Implications of the Klein tunneling times on high frequency graphene devices using Bohmian trajectories

Because of its large Fermi velocity, leading to a great mobility, graphene is expected to play an important role in (small signal) radio frequency electronics. Among other, graphene devices based on Klein tunneling phenomena are already envisioned. The connection between the Klein tunneling times of electrons and cut-off frequencies of graphene devices is not obvious. We argue in this paper that the trajectory-based Bohmian approach gives a very natural framework to quantify Klein tunneling times in linear band graphene devices because of its ability to distinguish, not only between transmitted and reflected electrons, but also between reflected electrons that spend time in the barrier and those that do not. Without such distinction, typical expressions found in the literature to compute dwell times can give unphysical results when applied to predict cut-off frequencies. In particular, we study Klein tunneling times for electrons in a two-terminal graphene device constituted by a potential barrier between two metallic contacts. We show that for a zero incident angle (and positive or negative kinetic energy), the transmission coefficient is equal to one, and the dwell time is roughly equal to the barrier distance divided by the Fermi velocity. For electrons incident with a non-zero angle smaller than the critical angle, the transmission coefficient decreases and dwell time can still be easily predicted in the Bohmian framework. The main conclusion of this work is that, contrary to tunneling devices with parabolic bands, the high graphene mobility is roughly independent of the presence of Klein tunneling phenomena in the active device region

J/ψ+jetJ/\psi + jet diffractive production in the direct photon process at HERA

We present a study of $J/\psi + jet$ diffractive production in the direct photon process at HERA based on the factorization theorem for lepton-induced hard diffractive scattering and the factorization formalism of the nonrelativistic QCD (NRQCD) for quarkonia production. Using the diffractive gluon distribution function extracted from HERA data on diffractive deep inelastic scattering and diffractive dijet photon production, we show that this process can be studied at HERA with present integrated luminosity, and can give valuable insights in the color-octet mechanism for heavy quarkonia production.Comment: Revtex, 21 pages, 7 EPS figure

Flat-band plasmons in twisted bilayer transition metal dichalcogenides

Twisted bilayer transition metal dichalcogenides are ideal platforms to study flat-band phenomena. In this paper, we investigate flat-band plasmons in the hole-doped twisted bilayer MoS$_2$ (tb-MoS$_2$) by employing a full tight-binding model and the random phase approximation. When considering lattice relaxations in tb-MoS$_2$, the flat band is not separated from remote valence bands, which makes the contribution of interband transitions in transforming the plasmon dispersion and energy significantly different. In particular, low-damped and quasi-flat plasmons emerge if we only consider intraband transitions in the doped flat band, whereas a $\sqrt q$ plasmon dispersion emerges if we also take into account interband transitions between the flat band and remote bands. Furthermore, the plasmon energies are tunable with twist angles and doping levels. However, in a rigid sample that suffers no lattice relaxations, lower-energy quasi-flat plasmons and higher-energy interband plasmons can coexist. For rigid tb-MoS$_2$ with a high doping level, strongly enhanced interband transitions quench the quasi-flat plasmons. Based on the lattice relaxation and doping effects, we conclude that two conditions, the isolated flat band and a properly hole-doping level, are essential for observing the low-damped and quasi-flat plasmon mode in twisted bilayer transition metal dichalcogenides. We hope that our study on flat-band plasmons can be instructive for studying the possibility of plasmon-mediated superconductivity in twisted bilayer transition metal dichalcogenides in the future

Tuning the flat bands by the interlayer interaction, spin-orbital coupling and electric field in twisted homotrilayer MoS2_2

Ultraflat bands have already been detected in twisted bilayer graphene (TBG) and twisted bilayer transition metal dichalcogenides (tb-TMDs), which provide a platform to investigate strong correlations. In this paper, the electronic properties of twisted trilayer molybdenum disulfide (TTM) are investigated via an accurate tight-banding Hamiltonian. We find that the highest valence bands are derived from $\Gamma$-point of the constituent monolayer, exhibiting a graphene-like dispersion or becoming isolated flat bands. The lattice relaxation, local deformation, and electric field can significantly tune the electronic structures of TTM with different starting stacking arrangements. After introducing the spin-orbital coupling (SOC) effect, we find a spin-valley-layer locking effect at the minimum of conduction band at K- and K$^\prime$-point of the Brillouin zone, which may provide a platform to study optical properties and magnetoelectric effects