High pressure studies of superconductivity and anomalous normal states in novel quantum materials
This dissertation contains a wide range of studies of many intriguing physical systems, including a novel incommensurate host-guest structure, strange metal, low-dimensional system, iron-based superconductivity, and a geometrically frustrated magnetic system. We used pressure as the primary tool in our studies, not only as one of the tuning parameters for accessing or tuning away from the quantum critical point but also to enter novel phases that cannot usually be found in any materials at ambient conditions.
Sb (Chapter 4): High-pressure phase Sb-II exhibits a novel incommensurate host-guest structure, giving rise to exotic sliding mode between host and guest chain. In principle, this sliding mode has a very flat dispersion relation perpendicular to the chain, enhancing electron-phonon coupling greatly. This unusual phonon spectrum causes the normal-state resistivity at low temperature to be linear, as previously observed in Bi-III phase. However, it is not the case in Sb-II as it shows a quadratic Fermi liquid-like relation, suggesting the pinning of the phason mode. The phason-pinning is supported by the estimation of electron- phason coupling parameter λ~0.18, which is surprisingly small. Furthermore, we observed an anomalous first-order transition at a high temperature in resistivity. Combining our results with the experiments from other literature that the interaction between host and guest chain is very strong in Sb-II, we propose that Sb-II may be the first material ever to exhibit Aubry’s transition.