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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.
The nature of the nematicity in iron pnictides is studied with a proposed magnetic fluctuation. The spin-driven order in the iron-based superconductor has been realized in two categories: stripe SDW state and nematic state. The stripe SDW order opens a gap in the band structure and causes a deformed Fermi surface. The nematic order does not make any gap in the band structure and still deforms the Fermi surface. The electronic mechanism of nematicity is discussed in an effective model by solving the self-consistent Bogoliubov-de Gennes equations. The nematic order can be visualized as crisscross horizontal and vertical stripes. Both stripes have the same period with different magnitudes. The appearance of the orthorhombic magnetic fluctuations generates two uneven pairs of peaks at ±π0 and 0±π in its Fourier transformation. In addition, the nematic order breaks the degeneracy of dxz and dyz orbitals and causes the elliptic Fermi surface near the Γ point. The spatial image of the local density of states reveals a dx2-y2-symmetry form factor density wave.
Since the discovery of superconductivity, a great number of theoretical and experimental efforts have been made to describe this new phase of matter that emerged in many body systems. In this regard, theoretical models have been presented; the most famous of which was the BCS theory that can only describe conventional superconductors. With the discovery of new class superconductors, the superconducting mechanism became a new challenge in the field of condensed matter physics. This unexpected discovery opened a new area in the history of superconductivity, and experimental researchers started trying to find new compounds in this class of superconductors. These superconductors are often characterized by the anisotropic character in the superconducting gap function with nodes along a certain direction in the momentum space. Since the pairing interaction has an important role in the superconducting gap structure, its determination is very important to explain the basic pairing mechanism.In this regard, this book includes valuable theoretical and experimental discussions about the properties of superconductors. Here you will find valuable research describing the properties of unconventional superconductors.
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.

Common Electronic Features and Electronic Nematicity in Parent Compounds of Iron-Based Superconductors and FeSe/SrTiO3 Films Revealed by Angle-Resolved Photoemission Spectroscopy*Supported by the National Natural Science Foundation of China Under Grant Nos 11190022, 11334010 and 11534007, the National Basic Research Program of China Under Grant No 2015CB921000, and the Strategic Priority Research Program (B) of Chinese Academy of Sciences Under Grant No XDB07020300

Published: 2016

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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.
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.
In this book the author presents two important findings revealed by high-precision magnetic penetration depth measurements in iron-based superconductors which exhibit high-transition temperature superconductivity up to 55 K: one is the fact that the superconducting gap structure in iron-based superconductors depends on a detailed electronic structure of individual materials, and the other is the first strong evidence for the presence of a quantum critical point (QCP) beneath the superconducting dome of iron-based superconductors. The magnetic penetration depth is a powerful probe to elucidate the superconducting gap structure which is intimately related to the pairing mechanism of superconductivity. The author discusses the possible gap structure of individual iron-based superconductors by comparing the gap structure obtained from the penetration depth measurements with theoretical predictions, indicating that the non-universal superconducting gap structure in iron-pnictides can be interpreted in the framework of A1g symmetry. This result imposes a strong constraint on the pairing mechanism of iron-based superconductors. The author also shows clear evidence for the quantum criticality inside the superconducting dome from the absolute zero-temperature penetration depth measurements as a function of chemical composition. A sharp peak of the penetration depth at a certain composition demonstrates pronounced quantum fluctuations associated with the QCP, which separates two distinct superconducting phases. This gives the first convincing signature of a second-order quantum phase transition deep inside the superconducting dome, which may address a key question on the general phase diagram of unconventional superconductivity in the vicinity of a QCP.