Torque Magnetometry and Transport Study on Topological Superconductors and Semimetals

[摘要] Recently, the topological features of materials have been intensively focused on not only due to its fundamental physical interest, but also its application to dissipationless devices and quantum computing.In this study, I investigated three materials with different topological classes: NbxBi2Se3, thin film WTe2 and LaNiO3 grown on LaAlO3.NbxBi2Se3 is a candidate of topological superconductors, which may host Majorana fermions that could be used for topological quantum computing. In the superconducting state of NbxBi2Se3, by torque magnetometry technique, I observed the in-plane angular dependence of susceptibility and supercurrent shows the two-fold symmetry although the crystal structure is trigonal. This in-plane rotational symmetry breaking is called the nematic superconducting state. On the other hand, in the normal state, the angular dependence of susceptibility follows six-fold symmetry, indicating that the nematicity occurs only in the superconducting state. However, de Haas-van Alphen study shows that the in-plane Fermi surface is also anisotropic. This would nail down the origin of the nematic order.Next, the bulk WTe2 is a type-II Weyl semimetal. However, when a thin film of WTe2 is grown on a sapphire substrate, it becomes superconducting. Moreover, there are a few exotic superconducting features in this system. First, the in-plane upper critical field Hc2 is more than 10 times larger than the Pauli limit Hp, indicating the Ising superconductivity in this system. Furthermore, the critical temperature Tc is enhanced by the in-plane magnetic field at both around T = Tc and T=0. This result may be associated with the unique Td crystal structure of this system, which also relates to the Weyl nature.Lastly, when LaNiO3 thin films are grown on LaAlO3 (111), the emergence of Dirac physics, as well as the multiferroic feature, is predicted due to the buckled honeycomb structure in (111) plane. We first observed the ferromagnetic state as predicted by many groups. This result is important not only because it gives further guidance to improve the theoretical modeling, but also because the ferromagnetic state can be coupled to the ferroelectricity for potential spintronics applications. Moreover, our results are consistent with the existence of a gapped Dirac point predicted by the theory. These results would be a significant step forward in the realization of a strongly correlated topological phase by geometrical engineering of buckled honeycomb lattice.These results will help further understand the exotic features of topological materials. In particular, the combination of topology and correlation or superconductivity is quite a new field. The observation of nematic superconductivity, field-enhanced Tc and magnetic Dirac physics would be a hallmark for this field.

[发布机构] University of Michigan

[效力级别] Weyl semimetals

[关键词] Topological superconductors;Weyl semimetals;Condensed matter physics;Physics;Science;Physics