No abstract

The quantum phase crossover in a spin ladder model with fourspin exchange is investigated. Previous studies show that the crossover cannot be detected by the singularity or finite-size analysis of ground-state observables for finite ladders. In this work, we find that the first-excited-state fidelity shows a sudden drop exactly at the crossover point, regardless of the length of the ladder. It suggests that the excited-state fidelity for finite systems is very effective in detecting the crossover in infinite systems. The underlying mechanism is understood through the analysis of re-construction of the low-lying energy levels of the systems.

The singularity of quantum correlation has been widely regarded as a valuable indicator for quantum phase transition (QPT) in low‐dimensional quantum systems. In this paper, in a two‐leg spin ladder with ring exchange, the critical points of the system cannot be detected by analyzing the singularity of the quantum discord (a kind of quantum correlation). Instead, the discord exhibits a novel odd–even effect in the symmetry‐breaking (SB) regions, which can be used to detect the two critical points between the SB regions and the non‐SB regions. The underlying mechanism of the odd–even effect is explained.