Charge-Density-Wave Order in 2H-NbSe${}_2$

Competition between collective states like charge density wave and superconductivity is played out in some of the transition metal dichalcogenides unencumbered by the spin degrees of freedom. Although 2H-NbSe${}_2$ has received much less attention than some of the other members of the family (like 1T-TiSe${}_2$ and 2H-TaSe${}_2$), it shows superconductivity at 7.2 K and incommensurate charge ordering at 33 K. Recent experiments, notably Angle Resolved Photoemission spectroscopy, have cast serious doubts on the mechanism of Fermi surface nesting via electron-phonon interaction. The normal state has been found to be a poor, incoherent metal and remarkably, the coherence increases in the broken symmetry state. From a preformed excitonic liquid scenario, we show that there exists a natural understanding of the experimental data on 2H-NbSe${}_2$ based on electron-electron interaction. The collective instabilities, in this scenario, are viewed as a condensation of an incoherent excitonic liquid already present at high temperature.