Hybrid k⋅p\mathbf{k\cdot p}-tight-binding model for intersubband optics in atomically thin InSe films

We propose atomic films of n-doped $\gamma$-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 $\sim 0.7$ eV for bilayer to $\sim 0.05$ eV for 15-layer InSe. We use a hybrid $\mathbf{k} \cdot \mathbf{p}$ theory and tight-binding model, fully parametrized using density functional theory, to predict their oscillator strengths and thermal linewidths at room temperature

Crossover from weakly indirect to direct excitons in atomically thin films of InSe

We perform a $\mathbf{k \cdot p}$ 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 In$_2$Se$_2$ layers, the exciton dispersion minimum shifts to $\Gamma$-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 MoSe2_2/MoSe2_2 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$^o$ twist angle MoSe$_2$/MoSe$_2$ 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 $\Gamma$-K interlayer excitons (X$_{I,1}$) can be flipped with electric fields, while higher-energy K-K interlayer excitons (X$_{I,2}$) undergo field-asymmetric hybridization with intralayer K-K excitons (X$_0$). 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 MoSe₂/MoSe₂ 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

The JWST Galactic Center Survey -- A White Paper

The inner hundred parsecs of the Milky Way hosts the nearest supermassive black hole, largest reservoir of dense gas, greatest stellar density, hundreds of massive main and post main sequence stars, and the highest volume density of supernovae in the Galaxy. As the nearest environment in which it is possible to simultaneously observe many of the extreme processes shaping the Universe, it is one of the most well-studied regions in astrophysics. Due to its proximity, we can study the center of our Galaxy on scales down to a few hundred AU, a hundred times better than in similar Local Group galaxies and thousands of times better than in the nearest active galaxies. The Galactic Center (GC) is therefore of outstanding astrophysical interest. However, in spite of intense observational work over the past decades, there are still fundamental things unknown about the GC. JWST has the unique capability to provide us with the necessary, game-changing data. In this White Paper, we advocate for a JWST NIRCam survey that aims at solving central questions, that we have identified as a community: i) the 3D structure and kinematics of gas and stars; ii) ancient star formation and its relation with the overall history of the Milky Way, as well as recent star formation and its implications for the overall energetics of our galaxy's nucleus; and iii) the (non-)universality of star formation and the stellar initial mass function. We advocate for a large-area, multi-epoch, multi-wavelength NIRCam survey of the inner 100\,pc of the Galaxy in the form of a Treasury GO JWST Large Program that is open to the community. We describe how this survey will derive the physical and kinematic properties of ~10,000,000 stars, how this will solve the key unknowns and provide a valuable resource for the community with long-lasting legacy value.Comment: This White Paper will be updated when required (e.g. new authors joining, editing of content). Most recent update: 24 Oct 202

Turbulent plumes from a glacier terminus melting in a stratified ocean

The melting of submerged faces of marine-terminating glaciers is a key contributor to the glacial mass budget via direct thermodynamic ablation and the impact of ablation on calving. This study considers the behavior of turbulent plumes of buoyant meltwater in a stratified ocean, generated by melting of either near-vertical calving faces or sloping ice shelves. We build insight by applying a turbulent plume model to describe melting of a locally planar region of ice face in a linearly stratified ocean, in a regime where subglacial discharge is insignificant. The plumes rise until becoming neutrally buoyant, before intruding into the ocean background. For strong stratifications, we obtain leading-order scaling laws for the flow including the height reached by the plume before intrusion, and the melt rate, expressed in terms of the background ocean temperature and salinity stratifications. These scaling laws provide a new perspective for parameterizing glacial melting in response to a piecewise-linear discretization of the ocean stratification