Fractional excitations in one-dimensional fermionic superfluids

Abstract: We study the soliton modes carrying fractional quantum numbers in one-dimensional superfluids. In the $s$-wave pairing superfluid with the phase of the order parameter twisted by opposite angles $\pm \varphi/2$ at the two ends there is an emergent complex $Z_2$ soliton mode carrying fractional spin number $\varphi /(2\pi)$ if there is only one pairing branch. We demonstrate that in finite systems of length $L$, the spin density for one pairing branch in the presence of a single soliton mode consists of two terms, a localized spin density profile carrying fractional quantum number $\varphi/(2\pi)$, and a uniform background $-\varphi /(2\pi L)$. The latter one vanishes in the thermodynamic limit leaving a single soliton mode carrying fractional excitation, however it is essential to keep the total quantum number conserved in finite systems. This analysis is also applicable to other systems with fractional quantum numbers, thus provides a mechanism to understand the compatibility of the emergence of fractional charges with the integral quantization of charges in a finite system. For the $p$-wave pairing superfluid with the chemical potential interpolating between the strong and weak pairing phases, the $Z_2$ soliton is associated with a Majorana zero mode. By introducing the dimension density, we argue that the Majorana zero mode may be understood as an object with 1/2 dimension of the single particle Hilbert space. We conjecture a connection of the dimension density of one-dimensional solitons with the quantum dimension of topological excitations.