Controlling phase diagram of finite spin-$1/2$ chains by tuning boundary interactions

Abstract

Searching for simple models that possess non-trivial controlling properties is one of the central tasks in the field of quantum technologies. In this work, we construct a quantum spin-$1/2$ chain of finite size, termed as controllable spin wire (CSW), in which we have ${\hat{S}}^z{\hat{S}}^z$ (Ising) interactions with a transverse field in the bulk, and ${\hat{S}}^x{\hat{S}}^z$ and ${\hat{S}}^z{\hat{S}}^z$ couplings with a canted field on the boundaries. The Hamiltonians on the boundaries, dubbed as tuning Hamiltonians (TH's), bear the same form as the effective Hamiltonians emerging in the so-called `quantum entanglement simulator' that is originally proposed for mimicking infinite models. We show that tuning the TH's (parametrized by $\alpha$) can trigger non-trivial controlling of the bulk properties, including the degeneracy of energy/entanglement spectra, and the response to the magnetic field $h_{bulk}$ in the bulk. A universal point dubbed as ${\alpha }^s$ emerges. For $\alpha > \alpha^s$,theground-statediagramversus$h_{bulk}$ consists of three `phases', which are Ne\'eL and polarized phases, and an emergent pseudo-magnet phase, distinguished by entanglement and magnetization. For $\alpha < \alpha^s$, the phase diagram changes completely, with no step-like behaviors to distinguish phases. Due to its controlling properties and simplicity, the CSW could potentially serve in future the experiments for developing quantum devices.