Giovanni Vignale
Degree: PhD
Position: Visiting Faculty
Affiliation: University of Missouri
Research Type: Theory
Email: vignaleg@missouri.edu
Website: https://faculty.missouri.edu/~vignaleg/
CA2DM Publications:
2024 |
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. |
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. |
2020 |
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. |
Song, Peng; Hsu, Chuang-Han; Vignale, Giovanni; Zhao, Meng; Liu, Jiawei; Deng, Yujun; Fu, Wei; Liu, Yanpeng; Zhang, Yuanbo; Lin, Hsin; Pereira, Vitor M; Loh, Kian Ping Coexistence of large conventional and planar spin Hall effect with long spin diffusion length in a low-symmetry semimetal at room temperature Journal Article NATURE MATERIALS, 19 (3), pp. 292-+, 2020, ISSN: 1476-1122. @article{ISI:000510823100005, title = {Coexistence of large conventional and planar spin Hall effect with long spin diffusion length in a low-symmetry semimetal at room temperature}, author = {Peng Song and Chuang-Han Hsu and Giovanni Vignale and Meng Zhao and Jiawei Liu and Yujun Deng and Wei Fu and Yanpeng Liu and Yuanbo Zhang and Hsin Lin and Vitor M Pereira and Kian Ping Loh}, doi = {10.1038/s41563-019-0600-4}, times_cited = {0}, issn = {1476-1122}, year = {2020}, date = {2020-02-03}, journal = {NATURE MATERIALS}, volume = {19}, number = {3}, pages = {292-+}, publisher = {NATURE PUBLISHING GROUP}, address = {MACMILLAN BUILDING, 4 CRINAN ST, LONDON N1 9XW, ENGLAND}, abstract = {A large spin Hall angle and long spin diffusion length are found in the low-symmetry, few-layer semimetal MoTe2 at room temperature, thus identifying this material as an excellent candidate for simultaneous spin generation, transport and detection.}, keywords = {}, pubstate = {published}, tppubtype = {article} } A large spin Hall angle and long spin diffusion length are found in the low-symmetry, few-layer semimetal MoTe2 at room temperature, thus identifying this material as an excellent candidate for simultaneous spin generation, transport and detection. |
Zarenia, Mohammad; Yudishtira, Indra; Adam, Shaffique; Vignale, Giovanni Enhanced hydrodynamic transport in near magic angle twisted bilayer graphene Journal Article PHYSICAL REVIEW B, 101 (4), 2020, ISSN: 2469-9950. @article{ISI:000507864100003, title = {Enhanced hydrodynamic transport in near magic angle twisted bilayer graphene}, author = {Mohammad Zarenia and Indra Yudishtira and Shaffique Adam and Giovanni Vignale}, doi = {10.1103/PhysRevB.101.045421}, times_cited = {3}, issn = {2469-9950}, year = {2020}, date = {2020-01-17}, journal = {PHYSICAL REVIEW B}, volume = {101}, number = {4}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {Using the semiclassical quantum Boltzmann theory and employing the Dirac model with twist angle-dependent Fermi velocity, we obtain results for the electrical resistivity, the electronic thermal resistivity, the Seebeck coefficient, and the Wiedemann-Franz ratio in near magic angle twisted bilayer graphene, as functions of doping density (around the charge-neutrality point) and modified Fermi velocity (v) over tilde. The (v) over tilde dependence of the relevant scattering mechanisms, i.e., electron-hole Coulomb, long-range impurities, and acoustic gauge phonons, is considered in detail. We find a range of twist angles and temperatures, where the combined effect of momentum-nonconserving collisions (long-range impurities and phonons) is minimal, opening a window for the observation of strong hydrodynamic transport. Several experimental signatures are identified, such as a sharp dependence of the electric resistivity on doping density and a large enhancement of the Wiedemann-Franz ratio and the Seebeck coefficient.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Using the semiclassical quantum Boltzmann theory and employing the Dirac model with twist angle-dependent Fermi velocity, we obtain results for the electrical resistivity, the electronic thermal resistivity, the Seebeck coefficient, and the Wiedemann-Franz ratio in near magic angle twisted bilayer graphene, as functions of doping density (around the charge-neutrality point) and modified Fermi velocity (v) over tilde. The (v) over tilde dependence of the relevant scattering mechanisms, i.e., electron-hole Coulomb, long-range impurities, and acoustic gauge phonons, is considered in detail. We find a range of twist angles and temperatures, where the combined effect of momentum-nonconserving collisions (long-range impurities and phonons) is minimal, opening a window for the observation of strong hydrodynamic transport. Several experimental signatures are identified, such as a sharp dependence of the electric resistivity on doping density and a large enhancement of the Wiedemann-Franz ratio and the Seebeck coefficient. |
2019 |
Trushin, Maxim; Neto, Antonio Castro H; Vignale, Giovanni; Culcer, Dimitrie Hidden anisotropy in the Drude conductivity of charge carriers with Dirac-Schrodinger dynamics Journal Article PHYSICAL REVIEW B, 100 (3), 2019, ISSN: 2469-9950. @article{ISI:000476681800007, title = {Hidden anisotropy in the Drude conductivity of charge carriers with Dirac-Schrodinger dynamics}, author = {Maxim Trushin and Antonio Castro H Neto and Giovanni Vignale and Dimitrie Culcer}, doi = {10.1103/PhysRevB.100.035427}, times_cited = {0}, issn = {2469-9950}, year = {2019}, date = {2019-07-22}, journal = {PHYSICAL REVIEW B}, volume = {100}, number = {3}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {We show that the conductivity of a two-dimensional electron gas can be intrinsically anisotropic despite isotropic Fermi surface, energy dispersion, and disorder configuration. In the model we study, the anisotropy stems from the interplay between Dirac and Schrodinger features combined in a special two-band Hamiltonian describing the quasiparticles similar to the low-energy excitations in phosphorene. As a result, even scalar isotropic disorder scattering alters the nature of the carriers and results in anisotropic transport. Solving the Boltzmann equation exactly for such carriers with pointlike random impurities, we find a hidden knob to control the anisotropy just by tuning either the Fermi energy or temperature. Our results are expected to be generally applicable beyond the model studied here, and should stimulate further search for the alternative ways to control electron transport in advanced materials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We show that the conductivity of a two-dimensional electron gas can be intrinsically anisotropic despite isotropic Fermi surface, energy dispersion, and disorder configuration. In the model we study, the anisotropy stems from the interplay between Dirac and Schrodinger features combined in a special two-band Hamiltonian describing the quasiparticles similar to the low-energy excitations in phosphorene. As a result, even scalar isotropic disorder scattering alters the nature of the carriers and results in anisotropic transport. Solving the Boltzmann equation exactly for such carriers with pointlike random impurities, we find a hidden knob to control the anisotropy just by tuning either the Fermi energy or temperature. Our results are expected to be generally applicable beyond the model studied here, and should stimulate further search for the alternative ways to control electron transport in advanced materials. |
Zarenia, Mohammad; Principi, Alessandro; Vignale, Giovanni Disorder-enabled hydrodynamics of charge and heat transport in monolayer graphene Journal Article 2D MATERIALS, 6 (3), 2019, ISSN: 2053-1583. @article{ISI:000467936300002, title = {Disorder-enabled hydrodynamics of charge and heat transport in monolayer graphene}, author = {Mohammad Zarenia and Alessandro Principi and Giovanni Vignale}, doi = {10.1088/2053-1583/ab1ad9}, times_cited = {0}, issn = {2053-1583}, year = {2019}, date = {2019-07-01}, journal = {2D MATERIALS}, volume = {6}, number = {3}, publisher = {IOP PUBLISHING LTD}, address = {TEMPLE CIRCUS, TEMPLE WAY, BRISTOL BS1 6BE, ENGLAND}, abstract = {Hydrodynamic behavior in electronic systems is commonly accepted to be associated with extremely clean samples such that electron-electron collisions dominate and total momentum is conserved. Contrary to this, we show that in monolayer graphene the presence of disorder is essential to enable an unconventional hydrodynamic regime which exists near the charge neutrality point and is characterized by a large enhancement of the Wiedemann-Franz ratio. Although the enhancement becomes more pronounced with decreasing disorder, the very possibility of observing the effect depends crucially on the presence of disorder. We calculate the maximum extrinsic carrier density n(c) below which the effect becomes manifest, and show that n(c) vanishes in the limit of zero disorder. For n > n(c), we predict that the Wiedemann-Franz ratio actually decreases with decreasing disorder. We complete our analysis by presenting a transparent picture of the physical processes that are responsible for the crossover from conventional to disorder-enabled hydrodynamics. Recent experiments on monolayer graphene are discussed and shown to be consistent with this picture.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Hydrodynamic behavior in electronic systems is commonly accepted to be associated with extremely clean samples such that electron-electron collisions dominate and total momentum is conserved. Contrary to this, we show that in monolayer graphene the presence of disorder is essential to enable an unconventional hydrodynamic regime which exists near the charge neutrality point and is characterized by a large enhancement of the Wiedemann-Franz ratio. Although the enhancement becomes more pronounced with decreasing disorder, the very possibility of observing the effect depends crucially on the presence of disorder. We calculate the maximum extrinsic carrier density n(c) below which the effect becomes manifest, and show that n(c) vanishes in the limit of zero disorder. For n > n(c), we predict that the Wiedemann-Franz ratio actually decreases with decreasing disorder. We complete our analysis by presenting a transparent picture of the physical processes that are responsible for the crossover from conventional to disorder-enabled hydrodynamics. Recent experiments on monolayer graphene are discussed and shown to be consistent with this picture. |
Song, Justin C W; Vignale, Giovanni Low-dissipation edge currents without edge states Journal Article PHYSICAL REVIEW B, 99 (23), 2019, ISSN: 2469-9950. @article{ISI:000470836000004, title = {Low-dissipation edge currents without edge states}, author = {Justin C W Song and Giovanni Vignale}, doi = {10.1103/PhysRevB.99.235405}, times_cited = {6}, issn = {2469-9950}, year = {2019}, date = {2019-06-06}, journal = {PHYSICAL REVIEW B}, volume = {99}, number = {23}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {We propose that bulk free carriers in topologically trivial multivalley insulators with nonvanishing Berry curvature can give rise to low-dissipation edge currents, which are squeezed within a distance on the order of the valley diffusion length from the edge. This happens even in the absence of edge states [topological (gapless) or otherwise], and when the bulk equilibrium carrier concentration is thermally activated across the gap. Physically, the squeezed edge current arises from the spatially inhomogeneous valley orbital magnetization that develops from valley-density accumulation near the edge. While this current possesses neither topology nor symmetry protection and, as a result, is not immune to dissipation, in clean enough devices it can mimic low-loss ballistic transport.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We propose that bulk free carriers in topologically trivial multivalley insulators with nonvanishing Berry curvature can give rise to low-dissipation edge currents, which are squeezed within a distance on the order of the valley diffusion length from the edge. This happens even in the absence of edge states [topological (gapless) or otherwise], and when the bulk equilibrium carrier concentration is thermally activated across the gap. Physically, the squeezed edge current arises from the spatially inhomogeneous valley orbital magnetization that develops from valley-density accumulation near the edge. While this current possesses neither topology nor symmetry protection and, as a result, is not immune to dissipation, in clean enough devices it can mimic low-loss ballistic transport. |
Zarenia, Mohammad; Smith, Thomas Benjamin; Principi, Alessandro; Vignale, Giovanni Breakdown of the Wiedemann-Franz law in AB-stacked bilayer graphene Journal Article PHYSICAL REVIEW B, 99 (16), 2019, ISSN: 2469-9950. @article{ISI:000466405300003, title = {Breakdown of the Wiedemann-Franz law in \textit{AB}-stacked bilayer graphene}, author = {Mohammad Zarenia and Thomas Benjamin Smith and Alessandro Principi and Giovanni Vignale}, doi = {10.1103/PhysRevB.99.161407}, times_cited = {0}, issn = {2469-9950}, year = {2019}, date = {2019-04-26}, journal = {PHYSICAL REVIEW B}, volume = {99}, number = {16}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {We present a simple theory of thermoelectric transport in bilayer graphene and report our results for the electrical resistivity, the thermal resistivity, the Seebeck coefficient, and the Wiedemann-Franz ratio as functions of doping density and temperature. In the absence of disorder, the thermal resistivity tends to zero as the charge neutrality point is approached; the electric resistivity jumps from zero to an intrinsic finite value, and the Seebeck coefficient diverges in the same limit. Even though these results are similar to those obtained for single-layer graphene, their derivation is considerably more delicate. The singularities are removed by the inclusion of a small amount of disorder, which leads to the appearance of a "window" of doping densities 0 < n < n(c) (with n(c) tending to zero in the zero-disorder limit) in which the Wiedemann-Franz law is severely violated.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We present a simple theory of thermoelectric transport in bilayer graphene and report our results for the electrical resistivity, the thermal resistivity, the Seebeck coefficient, and the Wiedemann-Franz ratio as functions of doping density and temperature. In the absence of disorder, the thermal resistivity tends to zero as the charge neutrality point is approached; the electric resistivity jumps from zero to an intrinsic finite value, and the Seebeck coefficient diverges in the same limit. Even though these results are similar to those obtained for single-layer graphene, their derivation is considerably more delicate. The singularities are removed by the inclusion of a small amount of disorder, which leads to the appearance of a "window" of doping densities 0 < n < n(c) (with n(c) tending to zero in the zero-disorder limit) in which the Wiedemann-Franz law is severely violated. |
Yudhistira, Indra; Chakraborty, Nilotpal; Sharma, Girish; Ho, Derek Y H; Laksono, Evan; Sushkov, Oleg P; Vignale, Giovanni; Adam, Shaffique Gauge-phonon dominated resistivity in twisted bilayer graphene near magic angle Journal Article PHYSICAL REVIEW B, 99 (14), 2019, ISSN: 2469-9950. @article{ISI:000465159300001, title = {Gauge-phonon dominated resistivity in twisted bilayer graphene near magic angle}, author = {Indra Yudhistira and Nilotpal Chakraborty and Girish Sharma and Derek Y H Ho and Evan Laksono and Oleg P Sushkov and Giovanni Vignale and Shaffique Adam}, doi = {10.1103/PhysRevB.99.140302}, times_cited = {0}, issn = {2469-9950}, year = {2019}, date = {2019-04-17}, journal = {PHYSICAL REVIEW B}, volume = {99}, number = {14}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {Recent experiments on twisted bilayer graphene (tBG) close to magic angle show that a small relative rotation in a van der Waals heterostructure greatly alters its electronic properties. We consider various scattering mechanisms and show that the carrier transport in tBG is dominated by a combination of charged impurities and acoustic gauge phonons. Charged impurities still dominate at low temperature and densities because of the inability of Dirac fermions to screen long-range Coulomb potentials at charge neutrality; however, the gauge phonons dominate for most of the experimental regime because, although they couple to current, they do not induce charge and are therefore unscreened by the large density of states close to magic angle. We show that the resistivity has a strong monotonically decreasing carrier density dependence at low temperature due to charged impurity scattering, and weak density dependence at high temperature due to gauge phonons. Away from charge neutrality, the resistivity increases with temperature, while it does the opposite close to the Dirac point. A nonmonotonic temperature dependence observed only at low temperature and carrier density is a signature of our theory that can be tested in experimentally available samples.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Recent experiments on twisted bilayer graphene (tBG) close to magic angle show that a small relative rotation in a van der Waals heterostructure greatly alters its electronic properties. We consider various scattering mechanisms and show that the carrier transport in tBG is dominated by a combination of charged impurities and acoustic gauge phonons. Charged impurities still dominate at low temperature and densities because of the inability of Dirac fermions to screen long-range Coulomb potentials at charge neutrality; however, the gauge phonons dominate for most of the experimental regime because, although they couple to current, they do not induce charge and are therefore unscreened by the large density of states close to magic angle. We show that the resistivity has a strong monotonically decreasing carrier density dependence at low temperature due to charged impurity scattering, and weak density dependence at high temperature due to gauge phonons. Away from charge neutrality, the resistivity increases with temperature, while it does the opposite close to the Dirac point. A nonmonotonic temperature dependence observed only at low temperature and carrier density is a signature of our theory that can be tested in experimentally available samples. |
2016 |
Trevisanutto, Paolo E; Vignale, Giovanni Ab initio electronic structure of quasi-two-dimensional materials: A "native" Gaussian-plane wave approach Journal Article JOURNAL OF CHEMICAL PHYSICS, 144 (20), 2016, ISSN: 0021-9606. @article{ISI:000377712700024, title = {\textit{Ab initio} electronic structure of quasi-two-dimensional materials: A "native" Gaussian-plane wave approach}, author = {Paolo E Trevisanutto and Giovanni Vignale}, doi = {10.1063/1.4951686}, times_cited = {0}, issn = {0021-9606}, year = {2016}, date = {2016-05-28}, journal = {JOURNAL OF CHEMICAL PHYSICS}, volume = {144}, number = {20}, publisher = {AMER INST PHYSICS}, address = {1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USA}, abstract = {Ab initio electronic structure calculations of two-dimensional layered structures are typically performed using codes that were developed for three-dimensional structures, which are periodic in all three directions. The introduction of a periodicity in the third direction (perpendicular to the layer) is completely artificial and may lead in some cases to spurious results and to difficulties in treating the action of external fields. In this paper we develop a new approach, which is "native" to quasi-2D materials, making use of basis function that are periodic in the plane, but atomic-like in the perpendicular direction. We show how some of the basic tools of ab initio electronic structure theory-density functional theory, GW approximation and Bethe-Salpeter equation-are implemented in the new basis. We argue that the new approach will be preferable to the conventional one in treating the peculiarities of layered materials, including the long range of the unscreened Coulomb interaction in insulators, and the effects of strain, corrugations, and external fields. Published by AIP Publishing.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Ab initio electronic structure calculations of two-dimensional layered structures are typically performed using codes that were developed for three-dimensional structures, which are periodic in all three directions. The introduction of a periodicity in the third direction (perpendicular to the layer) is completely artificial and may lead in some cases to spurious results and to difficulties in treating the action of external fields. In this paper we develop a new approach, which is "native" to quasi-2D materials, making use of basis function that are periodic in the plane, but atomic-like in the perpendicular direction. We show how some of the basic tools of ab initio electronic structure theory-density functional theory, GW approximation and Bethe-Salpeter equation-are implemented in the new basis. We argue that the new approach will be preferable to the conventional one in treating the peculiarities of layered materials, including the long range of the unscreened Coulomb interaction in insulators, and the effects of strain, corrugations, and external fields. Published by AIP Publishing. |