Comparison of the local structures of Ca0.82La0.18FeAs2 and Ba0.64K0.36Fe2As2 pnictide superconductors using atomic pair distribution function analysis

A comparative local structure study of pnictide superconductors Ca_0.82La_0.18FeAs₂ (112-type, T_c∼ 40 K) and Ba_0.64K_0.36Fe₂As₂ (122-type, T_c∼ 37 K), using room temperature x-ray total scattering measurements is reported. The Fe–As superconducting active layer is found to be globally similar in both the systems consisting of edge-sharing FeAs_4/4 tetrahedra as in all the iron-pnictide superconductors discovered so far. Although optimally superconducting, the active layer in these compounds is found to sustain a large local inhomogeneity. These results thus imply that a nanoscopic manipulation of the Fe–As active layer, rather than its isotropic structural tuning, is the key parameter to control the superconducting properties of the iron-based systems.     

Triplet superconducting correlations in oxide heterostructures with a composite ferromagnetic interlayer

The superconducting current induced by the penetration of the long-range triplet component of superconducting correlations into a composite ferromagnetic interlayer has been detected in mesa-heterostructures based on oxide cuprate superconductors YBa₂Cu₃O_{7 ? δ} and Au/Nb bilayer films with the composite oxide interlayer that is made of ferromagnetic films of manganite La_0.7Sr_0.3MnO₃ and ruthenate SrRuO₃ and has a thickness much larger than the length of correlations determined by the exchange field. The deviation of the superconducting current in the mesa-heterostructure with the fraction of the second harmonic of 13% from a sinusoidal current-phase relation has been detected; this deviation can also be due to the generation of the triplet component of superconducting correlations in the ferromagnet.     

Quantum-well structured high-Tc superconductors prepared by soft-chemical method

We have synthesized quantum-well structured high-T_c superconductors through soft-chemical processes, where Bi-based cuprate lattices are regularly interstratified with atom, molecule, and ionic salt. In these materials, superconducting (S) and nonsuperconducting (I) layers are alternately stacked to form superlattices with atomically clear-cut interfaces between them. The intercalation technique, which means a reaction of incorporating chemical species (guest) into the pre-synthesized layered compound (host), could provides an easy route to superconducting superlattices with molecular-level layer-by-layer structural feature. The joint analysis of temperature-dependent magnetic susceptibilities and X-ray absorption spectroscopy indicated that superconducting transition temperature is predominantly governed by the electronic structure of superconductive CuO₂ plane.     

Determination of transition curves of high-Tc superconductors by phase contrast microscopy

A new method which allows the detection of the superconducting phase transition of high-T_c superconductors (HTSC) on a microscopic scale is reported. Micro-size holes in thin foils of superconducting material are examined in a transmission electron microscope at varying temperatures. The superconducting transition induces small changes in the image intensity within the holes, which can be detected by using electronic image analysis. Superconducting transition curves are then obtained for various types of high-T_c superconductors and for given values of the applied magnetic field.     

Review of superconductivity in BiS2-based layered materials

In 2012, a new layered superconductor where BiS₂ layer is the superconducting layer was discovered. So far, seven types of BiS₂-based superconductors and two related superconductors have been discovered. In this article, the diversity of the crystal structure and the physical properties of the BiS₂-based superconductors are reviewed. Furthermore, notable characteristics of superconductivity in the BiS₂ family are introduced. The prospects for raising T_c in this family are proposed on the basis of experimental and theoretical studies.     

Superconductivity and properties of FeTeOx films

Films of the parent compound FeTe can be made superconducting via the addition of interstitial oxygen. The process is reversible. We have characterized the new superconductors with a variety of experiments. X-ray diffraction shows that the superconductor has the same overall structure but a small lattice constant change compared to pure FeTe. X-ray absorption shows that superconducting FeTeO_x has a nominal valence of 3+. DFT calculations show the most likely position for interstitial oxygen and confirm that such oxygen incorporation does not produce a large change in structure.     

A few points on iron-based superconductors

We highlight a few points on iron-based superconductors in this paper. Experimental evidence for intrinsic inhomogeneity and phase separation in Fe-based superconductors is reviewed first. The non-BCS feature and other similarities between the Fe-based superconductors and the cuprates are also presented. The pressure effect on the FeSe-based superconductors observed recently is then discussed in terms of the interplay between superconducting transition temperature and structural characters. It is suggested that there is room for exploring new superconductors with higher transition temperature and that the improvement on the sample quality is necessary for further investigation on the superconducting mechanism.     

Current issues in heavy fermion superconductivity

An attempt is made to summarize our current understanding of the superconductivity occuring in heavy fermion systems. The last three years have seen the discovery of two new superconductors (UNi₂Al₃ and UPd₂Al₃), much more use of directional probes to investigate the anisotropy of the gap structure, further experimental and theoretical inquiry into a possible coupling of magnetic and superconducting order parameters, wider application of pressure and uniaxial stress to examine the onset of ordering and some new indications of unconventional superconductivity. These topics will be reviewed along with others of current interest.     

Robustness of <Emphasis Type="Italic">s</Emphasis>-wave pairing symmetry in iron-based superconductors and its implications for fundamentals of magnetically driven high-temperature superconductivity

Based on the assumption that the superconducting state belongs to a single irreducible representation of lattice symmetry, we propose that the pairing symmetry in all measured iron-based superconductors is generally consistent with the A _1g s-wave. Robust s-wave pairing throughout the different families of iron-based superconductors at different doping regions signals two fundamental principles behind high-T _c superconducting mechanisms: (i) the correspondence principle: the short-range magnetic-exchange interactions and the Fermi surfaces act collaboratively to achieve high-T _c superconductivity and determine pairing symmetries; (ii) the magnetic-selection pairing rule: superconductivity is only induced by the magnetic-exchange couplings from the super-exchange mechanism through cation-anion-cation chemical bonding. These principles explain why unconventional high-T _c superconductivity appears to be such a rare but robust phenomena, with its strict requirements regarding the electronic environment. The results will help us to identify new electronic structures that can support high-T _c superconductivity.     

An investigation into density-wave propagation within high-temperature superconducting plasmas for use in creating traveling-wave devices

This paper theoretically investigates a novel application of high-temperature superconductors where the superconductor serves as the active component in a microwave or millimeter traveling-wave amplifier. A guided electromagnetic wave interacts with a dc superconducting electron current to set up charge-density gradients within the superconducting electron "plasma." The electromagnetic wave gradually extracts energy from the superconducting electrons by traveling in phase synchronism with these charge gradients. The interaction mechanism is similar to that of a conventional traveling-wave tube amplifier or oscillator. We have modeled the wave behavior of superconducting electrons using the London equations and a two-fluid approach. Our model includes dissipation within the superconductor, and it shows that traveling-wave devices may be possible using high-quality thin-film superconductors in which dissipation is kept low.     

Superconductivity in Shear Strained Semiconductors

Semiconductivity and superconductivity are remarkable quantum phenomena that have immense impact on science and technology,and materials that can be tuned,usually by pressure or doping,to host both types of quantum states are of great fundamental and practical significance.Here we show by first-principles calculations a distinct route for tuning semiconductors into superconductors by diverse large-range elastic shear strains,as demonstrated in exemplary cases of silicon and silicon carbide.Analysis of strain driven evolution of bonding structure,electronic states,lattice vibration,and electron-phonon coupling unveils robust pervading deformation induced mechanisms auspicious for modulating semiconducting and superconducting states under versatile material conditions.This finding opens vast untapped structural configurations for rational exploration of tunable emergence and transition of these intricate quantum phenomena in a broad range of materials.     

Ni2X2 (X = pnictide,chalcogenide, or B) based superconductors

We review the properties of Ni-based superconductors which contain Ni₂X₂ (X = As, P, Bi, Si, Ge, B) planes, a common structural element found also in the recently discovered FeAs superconductors. Strong evidence for the fully gapped nature of the superconducting state has come from field dependent thermal conductivity results on BaNi₂As₂. Coupled with the lack of magnetism, the majority of evidence suggests that the Ni-based compounds are conventional electron–phonon mediated superconductors. However, the increase in T_c in LaNiAsO with doping is anomalous, and mimics the behavior in LaFeAsO. Furthermore, comparisons of the properties of Ni- and Fe-based systems show many similarities, particularly with regards to structure–property relationships. This suggests a deeper connection between the physics of the FeAs superconductors and the related Ni-based systems which deserves further investigation.     

Strong-coupling superconductivity induced by calcium intercalation in bilayer transition-metal dichalcogenides

We theoretically investigate the possibility of achieving a superconducting state in transition-metal dichalcogenide bilayers through intercalation, a process previously and widely used to achieve metallization and superconducting states in novel superconductors. For the Ca-intercalated bilayers MoS₂ and WS₂, we find that the superconducting state is characterized by an electron–phonon coupling constant larger than 1.0 and a superconducting critical temperature of 13.3 and 9.3 K, respectively. These results are superior to other predicted or experimentally observed two-dimensional conventional superconductors and suggest that the investigated materials may be good candidates for nanoscale superconductors. More interestingly, we proved that the obtained thermodynamic properties go beyond the predictions of the mean-field Bardeen–Cooper–Schrieffer approximation and that the calculations conducted within the framework of the strong-coupling Eliashberg theory should be treated as those that yield quantitative results.     

Current status of iron-based superconductors

Current status of iron-based superconductors is summarized. Although short range magnetic ordering and magnetic phase separation of Fe are controversial, (long range) magnetic and electronic phase diagrams of iron based superconductors can be classified into two-type. Antiferromagnetic ordering of itinerant Fe does not coexist with superconducting phase of SmFeAsO_1???xF_x. The very large H _c2 of iron-based superconductors attract us to attempts at applications.     

Andreev states and shot noise in bicrystal junctions of cuprate superconductors

Experimentally observed features of the electrical and noise characteristics of bicrystal junctions of cuprate superconductors, such as linearity of the critical current density versus square root of the junction transparency and increase in the spectral density of shot noise for small bias voltages (below the superconducting gap), indicate that the superconducting current in cuprate bicrystal junctions is determined by the passage of quasi-particles through a potential barrier at the superconductor boundaries. This process involves bound states appearing as a result of multiple Andreev reflections in superconductors with dominant wavefunction components of the d _x ² ? _y ² symmetry type. At the same time, interpretation of the experimental current-phase and current-magnetic field curves requires that the character of faceting at the bicrystal junctions would be also taken into account.     

Coulomb repulsion-induced hyperbolic pairing as a mechanism of high-T c superconductivity

The hyperbolic metric of the dispersion law (the effective mass tensor components of carriers are opposite in sign) in the vicinity of the Fermi contour in high-T _c superconducting cuprates in the case of repulsive interaction gives rise to a superconducting state characterized by the condensate of pairs with a large total momentum (hyperbolic pairing). The gain in the energy of the superconducting state over the normal state is due to the fact that a change in the kinetic energy of pairs (because of the negative light component of the effective mass) dominates over the change in the potential energy (corresponding to energy loss). The shift of the chemical potential upon the transition to the superconducting phase is substantial in this case. With increasing repulsive interaction, the superconducting gap δ_K increases and the resulting gain in energy changes to an energy loss at a certain critical value of the repulsive potential. The low temperature T _c of the superconducting transition and the large value of δ _K in this region of potential values are the reasons for the high value of the 2δ_K/T _c ratio and for the developed quantum fluctuations that are observed in underdoped cuprate superconductors.     

Impurity induced resonance states at the superconducting interface LaAlO3/SrTiO3

Motivated by the recent discovery of superconductivity on the heterointerface LaAlO₃/SrTiO₃, we theoretically investigate the impurity-induced resonance states with coexisting spin singlet s- and triplet p-wave pairing symmetries by considering the influence of Rashba-type spin-orbit interaction (RSOI). Due to the nodal structure of the mixed gap function, we find single nonmagnetic impurity-induced resonance peaks appearing in the local density of state. We also analyze the evolutions of density of states and local density of states with the weight of triplet pairing component determined by the strength of RSOI, which will be widely observed in thin films of superconductors with surface or interface-induced RSOI, or various noncentrosymmetric superconductors in terms of point contact tunneling and scanning tunneling microscopy, and thus shed light on the admixture of the spin singlet and RSOI-induced triplet superconducting states.     

A brief review on <Emphasis Type="Italic">μ</Emphasis>SR studies of unconventional Fe- and Cr-based superconductors

Muon spin relaxation/rotation (μSR) is a vital technique for probing the superconducting gap structure, pairing symmetry and time reversal symmetry breaking, enabling an understanding of the mechanisms behind the unconventional superconductivity of cuprates and Fe-based high-temperature superconductors, which remain a puzzle. Very recently double layered Fe-based super- conductors having quasi-2D crystal structures and Cr-based superconductors with a quasi-1D structure have drawn considerable attention. Here we present a brief review of the characteristics of a few selected Fe- and Cr-based superconducting materials and highlight some of the major outstanding problems, with an emphasis on the superconducting pairing symmetries of these materials. We focus on μSR studies of the newly discovered superconductors ACa₂Fe₄As₄F₂ (A = K, Rb, and Cs), ThFeAsN, and A₂Cr₃As₃ (A = K, Cs), which were used to determine the superconducting gap structures, the presence of spin fluctuations, and to search for time reversal symmetry breaking in the superconducting states. We also briefly discuss the results of μSR investigations of the superconductivity in hole and electron doped BaFe₂As₂.     

高压下的铁基超导体:现象与物理

始于2008年的铁基超导体研究续写了高温超导发展史的新篇章.回顾过去十年对铁基超导体的研究,在理论、实验及应用方面都取得了辉煌的成绩,丰富了人们对高温超导电性的认识,为突破高温超导机理研究、最终实现超导材料的人工设计与更广泛的应用奠定了坚实的基础.本文主要介绍了通过高压实验研究手段在铁基超导体的研究中取得的一些重要进展及呈现出的新现象和新物理,例如压致超导现象、压力导致的超导再进入现象、压力对超导转变温度的提升效应、压力研究对铁基超导体超导转变温度的预测、相分离结构对超导电性的影响及反铁磁-超导双临界点的发现等.希望这些高压研究结果与本文报道的其他各类实验与理论研究成果一起,为全面、深入地理解铁基超导体勾画出一幅较为完整的物理图像.     

Effect of paramagnetic fluctuations on superconductivity in ferromagnetic superconductors

The superconducting transition temperature T_c is calculated for ferromagnetic superconductors taking into account explicitly elastic scattering between electrons and paramagnetic fluctuations. Depending on the strength and range of the Heisenberg interaction between localized magnetic moments, the superconducting gap may or may not remain finite at the onset of long range ferromagnetic ordering. In a dirty system, the mean free path reduces the depairing effect of fluctuations. The Heisenberg interaction is assumed to be of other than RKKY origin.