Dynamics and entanglement near quantum phase transitions

物质通过发生相变而改变状态。在我们的共同经验中，当温度超过一个阈值时就会发生这种情况，例如水在0°C时变成冰。然而，更奇特的转变可以通过首先将材料冷却到接近绝对零度（- 273°C）的最低温度来实现。在那里，控制电子行为的量子力学定律占据主导地位，由量子效应而不是热效应驱动的跃迁发生。我的研究将针对这种量子相变，它可以通过对材料施加磁场，改变其化学成分或压缩它来实现。例如，一些绝缘材料在非常低的温度下具有磁性，但当通过改变化学成分加入足够的移动电荷载流子时，它们就会变成金属并失去磁性。在这种绝缘体到金属的量子相变附近发生了一种非同寻常的现象：系统变成了超导体，其中电子形成对，没有阻力地移动。这种现象不能用超导的标准理论来解释，并且在材料被加热到绝对零度以上~ 100°C时仍然保持稳健。尽管潜在的机制还没有被很好地理解，但有证据表明，它与系统在磁性绝缘体和金属之间的“混乱”状态下的量子涨落有关。

Matter changes states by undergoing phase transitions. In our common experience, this occurs as the temperature goes through a threshold, such as water turning into ice at 0 °C. However, more exotic transitions can be realized by first cooling a material down to the lowest accessible temperatures, near absolute zero (−273 °C). There the laws of quantum mechanics governing the behavior of the electrons become dominant, and transitions driven by quantum effects instead of thermal ones occur. My research will target such quantum phase transitions, which can be realized by applying a magnetic field to the material, changing its chemical composition, or by compressing it. For example, some insulating materials possess magnetic order at very low temperatures but when enough mobile charge carriers are added by varying the chemical composition, they become metallic and loose their magnetism. An extraordinary phenomenon occurs near this insulator-to-metal quantum phase transition: the system becomes a superconductor in which the electrons form pairs that travel without resistance. This phenomenon cannot be explained with the standard theory of superconductivity, and remains robust as the material is heated up to ~ 100 °C above absolute zero. Although the underlying mechanism is still not well-understood, evidence suggests that it is connected to the quantum fluctuations of the system in its “confused” state between a magnetic insulator and a metal.