标题： 在体凝聚态系统中发现非厄米相变 显示英文标题

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

摘要： 相变在自然界中是基本的。 在临界点附近的小参数变化会导致系统性质的定性变化。 在常规相变中，系统保持热平衡，因此经历静态性质的变化，例如在低于居里温度冷却铁磁体时磁化的出现。 当将系统驱动远离平衡时，可能会出现新颖的、 otherwise 不可访问的量子物质状态。 这些状态通常是非厄米的，即它们的动力学破坏时间反演对称性，这是平衡物理中的基本定律。 非厄米系统中的相变具有根本性的新性质，因为在临界点，即所谓的异常点，可能发生动力学行为而非静态性质的定性变化。 在这里，我们实验实现了体凝聚态系统中的非厄米相变。 光学激发在铁磁半导体EuO中产生载流子。 在与厄米向铁磁序的温度依赖相互作用中，发生非厄米的弛豫动力学变化，在我们的时分辨反射数据中表现为从双指数实衰减到单指数复衰减的转变。 我们的理论模型对此行为进行了建模，并预测了大量凝聚态系统中的非厄米相变，其中它们可以用来敏感地控制体动态特性。 显示英文摘要

摘要： 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.