Interplay of charge density waves, disorder, and superconductivity in 2$H$-TaSe$_2$ elucidated by NMR

TL;DR

This study uses $^{77}$Se NMR to explore how charge density waves, disorder, and superconductivity interact in 2H-TaSe$_2$, revealing local lattice distortions, partial Fermi surface gaps, and pseudogap behavior related to CDW and superconductivity.

Contribution

It provides new insights into the local lattice distortions and pseudogap phenomena in 2H-TaSe$_2$, highlighting the role of CDW fluctuations in superconductivity.

Findings

Line narrowing indicates correlated local lattice distortions above CDW transition.

Partial Fermi surface gap opens during incommensurate CDW transition.

Pseudogap behavior in spin excitations is enhanced by superconductivity.

Abstract

Single crystals of pristine and 6% Pd-intercalated 2H-TaSe$_2$ have been studied by means of $^{77}$Se nuclear magnetic resonance (NMR). The temperature dependence of the $^{77}$Se spectrum, with an unexpected line narrowing upon Pd intercalation, unravels the presence of correlated local lattice distortions far above the transition temperature of the charge density wave (CDW) order, thereby supporting a strong-coupling CDW mechanism in 2H-TaSe$_2$. While, the Knight shift data suggest that the incommensurate CDW transition involves a partial Fermi surface gap opening. As for spin dynamics, the $^{77}$Se spin-lattice relaxation rate $T_1^{-1}$ as a function of temperature shows that a pseudogap behavior dominates the low-energy spin excitations even within the CDW phase, and gets stronger along with superconductivity in the Pd-6% sample. We discuss that CDW fluctuations may be responsible for the pseudogap as well as superconductivity, although the two phenomena are unlikely to be directly linked each other.