arXiv subjects

Xiaohui Yu

Publications and source records attributed to Xiaohui Yu.

At least 19 recordsLinked to original sources

arXiv2026

Magneto-Structural Coupling Enables Cryogenic Cation Redistribution in a Spinel Oxide

Ionic transport in oxides is generally frozen at cryogenic temperatures, where thermal energy lies far below typical cation-migration barriers. Neutron powder diffraction reveals progressive Fe/Mg redistribution between tetrahedral (A) and octahedral (B) sites in the spinel Mg0.5Fe0.5TiFeO4 upon cooling from 200 K to 5 K. A-site Fe occupancy increases toward near completion at 5 K within Rietveld resolution, while Ti remains on the B site. This exchange coincides with complex magnetic correlations rather than a classical thermally activated window. Low-temperature magnetostrictive volume changes indicate strong spin-lattice coupling, but do not identify magnetostriction as the sole thermodynamic driver. Room-temperature high-pressure X-ray diffraction produces the opposite occupancy trend, showing that volume contraction alone cannot explain the cryogenic site exchange. These results point to magneto-structural free-energy minimization as a plausible mechanism for unlocking cryogenic cation mobility in a correlated spinel oxide.

cond-mat.mtrl-sci↗

arXiv2026

Anomalous Structural Response of Quasi-One-Dimensional Antiferromagnetic Metal KMn6Bi5 under high pressure

We report high-pressure single-crystal X-ray diffraction measurements on the quasi-one-dimensional (Q1D) antiferromagnetic metal KMn6Bi5 up to 12.5 GPa, revealing the detailed pressure evolution of its atomic coordination environment. We find that the lattice exhibits pronounced anisotropic compressibility-the relative changes in the a and b lattice parameters reach a/a0=0.91 and b/b0 = 0.94 at 12.5 GPa-and a distinct structural anomaly emerges near 11 GPa without any symmetry-breaking. Detailed structural analysis further uncovers an anomalous hardening of the Mn nanotubes between 5 and 11 GPa, followed by a configuration optimization of the Mn/Bi nanotubes around 11 GPa. These features correlate closely with the reported pressure-temperature phase diagram of KMn6Bi5 and compare favorably with the chemical pressure effects induced by substituting K with Na, Rb, or Cs. Our findings provide key microscopic insights into how coordination environment modulation governs the stability of electronic orders in low-dimensional systems.

cond-mat.mtrl-sci↗

arXiv2026

Structural responses incipient to pressure-driven antiferromagnetic quantum critical point of van der Waals heavy-fermion metal CeSiI

CeSiI is a van der Waals heavy-fermion metal recently found to exhibit unconventional superconductivity near a pressure-induced antiferromagnetic quantum critical point (QCP) at Pc =6 GPa. Here, we report a comprehensive single-crystal X-ray diffraction study of CeSiI under high pressures up to 8.3 GPa at room temperature, revealing subtle structural responses that precede pressure-driven QCP. We find that the unit-cell volume decreases smoothly upon compression without showing any structural phase transition in the investigated pressure range. Intriguingly, we observe abrupt and concurrent anisotropic responses of the lattice parameters around Pc =6 GPa, i.e., the a-axis contracts while the c-axis enlongated suddenly, with the unit-cell volume smoothily varies with pressure. Structural refinements further show that these lattice anomalies primarily originate from changes of Ce-Ce and Ce-Si bond lengths, as well as a flattening of the inner honeycomb Si layer within the CeSiI monolayer around Pc. Our findings establish an interesting case linking pressure-driven electronic transition of QCP at low temperatures to incipient structural responses at room temperature, thereby providing fresh insight into the pressure-temperature phase diagram of CeSiI.

cond-mat.str-el↗

arXiv2026

Ultrafast decoupling of the pseudogap from superconductivity in a pressurized cuprate

The relationship between the pseudogap and superconductivity remains a central puzzle in the physics of cuprates. Hydrostatic pressure provides a clean tuning parameter free from chemical disorder, yet probing the microscopic energy scales of these phases under compression has remained experimentally challenging. Here, we utilize ultrafast optical spectroscopy to construct the high-pressure phase diagram of the underdoped cuprate Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ up to 37 GPa. Our results reveal a striking dichotomy within the pseudogap state: while the onset temperature $T^*$ rises monotonically with pressure, the energy gap $\Delta_{\mathrm{PG}}$ is continuously suppressed. In contrast, the critical temperature $T_{\mathrm{c}}$ and the superconducting gap $\Delta_{\mathrm{SC}}$ trace a correlated dome-like trajectory, demonstrating that superconductivity evolves independently from the pseudogap. Furthermore, an abrupt collapse of the gap ratio $2\Delta_{\mathrm{SC}}/k_{\mathrm{B}}T_{\mathrm{c}}$ near 8 GPa marks a pressure-driven dimensional crossover, quenching two-dimensional phase fluctuations to stabilize global three-dimensional coherence. Upon reaching 37 GPa, the superconducting condensate is completely quenched into an insulating-like state. By resolving the extended phase evolution, our findings disentangle the pseudogap and superconducting orders, establishing a rigorous experimental basis for the pairing mechanism of high-temperature superconductivity.

cond-mat.supr-con↗

arXiv2025

Flexible Keyword-Aware Top-$k$ Route Search

With the rise of Large Language Models (LLMs), tourists increasingly use it for route planning by entering keywords for attractions, instead of relying on traditional manual map services. LLMs provide generally reasonable suggestions, but often fail to generate optimal plans that account for detailed user requirements, given the vast number of potential POIs and possible routes based on POI combinations within a real-world road network. In this case, a route-planning API could serve as an external tool, accepting a sequence of keywords and returning the top-$k$ best routes tailored to user requests. To address this need, this paper introduces the Keyword-Aware Top-$k$ Routes (KATR) query that provides a more flexible and comprehensive semantic to route planning that caters to various user's preferences including flexible POI visiting order, flexible travel distance budget, and personalized POI ratings. Subsequently, we propose an explore-and-bound paradigm to efficiently process KATR queries by eliminating redundant candidates based on estimated score bounds from global to local levels. Extensive experiments demonstrate our approach's superior performance over existing methods across different scenarios.

cs.DB↗

arXiv2025

Compression-induced magnetic obstructed atomic insulator and spin singlet state in antiferromagnetic KV2Se2O

Among the complex many-body systems, the metal-insulator transition stands out as a cornerstone and a particularly fertile ground for scientific inquiry. The established models including Mott insulator, Anderson localization and Peierls transition, are still insufficient to capture the complex and intertwined phenomena observed in certain material systems. KV2Se2O, a newly discovered room-temperature altermagnetic candidate exhibiting a spin-density-wave transition below 100 K, provides a unique platform to investigate the interplay of many-body effects and unconventional magnetism, specifically the anticipated metal-insulator transition under extreme conditions. Here, we report a compression-induced insulator by suppressing the metallic behavior without structural phase transition. The newly opened gap is estimated to be 40 meV at around 43.5 GPa, given direct evidence for the insulating state. A concurrent switching of carrier type demonstrates the large Fermi surface reconstruction crossing the metal-insulator transition. The density functional theory calculations indicate that the discovered V+2.5-based insulator is a magnetic obstructed atomic insulator, being a spin-singlet state with bonding orbital order. This work not only presents an archetype of a pressure-driven metal-insulator transition decoupled from structural change but also delivers fundamental physical insights into the metal-insulator transition.

cond-mat.str-el↗

arXiv2025

Imaging magnetic flux trapping in lanthanum hydride using diamond quantum sensors

Lanthanum hydride has attracted significant attention in recent years due to its signatures of superconductivity at around 250 K (1, 2). However, the megabar pressures required for synthesize and maintain its state present extraordinary challenges for experiments, particularly in characterizing its Meissner effect (3, 4). The nitrogen-vacancy (NV) center in diamond has emerged as a promising quantum probe to address this problem (5-8), but a gap remains between its working pressure and the pressure required to study the superconducting state of lanthanum hydride (9-12). In this work, using neon gas as the pressure transmitting medium, the working pressure of NV centers is extended to nearly 200 GPa. This quantum probe is then applied to study the Meissner effect of a LaH$_{9.6}$ sample, synthesized by laser heating ammonia borane and lanthanum. A strong magnetic shielding effect is observed, with the transition temperature beginning at around 180 K and completing at 220 K. In addition, magnetic field imaging after field cooling reveals strong flux trapping and significant inhomogeneities within the sample. Our work provides compelling evidence for superconductivity in lanthanum hydride and highlights the importance of spatially resolved techniques in characterizing samples under ultrahigh pressure conditions.

cond-mat.supr-con↗

arXiv2025

Three-Dimensional Continuous Multi-Walled Carbon Nanotubes Network-Toughened Diamond Composite

Enhancing the fracture toughness of diamond while preserving its hardness is a significant challenge. Traditional toughening strategies have primarily focused on modulating the internal microstructural units of diamonds, including adjustments to stacking sequences, faults, nanotwinning, and the incorporation of amorphous phases, collectively referred to as intrinsic toughening. Here, we introduce an extrinsic toughening strategy to develop an unparalleled tough diamond composite with complex and abundant sp2-sp3 bonding interfaces, by incorporating highly dispersed multi-walled carbon nanotubes (MWCNTs) into the gaps of diamond grains to create a three-dimensional (3D) continuous MWCTNs network-toughen heterogeneous structure. The resultant composite exhibits a hardness of approximately 91.6 GPa and a fracture toughness of roughly 36.4 MPa.m1/2, which is six times higher than that of synthetic diamond and even surpasses that of tungsten alloys, surpassing the benefits achievable through intrinsic toughening alone. The remarkable toughening behavior can be attributed to the formation of numerous mixed sp2-sp3 bonding interactions at the 3D continuous network MWCNTs/diamond interfaces, which facilitate efficient energy dissipation. Our 3D continuous network heterogeneous structure design provides an effective approach for enhancing the fracture toughness of superhard materials, offering a new paradigm for the advanced composite ceramics.

cond-mat.other↗

arXiv2025

Orthorhombic nitride perovskite CeTaN3-{\delta} with switchable and robust ferroelectric polarization

Perovskite-type ternary nitrides with predicted exciting ferroelectricity and many other outstanding properties hold great promise to be an emerging class of advanced ferroelectrics for manufacturing diverse technologically important devices. However, such nitride ferroelectrics have not yet been experimentally identified, mainly due to the challenging sample synthesis by traditional methods at ambient pressure. Here we report the successful high-pressure synthesis of a high-quality ferroelectric nitride perovskite of CeTaN3-{\delta} with nitrogen deficiency, adopting an orthorhombic Pmn21 polar structure. This material is electrically insulating and exhibits switchable and robust electric polarization for producing ferroelectricity. Furthermore, a number of other extraordinary properties are also revealed in this nitride such as excellent mechanical properties and chemical inertness, which would make it practically useful for many device-relevant applications and fundamentally important for the study of condensed-matter physics.

cond-mat.mtrl-sci↗

arXiv2025

Unconventional charge density wave in Kagome metal BaFe2Al9

The charge density wave (CDW) is a macroscopic quantum state characterized by long-range lattice distortion and modulated charge density. Conventionally, CDWs compete with other electronic orders (e.g. superconductivity) and are suppressed under hydrostatic pressure. Intriguingly, the Kagome-variant metal BaFe2Al9, crystallized in a three-dimensional structure, exhibits pressure-enhanced CDW ordering, where the transition temperature (TCDW) rises from ~110 K to room temperature near 3.6 GPa. The lattice structure was checked by both powder and single crystal x-ray diffraction (XRD). The XRD data reveals an abnormal lattice expansion along a axis near 4-5 GPa upon compression. The strongly suppressed diffraction intensity and splitting diffraction spots from single crystal indicates cracking and breakdown to smaller pieces, indicative of an intrinsic first-order transition character. This anomalous response implies a CDW mechanism dominated by electron-electron and/or electron-phonon correlations, distinct from Fermi-surface nesting in conventional systems. Concomitant dome-shaped pressure-dependent resistance suggests competing electronic phases. Our work establishes BaFe2Al9 as a 3D Kagome platform with unconventional CDW behavior and strong electron-phonon coupling, which provides an alternative platform to explore the electron correlation induced exotic electronic states and other potential emergent quantum phenomenon.

cond-mat.str-el↗

arXiv2025

Revisiting Invariant Learning for Out-of-Domain Generalization on Multi-Site Mammogram Datasets

Achieving health equity in Artificial Intelligence (AI) requires diagnostic models that maintain reliability across diverse populations. However, breast cancer screening systems frequently suffer from domain overfitting, degrading significantly when deployed to varying demographics. While Invariant Learning algorithms aim to mitigate this by suppressing site-specific correlations, their efficacy in medical imaging remains underexplored. This study comprehensively evaluates domain generalization techniques for mammography. We constructed a multi-source training environment aggregating datasets from the United States (CBIS-DDSM, EMBED), Portugal (INbreast, BCDR), and Cyprus (BMCD). To assess global generalizability, we evaluated performance on unseen cohorts from Egypt (CDD-CESM) and Sweden (CSAW-CC). We benchmarked Invariant Risk Minimization (IRM) and Variance Risk Extrapolation (VREx) against a rigorously optimized Empirical Risk Minimization (ERM) baseline. Contrary to expectations, standard ERM consistently outperformed specialized invariant mechanisms on out-of-domain testing. While VREx showed potential in stabilizing attention maps, invariant objectives proved unstable and prone to underfitting. We conclude that engineering equitable AI is currently best served by maximizing multi-national data diversity rather than relying on complex algorithmic invariance.

cs.CV↗

arXiv2025

Observation of a $Pbca$ phase and robust metallicity in $\rm{RuO_2}$ under pressure

$\rm{RuO_2}$ stands as a quintessential rutile-type compound under ambient conditions, with its structural exploration under pressure bearing significant implications for both phase transition investigations and Earth science. Nonetheless, the precise phase transition sequence remains a debate. In this study, we disclose the emergence of the $Pbca$ phase alongside the enduring metallic character of $\rm{RuO_2}$ under megabar pressure. Employing state-of-the-art synchrotron X-ray diffraction, our observations delineate a phase transition trajectory progressing through rutile, $\rm{CaCl_2}$, and ultimately $Pbca$ phases. Notably, the $Pbca$ phase manifests immediately just after the rutile-$\rm{CaCl_2}$ transition, confining a narrow pressure regime for the pure $\rm{CaCl_2}$-type phase. Within the pressure range of 15.5 to 35.0 GPa, a coexistence of the $\rm{CaCl_2}$-type and $Pbca$ phases is observed, transforming to a sole presence of the $Pbca$ phase beyond 35.0 GPa. Electrical transport measurements conducted on both single crystal and powder samples confirm the enduring metallic conductivity of $\rm{RuO_2}$, persisting up to at least $\sim$120 GPa, albeit exhibiting a diminished conductivity at ultrahigh pressures due to a reduction in electronic density of states at the Fermi level. This study furnishes compelling evidence for the presence of the $Pbca$ phase across a broad pressure range, diverging from the previously widely acknowledged $Pa\bar{3}$ phase, thereby offering crucial insights into phase transition phenomena in other metal dioxides and advancing our comprehension of electronic behaviors within 4d and 5d electron systems.

cond-mat.mtrl-sci↗

arXiv2025

New phase space of hardness materials and synergic enhancement of hardness and toughness in superconducting Ti2Co and Ti4Co2X (X = B, C, N, O)

Compared to traditional superhard materials with high electron density and strong covalent bonds, alloy materials mainly composed of metallic bonding structures typically have great toughness and lower hardness. Breaking through the limits of alloy materials is a preface and long term topic, which is of great significance and value for improving the comprehensive mechanical properties of alloy materials. Here, we report on the discovery of a cubic alloy semiconducting material Ti2Co with large Vickers of hardness Hvexp = 6.7 GPa and low fracture toughness of KICexp =1.51 MPa m0.5. Unexpectedly, the former value is nearly triple of the Hvcal = 2.66 GPa predicted by density functional theory (DFT) calculations and the latter value is about one or two orders of magnitude smaller than that of ordinary titanium alloy materials (KICexp = 30-120 MPa m0.5).These specifications place Ti2Co far from the phase space of the known alloy materials, but close to medium hardness materials such as MgO or TiO2. Upon incorporation of oxygen into structural void positions, both values were simultaneously improved for Ti4Co2O to = 9.7 GPa and 2.19 MPa m0.5, respectively. Further DFT calculations on the electron localization function of Ti4Co2X (X = B, C, N, O) vs. the interstitial elements indicate that these simultaneous improvements originate from the coexistence of Ti-Co metallic bonds, the emergence of newly oriented Ti-X covalent bonds, and the increase of electron concentration. Moreover, the large difference between Hvexp and Hvcal of Ti2Co suggests underlying mechanism concerning the absence of the O(16d) or Ti2-O bonds in the O-(Ti2)6 octahedron.Our discovery expands the phase space of alloy materials and illuminates the path of exploring superconducting materials with excellent mechanical performances.

cond-mat.mtrl-sci↗

arXiv2025

Reliable Text-to-SQL with Adaptive Abstention

Large language models (LLMs) have revolutionized natural language interfaces for databases, particularly in text-to-SQL conversion. However, current approaches often generate unreliable outputs when faced with ambiguity or insufficient context. We present Reliable Text-to-SQL (RTS), a novel framework that enhances query generation reliability by incorporating abstention and human-in-the-loop mechanisms. RTS focuses on the critical schema linking phase, which aims to identify the key database elements needed for generating SQL queries. It autonomously detects potential errors during the answer generation process and responds by either abstaining or engaging in user interaction. A vital component of RTS is the Branching Point Prediction (BPP) which utilizes statistical conformal techniques on the hidden layers of the LLM model for schema linking, providing probabilistic guarantees on schema linking accuracy. We validate our approach through comprehensive experiments on the BIRD benchmark, demonstrating significant improvements in robustness and reliability. Our findings highlight the potential of combining transparent-box LLMs with human-in-the-loop processes to create more robust natural language interfaces for databases. For the BIRD benchmark, our approach achieves near-perfect schema linking accuracy, autonomously involving a human when needed. Combined with query generation, we demonstrate that near-perfect schema linking and a small query generation model can almost match SOTA accuracy achieved with a model orders of magnitude larger than the one we use.

cs.DB↗

arXiv2024

A Survey on Data Markets

Data is the new oil of the 21st century. The growing trend of trading data for greater welfare has led to the emergence of data markets. A data market is any mechanism whereby the exchange of data products including datasets and data derivatives takes place as a result of data buyers and data sellers being in contact with one another, either directly or through mediating agents. It serves as a coordinating mechanism by which several functions, including the pricing and the distribution of data as the most important ones, interact to make the value of data fully exploited and enhanced. In this article, we present a comprehensive survey of this important and emerging direction from the aspects of data search, data productization, data transaction, data pricing, revenue allocation as well as privacy, security, and trust issues. We also investigate the government policies and industry status of data markets across different countries and different domains. Finally, we identify the unresolved challenges and discuss possible future directions for the development of data markets.

cs.GT↗

arXiv2024

Coexistence of superconductivity and sliding polar metal state in HgPSe3

The simultaneous presence of polarity and metallicity in a material signifies an exotic polar metal state, but such materials are extremely rare, especially in bulk form, due to mutually exclusive nature of the fundamental defining properties. Here, we report experimental findings that HgPSe3 is a robust bulk polar metal at room temperature with a chiral structure stabilized by pressure and, remarkably, this polar metal hosts superconductivity with critical temperature Tc up to 11 K. Theoretical analysis reveals a two-step interlayer sliding-then-compressing mechanism for coexistence of polarity and metallicity in HgPSe3. This work unveils a new paradigm for creating the bulk polar metal state and simultaneous presence of coexisting quantum orders, raising the prospect of discovering novel emergent physics using pressure as a tuning knob.

cond-mat.supr-con↗

arXiv2024

Imaging the Meissner effect in pressurized bilayer nickelate with integrated multi-parameter quantum sensor

Recent reports on the signatures of high-temperature superconductivity with a critical temperature Tc close to 80 K have triggered great research interest and extensive follow-up studies. Although the zero resistance has been successfully achieved under improved hydrostatic pressure conditions, the Meissner effect of $\mathrm{La_{3}Ni_{2}O_{7-\delta}}$ under high pressure remains controversial. Here, using shallow nitrogen-vacancy centers implanted on the culet of diamond anvils as in-situ quantum sensors, we observe compelling evidence for the Meissner effect in polycrystalline bilayer nickelate samples: the magnetic field expulsion during both field cooling and field warming processes. In particular, we explore the multiparameter measurement capacity of the diamond quantum sensors to extract the weak demagnetization signal of $\mathrm{La_{3}Ni_{2}O_{7-\delta}}$. The correlated measurements of Raman spectra and magnetic imaging indicate an incomplete structural transformation related to the displacement of oxygen ions emerging in the non-superconducting region. Our work clarifies the controversy about the Meissner effect of $\mathrm{La_{3}Ni_{2}O_{7-\delta}}$ and contributes to the development of quantum sensing of weak signals under high-pressure conditions.

cond-mat.supr-con↗

arXiv2024

Density-wave-like gap evolution in La$_3$Ni$_2$O$_7$ under high pressure revealed by ultrafast optical spectroscopy

Density wave (DW) order is believed to be correlated with superconductivity in the recently discovered high-temperature superconductor La$_3$Ni$_2$O$_7$. However, experimental investigations of its evolution under high pressure are still lacking. Here, we explore the quasiparticle dynamics in bilayer nickelate La$_3$Ni$_2$O$_7$ single crystals using ultrafast optical pump-probe spectroscopy under high pressures up to 34.2 GPa. At ambient pressure, the temperature-dependent relaxation dynamics demonstrate a phonon bottleneck effect due to the opening of an energy gap around 151 K. The energy scale of the DW-like gap is determined to be 66 meV by the Rothwarf-Taylor model. Combined with recent experiential results, we propose that this DW-like transition at ambient pressure and low temperature is spin density wave (SDW). With increasing pressure, this SDW order is significantly suppressed up to 13.3 GPa before it completely disappears around 26 GPa. Remarkably, at pressures above 29.4 GPa, we observe the emergence of another DW-like order with a transition temperature of approximately 135 K, which is probably related to the predicted charge density wave (CDW) order. Our study provides the experimental evidence of the evolution of the DW-like gap under high pressure, offering critical insights into the correlation between DW order and superconductivity in La$_3$Ni$_2$O$_7$.

cond-mat.supr-con↗