Multifractality and excited-state quantum phase transition in ferromagnetic spin-1 Bose-Einstein condensates

Multifractality of quantum states plays an important role for understanding numerous complex phenomena observed in different branches of physics. The multifractal properties of the eigenstates allow for characterising various phase transitions. In this work, we perform a thorough analysis of the impacts of an excited-state quantum phase transition (ESQPT) on the fractal behavior of both static and dynamical wave functions in a ferromagentic spin-1 Bose-Einstein condensate. By studying the features of the fractal dimensions, we show how the multifractality of eigenstates and time evolved states are affected by the presence of ESQPT. Specifically, the underlying ESQPT leads to a strong localization effect, which in turn enables us to use it as an indicator of ESQPT. We verify the ability of the fractal dimensions to probe the occurrence of ESQPT through a detailed scaling analysis. We also discuss how the ESQPT manifests itself in the fractal dimensions of the long-time averaged state. Our findings further confirm that the multifractal analysis is a powerful tool for studying of phase transitions in quantum many-body systems and also hint an potential application of ESQPTs in the burgeoning field of state preparation engineering.