Non-Hermitian symmetry breaking and Lee-Yang theory for quantum XYZ and clock models.

Abstract

Lee-Yang theory offers a unifying framework for understanding classical phase transitions and dynamical quantum phase transitions through the analysis of partition functions and Loschmidt echoes. Recently, this framework was extended to characterize quantum phase transitions of quantum Ising models by introducing the concepts of non-Hermitian parity-symmetry breaking and fidelity zeros. Here, we generalize the theory by studying a broad class of quantum models, including the XY, the XXZ, the XYZ, and the Z_{p} clock models in one dimension, subject to a complex magnetic field. For the XY, XXZ, and XYZ models, we find that the complex field breaks parity symmetry and induces oscillations of the ground state between the two parity sectors, giving rise to fidelity zeros within the ordered phases. For the Z_{3} clock model, the complex field splits the real part of the ground-state energy between the neutral sector (q=0) and the charged sectors (q=1,2), while preserving the degeneracy within the charged sector. Fidelity zeros arise only after projecting out one of the charged sectors. For the Z_{4} and Z_{5} clock models, the ground states are instead projected to oscillate between the neutral sector (q=0) and the charged sectors q=2 and q=1, respectively, giving rise to fidelity zeros. Finite-size scaling of these zeros yields critical exponents in full agreement with analytical predictions, demonstrating that this approach is applicable not only to the Ising model with Z_{2} symmetry, but also to more general Heisenberg-type models and systems with higher discrete symmetries.