Advances in Cuprates and Iron-Based Superconductors: Physics, Properties, and Applications

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Introduction

The study of cuprates and iron-based superconductors continues to evolve rapidly, driven by the quest to understand unconventional pairing mechanisms and to harness their remarkable properties for high-field applications [1,2,3,4,5,6,7,8,9,10]. Despite major progress, key gaps remain in clarifying the microscopic origins of strange metallicity, the role of structural transitions in vortex dynamics, and the optimization of current-carrying capacity in practical conductors. This Special Issue on “Advances in Cuprates and Iron-Based Superconductors: Physics, Properties, and Applications”, addresses these challenges through six contributions that span theory, experimental study, and application. Together, they provide new insights into anomalous transport, vortex behavior, scalable fabrication routes, and interfacial phenomena, bridging fundamental physics with engineering solutions. Looking forward, future research should integrate advanced spectroscopic probes with theoretical modeling; explore heterostructures and interfacial effects to enhance superconducting performance; and develop scalable, cost-effective synthesis methods to fully realize the potential of these materials in next-generation quantum and energy technologies. This Editorial summarizes the six publications (five research articles and one review) included in this Special Issue.

The article on “The Shrinking Fermi Liquid Scenario for Cuprates Under the Scrutiny of Optical Conductivity Measurements” [11] analyzes the strange metallic state of cuprates. While optical conductivity data have often been interpreted through marginal Fermi liquid (MFL) theory [12,13], the authors propose a shrinking Fermi liquid (SFL) scenario [14,15] based on experimentally observed charge density fluctuations (CDFs) [16,17]. Within this framework, increased damping of CDFs at low temperatures shrinks the Fermi liquid regime, extending strange metal behavior down to very low temperatures [18]. Unlike MFL, SFL predicts finite quasiparticle mass and only approximate ω/T scaling, yet still reproduces optical conductivity features when combined with higher-energy excitations such as phonons and paramagnons [19]. The article’s conclusion states that optical experiments cannot uniquely distinguish MFL from SFL, making the latter a compelling, microscopically grounded alternative.

The article on “Magnetic Memory Effects in BaFe_2_(As_0.68_P_0.32_)2 Superconducting Single Crystal” [20] explores vortex dynamics in iron-based superconductors. The 122 family is notable for its high T_c_ and large upper critical fields [21,22], with P substitution inducing superconductivity and quantum critical behavior [23,24]. In slightly overdoped BaFe_2_(As_0.68_P_0.32_), besides the second magnetization peak (SMP), an anomalous peak effect in temperature appears, and is linked to the rhomb-to-square transition (RST) of the Bragg vortex glass [25,26]. In this work, multi-harmonic AC susceptibility under different cooling protocols revealed a distinct temperature region with a magnetic memory effect, reflecting the direction of the RST. Large differences between low- and high-frequency third harmonic signals show that the structural transition evolves on a slower timescale than vortex excitations. The paper concludes that RST strongly influences vortex dynamics beyond the SMP onset.

The article on “Iron-Based Superconductors for High-Field Applications: Realization of High Engineering Critical Current Density” [27] reports advances in Ba_0.6_K_0.4_Fe_2_As_2_ (Ba-122) tapes for high-field magnets. In general, iron-based superconductors combine high upper critical fields, low anisotropy, and cost-effective PIT fabrication [28,29,30]. In this work, by optimizing hot pressing pressure and increasing the filling factor to ~40%, silver-sheathed Ba-122 tapes achieved a record engineering critical current density (J_e_) of 4.1 × 10^4^ A/cm^2^ at 4.2 K and 10 T, the highest ever reported for Ba-122 conductors [31,32]. Microstructural analyses (XRD, SEM, and EBSD) showed improved grain size, density, and c-axis texture at optimal pressure, while excessive pressure introduced cracks. The results demonstrate that balancing the filling factor and microstructure is crucial, confirming Ba-122 tapes as strong candidates for practical applications in high-field technologies.

The paper on “Exploring Unconventional Electron Distribution Patterns: Contrasts Between FeSe and FeSe/STO Using an Ab Initio Approach” investigates the puzzling electron distribution observed in the ARPES spectra of iron-based superconductors [33,34]. Using ab initio calculations, the authors analyze the interplay of antiferromagnetism (AFM), spin-density waves (SDWs), charge-density waves (CDWs), and differential phonons [35,36]. In FeSe, these synergistic effects enhance electron–phonon coupling to values comparable with the ARPES energy range (~30–300 meV). In FeSe/SrTiO_3_, stronger interfacial phonons and SDW/CDW effects push the coupling to ~0.5 eV, matching experimental ARPES data [37,38]. The study concludes that magnetoelectric pulses from SDW/CDW phenomena may trigger the unconventional ARPES energy range, offering a microscopic explanation.

The article “Fe(Se,Te) Thin Films Deposited through Pulsed Laser Ablation from Spark Plasma Sintered Targets” presents Fe(Se,Te) thin films grown by pulsed laser deposition (PLD) using spark plasma-sintered (SPS) targets. As already reported, iron-based superconductors are promising for high-field applications due to thei high critical fields and low anisotropy, but the use of SPS enables the rapid densification of Fe(Se,Te) polycrystals, yielding dense, well-connected pellets with T_c,onset_ ≈ 16 K [39,40,41]. The authors prove that films deposited on CaF_2_ substrates show T_c,onset_ ≈ 18 K, high upper critical fields with low anisotropy, and critical current densities above 10^5^ A/cm^2^ and up to 16 T. Pinning analysis reveals point-like pinning centers, confirming the excellent film quality. The work demonstrates SPS to be a scalable route to reproducible Fe(Se,Te) targets for coated conductors [42,43,44].

The review “Properties and Applications of Iron-Chalcogenide Superconductors” summarizes the fabrication, properties, and applications of iron–chalcogenide superconductors. The 11 systems (FeSe, Fe(Se,Te)) offer simple, non-toxic structures and tunable superconductivity [45,46,47]. Techniques such as annealing, intercalation, ammonothermal synthesis, and ionic gating raised T_c_ values up to 48 K [48,49,50], while single-layer FeSe films on SrTiO_3_ reach record T_c_ ≈ 109 K [51]. Critical current density and flux pinning are discussed for single crystals and films, with coated conductors achieving meter-long lengths. Applications include superconducting radio frequency cavities, coils, and high-field magnets. The review highlights iron–chalcogenide as both a theoretical platform and as a practical candidate for next-generation superconducting technologies.

In conclusion, the six contributions collected in this Special Issue highlight both the fundamental advances and practical progress achieved in the study of cuprates and iron-based superconductors. From new theoretical frameworks for strange metallicity to experimental insights into vortex dynamics, and from innovative synthesis routes to record-setting current densities, these works collectively bridge the gap between microscopic understanding and technological application. They demonstrate how interdisciplinary approaches combining spectroscopy, modeling, and scalable fabrication are essential to addressing existing challenges. Looking ahead, future research should focus on integrating fundamental physics with engineering innovation, exploring heterostructures and interfacial phenomena, and developing cost-effective synthesis methods to fully unlock the transformative potential of these materials in quantum technologies and high-field applications.