A pseudo-goldstone mode in the exactly solvable model of phase transitions

Yu. M. Ivanchenko, Alexander A. Lisyansky, Alexander E. Filippov

Yu. M. Ivanchenko, Alexander A. Lisyansky, Alexander E. Filippov

Valerij V. Bryksin, Yu. A. Firsov, G. Ya. Yashin

V. V. Gorunovich

Yang-yang Tan, Yongrui Chen, Wei‐jie Fu +1 more

Real-time dynamics of strongly correlated systems, in particular its critical dynamics near phase transitions, have been always on the cutting edge of studies in diverse fields of physics, e.g., high energy physics, condensed matter, holography, etc. In this work, we investigate the critical damping of collective modes associated with spontaneous breaking of approximate symmetries, which are called pseudo-Goldstone modes, in strongly correlated systems. Using the Schwinger-Keldysh field theory, we find a universal pseudo-Goldstone damping via the critical O(N) model that has never been found before by other approaches. Different from the conventional damping found in holography and hydrodynamics, the new one is controlled by critical fluctuations, hence is invisible in mean-field systems or strongly correlated systems with classical gravity duals. Since the critical damping depends solely on the universalities of the critical point, irrespective of the microscopic details, our conclusion should be applicable to a wide class of interacting systems. Strongly correlated systems exhibit universal dynamics near phase transitions, from particle to condensed matter physics. However, despite the successes of various theoretical approaches, systematic treatments of fluctuations are scarce. Here the authors unveil a universal damping of pseudo-Goldstone modes in systems with approximate symmetries, advancing our understanding of near-critical dynamics in several contexts.

Myung-Joong Hwang, Martin Bodo Plenio

Phase transitions are commonly held to occur only in the thermodynamical limit of a large number of system components. Here, we exemplify at the hand of the exactly solvable Jaynes-Cummings (JC) model and its generalization to finite JC lattices that finite component systems of coupled spins and bosons may exhibit quantum phase transitions (QPTs). For the JC model we find a continuous symmetry-breaking QPT, a photonic condensate with a macroscopic occupation as the ground state, and a Goldstone mode as a low-energy excitation. For the two site JC lattice we show analytically that it undergoes a Mott-insulator to superfluid QPT. We identify as the underlying principle of the emergence of finite system QPTs the combination of increasing atomic energy and increasing interaction strength between the atom and the bosonic mode, which allows for the exploration of an increasingly large portion of the infinite dimensional Hilbert space of the bosonic mode. This suggests that finite system phase transitions will be present in a broad range of physical systems.