abstract: A theoretical study of a three-dimensional (3D) N/S interface with arbitrary spin polarization and interface geometry is presented. The 3D model gives the same intrinsic spin polarization and superconducting gap dependence as the 1D model. This demonstrates that the 1D model can be use to t 3D data. Using this model, a Heusler alloy is investigated. Andreev reflection measurements show that the spin polarization is 80% in samples sputtered on unheated MgO(100) substrates and annealed at high temperatures. However, the spin polarization is considerably smaller in samples deposited on heated substrates. Ferromagnetic FexSi􀀀x alloys have been proposed as potential spin injectors into silicon with a substantial spin polarization. Andreev Reflection Spectroscopy (ARS) is utilized to determine the spin polarization of both amorphous and crystalline Fe65Si35 alloys. The amorphous phase has a significantly higher spin polarization than that of the crystalline phase. In this thesis, (1111) Fe SmO0:82F0:18FeAs and Pb superconductors are used to measure the spin polarization of a highly spin-polarized material, La0:67Sr0:33MnO3. Both materials yield the same intrinsic spin polarization, therefore, Fe-superconductors can be used in ARS. Based on the behavior of the differential conductance for highly spin polarized LSMO and small polarization of Au, it can be concluded that the Fe-Sc is not a triplet superconductor. Zero bias anomaly (ZBA), in point contact Andreev reflection (PCAR), has been utilized as a characteristic feature to reveal many novel physics. Complexities at a normal metal/superconducting interface often cause nonessential ZBA-like features, which may be mistaken as ZBA. In this work, it is shown that an extrinsic ZBA, which is due to the contact resistance, cannot be suppressed by a highly spin-polarized current while a nonessential ZBA cannot be affected the contact resistance. Finally, Cu/Cu multilayer GMR structures were fabricated and the GMR% measured at 300 K and 4.5 K gave responses of 63% and 115% respectively. Not only do the GMR structures have a large enhancement of resistance, but by applying an external magnetic eld it is shown that, unlike most materials, the spin polarization can be tuned to values of 0.386 to 0.415 from H = 0 kOe to H = 15 kOe. / Dissertation/Thesis / Doctoral Dissertation Physics 2015

京都大学 / 0048 / 新制・論文博士 / 博士(理学) / 乙第12996号 / 論理博第1552号 / 新制||理||1604(附属図書館) / 32924 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 松田 祐司, 教授 前野 悦輝, 教授 石田 憲二 / 学位規則第4条第2項該当 / Doctor of Science / Kyoto University / DGAM

The fast progress in the exploration of 2D materials such as graphene became possible due to development of fabrication techniques that allowed these materials to be protected from e.g. undesirable doping and gave rise to new functionalities realized within van der Waals heterostructures. Attracted by van der Waals interaction the constituent layers of such heterostructures preserve their exceptional electronic quality and for example in graphene allow for high electron mobility to be achieved. However, the studies of atomically thin layers such as NbSe2 that exhibit metallic behavior have been impeded by their reactivity and hence oxidation during exposure to ambient or oxidizing agents such as solvents. In this thesis, the existing heterostructure assembly technique was improved by the introduction of exfoliation and re-stacking by a fully motorized system placed in an inert atmosphere. This approach allowed us to overcome the problem of environmental degradation and create Hall bars and planar tunnel junctions from atomically thin superconducting NbSe2. Furthermore, this versatile approach allowed us to study the thickness dependence of the normal and superconducting state transport properties of NbSe2, uncovering the reduction of the superconducting energy gap and transition temperature in the thinnest samples. On the other hand, 2D materials being just 1-3 atoms thick represent an ultimate example of a membrane - thin but laterally extended object. Consisting of such atomically thin membranes the van der Waals heterostructures can be used for purposes other than the studies of electronic transport. In this work, ubiquitous bubbles occurring during van der Waals heterostructure assembly are employed as a tool to explore 2D materials' mechanical properties and mutual adhesion. This allowed us to measure Young's modulus of graphene and other 2D materials under 1-2% strain and deduce the internal pressure that can reach up to 1 GPa in sub-nanometer size bubbles.