ABSTRACT
Quantized vorticity is a property of quantum fluids that has been widely studied in various types of superfluids, either with or without spin [77],[86],[87] and Chapter 6. Superfluids with a two component (spinor) order parameter are characterized by a different type of vortex than those found in conventional scalar superfluids [99]-[102]. This new type of vortex is the so called half quantum vortex (HQV). HQVs have lower energy with respect to full vortices and constitute the elementary excitations of spinor superfluids. When circumventing their core, the phase and the polarization vector experience a ±π rotation. In this sense, HQV can be understood as a half phase vortex combined with a half polarization vortex [99]. In 4He superfluids, the HQV cannot be formed as the spin degree of freedom is absent. However, in 3He [99],[100] or in triplet superconductors [101],[102], the order parameter has two or more components, so that the formation of HQV is possible. So far, experiments have not presented any direct evidence for HQV in 3He [103], while more reliable indications of existence of HQV have been reported in cuprate superconductors [104].
