Magnetotransport signatures of Weyl physics and discrete scale invariance in the elemental semiconductor tellurium.

The study of topological materials possessing nontrivial band structures enables exploitation of relativistic physics and development of a spectrum of intriguing physical phenomena. However, previous studies of Weyl physics have been limited exclusively to semimetals. Here, via systematic magnetotransport measurements, two representative topological transport signatures of Weyl physics, the negative longitudinal magnetoresistance and the planar Hall effect, are observed in the elemental semiconductor tellurium. More strikingly, logarithmically periodic oscillations in both the magnetoresistance and Hall data are revealed beyond the quantum limit and found to share similar characteristics with those observed in ZrTe5 and HfTe5 The log-periodic oscillations originate from the formation of two-body quasi-bound states formed between Weyl fermions and opposite charge centers, the energies of which constitute a geometric series that matches the general feature of discrete scale invariance (DSI). Our discovery reveals the topological nature of tellurium and further confirms the universality of DSI in topological materials. Moreover, introduction of Weyl physics into semiconductors to develop "Weyl semiconductors" provides an ideal platform for manipulating fundamental Weyl fermionic behaviors and for designing future topological devices.

Converting a Monolayered NbSe2 into an Ising Superconductor with Nontrivial Band Topology via Physical or Chemical Pressuring.

Two-dimensional transition metal dichalcogenides possessing superconductivity and strong spin-orbit coupling exhibit high in-plane upper critical fields due to Ising pairing. Yet to date, whether such systems can become topological Ising superconductors remains to be materialized. Here we show that monolayered NbSe2 can be converted into Ising superconductors with nontrivial band topology via physical or chemical pressuring. Using first-principles calculations, we first demonstrate that a hydrostatic pressure higher than 2.5 GPa can induce a p-d band inversion, rendering nontrivial band topology to NbSe2. We then illustrate that Te-doping can function as chemical pressuring in inducing nontrivial topology in NbSe2-xTex with x ≥ 0.8, due to a larger atomic radius and stronger spin-orbit coupling of Te. We also evaluate the upper critical fields within both approaches, confirming the enhanced Ising superconductivity nature, as experimentally observed. Our findings may prove to be instrumental in material realization of topological Ising superconductivity in two-dimensional systems.

Electric Field-Controlled Damping Switches of Coupled Dirac Plasmons.

For quasiparticle systems, the control of the quasiparticle lifetime is an important goal, determining whether the related fascinating physics can be revealed in fundamental research and utilized in practical applications. Here, we use double-layer graphene with a boron nitride spacer as a model system to demonstrate that the lifetime of coupled Dirac plasmons can be remotely tuned by electric field-controlled damping pathways. Essentially, one of the graphene layers serves as an external damping amplifier whose efficiency can be controlled by the corresponding doping level. Through this damping switch, the damping rate of the plasmon can be actively tuned up to 1.7 fold. This Letter provides a prototype design to actively control the lifetime of graphene plasmons and also broadens our horizon for the damping control of other quasiparticle systems.

Flat-Band-Induced Anomalous Anisotropic Charge Transport and Orbital Magnetism in Kagome Metal CoSn.

For solids, the dispersionless flat band has long been recognized as an ideal platform for achieving intriguing quantum phases. However, experimental progress in revealing flat-band physics has so far been achieved mainly in artificially engineered systems represented as magic-angle twisted bilayer graphene. Here, we demonstrate the emergence of flat-band-dominated anomalous transport and magnetic behaviors in CoSn, a paramagnetic kagome-lattice compound. By combination of angle-resolved photoemission spectroscopy measurements and first-principles calculations, we reveal the existence of a kagome-lattice-derived flat band right around the Fermi level. Strikingly, the resistivity within the kagome lattice plane is more than one order of magnitude larger than the interplane one, in sharp contrast with conventional (quasi-) two-dimensional layered materials. Moreover, the magnetic susceptibility under the out-of-plane magnetic field is found to be much smaller as compared with the in-plane case, which is revealed to be arising from the introduction of a unique orbital diamagnetism. Systematic analyses reveal that these anomalous and giant anisotropies can be reasonably attributed to the unique properties of flat-band electrons, including large effective mass and self-localization of wave functions. Our results broaden the already fascinating flat-band physics, and demonstrate the feasibility of exploring them in natural solid-state materials in addition to artificial ones.

Spontaneous Folding Growth of Graphene on h-BN.

Graphene has been the subject of much research, with structural engineering frequently used to harness its various properties. In particular, the concepts of graphene origami and kirigami have inspired the design of quasi-three-dimensional graphene structures, which possess intriguing mechanical, electronic, and optical properties. However, accurate controlling the folding process remains a big challenge. Here, we report the discovery of spontaneous folding growth of graphene on the h-BN substrate via adopting a simple chemical vapor deposition method. Folded edges are formed when two stacked graphene layers share a joint edge at a growth temperature up to 1300 °C. Using first-principles density functional theory calculations, the bilayer graphene with folded edges is demonstrated to be more stable than that with open edges. Utilizing this novel growth mode, hexagram bilayer graphene containing entirely sealed edges is eventually realized. Our findings provide a route for designing graphene devices with a new folding dimension.

Zero-Bias Conductance Peaks Effectively Tuned by Gating-Controlled Rashba Spin-Orbit Coupling.

Zero-bias conductance peaks (ZBCPs) can manifest a number of notable physical phenomena and thus provide critical characteristics to the underlying electronic systems. Here, we report observations of pronounced ZBCPs in hybrid junctions composed of an oxide heterostructure LaAlO_{3}/SrTiO_{3} and an elemental superconductor Nb, where the two-dimensional electron system (2DES) at the LaAlO_{3}/SrTiO_{3} interface is known to accommodate gate-tunable Rashba spin-orbit coupling (SOC). Remarkably, the ZBCPs exhibit a domelike dependence on the gate voltage, which correlates strongly with the nonmonotonic gate dependence of the Rashba SOC in the 2DES. The origin of the observed ZBCPs can be attributed to the reflectionless tunneling effect of electrons that undergo phase-coherent multiple Andreev reflection, and their gate dependence can be explained by the enhanced quantum coherence time of electrons in the 2DES with increased momentum separation due to SOC. We further demonstrate theoretically that, in the presence of a substantial proximity effect, the Rashba SOC can directly enhance the overall Andreev conductance in the 2DES-barrier-superconductor junctions. These findings not only highlight nontrivial interplay between electron spin and superconductivity revealed by ZBCPs, but also set forward the study of superconducting hybrid structures by means of controllable SOC, which has significant implications in various research fronts from superconducting spintronics to topological superconductivity.

Frictional Drag Effect between Massless and Massive Fermions in Single-Layer/Bilayer Graphene Heterostructures.

The emergence of two-dimensional layered materials offers an excellent opportunity for the fabrication of double-layer electron systems that are in close proximity but electronically isolated. In this work, heterostructures consisting of one single-layer graphene (SLG) and one bilayer graphene (BLG) separated by an ultrathin hBN layer are successfully fabricated, enabling the experimental investigation of the interlayer frictional drag effect between massless and massive fermions. With varying carrier densities, a giant positive peak of drag response emerges at the double neutrality point, around which nonmonotonic temperature dependent behaviors of drag resistance are further observed. These observations can be attributed to the anticorrelations in the distributions of e-h puddles between layers. More importantly, as the system shifts toward the strong coupling regime, the carrier density dependence of drag resistance Rdrag shows a crossover from 1/n3 to 1/n2 for the density matched cases, which is a unique characteristic predicted for massless-massive fermion systems. Consequently, a generalized carrier dependent expression (1/(|nS| + |nB|)2) is obtained for the strong coupling regime, where nS and nB are the carrier densities of SLG and BLG, respectively. Our study provides insight into the electronic frictional effects between massless and massive fermions and thus will promote the investigations of interlayer interactions in hybrid structures hosting different types of carriers.

Modulating the Electrical Transport in the Two-Dimensional Electron Gas at LaAlO_{3}/SrTiO_{3} Heterostructures by Interfacial Flexoelectricity.

Thin film flexoelectricity is attracting more attention because of its enhanced effect and potential application in electronic devices. Here we find that a mechanical bending induced flexoelectricity significantly modulates the electrical transport properties of the interfacial two-dimensional electron gas (2DEG) at the LaAlO_{3}/SrTiO_{3} (LAO/STO) heterostructure. Under variant bending states, both the carrier density and mobility of the 2DEG are changed according to the flexoelectric polarization direction, showing an electric field effect modulation. By measuring the flexoelectric response of LAO, it is found that the effective flexoelectricity in the LAO thin film is enhanced by 3 orders compared to its bulk. These results broaden the horizon of study on the flexoelectricity effect in the hetero-oxide interface and more research on the oxide interfacial flexoelectricity may be stimulated.

Quantum Control of Graphene Plasmon Excitation and Propagation at Heaviside Potential Steps.

Quantum mechanical effects of single particles can affect the collective plasmon behaviors substantially. In this work, the quantum control of plasmon excitation and propagation in graphene is demonstrated by adopting the variable quantum transmission of carriers at Heaviside potential steps as a tuning knob. First, the plasmon reflection is revealed to be tunable within a broad range by varying the ratio γ between the carrier energy and potential height, which originates from the quantum mechanical effect of carrier propagation at potential steps. Moreover, the plasmon excitation by free-space photos can be regulated from fully suppressed to fully launched in graphene potential wells also through adjusting γ, which defines the degrees of the carrier confinement in the potential wells. These discovered quantum plasmon effects offer a unified quantum-mechanical solution toward ultimate control of both plasmon launching and propagating, which are indispensable processes in building plasmon circuitry.

Gate Switching of Ultrafast Photoluminescence in Graphene.

The control of optical properties by electric means is the key to optoelectronic applications. For atomically thin two-dimensional (2D) materials, the natural advantage lies in that the carrier doping could be readily controlled through the electric gating effect, possibly affecting the optical properties. Exploiting this advantage, here we report the gate switching of the ultrafast upconverted photoluminescence from monolayer graphene. The luminescence can be completely switched off by the Pauli-blocking of one-photon interband transition in graphene with an on/off ratio exceeding 100, which is remarkable compared to other 2D semiconductors and 3D bulk counterparts. The chemical potential and pump fluence dependences of the luminescence are nicely described by a two-temperature model, including both the hot carrier dynamics and carrier-optical phonon interaction. This gate switchable and background-free photoluminescence can open up new opportunities for graphene-based ultrafast optoelectronic applications.

Flatbands and Emergent Ferromagnetic Ordering in Fe_{3}Sn_{2} Kagome Lattices.

A flatband representing a highly degenerate and dispersionless manifold state of electrons may offer unique opportunities for the emergence of exotic quantum phases. To date, definitive experimental demonstrations of flatbands remain to be accomplished in realistic materials. Here, we present the first experimental observation of a striking flatband near the Fermi level in the layered Fe_{3}Sn_{2} crystal consisting of two Fe kagome lattices separated by a Sn spacing layer. The band flatness is attributed to the local destructive interferences of Bloch wave functions within the kagome lattices, as confirmed through theoretical calculations and modelings. We also establish high-temperature ferromagnetic ordering in the system and interpret the observed collective phenomenon as a consequence of the synergetic effect of electron correlation and the peculiar lattice geometry. Specifically, local spin moments formed by intramolecular exchange interaction are ferromagnetically coupled through a unique network of the hexagonal units in the kagome lattice. Our findings have important implications to exploit emergent flat-band physics in special lattice geometries.

Quantum Percolation and Magnetic Nanodroplet States in Electronically Phase-Separated Manganite Nanowires.

One-dimensional (1D) confinement has been revealed to effectively tune the properties of materials in homogeneous states. The 1D physics can be further enriched by electronic inhomogeneity, which unfortunately remains largely unknown. Here we demonstrate the ultrahigh sensitivity to magnetic fluctuations and the tunability of phase stability in the electronic transport properties of self-assembled electronically phase-separated manganite nanowires with extreme aspect ratio. The onset of magnetic nanodroplet state, a precursor to the ferromagnetic metallic state, is unambiguously revealed, which is attributed to the small lateral size of the nanowires that is comparable to the droplet size. Moreover, the quasi-1D anisotropy stabilizes thin insulating domains to form intrinsic tunneling junctions in the low temperature range, which is robust even under magnetic field up to 14 T and thus essentially modifies the classic 1D percolation picture to stabilize a novel quantum percolation state. A new phase diagram is therefore established for the manganite system under quasi-1D confinement for the first time. Our findings offer new insight into understanding and manipulating the colorful properties of the electronically phase-separated systems via dimensionality engineering.

Optical Manipulation of Rashba Spin-Orbit Coupling at SrTiO3-Based Oxide Interfaces.

Spin-orbit coupling (SOC) plays a crucial role for spintronics applications. Here we present the first demonstration that the Rashba SOC at the SrTiO3-based interfaces is highly tunable by photoinduced charge doping, that is, optical gating. Such optical manipulation is nonvolatile after the removal of the illumination in contrast to conventional electrostatic gating and also erasable via a warming-cooling cycle. Moreover, the SOC evolutions tuned by illuminations with different wavelengths at various gate voltages coincide with each other in different doping regions and collectively form an upward-downward trend curve: In response to the increase of conductivity, the SOC strength first increases and then decreases, which can be attributed to the orbital hybridization of Ti 3d subbands. More strikingly, the optical manipulation is effective enough to tune the interferences of Bloch wave functions from constructive to destructive and therefore to realize a transition from weak localization to weak antilocalization. The present findings pave a way toward the exploration of photoinduced nontrivial quantum states and the design of optically controlled spintronic devices.

A Kinetic Pathway toward High-Density Ordered N Doping of Epitaxial Graphene on Cu(111) Using C5NCl5 Precursors.

Pristine graphene possesses high electrical mobility, but its low charge carrier density severely limits its technological significance. Past efforts to increase graphene's carrier density via chemical doping have shown limited successes, accompanied by substantial reductions in the mobility caused by disordered dopants. Here, based on first-principles calculations, we propose to grow graphene on Cu(111) via self-assembly of C5NCl5 molecular precursors to achieve high-density (1/6) and highly ordered nitrogen doping. Such a process relies on the elegant concerted roles played by the London dispersion, chemical, and screened Coulomb repulsive forces in enhancing molecular adsorption, facilitating easy dechlorination, and dictating the overall orientation of the C5N radicals, respectively. Further growth from the orientationally correlated graphene islands is accompanied by significantly minimized density of grain boundaries as the grains coalesce to form larger N-doped graphene sheets, which are further shown to possess superb electronic properties for future device applications. Initial kinetic processes involved in N-doped graphene growth using C5NH5 precursors are also investigated and contrasted with that of C5NCl5.

Substantially Enhancing Quantum Coherence of Electrons in Graphene via Electron-Plasmon Coupling.

The interplays between different quasiparticles in solids lay the foundation for a wide spectrum of intriguing quantum effects, yet how the collective plasmon excitations affect the quantum transport of electrons remains largely unexplored. Here we provide the first demonstration that when the electron-plasmon coupling is introduced, the quantum coherence of electrons in graphene is substantially enhanced with the quantum coherence length almost tripled. We further develop a microscopic model to interpret the striking observations, emphasizing the vital role of the graphene plasmons in suppressing electron-electron dephasing. The novel and transformative concept of plasmon-enhanced quantum coherence sheds new insight into interquasiparticle interactions, and further extends a new dimension to exploit nontrivial quantum phenomena and devices in solid systems.

Carbon Tetragons as Definitive Spin Switches in Narrow Zigzag Graphene Nanoribbons.

Precise spatial control of the spin propagation channels is of fundamental and practical importance in future graphene-based spintronic devices. Here we use first-principles calculations to show that when narrow zigzag graphene nanoribbons are connected to form junctions or superlattices, properly placed square-shaped carbon tetragons not only serve as effective bundles of the two incoming spin edge channels, but also act as definitive topological spin switches for the two outgoing channels. The nanoribbon segments are largely drawn from different acene molecules. We further show that such spin switches can lift the degeneracy between the two spin propagation channels, which enables tunability of different magnetic states upon charge doping. Preliminary experimental supports for the realization of such tetragons connecting nanoribbon segments are also presented.

Stabilization and manipulation of electronically phase-separated ground states in defective indium atom wires on silicon.

Exploration and manipulation of electronic states in low-dimensional systems are of great importance in the fundamental and practical aspects of nanomaterial and nanotechnology. Here, we demonstrate that the incorporation of vacancy defects into monatomic indium wires on n-type Si(111) can stabilize electronically phase-separated ground states where the insulating 8×2 and metallic 4×1 phases coexist. Furthermore, the areal ratio of the two phases in the phase-separated states can be tuned reversibly by electric field or charge doping, and such tunabilities can be quantitatively captured by first principles-based modeling and simulations. The present results extend the realm of electronic phase separation from strongly correlated d-electron materials typically in bulk form to weakly interacting sp-electron systems in reduced dimensionality.

Giant nonlinear Hall and wireless rectification effects at room temperature in the elemental semiconductor tellurium.

The second-order nonlinear Hall effect (NLHE) in non-centrosymmetric materials has recently drawn intense interest, since its inherent rectification could enable various device applications such as energy harvesting and wireless charging. However, previously reported NLHE systems normally suffer from relatively small Hall voltage outputs and/or low working temperatures. In this study, we report the observation of a pronounced NLHE in tellurium (Te) thin flakes at room temperature. Benefiting from the semiconductor nature of Te, the obtained nonlinear response can be readily enhanced through electrostatic gating, leading to a second-harmonic output at 300 K up to 2.8 mV. By utilizing such a giant NLHE, we further demonstrate the potential of Te as a wireless Hall rectifier within the radiofrequency range, which is manifested by the remarkable and tunable rectification effect also at room temperature. Extrinsic scattering is then revealed to be the dominant mechanism for the NLHE in Te, with symmetry breaking on the surface playing a key role. As a simple elemental semiconductor, Te provides an appealing platform to advance our understanding of nonlinear transport in solids and to develop NLHE-based electronic devices.

Ultrafast magnetization enhancement via the dynamic spin-filter effect of type-II Weyl nodes in a kagome ferromagnet.

The magnetic type-II Weyl semimetal (MWSM) Co3Sn2S2 has recently been found to host a variety of remarkable phenomena including surface Fermi-arcs, giant anomalous Hall effect, and negative flat band magnetism. However, the dynamic magnetic properties remain relatively unexplored. Here, we investigate the ultrafast spin dynamics of Co3Sn2S2 crystal using time-resolved magneto-optical Kerr effect and reflectivity spectroscopies. We observe a transient magnetization behavior, consisting of spin-flipping dominated fast demagnetization, slow demagnetization due to overall half-metallic electronic structures, and an unexpected ultrafast magnetization enhancement lasting hundreds of picoseconds upon femtosecond laser excitation. By combining temperature-, pump fluence-, and pump polarization-dependent measurements, we unambiguously demonstrate the correlation between the ultrafast magnetization enhancement and the Weyl nodes. Our theoretical modelling suggests that the excited electrons are spin-polarized when relaxing, leading to the enhanced spin-up density of states near the Fermi level and the consequently unusual magnetization enhancement. Our results reveal the unique role of the Weyl properties of Co3Sn2S2 in femtosecond laser-induced spin dynamics.

Signature of quantum interference effect in inter-layer Coulomb drag in graphene-based electronic double-layer systems.

The distinguishing feature of a quantum system is interference arising from the wave mechanical nature of particles which is clearly central to macroscopic electronic properties. Here, we report the signature of quantum interference effect in inter-layer transport process. Via systematic magneto-drag experiments on graphene-based electronic double-layer systems, we observe low-field correction to the Coulomb-scattering-dominated inter-layer drag resistance in a wide range of temperature and carrier density, with its characteristics sensitive to the band topology of graphene layers. These observations can be attributed to a new type of quantum interference between drag processes, with the interference pathway comprising different carrier diffusion paths in the two constituent conductors. The emergence of such effect relies on the formation of superimposing planar diffusion paths, among which the impurity potentials from intermediate insulating spacer play an essential role. Our findings establish an ideal platform where the interplay between quantum interference and many-body interaction is essential.