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A swarm of robots has advantages over a single robot, since it can explore larger areas much faster and is more robust to single-point failures. Accurate relative positioning is necessary to successfully carry out a collaborative mission without collisions. When Visual Simultaneous Localization and Mapping (VSLAM) is used to estimate the poses of each robot, inter-agent loop closing is widely applied to reduce the relative positioning errors. This technique can mitigate errors using the feature points commonly observed by different robots. However, it requires significant computing and communication capabilities to detect inter-agent loops, and to process the data transmitted by multiple agents. In this paper, we propose Collaborative SLAM using Visual Odometry and Range measurements (CoVOR-SLAM) to overcome this challenge. In the framework of CoVOR-SLAM, robots only need to exchange pose estimates, covariances (uncertainty) of the estimates, and range measurements between robots. Since CoVOR-SLAM does not require to associate visual features and map points observed by different agents, the computational and communication loads are significantly reduced. The required range measurements can be obtained using pilot signals of the communication system, without requiring complex additional infrastructure. We tested CoVOR-SLAM using real images as well as real ultra-wideband-based ranges obtained with two rovers. In addition, CoVOR-SLAM is evaluated with a larger scale multi-agent setup exploiting public image datasets and ranges generated using a realistic simulation. The results show that CoVOR-SLAM can accurately estimate the robots' poses, requiring much less computational power and communication capabilities than the inter-agent loop closing technique., Comment: Submitted to the IEEE Transactions on Intelligent Transportation Systems

Communication, Navigation and Surveillance (CNS) technologies are key enablers for future safe operation of drones in urban environments. However, the design of navigation technologies for these new applications is more challenging compared to e.g., civil aviation. On the one hand, the use cases and operations in urban environments are expected to have stringent requirements in terms of accuracy, integrity, continuity and availability. On the other hand, airborne sensors may not be based on high-quality equipment as in civil aviation and solutions need to rely on tighter multisensor solutions, whose safety is difficult to assess. In this work, we first provide some initial navigation requirements related to precision approach operations based on recently proposed vertiport designs. Then, we provide an overview of a possible multisensor navigation architecture solution able to support these types of operations and we comment on the challenges of each of the subsystems. Finally, initial proof of concept for some navigation sensor subsystems is presented based on flight trials performed during the German Aerospace Center (DLR) project HorizonUAM.

Combining the synthetic tunability of molecular compounds with the optical selection rules of transition metal dichalcogenides (TMDC) that derive from spin-valley coupling could provide interesting opportunities for the readout of quantum information. However, little is known about the electronic and spin interactions at such interfaces and the influence on spin-valley relaxation. In this work, vanadyl phthalocyanine (VOPc) molecular layers are thermally evaporated on WSe$_2$ to explore the effect of molecular layer thickness on excited-state spin-valley polarization. The thinnest molecular layer supports an interfacial state which destroys the spin-valley polarization almost instantaneously, whereas a thicker molecular layer results in longer-lived spin-valley polarization than the WSe$_2$ monolayer alone. The mechanism appears to involve a tightly-bound species at the molecule/TMDC interface that strengthens exchange interactions and is largely avoided in thicker VOPc layers that isolate electrons from WSe$_2$ holes.

The discovery of room-temperature single-photon emitters (SPEs) hosted by two-dimensional hexagonal boron nitride (2D hBN) has sparked intense research interest. Although emitters in the vicinity of 2 eV have been studied extensively, their microscopic identity has remained elusive. The discussion of this class of SPEs has centered on point defects in the hBN crystal lattice, but none of the candidate defect structures have been able to capture the great heterogeneity in emitter properties that is observed experimentally. Employing a widely used sample preparation protocol but disentangling several confounding factors, we demonstrate conclusively that heterogeneous single-photon emission ~2 eV associated with hBN originates from organic molecules, presumably aromatic fluorophores. The appearance of those SPEs depends critically on the presence of organic processing residues during sample preparation, and emitters formed during heat treatment are not located within the hBN crystal as previously thought, but at the hBN/substrate interface. We further demonstrate that the same class of SPEs can be observed in a different 2D insulator, fluorophlogopite mica., Comment: This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in ACS Nano, copyright \copyright\ 2023 American Chemical Society after peer review. To access the final edited and published work see https://pubs.acs.org/doi/10.1021/acsnano.3c02348

Although the magnetic order is suppressed by a strong magnetic frustration, it is maintained but appears in complex order forms such as a cycloid or spin density wave in weakly frustrated systems. Herein, we report a weakly magnetic-frustrated two-dimensional van der Waals material CrPSe$_3$. Polycrystalline CrPSe$_3$ was synthesized at an optimized temperature of 700$^\circ$C to avoid the formation of any secondary phases (e.g., Cr$_2$Se$_3$). The antiferromagnetic transition appeared at $T_N\sim 126$ K with a large Curie-Weiss temperature $T_{\rm CW} \sim -371$ via magnetic susceptibility measurements, indicating weak frustration in CrPSe$_3$ with a frustration factor $f (|T_{\rm CW}|/T_N) \sim 3$. Evidently, the formation of long-range incommensurate spin-density wave antiferromagnetic order with the propagation vector $k = (0, 0.04, 0)$ was revealed by neutron diffraction measurements at low temperatures (below 120K). The monoclinic crystal structure of C2/m symmetry is preserved over the studied temperature range down to 20K, as confirmed by Raman spectroscopy measurements. Our findings on the spin density wave antiferromagnetic order in two-dimensional (2D) magnetic materials, not previously observed in the MPX$_3$ family, are expected to enrich the physics of magnetism at the 2D limit, thereby opening opportunities for their practical applications in spintronics and quantum devices., Comment: 23 pages, 4 figures, 2 tables

Charge Density Waves (CDW) are commonly associated with the presence of near-Fermi level states which are separated from others, or "nested", by a wavector of $\mathbf{q}$. Here we use Angle-Resolved Photo Emission Spectroscopy (ARPES) on the CDW material Ta$_2$NiSe$_7$ and identify a total absence of any plausible nesting of states at the primary CDW wavevector $\mathbf{q}$. Nevertheless we observe spectral intensity on replicas of the hole-like valence bands, shifted by a wavevector of $\mathbf{q}$, which appears with the CDW transition. In contrast, we find that there is a possible nesting at $\mathbf{2q}$, and associate the characters of these bands with the reported atomic modulations at $\mathbf{2q}$. Our comprehensive electronic structure perspective shows that the CDW-like transition of Ta$_2$NiSe$_7$ is unique, with the primary wavevector $\mathbf{q}$ being unrelated to any low-energy states, but suggests that the reported modulation at $\mathbf{2q}$, which would plausibly connect low-energy states, might be more important for the overall energetics of the problem.

Purely quantum electron systems exhibit intriguing correlated electronic phases by virtue of quantum fluctuations in addition to electron-electron interactions. To realize such quantum electron systems, a key ingredient is dense electrons decoupled from other degrees of freedom. Here, we report the discovery of a pure quantum electron liquid, which spreads up to ~ 3 {\AA} in the vacuum on the surface of electride crystal. An extremely high electron density and its weak hybridisation with buried atomic orbitals evidence the quantum and pure nature of electrons, that exhibit a polarized liquid phase as demonstrated by our spin-dependent measurement. Further, upon enhancing the electron correlation strength, the dynamics of quantum electrons changes to that of non-Fermi liquid along with an anomalous band deformation, suggestive of a transition to a hexatic liquid crystal phase. Our findings cultivate the frontier of quantum electron systems, and serve as a platform for exploring correlated electronic phases in a pure fashion., Comment: 29 pages, 4 figures, 10 extended data figures

Temperature-linear resistance at low temperatures in strange metals is an exotic characteristic of strong correlation systems, as observed in high-TC superconducting cuprates, heavy fermions, Fe-based superconductors, ruthenates, and twisted bilayer graphene. Here, we introduce a hole-doped valley-spin insulator, V-doped WSe2, with hole pockets in the valence band. The strange metal characteristic was observed in VxW1-xSe2 at a critical carrier concentration of 9.5 x 10^20 cm-3 from 150 K to 1.8 K. The unsaturated magnetoresistance is almost linearly proportional to the magnetic field. Using the ansatz R(H,T) - R(0,0) ~ [(alpha.k.T)^2+(gamma.mu.B)^2]^1/2, the gamma/alpha ratio is estimated approximately to 4, distinct from that for the quasiparticles of LSCO, BaFe2(As1-xPx)2 (gamma/alpha=1) and bosons of YBCO (gamma/alpha=2). Our observation opens up the possible routes that induce strong correlation and superconductivity in two-dimensional materials with strong spin-orbit coupling., Comment: 8 pages, 4 figures + Supplemental Material

The electronic and magnetic properties of cobalt (Co) doped monolayer (ML) tungsten diselenide (WSe2) are investigated using the density functional theory with the on-site Hubbard potential correction (DFT+U) for the localized d orbitals of Co atom taking into account the spin orbit coupling (SOC) interaction. The results show that the substitution of Co at the W sites of ML WSe2 is energetically favorable under Se rich environment. We noticed that the Hund's exchange splitting (\Delta H_{ex}) is dominant over the crystal field splitting (\Delta_{cf}). The induced magnetic moment due to the Co-doped defect is ~3.00 \mu_B per Co atom. The magnetic interaction between two Co atoms at the nearest neighbor separation depends mainly on the concentration of the impurity atoms. The calculated value of curie temperature (TC) is increasing with increasing impurity concentration satisfying the Zener model. Based on the results, it can be proposed that the Co-doped WSe2 monolayer is potential candidate to apply in spintronics, optoelectronics, and magnetic storage devices.

The manipulation of magnetic properties using either electrical currents or gate bias is the key of future high-impact nanospintronics applications such as spin-valve read heads, non-volatile logic, and random-access memories. The current technology for magnetic switching with spin-transfer torque requires high current densities, whereas gate-tunable magnetic materials such as ferromagnetic semiconductors and multiferroic materials are still far from practical applications. Recently, magnetic switching induced by pure spin currents using the spin Hall and Rashba effects in heavy metals, called spin-orbit torque (SOT), has emerged as a candidate for designing next-generation magnetic memory with low current densities. The recent discovery of topological materials and two-dimensional (2D) van der Waals (vdW) materials provides opportunities to explore versatile 3D-2D and 2D-2D heterostructures with interesting characteristics. In this review, we introduce the emerging approaches to realizing SOT nanodevices including techniques to evaluate the SOT efficiency as well as the opportunities and challenges of using 2D topological materials and vdW materials in such applications.

Stochastic inhomogeneous oxidation is an inherent characteristic of copper (Cu), often hindering color tuning and bandgap engineering of oxides. Coherent control of the interface between metal and metal oxide remains unresolved. We demonstrate coherent propagation of an oxidation front in single-crystal Cu thin film to achieve a full-color spectrum for Cu by precisely controlling its oxide-layer thickness. Grain boundary-free and atomically flat films prepared by atomic-sputtering epitaxy allow tailoring of the oxide layer with an abrupt interface via heat treatment with a suppressed temperature gradient. Color tuning of nearly full-color RGB indices is realized by precise control of oxide-layer thickness; our samples covered ~50.4% of the sRGB color space. The color of copper/copper oxide is realized by the reconstruction of the quantitative yield color from oxide pigment (complex dielectric functions of Cu2O) and light-layer interference (reflectance spectra obtained from the Fresnel equations) to produce structural color. We further demonstrate laser-oxide lithography with micron-scale linewidth and depth through local phase transformation to oxides embedded in the metal, providing spacing necessary for semiconducting transport and optoelectronics functionality., Comment: 38 pages

Monolayers of transition metal dichalcogenides are semiconducting materials which offer many prospects in optoelectronics. A monolayer of molybdenum disulfide (MoS2) has a direct bandgap of 1.88 eV. Hence, when excited with optical photon energies below its bandgap, no photocarriers are generated and a monolayer of MoS2 is not of much use in either photovoltaics or photodetection. Here, we demonstrate that large size MoS2 monolayer sandwiched between two graphene layers makes this heterostructure optically active well below the band gap of MoS2. An ultrafast optical pump-THz probe experiment reveals in real-time, transfer of carriers between graphene and MoS2 monolayer upon photoexcitation with photon energies down to 0.5 eV. It also helps to unravel an unprecedented enhancement in the broadband transient THz response of this tri-layer material system. We propose possible mechanism which can account for this phenomenon. Such specially designed heterostructures, which can be easily built around different transition metal dichalcogenide monolayers, will considerably broaden the scope for modern optoelectronic applications at THz bandwidth., Comment: Manuscript with supplementary information

Although van der Waals (vdW) layered MoS2 shows the phase transformation from the semiconducting 2H-phase to the metallic 1T-phase through chemical lithium intercalation, vdW MoTe2 is thermodynamically reversible between the 2H- and 1T'-phases, and can be further transformed by energetics, laser irradiation, strain or pressure, and electrical doping. Here, thickness- and temperature-dependent optical properties of 1T'-MoTe2 thin films grown by chemical vapor depsition are investigated via Fourier-transformed infrared spectroscopy. An optical gap of 28 +/- 2 meV in a 3-layer (or 2 nm) thick 1T'-MoTe2 is clearly observed at a low temperature region below 50K. No discernible optical bandgap is observed in samples thicker than ~4 nm. The observed thickness-dependent bandgap results agree with the measured dc resistivity data; the thickness-dependent 1T'-MoTe2 clearly demonstrates the metal-semiconductor transition at a crossover below the 2 nm-thick sample., Comment: 18 pages, 4 figures

Room-temperature Fermi-Dirac electron thermal excitation in conventional three-dimensional (3D) or two-dimensional (2D) semiconductors generates hot electrons with a relatively long thermal tail in energy distribution. These hot electrons set a fundamental obstacle known as the "Boltzmann tyranny" that limits the subthreshold swing (SS) and therefore the minimum power consumption of 3D and 2D field-effect transistors (FETs). Here, we investigated a novel graphene (Gr)-enabled cold electron injection where the Gr acts as the Dirac source to provide the cold electrons with a localized electron density distribution and a short thermal tail at room temperature. These cold electrons correspond to an electronic cooling effect with the effective electron temperature of ~145 K in the monolayer MoS2, which enable the transport factor lowering and thus the steep-slope switching (across for 3 decades with a minimum SS of 29 mV/decade at room temperature) for a monolayer MoS2 FET. Especially, a record-high sub-60-mV/decade current density (over 1 {\mu}A/{\mu}m) can be achieved compared to conventional steep-slope technologies such as tunneling FETs or negative capacitance FETs using 2D or 3D channel materials. Our work demonstrates the great potential of 2D Dirac-source cold electron transistor as an innovative steep-slope transistor concept, and provides new opportunities for 2D materials toward future energy-efficient nanoelectronics., Comment: currently under review

THz conductivity of large area MoS2 and MoSe2 monolayers as well as their vertical heterostructure, MoSe2MoS2 is measured in the 0.3-5 THz frequency range. Compared to the monolayers, the ultrafast THz reflectivity of the MoSe2MoS2 heterobilayer is enhanced many folds when optically excited above the direct band gap energies of the constituting monolayers. The free carriers generated in the heterobilayer evolve with the characteristic times found in each of the two monolayers. Surprisingly, the same enhancement is recorded in the ultrafst THz reflectivity of the heterobilayer when excited below the MoS2 bandgap energy. A mechanism accounting for these observations is proposed., Comment: Contains Supplementary Information also

Although van der Waals layered transition metal dichalcogenides from transient absorption spectroscopy have successfully demonstrated an ideal carrier multiplication (CM) performance with an onset of nearly 2Eg,interpretation of the CM effect from the optical approach remains unresolved owing to the complexity of many-body electron-hole pairs. We demonstrate the CM effect through simple photocurrent measurements by fabricating the dual-gate P-N junction of a MoTe2 film on a transparent substrate. Electrons and holes were efficiently extracted by eliminating the Schottky barriers in the metal contact and minimizing multiple reflections. The photocurrent was elevated proportionately to the excitation energy. The boosted quantum efficiency confirms the multiple electron-hole pair generation of >2Eg, consistent with CM results from an optical approach, pushing the solar cell efficiency beyond the Shockley-Queisser limit.

Many-body effect and strong Coulomb interaction in monolayer transition metal dichalcogenides lead to shrink the intrinsic bandgap, originating from the renormalization of electrical/optical bandgap, exciton binding energy, and spin-orbit splitting. This renormalization phenomenon has been commonly observed at low temperature and requires high photon excitation density. Here, we present the augmented bandgap renormalization in monolayer MoS_2 anchored on CsPbBr_3 perovskite quantum dots at room temperature via charge transfer. The amount of electrons significantly transferred from perovskite gives rise to the large plasma screening in MoS_2. The bandgap in heterostructure is red-shifted by 84 meV with minimal pump fluence, the highest bandgap renormalization in monolayer MoS_2 at room temperature, which saturates with further increase of pump fluence. We further find that the magnitude of bandgap renormalization inversely relates to Thomas-Fermi screening length. This provides plenty of room to explore the bandgap renormalization within existing vast libraries of large bandgap van der Waals heterostructure towards practical devices such as solar cells, photodetectors and light-emitting-diodes.

Among transition metal dichalcogenides (TMdCs) as alternatives for Pt-based catalysts, metallic-TMdCs catalysts have highly reactive basal-plane but are unstable. Meanwhile, chemically stable semiconducting-TMdCs show limiting catalytic activity due to their inactive basal-plane. Here, we propose metallic vanadium sulfide (VSn) nanodispersed in a semiconducting MoS2 film (V-MoS2) as an efficient catalyst. During synthesis, vanadium atoms are substituted into hexagonal monolayer MoS2 to form randomly distributed VSn units. The V-MoS2 film on a Cu electrode exhibits Pt-scalable catalytic performance; current density of 1000 mA cm-2 at 0.6 V, overpotential of -0.06 V at a current density of 10 mA cm-2 and exchange current density of 0.65 mA cm-2 at 0 V with excellent cycle stability for hydrogen-evolution-reaction (HER). The high intrinsic HER performance of V-MoS2 is explained by the efficient electron transfer from the Cu electrode to chalcogen vacancies near vanadium sites with optimal Gibbs free energy (-0.02 eV). This study adds insight into ways to engineer TMdCs at the atomic-level to boost intrinsic catalytic activity for hydrogen evolution., Comment: 17 pages, 3 figures and a table

Growth of two-dimensional van der Waals layered single-crystal (SC) films is highly desired to manifest intrinsic material sciences and unprecedented devices for industrial applications. While wafer-scale SC hexagonal boron nitride film has been successfully grown, an ideal growth platform for diatomic transition metal dichalcogenide (TMdC) film has not been established to date. Here, we report the SC growth of TMdC monolayers in a centimeter scale via atomic sawtooth gold surface as a universal growth template. Atomic tooth-gullet surface is constructed by the one-step solidification of liquid gold, evidenced by transmission-electron-microscopy. Anisotropic adsorption energy of TMdC cluster, confirmed by density-functional calculations, prevails at the periodic atomic-step edge to yield unidirectional epitaxial growth of triangular TMdC grains, eventually forming the SC film, regardless of Miller indices. Growth using atomic sawtooth gold surface as a universal growth template is demonstrated for several TMdC monolayer films, including WS2, WSe2, MoS2, MoSe2/WSe2 heterostructure, and W1-xMoxS2 alloy. Our strategy provides a general avenue for the SC growth of diatomic van der Waals heterostructures in a wafer scale, to further facilitate the applications of TMdCs in post silicon technology., Comment: The supporting information is included in the manuscript file

Black TiO2 with/without noble metal has been proposed for visible photocatalyst, still leaving poor catalyst efficiency. Alternatively, phase-mixed TiO2 such as anatase and rutile has been commonly used for visible catalysts with the inevitable inclusion of noble metal. Here, we perform a noble metal-free visible photocatalyst blue TiO2 with type-II band-aligned ordered anatase/disordered rutile structure, via phase-selective reduction with alkali metals. The changed band alignment in this heterostructure was identified by absorption and ultraviolet photoemission spectroscopy, which was further confirmed by transient charge separation. The band alignment of type-I and type-II was clearly restructured by converting from ordered to disordered phase with a prolonged reduction period and as followed light absorbance enhancement also observed. Initiated type-I in a pristine sample, the type-II was organized from disordered rutile phase in 3-day Li-reduction. The type-II disordered rutile TiO2 heterostructure exhibits a remarkable photocatalytic performance by 55 times higher than conventional P25 TiO2 in solar-light driven hydrogen evolution reaction owing to an efficient electron and hole separation of type-II heterojunction. Furthermore, this restructured heterojunction type-II TiO2 demanded 10 times less Pt amount as a co-catalyst for the comparable photocatalytic performance, compared to Pt decorated type-I pristine anatase/rutile phase-mixed TiO2., Comment: 25 pages, 6 figures

We construct high-quality graphene-based van der Waals devices with narrow superconducting niobium nitride (NbN) electrodes, in which superconductivity and robust fqH coexist. We find crossed Andreev reflection (CAR) across the superconductor separating two fqH edges. Our observed CAR probabilities in the particle-like fractional fillings are markedly higher than those in the integer and hole-conjugate fractional fillings and depend strongly on temperature and magnetic field unlike the other fillings. Further, we find a filling-independent CAR probability in integer fillings, which we attribute to spin-orbit coupling in NbN allowing for Andreev reflection between spin-polarized edges. These results provide a route to realize novel topological superconducting phases in fqH-superconductor hybrid devices based on graphene and NbN., Comment: Revised text, additional main and appendix figures

An acoustic plasmonic mode in a graphene-dielectric-metal heterostructure has recently been spotlighted as a superior platform for strong light-matter interaction. It originates from the coupling of graphene plasmon with its mirror image and exhibits the largest field confinement in the limit of a nm-thick dielectric. Although recently detected in the far-field regime, optical near-fields of this mode are yet to be observed and characterized. Direct optical probing of the plasmonic fields reflected by the edges of graphene via near-field scattering microscope reveals a relatively small damping rate of the mid-IR acoustic plasmons in our devices, which allows for their real-space mapping even with unprotected, chemically grown, large-area graphene at ambient conditions. We show an acoustic mode that is twice as confined - yet 1.4 times less damped - compared to the graphene surface plasmon under similar conditions. We also image the resonant acoustic Bloch state in a 1D array of gold nanoribbons responsible for the high efficiency of the far-field coupling. Our results highlight the importance of acoustic plasmons as an exceptionally promising platform for large-area graphene-based optoelectronic devices operating in mid-IR.

Quantum interferometers are powerful tools for probing the wave-nature and exchange statistics of indistinguishable particles. Of particular interest are interferometers formed by the chiral, one-dimensional (1D) edge channels of the quantum Hall effect (QHE) that guide electrons without dissipation. Using quantum point contacts (QPCs) as beamsplitters, these 1D channels can be split and recombined, enabling interference of charged particles. Such quantum Hall interferometers (QHIs) can be used for studying exchange statistics of anyonic quasiparticles. In this study we develop a robust QHI fabrication technique in van der Waals (vdW) materials and realize a graphene-based Fabry-P\'erot (FP) QHI. By careful heterostructure design, we are able to measure pure Aharonov-Bohm (AB) interference effect in the integer QHE, a major technical challenge in finite size FP interferometers. We find that integer edge modes exhibit high visibility interference due to relatively large velocities and long phase coherence lengths. Our QHI with tunable QPCs presents a versatile platform for interferometer studies in vdW materials and enables future experiments in the fractional QHE.

Two-dimensional (2d) nano-electronics, plasmonics, and emergent phases require clean and local charge control, calling for layered, crystalline acceptors or donors. Our Raman, photovoltage, and electrical conductance measurements combined with \textit{ab initio} calculations establish the large work function and narrow bands of $\alpha$-RuCl$_3$ enable modulation doping of exfoliated, chemical vapor deposition (CVD), and molecular beam epitaxy (MBE) materials. Short-ranged lateral doping (${\leq}65\ \text{nm}$) and high homogeneity are achieved in proximate materials with a single layer of \arucl. This leads to the highest monolayer graphene (mlg) mobilities ($4,900\ \text{cm}^2/ \text{Vs}$) at these high hole densities ($3\times10^{13}\ \text{cm}^{-2}$); and yields larger charge transfer to bilayer graphene (blg) ($6\times10^{13}\ \text{cm}^{-2}$). We further demonstrate proof of principle optical sensing, control via twist angle, and charge transfer through hexagonal boron nitride (hBN).

One primary concern in diluted magnetic semiconductors (DMSs) is how to establish a long-range magnetic order with a low magnetic doping concentration to maintain the gate tunability of the host semiconductor, as well as to increase Curie temperature. Two-dimensional van der Waals semiconductors have been recently investigated to demonstrate the magnetic order in DMSs; however, a comprehensive understanding of the mechanism responsible for the gate-tunable long-range magnetic order in DMSs has not been achieved yet. Here, we introduce a monolayer tungsten diselenide (WSe2) semiconductor with V dopants to demonstrate the long-range magnetic order through itinerant spin-polarized holes. The V atoms are sparsely located in the host lattice by substituting W atoms, which is confirmed by scanning tunneling microscopy and high-resolution transmission electron microscopy. The V impurity states and the valence band edge states are overlapped, which is congruent with density functional theory calculations. The field-effect transistor characteristics reveal the itinerant holes within the hybridized band; this clearly resembles the Zener model. Our study gives an insight into the mechanism of the long-range magnetic order in V-doped WSe2, which can also be used for other magnetically doped semiconducting transition metal dichalcogenides., Comment: 19 pages, 4 figures

We demonstrate the gate-tunability of the long-range magnetic order in a p-type V-doped WSe2 monolayer using ab initio calculations. We found that at a low V-doping concentration limit, the long-range ferromagnetic order is enhanced by increasing the hole density. In contrast, the short-range antiferromagnetic order is manifested at a high electron density by full compensation of the p-type V doping concentration. The hole-mediated long-range magnetic exchange is ~70 meV, thus strongly suggesting the ferromagnetism in V-doped WSe2 at room temperature. Our findings on strong coupling between charge and spin order in V-doped WSe2 provide plenty of room for multifunctional gate-tunable spintronics., Comment: 13 pages, 4 figures, 1 table

We report the infrared transmission measurement on electrically gated twisted bilayer graphene. The optical absorption spectrum clearly manifests the dramatic changes such as the splitting of inter-linear-band absorption step, the shift of inter-van Hove singularity transition peak, and the emergence of very strong intra-valence (intra-conduction) band transition. These anomalous optical behaviors demonstrate consistently the non-rigid band structure modification created by the ion-gel gating through the layer-dependent Coulomb screening. We propose that this screening-driven band modification is an universal phenomenon that persists to other bilayer crystals in general, establishing the electrical gating as a versatile technique to engineer the band structures and to create new types of optical absorptions that can be exploited in electro-optical device application., Comment: 13 pages, 4 figures

Monocular vision-based Simultaneous Localization and Mapping (SLAM) is used for various purposes due to its advantages in cost, simple setup, as well as availability in the environments where navigation with satellites is not effective. However, camera motion and map points can be estimated only up to a global scale factor with monocular vision. Moreover, estimation error accumulates over time without bound, if the camera cannot detect the previously observed map points for closing a loop. We propose an innovative approach to estimate a global scale factor and reduce drifts in monocular vision-based localization with an additional single ranging link. Our method can be easily integrated with the back-end of monocular visual SLAM methods. We demonstrate our algorithm with real datasets collected on a rover, and show the evaluation results., Comment: Submitted to the International Conference on Robotics and Automation (ICRA) 2019

Diluted magnetic semiconductors including Mn-doped GaAs are attractive for gate-controlled spintronics but Curie transition at room temperature with long-range ferromagnetic order is still debatable to date. Here, we report the room-temperature ferromagnetic domains with long-range order in semiconducting V-doped WSe2 monolayer synthesized by chemical vapor deposition. Ferromagnetic order is manifested using magnetic force microscopy up to 360K, while retaining high on/off current ratio of ~105 at 0.1% V-doping concentration. The V-substitution to W sites keep a V-V separation distance of 5 nm without V-V aggregation, scrutinized by high-resolution scanning transmission-electron microscopy, which implies the possibility of the Ruderman-Kittel-Kasuya-Yoshida interaction (or Zener model) by establishing the long-range ferromagnetic order in V-doped WSe2 monolayer through free hole carriers. More importantly, the ferromagnetic order is clearly modulated by applying a back gate. Our findings open new opportunities for using two-dimensional transition metal dichalcogenides for future spintronics., Comment: 16 pages, 4 figures

Carrier multiplication (CM), a photo-physical process to generate multiple electron-hole pairs by exploiting excess energy of free carriers, is explored for efficient photovoltaic conversion of photons from the blue solar band, predominantly wasted as heat in standard solar cells. Current state-of-the-art approaches with nanomaterials have demonstrated improved CM but are not satisfactory due to high energy loss and inherent difficulties with carrier extraction. Here, we report ultra-efficient CM in van der Waals (vdW) layered materials that commences at the energy conservation limit and proceeds with nearly 100% conversion efficiency. A small threshold energy, as low as twice the bandgap, was achieved, marking an onset of quantum yield with enhanced carrier generation. Strong Coulomb interactions between electrons confined within vdW layers allow rapid electron-electron scattering to prevail over electron-phonon scattering. Additionally, the presence of electron pockets spread over momentum space could also contribute to the high CM efficiency. Combining with high conductivity and optimal bandgap, these superior CM characteristics identify vdW materials for third-generation solar cell., Comment: 17 pages, 4 figures

Excitons are electron-hole (e-h) pair quasiparticles, which may form a Bose-Einstein condensate (BEC) and collapse into the phase coherent state at low temperature. However, because of ephemeral strength of pairing, a clear evidence for BEC in electron-hole system has not yet been observed. Here, we report electron-hole pair condensation in graphene (Gr)/MoS2 heterointerface at 10K without magnetic field. As a direct indication of e-h pair condensation, we demonstrate a vanished Hall drag voltage and the resultant divergence of drag resistance. While strong excitons are formed at Gr/MoS2 heterointerface without insulating layer, carrier recombination via interlayer tunneling of carriers is suppressed by the vertical p-Gr/n-MoS2 junction barrier, consequently yielding high BEC temperature of 10K, ~1000 times higher than that of two-dimensional electron gas in III-V quantum wells. The observed excitonic transport is mainly governed by the interfacial properties of the Gr/MoS2 heterostructure, rather than the intrinsic properties of each layer. Our approach with available large-area monolayer graphene and MoS2 provides a high feasibility for quantum dissipationless electronics towards integration., Comment: 12 pages, 4 figures

Two-dimensional (2D) heterointerfaces often provide extraordinary carrier transport as exemplified by superconductivity or excitonic superfluidity. Recently, double-layer graphene separated by few-layered boron nitride demonstrated the Coulomb drag phenomenon: carriers in the active layer drag the carriers in the passive layer. Here, we propose a new switching device operating via Coulomb drag interaction at a graphene/MoS2 (GM) heterointerface. The ideal van der Waals distance allows strong coupling of the interlayer electron-hole pairs, whose recombination is prevented by the Schottky barrier formed due to charge transfer at the heterointerface. This device exhibits a high carrier mobility (up to ~3,700 cm^2V^-1s^-1) even at room temperature, while maintaining a high on/off current ratio (~10^8), outperforming those of individual layers. In the electron-electron drag regime, graphene-like Shubnikov-de Haas oscillations are observed at low temperatures. Our Coulomb drag transistor could provide a shortcut for the practical application of quantum-mechanical 2D heterostructures at room temperature., Comment: 14 pages, 4 figures