Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session R42: Superconductivity in 2D SytemsLive
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Sponsoring Units: DCMP Chair: Peter Rickhaus, ETH Zurich |
Thursday, March 18, 2021 8:00AM - 8:12AM Live |
R42.00001: Superconductivity and correlated states in twisted bilayer graphene-WSe2 heterostructures: Part 1 Yiran Zhang, Robert Polski, Harpreet Arora, Alex R Thomson, Youngjoon Choi, Hyunjin Kim, Zhong Lin, Xiaodong Xu, Jiun-Haw Chu, Kenji Watanabe, Takashi Taniguchi, Jason F. Alicea, Stevan Nadj-Perge Twisted bilayer graphene (TBG) near the magic angle (~1.1°) has been established to be a highly tunable platform that hosts correlated insulating and superconducting phases. However, the relation between these states remains elusive, owing to their sensitivity to microscopic details. Here we show that adding an insulating WSe2 monolayer to the TBG stabilizes superconductivity at twist angles much smaller than the magic angle. For the smallest twist angle of 0.79°, superconductivity and symmetry breaking states are still robust despite the metallic behavior across the whole electron density range. Our results demonstrate the impact of the dielectric substrate on correlated phases in TBG and put strict constraints for the theoretical explanation of superconductivity in this system. |
Thursday, March 18, 2021 8:12AM - 8:24AM Live |
R42.00002: Superconductivity and correlated states in twisted bilayer graphene-WSe2 heterostructures: Part 2 Robert Polski, Yiran Zhang, Harpreet Arora, Alex R Thomson, Youngjoon Choi, Hyunjin Kim, Zhong Lin, Xiaodong Xu, Jiun-Haw Chu, Kenji Watanabe, Takashi Taniguchi, Jason F. Alicea, Stevan Nadj-Perge Magic-angle twisted bilayer graphene (TBG) hosts a multitude of correlated insulating, superconducting, and topological phases. We show that adding an insulating monolayer of WSe2 to the TBG stabilizes superconductivity down to angles as small as 0.79°. At angles in this range, both the correlated insulating states and the band gaps between flat and dispersive bands disappear, leading to metallic behavior across the accessible range of electron density. We additionally observe weak antilocalization and the breaking of the fourfold spin-valley symmetry usually observed in TBG, consistent with strong spin-orbit coupling. Further, in-plane and out-of-plane magneto-transport measurements reveal that finite-field correlated phases also persist for twist angles well below the magic angle value. Our results constrain theoretical explanations for the superconductivity and correlated insulators in TBG and emphasize the importance of the dielectric substrates for engineering of the electronic states in moiré systems. |
Thursday, March 18, 2021 8:24AM - 8:36AM Live |
R42.00003: DC Josephson effect in Magic-Angle Twisted Bilayer Graphene Folkert De Vries, Elias Portoles, Giulia Zheng, Takashi Taniguchi, Kenji Watanabe, Thomas Ihn, Klaus Ensslin, Peter Rickhaus Novel electronic devices will be based on fundamentally different electronic phases in one and the same material. |
Thursday, March 18, 2021 8:36AM - 8:48AM Live |
R42.00004: AC Josephson effect in Magic-Angle Twisted Bilayer Graphene Elías Portolés, Folkert De Vries, Giulia Zheng, Takashi Taniguchi, Kenji Watanabe, Thomas Ihn, Klaus Ensslin, Peter Rickhaus Since the discovery of superconductivity in magic-angle twisted bilayer graphene (MATBG) [1] many aspects of twisted Van der Waals materials are being explored. Here, we electrostatically define a Josephson junction in MATBG by selectively tuning the Fermi energy on different parts of our device [2]. We observe both the DC and AC Josephson effects. We tune the leads of the junction to the superconducting regime and explore different chemical potential configurations for the junction area. Hysteresis measurements of the IV traces give access to the quality factors of the different junctions we are able to form. We measure Shapiro steps, the most characteristic signature of the AC Josephson effect. By comparing the data to simulations obtained in the framework of the RCSJ model, we gain insight on how tuning the chemical potential of the junction area to the 2nd and 3rd moiré bands of MATBG affects the capacitance and quasiparticle resistance across the junction. Finally, the Shapiro step pattern shows no evidence of topological superconductivity. |
Thursday, March 18, 2021 8:48AM - 9:00AM Live |
R42.00005: Independent Superconductors and Correlated Insulators in Twisted Bilayer Graphene Jingyuan Ge, Yu Saito, Takashi Taniguchi, Kenji Watanabe, Andrea Young Twisted bilayer graphene (tBLG) is a moiré heterostructure containing two sheets of graphene stacked on top of each other. At twist angle of θ ≈ 1.1°, theory predicts the formation of a nearly-flat electronic band in tBLG and experiments have successfully shown correlated insulating and superconducting states when the flat band is partially filled. The proximity of superconductivity to correlated insulators suggested a close relationship between these states. In this talk, I will describe our “cut-and-stack” technique to fabricate highly homogeneous tBLG devices, and survey our transport measurements of the intrinsic properties of these devices with a focus on superconductivity. Through transport measurements from five tBLG devices, we show that superconductivity can appear even in the absence of correlated insulating states. Although both superconductivity and correlated insulating behavior are strongest near the flat-band condition, superconductivity survives to larger detuning of the angle. |
Thursday, March 18, 2021 9:00AM - 9:12AM Live |
R42.00006: Superconductivity, correlated insulators, and Wess-Zumino-Witten terms in twisted bilayer graphene Maine Christos, Subir Sachdev, Mathias Scheurer Recent experiments on twisted bilayer graphene have shown a high-temperature parent state with massless Dirac fermions and broken electronic flavor symmetry. Superconductivity and correlated insulators are thought to emerge from this parent state at lower temperatures. 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 will present a comprehensive listing of plausible low-temperature orders for the correlated insulators, and the possible parent flavor symmetry breaking orders at charge neutrality and half-filling. We consider both singlet and triplet pairings for the superconducting state. We will also discuss how our classification may constrain possible order parameters for correlated insulators, superconductivity, and the high-temperature parent state in experiments. |
Thursday, March 18, 2021 9:12AM - 9:24AM Live |
R42.00007: Atlas of air-stable 2D metals: gapping conditions, alloying rules, and superconductivity Yuanxi Wang, Vincent Henry Crespi Metals and alloy engineering has enjoyed great design freedom from the immense chemical space spanned by metal elements – about 80% of the periodic table. The same chemical space appears difficult to access for atomically-thin two-dimensional metals, due to their fragility against oxidation. The recent realization of 2D metals intercalating graphene/SiC interfaces suggests a general platform for hosting air-stable 2D metals [1]. We present a first-principles high-throughput survey on the structures and stabilities of all metals in the periodic table that would intercalate a graphene/SiC interface [2]. Trends in the stabilities and structures of these systems follow general rules in metal cohesive energies and metal-Si bonding strengths. A robust bandgap opens for a subset of metals satisfying two conditions: appropriate electron filling and substrate-induced symmetry breaking. I discuss two consequences of this gap opening: alloying rules unique to 2D metals [2] and superconductivity with doped-semiconductor character [1]. |
Thursday, March 18, 2021 9:24AM - 9:36AM Live |
R42.00008: Plasmon-phonon interactions in twisted bilayer graphene Tommaso Cea The Fermi velocity in twisted bilayer graphene (TBG) near a magic angle can be comparable to the sound velocity of the acoustic phonons of graphene. As a result, the phonon acoustic bands coexist with a high density of electron-hole pairs in the momentum-frequency space, leading to two main effects: i) the renormalization of the phonon dispersion and ii) the intrinsic coupling between phonons and the plasmons of the 2d Coulomb gas. We compute the full bosonic excitation spectrum of TBG, accounting for both acoustic phonos and plasmons. We show that the Umklapp processes can not be neglected in magic-angle TBG, and strongly affect the dispersion of the excitations in the meV region of the spectrum. Unexpected features appear at these energy scale and we suggest that their signature can be detected via THz spectroscopic techniques. |
Thursday, March 18, 2021 9:36AM - 9:48AM Live |
R42.00009: Magic-angle Bilayer Phononic Graphene Yuanchen Deng, Mourad Oudich, Nikhil JRK Gerard, Jun Ji, Minghui Lu, Yun Jing Thanks to the recent discovery on the magic-angle bilayer graphene, twistronics is quickly becoming a burgeoning field in condensed matter physics. This work expands the realm of twistronics to acoustics by introducing twisted bilayer phononic graphene, which remarkably also harbors the magic angle, evidenced by the associated ultra-flat bands. Beyond mimicking quantum mechanical behaviors of twisted bilayer graphene, we show that their acoustic counterpart offers a considerably more straightforward and robust way to alter the interlayer hopping strength, enabling us to unlock magic angles (>3 degrees) inaccessible in classical twisted bilayer graphene. This study, not only establishes the acoustical analog of twisted (magic-angle) bilayer graphene, providing a testbed more easily accessible to probe the interaction and misalignment between stacked 2D materials, but also points out the direction to a new phononic crystal design paradigm that could benefit applications such as enhanced acoustic emission and sensing. |
Thursday, March 18, 2021 9:48AM - 10:00AM Live |
R42.00010: Interlayer electron-phonon coupling in Moire superlattices Tonghang Han, Tianyi Han, Jixiang Yang, Guorui Chen, Daniel Rodan-Legrain, Jeong Min Park, Lili Jiang, Hongyuan Li, Zhiwen Shi, Bosai Lv, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero, Yuanbo Zhang, feng wang, Long Ju Electron-phonon coupling is lying at the heart of many condensed matter phenomena. The van der Waals stacking of two-dimensional materials and the twisting-angle-controlled Moire superlattice offer new opportunities for studying and engineering interlayer electron-phonon coupling. I will present our infrared spectroscopy study of several graphene-based Moire superlattices, showing strong features of interlayer electron-phonon coupling. This coupling can be further tuned through controlling the Moire wavelength and gate electric fields. |
Thursday, March 18, 2021 10:00AM - 10:12AM Live |
R42.00011: Pairing in magic-angle twisted bilayer graphene: role of phonon and plasmon umklapp Cyprian Lewandowski, Debanjan Chowdhury, Jonathan Ruhman Identifying the microscopic mechanism for superconductivity in magic-angle twisted bilayer graphene (MATBG) is an outstanding open problem. While MATBG exhibits a rich phase-diagram, driven partly by the strong interactions relative to the electronic bandwidth, its single-particle properties, such as a non-trivial structure of the underlying Bloch wavefunctions, are unique and likely play an important role in its phenomenological complexity. We perform a theoretical study of the cooperative effects due to phonons and plasmons on pairing to disentangle their role played on superconductivity. We show that umklapp processes involving mini-optical phonon modes, arising as a result of the folding of the graphene acoustic branch due to the superlattice structure, contribute significantly towards enhancing pairing. We also investigate the role played by the dynamics of the screened Coulomb interaction on pairing, which leads to an enhancement in a narrow window of fillings and study the effect of external screening due to a metallic gate on superconductivity. We propose a smoking-gun experiment to detect resonant features associated with the phonon-umklapp processes in the differential conductance and also discuss experimental implications of a pairing mechanism relying on plasmons. |
Thursday, March 18, 2021 10:12AM - 10:24AM Live |
R42.00012: Atomistic Study on Electron-Phonon Coupling in Twisted Graphene Layers Young Woo Choi, Hyoung Joon Choi We investigate electron-phonon coupling in twisted graphene layers based on atomistic calculations of electronic structure, phonon dispersion, and electron-phonon matrix elements. First, we study electron-phonon coupling strength in twisted double bilayer graphene (TDBG). Then, we study effects of vertical electric field on electron-phonon coupling strength in TDBG. Also, we compare the electron-phonon coupling in TDBG and magic-angle twisted bilayer graphene from an atomistic point of view. Based on these results, we discuss roles of electron-phonon interaction for superconductivity in twisted graphene layers. |
Thursday, March 18, 2021 10:24AM - 10:36AM Live |
R42.00013: Van Hove driven symmetry breaking phases in twisted double bilayer graphene and superconductivity in a new twisted system Zeyu Hao, Andrew Zimmerman, Xiaomeng Liu, Yuval Ronen, Danial Haei Najafabadi, Kenji Watanabe, Takashi Taniguchi, Philip Kim Two dimensional moire superlattices have emerged as a new playground for studying correlated physics. Various correlated phases including supercondcutivity, correlated insulators and metals, ferromagnetism and generalized Wigner crystal have been observed in twisted systems. These correlated phases have been generally attributed to the flat bands formed by hybridization between the two twisting layers. Here we forward the understanding of this connection between strong correlation and flat bands by studying the correlated phases in twisted double bilayer graphene (TDBG). We show that in TDBG, not only do we have spin-polarizaed correlated insulators as shown in our previous study, but also we have clear signatures of different isospin symmetry breaking phases and magnetic field induced Chern insulators. By comparing the transport measurements and single particle band calculation, we find that these symmetry breaking phases are closely related to the evolution of van Hove singularities in TDBG energy bands. In addition, we will show superconductivity in a new twisted system. |
Thursday, March 18, 2021 10:36AM - 10:48AM Live |
R42.00014: Nonlocal Josephson Effect in Magic Angle Graphene Devices Daniel Rodan-Legrain, Yuan Cao, Jeong Min Park, Sergio de la Barrera, Mallika Randeria, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero The recent observation of superconductivity and correlated insulating states in ‘magic-angle’ twisted bilayer graphene (MATBG) featuring nearly-flat bands at twist angles close to 1.1 degrees presents a highly tunable two-dimensional (2-D) material platform capable of behaving as a metal, an insulator, or a superconductor. This tunability enables the creation of versatile quantum devices that were previously not achievable in other single material platforms. Here, we exploit the tunability of MATBG to engineer Josephson junctions and tunneling transistors all within one material, defined solely by electrostatic gates. Our multi-gated device geometry offers complete control of the weak link in the Josephson junction, with the ability to tune the barrier independently from the superconducting regions. We show that this purely 2-D MATBG Josephson junction exhibits nonlocal electrodynamics in a magnetic field, in agreement with the theory for ultrathin superconductors first proposed by J. Pearl. |
Thursday, March 18, 2021 10:48AM - 11:00AM Live |
R42.00015: Upper critical field and critical supercurrent in twisted bilayer graphene Wei Qin, Bo Zou, Allan Macdonald Superconductivity is observed in magic-angle twisted bilayer graphene over a wide range of moir\'e band fillings and electric gating conditions. Based on the optical-phonon-mediated mechanism assisted by adjustable Coulomb repulsion, we perform self-consistent Bogoliubov-de Gennes mean-field calculations to ascertain the characters of superconducting state generated at finite pairing momentum in twisted bilayer graphene. Our calculations show that the free energy of finite-momentum pairing state can be well described by the phenomenological Ginzburg-Landau theory of superconductivity with quadratic kinetic energy. The perpendicular upper critical field Hc2 possesses a sharp peak at the van Hove singularity of density of the states and decreases quickly away from it, with an averaged value comparable with experimental result. The critical supercurrent density exhibits a dome-like character in analogy to the mean-field superconducting transition temperature upon varying chemical potential. We discuss these results in connection with recent experimental observations, which may provide insights for understanding the exotic superconducting properties in twisted bilayer graphene. |
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