Theoretical study of electronic properties in strontium ruthenate

Abstract

Since the discovery of superconductivity in Sr2RuO4, there has been intense research interest and efforts on its unconventional pairing symmetry. Although its normal state can be qualitatively described as a quasi-two-dimensional Fermi liquid, surprisingly, Sr2RuO4 turns out to be the prime candidate of the chiral p + ip superconductor, analogous to 3He-A. Such a state is of great interest surrounding, since under a certain conditions it hosts such exotic objects as half-quantum vortices and Majorana bound states, one possible route to an enigmatic quantum computer. Nevertheless, although it is well established now that this superconducting state has odd-parity, and most likely breaks time-reversal symmetry, the negative result on the search of edge current is one of the critical challenges to its chiral p-wave order. These serious discrepancies have even triggered a debate on the primary source of its superconductivity.

Motivated by this debate, in this thesis we propose two independent methods to resolve this controversy via “smoking-gun" experiments. First, the vortex structure within the single-band and two-band models is studied within a mean-field theory. The pattern of the local density-of-state at zero bias shows significant anisotropy in the two-band model, while it is nearly isotropic in the single-band case. Also, the spin lattice relaxation rate at the vortex site is greatly enhanced in the single-band case but not in the two-band scenario. These important distinctions stem from the topology of different Fermi surfaces, and can be tested by using standard probes such as scanning tunneling microscope and nuclear magnetic resonance. In the second proposal, we focus on the two-band scenario, and apply a renormalization group theory to explain the form of spin density wave fluctuations. This theory not only reconciles the absence of long range spin density wave order with strongly enhanced fluctuations, but also unveils the mutual exclusion of these fluctuations and p-wave superconducting pairing. Such an exclusion is reflected in the suppression of the spin-spin correlation function at low energies, which can be measured in the inelastic neutron scattering experiment. This suppression, if not observed experimentally, would be a critical challenge to the two-band model, and an indirect but strong support to the assignment of the single γ-band as the primary source of the unconventional pairing.