Unlocking the Potential of Bilayer Graphene: Ballistic Electron Trajectories and Specular Reflection

Electronic devices with well-defined ballistic electron trajectories have garnered significant attention in the field of nanotechnology. Such devices have shown immense promise, but to fully harness their potential, it is crucial to achieve specular reflection of electron jets. In this context, graphene, with its exceptional electronic properties, emerges as a prime candidate for various gate-defined devices due to Klein tunneling, enabling new functionalities. However, the absence of a bandgap in graphene hinders the creation of collimated beams and specular mirrors. Nonetheless, the recent progress in fabrication has allowed observation of multiple focusing peaks, but reflection induced by disordered graphene edges remains a fundamental limitation.

In a new study published in the peer-reviewed Journal Nano Letters, Delft University of Technology researchers in the Netherlands: Dr. Josep Ingla-Aynés, Antonio  Manesco, Dr. Talieh   Ghiasi, Serhii Volosheniuk, Kenji Watanabe, Takashi Taniguchi, and led by Professor Herre  van der Zant   reported a novel approach involving bilayer graphene (BLG), a tunable-bandgap semiconductor with a trigonally distorted Fermi surface, to overcome these limitations and unleash the potential of ballistic electron trajectories. They explored the electrical tunability of the bandgap in BLG to create ballistic multiterminal BLG devices and measure transverse electron focusing (TEF) between gate-defined quantum point contacts (QPCs). The researchers observed up to eight focusing peaks with comparable amplitudes, indicating specular reflection at the gate-defined edges of BLG. Notably, temperature-dependent measurements revealed the persistence of the TEF signal at up to 100 K.

Fabrication of the BLG devices involved the use of double-gated, boron nitride (hBN)-encapsulated BLG heterostructures on few-layer graphene back gates, implemented through the dry transfer technique. Electrodes were defined using conventional e-beam lithography, and the BLG flakes were connected to Ti/Au electrodes. The top gates, deposited on the top hBN, allowed for electrically tunable bandgap in the double-gated BLG regions.

The researchers characterized the behavior of the quantum point contacts (QPCs) by analyzing the two-terminal resistance (R) as a function of top gate voltage (Vtg) and back gate voltage (Vbg). Three distinct features were identified: the charge neutrality point (CNP) of the non-top-gated BLG channel, the CNP of BLG near the Ti/Au contacts, and the CNP of the regions under the top gates. The slope of the diagonal line corresponding to the CNP regions provided valuable information about the ratio between the top gate capacitance (Ctg) and the back gate capacitance (Cbg).

The authors examined the conductance (G) of the QPCs to determine if it was quantized. For negative Vbg, G exhibited values higher than 7 × 4e2/h, while for positive Vbg, G showed four steps at G = N × 4e2/h with N = 1, 2, 3, and 4. Additionally, the researchers observed a beating pattern in the TEF spectra, which was attributed to the trigonal warping of the BLG Fermi surface.

The results obtained from the TEF measurements and the study of quantum interference effects demonstrated the robustness of the ballistic multiterminal BLG devices. The decay in peak amplitude for electrons and the quantization of G in specific QPC geometries revealed the significance of QPC width in peak amplitude decrease. The TEF temperature dependence suggested the role of electron-electron interactions in T-dependent scattering for electrons, but not for holes. The presence of a periodic modulation of peak size consistent with the effect of trigonal warping indicated that the TEF spectra were influenced by BLG crystallographic orientation.

In conclusion, The Josep Ingla-Aynés and colleagues study exemplifies an advancement in the field of nanotechnology, where BLG is leveraged to create ballistic multiterminal devices with specular electron reflection. The results open new possibilities for future valleytronic devices, with potential applications in quantum computing, spintronics, and valleytronics. By exploiting the tunable bandgap and unique electronic properties of BLG, researchers have laid the foundation for innovative gate-defined devices that hold promise for the advancement of nanotechnology and its wide-ranging applications.