Advanced Quasiparticle Interference Imaging for Complex Superconductors

Abstract

State-of-the-art low-temperature spectroscopic imaging scanning tunneling microscopy offers a powerful tool to study materials with unprecedented spatial (subangstrom) and energy (micro-electronvolts) resolution. Imaging the quasiparticle interference of eigenstates in a material, offers a window into the underlying Hamiltonian. In this thesis, we performed quasiparticle interference imaging under extreme (milliKelvin temperatures) conditions and developed novel analysis techniques to address important contemporary problems in superconductivity which have defied complete understanding due to their complex multiband nature and small energy scales. Discovered almost 25 years ago, the momentum space gap structure of Sr2RuO4 has remained a mystery despite being an intensely researched topic due to possibilities of correlated and topological superconductivity. The multiband nature of Sr2RuO4 makes the problem complicated because usual thermodynamical probes cannot directly reveal which bands the subgap quasiparticles are coming from. Our first advanced approach addresses this problem by applying milliKelvin spectroscopic imaging scanning tunneling microscopy (SISTM) to visualize the Bogoliubov quasiparticle interference (BQPI) pattern deep within the superconducting gap at T=90 mK. From T-matrix modeling of the subgap scattering, we are able to conclude that the major gap lies on alpha:beta bands and the minima/nodes lie along (1; 1) directions. Further angular analysis of the scattering features reveals that the nodes must be within 0.05 radians of the (0; 0) - (1; 1) lines. These observations, along with the other theoretical and experimental literature at the present moment, are most consistent with a B1g symmetry (dx2-y2-wave) gap on alpha:beta bands of Sr2RuO4. This conclusion indicates that Sr2RuO4 may not be a chiral odd-parity superconductor as believed for many years. While many techniques exist to measure the magnitude of the superconducting gap, only a few can probe its phase. High-quality BQPI patterns can be analyzed for phase information to determine the sign of the gap. Such techniques rely on centering a single atomic defect in the field of view (FOV). Since the defect concentration determines the BQPI signal intensity, this usually forces the choice of a smaller field of view, limiting the momentum-space resolution and the BQPI signal intensity. As a second advanced approach, we implement quantum phase-sensitive technique which can work with multiple atomic defects in the FOV. We first show that our multiple atom phase-analysis technique reproduces the results for FeSe, a material previously studied with single impurity phase-analysis. Then we implement our technique to study LiFeAs. Our multi-atom analysis reveals that the gap sign changes from hole band to electron band. This establishes that the gap symmetry in LiFeAs is s. This multi-atom innovation extends applicability to more disordered superconductors, enables its application to larger fields of view thereby enhancing k-space resolution, and greatly increases the signal to noise ratio by suppressing phase randomization due to scattering interference coming from multiple atomic centers. Overall, it represents a powerful and general new approach for determination of superconducting order parameter for complex superconductors.