High-temperature Iron-based Superconductors Research Articles

Phase Diagram of the Two-Orbital Model for Iron-Based HTSC: Variational Cluster Approximation

The phase diagram of the two-dimensional two-orbital Hubbard model, which is widely used to describe high-temperature iron-based superconductors, is calculated in the electron doping region using the method of the variational cluster approximation. It is found that the coexistence of the antiferromagnetic and superconducting phases is realized in a wide range of electron doping values. In the superconducting phase, the order parameter has s± symmetry.

Electronic Specific Heat of Iron Pnictides Based on Electron-Cooper Pair Interaction

The discovery of iron pnictides in 2006 added on the number of materials that have the potential to transmit electricity with close zero d.c resistance. High-temperature iron-based superconductors have been obtained through modification, mostly by doping, of the initially low-temperature iron-based superconductors. Unlike in LTSC, the energy gap in HTSC requires a theory, beyond spin fluctuations, to explain its anisotropy. This study seeks to establish a common ground between iron pnictides and cuprates towards explaining high temperature superconductivity. There is a general consensus on the existence of Cooper pairs in these systems. In addition to this, experimental results have revealed the existence of electron-boson coupling in iron pnictides. These results make it viable to study the interaction between an electron and a Cooper pair in iron based superconductors (IBSC). In this study, Bogoliubov-Valatini transformation has been used in determining the electronic specific heat based on the interaction between an electron and a Cooper pair in high-temperature IBSC, namely, Ca0.33Na0.6Fe2As2 and SmFeAsO0.8F0.2. We record the theoretical electronic specific heat of CeFeAsO0.84F0.16 and SmFeAsO0.8F0.2 as 164.3 mJ mol-1 K-2 and 101.6 mJ mol-1 K-2 respectively.

High-energy electronic interaction in the 3d band of high-temperature iron-based superconductors

Strong electron interactions in solids increase effective mass, and shrink the electronic bands [1]. One of the most unique and robust experimental facts about iron-based superconductors [2-4] is the renormalization of the conduction band by factor of 3 near the Fermi level [5-9]. Obviously related to superconductivity, this unusual behaviour remains unexplained. Here, by studying the momentum-resolved spectrum of the whole valence band in a representative material, we show that this phenomenon originates from electronic interaction on a much larger energy scale. We observe an abrupt depletion of the spectral weight in the middle of the Fe $3d$ band, which is accompanied by a drastic increase of the scattering rate. Remarkably, all spectral anomalies including the low-energy renormalization can be explained by coupling to excitations, strongly peaked at about 0.5 eV. Such high-energy interaction distinguishes all unconventional superconductors from common metals.

Iron Superconductivity Weathers Another Storm

A new class of high-temperature iron-based superconductors lacks features in its electronic band structure that, based on current theoretical understanding, were considered essential. In a field that has produced quite a few surprises, this is the biggest yet.

Cleavage surfaces of theBaFe2−xCoxAs2andFeySe1−xTexsuperconductors: A combined STM plus LEED study

We elucidate the termination surface of cleaved single crystals of the ${\text{BaFe}}_{2\ensuremath{-}x}{\text{Co}}_{x}{\text{As}}_{2}$ and ${\text{Fe}}_{y}{\text{Se}}_{1\ensuremath{-}x}{\text{Te}}_{x}$ families of the high-temperature iron-based superconductors. By combining scanning tunneling microscopic data with low-energy electron diffraction we prove that the termination layer of the ${\text{BaFe}}_{2}{\text{As}}_{2}$ systems is a remnant of the Ba layer, which exhibits a complex diversity of ordered and disordered structures. The observed surface topographies and their accompanying superstructure reflections in electron diffraction depend on the cleavage temperature. In stark contrast, ${\text{Fe}}_{y}{\text{Se}}_{1\ensuremath{-}x}{\text{Te}}_{x}$ possesses only a single termination structure---that of the tetragonally ordered ${\text{Se}}_{1\ensuremath{-}x}{\text{Te}}_{x}$ layer.

Superconductivity in Ir-dopedLaFe1−xIrxAsO

We report the realization of superconductivity by $5d$ element Ir doping in LaFeAsO, a prototype parent compound of high-temperature iron-based superconductors. X-ray diffraction patterns indicate that the material has formed the ZrCuSiAs-type structure with a space group $P4/\text{nmm}$. The systematic evolution of the lattice constants demonstrated that the Fe ions were successfully replaced by Ir. Both electrical resistance and magnetization measurements show superconductivity up to 11.8 K in ${\text{LaFe}}_{1\ensuremath{-}x}{\text{Ir}}_{x}\text{AsO}$. The superconducting transitions at different magnetic fields were also measured yielding a slope of $\ensuremath{-}d{H}_{c2}/dT=6.7\text{ }\text{T}/\text{K}$ near ${T}_{c}$. Using the Werthamer-Helfand-Hohenberg formula ${H}_{c2}=\ensuremath{-}0.69$ $(d{H}_{c2}/dT){\ensuremath{\mid}}_{{T}_{c}}{T}_{c}$, the upper critical field at zero K is found to be about 54 T. Hall-effect measurements indicate that the conduction in this material is dominated by electronlike charge carriers; the charge-carrier density determined at 100 K is about $3.71\ifmmode\times\else\texttimes\fi{}{10}^{20}/{\text{cm}}^{3}$, which is close to the ${\text{LaFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ system. This is the example of bulk superconductivity induced by replacing the Fe sites with higher $d$-orbital electrons in FeAs-1111 family, which should add more ingredients to the underlying physics of the iron-based superconductors.