Pressure-dependent topological superconductivity on the surface of FeTe0.5Se0.5

FeTe1-xSex is a family of iron-based superconductors with its critical temperature (Tc) dependent on the composition of Se. A well-known Tc is 14.5 K for x = 0.45, which exhibits an s-wave superconducting gap between the topological superconducting surfaces states. Exchange interaction between the electrons has been proposed as the mechanism behind the formation of Cooper pairs for the sample of FeTe0.5Se0.5. In this article we provide further proof that exchange interaction, and hence the associated Tc, depends on the applied pressure on FeTe0.5Se0.5. Using density functional calculations for electrons and phonons and the Bardeen-Cooper-Schrieffer (BCS) theory for superconductivity, we found that Tc and superconducting gap for FeTe0.5Se0.5 soars under increasing compression, consistent with the results of experiment.

Bulk and Single Crystal Growth Progress of Iron-Based Superconductors (FBS): 1111 and 1144

The discovery of iron-based superconductors (FBS) and their superconducting properties has generated huge research interest and provided a very rich physics high Tc family for fundamental and experimental studies. The 1111 (REFeAsO, RE = Rare earth) and 1144 (AEAFe4As4, AE = Ca, Eu; A = K, Rb) families are the two most important families of FBS, which offer the high Tc of 58 K and 36 K with doping and without doping, respectively. Furthermore, the crystal growth of these families is not an easy process, and a lot of efforts have been reported in this direction. However, the preparation of high-quality and suitable-sized samples is still challenging. In this short review, we will summarize the growth of materials with their superconducting properties, especially polycrystals and single crystals, for the 1111 and 1144 families, and make a short comparison between them to understand the developmental issues.

Iron-based magnetic superconductors AEuFe4As4 (A= Rb, Cs): Natural superconductor-ferromagnet hybrids

Superconductivity (SC) and ferromagnetism (FM) are normally antagonistic, and their coexistence in a single crystalline material appears to be very rare. Over a decade ago, the iron-based pnictides of doped EuFe2As2 were found to render such a coexistence primarily because of the Fe-3d multiorbitals which simultaneously satisfy the superconducting pairing of Fe-3d electrons and the ferromagnetic exchange interaction among Eu local spins. In 2016, the discovery of the iron-based superconductors AEuFe4As4 (A= Rb, Cs) provided an additional and complementary material basis for the study of the coexistence and interplay between SC and FM. The two sibling compounds, which can be viewed as the intergrowth or hybrid between AFe2As2 and EuFe2As2, show SC in the FeAs bilayers at T c = 35 – 37 K, followed by a magnetic ordering at T m ∼ 15 K in the sandwiched Eu2+-ion sheets. Below T m, the Eu2+ spins align ferromagnetically within each Eu plane, making the system as a natural atomic-thick superconductor-ferromagnet superlattice. This paper reviews the main research progress in the emerging topic during the past five years, and an outlook for the future research opportunities is also presented.

Effect of annealing on the structural and superconducting properties of FeSe1−xTex

<p><b>The term 'high-temperature superconductivity' has long been synonymous with copper oxide-based superconductors (cuprates) up until the recent discovery of the iron-based superconductors in 2008. This new family of superconductors exhibits fundamentally interesting properties such as the interplay between magnetism and superconductivity as well as the very recently discovered topological properties of FeSe1−xTex. Furthermore, from an application point of view, iron-based superconductors have the potential to become the new norm for low-temperature, high-field applications such as MRI and nuclear fusion.</b></p> <p>However, some of the post-processing procedures required to obtain high-quality samples, like the annealing process of FeSe1−xTex, are yet to be fully understood.</p> <p>This thesis reports on the effect of annealing on the structure and composition of FeSe1−xTex and how they manifest as changes in the superconducting properties.</p> <p>Overall, air annealing is shown to improve the critical temperature and critical current density of FeSe1−xTex for almost all investigated doping concentrations.</p> <p>These improvements are the result of a decrease in excess iron driven by the formation of thin iron oxide layers on exposed surfaces of the crystal.</p> <p>Further analysis suggests that the reduction in the excess iron concentration is largest in the region right underneath the oxide layers. Consequently, the improvement in the superconducting properties is also found to be largest in these regions. In terms of the annealing atmosphere, even in nitrogen and low-vacuum atmospheres, annealing still leads to the formation of an iron oxide layer and an improvement in the superconducting properties due to the presence of residual oxygen. In rare cases, annealing was found to induce asymmetric magnetic hysteresis loops as a result of weak bulk pinning and strong surface pinning. Whilst asymmetric hysteresis loops have occasionally been reported in the cuprates and polycrystalline iron-based superconductors, this work reports the first observation of such behaviour in FeSe1−xTex single crystals. This work has deepened the understanding of the annealing process on the intrinsic properties of FeSe1−xTexand facilitates the study of additional post-processing procedures that will further improve the properties of this family of superconductors.</p>

Effect of annealing on the structural and superconducting properties of FeSe1−xTex

<p><b>The term 'high-temperature superconductivity' has long been synonymous with copper oxide-based superconductors (cuprates) up until the recent discovery of the iron-based superconductors in 2008. This new family of superconductors exhibits fundamentally interesting properties such as the interplay between magnetism and superconductivity as well as the very recently discovered topological properties of FeSe1−xTex. Furthermore, from an application point of view, iron-based superconductors have the potential to become the new norm for low-temperature, high-field applications such as MRI and nuclear fusion.</b></p> <p>However, some of the post-processing procedures required to obtain high-quality samples, like the annealing process of FeSe1−xTex, are yet to be fully understood.</p> <p>This thesis reports on the effect of annealing on the structure and composition of FeSe1−xTex and how they manifest as changes in the superconducting properties.</p> <p>Overall, air annealing is shown to improve the critical temperature and critical current density of FeSe1−xTex for almost all investigated doping concentrations.</p> <p>These improvements are the result of a decrease in excess iron driven by the formation of thin iron oxide layers on exposed surfaces of the crystal.</p> <p>Further analysis suggests that the reduction in the excess iron concentration is largest in the region right underneath the oxide layers. Consequently, the improvement in the superconducting properties is also found to be largest in these regions. In terms of the annealing atmosphere, even in nitrogen and low-vacuum atmospheres, annealing still leads to the formation of an iron oxide layer and an improvement in the superconducting properties due to the presence of residual oxygen. In rare cases, annealing was found to induce asymmetric magnetic hysteresis loops as a result of weak bulk pinning and strong surface pinning. Whilst asymmetric hysteresis loops have occasionally been reported in the cuprates and polycrystalline iron-based superconductors, this work reports the first observation of such behaviour in FeSe1−xTex single crystals. This work has deepened the understanding of the annealing process on the intrinsic properties of FeSe1−xTexand facilitates the study of additional post-processing procedures that will further improve the properties of this family of superconductors.</p>

Two distinct superconducting states controlled by orientations of local wrinkles in LiFeAs

For iron-based superconductors, the phase diagrams under pressure or strain exhibit emergent phenomena between unconventional superconductivity and other electronic orders, varying in different systems. As a stoichiometric superconductor, LiFeAs has no structure phase transitions or entangled electronic states, which manifests an ideal platform to explore the pressure or strain effect on unconventional superconductivity. Here, we observe two types of superconducting states controlled by orientations of local wrinkles on the surface of LiFeAs. Using scanning tunneling microscopy/spectroscopy, we find type-I wrinkles enlarge the superconducting gaps and enhance the transition temperature, whereas type-II wrinkles significantly suppress the superconducting gaps. The vortices on wrinkles show a C2 symmetry, indicating the strain effects on the wrinkles. By statistics, we find that the two types of wrinkles are categorized by their orientations. Our results demonstrate that the local strain effect with different directions can tune the superconducting order parameter of LiFeAs very differently, suggesting that the band shifting induced by directional pressure may play an important role in iron-based superconductivity.