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In general, low-dimensional metallic oxides exhibit three kinds of electronic instabilities at low temperatures such as charge density waves (CDW), spin density waves (SDW) as well as superconductivity (SC).
The CDW phases observed in molybdenum as well as in tungsten bronzes have been generally explained, within the context of the hidden nesting model. A CDW takes place when a low-dimensional metal reduces its electronic energy by opening a gap, as a consequence of a structural rearrangement. This situation is energetically favorable only in low-dimensional systems and closely related to the anisotropy of the Fermi Surface (FS). The CDW, is characterized then by a structural modulation, which should be associated with a FS that present enough nesting, i.e., large parallel portions of the metal FS connected by such vector. In this scenario the electron-phonon coupling is strong enough to stabilize a CDW, leading to a Peierls distortion. In the hidden nesting model, the FS of the metallic phase of a low-dimensional compound can be decomposed in different one-dimensional (1D) Fermi surface sheets, formed by parallel lines that exhibit a large nesting. In real space this decomposition corresponds to identifying three sets of zig-za chains, forming three electronic sub-systems with quasi-1D character.
As it, was demonstrated the last decade, angle resolved photoemission spectroscopy (ARPES) is one of the most valuable technique to explore the topology of the FS, the location, symmetry and size of the CDW gap and nature of the transition, as seen in many bronzes and high temperature superconductors. ARPES also provides a very useful insight on the single particle spectral function, as the ARPES line shapes is able to provide detailed information about the nature of the single particle excitations of the system.
In this contribution, a comprehensive photoemission spectroscopy study of several systems, which have showed a CDW transformation will be presented. The electronic structure near the Fermi Level as well as the topology of the FS extended over several brillouin Zones(BZ) at the normal metallic and the CDW states will be compared with the last results of theoretical calculations.
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