Temperature-dependent resistivity, upper critical field H-c2 and its anisotropy in overdoped superconducting Ba1-xKxFe2As2 (x = 0.6-1) single crystals have been measured in steady magnetic fields up to 44 T and low temperatures down to 0.4 K. Analysis using both the quadratic term and power-law fitting demonstrates that the in-plane resistivity rho(ab)(T) progressively approaches the Fermi-liquid T-2 behavior with increasing K doping and reaches a saturation plateau at x approximate to 0.8. The temperature dependence of both and follows the Werthamer-Helfand-Hohenberg model, incorporating orbital and spin paramagnetic effects. For x <= 0.8, the orbital effect dominates for H parallel to ab, while the Pauli paramagnetic effect prevails for H parallel to c. For x > 0.8, the Pauli paramagnetic effect becomes dominant in both crystallographic directions. The anisotropy of H-c2(0) exhibits a discontinuity in its dependence on K doping concentration with a significant enhancement at x = 0.8 and a maximum at x = 0.9. These experimental results indicate that the electron correlation effect is enhanced in the heavily overdoped Ba1-xKxFe2As2 system where the underlying symmetries are broken due to the Fermi surface reconstruction before x = 0.9.

Spin fluctuations have been generally believed as the pairing glue of high-Tc superconductivity. Recent inelastic neutron scattering (INS) studies have revealed a weak flat spin-fluctuation signal around 45 meV in the bilayer nickelate La3Ni2O7−δ, suggesting strong interlayer and weak intralayer magnetic couplings (S J⊥ ≈ 60 meV, S J∥ ⩽ 3.5 meV) in contrast to cuprate and pnictide superconductors. Here, we report further INS studies on the Pr and Nd doped La3Ni2O7−δ powder samples at ambient pressure. Besides the crystalline electric field excitations at low energies, we have found that the 45 meV flat mode splits into two modes in doped compounds, along with another weak mode at about 60 meV, where the spin fluctuations in La2NdNi2O7−δ are stronger than La3Ni2O7−δ and La2PrNi2O7−δ. Our results are consistent with an enhanced interlayer coupling S J⊥ within the stripe-type Heisenberg model framework, where the estimated S J⊥ value is in the range of about 69 to 73 meV for the rare-earth doped bilayer nickelates.

We report a dynamic magnetostrictive effect in type-II superconductors (e.g., Nb, YBa2Cu3O7-x, Bi2Sr2CaCu2O8+delta and Ba0.6K0.4Fe2As2) and any type-II superconductor is expected to present a similar response to this new technique. Measured via a composite magnetoelectric technique, an ac field excites an in-phase, nondissipative strain response scaling linearly with vortex density in the vortex lattice phase. In the vortex liquid phase, the signal acquires an out-of-phase component before vanishing in the normal state. We propose the ac strain susceptibility a thermodynamic criterion for the vortex lattice, capturing vortex collective modes inaccessible from static measurements.

We present direct experimental evidence of a weakly coupled multiband superconducting state in the bilayer iron-based superconductor ACa$_2$Fe$_4$As$_4$F$_2$ (A = K, Cs) via ultrahigh-resolution angle-resolved photoemission spectroscopy (ARPES). Remarkably, the K-containing compound exhibits two distinct transition temperatures, corresponding to two separate sets of bilayer-split bands, as evidenced by temperature-dependent superconducting gap and spectral weight near the Fermi energy, while its Cs counterpart displays conventional single transition behavior. These experimental observations are well described by the weakly coupled two-band model of Eilenberger theory, which identifies suppressed interband pairing interactions between the bilayer-split bands as the key mechanism. By exploring quantum phenomena in the weak-coupling limit within a multiband system, our findings pave the way for engineering exotic superconductivity via band-selective pairing control.

Abstract Understanding the role of electron correlations in quasiparticle coherence is central to multiorbital iron-based superconductors. Here, we investigate the electronic structure and temperature-dependent quasiparticle dynamics of CsFe$_2$As$_2$ using polarization-resolved laser- and synchrotron-based angle-resolved photoemission spectroscopy combined with first-principles calculations. We resolve multiple hole-like Fermi surface sheets around the $\mathbf{\Gamma}$ and M points, with the $\alpha$($\alpha'$) and $\beta$ bands predominantly derived from Fe $3d_{xz/yz}$ orbitals and the $\gamma$ band mainly from $d_{xy}$. The calculations reveal a strongly three-dimensional and multiorbital Fermi surface topology, while discrepancies in the detailed orbital character indicate significant correlation effects. Temperature-dependent ARPES further reveals an orbital-selective coherence-incoherence crossover. The $\alpha$ band exhibits a gradual loss of coherence over approximately $50$-$85$\,K, accompanied by suppressed spectral weight, increasing linewidth, and evolving dispersion, whereas the $\beta$ band shows an abrupt collapse of coherent spectral weight near $50$-$60$\,K with little change in its dispersion. Together with the resistivity anomaly near $80$\,K, these results establish a microscopic connection between orbital-selective quasiparticle coherence and macroscopic transport behavior. Our findings provide direct momentum-resolved evidence for band-selective correlation effects in the strongly correlated multiorbital state of CsFe$_2$As$_2$.

Magnetic refrigeration in the sub-Kelvin regime requires refrigerant materials to retain a large magnetic entropy at low temperatures by suppressing magnetic ordering. Quantum spin liquids (QSLs), which evade long-range magnetic ordering while retaining strong quantum fluctuations to the lowest temperatures, therefore provide a promising platform for realizing high-performance magnetic refrigerants. Here, we investigate the magnetic ground state and the magnetocaloric effect of the hexaaluminate, NdMgAl_11O_19, in which the Nd^3+ ions form a network of triangular lattices. Magnetic susceptibility and specific heat measurements indicate a magnetically dynamic state down to 50 mK, consistent with a QSL state. Specific heat measurements further reveal substantial magnetic entropy retained below 50 mK. Quasi-adiabatic demagnetization measurements demonstrate a superior cooling performance of NdMgAl_11O_19, which can be cooled to 113 mK from 1.9 K by only a small magnetic field change of 2 T. The outstanding refrigeration performance is attributed to the persistent spin fluctuations associated with the QSL-like ground state, together with a large effective g factor and the smallness of the exchange interactions along the easy-axis direction. This study demonstrates that frustration, combined with strong spin-orbit coupling and crystal-electric-field effect in the rare earth magnets provides a promising design principle for next-generation cryogenic magnetic refrigerants.

We present direct experimental evidence of a weakly coupled multiband superconducting state in the bilayer iron-based superconductor ACa$_2$Fe$_4$As$_4$F$_2$ (A = K, Cs) via ultrahigh-resolution angle-resolved photoemission spectroscopy (ARPES). Remarkably, the K-containing compound exhibits two distinct transition temperatures, corresponding to two separate sets of bilayer-split bands, as evidenced by temperature-dependent superconducting gap and spectral weight near the Fermi energy, while its Cs counterpart displays conventional single transition behavior. These experimental observations are well described by the weakly coupled two-band model of Eilenberger theory, which identifies suppressed interband pairing interactions between the bilayer-split bands as the key mechanism. By exploring quantum phenomena in the weak-coupling limit within a multiband system, our findings pave the way for engineering exotic superconductivity via band-selective pairing control.

Abstract A macroscopic transport mechanism for superconductivity is lacking. In this work, we performed a complete impeditivity spectrum analysis of a 122-family electron-doped pnictide, i.e. BaFe 2− x Ni x As 2 ( x = 0.065, 0.085, 0.1, 0.12, 0.15, 0.2, and 0.25). Below the critical temperature, the complex impeditivity exhibited in-plane anisotropy, with the AC resistivity as its real part and AC inductive reactivity as its imaginary part. In terms of electron duality, we emphasize the wave nature rather than the particle nature. The vortex waves of paired electrons propagated along the c axis of the single crystals to pass through series-connected nanoslabs in the ab -plane. In each nano-slab, the quantum nanodomains (QNs) of the magnetic flux in the Abrikosov sublattice were surrounded by a conducting region. The nano-slab was identified using the geometric-phase coefficient of a positive integer. Parallel nanodomains were the same in the nano-slabs, and all of them were characterized by the nano-slab coefficient when the conducting region was zero. The complex impeditivity plot revealed conducting heterogeneity between superconducting QNs and the conducting region. When the QN contained a magnetic flux quantum, the superconducting current was amplified, where the quantum number was dependent not only on the crystal axis but also on the dopant. When we predicted the ultimate superconductor with zero impeditivity, the other nanodomains exhibited a magnetic flux with left-hand spiral symmetry, which could be characterized by the geometric-phase coefficient of the negative integer. The co-doping of electrons and holes in an ultimate superconductor may be a solution for practical power transmission at room temperature.

We present direct experimental evidence of a weakly coupled multiband superconducting state in the bilayer iron-based superconductor ACa2Fe4As4F2 (A = K, Cs) via ultrahigh-resolution angle-resolved photoemission spectroscopy (ARPES). Remarkably, the K-containing compound exhibits two distinct transition temperatures, corresponding to two separate sets of bilayer-split bands, as evidenced by temperature-dependent superconducting gap and spectral weight near the Fermi energy, while its Cs counterpart displays conventional single transition behavior. These experimental observations are well described by the weakly coupled two-band model of Eilenberger theory, which identifies suppressed interband pairing interactions between the bilayer-split bands as the key mechanism. By exploring quantum phenomena in the weak-coupling limit within a multiband system, our findings pave the way for engineering exotic superconductivity via band-selective pairing control.

Abstract We report the single crystal growth and characterization of a Ni-doped 12442-type iron-based superconductor KCa$_2$(Fe$_{1-x}$Ni$_x$)$_4$As$_4$F$_2$ (Ni-K12442) with $0 \leq x \leq 0.144$. By increasing the Ni concentration, the $c$-axis is linearly compressed, and the superconducting critical temperature ($T_c$) is systematically suppressed, much like the cases in RbCa$_2$(Fe$_{1-x}$Ni$_x$)$_4$As$_4$F$_2$ and KCa$_2$(Fe$_{1-x}$Co$_x$)$_4$As$_4$F$_2$. The nature of superconductivity is examined by electrical transport measurements under different geometries of applied magnetic field. While the superconducting anisotropy ($\gamma_H$) near $T_c$ decreases upon Ni doping, both the upper critical field ($H_{c2}$) and the thermally activated energy of vortices ($U_0$) exhibit a maximum at $x = 0.034$, indicating that moderate Ni doping initially enhances the flux pinning via disorder, while superconductivity is eventually suppressed above $x = 0.12$.

We report the synthesis, structure and physical properties of a new quaternary nitride LaCr_2Ge_2N. The compound crystallizes in the CeCr_2Si_2C-type structure (P4/mmm), featuring distinctive Cr_2N square sheets within Cr_2Ge_2N block layers. Physical characterizations reveal enhanced electron correlations evidenced by a Sommerfeld coefficient substantially larger than band calculations and pressure-induced deviation from Fermi-liquid behavior. Magnetic measurements show short-range antiferromagnetic correlations developing around 460 K, followed by long-range magnetic ordering at 14 K. Additionally, subtle anomalies at 378 K suggest possible electronic ordering. First-principles calculations reveal nearly-flat Cr-3d bands near the Fermi level and predict a striped antiferromagnetic ground state. This work demonstrates how electron count variation in the CeCr_2Si_2C-type structure family leads to magnetic ordering in LaCr_2Ge_2N, contrasting with the paramagnetic behavior of LnCr_2Si_2C compounds.

The van der Waals cluster magnet Nb3Cl8 has recently been shown to possibly host a quantum-spin-liquid ground state.The Nb ions in this compound form a breathing kagome structure,where the magnetic moment comes from three nearest Nb ions forming a molecular cluster with spin 1/2.Previous bulk measurements including magnetic susceptibility and specific heat suggested the existence of spinon Fermi surfaces.Here we further probe the spin system by nuclear magnetic resonance(NMR)and muon spin rotation and relaxation(μSR)techniques.We confirm that there is no magnetic long-range order and the dynamical spin fluctuations persist down to 0.075 K.These results provide further evidence that Nb3Cl8 may host a quantum spin liquid.

Electron-boson coupling in unconventional superconductors is one of the key parameters in understanding the superconducting pairing symmetry. Here, we report definitive photoemission evidence of electron-spin fluctuation coupling in the iron-based superconductor CaKFe_{4}As_{4}, obtained via high-resolution ARPES. Our study identifies a distinct kink structure on the α band, observable only in the superconducting phase and closely linked with the superconductivity, indicative of strong electron-boson interactions. Notably, this kink structure corresponds to two distinct bosonic modes at 11 meV and 13 meV, aligning with spin resonance modes previously observed in inelastic neutron-scattering experiments. This alignment underscores the significant role of antiferromagnetic fluctuations in the pairing mechanism of this superconductor. Furthermore, the unique momentum-dependent and orbital-selective properties of the coupling revealed by ARPES provide profound insights into the pairing symmetry, suggesting predominantly s_{±}-wave pairing facilitated by spin fluctuations. Our findings not only highlight the pivotal role of spin resonance in the superconductivity of CaKFe_{4}As_{4} but also enhance our understanding of the electron-spin fluctuation interactions in unconventional superconductors.

We report the crystal growth of a new hole-doped iron-based superconductor Ba(Fe_0.875Ti_0.125)_2As_2 by substituting Ti on the Fe site. The crystals are accidentally obtained in trying to grow Ni doped Ba_2Ti_2Fe_2As_4O. After annealing at 500 ℃ in vacuum for one week, superconductivity is observed with zero resistance at T_c0≈ 17.5 K, and about 20% diamagnetic volume down to 2 K. While both the small anisotropy of superconductivity and the temperature dependence of normal state resistivity are akin to the electron doped 122-type compounds, the Hall coefficient is positive and similar to the case in hole-doped Ba_0.9K_0.1Fe_2As_2. The density functional theory calculations suggest dominated hole pockets contributed by Fe/Ti 3d orbitals. Therefore, the Ba(Fe_1-xTi_x)_2As_2 system provides a new platform to study the superconductivity with hole doping on the Fe site of iron-based superconductors.

Altermagnetism has been proposed as a distinct class of antiferromagnets exhibiting momentum-dependent spin splitting band structures without requiring spin-orbit coupling. Recently, KV2Se2O has been identified as a metallic room-temperature altermagnet with d-wave spin-momentum locking. Here, we investigate the magnetic structure of KV2Se2O in both polycrystalline and single-crystal samples using neutron diffraction techniques. The system exhibits G-type antiferromagnetic structure with a N & eacute;el temperature TN approximate to 400 K. Notably, substantial broadening of magnetic peaks was observed along L at low temperatures in single crystals, consistent with the coexistence of G-type AFM order and a c-axis spin density wave. These results demonstrate that bulk KV2Se2O cannot host altermagnetism.

Electron pairing along with phase coherence generates superconductivity below the critical temperature (T_c). In underdoped high-T_c cuprates, these two quantum phenomena may occur at separate temperatures, which was lately confirmed in the quasi-two-dimensional (quasi-2D) iron chalcogenide superconductors. Here, we report a systematic investigation on the pre-pairing behavior in a triclinic iron pnictide superconductor (Ca_0.85La_0.15)_10(Pt_3As_8)(Fe_2As_2)_5 with T_c ≈ 30 K, where the superconductivity is quasi-2D manifested by the Berezinskii-Kosterlitz-Thouless behaviors. Inelastic neutron scattering experiments unambiguously reveal a spin resonance peak around E_R = 13 meV in the superconducting state, but its intensity continuously decreases when warming up across T_c, accompanied with an anomaly around T^*≈ 45 K in spin correlations, and a suppression by an in-plane magnetic field persisting to the same temperature. Below T^*, a significant Nernst signal and a reduction of density of states at the Fermi level are also observed. These results suggest that the precursor of spin resonance is highly related to the preformed Cooper pairs driven by phase fluctuations, much like the pseudogap case in cuprates.

Room temperature superconductivity, as one of the famous jewels on the crown of physics, has attracted continuous attention and unremitting efforts from numerous scientists. In recent years, more and more reports on room temperature superconductivity evoke many anticipations, but results remain controversial. Here, we introduce the characteristics of superconducting phenomena and propose 10 feasible paths to achieve room-temperature superconductivity in the future. This is an Editorial of The Innovation Materials in Feb. 2025.

Spin fluctuations are often considered the most likely candidates for superconducting electron pairing media in unconventional superconductors. The iron-based superconductors provide a wide range of opportunities for studying the mechanism of unconventional superconductivity, as they have many systems with different structures and rich magnetisms. Taking the iron pnictide superconductors for example, this review summarizes the inelastic neutron scattering results of the spin excitation spectrum of iron-based superconductors, especially for their common features. Firstly, we introduce the direct connection between the low-energy spin excitations and superconductivity, which is so called the neutron spin resonance mode. This mode widely exists in the superconducting states of all iron-based superconductors, where the resonance energy E (R) is linearly proportional to the critical temperature T- c : E- R = 4.9kBTc, and it has a universal c-axis preferred characteristic. The in-plane dispersion of spin resonance mode is not limited by the superconducting energy gap, which is in contrast to the traditional spin exciton model. The out-of plane dispersion of spin resonance mode is determined by the Fe-As interplanar distance, indicating that the three-dimensional spin correlation effect cannot be ignored, which may be the key to clarifying the role of spin fluctuations in superconductivity. Secondly, we summarize the energy dispersion, intensity distribution, and total fluctuating moment for high energy spin excitations. Although the Heisenberg model can roughly describe the similar dispersions in different systems based on the anisotropic in-plane nearest neighbor effective exchange couplings and the similar second nearest neighbor effective exchange coupling, the correlated Hubbard model based on itinerant magnetism can more accurately describe the spin wave behavior after degeneracy, thus the spin excitations are more likely to be understood from the perspective of itinerant magnetism. The spin excitation intensity varies greatly with energy in different systems, indicating a competitive relationship between itinerant and localized magnetic interactions. However, the total fluctuating moments are generally the same, indicating that the effective spin S = 1/2. The spin excitation bandwidth is in a range of 100-200 meV, probably is correlated with the height of As away from the Fe-Fe plane. Finally, we make a comprehensive comparison of the spin excitations in iron-based superconductors and copper oxide superconductors. The spin excitation spectra of iron-based superconductors have much richer physics than cuprates, due to the complex physics of multiple orbitals, Fermi surfaces, and energy gaps. These phenomena lead to the diversity of spin excitations, especially the prominent three-dimensional spin correlation effect. This indicates that interlayer pairing and intra layer pairing driven by spin interactions are equally important and must be fully considered in microscopic theories of high-Tc superconductivity.

Elucidating the relationship between spin excitations and fermiology is essential for clarifying the pairing mechanism in iron-based superconductors (FeSCs). Here, we report inelastic neutron scattering results on the hole overdoped Ba_0.4K_0.6Fe_2As_2 near a Lifshitz transition, where the electron pocket at M point is nearly replace by four hole pockets. In the normal state, the spin excitations are observed at incommensurate wave vectors with chimney-like dispersions. By cooling down to the superconducting state, a neutron spin resonance mode emerges with a peak energy of E_r= 14-15 meV weakly modulated along L-direction. The incommensurability notably increases at low energies, giving rise to downward dispersions of the resonance mode. This behavior contrasts sharply with the upward dispersions of resonance observed in optimally doped Ba_0.67K_0.33Fe_2As_2 contributed by the hole to electron scattering, but resembles with the cases in KFe_2As_2 and KCa_2Fe_4As_4F_2 where the fermiology are dominated by hole pockets. These results highlight the critical role of electronic structure modifications near the Fermi level, especially in governing interband scattering under imperfect nesting conditions, which fundamentally shape the spin dynamics of FeSCs.

A vortex is a topological defect in the superconducting condensate when a magnetic field is applied to a type-II superconductor, as elucidated by the Ginzburg-Landau theory. Because of the confinement of the quasiparticles by a vortex, it exhibits a circular-shaped pattern of bound states with discrete energy levels, as predicted by the Caroli–de Gennes–Matricon theory in 1964. Here, however, we report a completely new type of vortex pattern which is necklacelike in an iron-based superconductor KCa2Fe4As4F2. Our theoretical analysis shows that this necklacelike vortex pattern arises primarily from selective off-shell interference between vortex bound states of opposite angular momenta in the presence of rotational symmetry breaking due to disorders. This fascinating effect can be observed in a system with a small Fermi energy and wave vector, conditions fortuitously met in our samples. Our results not only disclose a novel vortex structure, but also unravel a completely new quantum phenomenon in the superconducting condensate. Published by the American Physical Society 2025