arXiv subjects

Donghui Lu

Publications and source records attributed to Donghui Lu.

At least 19 recordsLinked to original sources

arXiv2026

Unexpected stabilization of a single-$q$ charge density wave in pristine 1T-TiSe$_2$

Over the past decade, studies revisiting the classical charge-density-wave system 1T-TiSe$_2$, long understood to host the $2 \times 2$ triple-$q$ order with equivalent subcomponents, have suggested that these subcomponents may not be identical, challenging a 50-year-old paradigm. Here, we demonstrate not only that these subcomponents are non-identical but also that they develop sequentially rather than simultaneously. By tracking folded-band features, angle-resolved photoemission spectroscopy reveals that a unidirectional $2 \times 1$ single-$q$ order stabilizes first, while the other two components emerge sequentially at lower temperatures, completing the triple-$q$ phase. Our free-energy analysis explains this hierarchy through repulsive coupling among the three subcomponents, as supported by first-principles calculations incorporating phonon anharmonicity. Consequently, the three subcomponents are inequivalent in both phase and amplitude. These results reconcile reported symmetry breaking and differing characteristic temperatures and establish a framework in which ostensibly simple, high-symmetry charge-density order emerges from temperature-dependent competition and cooperation among inequivalent subcomponents.

cond-mat.str-el↗

arXiv2026

Framework for an AI-Ready ARPES Community Database

Condensed matter stands to gain profoundly from widespread adoption of artificial intelligence (AI). However, its current means of data organization impedes this endeavor. As a premier measure of electronic structure underlying material properties, Angle-Resolved Photoemission Spectroscopy (ARPES) has accumulated over four decades of spectra, yet the community's publication-centered workflow preserves only a highly selected fraction of its experimental record. A shared data ecosystem could transform these fragmented measurements and their contexts into a persistent, collective scientific memory, creating immediate value for reproducibility and reuse while laying the foundation for AI-enabled discovery. In this Perspective, we report outcomes of a recent community workshop and propose a practical path toward such an ecosystem. Its benefits unfold along three timescales. In the near term, standardized, searchable, and citable datasets strengthen analysis pipelines independently of AI. In the intermediate future, community-validated tools operating across laboratories could enable broader reuse and increasingly closed-loop automated experiments. In the long term, a diverse corpus of measured electronic spectra could support foundation models for quantum matter. Looking forward, a diverse corpus of measured electronic spectra could support foundation models for quantum matter and new discoveries. We discuss lessons from existing scientific databases, the current state of AI in ARPES, as well as areas of community convergence on data organization, access, credit, governance, and a staged pilot program.

cond-mat.mtrl-sci↗

arXiv2026

Luttinger Liquid Behavior in a Single-Layer Nickelate La1.4Sr0.6NiO4

The discovery of high-temperature superconductivity in bilayer and trilayer nickelates has spurred intense interest in the Ruddlesden-Popper nickelates; yet the fundamental properties of the NiO2 layer remain obscured by interlayer coupling. It is therefore imperative to investigate the electronic properties of their single-layer counterpart to isolate the intrinsic physics of the NiO layer. In this work, we present a systematic study of the single-layer nickelate La1.4Sr0.6NiO4 using high-resolution angle-resolved photoemission spectroscopy (ARPES) and theoretical calculations. We reveal strong electron correlation effects, manifested by high-energy kinks in band dispersions and a pronounced orbital-dependent band renormalization. Interestingly, we observe a quasi-one-dimensional electronic structure characterized by straight Fermi surface sheets along the diagonal momentum directions. Such square Fermi surface topology facilitates non-Fermi liquid behavior consistent with the Luttinger liquid model, as evidenced by the power-law spectral function, robust temperature scaling, and the observation of spin-charge separation. Our results therefore not only unveil an exotic Luttinger liquid behavior emerging from the unexpected dimensional reduction in an intrinsically quasi-two-dimensional nickelate but also provide a new perspective for understanding the intriguing physics in multilayer nickelates.

cond-mat.supr-con↗

arXiv2026

High-Temperature ferromagnetism from site-selective filling in (Fe,Ni)$_{6-δ}$GeTe$_2$

The discovery of high-temperature ferromagnetism in the metallic van der Waals (vdW) system Fe$_N$GeTe$_2$ has brought two-dimensional (2D) magnets into technologically relevant temperature scales. Specifically at N = 5, dilution of magnetic moments by nickel substitution counterintuitively achieves a record high Curie temperature of 478~K. Unraveling the origin of this nickel-substitution-induced enhancement is complicated by the compound's structural complexity, coexistent itinerant and local magnetic contributions, and mesoscopic compositional domains. Through coordinated structural and electronic characterization, we identify that the high-T$_C$ magnetic phase arises from a strain-stabilized Fe$_6$GeTe$_2$ nano-precipitate. Combining first-principles calculations and spin- and angle-resolved photoemission spectroscopy (ARPES), we uncover a site-specific electronic landscape in which interior iron atoms primarily host localized moments while the outer iron atoms neighboring the tellurium layers produce spin-polarized itinerant carriers that cross the vdW gap. The large energy cost associated with homogeneous nickel substitution is found to favor the spontaneous precipitation of the crystallographically and electronically ``clean'' high-T$_C$ phase. Finally, we compare metal-rich vdW magnets with binary magnetic alloys, and discuss the unifying roles of nano-precipitates in stabilizing otherwise unattainable bulk phases. Our work provides mechanistic insights into the record-high T$_C$ ferromagnetism in (Fe,Ni)$_{5+δ}$GeTe$_2$, establishing a rigorous foundation for the atomic engineering of vdW magnetic metals informed by direct electronic signatures.

cond-mat.mtrl-sci↗

arXiv2026

BCS-BEC crossover driven by small Fermi pockets of a high-Tc cuprate superconductor

Fermi arcs observed in underdoped cuprates have sparked debate over whether they represent segments of a large Fermi surface or small Fermi pockets. This ambiguity has long hindered their classification as either the conventional Bardeen-Cooper-Schrieffer (BCS) regime or the strongly coupled Bose-Einstein condensation (BEC) crossover limit. Here, using angle-resolved photoemission spectroscopy and quantum oscillations, we demonstrate the coexistence of a small Fermi pocket and a large superconducting gap in the clean inner CuO2 layers of the four-layer cuprate Ba2Ca3Cu4O8(F,O)2. This coexistence constitutes a hallmark of the BCS-BEC crossover and has remained elusive for decades. Despite the presence of antiferromagnetic (AF) order, the superconducting gap in the small pocket is remarkably large, yielding a gap-to-Fermi-energy ratio (Delta_pocket/e_F ~ 0.6) and a critical-to-Fermi-temperature ratio (Tc/TF ~ 1.3) that reach the theoretical upper bound for two-dimensional superconductivity. Unexpectedly, this BCS-BEC crossover emerges not as the carrier density decreases but as it increases, abruptly within a narrow doping range of less than 1%. These results provide a long-sought microscopic foundation for the d-wave pairing mechanism in doped AF-Mott insulators.

cond-mat.supr-con↗

arXiv2026

Evolution of the Saddle Point in Antimony Telluride Homologous Superlattices

Combining topological insulators with topological semimetals in the form of homologous superlattices is a promising approach for generating correlated quantum matter based upon Fermi level alignment with band extrema. For antimony telluride, a saddle point is predicted to occur at the M-point, while antimonene layering is predicted to move the M-point valence band towards the Fermi level. To date, the predicted saddle point at the M-point has not yet been demonstrated, and studies of antimony telluride homologous superlattices have been limited to one or two layers of antimonene added to antimony telluride. Here, we present scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy studies of a series of antimony telluride homologous superlattices with two to four layers of antimonene. In addition to demonstrating the presence of a saddle point and associated van Hove singularity near the M-point, we identify the key role of Sb and Te $p_z$ orbital hybridization in driving the van Hove singularity toward the Fermi level.

cond-mat.mtrl-sci↗

arXiv2026

Electronic origin of delicate antiferromagnetism in Fe$_{x}$NbS$_2$

Among the family of intercalated transition-metal dichalcogenides (TMDs), Fe$_{x}$NbS$_2$ is found to possess unique current-induced resistive switching behaviors, tunable antiferromagnetic states, and a commensurate charge order, all of which are tied to a critical Fe doping of $x_c$ = 1/3. However, the electronic origin of such extreme stoichiometry sensitivities remains unclear. Combining angle-resolved photoemission spectroscopy (ARPES) with density functional theory (DFT) calculations, we identify and characterize a dramatic eV-scale electronic restructuring that occurs across the $x_c$. Moment-carrying Fe 3$d_{z^2}$ electrons manifest as narrow bands within 200 meV of the Fermi level, distinct from other transition metal intercalated TMD magnets. These states strongly hybridize with itinerant electrons in TMD layer, rapidly lose coherence above $x_c$ due to correlation-driven effects. This sudden quasiparticle decoherence collapses the Fe-Nb hybridization, which explicitly suppresses the out-of-plane effective Fe-Fe exchange interaction, driving the transformation of the magnetic ground state from an antiferromagnetic stripe phase to a zigzag phase. These observations resemble the exceptional electronic and magnetic sensitivity of strongly correlated systems, and demonstrate that quantifying orbital-specific hybridization via ARPES offers an alternative pathway to evaluate effective magnetic exchange in metallic magnets, complementing inelastic neutron and resonant x-ray scattering probes.

cond-mat.str-el↗

arXiv2026

Probing Coordination Environments in Buried Oxides of Aluminum Josephson Junctions by Resonant X-ray Reflectivity

Decoherence remains a critical obstacle to achieving high-fidelity, scalable superconducting qubits, with the tunnel barrier of Josephson junctions a key source of loss. Here we apply resonant X-ray reflectivity to non-destructively probe the electronic structure of buried layers in Al/AlO$_x$/Al Josephson junctions. At the Al $K$-edge, energy-dependent modulations in the reflectivity maps enable Kramers-Kronig-constrained extraction of the layer-resolved atomic scattering factors. The analysis reveals that the barrier coordination evolves from more tetrahedral toward predominantly octahedral character with increasing oxidation pressure. At the interfaces, the lower metal-oxide boundary is comparatively under-coordinated and disordered relative to the upper interface. Comparison with simulated X-ray absorption spectra identifies the dominant coordination motifs within the oxide and its interfaces, providing depth-resolved structural insight that constrains microscopic models of two-level system formation. These results link growth conditions, local coordination environments, and junction electronic properties, demonstrating resonant X-ray reflectivity as a powerful tool for probing the microscopic materials properties of Josephson junctions and providing a materials-level framework for mitigating decoherence in superconducting qubits.

cond-mat.mtrl-sci↗

arXiv2026

Orbital-selective Mottness Driven by Geometric Frustration of Interorbital Hybridization in Pr4Ni3O10

The interplay among orbital-selective Mott physics, Hund's coupling, tunable structural motifs, and Kondo-like scattering establishes a compelling paradigm for understanding and engineering correlated multi-orbital systems, as vividly exemplified by nickelate superconductors. Here, using high-resolution angle-resolved photoemission spectroscopy combined with theoretical calculations, we systematically investigate the electronic properties of trilayer nickelates. In La4Ni3O10, we observe pronounced interorbital hybridization, whereas in Pr4Ni3O10, the flat d_(z^2 ) band becomes markedly incoherent and diminishes in spectral weight. By contrast, the dispersive d_(x^2-y^2 ) bands retain coherence in both compounds. This striking incoherence/coherence dichotomy identifies an orbital-selective Mott phase modulated by the interlayer Ni-O-Ni bonding angle. The depletion of the d_(z^2 ) orbitals further frustrates the interorbital hybridization and influences the density-wave transition in Pr4Ni3O10. Moreover, the density-wave gap is substantially reduced in Pr4Ni3O10, likely due to extra scattering channels provided by the local moments of Pr3+ cations. Our findings elucidate the intricate interplay among lattice, orbital, spin, and electronic degrees of freedom and reveal a feasible structural control parameter for the multi-orbital correlated state in trilayer nickelates, which provide a concrete framework for understanding the emergence of superconductivity under high pressure.

cond-mat.str-el↗

arXiv2025

Superconductivity suppression and bilayer decoupling in Pr substituted YBa$_2$Cu$_3$O$_{7-δ}$

The mechanism behind superconductivity suppression induced by Pr substitutions in YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) has been a mystery since its discovery: in spite of being isovalent to Y$^{3+}$ with a small magnetic moment, it is the only rare-earth element that has a dramatic impact on YBCO's superconducting properties. Using angle-resolved photoemission spectroscopy (ARPES) and DFT+$U$ calculations, we uncover how Pr substitution modifies the low-energy electronic structure of YBCO. Contrary to the prevailing Fehrenbacher-Rice (FR) and Liechtenstein-Mazin (LM) models, the low energy electronic structure contains no signature of any $f$-electron hybridization or new states. Yet, strong electron doping is observed primarily on the antibonding Fermi surface. Meanwhile, we reveal major electronic structure modifications to Cu-derived states with increasing Pr substitution: a pronounced CuO$_2$ bilayer decoupling and an enhanced CuO chain hopping, implying indirect electron-release pathways beyond simple 4$f$ state ionization. Our results challenge the long-standing FR/LM mechanism and establish Pr substituted YBCO as a potential platform for exploring correlation-driven phenomena in coupled 1D-2D systems.

cond-mat.supr-con↗

arXiv2025

Spin Excitations and Flat Electronic Bands in a Cr-based Kagome Superconductor

In the quest for topology- and correlation-driven quantum states, kagome lattice materials have garnered significant interest for their band structures, featuring flat bands (FBs) from the quantum destructive interference of the electronic wavefunction. Tuning an FB to the chemical potential could induce electronic instabilities and emergent orders. Despite extensive studies, direct evidence of FBs tuned to the chemical potential and their role in emergent orders in bulk materials remains lacking. Using angle-resolved photoemission spectroscopy, resonant inelastic X-ray scattering, and density functional theory, we show that the low-energy structure of the Cr-based kagome metal superconductor {\Cr} is dominated by FBs at the Fermi level. We also observe low-energy magnetic excitations evolving across the low-temperature transition, largely consistent with the FB shift. Our results suggest that the low-temperature order contains a magnetic origin and that the kagome FBs may play a role in the emergence of this order.

cond-mat.str-el↗

arXiv2025

Anomalous Nodal Gap in a Doped Spin-1/2 Antiferromagnetic Mott Insulator

Many emergent phenomena appear in doped Mott insulators near the insulator-to-metal transition. In high-temperature cuprate superconductors, superconductivity arises when antiferromagnetic (AFM) order is gradually suppressed by carrier doping, and a $\textit{d}$-wave superconducting gap forms when an enigmatic nodal gap evolves into a point node. Here, we examine electron-doped Sr$_{2}$IrO$_{4}$, the 5$\textit{d}$-electron counterpart of cuprates, using angle-resolved photoemission spectroscopy. At low doping levels, we observe the formation of electronic states near the Fermi level, accompanied by a gap at the AFM zone boundary, mimicking the AFM gap in electron-doped cuprates. With increasing doping, a distinct gap emerges along the (0,0)-($π$,$π$) nodal direction, paralleling that observed in hole-doped cuprates. This anomalous nodal gap persists after the collapse of the AFM gap and gradually decreases with further doping. It eventually vanishes into a point node of the reported $\textit{d}$-wave gap. These observations replicate the characteristic features in both electron- and hole-doped cuprates, indicating a unified route toward nodal metallicity in doped spin-1/2 AFM Mott insulators.

cond-mat.supr-con↗

arXiv2025

Hund's coupling assisted orbital-selective superconductivity in Ba1-xKxFe2As2

While the superconducting transition temperature of hole-doped Ba_{1-x}K_{x}Fe_{2}As_{2} decreases past optimal doping, superconductivity does not completely disappear even for the fully doped KFe_{2}As_{2} compound. In fact, superconductivity is robust through a Lifshitz transition where electron bands become hole-like around the zone corner at around x=0.7, thus challenging the conventional understanding of superconductivity in iron-based systems. High-resolution angle-resolved photoemission spectroscopy is used to investigate the superconducting gap structure, as well as the normal state electronic structure, around optimal doping and across the Lifshitz transition. Our findings reveal a largely orbital-dependent superconducting gap structure, where the more strongly correlated d_{xy} band has a vanishing superconducting gap at higher doping, aligning with the Hund's metal behavior observed in the normal state. Notably, the superconducting gap on the d_{xy} band disappears before the Lifshitz transition, suggesting that the Fermi surface topology may play a secondary role. We discuss how these results point to orbital-selective superconducting pairing and how strong correlations via Hund's coupling may shape superconducting gap structures in iron-based and other multiorbital superconductors.

cond-mat.supr-con↗

arXiv2025

Sub-tesla on-chip nanomagnetic metamaterial platform for angle-resolved photoemission spectroscopy

Magnetically controlled states in quantum materials are central to their unique electronic and magnetic properties. However, direct momentum-resolved visualization of these states via angle-resolved photoemission spectroscopy (ARPES) has been hindered by the disruptive effect of magnetic fields on photoelectron trajectories. Here, we introduce an \textit{in-situ} method that is, in principle, capable of applying magnetic fields up to 1 T. This method uses substrates composed of nanomagnetic metamaterial arrays with alternating polarity. Such substrates can generate strong, homogeneous, and spatially confined fields applicable to samples with thicknesses up to the micron scale, enabling ARPES measurements under magnetic fields with minimal photoelectron trajectory distortion. We demonstrate this minimal distortion with ARPES data taken on monolayer graphene. Our method paves the way for probing magnetic field-dependent electronic structures and studying field-tunable quantum phases with state-of-the-art energy-momentum resolutions.

cond-mat.mes-hall↗

arXiv2025

Non-monotonic band flattening near the magic angle of twisted bilayer MoTe$_2$

Twisted bilayer MoTe$_2$ (tMoTe$_2$) is an emergent platform for exploring exotic quantum phases driven by the interplay between nontrivial band topology and strong electron correlations. Direct experimental access to its momentum-resolved electronic structure is essential for uncovering the microscopic origins of the correlated topological phases therein. Here, we report angle-resolved photoemission spectroscopy (ARPES) measurements of tMoTe$_2$, revealing pronounced twist-angle-dependent band reconstruction shaped by orbital character, interlayer coupling, and moiré potential modulation. Density functional theory (DFT) captures the qualitative evolution, yet underestimates key energy scales across twist angles, highlighting the importance of electronic correlations. Notably, the hole effective mass at the K point exhibits a non-monotonic dependence on twist angle, peaking near 2°, consistent with band flattening at the magic angle predicted by continuum models. Via electrostatic gating and surface dosing, we further visualize the evolution of electronic structure versus doping, enabling direct observation of the conduction band minimum and confirm tMoTe$_2$ as a direct band gap semiconductor. These results establish a spectroscopic foundation for modeling and engineering emergent quantum phases in this moiré platform.

cond-mat.mes-hall↗

arXiv2025

Dichotomy of flat bands in the van der Waals ferromagnet Fe$_5$GeTe$_2$

Quantum materials with bands of narrow bandwidth near the Fermi level represent a promising platform for exploring a diverse range of fascinating physical phenomena, as the high density of states within the small energy window often enables the emergence of many-body physics. On one hand, flat bands can arise from strong Coulomb interactions that localize atomic orbitals. On the other hand, quantum destructive interference can quench the electronic kinetic energy. Although both have a narrow bandwidth, the two types of flat bands should exhibit very distinct spectral properties arising from their distinctive origins. So far, the two types of flat bands have only been realized in very different material settings and chemical environments, preventing a direct comparison. Here, we report the observation of the two types of flat bands within the same material system--an above-room-temperature van der Waals ferromagnet, Fe$_{5-x}$GeTe$_2$, distinguishable by a switchable iron site order. The contrasting nature of the flat bands is also identified by the remarkably distinctive temperature-evolution of the spectral features, indicating that one arises from electron correlations in the Fe(1) site-disordered phase, while the other geometrical frustration in the Fe(1) site-ordered phase. Our results therefore provide a direct juxtaposition of the distinct formation mechanism of flat bands in quantum materials, and an avenue for understanding the distinctive roles flat bands play in the presence of magnetism, topology, and lattice geometrical frustration, utilizing sublattice ordering as a key control parameter.

cond-mat.str-el↗

arXiv2025

Superconducting coherence boosted by outer-layer metallic screening in multilayered cuprates

In multilayered high-Tc cuprates with three or more CuO2 layers per unit cell, the inner CuO2 planes (IPs) are spatially separated from the dopant layers and thus remain cleaner than the outer planes (OPs). While both interlayer coupling and the presence of clean IPs have been proposed as key factors enhancing superconductivity, their individual roles have been difficult to disentangle, as IPs and OPs typically become superconducting simultaneously. Here we investigate five-layer (Cu,C)Ba2Ca4Cu5Oy (Cu1245) with Tc = 78 K and three-layer Ba2Ca2Cu3O6(F,O)2 (F0223) with Tc = 100 K using ARPES, and uncover an unprecedented situation, in which only the IPs become superconducting while the OPs remain metallic at low temperatures. Model calculations indicate that more than 95% of the OP wavefunction remains confined to OP itself, with minimal hybridization from the superconducting IPs. In particular, we experimentally realize an ideal configuration: a single superconducting CuO2 layer sandwiched between heavily overdoped metallic outer layers, which screen disorder originating from the dopant layers. Strikingly, this clean CuO2 layer exhibits the largest superconducting gap among all known cuprates and coherent Bogoliubov peaks extending beyond the antiferromagnetic zone boundary -- long regarded as the boundary beyond which coherence vanishes in heavily underdoped cuprates. Furthermore, a widely extended coherent flat band emerges at the Brillouin zone edge, overcoming the pseudogap damping effect. Our results introduce a new physical parameter, the degree of screening, to investigate the competition between superconductivity and the pseudogap, potentially shedding new light on its origin. The nearly disorder-free superconducting CuO2 layers offer a model platform for bridging the gap between disordered real materials and idealized theoretical models, which generally neglect disorder effects.

cond-mat.supr-con↗

arXiv2025

Pseudogap and Fermi arc induced by Fermi surface nesting in a centrosymmetric skyrmion magnet

Skyrmions in noncentrosymmetric materials are believed to occur due to the Dzyaloshinskii-Moriya interaction. By contrast, the skyrmion formation mechanism in centrosymmetric materials remains elusive. Here, we reveal the intrinsic electronic structure of the centrosymmetric GdRu2Si2 by selectively measuring magnetic domains using angle-resolved photoemission spectroscopy (ARPES). We found robust Fermi surface (FS) nesting, consistent with the magnetic modulation q-vector detected by the previous resonant x-ray scattering measurements. The pseudogap opens at the nested FS portions, which vary for different magnetic domains. The anomalous pseudogap disconnects the FS to generate Fermi arcs with twofold symmetry. These results indicate that the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction plays a decisive role in generating the screw spin modulation responsible for the skyrmion formation in GdRu2Si2. Furthermore, we demonstrate the flexible nature of magnetism in GdRu2Si2 by manipulating magnetic domains with magnetic field and temperature cyclings, providing potential future applications for data storage and processing devices.

cond-mat.mtrl-sci↗