Search for Topological Superconductivity in Doped Bi2 Se3

摘要

Bi2Se3 is a known topological insulator. When doped with certain metals, this compound becomes superconducting. To date, doping Bi2Se3 has been one of the leading avenues to search for topological superconductivity. In this dissertation, I present toque mangetometry studies on Cu-doped and Nb-doped Bi2Se3, which are candidates to be topological superconductors.;Quantum oscillations are generally studied to resolve the electronic structure of topological insulators. Using torque magnetometry, I observed quantum oscillations (the de Haas-van Alphen effect) in Cu-doped and Nb-doped Bi2Se3. The doping of Cu in Bi2Se3 increases the carrier density and the effective mass without increasing the scattering rate or decreasing the mean free path. As the magnetic field tilts from the crystalline c axis to the ab plane, the change of the oscillation period follows the prediction of the ellipsoidal Fermi surface. As the doping level changes, the 3D Fermi surface becomes quasicylindrical at high carrier density. Such a transition is potentially a Lifshitz transition of the electronic state in Cu-doped Bi2Se3. In addition, the Fermi velocity remains the same in Cu-doped Bi2Se3 as that in Bi 2Se3. These results imply that the insertion of Cu does not change the band structure and that conduction electrons in Cu doped Bi 2Se3 sit in the linear Dirac-like band.;In the fully superconducting Nb-doped Bi2Se3 crystal, two distinct quantum oscillations frequencies are observed, in sharp contrast to Bi2Se3 and Cu-doped Bi2Se3. The multiple frequencies observed in the quantum oscillations, combined with the electrical transport studies, indicate the multi-orbit nature of the electronic state of Nb-doped Bi2Se3. The observation of the multiple orbits in the superconducting Nb-doped Bi2Se3 also points to Fermi surface nesting as a possible superconducting mechanism, and reveals that the insertion of Nb radically changes the band structure of Bi2 Se3.;For topological superconductors, the coupling between the physical properties in the superconducting state and its underlying crystal symmetry is a crucial test for the topological nature of the superconductivity. The superconducting magnetic response in Nb-doped Bi2Se3 couples strongly to the underlying 3-fold crystal symmetry. More importantly, the magnetic response is greatly enhanced along one preferred direction spontaneously breaking the rotational symmetry. This confirms the presence of nematic order in the superconducting ground state of Nb-doped Bi2Se3. The observation of nematic order in the superconducting state provides a strong evidence of odd-parity topological superconductivity.