The dynamics of correlated two-dimensional materials: the 2H-TaSe₂ case

Two-dimensional (2D) layered transition-metal dichalcogenides (like, e.g. 1T-TaS₂ and 2H-TaSe₂) exhibit a range of CDW transitions. The pronounced quasi-2D nature of the crystallographic structure leads to strongly anisotropic physical properties and approximately cylindrical Fermi surfaces. Obviously, the Fermi surface-driven instabilities are generally weaker in 2D than in 1D systems. Nevertheless, under particular nesting conditions, or as a result of saddle-point singularities, the electronic susceptibility can be sufficiently enhanced for a CDW to develop. Our goal is to investigate the dynamics of the charge excitation spectrum in connection with the transport properties of 2H-TaSe₂, which undergoes a second-order transition to an incommensurate triple CDW at 122 K, followed by a first order lock-in transition to a 3x3 commensurate phase at 90 K. The main purpose of our study is to investigate the electrodynamic response of 2H-TaSe₂, in order to particularly emphasize the low energy scales associated with the CDW-induced phase transitions (Fig. 3.2). The motivation is twofold. First, the mechanism for the CDW formation is not yet well understood. Second, 2H-TaSe₂ can serve as a model system for understanding the still puzzling data of other highly correlated electronic systems, such as the high-T_c superconducting cuprates. For this purpose, 2H-TaSe₂ is indeed an excellent material, because it shares many physical properties with the high-T_c cuprates: like the layered-like structure and qualitatively similar resistivity and susceptibility.

We have also presented an optical study of 2H-TaSe₂ along the less conducting c-axis. This dichalcogenide compound belongs to a large class of conductors called "bad metals" (with mean free path smaller than the lattice constant along the c-axis), which also includes the superconducting cuprates. The optical response shows the progressive development of a pseudogap-like feature with decreasing temperature. (Fig. 3.3) The spectral weight lost by the opening of such a pseudogap goes into the narrow Drude component, developing at low frequency and temperature. (Fig. 3.4) There is no violation of the sum rule in 2H-TaSe₂ contrary to the cuprates.