Field-modulated Quantum Transport In Monolayer And Bilayer Graphene Heterostructures

Since the first experimental confirmation of two-dimensional graphene materials in 2004,many two-dimensional materials have been successfully prepared and studied.Due to the dimension limitation in these materials,new vitality has been injected into spintronics and superconducting electronics,which have rapidly become one of the key objects of attention in condensed matter physics and materials science.On one hand,the goal of spintronics is to fabricate novel information storage and logic devices by using the spin freedom regulation of electrons.Researchers expect to achieve high energy and low energy consumption spin logic devices by using the low-dimensional limited structure of graphene materials,and have shown great interest in the transport properties of heterostructures.On the other hand,by studying the reflection of graphene relativistic electrons on the normal-superconducting heterostructures,the researchers discovered the specular Andreev reflection,which adds a new understanding dimension for superconducting transport phenomenon.Although the basic properties of electrons and transport properties of heterostructures in graphene materials have been thoroughly studied,further exploration is still needed in antiferromagnetic-state spin manipulation and strain-field superconducting transport,which are the scientific issues focused on in this paper.First,we propose a spin valve that is based on van der Waals antiferromagnetism and is fully electrically controlled.The device is composed of two antiferromagnetic terminals that allow for vertical bias control and a linked central scattering potential region.When the bias direction of the electrodes at both ends changes from parallel to anti-parallel,the reluctance changes significantly,which results from the change of band spin polarization under the control of local electric field.Through first-principles calculations,we further prove that the ground state of the bilayer graphene structure,which is encapsulated by two monolayers of Cr₂Ge₂Te₆,is antiferromagnetic and remains unchanged under a small applied electric field,providing a feasible platform for the realization of van der Waals antiferromagnetic spin valve device.Then,we investigate the Andreev reflection across a uniaxial strained graphene-based superconducting junction.Compared with pristine graphene-based superconducting junction,three opposite properties are found.Firstly,in the regime of the intraband conversion of electron-hole,the Andreev retro-reflection happens.Secondly,in the regime of the interband conversion of electron-hole,the specular Andreev reflection happens.Thirdly,the perfect Andreev reflection,the electron-hole conversion with unit efficiency,happens at a nonzero incident angle of electron.These three exotic properties arise from the strain-induced anisotropic band structure of graphene,which breaks up the original relation between the direction of velocity of particle and the direction of the corresponding wavevector.Our finding gives an insight into the understanding of Andreev reflection and provides an alternative method to modulate Andreev reflection.At last,the applicant presents the conclusions and outlook.