Superconductivity presents itself many peculiar and interesting properties in different dimensionalities, which have attracted much attention ever since it has been discovered in bulk materials. At an early stage, theoretical models were established in 3D regimes to describe the conventional bulk superconductivity, while superconductivity was also gradually observed and well studied in 2D, 1D and 0D materials, both experimentally and theoretically. Nevertheless, new superconducting materials are emerging regularly, with novel phenomena, unknown mechanisms and the potential to future applications. This makes superconductivity so charming that people never let up the efforts to further excavate and explore the mystery of superconductors. In this thesis work, I will investigate sup...[ Read more ]

Superconductivity presents itself many peculiar and interesting properties in different dimensionalities, which have attracted much attention ever since it has been discovered in bulk materials. At an early stage, theoretical models were established in 3D regimes to describe the conventional bulk superconductivity, while superconductivity was also gradually observed and well studied in 2D, 1D and 0D materials, both experimentally and theoretically. Nevertheless, new superconducting materials are emerging regularly, with novel phenomena, unknown mechanisms and the potential to future applications. This makes superconductivity so charming that people never let up the efforts to further excavate and explore the mystery of superconductors. In this thesis work, I will investigate superconductivity in several materials, including a Nb doped bulk topological insulator (Nb_xBi₂Se₃), 2D interfacial superconductor (Bi₂Te₃/Fe_1+yTe) and 1D nanowires (Sn and Al).

The basic concepts of superconductivity and corresponding microscopic theories will be introduced in Chapter 1, including the London equation, the Ginzburg-Landau theory and the Bardeen-Cooper-Schrieffer (BCS) theory. Especially, the BCS theory is applicable to conventional superconductors, of which electrons can be weakly bonded as Cooper pairs and move without any resistance below the superconducting transition temperature T_c. However, in low dimensionality, thermal fluctuation activated phase slips of the order parameter will be enhanced and result in a finite resistance in 1D or 2D materials below T_c. For 1D superconductors, such a process has been well explained by the Langer-Ambegaokar-McCumber-Halperin (LAMH) theory. For 2D superconducting materials (thin films or Josephson junctions), vortex-antivortex bonded pairs can form a quasi-long-range-ordered phase coherent state in the 2D plane below a characteristic temperature T_BKT, which is slightly lower than T_c. Therefore, a zero resistance state could also be established in 2D films or Josephson junctions, which fall in the same universality class, and such a transition is described by the well-known Berezinskii-Kosterlitz-Thouless (BKT) theory.

Various instruments and methods have been involved in this thesis work, which will be briefly described in Chapter 2. Mainly, I focus on the transportation properties and magnetization investigations, and one of my frequently used techniques, the point-contact spectroscopy, will be thoroughly discussed with the corresponding theoretical model, the Blonder-Tinkham-Klapwijk (BTK) model, being presented.

In the following Chapter 3, a study of the upper critical field transition in Nb doped Bi₂Se₃ by field-angle-resolved resistive and magnetization measurements will be presented. 3D topological insulators (TI) could become superconducting by doping with intercalation of metallic ions, such as Cu_xBi₂Se₃, Sr_xBi₂Se₃, and Nb_xBi₂Se₃, which could be perfect candidates to study unconventional superconductivity, and may be of high chance to be topological superconductors which serve as platforms for the Majorana fermionc states. Our experimental data of Nb_xBi₂Se₃ clearly shows the two-fold symmetry in the superconducting state that could be well explained by the theoretical model of nematic superconductivity, which spontaneously breaks the three-fold crystalline symmetry of the normal state, in good agreement with reports on Cu_xBi₂Se₃ and Sr_xBi₂Se₃. More detailed discussions will be provided in the main text and a theoretical derivation is included in Appendix I.

For 2D thin films, studies of the resistivity, I-V curves and differential conductance on Bi₂Te₃/Fe_1+yTe will be displayed in Chapter 4, 5 and 6. The Bi₂Te₃/Fe_1+yTe is interfacial superconducting with a superconducting thickness of 7 nm and undergoes a BKT transition below the onset T_c, which is dependent on the thickness of the topological layer (Bi₂Te₃). Our point-contact spectroscopy analyzed by the BTK model show a smaller superconducting gap associated with the proximity induced superconductivity in Bi₂Te₃, a larger gap from the superconducting Fe_1+yTe layer at the interface, and a large pseudogap persisting up to 40 K. Pressure effect has been studied on a featured thickness of Bi₂Te₃/Fe_1+yTe, showing an enhanced onset T_c and different evolutions of the two energy scales, and we demonstrate that the pressure plays a role as the doping effect that pushes the interface from the under-doped towards the optimal doped regime by charge transfer. Further point-contact measurements on different thicknesses of the heterostructures reveal that the smaller gap only emerges in the phase-coherent superconducting state in Bi₂Te₃, while the larger gap occurs once the superconducting transition starts below the onset T_c. Our results display the unconventional superconductivity in Bi₂Te₃/Fe_1+yTe and show the irreplaceable character of TI to induce a phase-coherent superconducting state that opens the second gap.

In Chapter 7 and Chapter 8, I will first briefly review some recent work on 4 Å carbon nanotubes, ultrathin lead nanowire arrays and quasi-1D carbide Sc₃CoC₄, then focus on our studies of Sn nanowires and Al nanowire arrays. The Sn nanowires are ultrathin with a diameter of 60~70 nm, which are randomly distributed and form a quasi-2D network. By electrical transport, magnetization and specific heat characterization, we have observed a 1.8 K increase of the onset T_c and an enhancement of the upper critical field H_c2 about ~100 times with respect to bulk Sn. We attribute the enhancement of T_c and H_c in Sn nanowire networks to an enhanced effective electron-phonon interaction near the surface of a superconductor and the strong spin orbital coupling in an open Fermi surface without closed orbits in the 1D limit, respectively. The fingerprints of transverse Josephson coupling was also observed, since a zero resistance state emerged below T_BKT.

The Al nanowires were fabricated in AlPO₄-5 (AFI) zeolite single crystals of micrometer size with a pore diameter of 0.73 nm to form nanowire arrays. By resistance measurements, very high superconducting transition temperatures have been observed at T_c = 45 K and 75 K for two selected samples, while the point-contact study on the third sample reveals a superconducting gap opening below 40 K and the gap amplitude Δ is surprisingly large as 35 meV at 1.5 K. As origin of the strikingly high T_c and the large value of Δ, a possible presence of a Van Hove singularity in the electronic density of states of the Al nanowires and an enhanced Debye temperature due to a layer of Al₂O₃ at the surface of the nanowires are discussed in Chapter 8.