Superconductivity and charge density wave in transition metal chalcogenides: A first principle study

The transition metal chalcogenides NbTe4 has both a superconducting phase and a charge density wave (CDW) phase. It is very important to understand the relationship between the two phases, say, whether they are cooperating or competing with each other. Fortunately, these two quantum orders depend on the crystal structure and the electronic structure, which can be fine-tuned neatly by applying a pressure. Based on density functional theory, we find that the CDW order and the superconducting order respond to the pressure rather differently. Some unique properties of the CDW phase and its relationship with the superconducting phase of NbTe4 are revealed by increasing the pressure up to 80 GPa. The phonon dispersion of NbTe4 shows that there are Kohn anomalies at three different wavevectors. It seems like one of the Kohn anomalies is originated from the Fermi surface nesting, while the other two are coming from the electron-phonon interaction. The critical behave of the CDW order agrees well with the mean field theory. Importantly, the fitted critical temperature of phase transition is reduced while compressing and CDW instability disappears at room temperature above 8 GPa, indicating that the CDW instability is suppressed by the pressure effect. Eventually, at 80 GPa, the superconducting gap of NbTe4 is closed near 2.4 K, which is also in good agreements with recent experiment. The fact that NbTe4 starts to show superconductivity after the CDW instability has been suppressed suggests that the superconducting order and CDW order in NbTe4 could be competing with each other.