Shaffique Adam
Degree: PhD
Position: Associate Professor
Affiliation: Yale-NUS College
Research Type: Theory
Office: S16-06-15
Email: shaffique.adam@yale-nus.edu.sg
Contact: (65) 6601 3175
Website: https://sites.google.com/site/shaffiqueadam/home
Research Interests:
Effects of interactions and disorder in topological materials (e.g. graphene, MoS2, topological insulators, Weyl semimetals).
CA2DM Publications:
2025 |
Phong, Vo Tien; Kunkelmann, Kason; Beule, Christophe De; Ezzi, Mohammed Al M; Slager, Robert-Jan; Adam, Shaffique; Mele, E J Squeezing quantum states in three-dimensional twisted crystals Journal Article PHYSICAL REVIEW B, 111 (24), 2025, ISSN: 2469-9950. @article{ISI:001523902000002, title = {Squeezing quantum states in three-dimensional twisted crystals}, author = {Vo Tien Phong and Kason Kunkelmann and Christophe De Beule and Mohammed Al M Ezzi and Robert-Jan Slager and Shaffique Adam and E J Mele}, doi = {10.1103/cj2q-f9q2}, times_cited = {0}, issn = {2469-9950}, year = {2025}, date = {2025-06-25}, journal = {PHYSICAL REVIEW B}, volume = {111}, number = {24}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {Bloch's theorem provides a conventional starting point for describing wave propagation in periodic media, but in ordered materials where competing spatial periods coexist it is rendered ineffective, often with dramatic consequences. Here we develop an alternate approach that uses coherent free-particle vortex states to study quantum states in supertwisted crystals: three-dimensional stacks of atomically thin two-dimensional layers. This formalism leads naturally to the representation of the spectrum using squeezed coherent states, and it reveals the crucial role of a Coriolis coupling in the equations of motion. This identifies an underlying noncommutative geometry and novel edge state structure in a family of complex ordered structures.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Bloch's theorem provides a conventional starting point for describing wave propagation in periodic media, but in ordered materials where competing spatial periods coexist it is rendered ineffective, often with dramatic consequences. Here we develop an alternate approach that uses coherent free-particle vortex states to study quantum states in supertwisted crystals: three-dimensional stacks of atomically thin two-dimensional layers. This formalism leads naturally to the representation of the spectrum using squeezed coherent states, and it reveals the crucial role of a Coriolis coupling in the equations of motion. This identifies an underlying noncommutative geometry and novel edge state structure in a family of complex ordered structures. |
Swain, Nyayabanta; Tang, Ho-Kin; Foo, Darryl Chuan Wei; Khor, Brian J J; Lemarie, Gabriel; Assaad, Fakher F; Sengupta, Pinaki; Adam, Shaffique Engineering many-body quantum Hamiltonians with nonergodic properties using quantum Monte Carlo Journal Article PHYSICAL REVIEW B, 111 (22), 2025, ISSN: 2469-9950. @article{ISI:001511184900009, title = {Engineering many-body quantum Hamiltonians with nonergodic properties using quantum Monte Carlo}, author = {Nyayabanta Swain and Ho-Kin Tang and Darryl Chuan Wei Foo and Brian J J Khor and Gabriel Lemarie and Fakher F Assaad and Pinaki Sengupta and Shaffique Adam}, doi = {10.1103/PhysRevB.111.224201}, times_cited = {0}, issn = {2469-9950}, year = {2025}, date = {2025-06-02}, journal = {PHYSICAL REVIEW B}, volume = {111}, number = {22}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {We present a computational framework to identify Hamiltonians of interacting quantum many-body systems that host nonergodic excited states. We combine quantum Monte Carlo simulations with the recently proposed eigenstate-to-Hamiltonian construction, which maps the ground state of a specified parent Hamiltonian to a single nonergodic excited state of a new derived Hamiltonian. This engineered Hamiltonian contains nontrivial, systematically-obtained, and emergent features that are responsible for its nonergodic properties. We demonstrate this approach by applying it to quantum many-body scar states where we discover a previously unreported family of Hamiltonians with spatially oscillating spin exchange couplings that host scar-like properties, including revivals in the quantum dynamics, and towers in the inverse participation ratio; and to many-body localization, where we find a two-dimensional Hamiltonian with correlated disorder that exhibits nonergodic scaling of the participation entropy and inverse participation ratios of order unity. The method can be applied to other known ground states to discover new quantum many-body systems with nonergodic excited states.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We present a computational framework to identify Hamiltonians of interacting quantum many-body systems that host nonergodic excited states. We combine quantum Monte Carlo simulations with the recently proposed eigenstate-to-Hamiltonian construction, which maps the ground state of a specified parent Hamiltonian to a single nonergodic excited state of a new derived Hamiltonian. This engineered Hamiltonian contains nontrivial, systematically-obtained, and emergent features that are responsible for its nonergodic properties. We demonstrate this approach by applying it to quantum many-body scar states where we discover a previously unreported family of Hamiltonians with spatially oscillating spin exchange couplings that host scar-like properties, including revivals in the quantum dynamics, and towers in the inverse participation ratio; and to many-body localization, where we find a two-dimensional Hamiltonian with correlated disorder that exhibits nonergodic scaling of the participation entropy and inverse participation ratios of order unity. The method can be applied to other known ground states to discover new quantum many-body systems with nonergodic excited states. |
Chakraborty, Amarnath; Rodin, Aleksandr; Adam, Shaffique; Vignale, Giovanni Insulator-metal transition and magnetic crossover in bilayer graphene Journal Article PHYSICAL REVIEW B, 111 (12), 2025, ISSN: 2469-9950. @article{ISI:001458804200004, title = {Insulator-metal transition and magnetic crossover in bilayer graphene}, author = {Amarnath Chakraborty and Aleksandr Rodin and Shaffique Adam and Giovanni Vignale}, doi = {10.1103/PhysRevB.111.125130}, times_cited = {0}, issn = {2469-9950}, year = {2025}, date = {2025-03-11}, journal = {PHYSICAL REVIEW B}, volume = {111}, number = {12}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {In-plane magnetic fields offer a relatively unexplored opportunity to alter the band structure of stacks of two-dimensional (2D) materials so that they exhibit the desired physical properties. Here we show that an inplane magnetic field combined with a transverse electric field can induce an insulator-metal (IM) transition in bilayer graphene. Our study of the magnetic response reveals that the orbital magnetic susceptibility changes from diamagnetic to paramagnetic around the transition point. We discuss several strategies to observe the IM transition, switch the diamagnetism, and more generally control the band structure of stacked 2D materials at experimentally accessible magnetic fields.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In-plane magnetic fields offer a relatively unexplored opportunity to alter the band structure of stacks of two-dimensional (2D) materials so that they exhibit the desired physical properties. Here we show that an inplane magnetic field combined with a transverse electric field can induce an insulator-metal (IM) transition in bilayer graphene. Our study of the magnetic response reveals that the orbital magnetic susceptibility changes from diamagnetic to paramagnetic around the transition point. We discuss several strategies to observe the IM transition, switch the diamagnetism, and more generally control the band structure of stacked 2D materials at experimentally accessible magnetic fields. |
Yudhistira, Indra; Afrose, Ramal; Adam, Shaffique PHYSICAL REVIEW B, 111 (8), 2025, ISSN: 2469-9950. @article{ISI:001448500900003, title = {Nonmonotonic temperature dependence of electron viscosity and crossover to high-temperature universal viscous fluid in monolayer and bilayer graphene}, author = {Indra Yudhistira and Ramal Afrose and Shaffique Adam}, doi = {10.1103/PhysRevB.111.085433}, times_cited = {1}, issn = {2469-9950}, year = {2025}, date = {2025-02-28}, journal = {PHYSICAL REVIEW B}, volume = {111}, number = {8}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {Electrons in quantum matter behave like a fluid when the quantum-mechanical carrier-carrier scattering dominates over other relaxation mechanisms. By combining a microscopic treatment of electron-electron interactions within the random phase approximation with a phenomenological Navier-Stokes-like equation, we predict that in the limit of high temperature and strong Coulomb interactions, both monolayer graphene and bilayer graphene exhibit a universal behavior in dynamic viscosity. We find that the dynamic viscosity to entropy density ratio for bilayer graphene is closer to the holographic bound, suggesting that such a bound might be observable in a condensed matter system. We discuss how this could be observed experimentally using magnetoconductance measurements in a Corbino geometry for a realistic range of temperature and carrier density.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Electrons in quantum matter behave like a fluid when the quantum-mechanical carrier-carrier scattering dominates over other relaxation mechanisms. By combining a microscopic treatment of electron-electron interactions within the random phase approximation with a phenomenological Navier-Stokes-like equation, we predict that in the limit of high temperature and strong Coulomb interactions, both monolayer graphene and bilayer graphene exhibit a universal behavior in dynamic viscosity. We find that the dynamic viscosity to entropy density ratio for bilayer graphene is closer to the holographic bound, suggesting that such a bound might be observable in a condensed matter system. We discuss how this could be observed experimentally using magnetoconductance measurements in a Corbino geometry for a realistic range of temperature and carrier density. |
Jin, Shangjian; Foo, Darryl C W; Qu, Tingyu; Ozyilmaz, Barbaros; Adam, Shaffique Unified theoretical framework for Kondo superconductors: Periodic Anderson impurities with attractive pairing and Rashba spin-orbit coupling Journal Article PHYSICAL REVIEW B, 111 (1), 2025, ISSN: 2469-9950. @article{ISI:001416427700003, title = {Unified theoretical framework for Kondo superconductors: Periodic Anderson impurities with attractive pairing and Rashba spin-orbit coupling}, author = {Shangjian Jin and Darryl C W Foo and Tingyu Qu and Barbaros Ozyilmaz and Shaffique Adam}, doi = {10.1103/PhysRevB.111.014505}, times_cited = {1}, issn = {2469-9950}, year = {2025}, date = {2025-01-08}, journal = {PHYSICAL REVIEW B}, volume = {111}, number = {1}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {Magnetic superconductors manifest a fascinating interplay between their magnetic and superconducting properties. This becomes evident, for example, in the significant enhancement of the upper critical field observed in uranium-based superconductors, or the destruction of superconductivity well below the superconducting transition temperature Tc in cobalt-doped NbSe2. In this work, we argue that the Kondo interaction plays a pivotal role in governing these behaviors. By employing a periodic Anderson model, we study the Kondo effect in superconductors with either singlet or triplet pairing. In the regime of small impurity energies and high doping concentrations, we find the emergence of a Kondo resistive region below Tc. While a magnetic field suppresses singlet superconductivity, it stabilizes triplet pairing through the screening of magnetic impurities, inducing reentrant superconductivity at high fields. Moreover, introducing an antisymmetric spin-orbital coupling suppresses triplet superconductivity. This framework provides a unified picture to understand the observation of Kondo effect in NbSe2 as well as the phase diagrams in Kondo superconductors such as UTe2 and URhGe.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Magnetic superconductors manifest a fascinating interplay between their magnetic and superconducting properties. This becomes evident, for example, in the significant enhancement of the upper critical field observed in uranium-based superconductors, or the destruction of superconductivity well below the superconducting transition temperature Tc in cobalt-doped NbSe2. In this work, we argue that the Kondo interaction plays a pivotal role in governing these behaviors. By employing a periodic Anderson model, we study the Kondo effect in superconductors with either singlet or triplet pairing. In the regime of small impurity energies and high doping concentrations, we find the emergence of a Kondo resistive region below Tc. While a magnetic field suppresses singlet superconductivity, it stabilizes triplet pairing through the screening of magnetic impurities, inducing reentrant superconductivity at high fields. Moreover, introducing an antisymmetric spin-orbital coupling suppresses triplet superconductivity. This framework provides a unified picture to understand the observation of Kondo effect in NbSe2 as well as the phase diagrams in Kondo superconductors such as UTe2 and URhGe. |
2024 |
Ezzi, Mohammed Al M; Pallewela, Gayani N; Beule, Christophe De; Mele, E J; Adam, Shaffique Analytical Model for Atomic Relaxation in Twisted Moire<acute accent> Materials Journal Article PHYSICAL REVIEW LETTERS, 133 (26), 2024, ISSN: 0031-9007. @article{ISI:001386385200014, title = {Analytical Model for Atomic Relaxation in Twisted Moire author = {Mohammed Al M Ezzi and Gayani N Pallewela and Christophe De Beule and E J Mele and Shaffique Adam}, doi = {10.1103/PhysRevLett.133.266201}, times_cited = {5}, issn = {0031-9007}, year = {2024}, date = {2024-12-23}, journal = {PHYSICAL REVIEW LETTERS}, volume = {133}, number = {26}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {By virtue of being atomically thin, the electronic properties of heterostructures built from twodimensional materials are strongly influenced by atomic relaxation. The atomic layers behave as flexible membranes rather than rigid crystals. Here we develop an analytical theory of lattice relaxation in twisted moire keywords = {}, pubstate = {published}, tppubtype = {article} } By virtue of being atomically thin, the electronic properties of heterostructures built from twodimensional materials are strongly influenced by atomic relaxation. The atomic layers behave as flexible membranes rather than rigid crystals. Here we develop an analytical theory of lattice relaxation in twisted moire<acute accent> materials. We obtain analytical results for the lattice displacements and corresponding pseudo gauge fields, as a function of twist angle. We benchmark our results for twisted bilayer graphene and twisted WSe2 bilayers using large-scale molecular dynamics simulations. Our single-parameter theory is valid in graphene bilayers for twist angles B >= 0.7 degrees, and in twisted WSe2 for B >= 1.6 degrees. We also investigate how relaxation alters the electronic structure in twisted bilayer graphene, providing a simple extension to the continuum model to account for lattice relaxation. |
Afrose, Ramal; Keser, Aydin Cem; Sushkov, Oleg P; Adam, Shaffique Tunable viscous layers in Corbino geometry using density junctions Journal Article PHYSICAL REVIEW B, 110 (12), 2024, ISSN: 2469-9950. @article{ISI:001309683400002, title = {Tunable viscous layers in Corbino geometry using density junctions}, author = {Ramal Afrose and Aydin Cem Keser and Oleg P Sushkov and Shaffique Adam}, doi = {10.1103/PhysRevB.110.125409}, times_cited = {0}, issn = {2469-9950}, year = {2024}, date = {2024-09-06}, journal = {PHYSICAL REVIEW B}, volume = {110}, number = {12}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {In sufficiently clean materials where electron-electron interactions are strong compared to momentum-relaxing scattering processes, electron transport resembles the flow of a viscous fluid. We study hydrodynamic electron transport across density interfaces (n-n junctions) in a 2DEG in the Corbino geometry. From numerical simulations in COMSOL using realistic parameters, we show that we can produce tunable viscous layers at the density interface by varying the density ratio of charge carriers. We quantitatively explain this observation with simple analytic expressions together with boundary conditions at the interface. We also show signatures of these viscous layers in the magnetoresistance. Breaking down viscous and Ohmic contributions, we find that when the outer radial region of the Corbino has higher charge density compared to the inner region, the viscous layers at the interface serve to suppress the magnetoresistance produced by momentum-relaxing scattering. Conversely, the magnetoresistance is enhanced when the inner region has higher density than the outer. Our results add to the repertoire of techniques for engineering viscous electron flows, which hold a promise for applications in future electronic devices.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In sufficiently clean materials where electron-electron interactions are strong compared to momentum-relaxing scattering processes, electron transport resembles the flow of a viscous fluid. We study hydrodynamic electron transport across density interfaces (n-n junctions) in a 2DEG in the Corbino geometry. From numerical simulations in COMSOL using realistic parameters, we show that we can produce tunable viscous layers at the density interface by varying the density ratio of charge carriers. We quantitatively explain this observation with simple analytic expressions together with boundary conditions at the interface. We also show signatures of these viscous layers in the magnetoresistance. Breaking down viscous and Ohmic contributions, we find that when the outer radial region of the Corbino has higher charge density compared to the inner region, the viscous layers at the interface serve to suppress the magnetoresistance produced by momentum-relaxing scattering. Conversely, the magnetoresistance is enhanced when the inner region has higher density than the outer. Our results add to the repertoire of techniques for engineering viscous electron flows, which hold a promise for applications in future electronic devices. |
Trushin, Maxim; Peng, Liangtao; Sharma, Gargee; Vignale, Giovanni; Adam, Shaffique High conductivity from cross-band electron pairing in flat-band systems Journal Article PHYSICAL REVIEW B, 109 (24), 2024, ISSN: 2469-9950. @article{ISI:001247474400001, title = {High conductivity from cross-band electron pairing in flat-band systems}, author = {Maxim Trushin and Liangtao Peng and Gargee Sharma and Giovanni Vignale and Shaffique Adam}, doi = {10.1103/PhysRevB.109.245118}, times_cited = {0}, issn = {2469-9950}, year = {2024}, date = {2024-06-13}, journal = {PHYSICAL REVIEW B}, volume = {109}, number = {24}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {Electrons in condensed matter may transition into a variety of broken-symmetry phase states due to electronelectron interactions. Applying diverse mean-field approximations to the interaction term is arguably the simplest way to identify the phase states theoretically possible in a given setting. Here, we explore electron-electron attraction in a two-band system comprising symmetric conduction and valence bands touching each other at a single point. We assume a mean-field pairing between the electrons having opposite spins, momenta, and in contrast to the conventional superconducting pairing, residing in opposite bands, i.e., having opposite energies. We show that electrons transition into a correlated ground state if and only if the bands are flat enough, i.e., the transition is impossible in the case of conventional parabolic bands. Although this state is not superconducting in the usual sense and does not exhibit a gap in its excitation spectrum, it is nevertheless immune to elastic scattering caused by any kind of disorder and is therefore expected to exhibit high electric conductivity at low temperature, mimicking the behavior of a real superconductor. Having in mind the recent experimental realizations of flat-band electronic systems in twisted multilayers, we foresee an exciting opportunity for observing a class of highly conductive materials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Electrons in condensed matter may transition into a variety of broken-symmetry phase states due to electronelectron interactions. Applying diverse mean-field approximations to the interaction term is arguably the simplest way to identify the phase states theoretically possible in a given setting. Here, we explore electron-electron attraction in a two-band system comprising symmetric conduction and valence bands touching each other at a single point. We assume a mean-field pairing between the electrons having opposite spins, momenta, and in contrast to the conventional superconducting pairing, residing in opposite bands, i.e., having opposite energies. We show that electrons transition into a correlated ground state if and only if the bands are flat enough, i.e., the transition is impossible in the case of conventional parabolic bands. Although this state is not superconducting in the usual sense and does not exhibit a gap in its excitation spectrum, it is nevertheless immune to elastic scattering caused by any kind of disorder and is therefore expected to exhibit high electric conductivity at low temperature, mimicking the behavior of a real superconductor. Having in mind the recent experimental realizations of flat-band electronic systems in twisted multilayers, we foresee an exciting opportunity for observing a class of highly conductive materials. |
Qiu, Zhizhan; Han, Yixuan; Noori, Keian; Chen, Zhaolong; Kashchenko, Mikhail; Lin, Li; Olsen, Thomas; Li, Jing; Fang, Hanyan; Lyu, Pin; Telychko, Mykola; Gu, Xingyu; Adam, Shaffique; Quek, Su Ying; Rodin, Aleksandr; Neto, Castro A H; Novoselov, Kostya S; Lu, Jiong Evidence for electron-hole crystals in a Mott insulator Journal Article 13 NATURE MATERIALS, 23 (8), 2024, ISSN: 1476-1122. @article{ISI:001237790900002, title = {Evidence for electron-hole crystals in a Mott insulator}, author = {Zhizhan Qiu and Yixuan Han and Keian Noori and Zhaolong Chen and Mikhail Kashchenko and Li Lin and Thomas Olsen and Jing Li and Hanyan Fang and Pin Lyu and Mykola Telychko and Xingyu Gu and Shaffique Adam and Su Ying Quek and Aleksandr Rodin and Castro A H Neto and Kostya S Novoselov and Jiong Lu}, doi = {10.1038/s41563-024-01910-3}, times_cited = {13}, issn = {1476-1122}, year = {2024}, date = {2024-06-03}, journal = {NATURE MATERIALS}, volume = {23}, number = {8}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {The coexistence of correlated electron and hole crystals enables the realization of quantum excitonic states, capable of hosting counterflow superfluidity and topological orders with long-range quantum entanglement. Here we report evidence for imbalanced electron-hole crystals in a doped Mott insulator, namely, alpha-RuCl3, through gate-tunable non-invasive van der Waals doping from graphene. Real-space imaging via scanning tunnelling microscopy reveals two distinct charge orderings at the lower and upper Hubbard band energies, whose origin is attributed to the correlation-driven honeycomb hole crystal composed of hole-rich Ru sites and rotational-symmetry-breaking paired electron crystal composed of electron-rich Ru-Ru bonds, respectively. Moreover, a gate-induced transition of electron-hole crystals is directly visualized, further corroborating their nature as correlation-driven charge crystals. The realization and atom-resolved visualization of imbalanced electron-hole crystals in a doped Mott insulator opens new doors in the search for correlated bosonic states within strongly correlated materials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The coexistence of correlated electron and hole crystals enables the realization of quantum excitonic states, capable of hosting counterflow superfluidity and topological orders with long-range quantum entanglement. Here we report evidence for imbalanced electron-hole crystals in a doped Mott insulator, namely, alpha-RuCl3, through gate-tunable non-invasive van der Waals doping from graphene. Real-space imaging via scanning tunnelling microscopy reveals two distinct charge orderings at the lower and upper Hubbard band energies, whose origin is attributed to the correlation-driven honeycomb hole crystal composed of hole-rich Ru sites and rotational-symmetry-breaking paired electron crystal composed of electron-rich Ru-Ru bonds, respectively. Moreover, a gate-induced transition of electron-hole crystals is directly visualized, further corroborating their nature as correlation-driven charge crystals. The realization and atom-resolved visualization of imbalanced electron-hole crystals in a doped Mott insulator opens new doors in the search for correlated bosonic states within strongly correlated materials. |
Ezzi, Mohammed Al M; Hu, Junxiong; Ariando, Ariando; Guinea, Francisco; Adam, Shaffique Topological Flat Bands in Graphene Super-Moire<acute accent> Lattices Journal Article PHYSICAL REVIEW LETTERS, 132 (12), 2024, ISSN: 0031-9007. @article{ISI:001198615600004, title = {Topological Flat Bands in Graphene Super-Moire author = {Mohammed Al M Ezzi and Junxiong Hu and Ariando Ariando and Francisco Guinea and Shaffique Adam}, doi = {10.1103/PhysRevLett.132.126401}, times_cited = {0}, issn = {0031-9007}, year = {2024}, date = {2024-03-18}, journal = {PHYSICAL REVIEW LETTERS}, volume = {132}, number = {12}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {Moire ' -pattern -based potential engineering has become an important way to explore exotic physics in a variety of two-dimensional condensed matter systems. While these potentials have induced correlated phenomena in almost all commonly studied 2D materials, monolayer graphene has remained an exception. We demonstrate theoretically that a single layer of graphene, when placed between two bulk boron nitride crystal substrates with the appropriate twist angles, can support a robust topological ultraflat band emerging as the second hole band. This is one of the simplest platforms to design and exploit topological flat bands.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Moire ' -pattern -based potential engineering has become an important way to explore exotic physics in a variety of two-dimensional condensed matter systems. While these potentials have induced correlated phenomena in almost all commonly studied 2D materials, monolayer graphene has remained an exception. We demonstrate theoretically that a single layer of graphene, when placed between two bulk boron nitride crystal substrates with the appropriate twist angles, can support a robust topological ultraflat band emerging as the second hole band. This is one of the simplest platforms to design and exploit topological flat bands. |
Peng, Liangtao; Yudhistira, Indra; Vignale, Giovanni; Adam, Shaffique Theoretical determination of the effect of a screening gate on plasmon-induced superconductivity in twisted bilayer graphene Journal Article PHYSICAL REVIEW B, 109 (4), 2024, ISSN: 2469-9950. @article{ISI:001173887400003, title = {Theoretical determination of the effect of a screening gate on plasmon-induced superconductivity in twisted bilayer graphene}, author = {Liangtao Peng and Indra Yudhistira and Giovanni Vignale and Shaffique Adam}, doi = {10.1103/PhysRevB.109.045404}, times_cited = {4}, issn = {2469-9950}, year = {2024}, date = {2024-01-05}, journal = {PHYSICAL REVIEW B}, volume = {109}, number = {4}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {The microscopic pairing mechanism for superconductivity in magic-angle twisted bilayer graphene remains an open question. Recent experimental studies seem to rule out a purely electronic mechanism due to the insensitivity of the critical superconducting temperature to either a highly doped screening layer or the proximity to a metallic screening gate. In this theoretical work, we explore the role of external screening layers on the superconducting properties of twisted bilayer graphene within a purely electronic mechanism. Consistent with the experimental observations, we find that the critical temperature is unaffected by screening unless the screening layer is closer than 3 nm from the superconductor. Thus, the available transport data are not in contradiction with a plasmon-mediated mechanism. We also investigate other properties of this plasmon-mediated superconductivity, including signatures in the tunneling density of states as probed in spectroscopy experiments.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The microscopic pairing mechanism for superconductivity in magic-angle twisted bilayer graphene remains an open question. Recent experimental studies seem to rule out a purely electronic mechanism due to the insensitivity of the critical superconducting temperature to either a highly doped screening layer or the proximity to a metallic screening gate. In this theoretical work, we explore the role of external screening layers on the superconducting properties of twisted bilayer graphene within a purely electronic mechanism. Consistent with the experimental observations, we find that the critical temperature is unaffected by screening unless the screening layer is closer than 3 nm from the superconductor. Thus, the available transport data are not in contradiction with a plasmon-mediated mechanism. We also investigate other properties of this plasmon-mediated superconductivity, including signatures in the tunneling density of states as probed in spectroscopy experiments. |
2023 |
Hu, Junxiong; Tan, Junyou; Ezzi, Mohammed Al M; Chattopadhyay, Udvas; Gou, Jian; Zheng, Yuntian; Wang, Zihao; Chen, Jiayu; Thottathil, Reshmi; Luo, Jiangbo; Watanabe, Kenji; Taniguchi, Takashi; Wee, Andrew Thye Shen; Adam, Shaffique; Ariando, A Controlled alignment of supermoire lattice in double-aligned graphene heterostructures Journal Article 11 NATURE COMMUNICATIONS, 14 (1), 2023. @article{ISI:001029450400007, title = {Controlled alignment of supermoire lattice in double-aligned graphene heterostructures}, author = {Junxiong Hu and Junyou Tan and Mohammed Al M Ezzi and Udvas Chattopadhyay and Jian Gou and Yuntian Zheng and Zihao Wang and Jiayu Chen and Reshmi Thottathil and Jiangbo Luo and Kenji Watanabe and Takashi Taniguchi and Andrew Thye Shen Wee and Shaffique Adam and A Ariando}, doi = {10.1038/s41467-023-39893-5}, times_cited = {11}, year = {2023}, date = {2023-07-12}, journal = {NATURE COMMUNICATIONS}, volume = {14}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {The supermoire lattice, built by stacking two moire patterns, provides a platform for creating flat mini-bands and studying electron correlations. An ultimate challenge in assembling a graphene supermoire lattice is in the deterministic control of its rotational alignment, which is made highly aleatory due to the random nature of the edge chirality and crystal symmetry. Employing the so-called "golden rule of three", here we present an experimental strategy to overcome this challenge and realize the controlled alignment of double-aligned hBN/graphene/hBN supermoire lattice, where the twist angles between graphene and top/bottom hBN are both close to zero. Remarkably, we find that the crystallographic edge of neighboring graphite can be used to better guide the stacking alignment, as demonstrated by the controlled production of 20 moire samples with an accuracy better than similar to 0.2 degrees. Finally, we extend our technique to low-angle twisted bilayer graphene and ABC-stacked trilayer graphene, providing a strategy for flat-band engineering in these moirematerials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The supermoire lattice, built by stacking two moire patterns, provides a platform for creating flat mini-bands and studying electron correlations. An ultimate challenge in assembling a graphene supermoire lattice is in the deterministic control of its rotational alignment, which is made highly aleatory due to the random nature of the edge chirality and crystal symmetry. Employing the so-called "golden rule of three", here we present an experimental strategy to overcome this challenge and realize the controlled alignment of double-aligned hBN/graphene/hBN supermoire lattice, where the twist angles between graphene and top/bottom hBN are both close to zero. Remarkably, we find that the crystallographic edge of neighboring graphite can be used to better guide the stacking alignment, as demonstrated by the controlled production of 20 moire samples with an accuracy better than similar to 0.2 degrees. Finally, we extend our technique to low-angle twisted bilayer graphene and ABC-stacked trilayer graphene, providing a strategy for flat-band engineering in these moirematerials. |
2022 |
Tan, Cheng; Ho, Derek Y H; Wang, Lei; Li, Jia I A; Yudhistira, Indra; Rhodes, Daniel A; Taniguchi, Takashi; Watanabe, Kenji; Shepard, Kenneth; McEuen, Paul L; Dean, Cory R; Adam, Shaffique; Hone, James Dissipation-enabled hydrodynamic conductivity in a tunable bandgap semiconductor Journal Article 19 SCIENCE ADVANCES, 8 (15), 2022, ISSN: 2375-2548. @article{ISI:000786201300005, title = {Dissipation-enabled hydrodynamic conductivity in a tunable bandgap semiconductor}, author = {Cheng Tan and Derek Y H Ho and Lei Wang and Jia I A Li and Indra Yudhistira and Daniel A Rhodes and Takashi Taniguchi and Kenji Watanabe and Kenneth Shepard and Paul L McEuen and Cory R Dean and Shaffique Adam and James Hone}, doi = {10.1126/sciadv.abi8481}, times_cited = {19}, issn = {2375-2548}, year = {2022}, date = {2022-04-01}, journal = {SCIENCE ADVANCES}, volume = {8}, number = {15}, publisher = {AMER ASSOC ADVANCEMENT SCIENCE}, address = {1200 NEW YORK AVE, NW, WASHINGTON, DC 20005 USA}, abstract = {Electronic transport in the regime where carrier-carrier collisions are the dominant scattering mechanism has taken on new relevance with the advent of ultraclean two-dimensional materials. Here, we present a combined theoretical and experimental study of ambipolar hydrodynamic transport in bilayer graphene demonstrating that the conductivity is given by the sum of two Drude-like terms that describe relative motion between electrons and holes, and the collective motion of the electron-hole plasma. As predicted, the measured conductivity of gapless, charge-neutral bilayer graphene is sample- and temperature-independent over a wide range. Away from neutrality, the electron-hole conductivity collapses to a single curve, and a set of just four fitting parameters provides quantitative agreement between theory and experiment at all densities, temperatures, and gaps measured. This work validates recent theories for dissipation-enabled hydrodynamic conductivity and creates a link between semiconductor physics and the emerging field of viscous electronics.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Electronic transport in the regime where carrier-carrier collisions are the dominant scattering mechanism has taken on new relevance with the advent of ultraclean two-dimensional materials. Here, we present a combined theoretical and experimental study of ambipolar hydrodynamic transport in bilayer graphene demonstrating that the conductivity is given by the sum of two Drude-like terms that describe relative motion between electrons and holes, and the collective motion of the electron-hole plasma. As predicted, the measured conductivity of gapless, charge-neutral bilayer graphene is sample- and temperature-independent over a wide range. Away from neutrality, the electron-hole conductivity collapses to a single curve, and a set of just four fitting parameters provides quantitative agreement between theory and experiment at all densities, temperatures, and gaps measured. This work validates recent theories for dissipation-enabled hydrodynamic conductivity and creates a link between semiconductor physics and the emerging field of viscous electronics. |
2021 |
Sharma, Girish; Yudhistira, Indra; Chakraborty, Nilotpal; Ho, Derek Y H; Ezzi, Al M M; Fuhrer, Michael S; Vignale, Giovanni; Adam, Shaffique Carrier transport theory for twisted bilayer graphene in the metallic regime Journal Article 23 NATURE COMMUNICATIONS, 12 (1), 2021. @article{ISI:000702528800007, title = {Carrier transport theory for twisted bilayer graphene in the metallic regime}, author = {Girish Sharma and Indra Yudhistira and Nilotpal Chakraborty and Derek Y H Ho and Al M M Ezzi and Michael S Fuhrer and Giovanni Vignale and Shaffique Adam}, doi = {10.1038/s41467-021-25864-1}, times_cited = {23}, year = {2021}, date = {2021-09-30}, journal = {NATURE COMMUNICATIONS}, volume = {12}, number = {1}, publisher = {NATURE PORTFOLIO}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {The mechanisms responsible for the strongly correlated insulating and superconducting phases in twisted bilayer graphene are still debated. The authors provide a theory for phonon-dominated transport that explains several experimental observations, and contrast it with the Planckian dissipation mechanism.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The mechanisms responsible for the strongly correlated insulating and superconducting phases in twisted bilayer graphene are still debated. The authors provide a theory for phonon-dominated transport that explains several experimental observations, and contrast it with the Planckian dissipation mechanism. |
Keser, Aydin Cem; Wang, Daisy Q; Klochan, Oleh; Ho, Derek Y H; Tkachenko, Olga A; Tkachenko, Vitaly A; Culcer, Dimitrie; Adam, Shaffique; Farrer, Ian; Ritchie, David A; Sushkov, Oleg P; Hamilton, Alexander R Geometric Control of Universal Hydrodynamic Flow in a Two-Dimensional Electron Fluid Journal Article 45 PHYSICAL REVIEW X, 11 (3), 2021, ISSN: 2160-3308. @article{ISI:000684262800001, title = {Geometric Control of Universal Hydrodynamic Flow in a Two-Dimensional Electron Fluid}, author = {Aydin Cem Keser and Daisy Q Wang and Oleh Klochan and Derek Y H Ho and Olga A Tkachenko and Vitaly A Tkachenko and Dimitrie Culcer and Shaffique Adam and Ian Farrer and David A Ritchie and Oleg P Sushkov and Alexander R Hamilton}, doi = {10.1103/PhysRevX.11.031030}, times_cited = {45}, issn = {2160-3308}, year = {2021}, date = {2021-08-06}, journal = {PHYSICAL REVIEW X}, volume = {11}, number = {3}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {Fluid dynamics is one of the cornerstones of modern physics and has recently found applications in the transport of electrons in solids. In most solids, electron transport is dominated by extrinsic factors, such as sample geometry and scattering from impurities. However, in the hydrodynamic regime, Coulomb interactions transform the electron motion from independent particles to the collective motion of a viscous "electron fluid." The fluid viscosity is an intrinsic property of the electron system, determined solely by the electron-electron interactions. Resolving the universal intrinsic viscosity is challenging, as it affects the resistance only through interactions with the sample boundaries, whose roughness not only is unknown but also varies from device to device. Here, we eliminate all unknown parameters by fabricating samples with smooth sidewalls to achieve the perfect slip boundary condition, which has been elusive in both molecular fluids and electronic systems. We engineer the device geometry to create viscous dissipation and reveal the true intrinsic hydrodynamic properties of a 2D system. We observe a clear transition from ballistic to hydrodynamic electron motion, driven by both temperature and magnetic field. We directly measure the viscosity and electron-electron scattering lifetime (the Fermi quasiparticle lifetime) over a wide temperature range without fitting parameters and show they have a strong dependence on electron density that cannot be explained by conventional theories based on the random phase approximation.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Fluid dynamics is one of the cornerstones of modern physics and has recently found applications in the transport of electrons in solids. In most solids, electron transport is dominated by extrinsic factors, such as sample geometry and scattering from impurities. However, in the hydrodynamic regime, Coulomb interactions transform the electron motion from independent particles to the collective motion of a viscous "electron fluid." The fluid viscosity is an intrinsic property of the electron system, determined solely by the electron-electron interactions. Resolving the universal intrinsic viscosity is challenging, as it affects the resistance only through interactions with the sample boundaries, whose roughness not only is unknown but also varies from device to device. Here, we eliminate all unknown parameters by fabricating samples with smooth sidewalls to achieve the perfect slip boundary condition, which has been elusive in both molecular fluids and electronic systems. We engineer the device geometry to create viscous dissipation and reveal the true intrinsic hydrodynamic properties of a 2D system. We observe a clear transition from ballistic to hydrodynamic electron motion, driven by both temperature and magnetic field. We directly measure the viscosity and electron-electron scattering lifetime (the Fermi quasiparticle lifetime) over a wide temperature range without fitting parameters and show they have a strong dependence on electron density that cannot be explained by conventional theories based on the random phase approximation. |