THESIS
2015
xix, 107 pages : illustrations ; 30 cm
Abstract
Topological insulators (TIs) have a fully-insulating gap in the bulk and gapless conducting states on the edges or surfaces, which render the electrons travelling on the edges or surfaces insensitive to scattering by spin-independent impurities. A heterostructure that consists of such a topological insulator and a certain conventional material endows the heterostructure with novel quantum phenomena. In this thesis work, a number of novel quantum phenomena in two heterostructures, a transition metal/topological insulator (Pd/Bi
2Te
3) and a topological insulator/parent compound of iron-based superconductor (Bi
2Te
3/FeTe), were experimentally demonstrated along with corresponding theoretical analysis.
In the Pd/Bi
2Te
3 heterostructure, effective electron bath effect arising from the underlyi...[
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Topological insulators (TIs) have a fully-insulating gap in the bulk and gapless conducting states on the edges or surfaces, which render the electrons travelling on the edges or surfaces insensitive to scattering by spin-independent impurities. A heterostructure that consists of such a topological insulator and a certain conventional material endows the heterostructure with novel quantum phenomena. In this thesis work, a number of novel quantum phenomena in two heterostructures, a transition metal/topological insulator (Pd/Bi
2Te
3) and a topological insulator/parent compound of iron-based superconductor (Bi
2Te
3/FeTe), were experimentally demonstrated along with corresponding theoretical analysis.
In the Pd/Bi
2Te
3 heterostructure, effective electron bath effect arising from the underlying Bi
3Te
3 TI thin film was found to significantly enhance the surface reactivity of the top Pd layer in the presence of two oxidizing agents (O and Te). The surface reactivity of the adsorbed Te on this heterostructure was also intensified, which is likely also benefitted from the effective transfer of the bath electrons. A partially inserted Fe ferromagnetic layer at the interface of this heterostructure was found to play two competing roles arising from the higher-lying d-band center of the Pd/Fe bilayer and the interaction between the ferromagnetism and the surface spin texture of Bi
2Te
3 on the surface reactivity. The results obtained from the studies of these two competing roles also demonstrate that the electron bath effect is long-lasting against accumulated thickness of adsorbates.
In the Bi
2Te
3/FeTe heterostructure, a two-dimensional superconductivity at the interface of the heterostructure was found to arise even when the thickness of Bi
2Te
3 is as thin as one nanometer. Structural characterizations and transport measurements on this heterostructure demonstrate that superconductivity at the interface is induced by the Bi
2Te
3 epilayer though there is no clear-cut evidence that the observed superconductivity is induced by the topological surface states. The two-dimensional nature of the observed superconductivity with the highest superconducting transition temperature (T
c) around 12 K was verified by the existence of a Berezinsky–Kosterlitz–Thouless transition and the diverging ratio of in-plane to out-plane upper critical field on approaching T
c. We have investigated several possible underlying causes of the observed two-dimensional superconductivity of this heterostructure, including the extrinsic doping of Bi, intrinsic doping of Te, interdiffusion, and strain built-up across the interface, as well as the topological nature of the Bi
2Te
3 layer. The results of these investigations lead us to propose that the electrons from both the bulk and the topological surface states of the Bi
2Te
3 layer may increase the charge density of the FeTe layers near the interface and induce the observed superconductivity. However, this is far from conclusive and further experimental evidence is needed to test this hypothesis.
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