Family Of Iron-based Superconductors Research Articles

Oxidation in Ca/K-1144 iron-based superconductors polycrystalline compounds

Iron-based superconductors (IBSCs) are a class of material under investigation for the development of superconducting wires in the low-temperature-high magnetic fields power application. Among the various families of IBSCs, the 1144 CaKFe4As4 compound is a promising material able to achieve outstanding superconducting properties with a cheap and simple chemical composition. Oxidation, in these compounds, is considered an obstacle for high intergranular critical current density, Jc,GB. A study devoted to the evaluation of oxidation phenomena and their effects on the superconducting properties is thus needed in order to fully understand the involved mechanisms. From the evaluation of polycrystalline samples obtained by a mechanochemically assisted synthesis route, a degradation of the critical temperature and critical currents has been observed concurrently with oxygen accumulation at grain boundaries in open porosities. However, the crystalline structure at an atomic level seems not affected, as well as intragranular superconducting properties assessed by means of calorimetric methods. These results suggest that loss of superconducting properties in Ca/K-1144 compounds following oxidation is significantly associated with the worsening of grain connectivity.

Ubiquitous enhancement of nematic fluctuations across the phase diagram of iron based superconductors probed by the Nernst effect

The role of nematic fluctuations for unconventional superconductivity has been the subject of intense discussions for many years. In iron-based superconductors, the most established probe for electronic-nematic fluctuations, i.e. the elastoresistivity seems to imply that superconductivity is reinforced by electronic-nematic fluctuations, since the elastoresistivity amplitude peaks at or close to optimal Tc. However, on the over-doped side of the superconducting dome, the diminishing elastoresistivity suggests a negligible importance in the mechanism of superconductivity. Here we introduce the Nernst coefficient as a genuine probe for electronic nematic fluctuations, and we show that the amplitude of the Nernst coefficient tracks the superconducting dome of two prototype families of iron-based superconductors, namely Rh-doped BaFe2As2 and Co-doped LaFeAsO. Our data thus provide fresh evidence that in these systems, nematic fluctuations foster the superconductivity throughout the phase diagram.

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

FeTe1−x Se x is a family of iron-based superconductors with its critical temperature (T c) dependent on the composition of Se. A well-known T c 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 T c, 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 T c and superconducting gap for FeTe0.5Se0.5 soars under increasing compression, consistent with the results of experiment.

Nodal multigap superconductivity in the anisotropic iron-based compound RbCa2Fe4As4F2

The 12442 compounds are a recently discovered family of iron-based superconductors, that share several features with the cuprates due to their strongly anisotropic structure, but are so far poorly understood. Here, we report on the gap structure and anisotropy of RbCa2(Fe1−xNix)4As4F2 single crystals, investigated by a combination of directional point-contact Andreev-reflection spectroscopy and coplanar waveguide resonator measurements. Two gaps were identified, with clear signatures of d-wave-like nodal structures which persist upon Ni doping, well described by a two-band d − d state with symmetry-imposed nodes. A large London penetration depth anisotropy was revealed, weakly dependent on temperature and fully compatible with the d − d model.

Electronic and Superconducting Properties of the AFeAs (A = Li, Na) Family Alkali-Metal Pnictides: Current Stage of the Research (Brief Review)

The review is focused on one of the most exotic families of iron-based superconductors belonging to the AFeAs structural type, where A is alkali metal. We briefly concern physical and electron properties of the typical members of this family, LiFeAs and NaFeAs, discuss the theoretical models describing the multiple-gap superconducting state, and the experimental data available in literature. As well, we specify the main unsolved problems, that seem crucial for both the AFeAs family and for iron-based superconductors in general.

Hund's metal crossover and superconductivity in the 111 family of iron-based superconductors

We study LiFeP, LiFeAs and NaFeAs in their paramagnetic metallic phase including dynamical electronic correlations within a density functional theory + slave-spin mean-field framework. The three compounds are found to lie next to the crossover between a normal and a Hund's metal, where a region of enhanced electronic compressibility that may boost superconductivity is systematically present in this type of systems. We find that LiFeP lies in the normal metallic regime, LiFeAs at the crossover, and NaFeAs is in the Hund's metal regime, which possibly explains the different experimental trends for the pressure- and doping-dependence of superconductivity in these compounds. Our picture captures the orbitally-resolved mass renormalizations measured in these materials, while an analysis of the Sommerfeld specific-heat coefficient highlights some limitations of currently used implementations of density-functional theory for the correct prediction of the details of band structures in the iron-based superconductors.

Nematic quantum criticality in an Fe-based superconductor revealed by strain-tuning.

Quantum criticality may be essential to understanding a wide range of exotic electronic behavior; however, conclusive evidence of quantum critical fluctuations has been elusive in many materials of current interest. An expected characteristic feature of quantum criticality is power-law behavior of thermodynamic quantities as a function of a nonthermal tuning parameter close to the quantum critical point (QCP). Here, we observed power-law behavior of the critical temperature of the coupled nematic/structural phase transition as a function of uniaxial stress in a representative family of iron-based superconductors, providing direct evidence of quantum critical nematic fluctuations in this material. These quantum critical fluctuations are not confined within a narrow regime around the QCP but rather extend over a wide range of temperatures and compositions.

Electronic structure and coexistence of superconductivity with magnetism in RbEuFe4As4

In the novel stoichiometric iron-based material RbEuFe$_{4}$As$_{4}$ superconductivity coexists with a peculiar long-range magnetic order of Eu 4f states. Using angle-resolved photoemission spectroscopy, we reveal a complex three dimensional electronic structure and compare it with density functional theory calculations. Multiple superconducting gaps were measured on various sheets of the Fermi surface. High resolution resonant photoemission spectroscopy reveals magnetic order of the Eu 4f states deep into the superconducting phase. Both the absolute values and the anisotropy of the superconducting gaps are remarkably similar to the sibling compound without Eu, indicating that Eu magnetism does not affect the pairing of electrons. A complete decoupling between Fe- and Eu-derived states was established from their evolution with temperature, thus unambiguously demonstrating that superconducting and a long range magnetic orders exist independently from each other. The established electronic structure of RbEuFe$_{4}$As$_{4}$ opens opportunities for the future studies of the highly unorthodox electron pairing and phase competition in this family of iron-based superconductors with doping.

Revealing the intrinsic superconducting gap anisotropy in surface-neutralized BaFe2(As0.7P0.3)2

Alkaline-earth iron arsenide (122) is one of the most studied families of iron-based superconductors, especially for angle-resolved photoemission spectroscopy. While extensive photoemission results have been obtained, the surface complexity of 122 caused by its charge-non-neutral surface is rarely considered. Here, we show that the surface of 122 can be neutralized by potassium deposition. In potassium-coated BaFe2(As0.7P0.3)2, the surface-induced spectral broadening is strongly suppressed, and hence the coherent spectra that reflect the intrinsic bulk electronic state recover. This enables the measuring of superconducting gap with unpreceded precision. The result shows the existence of two pairing channels. While the gap anisotropy on the outer hole/electron pockets can be well fitted using an s± gap function, the gap anisotropy on the inner hole/electron shows a clear deviation. Our results provide quantitative constraints for refining theoretical models and also demonstrate an experimental method for revealing the intrinsic electronic properties of 122 in future studies.

Recent development of iron-based superconducting films

<p indent="0mm">The discovery of F-doped LaFeAsO in 2008 brought a worldwide research surge of iron-based superconductors. As a new type of high-temperature superconductors, iron-based superconductors have attracted much attention due to their unique properties and significant differences from copper oxide and traditional superconductors. Iron-based superconductors have small anisotropy, high superconducting transition temperature and upper critical field, which are preferred in film preparation and applications. At the same time, the features of iron-based superconductor films, such as higher superconducting transition temperature in epitaxial films and the ability to stabilize metastable equivalents, are interesting to the researchers. The studies on films are very useful for the basic research on superconducting physics and the search of new superconducting materials. The iron-based superconductors discovered so far can be divided into four systems: (1) 1111 system: LnFeAs (O, F) (Ln= lanthanides); (2) 122 system: AEFe₂As₂ (AE = alkaline earth metals); (3) 111 system: LiFeAs and NaFeAs; (4) 11 system: FeCh (Ch=chalcogenides). 1111 system is the first reported iron-based superconductor. It has a high critical transition temperature up to <sc>58 K</sc> as well as a higher upper critical field. The disadvantage of 1111 superconductors is the lager anisotropy and the broadened superconducting transition under the magnetic field. The critical transition temperature of 122 system is lower than 1111 system, but it can still reach <sc>38 K,</sc> and its anisotropy and elemental stability is better than 1111 system. The structure of 11 system is the simplest in iron-based superconductor family. It contains no toxic element As, but the transition temperature is low <sc>(8 K).</sc> 111 system is unstable in the air, limiting its application prospects. In addition, the critical current density is very important for the application of superconductors, which has been obtained in 122-system superconducting tapes. Meanwhile, the 11-system superconducting film stands out because of its easy growth, simple structure and high transport current density, which can exceed <sc>10⁵ A/cm²</sc> even at <sc>30 T.</sc> During the past years, researchers around the world have made great progress in the fabrication and synthesis of iron-based superconducting single crystals, bulk materials, tapes and thin films. Among them, the development of thin films cannot be ignored. The fabrication of high-quality thin films has three purposes: Basic research, tape application and electronic application. Basic and application-oriented research is developing quickly. Increasing the critical current density is an important strategy for realizing commercialized and cost-effective high-temperature superconductor technology, which can be improved by microstructure engineering. Naturally grown pinning centers and artificial pinning centers are able to pin on magnetic vortices in magnetic fields, which can effectively prevent the dissipation of energy and raise critical current density. Based on the results achieved in 1111 system, doped 122 system, and 11 system, we analyzed the possible connections between the growth technique, microstructure and superconducting properties of iron-based superconducting films.

Soft elastic constants from phonon spectroscopy in hole-doped Ba1−x(K,Na)xFe2As2 and Sr1−xNaxFe2As2

We report inelastic x-ray scattering measurements of the in-plane polarized transverse acoustic phonon mode propagating along $q\parallel$[100] in various hole-doped compounds belonging to the 122 family of iron-based superconductors. The slope of the dispersion of this phonon mode is proportional to the square root of the shear modulus $C_{66}$ in the $q \rightarrow 0$ limit and, hence, sensitive to the tetragonal-to-orthorhombic structural phase transition occurring in these compounds. In contrast to a recent report for Ba(Fe$_{0.94}$Co$_{0.06}$)$_2$As$_2$ [F. Weber et al., Phys. Rev. B 98, 014516 (2018)], we find qualitative agreement between values of $C_{66}$ deduced from our experiments and those derived from measurements of the Youngs modulus in Ba$_{1-x}$(K,Na)$_x$Fe$_2$As$_2$ at optimal doping. These results provide an upper limit of about 50 {\AA} for the nematic correlation length for the optimally hole-doped compounds. Furthermore, we also studied compounds at lower doping levels exhibiting the orthorhombic magnetic phase, where $C_{66}$ is not accessible by volume probes, as well as the C4 tetragonal magnetic phase.investigated

Superconductivity of underdoped PrFeAs(O,F) investigated via point-contact spectroscopy and nuclear magnetic resonance

Underdoped PrFeAs(O,F), one of the less known members of the 1111 family of iron-based superconductors, was investigated in detail by means of transport, SQUID magnetometry, nuclear magnetic resonance (NMR) measurements and point-contact Andreev-reflection spectroscopy (PCARS). PCARS measurements on single crystals evidence the multigap nature of PrFeAs(O,F) superconductivity, shown to host at least two isotropic gaps, clearly discernible in the spectra, irrespective of the direction of current injection (i.e., along the ab planes or along the c axis). Additional features at higher energy can be interpreted as signatures of a strong electron-boson coupling, as demonstrated by a model which combines Andreev reflection with the Eliashberg theory. Magnetic resonance measurements in the normal phase indicate the lack of a magnetic order in underdoped PrFeAs(O,F), while $^{75}$As NMR spin-lattice relaxation results suggest the presence of significant electronic spin fluctuations, peaking above $T_{c}$ and expected to mediate the superconducting pairing.

Developments of (Ba,Na)Fe2As2 and CaKFe4As4 HIP round wires

With excellent superconducting properties such as high Tc, large Hc2, small anisotropy, and large Jc, developments of superconducting wires and tapes of iron-based superconductors (IBSs) have been undertaken with promising results. So far, K-doped 122-type IBSs have been mainly developed for such purposes. Here, efforts to fabricate Na-doped 122-type IBS, (Ba,Na)Fe2As2, are presented. With the help of hot isostatic pressing (HIP) process, excellent Jc characteristics of up to 44 kA cm−2 at 4.2 K under 100 kOe have been achieved. Detailed characterizations of (Ba,Na)Fe2As2 HIP round wire are presented. On the other hand, discoveries of new IBSs continue, with CaKFe4As4 as one of recent examples. CaKFe4As4 belongs to a new family of IBSs with 1144 structure, where Ca and K occupy the Ba site in BaFe2As2 alternately along the c-axis. Studies on CaKFe4As4 single crystals have demonstrated moderately large Jc with peculiar anisotropic properties related to the presence of novel planar defects. We have also fabricated HIP round wires of CaKFe4As4, and reported their basic characterizations. Despite the instability of CaKFe4As4 at low temperatures, Jc values at 4.2 K close to 100 kA cm−2 at self-field and 7.6 kA cm−2 at 100 kOe are achieved. Attempts to introduce defects into CaKFe4As4 single crystals, chemically by Co substitution and physically by proton irradiation, are attempted, and the effects are compared. Attempts to fabricate Co-doped CaKFe4As4 HIP round wires with limited success are also reported.

Topological superconductivity facilitated by exchange interaction on the surface of FeTe0.5Se0.5

FeTe1−xSex is a family of iron-based superconductors with a well-known critical temperature (Tc) of 14.5 K for x = 0.45. Also well-established is the presence of topological surface states of FeTe0.55Se0.45 in which topological superconductivity sets in. By using density functional calculations and the Bardeen–Cooper–Schrieffer (BCS) theory for traditional superconductors we calculated the Tc of the surface layers of FeTe0.5Se0.5 with the exchange interaction as the source of attractive force to form the Cooper pairs. The estimated Tc is in reasonable agreement with the experimental value and suggests that the exchange interaction is behind the topological superconductivity on the surface of FeTe0.55Se0.45.

Probing the anisotropy of (Li0.84Fe0.16)OHFe0.98Se by angular-dependent torque measurements

A strongly correlated superconductor with reduced dimension attracts much interest since its critical temperature (${T}_{c}$) is often enhanced. Superconducting anisotropy is the key parameter to characterize the dimensional reduction of a superconductor. $({\mathrm{Li}}_{0.84}{\mathrm{Fe}}_{0.16}){\mathrm{OHFe}}_{0.98}\mathrm{Se}$ (FeSe11111) is a newly found iron-based superconductor by intercalating (Li/Fe)OH layers into FeSe. The large distance between the FeSe layers leads to a highly two-dimensional character of the electronic structure compared to bulk FeSe. By performing out-of-plane angular-dependent torque measurements on FeSe11111 single-crystal samples with ${T}_{c}=40$ K, we directly obtain the superconducting anisotropy parameter $\ensuremath{\gamma}$. It is found that $\ensuremath{\gamma}$ reaches about 20 at a reduced temperature $t(=T/{T}_{c})=0.8$, suggesting that FeSe11111 is more anisotropic than other families of iron-based superconductors. Further, $\ensuremath{\gamma}$ shows a temperature-dependent behavior, which points towards a multiband scenario. Our result provides thermodynamic evidence for the two-dimensional nature of the superconductivity in FeSe11111.

The Synthesis of a Quasi-One-Dimensional Iron-Based Telluride with Antiferromagnetic Chains and a Spin Glass State.

The report on the superconductivity of the two-legged spin ladders BaFe2S3 and BaFe2Se3 has established 123-type iron chalcogenides as a novel subgroup in the iron-based superconductor family and has stimulated the continuous exploration of other iron-based materials with new structures and potentially novel properties. In this paper, we report the systematic study of a new quasi-one-dimensional (1D) iron-based compound, Ba9Fe3Te15, including its synthesis and magnetic properties. The high-pressure synthesized Ba9Fe3Te15 crystallized in a hexagonal structure that mainly consisted of face-sharing FeTe6 octahedral chains running along the c axis, with a lattice constant of a = 10.23668 Å; this led to weak interchain coupling and an enhanced one-dimensionality. The systematic static and dynamic magnetic properties were comprehensively studied experimentally. The dc magnetic susceptibility showed typical 1D antiferromagnetic characteristics, with a Tmax at 190 K followed by a spin glass (SG) state with freezing at Tf ≈ 6.0 K, which were also unambiguously proved by ac susceptibility measurements. Additionally, X-ray magnetic circular dichroism (XMCD) experiments revealed an unexpected orbital moment for Fe2+, i.e., 0.84 μB per Fe in Ba9Fe3Te15. The transport property is electrically insulating, with a thermal activation gap of 0.32 eV. These features mark Ba9Fe3Te15 as an alternative type of iron-based compound, providing a diverse candidate for high-pressure studies in order to pursue some emerging physics.

Electronic structure and spatial inhomogeneity of iron-based superconductor FeS**Project supported by CAS-Shanghai Science Research Center, China (Grant No. CAS-SSRC-YH-2015-01), the National Key R&D Program of China (Grant No. 2017YFA0305400), the National Natural Science Foundation of China (Grant Nos. 11674229, 11227902, and 11604207), the EPSRC Platform Grant (Grant No. EP/M020517/1), Hefei Science Center, Chinese Academy of Sciences (Grant No. 2015HSC-UE013), Science

Iron-based superconductor family FeX (X = S, Se, Te) has been one of the research foci in physics and material science due to their record-breaking superconducting temperature (FeSe film) and rich physical phenomena. Recently, FeS, the least studied FeX compound (due to the difficulty in synthesizing high quality macroscopic crystals) attracted much attention because of its puzzling superconducting pairing symmetry. In this work, combining scanning tunneling microscopy and angle resolved photoemission spectroscopy (ARPES) with sub-micron spatial resolution, we investigate the intrinsic electronic structures of superconducting FeS from individual single crystalline domains. Unlike FeTe or FeSe, FeS remains identical tetragonal structure from room temperature down to 5 K, and the band structures observed can be well reproduced by our ab-initio calculations. Remarkably, mixed with the 1 × 1 tetragonal metallic phase, we also observe the coexistence of reconstructed insulating phase in the crystal, which not only helps explain the unusual properties of FeS, but also demonstrates the importance of using spatially resolved experimental tools in the study of this compound.

Distinct pressure evolution of coupled nematic and magnetic orders in FeSe

FeSe, despite being the structurally simplest compound in the family of iron-based superconductors, shows an astoundingly rich interplay of physical phenomena including nematicity and pressure-induced magnetism. Here, we present a microscopic study of these two phenomena by high-energy x-ray diffraction and time-domain M\"ossbauer spectroscopy on FeSe single crystals over a wide temperature and pressure range. The topology of the pressure-temperature phase diagram is a surprisingly close parallel to the well-known doping-temperature phase diagram of BaFe2As2 generated through partial Fe/Co and Ba/Na substitution. In FeSe with pressure p as a control parameter, the magneto-structural ground state can be tuned from "pure" nematic - paramagnetic with an orthorhombic lattice distortion - through a strongly coupled magnetically ordered and orthorhombic state to a magnetically ordered state without an orthorhombic lattice distortion. The magnetic hyperfine field increases monotonically over a wide pressure range. However, the orthorhombic distortion initially decreases under increasing pressure, but is stabilized by cooperative coupling to the pressure-induced magnetic order. Close to the reported maximum of the superconducting critical temperature Tc (occuring at p = 6.8 GPa), the orthorhombic distortion suddenly disappears and FeSe remains tetragonal down to the lowest temperature measured. Analysis of the structural and magnetic order parameters suggests an independent origin of the structural and magnetic ordering phenomena, and their cooperative coupling leads to the similarity with the canonical phase diagram of iron pnictides.

Plaquette instability competing with bicollinear ground state in detwinned FeTe

We use inelastic neutron scattering to show that long-range spin waves arising from the static bicollinear antiferromagnetic (AF) order in FeTe, which have twofold rotational symmetry in a fully detwinned crystal, rapidly dissolve above $E\ensuremath{\approx}26\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ into ridges of scattering with fourfold rotational symmetry and a nearly isotropic magnetic fluctuation spectrum. With increasing temperature above ${T}_{N}\ensuremath{\approx}68\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, the twofold spin waves change into broad regions of scattering with fourfold symmetry. Since the scattering patterns from plaquette magnetic order generated within a bilinear biquadratic Hamiltonian have fourfold rotational symmetry consistent with the high-energy, spin-isotropic spin waves of FeTe, we conclude that the bicollinear AF state in FeTe is quasidegenerate with plaquette magnetic order, providing evidence of the strongly frustrated nature of the local moments in the iron chalcogenide family of iron-based superconductors.

Energy scale of nematic ordering in the parent iron-based superconductor BaFe2As2

Nematicity plays an important role in the physics of iron-based superconductors (IBS). Its microscopic origin and in particular its importance for the mechanism of high-temperature superconductivity itself are highly debated. A crucial knowledge in this regard is the degree to which the nematic order influences the electronic structure of these materials. Earlier angle-resolved photoemission spectroscopy (ARPES) studies found that the effect is dramatic in three families of IBS including 11, 111 and 122 compounds: energy splitting reaches 70 meV and Fermi surface becomes noticeably distorted. More recent experiments, however, reported significantly lower energy scale in 11 and 111 families, thus questioning the degree and universality of the impact of nematicity on the electronic structure of IBS. Here we revisit the electronic structure of undoped parent BaFe2As2 (122 family). Our systematic ARPES study including the detailed temperature and photon energy dependencies points to the significantly smaller energy scale also in this family of materials, thus establishing the universal scale of this phenomenon in IBS. Our results form a necessary quantitative basis for theories of high-temperature superconductivity focused on the nematicity.