Oscillating Magnetoresistance in Graphene p-n Junctions at Intermediate Magnetic Fields

Preparing dangling bonds by nanoholes on graphene oxide nanosheets and their enhanced magnetism

The effects of dangling bonds on the magnetic properties of graphene oxide (GO) were studied experimentally by creating nanoholes on GO nanosheets. GO with more nanoholes (MHGO) and less nanoholes (LHGO) on graphene oxide nanosheets were synthesized. Results showed that nanoholes brought new dangling bonds for GO and the increase of the dangling bonds on GO could be adjusted by the amounts of the nanoholes on GO. The magnetism of GO was enhanced with increased density of nanoholes on GO (MHGO > LHGO > GO). Furthermore, the increased dangling bonds induced magnetic coupling between the spin units and so converted paramagnetism GO to ferromagnetism (MHGO, LHGO). The easy generation and adjustment of GO dangling bonds by nanoholes on GO nanosheets will promote the applications of GO.

The effects of dangling bonds on the magnetic properties of graphene oxide (GO) were studied experimentally by creating nanoholes on GO nanosheets.

Fabry-Pérot resonances and a crossover to the quantum Hall regime in ballistic graphene quantum point contacts

We report on the observation of quantum transport and interference in a graphene device that is attached with a pair of split gates to form an electrostatically-defined quantum point contact (QPC). In the low magnetic field regime, the resistance exhibited Fabry-Pérot (FP) resonances due to np'n(pn'p) cavities formed by the top gate. In the quantum Hall (QH) regime with a high magnetic field, the edge states governed the phenomena, presenting a unique condition where the edge channels of electrons and holes along a p-n junction acted as a solid-state analogue of a monochromatic light beam. We observed a crossover from the FP to QH regimes in ballistic graphene QPC under a magnetic field with varying temperatures. In particular, the collapse of the QH effect was elucidated as the magnetic field was decreased. Our high-mobility graphene device enabled observation of such quantum coherence effects up to several tens of kelvins. The presented device could serve as one of the key elements in future electronic quantum optic devices.

Gate-Controlled Transmission of Quantum Hall Edge States in Bilayer Graphene

The edge states of the quantum Hall and fractional quantum Hall effect of a two-dimensional electron gas carry key information of the bulk excitations. Here we demonstrate gate-controlled transmission of edge states in bilayer graphene through a potential barrier with tunable height. The backscattering rate is continuously varied from 0 to close to 1, with fractional quantized values corresponding to the sequential complete backscattering of individual modes. Our experiments demonstrate the feasibility to controllably manipulate edge states in bilayer graphene, thus opening the door to more complex experiments.

Mach-Zehnder interferometry using spin- and valley-polarized quantum Hall edge states in graphene

Confined to a two-dimensional plane, electrons in a strong magnetic field travel along the edge in one-dimensional quantum Hall channels that are protected against backscattering. These channels can be used as solid-state analogs of monochromatic beams of light, providing a unique platform for studying electron interference. Electron interferometry is regarded as one of the most promising routes for studying fractional and non-Abelian statistics and quantum entanglement via two-particle interference. However, creating an edge-channel interferometer in which electron-electron interactions play an important role requires a clean system and long phase coherence lengths. We realize electronic Mach-Zehnder interferometers with record visibilities of up to 98% using spin- and valley-polarized edge channels that copropagate along a pn junction in graphene. We find that interchannel scattering between same-spin edge channels along the physical graphene edge can be used to form beamsplitters, whereas the absence of interchannel scattering along gate-defined interfaces can be used to form isolated interferometer arms. Surprisingly, our interferometer is robust to dephasing effects at energies an order of magnitude larger than those observed in pioneering experiments on GaAs/AlGaAs quantum wells. Our results shed light on the nature of edge-channel equilibration and open up new possibilities for studying exotic electron statistics and quantum phenomena.