Renxin Xu's focus is compressed baryonic matter, such as pulsars especially, in astrophysics. Baryonic nuclei in the daily life are forbidden to fuse by compression due to the Coulomb repulse; nevertheless, it is usually unexpected in extraterrestrial extreme-environments: the gravity in a core of massive evolved star is so strong that all the other forces (including the Coulomb one) could be negligible. Compressed baryonic matter is then produced after supernova, manifesting itself as pulsars observed. The study of this compressed baryonic matter can not only be meaningful in fundamental physics (e.g., the fundamental strong interaction at low-energy scale, testing gravity theories, detecting nano-Hertz background gravitational waves), but has also profound implications in engineering applications (including time standard and navigation), and additionally, is focused by international as well as Chinese advanced facilities, either terrestrial or in space.
What's the state of gravity-compressed matter produced during supernova? Unfortunately/fortunately, this remains unknown, to be relevant to the nature of non-perturbative QCD (quantum chromo-dynamics). Historically, in 1930s, Lev Landau speculated that dense matter at supra-nuclear density in stellar cores could be considered as gigantic nuclei (the prototype of standard model of neutron star). A giant nucleus should be neutron-rich (renamed neutron star thus) via neutronization. However, Renxin Xu and his collaborators proposed a very different point of view: the rump left behind after a supernova could be a strangeon star. Nucleon is the constituent of a nucleus, while strangeon is named as the constituent of a gigantic nucleus if three flavor symmetry (u, d and s) restores there. They are both developing the strangeon star model and expecting to test it by further observations.
Besides pulsar-like compact objects, strangeon matter could also be manifested in the form of cosmic rays and even dark matter. Strangeon nuggets, with kinematic energy order of EeV, could be produced in the ejecta of merging strangeon stars, mixing in the ingredients of ultra-high energy cosmic ray. The nugget could possibly be left behind after the cosmic QCD-phase transition, as a candidate of dark matter.