Abstract
At present, the aim of superconductivity research is still expected to provide directions and a theoretical framework for the search for new superconductors, even high-temperature superconductors. Given the difficulty of discovering new superconductors, superconductivity research has focused on the alterations of existing of superconductors and the phases coexisting with superconductivity, such as a charge-density wave (CDW) state. The relationship between superconductivity and CDW has been an important research topic for the detailed understanding of superconductivity. In 2H polytypes of transition metal dichalcogenides (TMDs), the natural CDW and superconductivity coexist without any magnetic order, which provides an ideal platform to further explore the interplay between superconductivity and the CDW state. This thesis presents a systematic study of the 2H-TaS2 system with the introduction of external atoms and disorder to investigate the CDW state, superconductivity, and their interplay. In the lithium-intercalated 2H-TaS2 system, the intercalated lithium can tip the delicate balance between superconductivity and the CDW state, revealing the underlying interactions that give rise to them. The formation of superconductivity and the CDW state has been investigated by electrical and thermal transport properties, in which the CDW formation temperature is continuously suppressed, and the transition temperature of superconductivity (Tc) increases with increasing lithium intercalation, indicating that superconductivity and CDW compete with each other in lithium-intercalated 2H-TaS2. Furthermore, the electronic contribution to the specific heat and Hall resistivity data further demonstrate that the CDW weakens with lithium intercalation to indirectly increase the charge carrier density and boost superconductivity.
In the hydrated 2H-Lix(H2O)yTaS2 system, the intercalated lithium can absorb water into the interlayer of 2H-LixTaS2. The amount of water is related to the content of the interlayer lithium and remains constant when the lithium content is within a certain range, accompanied by the complete suppression of the CDW state. The superconductivity has been studied by electrical and thermal transport and magnetic properties, showing a dome-shaped dependence on the lithium content x. The Debye temperatures ΘD, the electron-phonon coupling λep and the electron density of states at the Fermi level (DOS(EF)), estimated from the heat capacity data, show a close relationship with the superconductivity, indicating that the lithium intercalation tunes Tc by changing the DOS(EF) in 2H-Lix(H2O)yTaS2, but the simultaneous changes of λep and ΘD may also play a certain role.
In the disordered 2H-TaS2-x system, the level of disorder can be well controlled by the number of structural defects induced by sulphur-vacancies, which influence the evolution of the long- range CDW and superconductivity. Measurements of complementary magnetization, electronic and thermal transport properties show that the long-range CDW is continuously suppressed, leading to strange-metal behaviour with linear resistivity at the endpoint of the long-range CDW, which is accompanied by the emergence of a short-range CDW phase. The superconductivity shows at first a two-step-like behaviour but reaches a maximum at the endpoint of long-range CDW with a single homogeneous phase, suggesting an interplay between superconductivity and CDW order. Moreover, the results suggest that the observed strange-metal behaviour, which could arise from the short-range charge density fluctuations, is a signature of quantum criticality with Planckian dissipation.
Liu, Huanlong