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; 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 = {0}, 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 |
Foo, D C W; Swain, N; Sengupta, P; Lemarie, G; Adam, S Stabilization mechanism for many-body localization in two dimensions Journal Article PHYSICAL REVIEW RESEARCH , 5 (3), 2023. @article{ISI:001050777700001, title = {Stabilization mechanism for many-body localization in two dimensions }, author = {D C W Foo and N Swain and P Sengupta and G Lemarie and S Adam}, doi = {10.1103/PhysRevResearch.5.L032011}, times_cited = {0}, year = {2023}, date = {2023-07-20}, journal = {PHYSICAL REVIEW RESEARCH }, volume = {5}, number = {3}, publisher = {AMER PHYSICAL SOC }, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA }, abstract = {Experiments in cold-atom systems see almost identical signatures of many-body localization (MBL) in both one-dimensional (d = 1) and two-dimensional (d = 2) systems despite the thermal avalanche hypothesis showing that the MBL phase is unstable ford > 1. Underpinning the thermal avalanche argument is the assumption of exponential localization of local integrals of motion (LIOM). In this Letter we demonstrate that the addition of a confining potential-as is typical in experimental setups-allows a noninteracting disordered system to have superexponentially (Gaussian) localized wave functions, and an interacting disordered system to undergo a localization transition. Moreover, we show that Gaussian localization of MBL LIOM shifts the quantum avalanche critical dimension from d = 1 to d = 2, potentially bridging the divide between the experimental demonstrations of MBL in these systems and existing theoretical arguments that claim that such demonstrations are impossible. }, keywords = {}, pubstate = {published}, tppubtype = {article} } Experiments in cold-atom systems see almost identical signatures of many-body localization (MBL) in both one-dimensional (d = 1) and two-dimensional (d = 2) systems despite the thermal avalanche hypothesis showing that the MBL phase is unstable ford > 1. Underpinning the thermal avalanche argument is the assumption of exponential localization of local integrals of motion (LIOM). In this Letter we demonstrate that the addition of a confining potential-as is typical in experimental setups-allows a noninteracting disordered system to have superexponentially (Gaussian) localized wave functions, and an interacting disordered system to undergo a localization transition. Moreover, we show that Gaussian localization of MBL LIOM shifts the quantum avalanche critical dimension from d = 1 to d = 2, potentially bridging the divide between the experimental demonstrations of MBL in these systems and existing theoretical arguments that claim that such demonstrations are impossible. |
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 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 = {0}, 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 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 = {0}, 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 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 = {0}, 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 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 = {0}, 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. |
Xu, Shuigang; Ezzi, Mohammed Al M; Balakrishnan, Nilanthy; Garcia-Ruiz, Aitor; Tsim, Bonnie; Mullan, Ciaran; Barrier, Julien; Xin, Na; Piot, Benjamin A; Taniguchi, Takashi; Watanabe, Kenji; Carvalho, Alexandra; Mishchenko, Artem; Geim, A K; Fal'ko, Vladimir I; Adam, Shaffique; Neto, Antonio Helio Castro; Novoselov, Kostya S; Shi, Yanmeng Tunable van Hove singularities and correlated states in twisted monolayer-bilayer graphene Journal Article NATURE PHYSICS, 17 (5), pp. 619-+, 2021, ISSN: 1745-2473. @article{ISI:000619417000001, title = {Tunable van Hove singularities and correlated states in twisted monolayer-bilayer graphene}, author = {Shuigang Xu and Mohammed Al M Ezzi and Nilanthy Balakrishnan and Aitor Garcia-Ruiz and Bonnie Tsim and Ciaran Mullan and Julien Barrier and Na Xin and Benjamin A Piot and Takashi Taniguchi and Kenji Watanabe and Alexandra Carvalho and Artem Mishchenko and A K Geim and Vladimir I Fal'ko and Shaffique Adam and Antonio Helio Castro Neto and Kostya S Novoselov and Yanmeng Shi}, doi = {10.1038/s41567-021-01172-9}, times_cited = {0}, issn = {1745-2473}, year = {2021}, date = {2021-02-18}, journal = {NATURE PHYSICS}, volume = {17}, number = {5}, pages = {619-+}, publisher = {NATURE RESEARCH}, address = {HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY}, abstract = {Understanding and tuning correlated states is of great interest and importance to modern condensed-matter physics. The recent discovery of unconventional superconductivity and Mott-like insulating states in magic-angle twisted bilayer graphene presents a unique platform to study correlation phenomena, in which the Coulomb energy dominates over the quenched kinetic energy as a result of hybridized flat bands. Extending this approach to the case of twisted multilayer graphene would allow even higher control over the band structure because of the reduced symmetry of the system. Here we study electronic transport properties of twisted monolayer-bilayer graphene (a bilayer on top of monolayer graphene heterostructure). We observe the formation of van Hove singularities that are highly tunable by changing either the twist angle or external electric field and can cause strong correlation effects under optimum conditions. We provide basic theoretical interpretations of the observed electronic structure.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Understanding and tuning correlated states is of great interest and importance to modern condensed-matter physics. The recent discovery of unconventional superconductivity and Mott-like insulating states in magic-angle twisted bilayer graphene presents a unique platform to study correlation phenomena, in which the Coulomb energy dominates over the quenched kinetic energy as a result of hybridized flat bands. Extending this approach to the case of twisted multilayer graphene would allow even higher control over the band structure because of the reduced symmetry of the system. Here we study electronic transport properties of twisted monolayer-bilayer graphene (a bilayer on top of monolayer graphene heterostructure). We observe the formation of van Hove singularities that are highly tunable by changing either the twist angle or external electric field and can cause strong correlation effects under optimum conditions. We provide basic theoretical interpretations of the observed electronic structure. |
Lee, Hae Yeon; Ezzi, Mohammed Al M; Raghuvanshi, Nimisha; Chung, Jing Yang; Watanabe, Kenji; Taniguchi, Takashi; Garaj, Slaven; Adam, Shaffique; Gradecak, Silvija Tunable Optical Properties of Thin Films Controlled by the Interface Twist Angle Journal Article NANO LETTERS, 21 (7), pp. 2832-2839, 2021, ISSN: 1530-6984. @article{ISI:000641160500018, title = {Tunable Optical Properties of Thin Films Controlled by the Interface Twist Angle}, author = {Hae Yeon Lee and Mohammed Al M Ezzi and Nimisha Raghuvanshi and Jing Yang Chung and Kenji Watanabe and Takashi Taniguchi and Slaven Garaj and Shaffique Adam and Silvija Gradecak}, doi = {10.1021/acs.nanolett.0c04924}, times_cited = {0}, issn = {1530-6984}, year = {2021}, date = {2021-02-16}, journal = {NANO LETTERS}, volume = {21}, number = {7}, pages = {2832-2839}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Control of materials properties has been the driving force of modern technologies. So far, materials properties have been modulated by their composition, structure, and size. Here, by using cathodoluminescence in a scanning transmission electron microscope, we show that the optical properties of stacked, >100 nm thick hexagonal boron nitride (hBN) films can be continuously tuned by their relative twist angles. Due to the formation of a moire superlattice between the two interface layers of the twisted films, a new moire ' sub-band gap is formed with continuously decreasing magnitude as a function of the twist angle, resulting in tunable luminescence wavelength and intensity increase of >40x. Our results demonstrate that moire ' phenomena extend beyond monolayer-based systems and can be preserved in a technologically relevant, bulklike material at room temperature, dominating optical properties of hBN films for applications in medicine, environmental, or information technologies.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Control of materials properties has been the driving force of modern technologies. So far, materials properties have been modulated by their composition, structure, and size. Here, by using cathodoluminescence in a scanning transmission electron microscope, we show that the optical properties of stacked, >100 nm thick hexagonal boron nitride (hBN) films can be continuously tuned by their relative twist angles. Due to the formation of a moire superlattice between the two interface layers of the twisted films, a new moire ' sub-band gap is formed with continuously decreasing magnitude as a function of the twist angle, resulting in tunable luminescence wavelength and intensity increase of >40x. Our results demonstrate that moire ' phenomena extend beyond monolayer-based systems and can be preserved in a technologically relevant, bulklike material at room temperature, dominating optical properties of hBN films for applications in medicine, environmental, or information technologies. |
2020 |
Trushin, Maxim; Sarkar, Soumya; Mathew, Sinu; Goswami, Sreetosh; Sahoo, Prasana; Wang, Yan; Yang, Jieun; Li, Weiwei; MacManus-Driscoll, Judith L; Chhowalla, Manish; Adam, Shaffique; Venkatesan, T Evidence of Rotational Frohlich Coupling in Polaronic Trions Journal Article PHYSICAL REVIEW LETTERS, 125 (8), 2020, ISSN: 0031-9007. @article{ISI:000560967100008, title = {Evidence of Rotational Frohlich Coupling in Polaronic Trions}, author = {Maxim Trushin and Soumya Sarkar and Sinu Mathew and Sreetosh Goswami and Prasana Sahoo and Yan Wang and Jieun Yang and Weiwei Li and Judith L MacManus-Driscoll and Manish Chhowalla and Shaffique Adam and T Venkatesan}, doi = {10.1103/PhysRevLett.125.086803}, times_cited = {0}, issn = {0031-9007}, year = {2020}, date = {2020-08-20}, journal = {PHYSICAL REVIEW LETTERS}, volume = {125}, number = {8}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {Electrons commonly couple through Frohlich interactions with longitudinal optical phonons to form polarons. However, trions possess a finite angular momentum and should therefore couple instead to rotational optical phonons. This creates a polaronic trion whose binding energy is determined by the crystallographic orientation of the lattice. Here, we demonstrate theoretically within the Frohlich approach and experimentally by photoluminescence emission that the bare trion binding energy (20 meV) is significantly enhanced by the phonons at the interface between the two-dimensional semiconductor MoS2 and the bulk transition metal oxide SrTiO3. The low-temperature binding energy changes from 60 meV in [001]-oriented substrates to 90 meV for [111] orientation, as a result of the counterintuitive interplay between the rotational axis of the MoS2 trion and that of the SrTiO3 phonon mode.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Electrons commonly couple through Frohlich interactions with longitudinal optical phonons to form polarons. However, trions possess a finite angular momentum and should therefore couple instead to rotational optical phonons. This creates a polaronic trion whose binding energy is determined by the crystallographic orientation of the lattice. Here, we demonstrate theoretically within the Frohlich approach and experimentally by photoluminescence emission that the bare trion binding energy (20 meV) is significantly enhanced by the phonons at the interface between the two-dimensional semiconductor MoS2 and the bulk transition metal oxide SrTiO3. The low-temperature binding energy changes from 60 meV in [001]-oriented substrates to 90 meV for [111] orientation, as a result of the counterintuitive interplay between the rotational axis of the MoS2 trion and that of the SrTiO3 phonon mode. |
Liu, Chang; Akhgar, Golrokh; Collins, James L; Hellerstedt, Jack; Tan, Cheng; Wang, Lan; Adam, Shaffique; Fuhrer, Michael S; Edmonds, Mark T Quantum Transport in Air-Stable Na3Bi Thin Films Journal Article ACS APPLIED MATERIALS & INTERFACES, 12 (31), pp. 35542-35546, 2020, ISSN: 1944-8244. @article{ISI:000558792700107, title = {Quantum Transport in Air-Stable Na_{3}Bi Thin Films}, author = {Chang Liu and Golrokh Akhgar and James L Collins and Jack Hellerstedt and Cheng Tan and Lan Wang and Shaffique Adam and Michael S Fuhrer and Mark T Edmonds}, doi = {10.1021/acsami.0c05832}, times_cited = {0}, issn = {1944-8244}, year = {2020}, date = {2020-08-05}, journal = {ACS APPLIED MATERIALS & INTERFACES}, volume = {12}, number = {31}, pages = {35542-35546}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {Na3Bi has attracted significant interest in both bulk form as a three-dimensional topological Dirac semimetal and ultrathin form as a wide-band gap two-dimensional topological insulator. Its extreme air sensitivity has limited experimental efforts on thin and ultrathin films grown via molecular beam epitaxy to ultrahigh vacuum environments. Here, we demonstrate air-stable Na3Bi thin films passivated with magnesium difluoride (MgF2) or silicon (Si) capping layers. Electrical measurements show that deposition of MgF2 or Si has minimal impact on the transport properties of Na3Bi while in ultrahigh vacuum. Importantly, the MgF2-passivated Na3Bi films are air-stable and remain metallic for over 100 h after exposure to air, as compared to near instantaneous degradation when they are unpassivated. Air stability enables transfer of films to a conventional high-magnetic field cryostat, enabling quantum transport measurements, which verify that the Dirac semimetal character of Na3Bi films is retained after air exposure.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Na3Bi has attracted significant interest in both bulk form as a three-dimensional topological Dirac semimetal and ultrathin form as a wide-band gap two-dimensional topological insulator. Its extreme air sensitivity has limited experimental efforts on thin and ultrathin films grown via molecular beam epitaxy to ultrahigh vacuum environments. Here, we demonstrate air-stable Na3Bi thin films passivated with magnesium difluoride (MgF2) or silicon (Si) capping layers. Electrical measurements show that deposition of MgF2 or Si has minimal impact on the transport properties of Na3Bi while in ultrahigh vacuum. Importantly, the MgF2-passivated Na3Bi films are air-stable and remain metallic for over 100 h after exposure to air, as compared to near instantaneous degradation when they are unpassivated. Air stability enables transfer of films to a conventional high-magnetic field cryostat, enabling quantum transport measurements, which verify that the Dirac semimetal character of Na3Bi films is retained after air exposure. |
Plumadore, Ryan; Ezzi, Mohammed Al M; Adam, Shaffique; Luican-Mayer, Adina Moire patterns in graphene-rhenium disulfide vertical heterostructures Journal Article JOURNAL OF APPLIED PHYSICS, 128 (4), 2020, ISSN: 0021-8979. @article{ISI:000559770700002, title = {Moire patterns in graphene-rhenium disulfide vertical heterostructures}, author = {Ryan Plumadore and Mohammed Al M Ezzi and Shaffique Adam and Adina Luican-Mayer}, doi = {10.1063/5.0015643}, times_cited = {0}, issn = {0021-8979}, year = {2020}, date = {2020-07-28}, journal = {JOURNAL OF APPLIED PHYSICS}, volume = {128}, number = {4}, publisher = {AIP Publishing}, address = {1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USA}, abstract = {Vertical stacking of atomically thin materials offers a large platform for realizing novel properties enabled by proximity effects and moire patterns. Here, we focus on mechanically assembled heterostructures of graphene and ReS2, a van der Waals layered semiconductor. Using scanning tunneling microscopy and spectroscopy, we image the sharp edge between the two materials as well as areas of overlap. Locally resolved topographic images revealed the presence of a striped superpattern originating in the interlayer interactions between graphene's hexagonal structure and the triclinic, low in-plane symmetry of ReS2. We compare the results with a theoretical model that estimates the shape and angle dependence of the moire pattern between graphene and ReS2. These results shed light on the complex interface phenomena between van der Waals materials with different lattice symmetries.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Vertical stacking of atomically thin materials offers a large platform for realizing novel properties enabled by proximity effects and moire patterns. Here, we focus on mechanically assembled heterostructures of graphene and ReS2, a van der Waals layered semiconductor. Using scanning tunneling microscopy and spectroscopy, we image the sharp edge between the two materials as well as areas of overlap. Locally resolved topographic images revealed the presence of a striped superpattern originating in the interlayer interactions between graphene's hexagonal structure and the triclinic, low in-plane symmetry of ReS2. We compare the results with a theoretical model that estimates the shape and angle dependence of the moire pattern between graphene and ReS2. These results shed light on the complex interface phenomena between van der Waals materials with different lattice symmetries. |
Zarenia, Mohammad; Adam, Shaffique; Vignale, Giovanni Temperature collapse of the electric conductivity in bilayer graphene Journal Article PHYSICAL REVIEW RESEARCH , 2 (2), 2020. @article{ISI:000603640300002, title = {Temperature collapse of the electric conductivity in bilayer graphene }, author = {Mohammad Zarenia and Shaffique Adam and Giovanni Vignale}, doi = {10.1103/PhysRevResearch.2.023391}, times_cited = {1}, year = {2020}, date = {2020-06-24}, journal = {PHYSICAL REVIEW RESEARCH }, volume = {2}, number = {2}, publisher = {AMER PHYSICAL SOC }, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA }, abstract = {Recent experiments have reported evidence of dominant electron-hole scattering in the electric conductivity of suspended bilayer graphene near charge neutrality. According to these experiments, plots of the electric conductivity as a function of mu/k(B)T (chemical potential scaled with temperature) obtained for different temperatures in the range of 12 K less than or similar to T less than or similar to 40 K collapse on a single curve independent of T. In a recent theory, this observation has been taken as an indication that the main subdominant scattering process is not electron impurity but electron-phonon. Here, we demonstrate that the collapse of the data on a single curve can be explained without invoking electron-phonon scattering but assuming that the suspended bilayer graphene is not a truly gapless system and by including the effect of electron-hole puddles in the subdominant charged impurity scattering mechanism. With a gap of similar to 5 meV, our theory produces excellent agreement with the observed conductivity over the full reported range of temperatures. These results are based on the hydrodynamic theory of conductivity, which, thus, emerges as a solid foundation for the analysis of experiments and the estimation of the band gap in multiband systems. }, keywords = {}, pubstate = {published}, tppubtype = {article} } Recent experiments have reported evidence of dominant electron-hole scattering in the electric conductivity of suspended bilayer graphene near charge neutrality. According to these experiments, plots of the electric conductivity as a function of mu/k(B)T (chemical potential scaled with temperature) obtained for different temperatures in the range of 12 K less than or similar to T less than or similar to 40 K collapse on a single curve independent of T. In a recent theory, this observation has been taken as an indication that the main subdominant scattering process is not electron impurity but electron-phonon. Here, we demonstrate that the collapse of the data on a single curve can be explained without invoking electron-phonon scattering but assuming that the suspended bilayer graphene is not a truly gapless system and by including the effect of electron-hole puddles in the subdominant charged impurity scattering mechanism. With a gap of similar to 5 meV, our theory produces excellent agreement with the observed conductivity over the full reported range of temperatures. These results are based on the hydrodynamic theory of conductivity, which, thus, emerges as a solid foundation for the analysis of experiments and the estimation of the band gap in multiband systems. |
Sharma, Girish; Trushin, Maxim; Sushkov, Oleg P; Vignale, Giovanni; Adam, Shaffique Superconductivity from collective excitations in magic-angle twisted bilayer graphene Journal Article 20 PHYSICAL REVIEW RESEARCH , 2 (2), 2020. @article{ISI:000603585200002, title = {Superconductivity from collective excitations in magic-angle twisted bilayer graphene }, author = {Girish Sharma and Maxim Trushin and Oleg P Sushkov and Giovanni Vignale and Shaffique Adam}, doi = {10.1103/PhysRevResearch.2.022040}, times_cited = {20}, year = {2020}, date = {2020-05-13}, journal = {PHYSICAL REVIEW RESEARCH }, volume = {2}, number = {2}, publisher = {AMER PHYSICAL SOC }, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA }, abstract = {A purely electronic mechanism is proposed for the unconventional superconductivity recently observed in twisted bilayer graphene (tBG) close to the magic angle. Using the Migdal-Eliashberg framework on a one-parameter effective lattice model for tBG we show that a superconducting state can be achieved by means of collective electronic modes in tBG. We posit robust features of the theory, including an asymmetrical superconducting dome and the magnitude of the critical temperature that are in agreement with experiments. }, keywords = {}, pubstate = {published}, tppubtype = {article} } A purely electronic mechanism is proposed for the unconventional superconductivity recently observed in twisted bilayer graphene (tBG) close to the magic angle. Using the Migdal-Eliashberg framework on a one-parameter effective lattice model for tBG we show that a superconducting state can be achieved by means of collective electronic modes in tBG. We posit robust features of the theory, including an asymmetrical superconducting dome and the magnitude of the critical temperature that are in agreement with experiments. |
Gu, Xingyu; Chen, Chuan; Leaw, Jia Ning; Laksono, Evan; Pereira, Vitor M; Vignale, Giovanni; Adam, Shaffique Antiferromagnetism and chiral d-wave superconductivity from an effective t-J-D model for twisted bilayer graphene Journal Article PHYSICAL REVIEW B, 101 (18), 2020, ISSN: 2469-9950. @article{ISI:000531445800003, title = {Antiferromagnetism and chiral \textit{d}-wave superconductivity from an effective \textit{t}-\textit{J}-\textit{D} model for twisted bilayer graphene}, author = {Xingyu Gu and Chuan Chen and Jia Ning Leaw and Evan Laksono and Vitor M Pereira and Giovanni Vignale and Shaffique Adam}, doi = {10.1103/PhysRevB.101.180506}, times_cited = {5}, issn = {2469-9950}, year = {2020}, date = {2020-05-11}, journal = {PHYSICAL REVIEW B}, volume = {101}, number = {18}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {We derive an effective tight-binding model that captures, in real space and with only two parameters, the dominant Coulomb interactions and superconducting pairing near half-filling of magic-angle twisted bilayer graphene. We show that, in an antiferromagnetic Mott insulating ground state with intervalley coherence, magnetic fluctuations and doping mediate superconducting pairing. We find the pairing wave function to have chiral d-wave symmetry and obtain a self-consistent mean-field phase diagram in line with experiments on the doping-induced insulator-to-superconductor transition. We further reveal the existence of chiral Majorana edge modes implied by the nontrivial pairing symmetry, which establishes twisted bilayer graphene as a potential platform for topological superconductivity. This effective model opens the door to systematic scrutiny of the competition between correlated states in this system.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We derive an effective tight-binding model that captures, in real space and with only two parameters, the dominant Coulomb interactions and superconducting pairing near half-filling of magic-angle twisted bilayer graphene. We show that, in an antiferromagnetic Mott insulating ground state with intervalley coherence, magnetic fluctuations and doping mediate superconducting pairing. We find the pairing wave function to have chiral d-wave symmetry and obtain a self-consistent mean-field phase diagram in line with experiments on the doping-induced insulator-to-superconductor transition. We further reveal the existence of chiral Majorana edge modes implied by the nontrivial pairing symmetry, which establishes twisted bilayer graphene as a potential platform for topological superconductivity. This effective model opens the door to systematic scrutiny of the competition between correlated states in this system. |