Ruoyu Chen

Surface transport and quantum Hall effect in ambipolar black phosphorus double quantum wells

Few-layer black phosphorus acts as tunable ambipolar wide quantum wells. Quantum wells (QWs) constitute one of the most important classes of devices in the study of two-dimensional (2D) systems. In a double-layer QW, the additional “which-layer” degree of freedom gives rise to celebrated phenomena, such as Coulomb drag, Hall drag, and exciton condensation. We demonstrate facile formation of wide QWs in few-layer black phosphorus devices that host double layers of charge carriers. In contrast to traditional QWs, each 2D layer is ambipolar and can be tuned into n-doped, p-doped, or intrinsic regimes. Fully spin-polarized quantum Hall states are observed on each layer, with an enhanced Landé g factor that is attributed to exchange interactions. Our work opens the door for using 2D semiconductors as ambipolar single, double, or wide QWs with unusual properties, such as high anisotropy.

Long-distance spin transport through a graphene quantum Hall antiferromagnet

Because of their ultrafast intrinsic dynamics and robustness against stray fields, antiferromagnetic insulators1–3 are promising candidates for spintronic components. Therefore, long-distance, low-dissipation spin transport and electrical manipulation of antiferromagnetic order are key research goals in antiferromagnetic spintronics. Here, we report experimental evidence of robust spin transport through an antiferromagnetic insulator, in our case the gate-controlled state that appears in charge-neutral graphene in a magnetic field4–6. Utilizing quantum Hall edge states as spin-dependent injectors and detectors, we observe large, non-local electrical signals across charge-neutral channels that are up to 5 μm long. The dependence of the signal on magnetic field, temperature and filling factor is consistent with spin superfluidity1,2,4,7–10 as the spin-transport mechanism. This work demonstrates the utility of graphene in the quantum Hall regime as a powerful model system for fundamental studies in antiferromagnetic spintronics.Spins are transmitted over a distance of 5 μm through a piece of antiferromagnetic graphene. This shows that graphene can be a platform to explore the fundamental physics of spin transport in antiferromagnets for application in spintronics.

Integer and Fractional Quantum Hall effect in Ultrahigh Quality Few-layer Black Phosphorus Transistors

As a high mobility two-dimensional semiconductor with strong structural and electronic anisotropy, atomically thin black phosphorus (BP) provides a new playground for investigating the quantum Hall (QH) effect, including outstanding questions such as the functional dependence of Landau level (LL) gaps on magnetic field B, and possible anisotropic fractional QH states. Using encapsulated few-layer BP transistors with mobility up to 55 000 cm2/(V s), we extracted LL gaps over an exceptionally wide range of B for QH states at filling factors -1 to -4, which are determined to be linear in B, thus resolving a controversy raised by its anisotropy. Furthermore, a fractional QH state at ν ≈ -4/3 and an additional feature at -0.56 ± 0.1 are observed, underscoring BP as a tunable 2D platform for exploring electron interactions.