Strain Engineering And Transport Study On ?-Sn Epitaxial Films

Gray tin(?-Sn),an allotrope of tin,has a diamond structure.It is predicted to be a topological material when the cubic symmetry is broken,for example,crystal structural deformation by appropriately applying strain.Compared to other topological materials,?-Sn has both simpler crystal structure and composition.The topologically nontrivial bandgap can be tuned by strain.Therefore ?-Sn has attracted more attention recently,especially stanene,the two dimensional form of ?-Sn,which is predicted to be a large-gap two-dimensional topological insulator.Though the topological band structure of epitaxial ?-Sn films has been investigated by angle-resolved photoemission spectroscopy(ARPES),the electrical characterization is seldom reported.The reasons are mainly the limitation caused by the instability of the ?-Sn films,and the difficulties in the analysis of electrical properties due to the InSb substrate contribution.In this thesis,we investigated the electrical transport properties at low temperatures and high magnetic fields of the Ca-Sn films grown by molecular beam epitaxy(MBE).In addition to InSb,GaAs substrates are used to reduce the InSb contribution in electrical measurements.We have confirmed the high qualities of the a-Sn samples and the interfaces through various structural characterizations before the electrical measurements.Temperature dependent X-ray diffraction(XRD)and Raman spectra indicate that the ?-Sn films have enhanced thermal stabilities.At low temperatures and high magnetic fields,the ?-Sn samples show a giant magnetoresistance effect with aperiodic oscillations.This giant magnetoresistance effect can persist up to 150K.Strong fields may modify the Fermi surface and introduce extra carriers.We have also observed multiple superconducting transitions in the a-Sn samples.Besides the P-Sn transition,there are additional transitions at higher temperatures,which evolve over time.These transitions show two-dimensional superconducting properties.We conclude that a-Sn may contribute to the superconducting behaviors after carefully analyzing all the possible superconducting phases in the samples.The relative thickness of the InSb layer to the a-Sn films can be altered to tune the strain in the films and therefore influence the superconducting properties.The measurements of Meissner effect indicates type-II superconducting features in the a-Sn samples containing P-Sn,which should be attributed to the flux pinning possibly induced by defects or disorders.The analysis of the magnetoresistance behavior of the ca-Sn/InSb heterostructures in this thesis demonstrates complex and novel quantum properties.The giant magnetoresistance at higher temperature is a promising property for applications such as magnetic sensitive or memory devices.We have investigated the superconducting behavior in the ?-Sn films which are orders of magnitude thicker than stanene,providing a reference for future study.With the improvements on growth conditions and characterization methods,more unique properties of a-Sn will be discovered and attract more attention in both fundamental investigations and device applications in the future.