Confinement-Induced Enhancement of Superconductivity in a Spin-$\frac{1}{2}$ Fermion Chain Coupled to a ${\mathbb{Z}}_2$ Lattice Gauge Field

Abstract

We investigate a spin-$\frac{1}{2}$ fermion chain minimally coupled to a ${\mathbb{Z}}_2$ gauge field. In the sector of the gauge generator ${\hat{G}}_j=-1$, the model reduces to the Hubbard model with repulsive onsite interaction coupled to a ${\mathbb{Z}}_2$ gauge field. We uncover how electric fields affect low-energy excitations by both analytical and numerical methods. In the half-filling case, despite electric fields, the system is still a Mott insulator, just like the Hubbard model. For hole-doped systems, holes are confined under nonzero electric fields, resulting in a hole-pair bound state. Furthermore, this bound state also significantly affects the superconductivity, which manifests itself in the emergence of attractive interactions between bond singlet Cooper pairs. Specifically, numerical results reveal that the dimension of the dominant superconducting order parameter becomes smaller when increasing the electric field, signaling an enhancement of the superconducting instability induced by lattice fermion confinement. The superconducting order can even be the dominant order of the system for suitable doping and large applied electric field. The confinement also induces a $\pi$ momentum for the dominant superconducting order parameter leading to a quasi-long-range pair density wave order. Our results provide insights for understanding unconventional superconductivity in ${\mathbb{Z}}_2$ LGTs and might be experimentally addressed in quantum simulators.

14 pages, 7+2 figures

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