Yao Shen

Intense light pulses can drive quantum materials away from equilibrium and access states that are absent under ordinary conditions. Whether resonant soft-x-ray free-electron laser (XFEL) pulses can persistently modify an ordered phase, rather than merely probe or damage it, remains an important question. Here we show that soft-x-ray XFEL exposure stabilizes a short-range incommensurate order in the itinerant helimagnet CrAs. Pristine CrAs exhibits only the native helical magnetic reflection, whereas after XFEL irradiation a second Cr-resonant peak appears at a distinct wave vector and remains observable by synchrotron resonant soft-x-ray scattering. This induced order develops below T∗ ≈ 230 K, has a correlation length of approximately 4 nm, and exhibits polarization-dependent resonant scattering with a modified Cr 3d spectral fingerprint. X-ray absorption and electron microscopy show that the CrAs framework is largely preserved while the near-surface lattice environment is locally modified. Time-resolved measurements further show that the induced order is optically suppressible and recovers on a picosecond timescale. These results establish soft-x-ray FEL pulses as an active perturbation for selecting competing orders in fragile correlated magnets.

The A- and B-site ordered quadruple perovskite oxide DyCu₃Fe₂Re₂O₁₂ with cubic Pn-3 symmetry was synthesized under high-pressure and high-temperature conditions. The material experiences a sharp long-range ferrimagnetic transition arising from the strong superexchange antiferromagnetic interactions of Cu-Re and Fe-Re at a high Curie temperature (T_C ≈ 660 K). Owing to the influence at lower temperatures of the antiferromagnetic ordering of the Dy³⁺ occupying the A-site, the susceptibility of DyCu₃Fe₂Re₂O₁₂ decreases continuously below 50 K, which is essentially different from that of the isostructural compound LaCu₃Fe₂Re₂O₁₂. Moreover, in the presence of a magnetic field, the A-site Dy³⁺ spins are readily transformed into the ferromagnetic state from the antiferromagnetic ground state. Thus, the saturated magnetic moment of DyCu₃Fe₂Re₂O₁₂ is sharply enhanced from 7.0 µ_B/f.u. at 300 K to 14 µ_B/f.u. at 2 K by applying a magnetic field of 7 T. Theoretical calculations suggest that DyCu₃Fe₂Re₂O₁₂ is a half-metallic ferrimagnet with a spin-up band gap of approximately 2.0 eV. The combination of the high Curie temperature, wide half-metallic energy gap, and large magnetic moment makes DyCu₃Fe₂Re₂O₁₂ promising for potential applications in advanced spintronic devices.

An A-site-ordered AA'₃B₄O₁₂-type quadruple perovskite oxide LaHg₃Mn₄O₁₂ was prepared by using high-pressure and high-temperature methods. The compound crystallizes in cubic Im3̅ symmetry with a lattice parameter a = 7.5949(2) Å. The introduction of Hg²⁺ at the A' site leads to a significant increase in the Mn-O-Mn bond angle, up to 159.5°. A ferromagnetic phase transition is found to occur at the Curie temperature: T_C ≈ 280 K, accompanied by a large saturation magnetic moment of 13.0 μ_B/f.u. A corresponding anomaly in the electrical resistivity is observed near T_C. First-principles calculations reveal a half-metallic electronic band structure with a wide minority-spin energy gap of ∼2.0 eV, while the majority-spin states contribute to metallic electrical transport. The current sample, LaHg₃Mn₄O₁₂ thus provides the first example of A'-site nonmagnetic quadruple perovskite oxide with half-metallic behavior. Moreover, compared with other isostructural RCu₃Mn₄O₁₂ (R = rare earth and Bi) half-metals with magnetic Cu²⁺ at the A' site, LaHg₃Mn₄O₁₂ exhibits significant enhancement in both the minority-spin energy gap and saturation magnetization, making it promising for potential spintronic applications.

Disorder in magnetic systems typically suppresses long-range order, promoting short-range states such as spin glasses and magnetic clusters. This is particularly prominent in high-entropy materials, characterized by the random distributions of local magnetic entities and exchange interactions. However, in rare exceptions, long-range magnetic order can persist in high-entropy systems, while the microscopic characters and underlying mechanisms remain elusive, especially the magnetic behaviors of individual elements. Here, combining neutron diffraction and resonant soft x-ray scattering, we have conducted an element-specific investigation into the magnetic order of a high-entropy honeycomb-lattice van der Waals material (Mn_1/4Fe_1/4Co_1/4Ni_1/4)PS₃. Despite significant atomic disorder, long-range zigzag antiferromagnetic order is observed below 72 K, with all four transition-metal elements participating in a unified phase transition. However, the spin orientations of various elements are distinct, attributed to the competition between single-ion anisotropies and exchange interactions. Our findings showcase a novel form of long-range magnetic order with disordered spin orientations, which is synergically stabilized by distinct magnetic elements in a high entropy magnet, offering a new paradigm for understanding complex magnetic systems.

Intertwined charge and spin correlations are ubiquitous in a wide range of transition metal oxides and are often perceived as intimately related to unconventional superconductivity. Theoretically envisioned as driven by strong electronic correlations, the intertwined order is usually found to be strongly coupled to the lattice as signaled by pronounced phonon softening. Recently, both charge and spin density waves (CDW and SDW) and superconductivity have been discovered in several Ruddlesden-Popper (RP) nickelates, in particular trilayer nickelates R E 4 Ni 3 O 10 ( R E = Pr , La). The nature of the intertwined order and the role of lattice-charge coupling are at the heart of the debate about these materials. Using inelastic x-ray scattering, we mapped the low-energy phonon dispersions in R E 4 Ni 3 O 10 and found no evidence of softening near the CDW wave vector over a wide temperature range, which contrasts with the pronounced anomalies frequently observed in cuprate superconductors. Calculations of the electronic susceptibility revealed a peak at the observed SDW ordering vector but not at the CDW wave vector. Our experimental and theoretical findings highlight the crucial role of the spin degree of freedom and establish a foundation for understanding the interplay between superconductivity and density-wave transitions in RP nickelate superconductors and beyond.

The discovery of nickelate superconductors has opened a new arena for studying the behavior of correlated electron liquids that give rise to unconventional superconductivity. While critical information about a material's charge dynamics is encoded in its plasmons, collective modes of the electron gas, these excitations have not yet been observed in nickelate materials. Here, we use resonant inelastic x-ray scattering (RIXS) to detect plasmons in the metallic, low-valence nickelate Pr4Ni3O8. Although qualitatively similar to those in cuprates, the nickelate plasmons are more heavily damped and have a lower velocity than those in a cuprate at comparable doping, which we attribute to reduced electronic hopping and enhanced screening of the long-range Coulomb interactions. Furthermore, the plasmons in Pr4Ni3O8 soften with increasing temperature, in contrast to the cuprate, where plasmons remain at nearly fixed energy but become more strongly damped. Taken together, these results reveal a distinct charge-screening landscape in nickelates and place quantitative constraints on analogies to cuprates.

The transition-metal-only A- and B-site-ordered perovskite oxide CuCu₃Fe₂Os₂O₁₂ was successfully synthesized under high-pressure (18 GPa) and high-temperature (1475 K) conditions. The compound adopts a cubic lattice with the charge configuration Cu⁺Cu₃²⁺Fe₂³⁺Os₂^5.5+O₁₂, in which Cu(1)⁺ and Cu(2)²⁺ exhibit a 1:3 ordering at the A-site and Fe³⁺ and Os^5.5+ arrange in a rock-salt-type ordered pattern at the B-site. A ferrimagnetic transition is found to occur at 290 K due to magnetic interactions among the corner-shared Cu(2)O₄ planar squares, FeO₆ octahedra, and OsO₆ octahedra. Low-temperature specific heat data suggest the presence of itinerant electronic behavior, in agreement with the half-metallic electronic band structure calculated in theory. Different from most itinerant electronic systems, the current CuCu₃Fe₂Os₂O₁₂ shows unusually low thermal conductivity (1.4 W K^-1 m^-1 at 300 K) since the small-size A-site Cu(1)⁺ ions "rattle" within large cuboctahedron units along the [111] direction. This study opens up a new way to design metallic conductive materials with unusually low thermal conductivity in perovskite oxides.

Many-body excitons in CrSBr are attracting intense interest in view of their highly anisotropic magneto-optical coupling and their potential for novel optical interfaces within spintronic and magnonic devices. Characterizing the orbital character and propagation of these electronic excitations is crucial for understanding and controlling their behavior; however, this information is challenging to access. High resolution resonant inelastic x-ray scattering is a momentum-resolved technique that can address these crucial questions. We present measurements collected at the Cr L_{3}-edge which show a rich spectrum of excitations with a variety of spin-orbital characters. While most of these excitations appear to be localized, the dispersion of the lowest energy dark exciton indicates that it is able to propagate along both the a and b directions within the planes of the crystal. This two-dimensional character is surprising as it contrasts with electrical conductivity and the behavior of the bright exciton, both of which are strongly one dimensional. The discovery of this propagating dark exciton highlights an unusual coexistence of one- and two-dimensional electronic behaviors in CrSBr.

Colossal magnetoresistance (CMR) effects have been extensively studied in ABO₃ perovskite manganites where the Mn³⁺-O-Mn⁴⁺ double-exchange mechanism plays a pivotal role. However, A-site-ordered AA'₃B₄O₁₂-type quadruple perovskite oxides exhibit significantly suppressed double exchange due to their extremely small B-O-B bond angles (≈140°), hindering the realization of intrinsic CMR effects. Here, we report the design and synthesis of a novel quadruple perovskite oxide Pb(Pb_1/3Hg_2/3)₃Mn₄O₁₂ (PPHMO) characterized by an unusually increased Mn-O-Mn bond angle of up to 153°. This compound crystallizes into a cubic Im3̅ structure with the charge distribution Pb²⁺(Pb_1/3^3.5+Hg_2/3²⁺)₃Mn₄^3.63+O₁₂. A ferromagnetic phase transition is observed at the Curie temperature T_C ≈ 120 K, accompanied by an insulator-to-metal transition. Furthermore, applying magnetic fields significantly reduces the resistivity, resulting in intrinsic CMR effects with an absolute MR value of 650% at 8 T, increasing to 2250% at 16 T near T_C. The large intrinsic MR is thereby realized unprecedentedly in an A-site-ordered quadruple perovskite oxide. Related origins for the intrinsic CMR effects presented in the current PPHMO are discussed in detail.