Spatially reconfigurable antiferromagnetic states in topologically rich free-standing nanomembranes.

Antiferromagnets hosting real-space topological textures are promising platforms to model fundamental ultrafast phenomena and explore spintronics. However, they have only been epitaxially fabricated on specific symmetry-matched substrates, thereby preserving their intrinsic magneto-crystalline order. This curtails their integration with dissimilar supports, restricting the scope of fundamental and applied investigations. Here we circumvent this limitation by designing detachable crystalline antiferromagnetic nanomembranes of α-Fe2O3. First, we show-via transmission-based antiferromagnetic vector mapping-that flat nanomembranes host a spin-reorientation transition and rich topological phenomenology. Second, we exploit their extreme flexibility to demonstrate the reconfiguration of antiferromagnetic states across three-dimensional membrane folds resulting from flexure-induced strains. Finally, we combine these developments using a controlled manipulator to realize the strain-driven non-thermal generation of topological textures at room temperature. The integration of such free-standing antiferromagnetic layers with flat/curved nanostructures could enable spin texture designs via magnetoelastic/geometric effects in the quasi-static and dynamical regimes, opening new explorations into curvilinear antiferromagnetism and unconventional computing.

Holographic imaging of antiferromagnetic domains with in-situ magnetic field.

Lensless coherent x-ray imaging techniques have great potential for high-resolution imaging of magnetic systems with a variety of in-situ perturbations. Despite many investigations of ferromagnets, extending these techniques to the study of other magnetic materials, primarily antiferromagnets, is lacking. Here, we demonstrate the first (to our knowledge) study of an antiferromagnet using holographic imaging through the 'holography with extended reference by autocorrelation linear differential operation' technique. Energy-dependent contrast with both linearly and circularly polarized x-rays are demonstrated. Antiferromagnetic domains and topological textures are studied in the presence of applied magnetic fields, demonstrating quasi-cyclic domain reconfiguration up to 500 mT.

Topological Flat Bands in Graphene Super-Moiré Lattices.

Moiré-pattern-based potential engineering has become an important way to explore exotic physics in a variety of two-dimensional condensed matter systems. While these potentials have induced correlated phenomena in almost all commonly studied 2D materials, monolayer graphene has remained an exception. We demonstrate theoretically that a single layer of graphene, when placed between two bulk boron nitride crystal substrates with the appropriate twist angles, can support a robust topological ultraflat band emerging as the second hole band. This is one of the simplest platforms to design and exploit topological flat bands.

Chemical Vapor Deposition of Superconducting FeTe1-xSex Nanosheets.

FeTe1-xSex, a promising layered material used to realize Majorana zero modes, has attracted enormous attention in recent years. Pulsed laser deposition (PLD) and molecular-beam epitaxy (MBE) are the routine growth methods used to prepare FeTe1-xSex thin films. However, both methods require high-vacuum conditions and polished crystalline substrates, which hinder the exploration of the topological superconductivity and related nanodevices of this material. Here we demonstrate the growth of the ultrathin FeTe1-xSex superconductor by a facile, atmospheric pressure chemical vapor deposition (CVD) method. The composition and thickness of the two-dimensional (2D) FeTe1-xSex nanosheets are well controlled by tuning the experimental conditions. The as-prepared FeTe0.8Se0.2 nanosheet exhibits an onset superconducting transition temperature of 12.4 K, proving its high quality. Our work offers an effective strategy for preparing the ultrathin FeTe1-xSex superconductor, which could become a promising platform for further study of the unconventional superconductivity in the FeTe1-xSex system.

Phase Diagram and Superconducting Dome of Infinite-Layer Nd_{1-x}Sr_{x}NiO_{2} Thin Films.

Infinite-layer Nd_{1-x}Sr_{x}NiO_{2} thin films with Sr doping level x from 0.08 to 0.3 are synthesized and investigated. We find a superconducting dome x between 0.12 and 0.235 accompanied by a weakly insulating behavior in both under- and overdoped regimes. The dome is akin to that in the electron-doped 214-type and infinite-layer cuprate superconductors. For x≥0.18, the normal state Hall coefficient (R_{H}) changes the sign from negative to positive as the temperature decreases. The temperature of the sign changes decreases monotonically with decreasing x from the overdoped side and approaches the superconducting dome at the midpoint, suggesting a reconstruction of the Fermi surface with the dopant concentration across the dome.

Fast Batch Production of High-Quality Graphene Films in a Sealed Thermal Molecular Movement System.

Chemical vapor deposition (CVD) growth of high-quality graphene has emerged as the most promising technique in terms of its integrated manufacturing. However, there lacks a controllable growth method for producing high-quality and a large-quantity graphene films, simultaneously, at a fast growth rate, regardless of roll-to-roll (R2R) or batch-to-batch (B2B) methods. Here, a stationary-atmospheric-pressure CVD (SAPCVD) system based on thermal molecular movement, which enables fast B2B growth of continuous and uniform graphene films on tens of stacked Cu(111) foils, with a growth rate of 1.5 µm s-1 , is demonstrated. The monolayer graphene of batch production is found to nucleate from arrays of well-aligned domains, and the films possess few defects and exhibit high carrier mobility up to 6944 cm2 V-1 s-1 at room temperature. The results indicate that the SAPCVD system combined with single-domain Cu(111) substrates makes it possible to realize fast batch-growth of high-quality graphene films, which opens up enormous opportunities to use this unique 2D material for industrial device applications.

Tunable Spin-Polarized States in Graphene on a Ferrimagnetic Oxide Insulator.

Spin-polarized two-dimensional (2D) materials with large and tunable spin-splitting energy promise the field of 2D spintronics. While graphene has been a canonical 2D material, its spin properties and tunability are limited. Here, this work demonstrates the emergence of robust spin-polarization in graphene with large and tunable spin-splitting energy of up to 132 meV at zero applied magnetic fields. The spin polarization is induced through a magnetic exchange interaction between graphene and the underlying ferrimagnetic oxide insulating layer, Tm3 Fe5 O12 , as confirmed by its X-ray magnetic circular dichroism (XMCD). The spin-splitting energies are directly measured and visualized by the shift in their Landau-fan diagram mapped by analyzing the measured Shubnikov-de-Haas (SdH) oscillations as a function of applied electric fields, showing consistent fit with the first-principles and machine learning calculations. Further, the observed spin-splitting energies can be tuned over a broad range between 98 and 166 meV by field cooling. The methods and results are applicable to other 2D (magnetic) materials and heterostructures, and offer great potential for developing next-generation spin logic and memory devices.

Controlled alignment of supermoiré lattice in double-aligned graphene heterostructures.

The supermoiré lattice, built by stacking two moiré patterns, provides a platform for creating flat mini-bands and studying electron correlations. An ultimate challenge in assembling a graphene supermoiré lattice is in the deterministic control of its rotational alignment, which is made highly aleatory due to the random nature of the edge chirality and crystal symmetry. Employing the so-called "golden rule of three", here we present an experimental strategy to overcome this challenge and realize the controlled alignment of double-aligned hBN/graphene/hBN supermoiré lattice, where the twist angles between graphene and top/bottom hBN are both close to zero. Remarkably, we find that the crystallographic edge of neighboring graphite can be used to better guide the stacking alignment, as demonstrated by the controlled production of 20 moiré samples with an accuracy better than ~ 0.2°. Finally, we extend our technique to low-angle twisted bilayer graphene and ABC-stacked trilayer graphene, providing a strategy for flat-band engineering in these moiré materials.

Graphene moiré superlattices with giant quantum nonlinearity of chiral Bloch electrons.

Graphene-based samples have shown a plethora of exotic characteristics and these properties may help the realization of a new generation of fast electronic devices. However, graphene's centrosymmetry prohibits second-order electronic transport. Here, we show giant second-order nonlinear transports in graphene moiré superlattices at zero magnetic field, both longitudinal and transverse to the applied current direction. High carrier mobility and inversion symmetry breaking by hexagonal boron nitride lead to nonlinear conductivities five orders of magnitude larger than those in WTe2. The nonlinear conductivity strongly depends on the gate voltage as well as on the stacking configuration, with a giant enhancement originating from the moiré bands. Longitudinal nonlinear conductivity cannot originate from Berry curvature dipoles. Our theoretical modelling highlights skew scattering of chiral Bloch electrons as the physical origin. With these results, we demonstrate nonlinear charge transport due to valley-contrasting chirality, which constitutes an alternative means to induce second-order transports in van der Waals heterostructures. Our approach is promising for applications in frequency-doubling and energy harvesting via rectification.

Superconductivity in infinite-layer nickelate La1-xCaxNiO2 thin films.

We report the observation of superconductivity in infinite-layer Ca-doped LaNiO2 (La1-xCaxNiO2) thin films and construct their phase diagram. Unlike the metal-insulator transition in Nd- and Pr-based nickelates, the undoped and underdoped La1-xCaxNiO2 thin films are entirely insulating from 300 K down to 2 K. A superconducting dome is observed at 0.15 < x < 0.3 with weakly insulating behavior at the overdoped regime. Moreover, the sign of the Hall coefficient RH changes at low temperature for samples with a higher doping level. However, distinct from the Nd- and Pr-based nickelates, the RH-sign-change temperature remains at around 35 K as the doping increases, which begs further theoretical and experimental investigation to reveal the role of the 4f orbital to the (multi)band nature of the superconducting nickelates. Our results also emphasize a notable role of lattice correlation on the multiband structures of the infinite-layer nickelates.

Observation of Anisotropic Magnetoresistance in Layered Nonmagnetic Semiconducting PdSe2.

Anisotropy in crystals usually has remarkable consequences in two-dimensional (2D) materials, for example, black phosphorus, PdSe2, and SnS, arising from different lattice periodicities along different crystallographic directions. Electrical anisotropy has been successfully demonstrated in 2D materials, but anisotropic magnetoresistance in 2D materials is rarely studied. Herein, we report anisotropic magnetoresistance in layered nonmagnetic semiconducting PdSe2 flakes. Anisotropic magnetoresistance along the two crystalline axes under a perpendicular magnetic field is demonstrated, and the magnetoresistance along the a-axis is apparently different from the magnetoresistance along the b-axis. The magnetoresistance can also be flexibly tuned by applying a gate voltage, leveraging the semiconductor properties of PdSe2. The computed anisotropic electronic density of states and electronic mobility with ab initio density functional calculations support the anisotropic and measured magnetoresistance. Our findings advance the understanding of magnetoresistance in anisotropic transition-metal dichalcogenides and pave the way for potential applications in anisotropic spintronic devices.

Room-Temperature Colossal Magnetoresistance in Terraced Single-Layer Graphene.

Disorder-induced magnetoresistance (MR) effect is quadratic at low perpendicular magnetic fields and linear at high fields. This effect is technologically appealing, especially in 2D materials such as graphene, since it offers potential applications in magnetic sensors with nanoscale spatial resolution. However, it is a great challenge to realize a graphene magnetic sensor based on this effect because of the difficulty in controlling the spatial distribution of disorder and enhancing the MR sensitivity in the single-layer regime. Here, a room-temperature colossal MR of up to 5000% at 9 T is reported in terraced single-layer graphene. By laminating single-layer graphene on a terraced substrate, such as TiO2 -terminated SrTiO3 , a universal one order of magnitude enhancement in the MR compared to conventional single-layer graphene devices is demonstrated. Strikingly, a colossal MR of >1000% is also achieved in the terraced graphene even at a high carrier density of ≈1012 cm-2 . Systematic studies of the MR of single-layer graphene on various oxide- and non-oxide-based terraced surfaces demonstrate that the terraced structure is the dominant factor driving the MR enhancement. The results open a new route for tailoring the physical property of 2D materials by engineering the strain through a terraced substrate.

Phase-controllable growth of ultrathin 2D magnetic FeTe crystals.

Two-dimensional (2D) magnets with intrinsic ferromagnetic/antiferromagnetic (FM/AFM) ordering are highly desirable for future spintronic devices. However, the direct growth of their crystals is in its infancy. Here we report a chemical vapor deposition approach to controllably grow layered tetragonal and non-layered hexagonal FeTe nanoplates with their thicknesses down to 3.6 and 2.8 nm, respectively. Moreover, transport measurements reveal these obtained FeTe nanoflakes show a thickness-dependent magnetic transition. Antiferromagnetic tetragonal FeTe with the Néel temperature (TN) gradually decreases from 70 to 45 K as the thickness declines from 32 to 5 nm. And ferromagnetic hexagonal FeTe is accompanied by a drop of the Curie temperature (TC) from 220 K (30 nm) to 170 K (4 nm). Theoretical calculations indicate that the ferromagnetic order in hexagonal FeTe is originated from its concomitant lattice distortion and Stoner instability. This study highlights its potential applications in future spintronic devices.

High-Performance, Room Temperature, Ultra-Broadband Photodetectors Based on Air-Stable PdSe2.

Photodetection over a broad spectral range is crucial for optoelectronic applications such as sensing, imaging, and communication. Herein, a high-performance ultra-broadband photodetector based on PdSe2 with unique pentagonal atomic structure is reported. The photodetector responds from visible to mid-infrared range (up to ≈4.05 µm), and operates stably in ambient and at room temperature. It promises improved applications compared to conventional mid-infrared photodetectors. The highest responsivity and external quantum efficiency achieved are 708 A W-1 and 82 700%, respectively, at the wavelength of 1064 nm. Efficient optical absorption beyond 8 µm is observed, indicating that the photodetection range can extend to longer than 4.05 µm. Owing to the low crystalline symmetry of layered PdSe2 , anisotropic properties of the photodetectors are observed. This emerging material shows potential for future infrared optoelectronics and novel devices in which anisotropic properties are desirable.

Roles of Oxygen and Hydrogen in Crystal Orientation Transition of Copper Foils for High-Quality Graphene Growth.

The high-quality graphene film can be grown on single-crystal Cu substrate by seamlessly stitching the aligned graphene domains. The roles of O2 and H2 have been intensively studied in the graphene growth kinetics, including lowering the nucleation sites and tailoring the domain structures. However, how the O2 and H2 influence Cu orientations during recrystallization prior to growing graphene, still remains unclear. Here we report that the oxidation of Cu surface tends to stabilize the Cu(001) orientation while impedes the evolution of Cu(111) single domain during annealing process. The crystal orientation-controlled synthesis of aligned graphene seeds is further realized on the long-range ordered Cu(111) substrate. With decreasing the thickness of oxide layer on Cu surface by introducing H2, the Cu(001) orientation changes into Cu(111) orientation. Meanwhile, the average domain size of Cu foils is increased from 50 µm to larger than 1000 µm. The density functional theory calculations reveal that the oxygen increases the energy barrier for Cu(111) surface and makes O/Cu(001) more stable than O/Cu(111) structure. Our work can be helpful for revealing the roles of O2 and H2 in controlling the formation of Cu single-crystal substrate as well as in growing high-quality graphene films.