For the separation of hydrogen isotopes (H2/D2), traditional kinetic quantum sieving (KQS) takes advantage of the diffusion barriers created by the flexibility of organic linkers and the breathing frameworks in porous solids. While the phenomena have been observed typically below 77 K, in this study, we present that a copper-based zeolite imidazolate framework (Cu-ZIF-gis) can show KQS above 120 K. Since Cu-ZIF-gis has narrow channels with ca. 2.4 Å in aperture, the small pore size itself acts as a diffusion barrier. This barrier changes with temperatures, leading to pore contraction or expansion through lattice-driven gating (LDG). The H2 adsorption isotherms measured at 40 – 150 K reflect the temperature sensitivity of the pore properties. Quasi-elastic neutron scattering (QENS) experiments indicate a notable difference in the molecular mobility of H2 and D2, even at temperatures exceeding 150 K. Temperature-variation powder X-ray diffraction measurements at 20 – 300 K show a small but gradual increase in the unit cell volume, indicating that LDG gives rise to the KQS at temperatures above 120 K. These findings can be applied to develop sustainable isotope separation technologies using existing LNG cryogenic infrastructure. Using lattice-driven gating, a copper-based zeolitic imidazolate framework (Cu-ZIF-gis) with ultra-narrow 2.4 Å channels demonstrates efficient hydrogen isotope separation at high temperatures (above 120 K). This innovation leverages temperature-induced aperture adjustments to achieve sustainable and energy-efficient isotope separation, which is suitable for integration with existing LNG cryogenic infrastructure.

FeSe_1-xS_x superconductors are commonly considered layered van der Waals materials with negligible inter-layer coupling. Here, using inelastic neutron scattering to study spin excitations in single-crystal samples, we reveal that the magnetic coupling between adjacent Fe layers is not only significant, as it affects excitations up to 15 meV, but also ferromagnetic in nature, making the system different from most unconventional superconductors including iron pnictides. Our observation provides a new standpoint to understand the absence of magnetic order in FeSe_1-xS_x. Since intercalating between the Fe layers is known to enhance superconductivity and suppress the inter-layer coupling, superconductivity appears to be a more robust phenomenon in the two-dimensional limit than antiferromagnetic order.

FeSe1-xSx superconductors are commonly considered layered van der Waals materials with negligible interlayer coupling. Here, using inelastic neutron scattering to study spin excitations in single-crystal samples, we reveal that the magnetic coupling between adjacent Fe layers is ferromagnetic in nature, making the system different from most unconventional superconductors including iron pnictides. The weak interlayer coupling is estimated to be Jc 0.2 meV, in agreement with the short spin-spin correlation length xi c 0.2c along the c axis. The results provide an experimental basis for establishing a microscopic theoretical model to describe the absence of magnetic order in FeSe1-xSx.

The structurally simplest high-temperature superconductor FeSe exhibits an intriguing superconducting nematic paramagnetic phase with unusual spin excitation spectra that are different from typical spin waves; thus, determining its effective magnetic exchange interactions is challenging. Here we report neutron scattering measurements of spin fluctuations of FeSe in the tetragonal paramagnetic phase. We show that the equal-time magnetic structure factor, $\mathcal{S}(\textbf{Q})$, can be effectively modeled using the self-consistent Gaussian approximation calculation with highly frustrated nearest-neighbor ($J_{1}$) and next-nearest-neighbor ($J_{2}$) exchange couplings, and very weak further neighbor exchange interaction. Our results elucidate the frustrated magnetism in FeSe, which provides a natural explanation for the highly tunable superconductivity and nematicity in FeSe and related materials.

The main driven force of the electronic nematic phase in iron-based superconductors is still under debate. Here, we report a comprehensive study on the nematic fluctuations in a non-superconducting iron pnictide system BaFe1.9−x Ni0.1Cr x As2 by electronic transport, angle-resolved photoemission spectroscopy (ARPES), and inelastic neutron scattering (INS) measurements. Previous neutron diffraction and transport measurements suggested that the collinear antiferromagnetism persists to x = 0.8, with similar Néel temperature T N and structural transition temperature T s around 32 K, but the charge carriers change from electron type to hole type around x = 0.5. In this study, we have found that the in-plane resistivity anisotropy also highly depends on the Cr dopings and the type of charge carriers. While ARPES measurements suggest possibly weak orbital anisotropy onset near T s for both x = 0.05 and x = 0.5 compounds, INS experiments reveal clearly different onset temperatures of low-energy spin excitation anisotropy, which is likely related to the energy scale of spin nematicity. These results suggest that the interplay between the local spins on Fe atoms and the itinerant electrons on Fermi surfaces is crucial to the nematic fluctuations of iron pnictides, where the orbital degree of freedom may behave differently from the spin degree of freedom, and the transport properties are intimately related to the spin dynamics.

The main driven force of the electronic nematic phase in iron-based superconductors is still under debate. Here, we report a comprehensive study on the nematic fluctuations in a non-superconducting iron pnictide system BaFe 1 . 9 − x Ni 0 . 1 Cr x As 2 by electronic transport, angle-resolved photoemission spectroscopy (ARPES) and inelastic neutron scattering (INS) measurements. Previous neutron di ff raction and transport measurements sug-gested that the collinear antiferromagnetism persists to x = 0 . 8, with similar N´eel temperature T N and structural transition temperature T s around 32 K, but the charge carriers change from electron type to hole type around x = 0.5. In this study, we have found that the in-plane resistivity anisotropy also highly depends on the Cr dopings and the type of charge carriers. While ARPES measurements suggest possibly weak orbital anisotropy onset near T s for both x = 0 . 05 and x = 0 . 5 compounds, INS experiments reveal clearly di ff erent onset temperatures of low-energy spin excitation anisotropy, which is likely related to the energy scale of spin nematicity. These results suggest that the interplay between the local spins on Fe atoms and the itinerant electrons on Fermi surfaces is crucial to the nematic fluctuations of iron pnictides, where the orbital degree of freedom may behave di ff erently from the spin degree of freedom, and the transport properties are intimately related to the spin dynamics.

The metallic compound FeP belongs to the class of materials that feature a complex noncollinear spin order driven by magnetic frustration. While its double-helix magnetic structure with a period lambda(s) approximate to 5c, where c is the lattice constant, was previously well determined, the relevant spin-spin interactions that lead to that ground state remain unknown. By performing extensive inelastic neutron scattering measurements, we obtained the spin-excitation spectra in a large part of the momentum-energy space. The spectra show that the magnons are gapped with a gap energy of similar to 5 meV. Despite the 3D crystal structure, the magnon modes display strongly anisotropic dispersions, revealing a quasi-one-dimensional character of the magnetic interactions in FeP. The physics of the material, however, is not determined by the dominating exchange, which is ferromagnetic. Instead, the weaker two-dimensional antiferromagnetic interactions between the rigid ferromagnetic spin chains drive the magnetic frustration. Using linear spin-wave theory, we were able to construct an effective Heisenberg Hamiltonian with an anisotropy term capable of reproducing the observed spectra. This enabled us to quantify the exchange interactions in FeP and determine the mechanism of its magnetic frustration.

In article number 2007412, Jitae T. Park, Hyunchul Oh, and co-workers report a hydrogen-isotope separation mechanism in the flexible framework of [MIL-53(Al)] through a diffusion dynamics study by quasielastic neutron scattering measurements. The dynamic breathing of the flexible pores allows faster diffusion of the heavier isotope at high temperature, unlike in rigid frameworks.

We report polarized neutron scattering measurements of the low energy spin fluctuations of the iron-selenide superconductor Li$_{0.8}$Fe$_{0.2}$ODFeSe below and above its superconducting transition temperature $T_c=41$ K. Our experiments confirmed that the resonance mode near 21 meV is magnetic. Moreover, the spin excitations are essentially isotropic in spin space at 5$\leq E\leq$ 29 meV in the superconducting and normal states. Our results suggest that the resonance mode in iron-based superconductors becomes isotropic when the influence of spin-orbit coupling and magnetic/nematic order is minimized, similar to those observed in cuprate superconductors.

Kinetic‐quantum‐sieving‐assisted H 2 :D 2 separation in flexible porous materials is more effective than the currently used energy‐intensive cryogenic distillation and girdle‐sulfide processes for isotope separation. It is believed that material flexibility results in a pore‐breathing phenomenon under the influence of external stimuli, which helps in adjusting the pore size and gives rise to the optimum quantum‐sieving phenomenon at each stage of gas separation. However, only a few studies have investigated kinetic‐quantum‐sieving‐assisted isotope separation using flexible porous materials. In addition, no reports are available on the microscopic observation of isotopic molecular transportation during the separation process under dynamic transition. Here, the experimental observation of a significantly faster diffusion of deuterium than hydrogen in a flexible pore structure, even at high temperatures, through quasi‐elastic neutron scattering, is reported. Unlike rigid structures, the extracted diffusion dynamics of hydrogen isotopes within flexible frameworks show that the diffusion difference between the isotopes increases with an increase in temperature. Owing to this unique inverse trend, a new strategy is suggested for achieving higher operating temperatures for efficient isotope separation utilizing a flexible metal–organic framework system.

The bosonic analogs of topological insulators have been proposed in numerous theoretical works, but their experimental realization is still very rare, especially for spin systems. Recently, two-dimensional (2D) honeycomb van der Waals ferromagnets have emerged as a new platform for topological spin excitations. Here, via a comprehensive inelastic neutron scattering study and theoretical analysis of the spin-wave excitations, we report the realization of topological magnon insulators in CrXTe3 (X = Si, Ge) compounds. The nontrivial nature and intrinsic tunability of the gap opening at the magnon band-crossing Dirac points are confirmed, while the emergence of the corresponding in-gap topological edge states is demonstrated theoretically. The realization of topological magnon insulators with intrinsic gap-unability in this class of remarkable 2D materials will undoubtedly lead to new and fascinating technological applications in the domain of magnonics and topological spintronics.

This corrects the article DOI: 10.1103/PhysRevLett.123.027601.

This corrects the article DOI: 10.1103/PhysRevLett.123.027601.

An isotope-selective responsive system based on molecular recognition in porous materials has potential for the storage and purification of isotopic mixtures but is considered unachievable because of the almost identical physicochemical properties of the isotopes. Herein, a unique isotope-responsive breathing transition of the flexible metal-organic framework (MOF), MIL-53(Al), which can selectively recognize and respond to only D-2 molecules through a secondary breathing transition, is reported. This novel phenomenon is examined using in situ neutron diffraction experiments under the same conditions for H-2 and D-2 sorption experiments. This work can guide the development of a novel isotope-selective recognition system and provide opportunities to fabricate flexible MOF systems for energy-efficient purification of the isotopic mixture.

A small in-plane external uniaxial pressure has been widely used as an effective method to acquire single domain iron pnictide BaFe 2 As 2 , which exhibits twin-domains without uniaxial strain below the tetragonal-to-orthorhombic structural (nematic) transition temperature T s . Although it is generally assumed that such a pressure will not affect the intrinsic electronic/magnetic properties of the system, it is known to enhance the antiferromagnetic (AF) ordering temperature T N ( < T s ) and create in-plane resistivity anisotropy above T s . Here we use neutron polarization analysis to show that such a strain on BaFe 2 As 2 also induces a static or quasi-static out-of-plane ( c -axis) AF order and its associated critical spin fluctuations near T N / T s . Therefore, uniaxial pressure necessary to detwin single crystals of BaFe 2 As 2 actually rotates the easy axis of the collinear AF order near T N / T s , and such effects due to spin-orbit coupling must be taken into account to unveil the intrinsic electronic/magnetic properties of the system.

Spin-resonance modes (SRM) are taken as evidence for magnetically driven pairing in Fe-based superconductors, but their character remains poorly understood. The broadness, the splitting and the spin-space anisotropies of SRMs contrast with the mostly accepted interpretation as spin excitons. We study hole-doped Ba1−xNaxFe2As2 that displays a spin reorientation transition. This reorientation has little impact on the overall appearance of the resonance excitations with a high-energy isotropic and a low-energy anisotropic mode. However, the strength of the anisotropic low-energy mode sharply peaks at the highest doping that still exhibits magnetic ordering resulting in the strongest SRM observed in any Fe-based superconductor so far. This remarkably strong SRM is accompanied by a loss of about half of the magnetic Bragg intensity upon entering the SC phase. Anisotropic SRMs thus can allow the system to compensate for the loss of exchange energy arising from the reduced antiferromagnetic correlations within the SC state.

Inelastic neutron scattering measurements on the molecular dimer-Mott insulator κ-(BEDT-TTF)_{2}Cu[N(CN)_{2}]Cl reveal a phonon anomaly in a wide temperature range. Starting from T_{ins}∼50-60  K where the charge gap opens, the low-lying optical phonon modes become overdamped upon cooling towards the antiferromagnetic ordering temperature T_{N}=27  K, where also a ferroelectric ordering at T_{FE}≈T_{N} occurs. Conversely, the phonon damping becomes small again when spins and charges are ordered below T_{N}, while no change of the lattice symmetry is observed across T_{N} in neutron diffraction measurements. We assign the phonon anomalies to structural fluctuations coupled to charge and spin degrees of freedom in the BEDT-TTF molecules.

Superconductivity in FeSe emerges from a nematic phase that breaks four-fold rotational symmetry in the iron plane. This phase may arise from orbital ordering, spin fluctuations or hidden magnetic quadrupolar order. Here we use inelastic neutron scattering on a mosaic of single crystals of FeSe, detwinned by mounting on a BaFe2As2 substrate to demonstrate that spin excitations are most intense at the antiferromagnetic wave vectors QAF = (±1, 0) at low energies E = 6–11 meV in the normal state. This two-fold (C2) anisotropy is reduced at lower energies, 3–5 meV, indicating a gapped four-fold (C4) mode. In the superconducting state, however, the strong nematic anisotropy is again reflected in the spin resonance (E = 3.6 meV) at QAF with incommensurate scattering around 5–6 meV. Our results highlight the extreme electronic anisotropy of the nematic phase of FeSe and are consistent with a highly anisotropic superconducting gap driven by spin fluctuations. Extreme electronic anisotropy is revealed in the high-temperature superconductor FeSe through tour de force experiments on detwinned crystals.

2 dimerized antiferromagnetic-ferromagnetic quantum spin-chain BaCu2V2O8 E. S. Klyushina, 2, ∗ A.T.M.N. Islam, J. T. Park, E. A. Goremychkin, † E. Wheeler, B. Klemke, and B. Lake 2 Helmholtz-Zentrum Berlin für Materialien und Energie, 14109 Berlin, Germany Institut für Festkörperphysik, Technische Universität Berlin, 10623 Berlin, Germany Heinz Maier-Leibnitz Zentrum, TU München, 85747 Garching, Germany Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, United Kingdom Institut Laue-Langevin, Boite Postale 156X, F-38042 Grenoble Cedex 9, France