scholarly journals Strong electron-phonon coupling, electron-hole asymmetry, and nonadiabaticity in magic-angle twisted bilayer graphene

2018 ◽  
Vol 98 (24) ◽  
Author(s):  
Young Woo Choi ◽  
Hyoung Joon Choi
Keyword(s):  
Bilayer Graphene ◽  
Magic Angle ◽  
Phonon Coupling ◽  
Electron Hole ◽  
Strong Electron ◽  
Electron Phonon Coupling ◽  
Twisted Bilayer Graphene

scholarly journals Does filling-dependent band renormalization aid pairing in twisted bilayer graphene?

2021 ◽  
Vol 6 (1) ◽  
Author(s):  
Cyprian Lewandowski ◽  
Stevan Nadj-Perge ◽  
Debanjan Chowdhury
Keyword(s):  
Density Of States ◽  
Bilayer Graphene ◽  
Spatial Extent ◽  
Magic Angle ◽  
Many Body ◽  
Wannier Functions ◽  
Electron Phonon Coupling ◽  
Wide Range ◽  
Twisted Bilayer Graphene ◽  
Superconducting Pairing

AbstractMagic-angle twisted bilayer graphene (MATBG) exhibits a panoply of many-body phenomena that are intimately tied to the appearance of narrow and well-isolated electronic bands. The microscopic ingredients that are responsible for the complex experimental phenomenology include electron–electron (phonon) interactions and nontrivial Bloch wavefunctions associated with the narrow bands. Inspired by recent experiments, we focus on two independent quantities that are considerably modified by Coulomb interaction-driven band renormalization, namely the density of states and the minimal spatial extent associated with the Wannier functions. First, we show that a filling-dependent enhancement of the density of states, caused by band flattening, in combination with phonon-mediated attraction due to electron-phonon umklapp processes, increases the tendency towards superconducting pairing in a range of angles around magic-angle. Second, we demonstrate that the minimal spatial extent associated with the Wannier functions, which contributes towards increasing the superconducting phase stiffness, also develops a nontrivial enhancement due to the interaction-induced renormalization of the Bloch wavefunctions. While our modeling of superconductivity (SC) assumes a weak electron-phonon coupling and does not consider many of the likely relevant correlation effects, it explains simply the experimentally observed robustness of SC in the wide range of angles that occurs in the relevant range of fillings.

scholarly journals Structural phase transition and superconductivity hierarchy in 1T-TaS2 under pressure up to 100 GPa

2021 ◽  
Vol 6 (1) ◽  
Author(s):  
Qing Dong ◽  
Quanjun Li ◽  
Shujia Li ◽  
Xuhan Shi ◽  
Shifeng Niu ◽  
...  
Keyword(s):  
Structural Phase ◽  
Transition Metal Dichalcogenides ◽  
Phonon Coupling ◽  
Structural Phase Transitions ◽  
Strong Electron ◽  
Electron Phonon Coupling ◽  
Increasing Trend ◽  
Metal Dichalcogenides ◽  
Dome Shape ◽  
Enhanced Superconductivity

AbstractThe adoption of high pressure not only reinforces the comprehension of the structure and exotic electronic states of transition metal dichalcogenides (TMDs) but also promotes the discovery of intriguing phenomena. Here, 1T-TaS2 was investigated up to 100 GPa, and re-enhanced superconductivity was found with structural phase transitions. The discovered I4/mmm TaS2 presents strong electron–phonon coupling, revealing a good superconductivity of the nonlayered structure. The P–T phase diagram shows a dome shape centered at ~20 GPa, which is attributed to the distortion of the 1T structure. Accompanied by the transition to nonlayered structure above 44.5 GPa, the superconducting critical temperature shows an increasing trend and reaches ~7 K at the highest studied pressure, presenting superior superconductivity compared to the original layered structure. It is unexpected that the pressure-induced re-enhanced superconductivity was observed in TMDs, and the transition from a superconductor with complicated electron-pairing mechanism to a phonon-mediated superconductor would expand the field of pressure-modified superconductivity.

scholarly journals Gate-defined Josephson junctions in magic-angle twisted bilayer graphene

2021 ◽  
Author(s):  
Folkert K. de Vries ◽  
Elías Portolés ◽  
Giulia Zheng ◽  
Takashi Taniguchi ◽  
Kenji Watanabe ◽  
...  
Keyword(s):  
Josephson Junctions ◽  
Bilayer Graphene ◽  
Magic Angle ◽  
Twisted Bilayer Graphene

scholarly journals Direct observation of excitonic instability in Ta2NiSe5

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Kwangrae Kim ◽  
Hoon Kim ◽  
Jonghwan Kim ◽  
Changil Kwon ◽  
Jun Sung Kim ◽  
...  
Keyword(s):  
Phase Transition ◽  
Direct Observation ◽  
Structural Phase ◽  
Temperature Phase ◽  
Phonon Coupling ◽  
Strong Electron ◽  
Weiss Temperature ◽  
Excitonic Insulator ◽  
Electron Phonon Coupling ◽  
Low Temperature Phase

AbstractCoulomb attraction between electrons and holes in a narrow-gap semiconductor or a semimetal is predicted to lead to an elusive phase of matter dubbed excitonic insulator. However, direct observation of such electronic instability remains extremely rare. Here, we report the observation of incipient divergence in the static excitonic susceptibility of the candidate material Ta2NiSe5 using Raman spectroscopy. Critical fluctuations of the excitonic order parameter give rise to quasi-elastic scattering of B2g symmetry, whose intensity grows inversely with temperature toward the Weiss temperature of TW ≈ 237 K, which is arrested by a structural phase transition driven by an acoustic phonon of the same symmetry at TC = 325 K. Concurrently, a B2g optical phonon becomes heavily damped to the extent that its trace is almost invisible around TC, which manifests a strong electron-phonon coupling that has obscured the identification of the low-temperature phase as an excitonic insulator for more than a decade. Our results unambiguously reveal the electronic origin of the phase transition.

Strong electron-phonon coupling at a metal/organic interface: PTCDA/Ag(111)

2002 ◽  
Vol 65 (12) ◽  
Author(s):  
F. S. Tautz ◽  
M. Eremtchenko ◽  
J. A. Schaefer ◽  
M. Sokolowski ◽  
V. Shklover ◽  
...  
Keyword(s):  
Phonon Coupling ◽  
Strong Electron ◽  
Electron Phonon Coupling ◽  
Metal Organic
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