Master's Defense: Yi Dai

Unconventional superconductivity and the Hebel-Slichter peak of the kagome lattice

To determine the symmetry of the superconducting ground state is a crucial task for understanding unconventional superconductivity. The spin-lattice relaxation rate measured by nuclear magnetic resonance on superconductors is usually thought to be able to probe the symmetry of superconducting order parameter. Materials with kagome lattice structure, such as the AV₃Sb₅ family are possible candidates for unconventional superconductivity. They are already very interesting in the normal state because of the rich band structure and the unique sublattice interference effect. The kagome superconductor was reported to exhibit a Hebel-Slichter peak in the spin-lattice relaxation rate in the superconducting state and this result was interpreted as an evidence for s-wave conventional superconductivity. In this thesis, a theoretical investigation of the spin-lattice relaxation rate of unconventional kagome superconductors is performed.  The result shows that despite the existence of a sign-changing gap structure, which sums to zero over the Fermi surface, the d-wave unconventional pairing of the kagome lattice will exhibit a Hebel-Slichter peak in the spin-lattice relaxation rate. This result is further explained in the thesis as an effect of the sublattice interference effect. This thesis, together with the previous study of the robustness of superconducting critical temperature to disorder on the kagome lattice with d-wave order parameter, shows that the unconventional superconductivity of kagome lattice might appear similar to conventional non-sign-changing superconductivity, and further investigation is needed to determine the nature of the superconductivity on the kagome lattice.