Explorations of domain walls and half quantum vortices in unconventional superconductors

Most known superconductors are characterized by spin-singlet superconducting order. An exception is Sr2RuO4, for which evidence is accumulating in favor of superconducting order of spin-triplet character, specifically of the equal-spin type of pairing and, furthermore, which may exibit spontaneously broken time reversal symmetry. Triplet superconductors have been proposed to host half quantum vortices, whereas superconductors with spontaneously broken time-reversal symmetry are proposed to admit domain walls that separate regions of opposing chirality. Thus---in addition to conventional vortices---the topological structure of the unconventional superconducting order of Sr2RuO4 may allow for at least two additional topologically stable defects. This thesis explores aspects of two unconventional topological defects: In Part I of the thesis we focus on domain walls, and in Part II we turn to half quantum vortices.;In Part I, via a general phenomenological and symmetry based approach, we derive an effective description of superconductivity that spontaneously breaks time reversal symmetry, in terms of the relevant topological coordinates for domain walls and vortices. One of the key consequences expected of broken time-reversal symmetry superconductivity is that in its ground state the system should exhibit chiral currents of charge that are localized near the core of any domain wall and near the boundaries of the sample. However, signatures of such currents, in the form of magnetic fields, have not been observed in Sr2RuO4, to date, despite considerable efforts. In this thesis, we explore alternative magnetic signatures of the existence of walls between domains of opposing chirality. We show that, in the limit in which the superconducting system is taken to have in-plane rotational invariance, a domain wall that is translationally invariant along the z axis (which runs perpendicular to the aforementioned planes) and includes an isolated bend through an angle theta is accompanied by a nonintegral magnetic bend flux of value ((theta/pi)+n)phi 0, with integral n, that penetrates the superconductor and is localized near the bend. We find this result to be independent of the magnitude of chiral-charge currents that are predicted to flow along the core of domain walls. On the basis of this specialized result and its generalization to cases of discrete crystalline symmetry, we note that the observation of localized, nonintegral flux penetrating a z-axis surface (detected, e.g., via scanned-probe magnetic imaging) can potentially be interpreted in terms of the presence of bent walls between domains of opposing chirality, and hence would be suggestive of the existence of time-reversal symmetry-breaking superconductivity.;In Part II of this thesis we consider half quantum vortices. We begin by developing a classification of the topologically stable line and point defects that are allowed for layered superconductors having triplet equal-spin pairing, under various assumptions about the structure of the superconducting pairing. Here, the central result is that, in contrast to the case of bulk superfluid 3He, half quantum vortices are shown to exist for both the time-reversal symmetry broken and time-reversal intact forms of superconducting order. Then, after introducing the structure of the London free energy that effectively describes the energetics of half quantum vortices in such systems, we study the influence of sample geometry on the stability of half quantum vortices, finding that half quantum vortices are expected to be stable for a range of parameters that is wider than previously expected.;An "annular," ring-shaped, geometry is useful in the experimental study of topological line defects such as half quantum vortices because such a geometry is expected to yield a discrete family of low-energy "fluxoid" states in which the superconducting order parameter winds around the annulus as it would around the line defect. Recently, evidence for half integer fluxoid states (the fluxoid analog of half quantum vortices) has been obtained in experiments on mesoscopic, ring-shaped samples of Sr2RuO4 , using cantilever torque magnetometry [30]. We briefly review the experimental technique and the experimental results that have emerged from its application to Sr2RuO4. We then discuss two possible scenarios for the theoretical interpretation of the observed half integer fluxoid behavior: a half quantum vortex scenario, and a wall vortex scenario. We find the first scenario to be more consistent with the observations, and we suggest further experiments that could provide even more stringent tests of the consistency of this exciting, half quantum vortex scenario.