation gaps are characterized. Moreover, we also detected even-denominator FQHs in the lowest Landau levels, highlighting the extra orbital degeneracy for the many-body states. With the imaging power of STM, we can study the spatial variation of these FQH states, and the local excitations when these FQHs experience a defect potential. Our study helps with understanding the charge excitations of these FQHs and opens an opportunity to image anyons. |
Monday, March 6, 2023 11:54AM - 12:06PM |
B20.00003: Quantum cascade of correlated phases in natural bilayer graphene Anna M Seiler, Fan Zhang, R. Thomas R Weitz Diverging density of states can lead to correlated phases in low dimensional systems. This includes the graphene family that hosts electric-field controlled Lifshitz transitions and concomitant van Hove singularities in the density of states. Here, we present the observation of experimental signatures consistent with various interaction-driven phases in AB bilayer graphene including the fractional metals of Stoner type [1-3]. More prominently, we have found competing phases that exhibit intriguing temperature dependences and nonlinear I-V characteristics at zero magnetic field [1]. Evidencing interacting physics in this simple and reproducible system offers a fertile ground for exploring intricating many-body phases and for controlling quantum phase transitions, which can be leveraged to unravel dissipation pathways in correlated graphene systems. |
Monday, March 6, 2023 12:06PM - 12:18PM |
B20.00004: Enhanced Superconductivity in Spin-Orbit Proximitized Bernal Bilayer Graphene Yiran Zhang, Robert M Polski, Alex R Thomson, Etienne Lantagne-Hurtubise, Cyprian K Lewandowski, Haoxin Zhou, Kenji Watanabe, Takashi Taniguchi, Jason F Alicea, Stevan Nadj-Perge In the presence of a large electric field, Bernal bilayer graphene (BLG) features several broken-symmetry phases and magnetic-field-induced superconductivity with a critical temperature T≈ 30 mK. Here, we show that placing monolayer tungsten diselenide (WSe2) on BLG promotes Cooper pairing in several ways: superconductivity appears at zero magnetic field, exhibits an order of magnitude enhancement in Tc, and occurs over a wide density range. By mapping quantum oscillations, we establish that superconductivity emerges from a polarized normal state, with two out of four spin-valley flavors predominantly populated. For BLG-WSe2 with moderate Ising spin-orbit coupling, in-plane magnetic field measurements reveal that the critical field roughly obeys the Pauli limit on one end of the superconducting dome yet sharply violates on the other. Moreover, the superconductivity arises only for electric fields that push BLG hole wavefunctions towards WSe2—suggesting that proximity-induced Ising spin-orbit coupling plays a key role in stabilizing the pairing. These results provide an essential step towards engineering robust, highly tunable, and ultra-clean graphene-based superconductors. |
Monday, March 6, 2023 12:18PM - 12:30PM |
B20.00005: Ising Superconductivity in Bernal Bilayer Graphene WSe2 Heterostructures Ludwig F Holleis, Caitlin L Patterson, Yiran Zhang, Stevan Nadj-Perge, Andrea Young Bernal Bilayer Graphene features a gate-tuned van Hove singularity, leading to a cascade of symmetry breaking states [1]. Surprisingly, superconductivity emerges above a critical in plane magnetic field, and is not Pauli limited, suggesting a spin triplet order parameter. Recently, it has been shown that superconductivity can be stabilized at zero magnetic field—and ten times higher temperature—by supporting the graphene bilayer on a monolayer of WSe2 [2]. WSe2 substrates are known to introduce Ising spin-orbit coupling (SOC), raising questions about possible changes to the magnetic phase diagram and superconducting order parameter that might account for the change in critical temperature. I will describe transport and capacitance measurements of WSe2 supported BBG with large spin orbit coupling. We report the observation of several distinct superconducting states; for all of these, the magnetic field dependence of the critical temperature is in good agreement with expectations for Ising superconductivity. Using penetration field capacitance and quantum oscillation measurements, we analyze the underlying phases of both superconductors and their Fermiology to construct a full phase diagram of BBG with monolayer WSe2. We will discuss these results in the context of general mechanisms for superconductivity in graphene multilayers without twist. |
Monday, March 6, 2023 12:30PM - 12:42PM |
B20.00006: Engineering Correlated Insulators in Bilayer Graphene with a Remote Coulomb Superlattice Jingxu Xie Electron superlattices provide a powerful way to engineer novel correlated and topological quantum phenomena. Recently, the moiré pattern was discovered to offer an almost perfect nanometer-scale electronic superlattice. However, the requirement of the moiré pattern poses a stringent limit on the material selection, and the moiré potential is fixed for a given moiré heterostructure. Here we solve these problems by engineering tunable correlated states in bilayer graphene with a remote Coulomb superlattice. The Coulomb superlattice is realized by localized electrons in a twisted bilayer WS2 which is around 3 nm apart. The period of the Coulomb superlattice is determined by the moiré period of the twisted bilayer WS2, and the strength is controlled by the number of localized electrons at the bilayer WS2 moiré lattice. We demonstrate that the 2DEG in bilayer graphene is described by the Fermi liquid when the remote Coulomb superlattice is turned off. Electron correlation increases dramatically when the remote Coulomb superlattice is turned on, resulting in a series of correlated insulating states at both integer and fractional filling factors. This remote Coulomb superlattice can be applied to any 2D materials hosting a 2DEG. It opens a new route for in-situ control of correlated quantum phenomena in a wide variety of 2D systems. |
Monday, March 6, 2023 12:42PM - 12:54PM |
B20.00007: Thermopower Measurement in the Fractional Quantum Hall Regime in Bilayer Graphene Andrew Zimmerman, Isabelle Y Phinney, Jeffrey Kwan, Kenji Watanabe, Takashi Taniguchi, Philip Kim Measurements of thermoelectric voltages can provide a sensitive probe of the entropy carried by charge carriers in a system, probing the properties of interesting electrical ground states. In particular, attempts have been made to use magnetothermopower measurements to identify the ground state of even denominator fractional quantum hall (FQH) states, such as the 5/2 FQH state in GaAs 2D electron gases [1,2]. Theory predicts that the filling fraction dependent magnetothermopower can probe the charge of quasi-particles in this strongly correlated quantum system and provide evidence for the possible presence of a non-abelian ground state [3]. In this presentation, we will present magnetothermopower measurements in Bernal stacked bilayer graphene in the quantum limit, where measure the magneto-conductance and thermopower in the integer quantum Hall ferromagnetism and FQH regimes. We will discuss our progress on performing thermoelectric measurements on high quality bilayer graphene samples at high field and low temperature where fractional quantum Hall states are present, with particular focus on determining the entropy of the quasiparticles of even denominator fractions to determine the ground state. |
Monday, March 6, 2023 12:54PM - 1:06PM |
B20.00008: Uniaxial Strain Control of Bernal Bilayer Graphene Xuetao Ma, Zhaoyu Liu, Kenji Watanabe, Takashi Taniguchi, Jiun-Haw Chu, Matthew A Yankowitz Bernal-stacked bilayer graphene has recently been found to exhibit spin-triplet superconductivity and an array of symmetry-broken states at zero magnetic field. These phases emerge at low carrier concentration in a perpendicular displacement field owing to the large density of states at van Hove singularities near the band extrema. Here, we develop a new experimental technique to apply in-plane uniaxial strain to bilayer graphene. We use a custom-built strain apparatus assembled from three piezo-stacks arranged in parallel, allowing us to exert continuous compressive or tensile strain at cryogenic temperatures. The application of strain is expected to deform the bilayer graphene band structure, potentially enabling a new means of studying and controlling its emergent correlated states. We will discuss ongoing efforts to measure transport in bilayer graphene as a function of strain both at zero field and in the quantum Hall regime. |
Monday, March 6, 2023 1:06PM - 1:18PM |
B20.00009: Quasiparticle Interference in Trigonally Warped Graphene Systems Nuno V Castanheira, Fan Zhang Lifshitz transitions and divergent density of states (DOS) in trigonally warped graphene systems have been pointed out to be responsible for the unexpected magnetism and superconductivity discovered in recent transport experiments. However, direct evidence is yet to be provided in any experiment. Here we show that quasiparticle interference, imaged by spectroscopic imaging scanning tunneling microscopy, provides direct information of the trigonally warped Fermi surfaces and hence a starting point for examining their instabilities under electron-electron interactions. |
Monday, March 6, 2023 1:18PM - 1:30PM |
B20.00010: Broken-Symmetry Shubnikov–de Haas Oscillations via a Bilayer Graphene Quantum Point Contact Konstantin Davydov, Xi Zhang, Matthew Coles, Logan Kline, Bryan Zucker, Kenji Watanabe, Takashi Taniguchi We study low temperature magneto-transport across a dual-gated bilayer graphene quantum point contact (QPC). The QPC is gate-defined away from the physical edge of the devices, and highly-tunable by local electrostatics. By applying an out-of-plane magnetic field, we study the broken-symmetry Shubnikov–de Haas (SdH) oscillations across the quantum point contact, and their dependence on out-of-plane electric field. |
Monday, March 6, 2023 1:30PM - 1:42PM |
B20.00011: Electron-jet collimation by electrostatically defined quantum point contacts in bilayer graphene Josep Ingla-Aynés, Talieh Ghiasi, Antonio L Manesco, Herre van der Zant An electrostatically-defined quantum point contact (QPC) is a powerful tool to access the valley degree of freedom in bilayer graphene (BLG). As shown by recent scanning gate microscopy experiments [1], QPCs emit angularly-separated current streams in BLG. These ballistic current streams, which occur due to trigonal warping, are predicted to be valley polarized, thus, they can pave the way for applications of valleytronic devices. Here we address these valley-polarized streams by performing collimation experiments between two opposite QPCs that are connected by a 4-um-long ballistic BLG channel. We observe two distinct peaks in nonlocal resistance versus magnetic field which indicates that two current jets have been injected and detected. This is in contrast with previous collimation experiments performed in monolayer graphene [2] and GaAs/AlGaAs 2DEGs [3] where a single peak was observed. Our observations represent a step forward toward a new generation of all-electrical valleytronic devices. |
Monday, March 6, 2023 1:42PM - 1:54PM |
B20.00012: Characterizing Magnon Thermal Transport of a Quantum Hall Antiferromagnet in Bilayer Graphene Zhongying Yan, Jonah Waissman, Artem V Talanov, Young Jae Shin, Danial Haei, Philip Kim In the half-filled zero-energy Landau level of graphene, competing spin and valley orders give rise to multiple phases of quantum Hall ferromagnetism (QHFM) with spontaneously broken symmetry. Using a non-local noise thermometry technique, we measure electronic thermal conduction in bilayer graphene near charge neutrality as a function of external field strengths to explore the QHFM phase diagram. Our data show clear signatures of a phase transition between canted-antiferromagnetism (CAF) and valley-polarized states. We estimate the quantitative thermal conduction of magnons in the CAF state and study its behavior with respect to changes in different experimental parameters. Our findings constitute compelling evidence for magnon transport in quantum Hall graphene and prove that thermal transport is a powerful tool in studying strongly correlated states in mesoscale systems. |
Monday, March 6, 2023 1:54PM - 2:06PM |
B20.00013: Spin valves based on bilayer graphene quantum point contacts Eike Icking, Christian Volk, Luca Banszerus, Christoph Schattauer, Kenji Watanabe, Takashi Taniguchi, Florian Liebisch, Bernd Beschoten, Christoph Stampfer Bernal bilayer graphene (BLG) is a unique material as it allows to open and electrostatically tune a sizeable band gap by applying a perpendicular electric field [1,2,3]. It has been demonstrated recently that it is possible to confine charge carriers in one dimension and form quantum point contacts (QPC) based on split gates separated by a channel of a few hundred nm [3]. Moreover, it has been shown that in such structures transport through a QPC can be spin-polarized up to 6 e2/h with a high in-plane magnetic field. The critical field at which the lowest modes are spin-polarized, depends on the subband spacing and thus on the width of the split gate channel. In this work, we combine two QPCs of different widths, resulting in different critical magnetic fields, so that we can spin-polarize the first QPC and use it as a filter for the second QPC. More precisely, we report on the realization of such a spin-valve, where we achieved a spin-polarized channel of up to 10 e2/h. |
Monday, March 6, 2023 2:06PM - 2:18PM |
B20.00014: Coupled Spin and Orbital Magnetism in ABC Trilayer Graphene Trevor B Arp, Charles L Tschirhart, Owen I Sheekey, Evgeny Redekop, Haoxin Zhou, Martin E Huber, Andrea Young ABC trilayer graphene under an applied perpendicular electric field hosts a van Hove singularity that can be accessed by field-effect doping, where a variety of spin and valley symmetry breaking ferromagnetic phases have been observed. Among these are a ‘quarter metal’ phase, where quantum oscillations show the existence of a single Fermi surface [1]. Using both tilted-magnetic field transport, capacitance, and nanoSQUID on Tip (nSOT) microscopy, I will show that the quarter metal regime actually hosts two distinct phases, separated by a first order phase transition and distinguished by different orbital and spin magnetic moments. Surprisingly, the orbital magnetic moment of one of these phases can be tuned by an in-plane magnetic field, implying that the orbital motion is coupled to the electron spin despite the lack of atomic spin orbit coupling. I will discuss our results in the context of novel mechanisms for emergent spin orbit coupling arising from the interplay of Coulomb interactions, spin textures, and orbital magnetism [2]. [1] Zhou, H., Xie, T., Ghazaryan, A. et al. Nature 598, 429–433 (2021). [2] Dong, Z., Levitov, L., arXiv:2208.02051. |
Monday, March 6, 2023 2:18PM - 2:30PM |
B20.00015: Hartree-Fock study of spin-orbit-coupled rhombohedral trilayer graphene Jin Ming Koh, Etienne Lantagne-Hurtubise, Jason F Alicea Recent experiments indicate that Bernal bilayer graphene and rhombohedral trilayer graphene exhibit much of the richness of their twisted counterparts, including a cascade of broken-symmetry states and superconductivity. Interestingly, interfacing Bernal bilayers with WSe2 was shown to dramatically enhance superconductivity—suggesting that proximity-induced spin-orbit coupling plays a key role in promoting Cooper pairing. Motivated by this observation, we study the phase diagram of spin-orbit-coupled rhombohedral trilayer graphene via self-consistent Hartree-Fock simulations, elucidating the interplay between displacement fields, long-range Coulomb repulsion, short-range Hund's coupling, and substrate-induced Ising spin-orbit interaction. We pay particular attention to broken-symmetry phases that yield band structures compatible with zero-momentum Cooper pairing, with the goal of identifying regimes that potentially support spin-orbit enhanced superconductivity as found in bilayers. |
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