Danfeng Li

Intertwined superconducting and magnetic orders may give rise to exotic quantum phases^1-7, including field-induced and re-entrant superconductivity^8-10. However, such magnetism-enhanced superconductivity has remained elusive in superconductors with higher transition temperatures^1-3. While infinite-layer nickelates represent a new class of unconventional superconductors^11-20, the impact of rare-earth magnetism on superconducting properties remains largely unexplored. Here, we show that Eu-doped infinite-layer nickelate Sm_0.95-xCa_0.05Eu_xNiO₂ 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.

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 / 2 e provide direct evidence of 2 e 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.

Rare-earth infinite-layer nickelates are emerging unconventional superconductors, with materials synthesis largely limited to early lanthanide compounds. Here, we report phase-pure samarium-based nickelate thin films on (LaAlO₃)_0.3(Sr₂TaAlO₆)_0.7 (001) substrates, including the first demonstration of Sm_1-xSr_xNiO₂. Co-doped compounds achieve a record-small c-axis parameter (3.26 Å) and superconducting transitions up to 32.5 K, revealing a clear correlation between decreasing c-axis parameter and increasing critical temperature across different rare-earth systems. Angle-dependent magnetoresistance shows a hybrid 2D/3D superconductivity with enhanced rare-earth 5d–Ni 3 d orbital coupling, confirmed by resonant inelastic X-ray scattering. In addition, increasing Eu concentration drives a shift toward 3D superconductivity, and Eu-containing samples exhibit distinctive negative magnetoresistance even in the superconducting state. 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.

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 magnetic penetration depth in optimally doped La _0.8 Sr _0.2 NiO ₂ , Pr _0.8 Sr _0.2 NiO ₂ , Nd _0.8 Sr _0.2 NiO ₂ , and Nd _0.7875 Sr _0.2125 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 associated with Nd 4 f magnetism, which precludes a direct measure of the nodal structure.

This study investigates the impact of 1302 MeV krypton ion irradiation on the structural, optical, and electrical properties of β-Ga₂O₃/4H-SiC heterostructures. Through a combination of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), photoluminescence (PL), and current-voltage (I-V) measurements, we systematically analyzed irradiation-induced defect formation and its effects on device performance. The results reveal that irradiation introduces a significant density of gallium vacancies (V_Ga), oxygen vacancies (V_O), and Ga-O vacancy pairs (V_Ga-V_O), which considerably alter the material's optoelectronic properties. PL intensity at specific wavelengths was enhanced after irradiation, while changes in dark and photocurrent characteristics indicate increased carrier recombination and the formation of leakage current channels. Despite the rise in dark current, the irradiated sample exhibited a significantly enhanced responsivity of 288.20 A/W under a 10 V bias and 75 μW/cm² illumination (compared to 21.70 A/W in the control sample). These findings contribute to the understanding of radiation-induced defect dynamics in β-Ga₂O₃ heterojunctions in extreme environments and provide insights for designing radiation-tolerant optoelectronic devices for space applications, nuclear safety systems, and advanced communication technologies.

Superconductivity in infinite-layer nickelates has stirred much research interest, to which questions regarding the nature of superconductivity remain elusive. A critical leap forward to address these intricate questions is through the growth of high-crystallinity infinite-layer nickelates, including the "parent" phase. Here, we report the synthesis of a high-quality thin-film nickelate, NdNiO₂. This is achieved through the growth of a perovskite precursor phase (NdNiO₃) of superior crystallinity on the NdGaO₃ substrate by off-axis RF magnetron sputtering and a low-temperature topochemical reduction using NaH. We observe a nonlinear Hall effect at low temperatures in this "non-doped" phase. We further study the electronic properties using advanced X-ray scattering and first-principles calculations. We observe spectroscopic indications of the enhanced two-dimensionality and a reduced hybridization of Nd 5d and Ni 3d orbitals. These findings unlock new pathways for preparing high-quality infinite-layer nickelates and provide new insights into the intrinsic features of these compounds.

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.

Emergent phenomena such as two-dimensional electron gas (2DEG) and interfacial superconductivity and ferromagnetism are generally built on the interface between insulating oxide thin films and substrates, e.g., LaAlO₃/SrTiO₃, where the 2D profiles of these electronic states are precisely confined at the interface of two insulators. Herein we report a high-mobility electron gas state with unusual symmetry at the interface of the Sr₂CrMoO₆/SrTiO₃ (110) heterostructures, the fermiology of which follows the cubic crystallographic symmetry rather than the two-dimensional interface itself, resulting in the identical Shubnikov-de Haas oscillations with applied magnetic field along all the twelve equivalent [110] crystallographic directions of SrTiO₃, distinctly different from the 2D nature of the electron gas reported previously. Neutron diffraction verifies the predicted ferrimagnetic ordering between Cr and Mo moments. This, together with the magnetic hysteresis loops and negative magnetoresistance in low-field region, suggests possible spin polarization of itinerant electrons. Therefore, a quasi-3D profile, high mobility (up to 10⁴ cm² V⁻¹ s⁻¹) and possibly spin polarized electronic state is observed in the double-perovskite-based oxide heterostructures. This finding of the electronic properties in Sr₂CrMoO₆/SrTiO₃ (110) heterostructure expands the knowledge of interfacial physics, as well as shines light on oxide-based electronics and spintronics research.