Lattice reconstruction in twisted transition-metal dichalcogenide (TMD) bilayers gives rise to piezo- and ferroelectric moire potentials for electrons and holes, as well as a modulation of the hybridisation across the bilayer. Here, we develop hybrid
$mathbf{k}cdot mathbf{p}$ tight-binding models to describe electrons and holes in the relevant valleys of twisted TMD homobilayers with parallel (P) and anti-parallel (AP) orientations of the monolayer unit cells. We apply these models to describe moire superlattice effects in twisted WSe${}_2$ bilayers, in conjunction with microscopic emph{ab initio} calculations, and considering the influence of encapsulation, pressure and an electric displacement field. Our analysis takes into account mesoscale lattice relaxation, interlayer hybridisation, piezopotentials, and a weak ferroelectric charge transfer between the layers, and describes a multitude of possibilities offered by this system, depending on the choices of P or AP orientation, twist angle magnitude, and electron/hole valley.
We demonstrate that spin-orbit coupling (SOC) strength for electrons near the conduction band edge in few-layer $gamma$-InSe films can be tuned over a wide range. This tunability is the result of a competition between film-thickness-dependent intrins
ic and electric-field-induced SOC, potentially, allowing for electrically switchable spintronic devices. Using a hybrid $mathbf{kcdot p}$ tight-binding model, fully parameterized with the help of density functional theory computations, we quantify SOC strength for various geometries of InSe-based field-effect transistors. The theoretically computed SOC strengths are compared with the results of weak antilocalization measurements on dual-gated multilayer InSe films, interpreted in terms of Dyakonov-Perel spin relaxation due to SOC, showing a good agreement between theory and experiment.
We use dispersion-corrected density-functional theory to determine the relative energies of competing polytypes of bulk layered hexagonal post-transition-metal chalcogenides, to search for the most stable structures of these potentially technological
ly important semiconductors. We show that there is some degree of consensus among dispersion-corrected exchange-correlation functionals regarding the energetic orderings of polytypes, but we find that for each material there are multiple stacking orders with relative energies of less than 1 meV per monolayer unit cell, implying that stacking faults are expected to be abundant in all post-transition-metal chalcogenides. By fitting a simple model to all our energy data, we predict that the most stable hexagonal structure has P$6_3$/mmc space group in each case, but that the stacking order differs between GaS, GaSe, GaTe, and InS on the one hand and InSe and InTe on the other. At zero pressure, the relative energies obtained with different functionals disagree by around 1-5 meV per monolayer unit cell, which is not sufficient to identify the most stable structure unambiguously; however, multi-GPa pressures reduce the number of competing phases significantly. At higher pressures, an AB$$-stacked structure of the most stable monolayer polytype is found to be the most stable bulk structure; this structure has not been reported in experiments thus far.
We report diffusion quantum Monte Carlo (DMC) and many-body $GW$ calculations of the electronic band gaps of monolayer and bulk hexagonal boron nitride (hBN). We find the monolayer band gap to be indirect. $GW$ predicts much smaller quasiparticle gap
s at both the single-shot $G_0W_0$ and the partially self-consistent $GW_0$ levels. In contrast, solving the Bethe-Salpeter equation on top of the $GW_0$ calculation yields an exciton binding energy for the direct exciton at the $K$ point in close agreement with the DMC value. Vibrational renormalization of the electronic band gap is found to be significant in both the monolayer and the bulk. Taking vibrational effects into account, DMC overestimates the band gap of bulk hBN, while $GW$ theory underestimates it.
We apply a multiscale modeling approach to study lattice reconstruction in marginally twisted bilayers of transition metal dichalcogenides (TMD). For this, we develop DFT-parametrized interpolation formulae for interlayer adhesion energies of MoSe$_2
$, WSe$_2$, MoS$_2$, and WS$_2$, combine those with elasticity theory, and analyze the bilayer lattice relaxation into mesoscale domain structures. Paying particular attention to the inversion asymmetry of TMD monolayers, we show that 3R and 2H stacking domains, separated by a network of dislocations develop for twist angles $theta^{circ}<theta^{circ}_Psim 2.5^{circ}$ and $theta^{circ}<theta^{circ}_{AP}sim 1^{circ}$ for, respectively, bilayers with parallel (P) and antiparallel (AP) orientation of the monolayer unit cells and suggest how the domain structures would manifest itself in local probe scanning of marginally twisted P- and AP-bilayers.
We show that spin-orbit coupling (SOC) in InSe enables the optical transition across the principal band gap to couple with in-plane polarized light. This transition, enabled by $p_{x,y}leftrightarrow p_z$ hybridization due to intra-atomic SOC in both
In and Se, can be viewed as a transition between two dominantly $s$- and $p_z$-orbital based bands, accompanied by an electron spin-flip. Having parametrized $mathbf{kcdot p}$ theory using first principles density functional theory we estimate the absorption for $sigma^{pm}$ circularly polarized photons in the monolayer as $sim 1.5%$, which saturates to $sim 0.3%$ in thicker films ($3-5$ layers). Circularly polarized light can be used to selectively excite electrons into spin-polarized states in the conduction band, which permits optical pumping of the spin polarization of In nuclei through the hyperfine interaction.
We present a tight-binding (TB) model and $mathbf{kcdot p}$ theory for electrons in monolayer and few-layer InSe. The model is constructed from a basis of all $s$ and $p$ valence orbitals on both indium and selenium atoms, with tight-binding paramete
rs obtained from fitting to independently computed density functional theory (DFT) band structures for mono- and bilayer InSe. For the valence and conduction band edges of few-layer InSe, which appear to be in the vicinity of the $Gamma$ point, we calculate the absorption coefficient for the principal optical transitions as a function of the number of layers, $N$. We find a strong dependence on $N$ of the principal optical transition energies, selection rules, and optical oscillation strengths, in agreement with recent observations cite{Bandurin2016}. Also, we find that the conduction band electrons are relatively light ($m propto 0.14-0.18 m_e$), in contrast to an almost flat, and slightly inverted, dispersion of valence band holes near the $Gamma$-point, which is found for up to $N propto 6$.
Recently nanomechanical devices composed of a long stationary inner carbon nanotube and a shorter, slowly-rotating outer tube have been fabricated. In this Letter, we study the possibility of using such devices as adiabatic quantum pumps. Using the B
rouwer formula, we employ a Greens function technique to determine the pumped charge from one end of the inner tube to the other, driven by the rotation of a chiral outer nanotube. We show that there is virtually no pumping if the chiral angle of the two nanotubes is the same, but for optimal chiralities the pumped charge can be a significant fraction of a theoretical upper bound.
