| Graphene is a single layer carbon atoms arranged in hexagonal honeycomb structure based on sp2 hybridization.It is a new two-dimensional allotropy of carbon,in addition to the three-dimensional diamond,graphite and amorphous carbon,zero-dimensional fullerenes,and one-dimensional carbon nanotube.The structure can be seen as a triangular lattice with a basis of two atoms(A-site atom and B-site atom)per unit cell,in which there is spatial inversion symmetry between A-site atom and B-site atom.And the band structure of graphene contains two sets of valleys(around K or K' points),which are energy degenerate,but not equivalent.Therefore,the electrons in graphene have the additional valley degrees of freedom.The valence and conduction band in graphene touch at K and K' points,therefore the carrier can be holes or electrons.The energy dispersive relationship around K and K' points in graphene is linear and should be described by the Dirac equation.The Fermi velocity vF is about 106m/s,and the electron dynamics in graphene are thus effectively"relativistic".In summary,unlike two-dimensional electron gas based on conventional semiconductor heterostructures,graphene is a Dirac semimetal with chirality and relativistic effects.Thus,many unique physical effects have been revealed in graphene,such as half-integer quantum Hall effect and Klein tunneling.As a natural two-dimensional material,graphene has a wide range of applications,due to its numerous excellent electrical,optical,mechanical and thermal properties.However,there are also some characteristics limiting its application,such as the zero band gap limiting its application in field effect transistor,no intrinsic spin polarized electrons limiting its application in the magnetic electronics,the very weak intrinsic spin coupling strength limiting the application in the spin electronics.Therefore,how to control these characteristics in graphene is one of the current research focuses.Based on this purpose,the influence of using the type-? superconductor NbSe2 as the substrate,and the ferromagnetic and insulated EuS as capping layer on the electromagnetic transport properties of graphene was studied,respectivly.The dissertation is composed of 5 chapters.In the first chapter,the lattice structure,energy band structure,preparation method and some unique electrical transport characteristics in graphene,as well as some physical basis related to the dissertation,are briefly introduced.In the second chapter,the preparation and the measuring method of the devices are briefly described.It mainly includes cleavage and accurate transfer of two-dimensional materials,nanofabrication technology,sample characterization method,devices measurement system and measurement method.In the third chapter,we fabricate the hybrid structure of graphene/Al2O3/NbSe2 and study the modulation effect of the vortex dynamics on the quantum transport of graphene.Asymmetric magnetoresistivity plateaus accompanied by hysteresis loops are revealed at low magnetic field below the critical temperature of NbSe2.Such magnetoresistivity plateaus and hysteresis are attributed to the vortex pinning effect at the edge defects of the single-crystalline NbSe2 thin flakes.In the fourth chapter,we fabricate the hybrid structure of EuS/Graphene.By developing a substantially improved technique,a high carrier mobility was achieved in graphene in proximity to the ferromagnetic insulating EuS.The improvement of our technique is reflected in two aspects:(1)a ferromagnetic insulating EuS capping layer is deposited on graphene at a low temperature(?80 K);(2)a contamination free method is subsequently adopted to fabricate the electrodes for transport.Because of the protection from contamination and the dielectric environment improved by the EuS capping layer,as well as the minimized degradation of graphene during EuS deposition,the graphene mobility of?18000-26000 cm2V-1s-1 was achieved at 1.5 K.In the fifth chapter,we study the spin orbit coupling effect and the exchange field in EuS/Graphene devices by means of nonlocal measurement,respectively.At zero magnetic field,we have not observed the nonlocal signal caused by the spin-Hall effect based on spin-orbit coupling,which means that the spin orbital coupling in graphene in the EuS/Graphene device is very weak.In the presence of external magnetic field,we observe that the nonlocal signal in the EuS/Graphene device is stronger and grows faster with the magnetic field than the non-local signal in the referenced BN/Graphene device.Based on the theory of Zeeman Spin Hall effect,we calculate the exchange field in the graphene in EuS/Graphene device introduced by the magnetic proximity effect,and obtain the exchange field up to 3 T when the external magnetic field is 2.5 T at 1.5 K. |
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