System-environmental entanglement in critical spin systems under $ZZ$-decoherence and its relation to strong and weak symmetries

Abstract: Open quantum many-body system exhibits nontrivial behavior under decoherence. In particular, system-environmental entanglement is one of quantities to characterize mixed state properties under decoherence. In this study, we investigate the behavior of the system-environmental entanglement for critical spin chains under nearest-neighbor $ZZ$ -decoherence. We numerically find that the system-environmental entanglement exhibits a specific scaling law including a system-independent universal term ("$g$-function"). For the critical XXZ model, transition to strong-to-weak spontaneously symmetry breaking mixed state takes place. In that case, the $g$-function changes its value at decoherent transition point and gets double the value of system under single-site $Z$-decoherence, which was recently studied by conformal field theory. By studying Shannon entropy, we clarify origin of this $g$-function behavior.