Strange Metal Behavior in Electron Doped Cuprates Superconductor

Abstract

Unraveling the origin of unconventional superconductivity in cuprates has remained a long-standing enigma in the field of highly correlated systems since the discovery of high-temperature superconductors in 1986. The normal state properties of cuprates, from which superconductivity emerges, exhibit numerous anomalous features that defy conventional condensed matter theory. One such anomaly is the strange metal behavior of cuprates, characterized by a T-linear resistivity trend over all temperature ranges in hole-doped cuprates (HDCs) and a T-linear behavior at low temperatures transitioning to T-quadratic at high temperatures for electron-doped cuprates (EDCs), which lacks a comprehensive theoretical understanding. While the T-linear behavior has been empirically linked to superconductivity, a coherent theory remains elusive. Similarly, the origin of the T^2 behavior observed at high temperatures for EDCs is not well understood. The mechanisms driving the crossover from linear to quadratic temperature dependence in the resistivity of EDCs at elevated temperatures remain unclear. Furthermore, the distinct high-temperature resistivity behaviors of HDC (T-linear) and EDC (T^2), despite their similar low-temperature characteristics (T-linear), present a compelling puzzle. The focus of this thesis is centered specifically on the T^2 behavior. Previous studies on EDCs have reported that this T^2 behavior is not universally observed in all samples, with its observation being limited to some single crystal and some thin film studies. In our work, we have studied polycrystalline NCCO samples over a wide range of doping concentrations. Our detailed resistivity analysis suggests that the T^2 behavior is an inherent feature present in all types of EDC samples; however, its clear observation in experimental data is masked by the grain boundary scattering in the polycrystalline sample. Our resistivity analysis is supported by Seebeck studies and sample conditioning performed using hot-pressing techniques. In the second chapter, we propose a simplistic model to understand the strange metal phase, consistent with previous experimental observations. This model highlights the importance of competing contributions leading to unique resistivity trends in different samples and temperature ranges. This model facilitates a qualitative understanding of the transition from linear to quadratic temperature dependence in the resistivity of EDCs at high temperatures. Finally, we studied the thermal resistivity of our samples within the framework of the Planckian dissipation limit theorem to understand the correlation between thermal and electrical transport properties. Our studies suggest an inherent, yet not straightforward, connection between these two crucial physical parameters at high temperatures. The analysis of thermal resistivity has been carried out in parallel with electrical resistivity to elucidate the connection between electronic and phononic transport.