High pressure studies of superconductivity and anomalous normal states in novel quantum materials

Abstract

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.

Ca$_2$RuO$_4$ (Chapter 5): Calcium ruthenate has a long history of studies due to its similarity in structure to cuprates. There are series of transitions in the crystal structure and magnetic ground state under pressure, going from an antiferromagnetic Mott insulator, itinerant ferro- magnetic, to unconventional superconductivity. Our experiment has revealed a possibility of a new phase in Ca$_2$RuO$_4$ . The most prominent features that we observed are (i) a cross-over from the power-law exponent n = 4/3 to n = 1 in resistivity from below and above 100 kbar (ii) a coexistence between a magnetic ordering and superconductivity. We interpret this to be a cross-over between itinerant ferromagnetic state and itinerant antiferromagnetic state, or more general, a spin texture state.

YFe$_2$Ge$_2$ (Chapter 6): The anomaly in the low-temperature resistivity power-law exponent of the iron-based superconductor YFe$_2$Ge$_2$ has posed a question since its discovery. This work aims to answer this question. We found that the application of pressure can completely suppress superconductivity in YFe$_2$Ge$_2$. Moreover, the power-law study demonstrates the recovery of Fermi-liquid behaviour at high pressure. Our results support the picture that the exponent 3/2 in the resistivity power-law temperature dependence emerges from its proximity to the quantum critical regime.

PdCrO$_2$ (Chapter 6): This work aims to explore any possible quantum critical point that may arise in the geometrically frustrated magnet PdCrO$_2$. It is shown that the transition temperature of antiferromagnetic ordering that arises in this system is very much pressure independent. However, resistivity measurement suggests that there may be a structural phase transition to some unknown phase above 80 kbar. The physics of the high-pressure phase are unknown and requires further studies.