Topological nodal line phonons: Recent advances in materials realization

The topological phenomenon has been extensively studied in condensed matter physics and has attracted much attention in many different fields. Like electrons, phonons can also be studied using critical theorems and topology concepts, giving impetus to the birth of topological phonons. Among the topological phonons, the topological nodal line phonons in crystalline materials have emerged as a new area of rapid development in both experiment and theory. Researchers have been hunting for realistic materials with nodal line phonons for the last four years. To date, however, a timely review of recent advances in topological nodal line phonons, and especially their material realization, is still lacking. Considering the classification of the nodal line states, in this review, we will first review the identification of the materials hosting the nodal line phonons with different degeneracies, different types of dispersion, and different geometric shapes in theory. Moreover, we will review and discuss the reasons for the appearance of the nodal line phonons from the viewpoint of symmetry. Second, we will review the case where the nodal line appears in the projected surface phonon states instead of the bulk phonon states. Third, we will review the experimental material realization of nodal line phonons. Finally, we will present a summary, outlook, and additional notes. We believe that this review will contribute to a more advanced understanding of topological nodal line phonons in solid materials and will offer new perspectives on this rapidly expanding field.