Steering the current flow in twisted bilayer graphene

A nanoelectronic device made of twisted bilayer graphene (TBLG) is proposed to steer the direction of the current flow. The ballistic electron current, injected at one edge of the bottom layer, can be guided predominantly to one of the lateral edges of the top layer. The current is steered to the opposite lateral edge, if either the twist angle is reversed or the electrons are injected in the valence band instead of the conduction band, making it possible to control the current flow by electric gates. When both graphene layers are aligned, the current passes straight through the system without changing its initial direction. The observed steering angle exceeds well the twist angle and emerges for a broad range of experimentally accessible parameters. It is explained by the twist angle and the trigonal shape of the energy bands beyond the van Hove singularity due to the Moiré interference pattern. As the shape of the energy bands depends on the valley degree of freedom, the steered current is valley polarized. Our findings show how to control and manipulate the current flow in TBLG. Technologically, they are of relevance for applications in twistronics and valleytronics.

Plasmonic Field-Effect Transistors (TeraFETs) for 6G Communications

Ever increasing demands of data traffic makes the transition to 6G communications in the 300 GHz band inevitable. Short-channel field-effect transistors (FETs) have demonstrated excellent potential for detection and generation of terahertz (THz) and sub-THz radiation. Such transistors (often referred to as TeraFETs) include short-channel silicon complementary metal oxide (CMOS). The ballistic and quasi-ballistic electron transport in the TeraFET channels determine the TeraFET response at the sub-THz and THz frequencies. TeraFET arrays could form plasmonic crystals with nanoscale unit cells smaller or comparable to the electron mean free path but with the overall dimensions comparable with the radiation wavelength. Such plasmonic crystals have a potential of supporting the transition to 6G communications. The oscillations of the electron density (plasma waves) in the FET channels determine the phase relations between the unit cells of a FET plasmonic crystal. Excited by the impinging radiation and rectified by the device nonlinearities, the plasma waves could detect both the radiation intensity and the phase enabling the line-of-sight terahertz (THz) detection, spectrometry, amplification, and generation for 6G communication.

Spatiotemporal imaging of anisotropic charge transfer in photocatalyst particles

Water-splitting reactions using photocatalyst particles are promising routes for solar fuel production1-4. Photoinduced charge transfer from a photocatalyst to catalytic surface sites is key in ensuring photocatalytic efficiency5; however, it is challenging to understand this process, which spans a wide spatiotemporal range from nanometers to micrometers and from femtoseconds to seconds6-8. Although the steady-state charge distribution on single photocatalyst particles has been mapped using microscopic techniques9-11 and the averaged charge transfer dynamics in photocatalyst aggregations have been revealed via time-resolved spectroscopy12,13, spatiotemporally evolving charge transfer processes in single photocatalyst particles cannot be tracked, and the mechanism of charge transfer is unknown. Here, we report spatiotemporally resolved surface photovoltage measurements on Cu2O photocatalyst particles to map complete charge transfer processes throughout the femtosecond to second time scale at the single-particle level. We found that photogenerated electrons are transferred to the catalytic surface ballistically on a sub-picosecond timescale and are retained at this location for the duration, whereas photogenerated holes are transferred to a spatially separated surface and stabilized via selective trapping on a microsecond timescale. We demonstrate that these ballistic electron transfer and anisotropic trapping regimes, which challenge the classical perception of the drift–diffusion model, contribute to efficient charge separation in photocatalysis and improve the photocatalytic performance. We anticipate our findings to demonstrate the universality of other photoelectronic devices and facilitate the rational design of photocatalysts.

Relativistic electric potential near a resting straight carbon nanotube of a finite-length with stationary current

Based on the Lienard – Wiechert potentials for a uniformly and rectilinearly moving electron, a relativistic electric field is studied near a densely filled with potassium atoms single-walled carbon nanotube (K@CNT) with a stationary electric current inside it. The relativistic electric field in the laboratory coordinate system arises (due to the Lorentz transformations) only for a nanotube of a finite length. This field is a result of summation of the Coulomb fields of stationary positively charged ionic cores of potassium and an equal number of ballistically moving valence electrons of potassium that create a current. It is shown that the magnitude of the negative relativistic electric potential of K@CNT in the direction perpendicular to the nanotube does not depend on the direction of the current density. The relationship is obtained between the K@CNT radius and the number of open channels of ballistic electron transfer over potassium atoms. The Landauer formula is used, which relates the number of open quasi-one-dimensional channels and the direct current electrical conduction. For the first time, analytical formulas are obtained for the dependence of the relativistic potential near K@CNT on the electric voltage between the ends of the nanotube and on its radius in the limit of zero absolute temperature. The case is considered when the distance from the point of registration of the relativistic potential above the center of the nanotube is much less than its length. For nanotube with diameter of 2 nm and length of 100 mm, under an external electric field strength of 5 mV/mm, the magnitude of the potential of the relativistic electric field is of about 2 mV. Modern measurement techniques make it possible to register the predicted relativistic potential.

Kramers Degeneracy and Spin Inversion in a Lateral Quantum Dot

We show that the axial symmetry of the Bychkov–Rashba interaction can be exploited to produce electron spin-flip in a circular quantum dot, without lifting the time reversal symmetry. In order to elucidate this effect, we consider ballistic electron transmission through a two-dimensional circular billiard coupled to two one-dimensional electrodes. Using the tight-binding approximation, we derive the scattering matrix and the effective Hamiltonian for the considered system. Within this approach, we found the conditions for the optimal realization of this effect in the transport properties of the quantum dot. Numerical analysis of the system, extended to the case of two-dimensional electrodes, confirms our findings. The relatively strong quantization of the quantum dot can make this effect robust against the temperature effects.

On Symmetry Properties of The Corrugated Graphene System

The properties of the ballistic electron transport through a corrugated graphene system are analysed from the symmetry point of view. The corrugated system is modelled by a curved surface (an arc of a circle) connected from both sides to flat sheets. The spin–orbit couplings, induced by the curvature, give rise to equivalence between the transmission (reflection) probabilities of the transmitted (reflected) electrons with the opposite spin polarisation, incoming from opposite system sides. We find two integrals of motion that explain the chiral electron transport in the considered system.