Bulletin of the American Physical Society
2024 APS March Meeting
Monday–Friday, March 4–8, 2024; Minneapolis & Virtual
Session T09: Heavy Fermions and Flat Bands in Twisted Graphene Bilayers |
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Sponsoring Units: DCMP Chair: angiolo huaman gutierrez, University of Arkansas Room: L100J |
Thursday, March 7, 2024 11:30AM - 11:42AM |
T09.00001: Sub-system resolved physics in moiré heterostructures Mohammed M Al Ezzi, Alexandra Carvalho, Giovanni Vignale, Francisco Guinea, Shaffique Adam Emergent phenomena in moiré van der Waals structures are commonly ascribed to the collective moiré spectrum, characterizing the entire system, not the individual layers. Nevertheless, recent experiments are now able to identify correlated features resolved at the subsystem level. In this study, we theoretically show that some correlated phenomena in moiré systems can be attributed to individual subsystems. Using a combination of large-scale first-principles DFT calculations and effective models, we demonstrate that the correlated features in general moiré systems can be attributed to subsystems. This general idea is particularly demonstrated by analysing correlated features observed in twisted monolayer-bilayer and twisted bilayer-bilayer systems and we show that they stem from the bilayer subsystem. Furthermore, we present a series of toy models to provide physical insights into the asymmetric nature of the correlated regions in twisted monolayer-bilayer compared to that in twisted bilayer-bilayer graphene. |
Thursday, March 7, 2024 11:42AM - 11:54AM |
T09.00002: Unconventional states of matter in a domain-wall network of moiré bilayer systems Chen-Hsuan Hsu, Daniel Loss, Jelena Klinovaja Moiré bilayer systems have attracted significant attention for their potential to host unconventional states of matter [1]. When subjected to an interlayer bias, these systems reveal gapless domain-wall modes between AB- and BA-stacking domains, forming a triangular network of one-dimensional channels [2]. Here, we investigate correlated phenomena in the domain-wall network by incorporating electron-electron interactions through bosonization [3]. We introduce a general operator that accounts for various scatterings based on conservation laws, which have potential to destabilize the network, paving the way for unconventional states of matter. Within the renormalization-group framework, we classify the scattering operators and the emergent correlated states. Our analysis uncovers generalized umklapp scatterings facilitated by the moiré patterns in twisted bilayer structures, leading to correlated states at fractional fillings. We identify a set of scatterings leading to a gapped bulk while preserving gapless edge modes, reminiscent of the observed (quantum) anomalous Hall states [4]. We show that our description can offer insights in predicting observable features in spectroscopic probes and edge transport measurements. |
Thursday, March 7, 2024 11:54AM - 12:06PM |
T09.00003: Interplay between Rydberg excitons and moiré superlattices Qianying Hu, Zhen Zhan, Yalei Zhang, Shengjun Yuan, Yang Xu The Rydberg excitons are the excited Coulomb-bound states of electron-hole pairs. Their solid-state nature, in conjunction with the large dipole moments and enhanced interactions with the surroundings, makes them promising candidates for a wide range of applications. Here we study the optical response of Rydberg excitons in a monolayer semiconductor WSe2 placed on twisted bilayer graphene (TBG). First, the TBG with gate-tunable periodic potentials gives a pathway for spatial confinement and manipulation of Rydberg excitons. We experimentally demonstrate this capability through the spectroscopic evidence of Rydberg moiré excitons. Second, the optical response of Rydberg excitons in WSe2 could serve as a delicate dielectric sensor to probe the density-of-state evolution in the neighboring TBG near the magic angle, allowing us to reveal novel physics that combines both strong electronic correlations and non-trivial band topology. |
Thursday, March 7, 2024 12:06PM - 12:18PM |
T09.00004: Probing collective magnetic excitations in twisted bilayer graphene via electron spin resonance spectroscopy Rupini Kamat, Sandesh S Kalantre, Aaron L Sharpe, Kenji Watanabe, Takashi Taniguchi, Marc Kastner, David Goldhaber-Gordon In magic-angle twisted bilayer graphene (MATBG), correlated insulating (CI) states emerge at an integer number of electrons per moiré unit cell. These states are suspected to emerge from successive filling of the fourfold degenerate spin and valley states of the nearly-flat miniband [1][2]. However, the order in which the spin/valley flavor states are populated remains unknown, as does the nature of how electrons in these states couple to one another and to external magnetic fields. The nature of these states could be clarified by probing their excitations. We implement this by measuring near-DC transport through an encapsulated TBG device, noting the change in resistivity as a function of an externally applied static magnetic field in the presence of oscillating microwave magnetic fields. We couple in microwave magnetic fields from a nearby microfabricated superconducting coplanar waveguide. Similar measurements have been performed on several CVD graphene samples [3][4][5][6] as well as on MATBG with spin-orbit coupling induced by a proximal TMDC layer [7], but no such signal has been reported in canonical hBN encapsulated MATBG with clear CI states. |
Thursday, March 7, 2024 12:18PM - 12:30PM |
T09.00005: Quantum breakdown of superconductivity in mirror-symmetric twisted-trilayer graphene Phanibhusan S Mahapatra, Takashi Taniguchi, Kenji Watanabe, Eva Y Andrei Two-dimensional (2D) superconductors offer a promising opportunity to investigate the interplay between localization and long-range superconducting order since both phenomena have a critical dimension of 2. In the zero-temperature limit in 2D systems, the quasi-particles are not expected to diffuse due to the enhanced effect of disorder, making a metallic state forbidden. However, in the quantum phase transition (QPT) between a superconductor and a high-field insulating state, an intermediate metallic state was observed in the earlier experiments in disordered 2D films. The nature of this field-tuned anomalous metallic state is intensely debated and discussed in the context of strong phase fluctuations and localization of the Cooper pairs. In this talk, We shall present our recent experimental observation of the magnetic field-tuned QPT in mirror-symmetric twisted tri-layer graphene near magic angle. The superconductor-to-insulator transition (SIT) is characterized by the finite size scaling analysis with a scaling exponent of 4/3, resembling the classical percolation model. Under a small magnetic field, the superconducting state transforms to an anomalous metallic phase with finite resistance which increases with increasing magnetic field and eventually switches to an insulating state at the QPT. We shall discuss these results in the context of field-induced vortices and Josephson-coupled superconducting puddles. Our results provide new insights into the interplay between disorder and superconductivity in the flat bands of moire superconductors. |
Thursday, March 7, 2024 12:30PM - 12:42PM |
T09.00006: Critical behavior of fractionalized excitations in trimer model of twisted bilayer graphene Kevin Zhang, Dan Mao, Roderich Moessner, Eun-Ah Kim Moiré graphene systems provide an exciting platform to realize new physics, including a previously proposed suite of fractionalized excitations arising from fractional correlated insulating states at fractional fillings [1] of twisted bilayer graphene (TBG). However, rigorous analysis of the physics of quasiparticles has been difficult due to geometric frustration in the trimer model of TBG, arising in a ground state manifold with highly constrained local moves that still retains extensive entropy. In this work, we perform classical Monte Carlo simulations making use of the "pocket algorithm" [2] to achieve ergodicity with global updates even for highly frustrated models. We numerically evaluate critical exponents of monomer (1/3 charged quasiparticle) and trimer (electron) correlations to be 1/4 and 2, respectively, which is consistent with a proposed effective field theory. Furthermore, we investigate the confinement and restricted mobility of quasiparticles in the context of our numerical simulations and their implications for electronic transport in fractional fillings of twisted bilayer graphene. |
Thursday, March 7, 2024 12:42PM - 12:54PM |
T09.00007: Exact results for low-energy many-body optical sum-rule in moir'e graphene Juan F Mendez-Valderrama, Dan Mao, Debanjan Chowdhury Starting from the strong-coupling limit in twisted bilayer graphene, we present an exact analytical computation of the interaction-induced low-energy optical spectral weight at integer fillings. Remarkably, the emergent symmetries in this limit lead to the complete vanishing of the low-energy spectral-weight. Away from this solvable limit, we systematically analyze the effects of strain and finite interlayer tunnelings between the same sublattice sites, and comment on the role of electron-phonon interactions. Our results have direct implications for the physics of superconductivity and non-Fermi-liquid-like phases obtained upon doping these ``parent" correlated insulating states. |
Thursday, March 7, 2024 12:54PM - 1:06PM |
T09.00008: Kondo phase in twist bilayer graphene Gengdong Zhou, Zhida Song, Ninghua Tong, Yijie Wang While the gapped phases in magic-angle twisted bilayer graphene (MATBG) are believed to be symmetry-breaking states described by mean-field theories, the gapless phases exhibit features beyond the mean field. This work, based on the recently proposed topological heavy fermion model for MATBG, combining poor man's scaling, numerical renormalization group, and dynamic mean-field theory, demonstrates that the gapless phases are the heavy Fermi liquid state with some symmetries broken and the others preserved. At zero temperature and most non-integer fillings, the ground states are found to be heavy Fermi liquids with the Kondo temperature $T_K$ at the order of 1meV. A higher temperature than $T_K$ drives the system into a metallic LM phase where disordered local moments and a Fermi liquid coexist. At integer fillings ±1,±2, $T_K$ is suppressed to zero or a value weaker than RKKY interaction, leading to Mott insulators or symmetry-breaking states. This theory explains experimental observations, including zero-energy peaks and quantum-dot-like behaviors in STM, the Pomeranchuk effect, the saw-tooth feature of inverse compressibility, etc. We predict that the Fermi surface in the gapless phase will shrink upon heating. We also conjecture that the heavy Fermi liquid is the parent state of the observed unconventional superconductivity since the Kondo screening effectively reduces the Coulomb interaction (~60meV) to a small value (~1meV) comparable to possible weak attractive interactions. |
Thursday, March 7, 2024 1:06PM - 1:18PM |
T09.00009: Apparent strange metal behavior in small angle twisted bilayer graphene Shaffique Adam, Indra Yudhistira, Liangtao Peng Strange metals are an intriguing class of conductors known for their unconventional electronic properties. These materials, while not fully understood, are typically characterized by their linear temperature-dependent resistivity and linear field-dependent magnetotransport. In this theoretical work we focus on the electronic transport properties of twisted bilayer graphene, a material that is believed to exhibit strange metal behavior. We adapt and develop a version of the Sachdev-Ye-Kitaev model [A. Patel, et al. Phys. Rev. X 8, 021049 (2018)] to determine how a true strange metal will behave, and contrast this with some of our previous work on linear electrical transport in twisted bilayer graphene [G. Sharma et al. Nature Communications 12, 5737 (2021)] and linear magnetotransport in disordered 2D materials [J. Ping et al. Phys. Rev. Lett. 113 047206 (2014)]. Our results highlight the possibility of ordinary metals mimicking strange metal characteristics and underscore the necessity for careful interpretation of experimental data. |
Thursday, March 7, 2024 1:18PM - 1:30PM |
T09.00010: Ferromagnetic Ground States in Twisted N-Layer Graphene Kevin D Stubbs, Simon Becker, Lin Lin Magic angle twisted bilayer graphene exhibits a rich variety of correlated electronic phases, including the correlated insulator phase at integer fillings. Analytic calculations within the chiral limit indicate that certain Slater determinants with a non-zero charge gap can serve as exact many-body ground states for this phase. In a valleyless and spinless model at half filling, two distinct ferromagnetic Slater determinant states are identified as exact ground states. We investigate twisted N-layer graphene systems with an added layer symmetry, and define conditions where the ferromagnetic Slater determinants are the only Slater determinant ground states. Through analytical methods, we confirm that these conditions are met in twisted bilayer and equal twist angle trilayer graphene. |
Thursday, March 7, 2024 1:30PM - 1:42PM |
T09.00011: Novel assembly process for reducing twist disorder in graphene moiré heterostrutures Chaitrali Duse, Aaron L Sharpe, Joe Finney, Kenji Watanabe, Takashi Taniguchi, Marc Kastner, David Goldhaber-Gordon Fascinating correlated electronic phases emerge at special ‘magic’ values of interlayer twist angles in twisted graphene moiré systems. However, exploration of the underlying physics has been limited by an inability to achieve a precise twist angle uniformly across a sample and repeatably in different samples. Since electronic structure is highly sensitive to twist angle, samples intended to be identical show slight to dramatic differences in the electronic phase diagram; and some important phenomena have been reproduced only a few times. |
Thursday, March 7, 2024 1:42PM - 1:54PM |
T09.00012: Intrinsic and extrinsic photogalvanic effects in twisted bilayer graphene Fernando Peñaranda, Hector Ochoa, Fernando De Juan Photogalvanic effects have been widely used as a direct probe to access the geometrical properties of the electronic structure in many topological systems. In this work, we investigate the symmetry constraints imposed by particle-hole and point group symmetries on the non-linear photocurrents in twisted bilayer graphene. Under this analysis, we first show that the chiral lattice structure with D6 symmetry allows only for oblique incidence photocurrents which keep track of the magic angle band inversion. A detailed study of the extrinsic effects, due to substrates or gate potentials, is also presented to show that normal incidence components are constrained to vanish by the approximate particle-hole symmetry of the twisted bilayer Hamiltonian, while off-normal components are not. We conclude that a detailed comparison between intrinsic and extrinsic photocurrents discloses a wealth of information about the band structure and can also serve as a benchmark to constrain the symmetry breaking patterns of correlated states. |
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