The colour of charge density wave order

Charge density wave (CDW) orders are ubiquitous in the transition metal dichalcogenide (TMD) system, where the CDW orders could be quite different even though the host materials have the same structure and symmetry. This thesis focuses on the CDW orders in two TMD materials, whose temperature evolutions of the electronic structures are obtained by infrared spectroscopy.
1T-VSe₂ hosts a weakly-coupled CDW phase. The rearrangement of the spectral weight is observed within the low energy range. The Drude response present at high temperature is suppressed during the CDW transition, while a new interband response appears around 100 meV. The spectral weight redistribution shows that part of the spectral weight is transferred to the collective excitation of the CDW order, which is also supported by the asymptotic behaviour of the optical functions. The IR-active collective mode of the CDW, the sliding mode, was rarely reported in TMD materials before.
For comparison, 1T-TaS₂-_xSe_x hosts strongly-coupled CDW orders with non-equilibrium features. Two doping levels x = 0.8 and 1.0 are investigated. We obtain the phase diagram of TaS₂-_xSe_x containing an island of the meta-stable CDW phase induced by rapid cooling. Within this island, the optical response shows a bulk pseudogap of 200 meV. The stable commensurate CDW is obtained by heating from the meta-stable phase, where an optical gap of 100 meV is opened. Our data suggest that the formation of the meta-stable phase is accompanied by the appearance of charged defects as domain walls and the absence of long-range commensurability.