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Spectroscopy in broken symmetry ground states


Electronic instabilities are at the origin of phenomena as diverse as the formation of spin and charge density waves (SDW and CDW) or superconductivity, the interplay of which is among the most intriguing open questions of modern solid state physics. Besides determining the ground state of the quantum system in which they occur, electronic instabilities also fundamentally affect its excitation spectrum. In particular, they induce new types of collective behavior, the investigation of which can be used to identify the kind of electronic order involved. In the CDW state, on which I focus here, a gap opens up in the single particle spectrum, and two new collective modes, associated with the oscillations of the amplitude and of the phase of the CDW, respectively, appear. The paradigm of CDW forming materials are the quasi one-dimensional compounds. But electronically driven CDW states were also found and thoroughly investigated in novel two-dimensional (2D) layered compounds, an effort motivated in part by the fact that high temperature superconductivity in the copper-oxide systems may indeed emerge from a peculiar charge-ordering through the tuning of relevant parameters. In this respect, a family of layered compounds which have attracted a lot of attention recently are the rare-earth ( R ) tri-tellurides R Te 3 . They host an unidirectional, incommensurate CDW already well above room temperature for all R elements lighter than Dy, while in the heavy rare-earth tri-tellurides (i.e., R =Tm, Er, Ho, Dy) the corresponding transition temperature, T CDW1 , lies below 300 K and decreases with increasing R mass. In the latter systems, a further transition to a bidirectional CDW state occurs at T CDW2 , ranging from 180 K for TmTe 3 to 50 K for DyTe 3 . In my talk I will present a large wealth of data collected with different spectroscopic methods, as x-ray diffraction, and infrared and Raman spectroscopy as a function of both temperature and externally applied pressure. I will specifically address the above issues from the perspective of their impact on the electronic properties and lattice dynamics.