Preface

To unravel the nature of the emergent phenomena in strongly correlated many-body systems is one of the central challenges in theoretical and experimental research. Unexpected and exotic properties of these systems open active areas of study in the field of condensed matter physics. One of the remarkable phenomenon of these correlated systems was the discovery of superconductivity by Onnes in 1911. Since then, a lot of experimental and theoretical research has been done to describe this phenomenon. The discovery of modern superconductors, such as high-Tc, organic, two band, and some others, presented another criticism for the theoretical physicist. The possible superconductivity mechanism of these systems remains one of the most challenging problems in physics.

Most researchers believe that their superconductivity mechanism could not be explained within the BCS theory. At present, there is no mechanism to describe all the properties of these superconductors. Many different models such as the exciton model, plasmon model, bipolaron model, magnon model, spin fluctuation model etc., have been proposed. All of these models are evaluated by presenting new theoretical efforts and experimental results.

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 high-Tc and iron-based superconductors.

The main topics of the book are organized in five chapters, as described below.

Chapter 1 contains a brief discussion about the progress in superconductivity.

Chapter 2 discusses the experimental techniques to measure microwave absorption.

Chapter 3 deals with the electromagnetic properties of bulk high-temperature superconductors (HTS), which can be used in magnetic systems for various applications.

Chapter 4 focuses on the generalized BCS equations to discuss high Tc superconductors.

Chapter 5 covers the study of iron-pnictides using self-consistent Bogoliubov-de Gennes equations.

I hope this book is useful for theoretical and experimental researchers.

**Heshmatollah Yavari** University of Isfahan, Iran

**1**

**Chapter 1**

*Heshmatollah Yavari*

tor from a perfect conductor.

**1. Introduction**

Introductory Chapter:

of study in the field of condensed matter physics.

theory was formulated by Bardeen et al. [5].

conventional superconductors.

Superconductivity in Progress

Describing the behavior of strongly correlated systems is a challenge for every area of physics. The many-body ground state that emerges in these systems can have exotic and unexpected properties. As a remarkable of these correlated systems was the discovery of superconductivity by Onnes in 1911 [1], which opens active areas

This state is a quantum phenomenon that is displayed by certain materials under particular magnetic and temperature regimes. The Meissner effect discovered in 1933 shows that superconductivity is more than just the disappearance of resistance and it is a true thermodynamic state of matter because it leads to the superconductors as perfect diamagnetism materials [2]. This effect distinguishes a superconduc-

To describe these phenomena, a lot of theoretical works were proposed. The first phenomenological theory which is known as London theory was proposed by the London brothers [3]. This theory described well the magnetic properties and magnetic penetration depth of Type I superconductors in weak magnetic fields. By extending the London theory, Ginzburg and Landau proposed a phenomenological theory, which enables the study of Type II superconductors in a strong magnetic field [4]. By assuming the second-order phase using physical intuition and using variational principle of quantum mechanics, the Ginzburg-Landau (GL) theory allowed the calculation of macroscopic quantities of the material in the superconducting state. The most beautiful and successful theory in condensed matter physics, i.e. the microscopic theory of superconductivity which is known as BCS

The starting point of this theory was an effective Hamiltonian of fermionic quasiparticle excitations that interact via a weak phonon-mediated potential close to the Fermi surface. This attractive interaction leads to bound electron pair states known as Cooper pairs. While the single electrons are fermions, not being able to occupy the same state due to Pauli exclusion principle, Cooper pairs are no longer obliged to obey the Fermi-Dirac statistics, and like bosons, they can enter the same state. Superconductors that can be described by the BCS theory are known as

The discovery of high-temperature superconductors (HTS) in 1986 [6] presented another challenge for theoretical physicist. Their superconductivity mechanism remains one of the most challenging problems in physics. It is believed that the BCS theory alone cannot explain properties of these classes of superconductors and new theoretical concepts will be required. 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. In the following

## **Chapter 1**
