Tunneling between parallel two-dimensional electron gases

Quantum Lifetime Spectroscopy and Magnetotunneling in Double Bilayer Graphene Heterostructures

We describe a tunneling spectroscopy technique in a double bilayer graphene heterostructure where momentum-conserving tunneling between different energy bands serves as an energy filter for the tunneling carriers, and allows a measurement of the quasiparticle state broadening at well-defined energies. The broadening increases linearly with the excited state energy with respect to the Fermi level and is weakly dependent on temperature. In-plane magnetotunneling reveals a high degree of rotational alignment between the graphene bilayers, and an absence of momentum randomizing processes.

Resonant tunnelling spectroscopy of van der Waals heterosystems

The review concerns the most interesting aspects of (mainly experimental) resonance tunnelling spectroscopy studies of a new type of heterosystems called van der Waals heterostructures. The possibility to compose such systems is a result of the recent discovery of two-dimensional crystals, a new class of materials derived from graphene. The role of the angular mismatch of the crystal lattices of conductive graphene electrodes in the tunnelling of charge carriers between them, as well as the closely related issues associated with fulfillment of the conservation laws during tunnelling transitions are considered. The experimental results on inelastic tunnelling in the graphene/h-BN/graphene heterosystems with strong angular mismatch are discussed. The experiments made it possible to determine the phonon density of states spectra of the constituent layers and to detect and describe tunnelling transitions involving localized states of structural defects in the h-BN barrier. We consider new results of studies on tunnelling and magnetotunnelling in van der Waals heterosystems that demonstrate the possibilities of practical application of resonant tunnelling effects in, e.g., microwave engineering, based on realization of electronic devices having I – V curves with negative differential conductance (NDC) regions at tunnelling through defect levels of the barrier layers in such systems. These studies revealed two new types of heterosystems characterized by the formation of NDC regions as a result of resonant tunnelling through the defect levels in the h-BN barrier and by defect-assisted generation of tunnelling current.

The bibliography includes 40 references.

Spin-Conserving Resonant Tunneling in Twist-Controlled WSe2-hBN-WSe2 Heterostructures

We investigate interlayer tunneling in heterostructures consisting of two tungsten diselenide (WSe₂) monolayers with controlled rotational alignment, and separated by hexagonal boron nitride. In samples where the two WSe₂ monolayers are rotationally aligned we observe resonant tunneling, manifested by a large conductance and negative differential resistance in the vicinity of zero interlayer bias, which stem from energy- and momentum-conserving tunneling. Because the spin-orbit coupling leads to coupled spin-valley degrees of freedom, the twist between the two WSe₂ monolayers allows us to probe the conservation of spin-valley degree of freedom in tunneling. In heterostructures where the two WSe₂ monolayers have a 180° relative twist, such that the Brillouin zone of one layer is aligned with the time-reversed Brillouin zone of the opposite layer, the resonant tunneling between the layers is suppressed. These findings provide evidence that, in addition to momentum, the spin-valley degree of freedom is also conserved in vertical transport.

Coherent Interlayer Tunneling and Negative Differential Resistance with High Current Density in Double Bilayer Graphene-WSe2 Heterostructures

We demonstrate gate-tunable resonant tunneling and negative differential resistance between two rotationally aligned bilayer graphene sheets separated by bilayer WSe₂. We observe large interlayer current densities of 2 and 2.5 μA/μm² and peak-to-valley ratios approaching 4 and 6 at room temperature and 1.5 K, respectively, values that are comparable to epitaxially grown resonant tunneling heterostructures. An excellent agreement between theoretical calculations using a Lorentzian spectral function for the two-dimensional (2D) quasiparticle states, and the experimental data indicates that the interlayer current stems primarily from energy and in-plane momentum conserving 2D-2D tunneling, with minimal contributions from inelastic or non-momentum-conserving tunneling. We demonstrate narrow tunneling resonances with intrinsic half-widths of 4 and 6 meV at 1.5 and 300 K, respectively.

Gate-tunable resonant tunneling in double bilayer graphene heterostructures

We demonstrate gate-tunable resonant tunneling and negative differential resistance in the interlayer current-voltage characteristics of rotationally aligned double bilayer graphene heterostructures separated by hexagonal boron nitride (hBN) dielectric. An analysis of the heterostructure band alignment using individual layer densities, along with experimentally determined layer chemical potentials indicates that the resonance occurs when the energy bands of the two bilayer graphene are aligned. We discuss the tunneling resistance dependence on the interlayer hBN thickness, as well as the resonance width dependence on mobility and rotational alignment.

Tunneling spectroscopy of a two-dimensionally periodic electron system

The tunneling current between an electron gas with a periodic potential in two dimensions and a plain two-dimensional electron system (2DES) has been studied. The strength of the periodic potential, the subband energy of the plain 2DES, and an applied in-plane magnetic field were varied, mapping the Fourier transform of the periodic wave function. Periodic peaks were observed and explained by translations in the reciprocal lattice. When the potential was strongly modulated to form an array of antidots, commensurability peaks were seen in lateral transport, but, as expected, not in tunneling.