•Editorial• December 2018 Vol. 61 No. 12: 127401 Special Topic: Superconductivity and Magnetism in Transition-Metal Compounds https://doi.org/10.1007/s11433-018-9306-0 Preface to the topical issue Transition metals contain d-orbital electrons, and compounds based on transition metals often exhibit a duality of magnetism and superconductivity. To date, a large number of interesting physics phenomena exhibited by transition-metal compounds have been discovered; for example, high-temperature superconductivity has been found to emerge in compounds such as copper-oxide (cuprate) compounds, iron pnictides/chalcogenides. Understanding the relationship between superconductivity and magnetism is essential for realizing these unconventional superconductors. In this topic issue of SCIENCE CHINA Physics, Mechanics & Astronomy, we collect eight articles that discuss some of the frontier studies that have been conducted in the field of unconventional superconductors. First, Bhattacharyya et al. [1] from the ISIS Pulsed Neutron and Muon Facility in the United Kingdom present a brief review on two unconventional superconducting systems—the iron- and chromium-based superconductors—on the basis of muon spin relaxation/rotation (μSR) measurements. This review provides insight into the superconducting gap structures in this material and explains the role of antiferromagnetic spin fluctuations. The next two pieces of work are regarding the recent progress in the field of cuprate superconductors. Through angle-resolved photoemission spectroscopy studies, Ding’s group [2] at the Institute of Physics (IOP), Chinese Academy of Sciences (CAS) in China reports the band parameters of the cuprate superconductor bismuth (Bi)-2212 over a wide range of doping levels. Scanning tunneling spectroscopy studies conducted by Yin’s group [3] at Zhejiang University, China focus on the effects of differently positioned oxygen dopants on the local electronic properties of two types of Bi-based cuprate families: Bi-2212 and Bi-2201. We feature three research articles regarding iron-based superconductors. Cao’s group [4] at Zhejiang University in China targets the coexistence of and interplay between superconductivity and possible ferromagnetism in the compound Eu(Fe 0.96 Ni 0.04 )As 2 , which contains europium (Eu), iron (Fe), nickel (Ni), and arsenic (As). The next article is authored by Shi’s group [5] of Southeast University in China, who focus on the basic parameters of vortex dynamics and the vortex phase diagram in a recently discovered iron-based 112-type superconductor. The third article, by Mu’s group [6] from the Shanghai Institute of Microsystem and Information Technology, CAS in China, details the growth of high-quality, single-crystal samples of calcium, iron, arsenic, and fluorine (CaFeAsF) and the measurement of high-field resistivity. Upon applying a high magnetic field, they observe an insulator-metal transition, and through a scaling analysis, they conclude that a quantum phase transition is induced in this material by tuning the magnetic field. Finally, we present two papers that explore new types of superconductors or strongly correlated materials. First, Wu and Luo [7] of IOP, CAS report the growth of new types of FeAs-based crystals—the cerium (Ce)-containing Ce 12 Fe 57.5 As 41 and lanthanum (La)-containing La 12 Fe 57.5 As 41 —and the discovery of multiple magnetic transitions in these correlated materials. Next, another group at IOP, CAS that is led by Prof. Ren [8] reports the discovery of superconductivity in a compound containing lanthanum, palladium (Pd), and bismuth, LaPd 2 Bi 2 . All studies collected in this special topic issue strongly suggest that the d-orbital electrons in related compounds illustrate the rich physics concerning electron localization and itinerancy, which lead to the emergence of cooperative interactions and phenomena. Hai-Hu Wen 1 , HuiQiu Yuan 2 , and ShiPing Feng 3 1 Nanjing University, Nanjing 210093, China; 2 Zhejiang University, Hangzhou 310058, China; 3 Beijing Normal University, Beijing 100875, China © Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018 phys.scichina.com link.springer.com SCIENCE CHINA Physics, Mechanics & Astronomy