Discovery of a non-Hermitian phase transition in a bulk condensed-matter system

• URL: http://arxiv.org/abs/2412.16012v1
• Date: Fri, 20 Dec 2024 16:03:45 GMT
• Title: Discovery of a non-Hermitian phase transition in a bulk condensed-matter system
• Authors: Jingwen Li, Michael Turaev, Masakazu Matsubara, Kristin Kliemt, Cornelius Krellner, Shovon Pal, Manfred Fiebig, Johann Kroha,
• Abstract summary: We realize a non-Hermitian phase transition in a bulk condensed-matter system. In a temperature-dependent interplay with the Hermitian transition to ferromagnetic order, a non-Hermitian change of the relaxation dynamics occurs. Our theory predicts non-Hermitian phase transitions for a large class of condensed-matter systems.
• Score: 1.4436722857763085
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• Abstract: Phase transitions are fundamental in nature. A small parameter change near a critical point leads to a qualitative change in system properties. Across a regular phase transition, the system remains in thermal equilibrium and, therefore, experiences a change of static properties, like the emergence of a magnetisation upon cooling a ferromagnet below the Curie temperature. When driving a system far from equilibrium, novel, otherwise inaccessible quantum states of matter may arise. Such states are typically non-Hermitian, that is, their dynamics break time-reversal symmetry, a basic law of equilibrium physics. Phase transitions in non-Hermitian systems are of fundamentally new nature in that the dynamical behaviour rather than static properties may undergo a qualitative change at a critical, here called exceptional point. Here we experimentally realize a non-Hermitian phase transition in a bulk condensed-matter system. Optical excitation creates charge carriers in the ferromagnetic semiconductor EuO. In a temperature-dependent interplay with the Hermitian transition to ferromagnetic order, a non-Hermitian change of the relaxation dynamics occurs, manifesting in our time-resolved reflection data as a transition from bi-exponential real to single-exponential complex decay. Our theory models this behavior and predicts non-Hermitian phase transitions for a large class of condensed-matter systems, where they may be exploited to sensitively control bulk-dynamic properties.

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