Superconductor–insulator/metal transition (SMT) as a paradigm of quantum phase transition has been a research highlight over the last three decades. Benefit from recent developments in the fabrication and measurements of two-dimensional (2D) superconducting films and nanodevices, unprecedented quantum phenomena have been revealed in the quantum phase transitions of 2D superconductors. In this review, we introduce the recent progress on quantum phase transitions in 2D superconductors, focusing on the quantum Griffiths singularity (QGS) and anomalous metal state. Characterized by a divergent critical exponent when approaching zero temperature, QGS of SMT is discovered in ultrathin crystalline Ga films and subsequently detected in various 2D superconductors. The universality of QGS indicates the profound influence of quenched disorder on quantum phase transitions. Besides, in a 2D superconducting system, whether a metallic ground state can exist is a long-sought mystery. Early experimental studies indicate an intermediate metallic state in the quantum phase transition of 2D superconductors. Recently, in high-temperature superconducting films with patterned nanopores, a robust anomalous metal state (i.e. quantum metal or Bose metal) has been detected, featured as the saturated resistance in the low temperature regime. Moreover, the charge-2e quantum oscillations are observed in nanopatterned films, indicating the bosonic nature of the anomalous metal state and ending the debate on whether bosons can exist as a metal. The evidences of the anomalous metal states have also been reported in crystalline epitaxial thin films and exfoliated nanoflakes, as well as granular composite films. High quality filters are used in these works to exclude the influence of external high frequency noises in ultralow temperature measurements. The observations of QGS and metallic ground states in 2D superconductors not only reveal the prominent role of quantum fluctuations and dissipations but also provide new perspective to explore quantum phase transitions in superconducting systems.

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二维超导体中的量子相变：近期实验进展综述

超导-绝缘体/金属转变（SMT）作为量子相变的范例一直是过去三十年的研究亮点。受益于二维（2D）超导薄膜和纳米器件制造和测量的最新进展，二维超导体的量子相变揭示了前所未有的量子现象。在这篇综述中，我们介绍了二维超导体中量子相变的最新进展，重点关注量子格里菲斯奇点（QGS）和反常金属态。SMT 的 QGS 特点是接近零温度时临界指数发散，在超薄晶体 Ga 薄膜中被发现，随后在各种二维超导体中也被检测到。QGS 的普遍性表明了猝灭无序对量子相变的深远影响。此外，在二维超导系统中，是否可以存在金属基态也是一个长期以来的谜团。早期的实验研究表明二维超导体的量子相变中存在中间金属态。最近，在具有图案化纳米孔的高温超导薄膜中，检测到了鲁棒的反常金属态（即量子金属或玻色金属），其特征是低温状态下的饱和电阻。此外，电荷2e在纳米图案薄膜中观察到量子振荡，表明异常金属态的玻色子性质，并结束了玻色子是否可以作为金属存在的争论。在晶体外延薄膜和剥离纳米薄片以及颗粒复合薄膜中也报道了异常金属态的证据。这些工作中使用了高质量滤波器，以排除超低温测量中外部高频噪声的影响。对二维超导体中QGS和金属基态的观察不仅揭示了量子涨落和耗散的突出作用，而且为探索超导系统中的量子相变提供了新的视角。