Iron-based Superconductors Research Articles

Unveiling Electron‐Phonon and Electron‐Magnon Interactions in the Weak Itinerant Ferromagnet LaCo2P2

AbstractStudying and understanding many‐body interactions, particularly electron‐boson interactions, is essential for a deeper elucidation of fundamental physical phenomena and the development of novel material functionalities. Here, this aspect is explored in the weak itinerant ferromagnet LaCo2P2 by means of momentum‐resolved photoelectron spectroscopy (ARPES) and first‐principles calculations. The detailed ARPES patterns enable to unveil bulk and surface bands, spin splittings due to Rashba and exchange interactions, as well as the evolution of bands with temperature, which altogether creates a solid foundation for theoretical studies. The latter has allowed to establish the impact of electron‐boson interactions on the electronic structure, that are reflected in its strong renormalization driven by electron‐magnon interaction and the emergence of distinctive kinks of surface and bulk electron bands due to significant electron‐phonon coupling. Our results highlight the distinct impact of electron‐boson interactions on the electronic structure, particularly on the itinerant d states. Similar electronic states are observed in the isostructural iron pnictides, where electron‐boson interactions play a crucial role in the emergence of superconductivity. It is believed that further studies of material systems involving both magnetically active d‐ and f‐sublattices will reveal more advanced phenomena in the bulk and at distinct surfaces, driven by a combination of factors including Rashba and Kondo effects, exchange magnetism, and electron‐boson interactions.

Microstructure Morphology of Chemical and Structural Phase Separation in Thermally Treated KxFe2-ySe2 Superconductor.

The iron-based KxFe2-ySe2 superconductor displays chemical, structural and electronic phase separation at nanoscale to microscale, leading to the coexisting metallic phase embedded in an antiferromagnetic host matrix. The metallic character of the system is believed to arise from a percolative granular network affecting its transport in the normal as well as in the superconducting state. This percolative network can be manipulated and controlled through thermal treatments. In this study, we have used scanning X-ray micro-fluorescence to visualize morphology of the chemical phase separation coupled to the percolation in KxFe2-ySe2, manipulated by two distinct thermal treatments, i. e., fast quenching and slow cooling. We find a differing spatial correlation between Fe and K distributions in the two samples, ascribed to a different degree of Fe vacancy ordering. We have also identified an intermediate phase that acts as an interface between the two main phases. The high temperature quenching produces directionally oriented clustered microstructure in which the percolation threshold is lower and hence a more effective transport networks. Instead, the slow cooling results in larger interfaces around the percolation threshold that seems to affect the superconducting properties of the system. The results provide a quantitative characterization of microstructural morphology of differently grown KxFe2-ySe2 showing potential for the design of electronic devices based on sub-micron scale chemical phase separation, thus opening avenues for further studies of complex heterogeneous functional structures.

Dispersion kinks from electronic correlations in an unconventional iron-based superconductor

The attractive interaction in conventional BCS superconductors is provided by a bosonic mode. However, the pairing glue of most unconventional superconductors is unknown. The effect of electron-boson coupling is therefore extensively studied in these materials. A key signature is dispersion kinks that can be observed in the spectral function as abrupt changes in velocity and lifetime of quasiparticles. Here, we show the existence of two kinks in the unconventional iron-based superconductor RbFe2As2 using angle-resolved photoemission spectroscopy (ARPES) and dynamical mean field theory (DMFT). In addition, we observe the formation of a Hubbard band multiplet due to the combination of Coulomb interaction and Hund’s rule coupling in this multiorbital system. We demonstrate that the two dispersion kinks are a consequence of these strong many-body interactions. This interpretation is in line with a growing number of theoretical predictions for kinks in various general models of correlated materials. Our results provide a unifying link between iron-based superconductors and different classes of correlated, unconventional superconductors such as cuprates and heavy-fermion materials.

Crystal growth and pressure effect on the transport properties in 122-type (La,Na,K)Fe2As2 superconductors

Abstract High-quality single crystals are essential for probing the intrinsic properties of unconventional superconductors. In this work, we report the successful growth of (La,Na,K)Fe2As2 single crystals, a novel 122-type iron-based superconductor, using the self-flux method. The crystal structure and chemical composition were characterized through x-ray diffraction and energy-dispersive x-ray spectroscopy. The (La,Na,K)Fe2As2 single crystals exhibit bulk superconductivity, with a critical transition temperature Tc of approximately 22 K. Magnetization measurements reveal a second peak effect and a critical current density Jc ~ 5×105 A/cm2 at 2 K in a self-field. The upper critical field anisotropy was systematically studied within the framework of the anisotropic Ginzburg–Landau theory, yielding an anisotropy value of approximately 2.6. Furthermore, we employ the diamond anvil cell technique to investigate the pressure effect on (La,Na,K)Fe2As2. Our results demonstrate that superconductivity is gradually suppressed with increasing pressure, while the normal state resistivity at low temperatures evolves from non-Fermi liquid to Fermi liquid behavior. This observation suggests that (La,Na,K)Fe2As2 may be situated on the right of the quantum critical point or, at the very least, within the over-doped region. These findings provide a critical sample platform and experimental insights for advancing the understanding of the physical properties of the newly discovered (La,Na,K)Fe2As2 superconductors.

Protracted Kondo coherence with dilute carrier density in cerium based nickel pnictides

Nozières' exhaustion theory argues that the temperature for coherently screening of all local moments in Kondo lattice could be much lower than the temperature of single moment screening in case of insufficient conduction electrons. Experiments indicate that cerium based nickel pnictides CeNi2−δAs2(δ≈0.28) is an ideal material to exam such protracted Kondo screening. Using density functional theory plus dynamical mean-field theory, we found CeNi2As2 presents a transition from the local moment states to the coherent Kondo screening states under compression, accompanied by topological changes of the Fermi surface. Similar phase transition could be driven by chemical pressure via the isovalence As→P substitution. The derived coherent Kondo temperature Tcoh is much lower than the single-impurity Kondo temperature TK due to the diluted Ni-3d conduction electrons, indicating the protraction of Kondo screening in crystal. In contrast to structurally analogous layered iron pnictides, the electronic structures of the present systems show strong three-dimensionality.

Iron-Based Superconductors for High-Field Applications: Realization of High Engineering Critical Current Density.

Iron-based superconductors have strong potential for magnet applications through their very high upper critical field, low anisotropy and manufacturability through the powder-in-tube (PIT) route. The engineering critical current density (Je) is a key parameter for measuring the maximum current density that superconducting materials can withstand in practical applications. It serves as a bridge between theoretical research and practical applications of superconductors and has great significance in promoting the development and application of superconducting technology. In this study, Ag sheathed Ba0.6K0.4Fe2As2 (Ba-122) iron-based superconducting tapes were prepared by using the process of drawing, flat rolling and heat treatment by hot pressing (HP). For the first time, the filling factor of the tapes increased to about 40%, leading to a reduction in the volume fraction of Ag, consequently lowering the overall cost. The optimal parameters for achieving high transport Je were obtained by comparing the effects of different HP pressures on the properties and micro-morphology of the tapes. The prepared mono-filament tapes are capable of carrying the transport Je of 4.1 × 104 A/cm2 (Ic = 350 A) at 4.2 K, 10 T, marking the highest Je reported for Ba-122 wires and tapes to date. Our results show that high transport Je can be obtained in Ba-122 superconducting tapes, and iron-based superconductors have a promising future in practical applications.

Magnetic Memory Effects in BaFe2(As0.68P0.32)2 Superconducting Single Crystal.

Among many iron-based superconductors, isovalently substituted BaFe2(As1-xPx)2 displays, for x ≈ 0.3, apart from the quite usual Second Magnetization Peak (SMP) in the field dependence of the critical current density, an unusual peak effect in the temperature dependence of the critical current density in the constant field, which is related to the rhombic-to-square (RST) structural transition of the Bragg vortex glass (BVG). By using multi-harmonic AC susceptibility investigations in three different cooling regimes-field cooling, zero-field cooling, and field cooling with measurements during warming up-we have discovered the existence of a temperature region in which there is a pronounced magnetic memory effect, which we attributed to the direction of the structural transition. The observed huge differences in the third harmonic susceptibility at low and high AC frequencies indicates the difference in the time-scale of the structural transition in comparison with the timescale of the vortex excitations. Our findings show that the RST influence on the vortex dynamics goes beyond the previously observed influence on the onset of the SMP.

Persistent flat band splitting and strong selective band renormalization in a kagome magnet thin film.

Magnetic kagome materials provide a fascinating playground for exploring the interplay of magnetism, correlation and topology. Many magnetic kagome systems have been reported including the binary FemXn (X = Sn, Ge; m:n = 3:1, 3:2, 1:1) family and the rare earth RMn6Sn6 (R = rare earth) family, where their kagome flat bands are calculated to be near the Fermi level in the paramagnetic phase. While partially filling a kagome flat band is predicted to give rise to a Stoner-type ferromagnetism, experimental visualization of the magnetic splitting across the ordering temperature has not been reported for any of these systems due to the high ordering temperatures, hence leaving the nature of magnetism in kagome magnets an open question. Here, we probe the electronic structure with angle-resolved photoemission spectroscopy in a kagome magnet thin film FeSn synthesized using molecular beam epitaxy. We identify the exchange-split kagome flat bands, whose splitting persists above the magnetic ordering temperature, indicative of a local moment picture. Such local moments in the presence of the topological flat band are consistent with the compact molecular orbitals predicted in theory. We further observe a large spin-orbital selective band renormalization in the Fe spin majority channel reminiscent of the orbital selective correlation effects in the iron-based superconductors. Our discovery of the coexistence of local moments with topological flat bands in a kagome system echoes similar findings in magic-angle twisted bilayer graphene, and provides a basis for theoretical effort towards modeling correlation effects in magnetic flat band systems.

Exploring Unconventional Electron Distribution Patterns: Contrasts Between FeSe and FeSe/STO Using an Ab Initio Approach.

For more than a decade, the unusual distribution of electrons observed in ARPES (angle-resolved photoemission spectroscopy) data within the energy range of ~30 meV to ~300 meV below the Fermi level, known as the ARPES energy range, has remained a puzzle in the field of iron-based superconductivity. As the electron-phonon coupling of FeSe/SrTiO3 is very strong, our investigation is centered on exploring the synergistic interplay between spin-density waves (SDW) and charge-density waves (CDW) with differential phonons at the interface between antiferromagnetic maxima and minima under wave interference. Our analysis reveals that the synergistic energy is proportional to the ARPES energy range, as seen in the comparison between FeSe and FeSe/SrTiO3. This finding may suggest that the instantaneous interplay between these intricate phenomena may play a role in triggering the observed energy range in ARPES.

Coherence Length of Electronic Nematicity in Iron-Based Superconductors

Coherence Length of Electronic Nematicity in Iron-Based Superconductors

Review of the research status of practical superconducting materials and their current carrying performance

Abstract Superconducting materials hold great potential in high field magnetic applications compared to traditional conductive materials. At present, practical superconducting materials include low-temperature superconductors such as NbTi and Nb3Sn, hightemperature superconductors such as Bi-2212, Bi-2223, YBCO, iron-based superconductors and MgB2. The development of low-temperature superconducting wires started earlier and has now entered the stage of industrialized production, showing obvious advantages in mechanical properties and cost under low temperature and middle-low magnetic field. However, due to the insufficient intrinsic superconducting performance, low-temperature superconductors are unable to exhibit excellent performance at high temperature or high fields. Further improvement of supercurrent carrying performance mainly depends on the enhancement of pinning ability. Hightemperature superconductors have greater advantages in high temperature and high field, but many of them are still in the stage of further performance improvement. Many high-temperature superconductors are limited by the deficiency in their polycrystalline structure, and further optimization of intergranular connectivity is required. In addition, it is also necessary to further enhance their pinning ability. The numerous successful application instances of high-temperature superconducting wires and tapes also prove their tremendous potential in electric power applications.

Collapse of metallicity and high-Tc superconductivity in the high-pressure phase of FeSe0.89S0.11

We investigate the high-pressure phase of the iron-based superconductor FeSe0.89S0.11 using transport and tunnel diode oscillator studies using diamond anvil cells. We construct detailed pressure-temperature phase diagrams that indicate that the superconducting critical temperature is strongly enhanced by more than a factor of four towards 40 K above 4 GPa. The resistivity data reveal signatures of a fan-like structure of non-Fermi liquid behaviour which could indicate the existence of a putative quantum critical point buried underneath the superconducting dome around 4.3 GPa. With further increasing the pressure, the zero-field electrical resistivity develops a non-metallic temperature dependence and the superconducting transition broadens significantly. Eventually, the system fails to reach a fully zero-resistance state, and the finite resistance at low temperatures becomes strongly current-dependent. Our results suggest that the high-pressure, high-Tc phase of iron chalcogenides is very fragile and sensitive to uniaxial effects of the pressure medium, cell design and sample thickness. This high-pressure region could be understood assuming a real-space phase separation caused by nearly concomitant electronic and structural instabilities.

Effect of substrate temperature on the growth mechanism of FeSe superconducting films

Abstract Among all the iron-based superconductors (IBSs), Fe-chalcogenides (Fe-Ch) possess the advan tages of simple structure, non-toxicity, low anisotropy and the potential for high-field applications.One of the most effective methods for preparing Fe-Ch films is pulsed laser deposition (PLD). However, high quality film growth is challenged by the significant volatility difference between the iron
and chalcogens (S, Se, Te), which affects stoichiometric transfer from target to substrate and subsequently impacts superconductivity. Currently, there is limited research on the correlation between the growth mechanism and preparation conditions of Fe-Ch films, particularly in explaining chalcogen volatility during deposition. Technically, PLD offers various adjustable parameters, among
which the substrate temperature (Ts) is the most critical one affecting film quality. It governs particle behavior on the substrate including adsorption, diffusion, bonding reactions, crystallization processes while also strongly affecting volatilization rates. In this study, we fabricated a series of FeSe films using PLD at different Ts ranging from 25 ℃ to 750 ℃.We reveal in detail the impact of
competitive processes between Se volatilization and reactive crystallization of Fe/Se on film quality. The optimal film was obtained at Ts=500 ℃ with a composition ratio of Fe1.01Se. We also investigate the correlations between Ts and film crystallinity, surface morphology, along with their influence on superconductivity. Additionally, the pinning mechanism in FeSe film grown at the
optimal Ts was briefly analyzed. Our results provide reliable experimental evidence regarding the growth mechanism of FeSe films while revealing comprehensive insights into how Ts affects both film quality and performance.

Enhanced mechanical strength and texture of (Ba,K)Fe2As2 Cu/Ag composite sheathed tapes with Nb barrier layer

Abstract Iron-based superconductors with ultra-high upper critical fields and low anisotropy have attracted much attention for superconducting mechanisms and high-field applications. In practical applications, improving the mechanical strength and heat treatment temperature of superconducting tapes is of great significance for the improvement of transport current as well as stability. In this paper, (Ba, K)Fe2As2(Ba-122) superconducting tapes with Cu/Nb/Ag composite sheaths were successfully fabricated using a pre-composite process, which provides a feasible method for the fabrication of high-strength superconducting wires and tapes. It is shown that Cu/Nb/Ag composite sheathed tapes can be sintered at 880 °C, and tapes sintered at 880 °C have the highest transport properties as well as excellent superconductivity of the superconducting cores, as demonstrated by a series of characterizations. In addition, other superconducting properties of the tapes sintered at 880 °C, including grain orientation, flux pinning, upper critical field and irreversible field, were also studied. It was found that none of the three sheaths fractured after sintering and the superconducting core had a high c-axis texture and densities. The high mechanical strength of the Cu/Nb/Ag composite sheathed tape was also demonstrated by comparative tensile experiments. The results indicate that the low-cost Ba-122 tapes with Cu/Nb/Ag composite sheaths hold great promise for future practical applications.

Influence of Pressure on Electronic, Magnetic Behavior, and Fermi Surface Studies of SrFe2X2 (X = P, As, Sb) Iron-Based Superconductors

Influence of Pressure on Electronic, Magnetic Behavior, and Fermi Surface Studies of SrFe2X2 (X = P, As, Sb) Iron-Based Superconductors

Cryogenic Digital Image Correlation as a Probe of Strain in Iron-Based Superconductors

Abstract Uniaxial strain is a powerful tuning parameter that can control symmetry and anisotropic electronic properties in iron-based superconductors. However, accurately characterizing anisotropic strain can be challenging and complex. Here, we utilize a cryogenic optical system equipped with a high-spatial-resolution microscope to characterize surface strains in iron-based superconductors using the digital image correlation method. Compared with other methods such as high-resolution X-ray diffraction, strain gauge, and capacitive sensor, digital image correlation offers a non-contact, full-field measurement approach, acting as an optical virtual strain gauge that provides high spatial resolution. The results measured on detwinned BaFe2As2 are quantitatively consistent with the distortion measured by X-ray diffraction and neutron Larmor diffraction. These findings highlight the potential of cryogenic digital image correlation as an effective and accessible tool for probing the isotropic and anisotropic strains, facilitating the application of uniaxial strain tuning in the study of quantum materials.

A mesoscopic device for a realization of the topological Kondo effect

Abstract The search for anyons is a field of immense interest owing to its potential application in the field of quantum information. Quantum critical Kondo impurities constitute one possible platform for their realization and topological Kondo effect (TKE) by virtue of remaining critical in the presence of perturbations, seems to be especially promising in this regard. In this paper we discuss practical steps for a realization of TKE with a relatively high Kondo temperature T K . Its central feature is the so-called Majorana-Cooper box (MCB) and we argue that a particular type of iron-based topological superconductor is especially suitable for realization of TKE. Once MCB is available one needs to connect it to external metallic leads to produce TKE. A relatively high value of the Kondo temperature T K is then aided by a large superconducting gap of the iron-based superconductor. We give estimates for T K , for the cases of both isotropic and anisotropic exchange couplings of MCB with the leads.

Multiple magnetic orders discovered in the superconducting state of EuFe2(As1−xPx)2

Abstract The interplay between superconductivity and magnetism is an important subject in condensed matter physics. EuFe2As2-based iron pnictides could offer an interesting plateau to study their relationship that has attracted considerable attention. So far, two magnetic phase transitions were observed in EuFe2As2-based crystal, which were deemed to originate from the itinerant Fe moments ( 190 K) and the localized Eu2+ moments ( 19 K), respectively. Here, we systematically studied the heat capacity for the EuFe2(As1−xPx)2 crystals with x = 0.21 (optimally doped) and x = 0.29 (overdoped). We have found two new magnetic orders in the superconducting state (ranging from 0.4 to 1.2 K) in the optimally doped crystal. As more P was introduced into the As site, one of the magnetic orders becomes absent in the overdoped crystal. Additionally, we observed strong field and orientation dependence in heat capacity. The present findings in EuFe2(As1−xPx)2 have detected the new low-temperature magnetic orders, which may originate from the localized Eu2+ spins order or the spin reorientation.

High tolerance of the superconducting current to large grain boundary angles in potassium-doped BaFe2As2

Superconducting magnets based on high-temperature superconductors (HTSs) have become critical components in cutting-edge technologies such as advanced medical applications. In HTSs, weak links of superconductivity are inevitable at high-angle grain boundaries (GBs). Thus, two adjacent grains should be crystallographically aligned within the critical angle (θc), for which the intergrain critical current density (Jc) starts to decrease exponentially. The θc of several iron-based superconductors (IBSs) is larger than that of cuprates. However, the decreases in both θc and intergrain Jc under magnetic fields for IBSs are still substantial, hampering their applications in polycrystalline forms. Here, we report that potassium-doped BaFe2As2 (Ba122:K) exhibits superior GB performance to that of previously reported IBSs. A transport Jc of over 0.1 MA/cm2 across [001]-tilt GBs with misorientation angles up to θGB = 24° was recorded even at 28 K, which is a required level for practical applications. Additionally, even in an applied magnetic field, θc was unaltered, and the decay of the intergrain Jc was small. Our results highlight the exceptional potential of Ba122:K for polycrystalline applications and pave the way for next-generation superconducting magnets.

Effects of K excess in microstructure of (Ba0.6K0.4)Fe2As2 superconducting powders

Abstract Iron-based superconductors (IBSs) are promising for high-field applications due to their exceptional characteristics, like ultrahigh upper critical field and minimal electromagnetic anisotropy. Creating multifilamentary superconducting wires with elevated transport critical current density is essential for practical use. The Powder in Tube (PIT) technique is commonly used for this purpose, but achieving optimal results requires careful exploration of powder microstructural properties. This is particularly crucial for superconductors like (Ba,K)122, the IBS most promising from an applicative point of view, where factors such as reactivity, volatility, and toxicity of constituent elements affect phase formation. Potassium volatility often leads to nonstoichiometric conditions, introducing excess potassium in the formulation. This study focuses on the impact of potassium excess δ on the microstructural properties of the ‘optimally doped’ (Ba0.6K0.4+δ )Fe2As2 phase (0 ⩽ δ ⩽ 0.08). Using techniques like Scanning Electron Microscopy, x-ray diffraction, and temperature-dependent magnetization measurements, we demonstrate the ability to produce nearly pure powders of the superconducting phase with controlled grain size. Our findings are relevant for PIT wire fabrication, where grain size strongly affects mechanical deformation. Grain size also influences transport properties, as observed in previous studies, where reducing grain size enhanced current-carrying capability at high magnetic fields.