arXiv subjects

Danfeng Li

Publications and source records attributed to Danfeng Li.

At least 19 recordsLinked to original sources

arXiv2026

Competing magnetic states in a non-coplanar Kagome magnet

Non-collinear Kagome antiferromagnets (AFMs) Mn3X (X = Sn, Ga, Ge, Ir, Pt) can generate an anomalous Hall effect (AHE) despite vanishing net magnetization, enabled by broken time-reversal and inversion symmetries. However, strong in-plane anisotropy has limited studies of the AFM-AHE and electronic applications to coplanar spin configurations. Non-coplanar spin textures in these systems have been realized only in low temperature spin-glass states or at interfaces with heavy metals. Here, we report an intrinsic non-coplanar spin configuration persisting up to 400 K in cubic-phase Mn3Ge, originating from coexisting symmetric and antisymmetric exchange interactions. Competing magnetic states associated with this non-coplanar spin configuration give rise to an unconventional AHE with a magnetic-field-induced sign reversal and a hump-like feature. Our findings establish a platform for non-coplanar magnetism in AFM spintronics.

cond-mat.mtrl-sci↗

arXiv2026

Bosonic phases across the superconductor-insulator transition in infinite-layer samarium nickelate

Superconductivity arises from the global phase coherence of Cooper pairs. Modulation of phase coherence leads to quantum phase transitions, serving as an important tool for studying unconventional superconductivity. Here, we demonstrate bosonic phases across the superconductor-insulator transition in infinite-layer nickelate superconducting films by the control of spatially periodic network patterns. Magnetoresistance oscillations with a periodicity of h/2e provide direct evidence of 2e Cooper pairing in nickelates. The phase transition is predominantly driven by enhanced superconducting fluctuations, and Cooper pairs are involved in charge transport across the transition. Notably, we observe two types of anomalous metallic phases, emerging respectively at finite magnetic fields and down to zero magnetic field. They can be characterized by bosonic excitations, suggesting the dynamic roles of vortices in the ground states. Our work establishes nickelates as a key platform for investigating the rich landscape of bosonic phases controlled via the phase coherence of Cooper pairs.

cond-mat.supr-con↗

arXiv2026

Superconductor-insulator transitions in infinite-layer nickelates controlled via ${operando}$ monitored reduction

Nickelates represent an emerging class of superconductors that demand innovative approaches for structural and electronic phase modulations. Continuous control over superconductor-insulator transition (SIT) in nickelates remains particularly challenging, hindering both fundamental understanding and potential applications. Here, we demonstrate SIT in infinite-layer nickelate superconductors utilizing multiple techniques, including an ${operando}$ monitored reduction (OMR) method. OMR enables ultrawide-range continuous modulation of the Ni 3${d}$ orbital electron occupancy from ~3${d}^7$ to ~3${d}^9$. The 3${d}$ occupancy is calibrated through systematic synchrotron X-ray absorption (XAS), combined with scanning transmission electron microscopy (STEM) annular bright field (ABF) analysis of oxygen atoms. SIT is further modulated via ionic liquid gating and magnetic field. Strikingly different from cuprates, our Nernst effect measurements show that pairing initiates at the onset of the resistive drop. The subsequent emergence of the Meissner effect at zero resistance marks the establishment of global phase coherence. Angle-dependent magnetotransport within the transition temperature regime indicates a mixture of two-dimensional (2D) and three-dimensional (3D) superconducting characters, suggesting the observed SIT deviates from the canonical 2D model. Our results provide a unique perspective on the interplay of structural and electronic phase transitions in the infinite-layer nickelates across the oxygen content-magnetic field-temperature parameter space.

cond-mat.supr-con↗

arXiv2025

Field re-entrant superconductivity in Eu-doped infinite-layer nickelates

Intertwined superconducting and magnetic orders may give rise to exotic quantum phases, including field-induced and re-entrant superconductivity. However, such magnetism-enhanced superconductivity has remained elusive in superconductors with higher transition temperatures. While infinite-layer nickelates represent a new class of unconventional superconductors, the impact of rare-earth magnetism on superconducting properties remains largely unexplored. Here, we show that Eu-doped infinite-layer nickelate Sm$_{0.95-x}$Ca$_{0.05}$Eu$_x$NiO$_2$ exhibits a magnetic-field-induced re-entrant superconducting phase in the Eu-rich over-doped regime. Zero-resistance transport and high-field diamagnetic screening confirm the superconducting nature of this phase, which emerges after the initial suppression of low-field superconductivity and remains robust across a broad range of temperatures, fields and field orientations. In the same doping range, we observe nonlinear Hall transport and hysteretic magnetoresistance, indicating the unconventional nature of the re-entrant behaviour. While partially consistent with a compensation mechanism between the Eu-derived exchange field and the applied field, our data reveal pronounced deviations from this model at the highest-doping levels. Our findings establish infinite-layer nickelates as a fertile platform for exploring magnetically driven high-field superconductivity in strongly correlated oxides.

cond-mat.supr-con↗

arXiv2025

Thermoelectricity evidence for quantum criticality in clean infinite-layer nickelate films

We investigate the Seebeck coefficient ($S$) in infinite-layer nickelate films with different disorder levels. The disordered NdNiO$_{2}$ film exhibits a flat $S/T$ curve, whereas cleaner samples display a logarithmic divergence with decreasing temperature, followed by a pronounced ``hump'' near 25 K. These distinct behaviors reveal a disorder-driven transition from band-structure-dominated transport to quantum-critical-dominated transport. Below the ``hump'' temperature, four-fold symmetry breaking is observed in the in-plane angular magnetoresistance, indicating the presence of short-range antiferromagnetic order in parent infinite-layer nickelate films. Furthermore, the logarithmic divergence in $S/T$ is also observed in a clean superconducting Sm$_{0.73}$Ca$_{0.05}$Eu$_{0.22}$NiO$_{2}$ film, where it coexists with linear-in-temperature resistivity over the same temperature range. These findings demonstrate the existence of quantum criticality over a wide doping range in clean infinite-layer nickelate films, similar to cuprates, which highlights the central role of antiferromagnetic spin correlations in their superconducting pairing mechanisms.

cond-mat.supr-con↗

arXiv2025

Seg2Any: Open-set Segmentation-Mask-to-Image Generation with Precise Shape and Semantic Control

Despite recent advances in diffusion models, top-tier text-to-image (T2I) models still struggle to achieve precise spatial layout control, i.e. accurately generating entities with specified attributes and locations. Segmentation-mask-to-image (S2I) generation has emerged as a promising solution by incorporating pixel-level spatial guidance and regional text prompts. However, existing S2I methods fail to simultaneously ensure semantic consistency and shape consistency. To address these challenges, we propose Seg2Any, a novel S2I framework built upon advanced multimodal diffusion transformers (e.g. FLUX). First, to achieve both semantic and shape consistency, we decouple segmentation mask conditions into regional semantic and high-frequency shape components. The regional semantic condition is introduced by a Semantic Alignment Attention Mask, ensuring that generated entities adhere to their assigned text prompts. The high-frequency shape condition, representing entity boundaries, is encoded as an Entity Contour Map and then introduced as an additional modality via multi-modal attention to guide image spatial structure. Second, to prevent attribute leakage across entities in multi-entity scenarios, we introduce an Attribute Isolation Attention Mask mechanism, which constrains each entity's image tokens to attend exclusively to themselves during image self-attention. To support open-set S2I generation, we construct SACap-1M, a large-scale dataset containing 1 million images with 5.9 million segmented entities and detailed regional captions, along with a SACap-Eval benchmark for comprehensive S2I evaluation. Extensive experiments demonstrate that Seg2Any achieves state-of-the-art performance on both open-set and closed-set S2I benchmarks, particularly in fine-grained spatial and attribute control of entities.

cs.CV↗

arXiv2025

Enhanced Superconductivity and Mixed-dimensional Behaviour in Infinite-layer Samarium Nickelate Thin Films

Rare-earth infinite-layer nickelates represent an emerging class of unconventional superconductors, with materials synthesis largely limited to early lanthanide compounds. Here, we report the synthesis and characterization of phase-pure superconducting samarium-based infinite-layer nickelate thin films, including the first demonstration of Sm$_{1-x}$Sr$_x$NiO$_2$, along with co-doped variants incorporating europium and calcium. These films, grown on LSAT (001) substrates, exhibit coherent lattice structures up to $\sim$ 9 nm thickness with minimal stacking faults. The co-doped compounds achieve a record-small $c$-axis parameter of 3.26 {\AA} and display remarkable superconducting transition temperatures up to 32.5 K. These results establish a clear correlation between decreasing $c$-axis parameter and increasing critical temperature across different rare-earth systems. In addition, angle-dependent magnetoresistance investigations reveal the existence of a hybrid mixture of 2D and 3D superconductivity in this novel system with enhanced coupling between the rare-earth 5d and Ni 3d orbitals, confirmed by resonant inelastic X-ray scattering experiments. As the concentration of Eu increases, the system exhibits a clear tendency towards 3D superconductivity. Furthermore, we observe distinctive negative magnetoresistance in the europium-containing samples. These findings advocate clear materials design principles for higher transition temperatures and exotic physics in infinite-layer nickelate superconductors through structural engineering of the rare-earth site.

cond-mat.supr-con↗

arXiv2024

Gigantic-oxidative atomic-layer-by-layer epitaxy for artificially designed complex oxides

In designing material functionalities for transition metal oxides, lattice structure and d-orbital occupancy are key determinants. However, the modulation of these two factors is inherently limited by the need to balance thermodynamic stability, growth kinetics, and stoichiometry precision, particularly for metastable phases. We introduce a methodology, namely the gigantic-oxidative atomic-layer-by-layer epitaxy (GOALL-Epitaxy), enhancing oxidation power 3-4 orders of magnitude beyond conventional pulsed laser deposition (PLD) and oxide molecular beam epitaxy (OMBE), while ensuring atomic-layer-by-layer growth of designed complex structures. Thermodynamic stability is markedly augmented with stronger oxidation at elevated temperatures, whereas growth kinetics is sustained by laser ablation at lower temperatures. We demonstrate the accurate growth of complex nickelates and cuprates, especially an artificially designed structure with alternating single and double NiO2 layers possessing distinct nominal d-orbital occupancy, as a parent of high-temperature superconductor. The GOALL-Epitaxy enables material discovery within the vastly broadened growth parameter space.

cond-mat.str-el↗

arXiv2023

The vortex-Nernst effect in a superconducting infinite-layer nickelate

We report measurements of the Nernst and Seebeck effects in Nd$_{1-x}$Sr$_x$NiO$_2$ thin films near the superconducting transition temperature, $T$ = 6.5 - 15 K. Our main result is the observation of a vortex-Nernst signal $S_{yx}(T,H)$ with a maximum at $\mu_0H$ = 5 T and a tail that extends to $\mu_0H$ $\approx$ 15 T, which we identify as the upper-critical field $H_{c2}$. At $T > T_{\rm c} =$ 6.1 K, $H_{c2}$ remains large (15 T), up to the highest temperature we can resolve from $S_{yx}$ (11 K). These results indicate the existence of a vortex-liquid state over a wide range of finite-resistance temperatures, as in the high-$T_{\rm c}$ cuprates.

cond-mat.supr-con↗

arXiv2023

Emergent electronic landscapes in a novel valence-ordered nickelate with tri-component nickel coordination

The metal-hydride-based topochemical reduction process has produced novel thermodynamically unstable phases across various transition metal oxide series with unusual crystal structures and non-trivial ground states. Here, by such an oxygen (de-) intercalation method we synthesis a novel samarium nickelate with ordered nickel valences associated with tri-component coordination configurations. This structure, with a formula of Sm$_{9}$Ni$_{9}$O$_{22}$ as revealed by four-dimensional scanning transmission electron microscopy, emerges from the intricate planes of {303}$_{\text{pc}}$ ordered apical oxygen vacancies. X-ray spectroscopy measurements and ab-initio calculations show the coexistence of square-planar, pyramidal and octahedral Ni sites with mono-, bi- and tri-valences. It leads to an intense orbital polarization, charge-ordering, and a ground state with a strong electron localization marked by the disappearance of ligand-hole configuration at low-temperature. This new nickelate compound provides another example of previously inaccessible materials enabled by topotactic transformations and presents a unique platform where mixed Ni valence can give rise to exotic phenomena.

cond-mat.mtrl-sci↗

arXiv2023

Observation of Coulomb blockade and Coulomb staircases in superconducting Pr0.8Sr0.2NiO2 films

Motivated by the discovery of superconductivity in the infinite-layer nickelate family, we report an experimental endeavor to clean the surface of nickelate superconductor Pr0.8Sr0.2NiO2 films by Ar+ ion sputtering and subsequent annealing, and we study their electronic structures by cryogenic scanning tunneling microscopy and spectroscopy. The annealed surfaces are characterized by nano-sized clusters and Coulomb staircases with periodicity inversely proportional to the projected area of the nanoclusters, consistent with a double-barrier tunneling junction model. Moreover, the dynamical Coulomb blockade effects are observed and result in well-defined energy gaps around the Fermi level, which correlate closely with the specific configuration of the junctions. These Coulomb blockade-related phenomena provide an alternative plausible cause of the observed gap structure that should be considered in the spectroscopic understanding of nickelate superconductors with the nano-clustered surface.

cond-mat.supr-con↗

arXiv2022

Rare-Earth Control of the Superconducting Upper Critical Field in Infinite-Layer Nickelates

The consequences of varying the rare-earth element in the superconducting infinite-layer nickelates have been much debated. Here we show striking differences in the magnitude and anisotropy of the superconducting upper critical field across the La-, Pr-, and Nd-nickelates. These 5 distinctions originate from the 4f electron characteristics of the rare-earth ions in the lattice: they are absent for La3+, nonmagnetic for the Pr3+ singlet ground state, and magnetic for the Nd3+ Kramer's doublet. The unique polar and azimuthal angle-dependent magnetoresistance found in the Nd-nickelates can be understood to arise from the magnetic contribution of the Nd3+ 4f moments. In the absence of rare-earth effects, we find that the nickelates broadly violate the Pauli limit. Such robust and tunable superconductivity suggests potential in future high-field applications.

cond-mat.supr-con↗

arXiv2022

Evidence for nodal superconductivity in infinite-layer nickelates

Infinite-layer nickelates present a new family of potential unconventional superconductors. A key open question is the superconducting pairing symmetry. We present low-temperature measurements of the London penetration depth in optimally doped La_{0.8}Sr_{0.2}NiO_{2}, Pr_{0.8}Sr_{0.2}NiO_{2}, and Nd_{0.8}Sr_{0.2}NiO_{2}. For La and Pr-nickelates, the superfluid density shows a quadratic temperature dependence, indicating nodal superconductivity in the presence of disorder. Nd-nickelate exhibits complex low-temperature behavior, which we attribute to magnetic impurities. These results are consistent with d-wave pairing.

cond-mat.supr-con↗

arXiv2022

Intrinsic magnetism in superconducting infinite-layer nickelates

The discovery of superconductivity in Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ [1] introduced a new family of layered nickelate superconductors that has now been extended to include a range of Sr-doping [2, 3], Pr or La in place of Nd [4-6], and the 5-layer Nd$_6$Ni$_5$O$_{12}$ [7]. A number of studies indicate that electron correlations are strong in these materials [8-14], and hence a central question is whether or not magnetism is present as a consequence of these interactions. Here we report muon spin rotation/relaxation studies of a series of superconducting infinite-layer nickelates. In all cases we observe an intrinsic magnetic ground state, regardless of the rare earth ion or doping, arising from local moments on the nickel sublattice. The coexistence of magnetism - which is likely to be antiferromagnetic and short-range ordered - with superconductivity is reminiscent of some iron pnictides [15] and heavy fermion compounds [16], and qualitatively distinct from the doped cuprates [17].

cond-mat.supr-con↗

arXiv2022

Reconstructing the polar interface of infinite-layer nickelate thin films

Nickel-based superconductors provide a long-awaited experimental platform to explore possible cuprate-like superconductivity. Despite similar crystal structure and $d$ electron filling, these systems exhibit several differences. Nickelates are the most polar layered oxide superconductor, raising questions about the interface between substrate and thin film -- thus far the only sample geometry to successfully stabilize superconductivity. We conduct a detailed experimental and theoretical study of the prototypical interface between Nd$_{1-x}$Sr$_x$NiO$_2$ and SrTiO$_3$. Atomic-resolution electron energy loss spectroscopy in the scanning transmission electron microscope reveals the formation of a single intermediate Nd(Ti,Ni)O$_3$ layer. Density functional theory calculations with a Hubbard $U$ term show how the observed structure alleviates the strong polar discontinuity. We explore effects of oxygen occupancy, hole doping, and cation structure to disentangle the contributions of each for reducing interface charge density. Resolving the nontrivial interface structure will be instructive for future synthesis of nickelate films on other substrates and in vertical heterostructures.

cond-mat.supr-con↗

arXiv2021

Insulator-to-Metal Crossover near the Edge of the Superconducting Dome in Nd$_{1-x}$Sr$_x$NiO$_2$

We report a systematic magnetotransport study of superconducting infinite-layer nickelate thin films Nd$_{1-x}$Sr$_x$NiO$_2$ with $0.15 \leq x \leq 0.225$. By suppressing superconductivity with out-of-plane magnetic fields up to 37.5 T, we find that the normal state resistivity of Nd$_{1-x}$Sr$_x$NiO$_2$ is characterized by a crossover from a metallic $T^2$-behavior to an insulating log(1/$T$)-behavior for all $x$ except $x = 0.225$, at which the resistivity is predominantly metallic. The log(1/$T$)-behavior is found to be robust against magnetic fields, inconsistent with scenarios involving localization or Kondo scattering, and points to an anomalous insulating state possibly driven by strong correlations. In the metallic state, we find no evidence for non-Fermi-liquid behavior arising from proximity to a putative quantum critical point located inside the superconducting dome.

cond-mat.supr-con↗

arXiv2021

Electronic structure of superconducting nickelates probed by resonant photoemission spectroscopy

The discovery of infinite-layer nickelate superconductors has spurred enormous interest. While the Ni$^{1+}$ cations possess nominally the same 3$d^9$ configuration as Cu$^{2+}$ in cuprates, the electronic structure variances remain elusive. Here, we present a soft x-ray photoemission spectroscopy study on parent and doped infinite-layer Pr-nickelate thin films with a doped perovskite reference. By identifying the Ni character with resonant photoemission and comparison to density functional theory + U (on-site Coulomb repulsion energy) calculations, we estimate U ~5 eV, smaller than the charge transfer energy $\Delta$ ~8 eV, confirming the Mott-Hubbard electronic structure in contrast to charge-transfer cuprates. Near the Fermi level ($E_F$), we observe a signature of occupied rare-earth states in the parent compound, which is consistent with a self-doping picture. Our results demonstrate a correlation between the superconducting transition temperature and the oxygen 2$p$ hybridization near $E_F$ when comparing hole-doped nickelates and cuprates.

cond-mat.supr-con↗

arXiv2021

Nickelate superconductivity without rare-earth magnetism: (La,Sr)NiO$_{2}$

The observation of superconductivity in infinite layer nickelate (Nd,Sr)NiO$_{2}$ thin films has led to rapid theoretical and experimental investigations of these copper-oxide-analogue systems [1-15]. Superconductivity has also been found in (Pr,Sr)NiO$_{2}$ [16,17], but not previously in (La,Sr)NiO$_{2}$ [2], raising a fundamental question whether superconductivity is associated with the presence of rare-earth moments [18,19]. Here we show that with significant materials optimization, substantial portions of the La$_{1-x}$Sr$_{x}$NiO$_{2}$ phase diagram can enter the regime of coherent low-temperature transport ($x$ = 0.14 - 0.20), with subsequent superconducting transitions and a maximum onset of ~ 9 K at $x$ = 0.20. Additionally, we observe the unexpected indication of a superconducting ground state in undoped LaNiO$_{2}$, which likely reflects the self-doped nature of the electronic structure. Combining the results of (La/Pr/Nd)$_{1-x}$Sr$_{x}$NiO$_{2}$ reveals a generalized superconducting dome, characterized by systematic shifts in the unit cell volume and in the relative electron-hole populations across the lanthanides.

cond-mat.supr-con↗