Magnetic fields force ballistic electrons injected from a narrow contact to move along skipping orbits and form caustics. This leads to pronounced resistance peaks at nearby voltage probes as electrons are effectively focused inside them, a phenomenon known as magnetic focusing. This can be used not only for the demonstration of ballistic transport but also to study the electronic structure of metals. Here we use magnetic focusing to probe narrow bands in graphene bilayers twisted at ~2°. Their minibands are found to support long-range ballistic transport limited at low temperatures by intrinsic electron-electron scattering. A voltage bias between the layers causes strong valley splitting and allows selective focusing for different valleys, which is of interest for using this degree of freedom in frequently-discussed valleytronics.Crystallographic alignment of atomically thin crystals stacked together in a van der Waals heterostructure is a powerful tool that enables fine tuning of their electronic spectra. For crystals with similar honeycomb lattices the spectra are modified by the presence of a long-range interference (moiré) pattern with a period lS dependent on the twist angle θ between the layers (1-18), see Fig 1A. The additional spatial periodicity reduces the size of the Brillouin zone and introduces secondary Dirac points, as illustrated in Fig. 1B. So far, the most pronounced twist-engineered changes in the electronic properties of 2D crystals have been achieved in twisted bilayer graphene (TBG), where the twist at discrete 'magic' angles results in narrow bands, periodically modulated interlayer hybridisation and strong enhancement of electron correlations, leading to superconductivity and Mott insulator transitions (6-8). At larger θ, the TBG spectrum corresponds to a metal with several minibands at each K and K' valley in the Brillouin zone (Fig. 1B). Electronic properties of such a metal are expected to be quite different from the behaviour of Dirac electrons in monolayer or bilayer (aligned to Bernal stacking) graphene but so far remain largely unexplored. Here we use transverse focusing of electrons in a perpendicular magnetic field (TMF) (12,(19)(20)(21)(22)(23) to probe the properties of moiré minibands in TBG and demonstrate an exceptionally high quality of the 'artificial metal' in TBG, as well as a possibility to use vertical displacement field, D, to break the valley degeneracy in the two constituent layers and selectively enhance transport in one of the valleys.

We propose atomic films of n-doped γ-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) and amount of n-doping of the InSe film these transitions span from ∼ 0.7 eV for bilayer to ∼ 0.05 eV for 15-layer InSe. We use a hybrid k · p theory and tight-binding model, fully parametrized using density functional theory, to predict their oscillator strengths and thermal linewidths at room temperature.

We perform a k • 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 point.