Uncovering the Hidden Ferroaxial Density Wave as the Origin of the Axial Higgs Mode in RTe₃

The recent discovery of an axial amplitude (Higgs) mode in the long-studied charge density wave (CDW) systems GdTe₃ and LaTe₃ suggests a heretofore unidentified hidden order. A theoretical study proposed that the axial Higgs results from a hidden ferroaxial component of the CDW, which could arise from non-trivial orbital texture. Here, we report extensive experimental studies on RTe₃ (R = rare-earth) to answer the question: What is the true nature of unconventional CDW and the microscopic physics producing the Axial Higgs mode? Combining Raman spectroscopy with large-angle convergent beam electron diffraction (LACBED), rotational anisotropy second-harmonic generation (RA-SHG), and muon-spin relaxation (μSR), we provide unambiguous evidence that the Axial Higgs mode originates from a previously hidden, ferroaxial, unconventional density wave that exists throughout the whole RTe₃ series. Thus, our study provides a new standard for uncovering unconventional orders and confirms the power of Higgs modes to reveal them.