27 research outputs found
Hybrid -tight-binding model for intersubband optics in atomically thin InSe films
We propose atomic films of n-doped -InSe as a platform for
intersubband optics in the infrared (IR) and far infrared (FIR) range, coupled
to out-of-plane polarized light. Depending on the film thickness (number of
layers) of the InSe film these transitions span from eV for bilayer
to eV for 15-layer InSe. We use a hybrid theory and tight-binding model, fully parametrized using density
functional theory, to predict their oscillator strengths and thermal linewidths
at room temperature
Structures of bulk hexagonal post transition metal chalcogenides from dispersion-corrected density functional theory
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 technologically 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 the P63/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, multigigapascal 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
Crossover from weakly indirect to direct excitons in atomically thin films of InSe
We perform a theory analysis of the spectra of the
lowest energy and excited states of the excitons in few-layer atomically thin
films of InSe taking into account in-plane electric polarizability of the film
and the influence of the encapsulation environment. For the thinner films, the
lowest-energy state of the exciton is weakly indirect in momentum space, with
its dispersion showing minima at a layer-number-dependent wave number, due to
an inverted edge of a relatively flat topmost valence band branch of the InSe
film spectrum and we compute the activation energy from the momentum dark
exciton ground state into the bright state. For the films with more than seven
InSe layers, the exciton dispersion minimum shifts to -point.Comment: 12 pages, 7 figure
Ghost anti-crossings caused by interlayer umklapp hybridization of bands in 2D heterostructures
In two-dimensional heterostructures, crystalline atomic layers with differing lattice parameters can stack directly one on another. The resultant close proximity of atomic lattices with differing periodicity can lead to new phenomena. For umklapp processes, this opens the possibility for interlayer umklapp scattering, where interactions are mediated by the transfer of momenta to or from the lattice in the neighbouring layer. Using angle-resolved photoemission spectroscopy to study a graphene on InSe heterostructure, we present evidence that interlayer umklapp processes can cause hybridization between bands from neighbouring layers in regions of the Brillouin zone where bands from only one layer are expected, despite no evidence for Moiré-induced replica bands. This phenomenon manifests itself as ‘ghost’ anti-crossings in the InSe electronic dispersion. Applied to a range of suitable two-dimensional material pairs, this phenomenon of interlayer umklapp hybridization can be used to create strong mixing of their electronic states, giving a new tool for twist-controlled band structure engineering
Band alignment and interlayer hybridisation in transition metal dichalcogenide/hexagonal boron nitride heterostructures
In van der Waals heterostructures, the relative alignment of bands between layers, and the resulting band hybridisation, are key factors in determining a range of electronic properties. This work examines these effects for heterostructures of transition metal dichalcogenides (TMDs) and hexagonal boron nitride (hBN), an ubiquitous combination given the role of hBN as an encapsulating material. By comparing results of density functional calculations with experimental angle-resolved photoemission spectroscopy (ARPES) results, we explore the hybridisation between the valence states of the TMD and hBN layers, and show that it introduces avoided crossings between the TMD and hBN bands, with umklapp processes opening ‘ghost’ avoided crossings in individual bands. Comparison between DFT and ARPES spectra for the MoSe2/hBN heterostructure shows that the valence bands of MoSe2 and hBN are significantly further separated in energy in experiment as compared to DFT. We then show that a novel scissor operator can be applied to the hBN valence states in the DFT calculations, to correct the band alignment and enable quantitative comparison to ARPES, explaining avoided crossings and other features of band visibility in the ARPES spectra
Broken mirror symmetry in excitonic response of reconstructed domains in twisted MoSe/MoSe bilayers
Structural engineering of van der Waals heterostructures via stacking and
twisting has recently been used to create moir\'e superlattices, enabling the
realization of new optical and electronic properties in solid-state systems. In
particular, moir\'e lattices in twisted bilayers of transition metal
dichalcogenides (TMDs) have been shown to lead to exciton trapping, host Mott
insulating and superconducting states, and act as unique Hubbard systems whose
correlated electronic states can be detected and manipulated optically.
Structurally, these twisted heterostructures also feature atomic reconstruction
and domain formation. Unfortunately, due to the nanoscale sizes (~10 nm) of
typical moir\'e domains, the effects of atomic reconstruction on the electronic
and excitonic properties of these heterostructures could not be investigated
systematically and have often been ignored. Here, we use near-0 twist angle
MoSe/MoSe bilayers with large rhombohedral AB/BA domains to directly
probe excitonic properties of individual domains with far-field optics. We show
that this system features broken mirror/inversion symmetry, with the AB and BA
domains supporting interlayer excitons with out-of-plane (z) electric dipole
moments in opposite directions. The dipole orientation of ground-state
-K interlayer excitons (X) can be flipped with electric fields,
while higher-energy K-K interlayer excitons (X) undergo
field-asymmetric hybridization with intralayer K-K excitons (X). Our study
reveals the profound impacts of crystal symmetry on TMD excitons and points to
new avenues for realizing topologically nontrivial systems, exotic
metasurfaces, collective excitonic phases, and quantum emitter arrays via
domain-pattern engineering.Comment: 29 pages, 4 figures in main text, 6 figures in supplementary
informatio
Broken mirror symmetry in excitonic response of reconstructed domains in twisted MoSeâ‚‚/MoSeâ‚‚ bilayers
Van der Waals heterostructures obtained via stacking and twisting have been used to create moiré superlattices, enabling new optical and electronic properties in solid-state systems. Moiré lattices in twisted bilayers of transition metal dichalcogenides (TMDs) result in exciton trapping, host Mott insulating and superconducting states6 and act as unique Hubbard systems whose correlated electronic states can be detected and manipulated optically. Structurally, these twisted heterostructures feature atomic reconstruction and domain formation. However, due to the nanoscale size of moiré domains, the effects of atomic reconstruction on the electronic and excitonic properties have not been systematically investigated. Here we use near-0°-twist-angle MoSe₂/MoSe₂ bilayers with large rhombohedral AB/BA domains to directly probe the excitonic properties of individual domains with far-field optics. We show that this system features broken mirror/inversion symmetry, with the AB and BA domains supporting interlayer excitons with out-of-plane electric dipole moments in opposite directions. The dipole orientation of ground-state Γ–K interlayer excitons can be flipped with electric fields, while higher-energy K–K interlayer excitons undergo field-asymmetric hybridization with intralayer K–K excitons. Our study reveals the impact of crystal symmetry on TMD excitons and points to new avenues for realizing topologically non-trivial systems, exotic metasurfaces, collective excitonic phases and quantum emitter arrays via domain-pattern engineering