The Experts below are selected from a list of 3861 Experts worldwide ranked by ideXlab platform
Ruhman Jonathan - One of the best experts on this subject based on the ideXlab platform.
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Pairing in magic-angle twisted bilayer graphene: role of phonon and plasmon umklapp
'American Physical Society (APS)', 2021Co-Authors: Lewandowski Cyprian, Chowdhury Debanjan, Ruhman JonathanAbstract:Identifying the microscopic mechanism for superconductivity in magic-angle twisted bilayer graphene (MATBG) is an outstanding open problem. While MATBG exhibits a rich phase-diagram, driven partly by the strong interactions relative to the electronic bandwidth, its single-particle properties are unique and likely play an important role in some of the phenomenological complexity. Some of the salient features include an electronic bandwidth smaller than the characteristic phonon bandwidth and a non-trivial structure of the underlying Bloch wavefunctions. We perform a theoretical study of the cooperative effects due to phonons and plasmons on pairing in order to disentangle the distinct role played by these modes on superconductivity. We consider a variant of MATBG with an enlarged number of fermion flavors, $N \gg 1$, where the study of pairing instabilities reduces to the conventional (weak-coupling) Eliashberg framework. In particular, we show that certain umklapp processes involving mini-optical phonon modes, which arise physically as a result of the folding of the original Acoustic Branch of graphene due to the moir\'e superlattice structure, contribute significantly towards enhancing pairing. We also investigate the role played by the dynamics of the screened Coulomb interaction on pairing, which leads to an enhancement in a narrow window of fillings, and study the effect of external screening due to a metallic gate on superconductivity. At strong coupling the dynamical pairing interaction leaves a spectral mark in the single particle tunneling density of states. We thus predict such features will appear at specific frequencies of the umklapp phonons corresponding to the sound velocity of graphene times an integer multiple of the Brillouin zone size.Comment: 20 pages, 8 figure
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Pairing in magic-angle twisted bilayer graphene: Role of phonon and plasmon umklapp
'American Physical Society (APS)', 2021Co-Authors: Lewandowski Cyprian, Chowdhury Debanjan, Ruhman JonathanAbstract:Identifying the microscopic mechanism for superconductivity in magic-angle twisted bilayer graphene (MATBG) is an outstanding open problem. While MATBG exhibits a rich phase-diagram, driven partly by the strong interactions relative to the electronic bandwidth, its single-particle properties are unique and likely play an important role in some of the phenomenological complexity. Some of the salient features include an electronic bandwidth smaller than the characteristic phonon bandwidth and a nontrivial structure of the underlying Bloch wave functions. We perform a theoretical study of the cooperative effects due to phonons and plasmons on pairing in order to disentangle the distinct role played by these modes on superconductivity. We consider a variant of MATBG with an enlarged number of fermion flavors, N≫1, where the study of pairing instabilities reduces to the conventional (weak-coupling) Eliashberg framework. In particular, we show that certain umklapp processes involving minioptical phonon modes, which arise physically as a result of the folding of the original Acoustic Branch of graphene due to the moiré superlattice structure, contribute significantly towards enhancing pairing. We also investigate the role played by the dynamics of the screened Coulomb interaction on pairing, which leads to an enhancement in a narrow window of fillings, and study the effect of external screening due to a metallic gate on superconductivity. At strong coupling, the dynamical pairing interaction leaves a spectral mark in the single-particle tunneling density of states. We thus predict such features will appear at specific frequencies of the umklapp phonons corresponding to the sound velocity of graphene times an integer multiple of the Brillouin zone size
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Pairing in magic-angle twisted bilayer graphene: role of phonon and plasmon umklapp
2020Co-Authors: Lewandowski Cyprian, Chowdhury Debanjan, Ruhman JonathanAbstract:Identifying the microscopic mechanism for superconductivity in magic-angle twisted bilayer graphene (MATBG) is an outstanding open problem. While MATBG exhibits a rich phase-diagram, driven partly by the strong interactions relative to the electronic bandwidth, its single-particle properties are unique and likely play an important role in some of the phenomenological complexity. Some of the salient features include an electronic bandwidth smaller than the characteristic phonon bandwidth and a non-trivial structure of the underlying Bloch wavefunctions. We perform a systematic theoretical study of the cooperative effects due to phonons and plasmons on pairing in order to disentangle the distinct role played by these modes on superconductivity. We consider a variant of MATBG with an enlarged number of fermion flavors, $N \gg 1$, where the study of pairing instabilities reduces to the conventional (weak-coupling) Eliashberg framework. In particular, we show that certain umklapp processes involving mini-optical phonon modes, which arise physically as a result of the folding of the original Acoustic Branch of graphene due to the moir\'e superlattice structure, contribute significantly towards enhancing pairing. We also investigate the role played by the dynamics of the screened Coulomb interaction on pairing, which leads to an enhancement in a narrow window of fillings, and study the effect of external screening due to a metallic gate on superconductivity. We propose a smoking-gun experiment to detect resonant features associated with the phonon-umklapp processes in the differential conductance and also discuss experimental implications of a pairing mechanism relying on plasmons.Comment: 19 pages, 7 figure
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Pairing in magic-angle twisted bilayer graphene: role of phonon and plasmon umklapp
2020Co-Authors: Lewandowski Cyprian, Chowdhury Debanjan, Ruhman JonathanAbstract:Identifying the microscopic mechanism for superconductivity in magic-angle twisted bilayer graphene (MATBG) is an outstanding open problem. While MATBG exhibits a rich phase-diagram, driven partly by the strong interactions relative to the electronic bandwidth, its single-particle properties are unique and likely play an important role in some of the phenomenological complexity. Some of the salient features include an electronic bandwidth smaller than the characteristic phonon bandwidth and a non-trivial structure of the underlying Bloch wavefunctions. We perform a systematic theoretical study of the cooperative effects due to phonons and plasmons on pairing in order to disentangle the distinct role played by these modes on superconductivity. We consider a variant of MATBG with an enlarged number of fermion flavors, N≫1, where the study of pairing instabilities reduces to the conventional (weak-coupling) Eliashberg framework. In particular, we show that certain umklapp processes involving mini-optical phonon modes, which arise physically as a result of the folding of the original Acoustic Branch of graphene due to the moiré superlattice structure, contribute significantly towards enhancing pairing. We also investigate the role played by the dynamics of the screened Coulomb interaction on pairing, which leads to an enhancement in a narrow window of fillings, and study the effect of external screening due to a metallic gate on superconductivity. We propose a smoking-gun experiment to detect resonant features associated with the phonon-umklapp processes in the differential conductance and also discuss experimental implications of a pairing mechanism relying on plasmons
Gurudas Ganguli - One of the best experts on this subject based on the ideXlab platform.
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parallel inhomogeneous flows in a thermally anisotropic plasma the electrostatic ion Acoustic Branch
Physics of Plasmas, 2002Co-Authors: Robert Spangler, Earl Scime, Gurudas GanguliAbstract:The linearized dispersion relation describing waves in a plasma having a uniform magnetic field, uniform density, and inhomogeneous parallel (to the magnetic field) flow is generalized to include thermal anisotropy (Ti⊥/Ti∥), a key feature existing in many space and laboratory plasmas. The growth rate and the real frequency at which the maximum growth rate occurs for the ion Acoustic mode increases with increasing Ti⊥/Ti∥. The propagation angle, with respect to the background magnetic field, for ion Acoustic waves is shown to depend on Ti⊥/Ti∥. Also presented is a generalized calculation of experimentally relevant perturbed distribution functions to include shear in the field-aligned flow.
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electrostatic oscillations due to filamentary structures in the magnetic field aligned flow the ion Acoustic Branch
Journal of Geophysical Research, 1999Co-Authors: Valeriy V Gavrishchaka, Supriya B Ganguli, Gurudas GanguliAbstract:Recent space missions such as FAST and Freja report highly structured plasma flows along the magnetic field. Electrostatic fluctuations that can be supported by such inhomogeneous parallel flows are investigated. It is found that even a small transverse gradient in parallel flow can significantly reduce the critical value of the relative ion-electron field-aligned drift for the current-driven electrostatic ion Acoustic modes. It is also shown that the shear-modified ion Acoustic mode can be excited without any relative field-aligned drift provided that the flow gradient is sufficiently strong. The instability mechanism can be described in a local limit. The new shear-modified ion-Acoustic modes considered in this paper are shown to be different from both the nonresonant mode due to a velocity shear in the parallel flow [D'Angelo, 1965] and the resonant classical current-driven ion-Acoustic mode [Fried and Gould, 1961]. The new modes discussed in this paper can be excited for typical ionospheric conditions and a wide range of ion-electron temperature ratios. The reported results may explain ionospheric observations of low-frequency ion-Acoustic-like waves, especially for ion-electron temperature ratios of the order of unity and larger when the critical current for the homogeneous current-driven ion Acoustic mode is significantly above the observed values. Other possible applications of our results to space plasmas are also discussed.
S Nematnasser - One of the best experts on this subject based on the ideXlab platform.
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inherent negative refraction on Acoustic Branch of two dimensional phononic crystals
Mechanics of Materials, 2019Co-Authors: S NematnasserAbstract:Abstract Guided by theoretical predictions, we have demonstrated experimentally the existence of negative refraction on the two lowest Acoustic-Branch passbands (shear and longitudinal modes) of a simple two-dimensional phononic crystal consisting of an isotropic stiff (aluminum) matrix and square-patterned isotropic compliant (PMMA) circular inclusions. We experimentally distinguished the shear and longitudinal modes to ensure that there are no couplings between the two modes and that the shear-mode negative refraction is an inherent property of periodic composites with stiff matrix and compliant inclusions. We have also discovered that a composite with compliant (PMMA) matrix and stiff (aluminum) inclusions does not display negative shear-mode refraction on its first Branch. At frequencies and wave vectors where the refraction on the Acoustic-Branch passbands is negative, the effective mass-density and the effective stiffness tensors of the crystal are positive-definite, and this is an inherent property of such phononic crystals.The equi-frequency contours and energy flux vectors as functions of the phase-vector components, reveal a rich body of refractive properties that can be exploited to realize, for example, beam splitting, focusing, and frequency filtration on the lowest passbands of the crystal where the dissipation is minimum. By proper selection of material and geometric parameters these phenomena can be realized at remarkably low frequencies (large wave lengths) using rather small simple two-phase unit cells.
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inherent negative refraction on Acoustic Branch of two dimensional phononic crystals
arXiv: Classical Physics, 2017Co-Authors: S NematnasserAbstract:Guided by theoretical predictions, we have demonstrated experimentally the existence of negative refraction on the lowest two (Acoustic) passbands (shear and longitudinal modes) of a simple two dimensional phononic crystal consisting of an isotropic stiff (aluminum) matrix and square- patterned isotropic compliant (PMMA) circular inclusions. At frequencies and wave vectors where the refraction is negative, the effective mass density and the effective stiffness tensors of the crystal can be positive-defnite, and that, this is an inherent property of phononic crystals with an isotropic stiff matrix containing periodically distributed isotropic compliant inclusions. The equi-frequency contours and energy ux vectors as fuctions of the phase-vector components, reveal a rich body of refractive properties that can be exploited to realize, for example, beam splitting, focusing, and frequency filtration on the lowest passbands of the crystal where the dissipation is the least. By proper selection of material and geometric parameters these phenomena can be realized at remarkably low frequencies (large wave lengths) using rather small simple two-phase unit cells. Keywords: Doubly periodic phononic crystals, Acoustic Branch negative refraction, beam splitting, focusing, imaging, frequency filtration at large wave lengths
G Monaco - One of the best experts on this subject based on the ideXlab platform.
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merging of the Acoustic Branch with the boson peak in densified silica glass
Physical Review B, 2002Co-Authors: Marie Foret, Eric Courtens, Rene Vacher, G MonacoAbstract:Both high-frequency Acoustic modes and the boson peak related to SiO 4 librations are observed in a single inelastic x-ray scattering experiment. The experimental data are consistent with a picture where the Acoustic modesexperience a crossover at a frequency Ω c o beyond which plane waves cease to exist. The spectra evolve with the scattering vector to merge into a broad boson peak at Ω B P Ω c o ). The latter, although essentially optic in nature, might hybridize with the resonant Acousticlike modes, which can be crucial to their strong scattering.
Gogoi Papori - One of the best experts on this subject based on the ideXlab platform.
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From confined spinons to emergent fermions: Observation of elementary magnetic excitations in a transverse-field Ising chain
'American Physical Society (APS)', 2016Co-Authors: Wang Zhe, Wu Jianda, Xu Shenglong, Yang Wang, Wu Congjun, Bera, Anup Kumar, Islam A. T. M. Nazmul, Lake Bella, Kamenskyi Dmytro, Gogoi PaporiAbstract:We report on spectroscopy study of elementary magnetic excitations in an Ising-like antiferromagnetic chain compound SrCo$_2$V$_2$O$_8$ as a function of temperature and applied transverse magnetic field up to 25 T. An optical as well as an Acoustic Branch of confined spinons, the elementary excitations at zero field, are identified in the antiferromagnetic phase below the N\'{e}el temperature of 5 K and described by a one-dimensional Schr\"{o}dinger equation. The confinement can be suppressed by an applied transverse field and a quantum disordered phase is induced at 7 T. In this disordered paramagnetic phase, we observe three emergent fermionic excitations with different transverse-field dependencies. The nature of these modes is clarified by studying spin dynamic structure factor of a 1D transverse-field Heisenberg-Ising (XXZ) model using the method of infinite time evolving block decimation. Our work reveals emergent quantum phenomena and provides a concrete system for testifying theoretical predications of one-dimension quantum spin models.Comment: 8 pages and 6 figure
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From confined spinons to emergent fermions: Observation of elementary magnetic excitations in a transverse-field Ising chain
PHYSICAL REVIEW B, 2016Co-Authors: Wang Zhe, Wu Jianda, Xu Shenglong, Yang Wang, Wu Congjun, Bera, Anup Kumar, Islam A. T. M. Nazmul, Lake Bella, Kamenskyi Dmytro, Gogoi PaporiAbstract:We report on spectroscopy study of elementary magnetic excitations in an Ising-like antiferromagnetic chain compound SrCo2V2O8 as a function of temperature and applied transverse magnetic field up to 25 T. An optical as well as an Acoustic Branch of confined spinons, the elementary excitations at zero field, are identified in the antiferromagnetic phase below the Neel temperature of 5 K and described by a one-dimensional Schrodinger equation. The confinement can be suppressed by an applied transverse field and a quantum disordered phase is induced at 7 T. In this disordered paramagnetic phase, we observe three emergent fermionic excitations with different transverse-field dependencies. The nature of these modes is clarified by studying spin dynamic structure factor of a 1D transverse-field Heisenberg-Ising (XXZ) model using the method of infinite time evolving block decimation. Our work reveals emergent quantum phenomena and provides a concrete system for testifying theoretical predications of one-dimension quantum spin models.Deutsche Forschungsgemeinschaft via the Transregional Research Collaboration: From Electronic Correlations to Functionality (Augsburg - Munich - Stuttgart) [TRR 80]; Chinesisch-Deutsches Zentrum fur Wissenschaftsforderung; HFML-RU/FOM; NSF [DMR-1410375]; AFOSR [FA9550-14-1-0168]; University of California Office of the President [CA-15-327861]; National Natural Science Foundation of China [11328403]; CAS/SAFEA International Partnership Program for Creative Research Teams of ChinaSCI(E)ARTICLEzhe.wang@physik.uni-augsburg.de; jdwu@physics.ucsd.edu129
