Modulation Of Transport Properties In Graphene Via Proximity Effect

The transport behaviors of solids often depend on their effective dimensionality,especially for two-dimensional materials. For instance, Si MOSFETs, as the base of modern technology, utilize the charges confined at the surface layers. The quantum Hall effect and fractional quantum Hall effect are peculiar to two-dimensional systems. It is also believed that the exceptional high-temperature superconductivity of cuprates is related to its two-dimensional nature. Every time a new kind of two-dimensional system is brought into sight, a floodgate of research will be opened in condensed matter physics. Until a few years ago, the two-dimensional paradigms were thought to be electronic systems confined in quantum wells, semiconductor heterostructures, or surface inversion layers. This situation has been changed since 2004 when an individual atomic layer could be pulled out from the graphite bulk. In spite of being only one atom in thickness, this kind of two-dimensional crystal remains stable at ambient atmosphere. It is called graphene, the primary target material of this research.Graphene is a rapidly rising star in material science and condensed matter physics. This kind of rigid two-dimensional flake has exhibited fascinating crystal quality and electronic properties. Although its history is not very long, graphene has assembled a great deal of new physics and potential applications. For example, with its unique linear dispersion, graphene is leading a new paradigm in relativity condensed matter physics. Those quantum relativity phenomena unable to be realized in high energy physics, are now attracting intensive research based on graphene system. Moreover, graphene represents a new type of single-layer materials, providing novel platforms for fundamental research and applications in low dimensional physics.In Chapter ?, we briefly introduce the crystal structure of graphene and its extraordinary linear dispersion in momentum space. Then we bring in a general description to the electronic transport properties of graphene, including its bipolar field tuning, quantum interference, and half integer quantum Hall effect. Besides, we also give a clear interpretation to its anomalous Hall effect and quantum anomalous Hall effect.In Chapter ?,the preparations of graphene samples and low-temperature transport techniques are presented, especially some lab experiences. Descriptions include flakes exfoliation, transfer technique, characteristic methods,and nanofabrications to build graphene devices. Then, Laser Molecular Beam Epitaxy(Laser-MBE), a typical method for thin-film deposition of single crystal, is described in brief. The low-temperature high-magnetic-field system used to carry out transport measurements would be introduced at last.In Chapter ?, we devote to inducing magnetism in graphene system through proximity effect, with the final goal to realize the quantum anomalous Hall effect.First, we chose the magnetic insulating Pr0.8Ca0.2MnO3 as the proximity substrate.However, we found it is difficult to tune graphene's carrier densities on Pr0.8Ca0.2MnO3, Then we turned to another magnetic insulating LaMnO3 and discovered graphene on this device can work properly. We observed the nonlinearity of the Hall resistance which cannot be well understood by two-carrier model,especially when Fermi levels are away from the Dirac point. This anomalous Hall curves may come from the establishment of long-rang magnetic order in graphene.Meanwhile, the longitudinal resistance displayed an asymmetry behavior which may result from spin-orbit coupling enhancement at the interface of graphene and LaMnO3.In Chapter IV, we focused on the transport properties of graphene-plasmonics hybrid structures. The interplays between different quasiparticles in solids lay the foundation for a broad spectrum of intriguing quantum effects, yet how the collective plasmon excitations affect the quantum transport of electrons remains largely unexplored. In the present study, plasmons were induced in graphene by nanocavity coupling from proximal Au nanoparticles. We provide the first demonstration that when the electron-plasmon coupling is introduced, the quantum coherence of electrons in graphene is substantially enhanced with the quantum coherence length almost tripled. We further develop a microscopic model to interpret the striking observations, emphasizing the vital role of the graphene plasmons in suppressing electron-electron dephasing. The novel and transformative concept of plasmon-enhanced quantum coherence sheds new insight into inter-quasiparticle interactions,and further extend a new dimension to exploit nontrivial quantum phenomena and devices in solid systems.In Chapter V, we investigate the proximal spin-orbit coupling in graphene by transport measuement and near-field optics. We fabricate graphene/KTaO3 heterostructures expecting to induce spin-orbit coupling in graphene by heavy 5d element of Ta. In the first part, we found giant nonlocal signals which can not be simply attributed to spin Hall effect because we did not observe the signal oscillations which is the hallmark of spin precession under parallel magnetic field.In the second part, the recently rising near-field optics of graphene indicates the possibility of tailoring the excitation and propagation by alternative substrate environment. Therefore we detected the surface plasmon of graphene/KTaO3 system using near-field optical microscope (SNOM) and found peculiar standing wave patterns showing double plasmon modes behavior which is different from the observation on graphene/SiO2 system. This phenomenon illustrates the multimode feature, verifying the feasibility for edge-excited graphene plasmon mode.