Abstract

Itinerant electrons are both very sensitive to and prone to cause different ordered states. These include charge order like a charge-density-wave (CDW) state, Mott metal-insulator transitions, and superconductivity. Intriguingly, these different ordered states arise from the same fundamental interactions in metals. For instance, in the presence of strong electron-phonon interaction, superconductivity gaps out all conduction electrons. Similarly, a combination of electron-phonon interaction and Fermi-surface nesting leads to CDW order with the gapping out of some electronic states. Thus, both superconductivity and CDW order are linked to the number of electrons involved and often also linked to the strength of the same underlying interaction, e.g. electron-phonon interaction. Consequently, one can expect a competition. However, the modifications to the electronic structure and phonon spectrum of the CDW order may also give rise to promotion of superconductivity. Here, I discuss recent results on the Fermi surface reconstruction as well as the interplay with as well as the interplay with superconductivity in NbSe2, TiSe2, and YBa2Cu3O7−δ.