Exploration of exotic quantum phase transitions has always been a focus in condensed matter physics. Critical phenomenon in a phase transition is entirely determined by the universality class, which is controlled by the spatial and/or order-parameters and is independent of microscopic details. The quantum phase transition is a class of phase transitions that occurs due to quantum fluctuations, tuned by certain parameters of the system at the zero-temperature limit. The superconductor-insulator/metal phase transition is a classic example of quantum phase transition, which has been intensely studied for more than 40 years. Disorder is considered one of the most important influencing factors, and therefore has received widespread attention. During the phase transitions, the system usually satisfies scaling invariance, so the universality class will be characterized by a single critical exponent. In contrast, the peculiarity of quantum Griffith singularity is that it breaks the traditional scaling invariance, where exotic physics emerges.

The physics of Griffiths singularity dates back to 1969, when American physicist Griffiths proposed a type of phase transitions in which the scaling invariance is broken. In this case, the critical exponent tends to diverge rather than remain constant. The quantum Griffith singularity refers to the Griffith singularity in a quantum phase transition. Since the proposal of quantum Griffith singularity, it has only been observed in conventional low-dimensional superconducting films and in a few three-dimensional ferromagnets. The existence of quantum Griffith singularity in three-dimensional superconductors and in unconventional high-temperature superconductors has yet to be confirmed experimentally. Such confirmation will shed light on the understanding of mechanisms in unconventional high-temperature superconductivity.

Recently, a research group led by Jian-Hao Chen, a researcher at the International Center for Quantum Materials at the School of Physics at Peking University, the Beijing Academy of Quantum Information Sciences, and the Key Laboratory for the Physics and Chemistry of Nanodevices of Peking University, conducted a study on the quantum Griffith singularity in unconventional high-temperature superconducting bulk single crystal CaFe_1-xNi_xAsF. They and their collaborators grew a series of high-quality underdoped CaFe_1-xNi_xAsF bulk single crystals for the first time, and observed the evolution of quasi two-dimensional to three-dimensional anisotropic quantum Griffith singularities in the superconductor-metal phase transitions driven by magnetic fields. They found robust quantum Griffith singularity which can last up to 5.3 K, and it can be induced in the crystals by both parallel and vertical magnetic fields. This study not only reveals the universality of quantum Griffith singularity in three-dimensional and unconventional high-temperature superconducting systems, but also predicts the possibility of finding quantum Griffith states in more unconventional high-temperature superconducting families (i.e., nickel-based and copper-based superconductors). The findings were published in National Science Review in 2024, titled "Three-dimensional quantum Griffiths singularity in bulk iron-pnictide superconductors". Jian-Hao Chen researcher was the corresponding author of the article, and the co-first authors include Shao-Bo Liu, a postdoctoral fellow, Congkuan Tian, a Ph.D. student, both from the International Center for Quantum Materials at the School of Physics at Peking University, and Yongqing Cai, an assistant professor at the School of Physics at Dalian University of Technology.

Figure 1：Magnetic-field-driven superconductor-metal phase transition with multiple quantum critical points in CaFe_1-xNi_xAsF.

Figure 2：B–T phase diagram of quantum Griffiths singularity in a three-dimensional anisotropic superconductor.

Link: https://doi.org/10.1093/nsr/nwae220