The Synergetic Extreme Condition User Facility (SECUF) is a comprehensive, state-of-the-art user facility designed to provide integrated extreme physical conditions-including ultrahigh pressure, ultralow temperature, strong magnetic fields, and ultrafast optical fields-for frontier research in condensed matter physics and materials science. Within SECUF, the F2 Sample Pre-selection and Characterization Station plays a pivotal supporting role. Its mission is to provide comprehensive sample synthesis, processing, pre-screening, and characterization services to prepare high-quality specimens for subsequent experiments under extreme conditions. This paper details the specifications and performance of ten core instrument systems within these units. Furthermore, we highlight several breakthrough scientific achievements enabled by the F2 Station, encompassing the discovery of novel quantum spin supersolid states, pressure-induced high-temperature superconductivity in nickelates, giant anomalous Hall angles, and molecular water in lunar soil. We also outline ongoing technical developments that expand the station's capabilities, such as integrated high-pressure cells and self-built ancillary measurement systems.

The acidic oxygen evolution reaction (OER) in proton-exchange membrane water electrolyzers heavily relies on Ir-based catalysts, yet their widespread application is hindered by the trade-off between activity and stability. Recent studies have revealed that the active species formed during OER is not simply amorphous but possesses a distinct paracrystalline structure featuring short-to-medium-range order. However, directly constructing such metastable configurations via simple synthetic routes remains challenging. In this study, we present a rationally designed paracrystalline Ce-doped IrO x catalyst (p-IrO x :Ce), synthesized through a simple one-step pyrolysis method. Physical characterizations reveal that this unique paracrystalline architecture featured moderate Ir oxidation states (+3.5) and a short-range disordered arrangement, together improving the active site exposure and the electronic structure. As a result, the as-prepared p-IrO x :Ce exhibits outstanding acidic OER activity, achieving a low overpotential of 241 mV at 10 mA cm-2. Mechanistic studies demonstrate that Ce-induced structural distortion activates the lattice-oxygen oxidation mechanism of OER involving decoupled proton-electron transfer, thereby contributing to the enhanced catalytic performance. Notably, p-IrO x :Ce also delivers exceptional long-term durability, sustaining continuous operation at 100 mA cm-2 for over 300 hours with negligible degradation. This work establishes a direct synthetic strategy for constructing metastable paracrystalline Ir-based catalysts and highlights the important role of paracrystalline engineering in overcoming the activity-stability limitations. These findings provide valuable insights into the rational design of high-performance electrocatalysts for sustainable energy conversion processes.

In a recent preprint (arXiv:2602.19282) [1], the authors questioned the procedure we used to evaluate the demagnetization-corrected superconducting shielding volume fraction in pressurized Ruddlesden-Popper nickelates [2-5]. They further claimed that this methodology has neither been derived nor used previously, and they proposed an alternative normalization scheme. Here we clarify that our evaluation follows directly from the standard magnetostatic self-consistency relation for finite samples and has been widely adopted in the superconductivity literature for decades. We also demonstrate that the discrepancies claimed in Ref. [1] stem from a fundamental flaw in their approach, namely, the assumption that the measured diamagnetic moment is linearly proportional to the superconducting shielding volume fraction in the presence of a finite demagnetization factor N. This assumption is not valid for strongly demagnetized, thin disk-like specimens, where the internal field and the measured moment are coupled self-consistently through the demagnetizing field.

The two-dimensional (2D) limit in macroscopic bulk crystals provides a powerful platform for exploring exotic quantum phases. Here, we report the synthesis of a Sr0.75ClNbS2 superconductor that achieves unidirectional, parallel AA stacking-a configuration never before realized in a bulk crystal. Unlike conventional intercalation, which merely expands the interlayer spacing, our approach employs a planar Sr-Cl network to enforce a complete stacking reorganization, driving all NbS2 layers from the native antiparallel AB stacking into a unidirectional, parallel AA arrangement. This stacking switch globally breaks inversion symmetry, transforming centrosymmetric 2H-NbS2 into a noncentrosymmetric bulk crystal with D3h point group symmetry. Crucially, this structural design reproduces, in three dimensions, the electronic environment of an isolated monolayer, thereby preventing cancellation of the local Ising fields. As a result, strong Ising spin-orbit coupling and spin-split bands persist throughout the bulk. Transport measurements reveal extreme superconducting anisotropy (γ 77), an in-plane upper critical field ( 10.65 T) that far exceeds the Pauli paramagnetic limit, and clean-limit superconductivity indicative of high crystalline quality. Moreover, magnetotransport uncovers a novel magnetic-field-induced anomalous metallic state characterized by finite dissipation yet a vanishing Hall response. Direct band-structure measurements corroborate the layer-decoupled, quasi-2D electronic nature of the system. This work establishes stacking-geometry engineering as a powerful strategy to artificially enforce a globally noncentrosymmetric, quasi-2D superconducting state in bulk crystals, paving the way for designing quantum materials with tunable crystalline symmetry and electronic band topology.

As a promising alternative to conventional lenses, terahertz (THz) achromatic metalenses with planar configurations are of great significance in broadband optical applications. However, as the application scenarios extending to macro-scale far-field optical imaging, an inherent conflict arises between the increasing demand for large-aperture, high focusing efficiency, broadband achromatic metalenses and the steep rise in design complexity and fabrication costs. Herein, a multifunctional achromatic metalenses design scheme is proposed to realize large-aperture, high focusing efficiency, and polarization-insensitive THz all-dielectric single-layer metalenses, by integrating the discrete multi-wavelength achromatic design method and the global optimization algorithm. The designed metalens is successfully fabricated with a diameter up to 2.21 cm, which is the largest reported THz achromatic metalens to our knowledge. Experimental measurement results demonstrate that it has polarization-insensitive characteristics and excellent achromatic focusing performance from 0.80 to 1.20 THz, with achromatic coefficient values below 2.9% and average focusing efficiencies greater than 46% under both x- and y-polarized incident waves. Furthermore, the broadband achromatic imaging performance is characterized and validated, confirming its remarkable achromatic imaging capability. This study not only showcases an outstanding THz achromatic metalens but also presents a generalizable design strategy applicable across the entire electromagnetic spectrum.

The charge density wave (CDW) transition in the PrSb2 compound has been systematically investigated through variable-temperature Raman scattering, x-ray diffraction, and electrical resistivity measurements. A non-monotonic anomaly in resistivity is observed near 100 K. Below this temperature, an amplitude mode appears in Raman spectra-a well-established fingerprint of CDW order-that redshifts and weakens upon warming. X-ray diffraction reveals a change in lattice parameter slope at 100 K, evidencing electron-lattice coupling and confirming CDW instability without structural phase transition. Notably, no anomalies were found in magnetization or specific heat near 100 K. Furthermore, at T = 1.8 K and with H perpendicular to c axis, a series of fractional magnetization plateaus are observed, resembling the "devil's staircase." These findings show that PrSb2 hosts coexisting CDW instability and antiferromagnetic order with strong magneto-transport responses, making it a rare platform for studying quantum order interactions in correlated electron systems.

Orderings in charge and spin have been extensively studied to unravel their correlation to emergent superconductivity over the past decades. Bragg-Williams order (BWO), a classical structural order parameter describing site occupancy in alloys, has long been speculated to influence superconducting behavior. Yet, its role still remains ambiguous, largely due to the difficulty of isolating BWO from concomitant charge doping or competing electronic instabilities. Here, we establish In2/3PSe3 as a platform wherein indium vacancies are reversibly configurable between ordered and disordered states via thermal treatment. We show that the disordered phase undergoes a pressure-induced superconducting transition with a Tc of 11 K, significantly higher than the 7 K observed in its ordered counterpart. This constitutes a rare instance in which pure BWO variation drives a substantial shift in Tc. By combining a Ginzburg-Landau phenomenological analysis with a BCS-McMillan microscopic description, we demonstrate that BWO naturally suppresses superconductivity through electron-phonon interactions, a mechanism supported by ultra-low-wavenumber Raman measurements. Our findings support BWO as an independent order parameter that competes directly with superconductivity, extending the concept of competing orders beyond conventional electronic and magnetic degrees of freedom.

The spatial modulation of electron density into a wave-like pattern, known as charge density wave (CDW), represents a fundamental quantum state that often coexists with superconductivity, quantum Hall states and axion insulating phases. Conventional CDWs are mediated by longitudinal acoustic phonons, exhibit picometer-scale lattice distortions (10-12-10-11 m), and typically vanish upon approaching the atomic limit. Here, we report a series of anomalous CDW behaviors in the 2D superatomic superconductor Au6Te12Se8. Remarkably, its CDW is governed by transverse phonons and exhibits a real-space displacement of ~ 4 Ångström, which is an order of magnitude larger than that in conventional CDW. Furthermore, we observe a dimensional response persisting up to micrometer-scale thickness, a regime where other materials are already considered as bulk. Through liquid helium-temperature transmission electron microscopy, ultrafast pump-probe spectroscopy and transport measurements, we demonstrate a prominent enhancement of the CDW transition temperature (TCDW) from < 2 K in the bulk to 110 K upon approaching the "superatomic limit". Our findings not only reveal anomalous facets of both CDW and superatomic materials, but the competition between this anomalous CDW and superconductivity opens avenues for exploring unconventional electron-phonon interactions.

The intrinsic trade-off between electrical conductivity and hardness has been a longstanding obstacle in simultaneously enhancing both properties in materials. Here, we report a strategy to overcome this dilemma by utilizing the striking disparity effects of heavy doping on electrical and mechanical properties. A high room-temperature electrical conductivity of 3.03 x 105 S/m is achieved alongside a high nanoindentation hardness of 39.9 +/- 1.9 GPa and an elastic modulus of 525.9 +/- 13.0 GPa in cubic silicon carbide (3C-SiC) single crystals with a N-doping level of similar to 0.5 at %. The unprecedented combination of these two properties in compound semiconductors confirms the disparity effects of N doping, as manifested by a significant increase in electrical conductivity due to a large nitrogen solubility (similar to 0.5 at %), while there is a negligible impact (similar to 1% decrease) on hardness upon N doping. This strategy may find applications in other covalent compounds or semiconductors for acquiring both electrical and mechanical properties.

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) are promising platforms for low-dimensional superconductivity. However, in conventional intercalated systems, achieving a high superconducting transition temperature (Tc) often comes at the expense of reduced interlayer spacing and weakened 2D character. Here, we overcome this long-standing compromise through a unique chain-like intercalation strategy. We report the synthesis and properties of a new polymorph, (BaS)1/3TaS2, in which a distinctive Ba-S-S-Ba chain structure is inserted between TaS2 bilayers. This unique configuration breaks the bulk c-axis mirror symmetry while achieving exceptional interlayer decoupling, with an interbilayer spacing of 12.75 Å─more than three times that of pristine 2H-TaS2. By suppressing interlayer electronic coupling, this structural evolution allows local inversion symmetry breaking within individual TaS2 layers to dominate. This prevents compensation of the Ising spin-orbit fields typical of centrosymmetric bulk phases, enabling robust 2D Ising superconductivity. Remarkably, the compound exhibits an enhanced Tc without sacrificing its large interlayer spacing, thereby breaking the conventional trade-off between "large spacing/high anisotropy" and "high Tc". Comprehensive transport, magnetic, and thermodynamic measurements confirm its robust superconducting state. Our work establishes a versatile intercalation framework for designing bulk-like 2D Ising superconductors, providing a new route to reconcile competing material demands and expanding the scope of Ising superconductivity research.

The Hundred-deg^2 HI Deep (HD^2) survey carried out with the Five-hundred-meter Aperture Spherical Telescope (FAST) is planned to map a contiguous region within the DESI DR1 footprint, achieving an effective integration time of 20 minutes for each pointing and a uniform detection sensitivity of 0.28 mJy beam^-1 at 4.8 km s^-1 resolution. We present early results from the pilot HD^2 survey: a 10 deg^2 field overlapping with HSC-SSP and the DESI EDR SV3, observed with an integration time of 7.3 minutes per beam and the rms of 0.45 mJy beam^-1 at 4.8 km s^-1 resolution. We identify 339 HI sources at z<0.09, corresponding to ∼34 detections per deg^2, nearly six times higher than the detection rate of the wide-field surveys. Optical counterparts are primarily identified using DESI redshifts, yielding a matching rate and correctness exceeding 90

Layered transition metal dichalcogenides (TMDCs) such as TaSe2 provide a compelling platform to study the interplay between charge density wave (CDW) order and superconductivity, two collective electronic states that often compete for the ground state. However, effectively tuning this competition and enhancing superconductivity in such systems remain a significant challenge. Here, we report the successful synthesis of Cu-intercalated TaSe2 single crystals and demonstrate that Cu intercalation leads to a remarkable enhancement of the superconducting transition temperature (TC), increasing from 0.14 K in pristine TaSe2 to a maximum of 3.03 K. Simultaneously, the CDW transition is noticeably suppressed, indicating a strong competition between the two phases. X-ray photoelectron spectroscopy (XPS) and Hall measurements reveal that Cu atoms donate electrons to the TaSe2 layers, increasing the carrier density and thereby driving the observed enhancement of superconductivity. Furthermore, upper critical field (Hc2) measurements exhibit a clear temperature-dependent anisotropy, consistent with an anisotropic superconducting state, while the overall behaviour is well described by a single-gap model. Our work highlights intercalation as an effective strategy to engineer superconducting properties and provides new insights into the intertwined nature of CDW and superconductivity in TMDCs.

Multiple CDW-ordered layered rare-earth tellurides have increasingly emerged as a research hotspot, owing to their unconventional CDW formation, high transition temperature, and confirmed existence of axial Higgs modes. Recently, interweaved CDW in LaTe2 and its element-substituted phase LaTe2-xSbx have been investigated through TEM and ARPES measurements, revealing their distinct origins. Nevertheless, several complex diffraction features observed in TEM patterns remain unelucidated. In this work, we carried out scanning tunneling microscopy (STM) on LaTe1.6Sb0.4 crystals at 9 K. Three interweaved CDW wave vectors, q1=8/11a*, q2=5/11a* and q3=3/11a* were observed, which are induced by hole doping in unoccupied states. The q1 and q3 are theoretically verified to be nesting vectors connecting px- and py- bands. Furthermore, the satellite spots relative to the main Bragg spots p corresponding to a 11-a-superlattice have also been detected. Our findings provide critical insights for further exploring the origin of the interweaved CDW in hole/electron doping materials.

Superconductivity in the two-dimensional (2D) limit is a fertile ground for exotic quantum phenomena-many of which remain elusive in their 3D counterparts. While studies of 2D superconductivity have predominantly focused on mono- or few-layer systems, we demonstrate an alternative route-interlayer sliding in bulk crystals. Through a precisely controlled growth strategy, we engineer interlayer sliding in bulk 3R-NbSe2, deliberately disrupting [001] mirror symmetry and drastically suppressing interlayer coupling. Remarkably, this structural manipulation stabilizes Ising-type superconductivity coexisting with an unconventional charge-density-wave (CDW) state akin to that of monolayer 2H-NbSe2. The sliding phase exhibits a pronounced suppression of the upper critical field at low temperatures, revealing a delicate competition between Ising and Rashba spin-orbit coupling (SOC) in the globally noncentrosymmetric lattice. Intriguingly, the superconducting state displays two-fold symmetry, a signature that may arise from asymmetric SOC or a multi-component pairing order parameter. Our work establishes interlayer sliding as a symmetry-breaking tool to promote 2D superconductivity in bulk materials-without resorting to extrinsic intercalation or doping. More broadly, this approach sets a paradigm for unlocking hidden quantum states in layered materials, offering a new dimension in design of quantum matter.

Exploring various unexpected new quantum states and their corresponding extraordinary physics in low dimensional quantum materials, and investing them into application fields, is a primary concern of condensed matter physics. Previously, we performed joint experiments and theoretical calculations to investigate a superatomic crystal of Au6Te12Se8 with low symmetry, which stacks through non-covalent inter-cube quasi-bonds. Au6Te12Se8 exhibits a triple-cube charge density wave and spatially polarized metallic states interweaving at 9 K. In addition, it undergoes a BKT phase transition at 2.8 K to form a quasi-two-dimensional superconducting state. The subsequent high-pressure experimental results indicate that the two quantum states above compete with superconductivity. Here, we experimentally revealed competition and coexistence among superconductivity, triple-cube charge density waves, and polarized metallic states during further temperature-decreasing process, as examined using ultra-low temperature scanning tunneling microscopy/spectroscopy, transport measurement, and Raman spectra. An extraordinary inversion symmetry broken emerged inside the triple-cube-period of the original CDW at 300 mK, leading to the polarized metallic states transforming into parallel polarization states. The evidence of spontaneous polarization coexisting with superconductivity in real space is extremely rare and has been revealed by STM. In addition, transport and Raman measurements indicate that there are multiple phase transition temperatures from 80K to that below the superconducting transition temperature of Au6Te12Se8, which may also contain more unknown exotic quantum states, providing a platform for further exploration of intriguing physical properties.

The China Spallation Neutron Source (CSNS) is the fourth pulsed accelerator-driven neutron source in the world, and it achieved its design target of 100 kW in 2020. The planned China Spallation Neutron Source Phase II (CSNS-II) commenced in 2024. The CSNS-II linac design primarily involves the addition of a radio-frequency ion source and a section of a superconducting linear accelerator composed of two types of superconducting cavities, namely double-spoke and six-cell elliptical cavities, after the drift tube linac (DTL). The development of the double-spoke superconducting cavity began in early 2021, and by January 2023, the welding, post-processing, and vertical tests of two 324 MHz double-spoke cavity prototypes were completed, with vertical test gradients of 11.6 and 15 MV/m, and Q_0 ≥ 3× 10^10 @ E_acc≤10 MV/m . The R D of the cryomodule began in January 2022. In October 2023, the clean assembly of the double-spoke cavity string and cold mass installation of the cryomodule commenced, with the installation of the cryomodule and valve box completing in two months. In January 2024, a horizontal test of the cryomodule was completed, making it the first double-spoke cavity cryomodule in China. The test results showed that the maximum gradients of the two superconducting cavities at a pulse width of 4 ms and repetition frequency of 25 Hz were 12.8 and 15.2 MV/m, respectively. This article provides a detailed introduction to the double-spoke superconducting cavity, tuner, coupler, and cryomodule, elaborates on the clean assembly of the cavity string and cold mass installation of the cryomodule, and provides a detailed analysis of the horizontal test results.

The simultaneous presence of polarity and metallicity or superconductivity in a material signifies the exotic polar metallic or superconducting (SC) state, while such materials are extremely rare due to their exclusive nature. Recently, the interweaved CDW and antipolar charge orders have been discovered in a metallic superatomic crystal of Au6Te12Se8 (ATS), while their interplay and competition with the following emergent SC state remains elusive. Here, we report a further experimental investigation of the SC state emerged from the preformed CDW and antipolar order states using scanning tunneling microscopy/spectroscopy in combination with transport and Raman measurements. The temperature-dependent pre-formation and condensation of Cooper pairs are experimentally identified. The pre-existent CDW is gradually suppressed by the preformed Cooper pairs, and then the antipolar charge order is spatially suppressed into a ferrielectric-like polar order by the condensed Cooper pairs of SC state. The exotic charge-polarized superconducting state is discovered in the polar metal of ATS, suggesting a valuable platform for the exploration of intriguing polar superconducting properties.

The emergence of superconductivity in two-dimensional transition metal dichalcogenides with strong spin orbit coupling (SOC) has opened new avenues for exploring exotic superconducting states. Here, we report experimental observation of an anisotropic Fulde-Ferrell (FF) phase in few-layer NbSe_2/CrSiTe_3 heterostructures under in-plane magnetic fields. Through combined magnetoresistance and nonreciprocal transport measurements, we find that due to the couplings from the ferromagnetic CrSiTe_3, a half-dome-shaped region emerges in the magnetic field-temperature (B-T) diagram. Importantly, the half-dome-shaped region exhibits finite second harmonic resistance with in-plane anisotropy, indicating that the superconducting state is an anisotropic FF phase. Through a symmetry analysis combined with mean field calculations, we attribute the emergent anisotropic FF phase to the CrSiTe_3 layer induced Rashba SOC and three-fold rotational symmetry breaking. These results demonstrate that heterostructure stacking is a powerful tool for symmetry engineering in superconductors, which can advance the design of quantum devices in atomically thin superconducting materials.