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:
2024 |
Ezzi, Mohammed Al M; Pallewela, Gayani N; Beule, Christophe De; Mele, E J; Adam, Shaffique Analytical Model for Atomic Relaxation in Twisted Moire 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 = {0}, 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 |
Tang, Ho-Kin; Yudhistira, Indra; Chattopadhyay, Udvas; Ulybyshev, Maksim; Sengupta, P; Assaad, F F; Adam, S Spectral functions of lattice fermions on the honeycomb lattice with Hubbard and long-range Coulomb interactions Journal Article PHYSICAL REVIEW B, 110 (15), 2024, ISSN: 2469-9950. @article{ISI:001334829900007, title = {Spectral functions of lattice fermions on the honeycomb lattice with Hubbard and long-range Coulomb interactions}, author = {Ho-Kin Tang and Indra Yudhistira and Udvas Chattopadhyay and Maksim Ulybyshev and P Sengupta and F F Assaad and S Adam}, doi = {10.1103/PhysRevB.110.155120}, times_cited = {1}, issn = {2469-9950}, year = {2024}, date = {2024-10-09}, journal = {PHYSICAL REVIEW B}, volume = {110}, number = {15}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {The absence of screening of the nonlocal Coulomb interaction in Dirac systems at charge neutrality leads to the breakdown of the Fermi liquid and divergence of the Fermi velocity. On the other hand, Mott-Hubbard physics and the concomitant formation of local moments is dominated by the local effective Hubbard interaction. Using quantum Monte Carlo methods combined with stochastic analytical continuation, we compute the single particle spectral function of fermions on the honeycomb lattice for a realistic interaction that includes both the Hubbard interaction and long-ranged Coulomb repulsion. To a first approximation, we find that the generic high-energy features, such as the formation of the upper Hubbard band near the phase transition, are primarily determined by the local effective Hubbard interaction. In the weakly interacting regime, the long-range Coulomb interaction enhances the bandwidth of quasiparticles and suppresses their lifetime. Conversely, near the phase transition, the long-range Coulomb interaction suppresses the background antiferromagnetic fluctuation, which potentially promotes the propagation of spin polarons, leading to a slight enhancement of the quasiparticle spectral weight and lifetime.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The absence of screening of the nonlocal Coulomb interaction in Dirac systems at charge neutrality leads to the breakdown of the Fermi liquid and divergence of the Fermi velocity. On the other hand, Mott-Hubbard physics and the concomitant formation of local moments is dominated by the local effective Hubbard interaction. Using quantum Monte Carlo methods combined with stochastic analytical continuation, we compute the single particle spectral function of fermions on the honeycomb lattice for a realistic interaction that includes both the Hubbard interaction and long-ranged Coulomb repulsion. To a first approximation, we find that the generic high-energy features, such as the formation of the upper Hubbard band near the phase transition, are primarily determined by the local effective Hubbard interaction. In the weakly interacting regime, the long-range Coulomb interaction enhances the bandwidth of quasiparticles and suppresses their lifetime. Conversely, near the phase transition, the long-range Coulomb interaction suppresses the background antiferromagnetic fluctuation, which potentially promotes the propagation of spin polarons, leading to a slight enhancement of the quasiparticle spectral weight and lifetime. |
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 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 = {2}, 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 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 = {1}, 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. |