We report strong electron-phonon coupling in magic-angle twisted bilayer graphene (MA-TBG) obtained from atomistic description of the system including more than 10 000 atoms in the moiré supercell. Electronic structure, phonon spectrum, and electron-phonon coupling strength λ are obtained before and after atomicposition relaxation both in and out of plane. Obtained λ is very large for MA-TBG, with λ > 1 near the half-filling energies of the flat bands, while it is small (λ ∼ 0.1) for monolayer and unrotated bilayer graphene. Significant electron-hole asymmetry occurs in the electronic structure after atomic-structure relaxation, so λ is much stronger with hole doping than electron doping. Obtained electron-phonon coupling is nearly isotropic and depends very weakly on electronic band and momentum, indicating that electron-phonon coupling prefers single-gap s-wave superconductivity. Relevant phonon energies are much larger than electron energy scale, going far beyond adiabatic limit. Our results provide a fundamental understanding of the electron-phonon interaction in MA-TBG, highlighting that it can contribute to rich physics of the system.

We present atomistic calculations on structural and electronic properties of twisted double bilayer graphene (TDBG) consisting of two sets of rotationally misaligned Bernal-stacked bilayer graphene. Obtained equilibrium atomic structures exhibit in-plane strains and the modulation of the interlayer distances at the rotationally mismatched interface layers. We find that the electronic structure of TDBG can have an intrinsic band gap at the charge neutral point for a large range of the twist angle θ. Near θ = 1.25 • , the intrinsic band gap disappears and TDBG hosts flat bands at the Fermi level that are energetically well separated from higher and lower energy bands. We also show that the flat bands are easily tunable by applying vertical electric fields, and extremely narrow bandwidths less than 10 meV can be achieved for the electron-side flat bands in a wide range of the twist angle. Our results serve as a theoretical guide for exploring emergent correlated electron physics in this versatile moiré superlattice system. arXiv:1903.00852v1 [cond-mat.mes-hall]

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The application of two-dimensional materials has been expanded by introducing the twisted bilayer (TBL) system. However, the landscape of the interlayer interaction in hetero-TBLs has not yet been fully understood, while that in homo-TBLs has been extensively studied, with the dependence on the twist angle between the constituent layers. Here, we present detailed analyses on the interlayer interaction that depends on the twist angle in WSe 2 /MoSe 2 hetero-TBL via Raman and photoluminescence studies combined with first-principles calculation. We observe interlayer vibrational modes, moiréphonons, and the interlayer excitonic states that evolve with the twist angle and identify different regimes with distinct characteristics of such features. Moreover, the interlayer excitons that appear strong in the hetero-TBLs with twist angles near 0°or 60°have different energies and photoluminescence excitation spectra for the two cases, which results from different electronic structures and carrier relaxation dynamics. These results would enable a better understanding of the interlayer interaction in hetero-TBLs.

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We investigate the role of pseudospin structure of few-layer black phosphorus (BP) in interband tunneling properties in lateral BP junctions. We find that interband tunneling is critically dependent on junction directions because of the anisotropic pseudospin structure of BP. When the armchair direction of BP is normal to the interface, pseudospins of incident and transmitted carriers are nearly aligned so that interband tunneling is highly effective, analogous to the Klein tunneling in graphene. However, when the zigzag direction is normal to the interface, interband tunneling is suppressed by misaligned pseudospins. We also study junctions of band-gap inverted BP where the electronic structure is characterized by two Dirac cones. In this case, intervalley tunneling is prohibited either by momentum conservation or by pseudospin mismatch while intravalley tunneling is Klein-like irrespective of the junction direction. These results provide a foundation for developing high-performance devices from BP and other pseudospin materials.

Electron-doped high-Tc FeSe reportedly has a strong electron correlation that is enhanced with doping. It has been noticed that significant electric fields exist inevitably between FeSe and external donors along with electron transfer. However, the effects of such fields on electron correlation are yet to be explored. Here we study potassium-(K-) dosed FeSe layers using density-functional theory combined with dynamical mean-field theory to investigate the roles of such electric fields on the strength of the electron correlation. We find, very interestingly, the electronic potential-energy difference between the topmost Se and Fe atomic layers, generated by local electric fields of ionized K atoms, weakens the Se-mediated hopping between Fe d orbitals. Since it is the dominant hopping channel in FeSe, its reduction narrows the Fe d bands near the Fermi level, enhancing the electron correlation. This effect is orbital dependent and occurs in the topmost FeSe layer only. We also find the K dosing may increase the Se height, enhancing the electron correlation further. These results shed new light on the comprehensive study of high-Tc FeSe and other low-dimensional systems.

We have conducted a terahertz spectroscopic study and a density functional theory analysis of the phonon dynamics of the layered van der Waals semiconductors Nb3Cl8 and Nb3I8. Several infrared-active phonon modes were observed in the terahertz region, and their frequencies were found to be in excellent agreement with our first-principles lattice dynamics calculations. For Nb3Cl8, the observed phonon spectra are consistent with a structural transition at 90 K from the high-temperature P3̅m1 phase to the low-temperature R3̅m phase. Also, our study confirmed that the structural and magnetic transitions were coupled in Nb3Cl8. For Nb3I8, which is nonmagnetic at and below room temperature, no significant temperature or magnetic field dependence was observed in the phonon spectra. Our study provides an intriguing connection between the structural properties and the paramagnetic–nonmagnetic transitions in Nb3Cl8 and Nb3I8.