Tuning Superconductivity and Competing Phases in an Iron-based Superconductor with Uniaxial Stress

Abstract

Unconventional superconductors exhibit strong electronic correlations, quantum criticality, and numerous broken-symmetry collectively ordered phases involving a complex combination of charge, spin, and lattice degrees of freedom that manifest in an intertwined phenomenology where different phases and their fluctuations both compete and cooperate with each other. Understanding the role of the competing phases and quantum critical phenomena in the formation of superconductivity is a central question in condensed matter physics. In this thesis, I investigate the effect of uniaxial stress inducing strains in different symmetry channels on an unconventional Fe-based superconductor Ba(Fe1-xCox)2As2. I will first show that anisotropic strain enhancing nematic order is extremely efficient in suppressing the high-temperature superconductivity, which provides evidence for the role of nematic fluctuations in the superconducting pairing. I will then show that non-symmetry breaking strain and chemical composition are remarkably equivalent tuning parameters for superconductivity, which strongly favors the scenario in which the formation of superconductivity is driven by a quantum critical point. Finally, I will present Hall effect measurements revealing a new large and strongly temperature dependent elastoresistivity coefficient which reflects an extreme sensitivity of the conduction electrons to a strain-induced enhancement of spin fluctuations.