Iron-based superconductors
Introduction
Iron-based superconductors are a class of high-temperature superconductors that have garnered significant attention since their discovery in 2008. These materials are characterized by their layered crystal structures, which include iron and a pnictogen (such as arsenic) or a chalcogen (such as selenium or tellurium). The discovery of iron-based superconductors has opened new avenues for research in condensed matter physics, particularly in understanding the mechanisms behind high-temperature superconductivity.
Historical Background
The discovery of iron-based superconductors marked a significant milestone in the field of superconductivity. Prior to this, the focus was primarily on copper oxide-based superconductors, known as cuprates. In 2008, a team led by Hideo Hosono at the Tokyo Institute of Technology discovered superconductivity in LaFeAs(O,F) with a transition temperature (T_c) of 26 K. This discovery was quickly followed by the identification of other iron-based superconductors with higher transition temperatures, sparking a surge of research activity worldwide.
Crystal Structure and Composition
Iron-based superconductors typically possess a layered crystal structure, which is crucial to their superconducting properties. The most common structural motif is the FeAs layer, where iron atoms form a square planar lattice, and arsenic atoms are positioned above and below this plane. These layers are separated by spacer layers, which can be composed of rare earth oxides or other elements. The general formula for these materials is RFeAsO, where R represents a rare earth element.
1111-Type Compounds
The 1111-type compounds, such as LaFeAsO, are among the first discovered iron-based superconductors. These materials consist of alternating layers of FeAs and RO, where R is a rare earth element. The substitution of oxygen with fluorine or the application of pressure can enhance the superconducting transition temperature.
122-Type Compounds
The 122-type compounds, such as BaFe_2As_2, have a slightly different structure, where the FeAs layers are separated by layers of alkaline earth metals like barium. These materials can also exhibit superconductivity upon doping or applying pressure.
111-Type and 11-Type Compounds
The 111-type compounds, such as LiFeAs, and the 11-type compounds, such as FeSe, represent other structural variations. The 11-type compounds are particularly interesting because they lack spacer layers, consisting solely of FeSe layers.
Superconducting Mechanism
The mechanism of superconductivity in iron-based superconductors remains an active area of research. Unlike conventional superconductors, where electron pairing is mediated by lattice vibrations (phonons), iron-based superconductors are believed to involve more complex interactions. The presence of multiple Fermi surfaces and the role of spin fluctuations are thought to be crucial in the pairing mechanism.
Role of Magnetism
Magnetism plays a significant role in the properties of iron-based superconductors. Many of these materials exhibit antiferromagnetic order in their parent, non-superconducting state. The suppression of this magnetic order, either through chemical doping or pressure, often coincides with the emergence of superconductivity.
Electronic Structure
The electronic structure of iron-based superconductors is characterized by multiple bands crossing the Fermi level. The interplay between these bands, along with the presence of electron and hole pockets, is believed to be essential for understanding the superconducting state.
Experimental Techniques
Various experimental techniques are employed to study iron-based superconductors, providing insights into their structural, electronic, and magnetic properties.
Angle-Resolved Photoemission Spectroscopy (ARPES)
ARPES is a powerful tool for probing the electronic structure of materials. In iron-based superconductors, ARPES has been used to map out the Fermi surface and investigate the nature of the superconducting gap.
Neutron Scattering
Neutron scattering experiments have been instrumental in exploring the magnetic properties of iron-based superconductors. These studies have revealed the presence of spin fluctuations, which are believed to be linked to the superconducting mechanism.
Scanning Tunneling Microscopy (STM)
STM provides real-space imaging of the surface electronic structure. In iron-based superconductors, STM has been used to visualize the superconducting gap and investigate the effects of impurities and defects.
Applications and Future Directions
While the practical applications of iron-based superconductors are still in the early stages, their potential for technological advancements is significant. These materials could be used in applications requiring high magnetic fields and low energy losses, such as in magnetic resonance imaging (MRI) and power transmission.
Challenges
Several challenges remain in the development of iron-based superconductors for practical applications. These include improving the critical current density, enhancing the transition temperature, and understanding the effects of impurities and defects.
Future Research
Future research is likely to focus on optimizing the properties of iron-based superconductors through chemical doping, pressure, and strain. Additionally, theoretical studies aimed at unraveling the superconducting mechanism will continue to be a major area of interest.