Strong electronic nematic fluctuations have been discovered near optimal doping for several families of Fe-based superconductors, raising the question of what role, if any, these fluctuations might play in the superconducting pairing interaction. Underdoped Fe-based superconductors undergo a second order tetragonal-to-orthorhombic structural transition driven by electronic nematic order that accompanies or precedes an antiferromagnetic transition and borders the superconducting dome. In the tetragonal phase Fe-based superconductors are highly sensitive to perturbations of the same symmetry as the nematic order, such as antisymmetric (in this case B2g) strain. Elastoresistivity is described by a high rank tensor (fourth-rank +) as it relates changes in resistivity (second-rank) to strain (second-rank) experienced by a material and is a sensitive probe of broken symmetries. Previously, elastoresistivity measurements of the linear resistivity response to antisymmetric strain have revealed a divergent electronic nematic susceptibility in underdoped materials and has been used to characterize the nematic fluctuations in the tetragonal phase above the zero-field superconducting dome. In this work, I develop two new extensions to elastoresistivity measurements in order to probe the effects of electronic nematic fluctuations in greater detail in a representative family of these materials, the prototypical electron-doped pnictide Ba(Fe(1-x)Cox)2As2. First, I go beyond the first order response via a measurement of the nonlinear elastoresistivity. I find that the strong nematic fluctuations play a large role in the isotropic electronic response of these materials--as evidenced by a diverging nonlinear symmetric elastoresistivity response to antisymmetric strain. Second, I performed elastoresistivity measurements in magnetic fields large enough to suppress superconductivity to investigate the potential that the nematic fluctuations are related to a quantum critical point. I do not observe a magnetic field dependence of the nematic fluctuations and find that they continue to grow with decreasing temperature beneath the zero-field superconducting dome. I performed high magnetic field measurements on samples with a fine distribution of dopings and find that close to the putative quantum critical point, the nematic susceptibility appears to obey power law behavior over almost a decade of variation in composition. This is consistent with basic notions of nematic quantum criticality which, for clean systems, is associated with power law scaling in both doping and temperature. Paradoxically, however, I also find that the temperature dependence of the nematic susceptibility for compositions close to the critical value cannot be described by a single power law.

From fundamental physics point of view, iron-based superconductors have properties that are more amenable to band structural calculations. This book reviews the progress made in this fascinating field. With contributions from leading experts, the book provides a guide to understanding materials, physical properties, and superconductivity mechanism aspects, and is important for students and beginners to have an overall view of the recent progress in this active field.

Within this work, the pairing mechanism of conventional (Pb) and unconventional superconductors (SrFe2(As1-xPx )2, FeSe, FeSe/STO) was investigated experimentally by means of elastic and inelastic tunneling spectroscopy at temperatures down to 30 mK. The distinction between elastic and inelastic contributions to tunneling data was elaborated. The results help to identify conventional (phonon-mediated) and unconventional (e.g. spin-?uctuation mediated) superconductivity. This work was published by Saint Philip Street Press pursuant to a Creative Commons license permitting commercial use. All rights not granted by the work's license are retained by the author or authors.

This volume presents an in-depth review of experimental and theoretical studies on the newly discovered Fe-based superconductors. Following the Introduction, which places iron-based superconductors in the context of other unconventional superconductors, the book is divided into three sections covering sample growth, experimental characterization, and theoretical understanding. To understand the complex structure-property relationships of these materials, results from a wide range of experimental techniques and theoretical approaches are described that probe the electronic and magnetic properties and offer insight into either itinerant or localized electronic states. The extensive reference lists provide a bridge to further reading. Iron-Based Superconductivity is essential reading for advanced undergraduate and graduate students as well as researchers active in the fields of condensed matter physics and materials science in general, particularly those with an interest in correlated metals, frustrated spin systems, superconductivity, and competing orders.

Emerging evidence for the presence of strongly anisotropic electronic states in the underdoped regime of both cuprate and iron-based high temperature superconductors suggests the possibility of an important role for electronic nematic order in these materials. The central theme of my thesis work has been the experimental study of electronic nematicity in iron-based superconductors via measurement of resistivity anisotropy. To do this, I have developed several new experimental techniques, on the one hand enabling detwinning of sub-mm size single crystals in the broken-symmetry orthorhombic state, and on the other hand revealing the nematic susceptibility in the high-symmetry tetragonal state. A major part of my thesis work has involved measurement of the elastoresistance; that is, the change in the resistance of a material as a consequence of the strains that it experiences. In this thesis, I will show how differential elastoresistance measurements can directly reveal the nematic susceptibility of a material in the tetragonal state. I will introduce the appropriate tensor formalism necessary to describe these measurements, and describe an experimental technique to determine these coefficients using piezoelectric stacks to provide anisotropic bi-axial strain. Results in the tetragonal state of various underdoped families based on the parent compound BaFe2As2 explicitly demonstrate that the tetragonal-to-orthorhombic structural transition in these materials is fundamentally driven by an electronic nematic instability. These results also suggest that the resistivity anisotropy in the paramagnetic orthorhombic state is dominated by the Fermi surface anisotropy, rather than an anisotropy in the scattering rate. Finally, similar measurements of a wide variety of optimally doped iron-pnictides and iron-chalcogenides reveal that a divergence of the nematic susceptibility in the B2g symmetry channel appears to be a generic feature of optimally-doped iron-based superconductors. In addition to the above, I also employ a mechanical detwinning technique to reveal the resistivity anisotropy in the orthorhombic state of the same Fe-based superconductors. For the isovalently-substituted material BaFe2(As1-xPx)2, these measurements reveal a strong coupling between external stress and both the Neel temperature and the superconducting critical temperature.

This book covers different aspects of the physics of iron-based superconductors ranging from the theoretical, the numerical and computational to the experimental ones. It starts from the basic theory modeling many-body physics in Fe-superconductors and other multi-orbital materials and reaches up to the magnetic and Cooper pair fluctuations and nematic order. Finally, it offers a comprehensive overview of the most recent advancements in the experimental investigations of iron based superconductors.