Quantum Griffiths singularity (QGS) reveals the profound influence of quenched disorder on superconductor-metal quantum transitions. The main characteristic of QGS is the divergent critical exponent zν when approaching the quantum critical point. We firstly observed QGS in Ga films [1] and subsequently it was detected in various 2D superconductors [2-5] under perpendicular magnetic field, indicating the universality of QGS. The studies on QGS in diverse 2D superconductors shed new light on the superconductor-metal transitions and may change the paradigm of quantum phase transitions. After decades of explorations, suffering from the subtle nature, whether a metallic ground state exists in a 2D superconducting system is a mystery. Our findings [6-8] offer the reliable evidences on the existence of anomalous quantum metallic ground states dominated by bosons (Cooper pairs) in 2D superconducting systems and will stimulate further investigations on the origin of this intriguing phenomenon.

The pair density wave (PDW) is an extraordinary superconducting state where Cooper pairs carry nonzero momentum. Benefited from the successful growth of monolayer iron-based high-Tc Fe(Te,Se) films on SrTiO3(001) substrates [9-12], we report the discovery of the emergent PDW state on domain walls along Fe-Fe direction by using scanning tunneling microscopy/spectroscopy [13]. The PDW state with a period of λ ~ 3.6a_Fe (a_Fe is the distance between neighboring Fe atoms) is observed at the domain walls by the spatial electronic modulations of the local density of states, superconducting coherence peak height and gap energy. Moreover, the π-phase shift boundaries of the PDW state are observed near the vortices of the induced secondary charge density wave state, further demonstrating that the PDW state at the domain wall is a primary state. The discovery of the primary PDW state in the monolayer Fe(Te,Se) film provides a low-dimensional platform to study the PDW state and its interplay with the topological electronic states and unconventional high-Tc superconductivity.
 

[1] Y. Xing et al., Science 350, 542-545 (2015).
[2] S. C. Shen et al., Phys. Rev. B 94, 144517 (2016).
[3] Y. Xing et al., Nano Lett. 17, 6802-6807 (2017).
[4] Y. Liu et al., Nat. Commun. 10, 3633 (2019).
[5] Y. Liu et al., Phys. Rev. Lett. 127, 137001 (2021).
[6] C. Yang et al., Science 366, 1505-1509 (2019).
[7] Y. Liu et al., Nano Lett. 20, 5728-5734 (2020).
[8] Y. Xing et al., Nano Lett. 21, 7486-7494 (2021).
[9] C. Liu et al., Sci. Adv. 6, eaax7547 (2020).
[10] C. Chen et al., Nat. Phys. 16, 536 (2020).
[11] C. Chen et al., Nano Lett. 20, 2056 (2020).
[12] Y. Li et al., Nano Lett. 23, 140 (2023).
[13] Y. Liu et al., Nature 618, 934-939 (2023).

Dr. Jian Wang is a Boya Distinguished Professor of Physics at Peking University. He developed two laboratories at Peking University to perform ultralow temperature-high magnetic field transport measurements and low temperature scanning tunneling microscopy/spectroscopy-molecular beam epitaxy investigations. Dr. Jian Wang’s current research interests are quantum transport properties of low dimensional superconductors and topological materials. He has discovered a series of emergent quantum phase transitions and quantum states in both low dimensional superconductors and topological materials, including quantum Griffiths singularity, bosonic anomalous (quantum) metal state, topological zero energy bound states and pair density wave states in two dimensional superconductors, as well as unconventional interface superconductivity, log-periodic quantum oscillations, high Chern number and high temperature Chern insulator states in topological materials. Dr. Jian Wang authored more than 130 papers in Science, Nature, Nature Physics, Nature Materials, Nature Nanotechnology, Science Advances, Nature Communications, PNAS, Physical Review X, Physical Review Letters, etc. He has given more than 100 invited talks and was awarded Sir Martin Wood China Prize in 2015, Changjiang Distinguished Professor of China's Ministry of Education in 2016, Outstanding Achievement Award for Research in Institutes of Higher Education of China (Young Scientists) in 2019, and the Achievement in Asia Award (Robert T. Poe Prize) in 2022. Homepage: https://faculty.pku.edu.cn/JianWangGroup

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