Hidden order in a charge density wave system

Rare-earth tritellurides (RTe₃) are a series of compounds that features multiple charge density wave (CDW) behaviors. They share a similar orthorhombic crystal structure, consisting of double layers of Te square net sheets, separated by a RTe slab. Although there are some reports that emphasized the role of momentum-dependent electron-phonon coupling, the system has been recognized as a prototype CDW system exhibiting Fermi surface nesting, originating from the parallel Fermi surface sheets dominated by the Te and orbitals in the square nets. With lighter rare-earth elements, the system shows a unidirectional CDW along the axis, while with heavier elements, an additional CDW order that is orthogonal to the first one forms at a lower temperature. A recent Raman study has shown that there is a hidden axial Higgs mode when the system crosses the first CDW transition. Here we present a study on the RTe₃ (R=La, Tb, Er) systems to probe the broken symmetries as measured from the electronic structure to understand the microscopic origin of the axial Higgs mode. We observed a clear mirror symmetry breaking relative to the and axes on the Fermi surface and studied the temperature dependence of the ferro-axial order across the CDW transitions. Further we will discuss the order response under an external uniaxial strain.