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Monolayer transition-metal dichalcogenide (TMD) semiconductors exhibit strong excitonic effects and hold promise for optical and optoelectronic applications. Yet, electron doping of TMDs leads to the conversion of neutral excitons into negative trions, which recombine predominantly non-radiatively at room temperature. As a result, the photoluminescence (PL) intensity is quenched. Here we study the optical and electronic properties of a MoS2/WSe2 heterostructure as a function of chemical doping by Cs atoms performed under ultra-high vacuum conditions. By PL measurements we identify two interlayer excitons and assign them to the momentum-indirect Q-Gamma and K-Gamma transitions. The energies of these excitons are in a very good agreement with ab initio calculations. We find that the Q-Gamma interlayer exciton is robust to the electron doping and is present at room temperature even at a high charge carrier concentration. Submicrometer angle-resolved photoemission spectroscopy (micro-ARPES) reveals charge transfer from deposited Cs adatoms to both the upper MoS2 and the lower WSe2 monolayer without changing the band alignment. This leads to a small (10 meV) energy shift of interlayer excitons. Robustness of the momentum-indirect interlayer exciton to charge doping opens up an opportunity of using TMD heterostructures in light-emitting devices that can work at room temperature at high densities of charge carriers.

The rise of quantum science and technologies motivates photonics research to seek new platforms with strong light-matter interactions to facilitate quantum behaviors at moderate light intensities. Topological polaritons (TPs) offer an ideal platform in this context, with unique properties stemming from resilient topological states of light strongly coupled with matter. Here we explore polaritonic metasurfaces based on 2D transition metal dichalcogenides (TMDs) as a promising platform for topological polaritonics. We show that the strong coupling between topological photonic modes of the metasurface and excitons in TMDs yields a topological polaritonic Z2 phase. We experimentally confirm the emergence of one-way spin-polarized edge TPs in metasurfaces integrating MoSe2 and WSe2. Combined with the valley polarization in TMD monolayers, the proposed system enables an approach to engage the photonic angular momentum and valley and spin of excitons, offering a promising platform for photonic/solid-state interfaces for valleytronics and spintronics., In this work light and matter have been coupled in a strong interaction between exciton resonances and topological photonic bands. The authors herein demonstrate a Z2 spin-Hall topological polaritonic phase enabling new coupling schemes in valleytronics and spintronics.

Emerging photo-induced excitonic processes in transition metal dichalcogenide (TMD) heterobilayers, e.g., interplay of intra- and inter-layer excitons and conversion of excitons to trions, allow new opportunities for ultrathin hybrid photonic devices. However, with the associated large degree of spatial heterogeneity, understanding and controlling their complex competing interactions in TMD heterobilayers at the nanoscale remains a challenge. Here, we present an all-round dynamic control of interlayer-excitons and -trions in a WSe2/Mo0.5W0.5Se2 heterobilayer using multifunctional tip-enhanced photoluminescence (TEPL) spectroscopy with < 20 nm spatial resolution. Specifically, we demonstrate the bandgap tunable interlayer excitons and the dynamic interconversion between interlayer-trions and -excitons, through the combinational tip-induced engineering of GPa-scale pressure and plasmonic hot electron injection, with simultaneous spectroscopic TEPL measurements. This unique nano-opto-electro-mechanical control approach provides new strategies for developing versatile nano-excitonic/trionic devices using TMD heterobilayers.

Complex oxides show a vast range of functional responses, unparalleled within the inorganic solids realm, making them promising materials for applications as varied as next-generation field-effect transistors, spintronic devices, electro-optic modulators, pyroelectric detectors, or oxygen reduction catalysts. Their stability in ambient conditions, chemical versatility, and large susceptibility to minute structural and electronic modifications make them ideal subjects of study to discover emergent phenomena and to generate novel functionalities for next-generation devices. Recent advances in the synthesis of single-crystal, freestanding complex oxide membranes provide an unprecedented opportunity to study these materials in a nearly-ideal system (e.g. free of mechanical/thermal interaction with substrates) as well as expanding the range of tools for tweaking their order parameters (i.e. (anti-)ferromagnetic, (anti-)ferroelectric, ferroelastic), and increasing the possibility of achieving novel heterointegration approaches (including interfacing dissimilar materials) by avoiding the chemical, structural, or thermal constraints in synthesis processes. Here, we review the recent developments in the fabrication and characterization of complex-oxide membranes and discuss their potential for unraveling novel physicochemical phenomena at the nanoscale and for further exploiting their functionalities in technologically relevant devices., D P acknowledges the support of the European Union's Horizon 2020 research and innovation programmes under Grant agreements No. 797123 (Marie Skłodowska-Curie FERROENERGY) and No. 964931 (TSAR), funding from 'la Caixa' Foundation fellowship (ID 100010434), the Spanish Ministry of Industry, Economy and Competitiveness (MINECO) through Grant No. PID2019-108573GB-C21, the CERCA programme (Generalitat de Catalunya) and the 'Severo Ochoa' programme for Centers of Excellence in R&D of MINECO (Grant No. SEV-2017-0706). E K acknowledges funding from BIST-FBA fellowship. D P and E K acknowledge support of BIST Ignite Grant (TeraFox). The work at Berkeley acknowledges the support of the Army Research Office under Grants W911NF-21-1-0118, W911NF-21-1-0126, and under the ETHOS MURI via cooperative agreement W911NF-21-2-0162, the Army Research Laboratory via the Collaborative for Hierarchical Agile and Responsive Materials (CHARM) under cooperative agreement W911-NF-19-2-0119, the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under Contract No. DE-AC02-05-CH11231 (Materials Project program KC23MP), the U.S. Department of Energy, Office of Science for support of microelectronics research under Contract No. DE-AC02-05-CH11231, the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC-0012375, and the National Science Foundation under Grants DMR-1708615 and DMR-2102895

Typically, topological superconductivity is reachable via proximity effect by a direct deposition of superconductor (S) on top of a topological insulator (TI) surface. Here we observed and analyzed the double critical current in the Josephson junctions based on the topological insulator in the fabricated planar Superconducting Quantum Interference Device. By measuring critical currents as a function of temperature and magnetic field, we show that the second critical current stems from the intrinsic superconductivity of the S/TI interface, which is supported by the modified Resistively Shunted Junction model and Transmission Electron Microscopy studies. This complex structure of the interface should be taken into account when technological process involves Ar-plasma cleaning.

Epitaxial films may be released from growth substrates and transferred to structurally and chemically incompatible substrates, but epitaxial films of transition metal perovskite oxides have not been transferred to electroactive substrates for voltage control of their myriad functional properties. Here we demonstrate good strain transmission at the incoherent interface between a strain-released film of epitaxially grown ferromagnetic La0.7Sr0.3MnO3 and an electroactive substrate of ferroelectric 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 in a different crystallographic orientation. Our strain-mediated magnetoelectric coupling compares well with respect to epitaxial heterostructures, where the epitaxy responsible for strong coupling can degrade film magnetization via strain and dislocations. Moreover, the electrical switching of magnetic anisotropy is repeatable and non-volatile. High-resolution magnetic vector maps reveal that micromagnetic behaviour is governed by electrically controlled strain and cracks in the film. Our demonstration should inspire others to control the physical/chemical properties in strain-released epitaxial oxide films by using electroactive substrates to impart strain via non-epitaxial interfaces., Key properties of transition metal perovskite oxides are degraded after epitaxial growth on ferroelectric substrates due to lattice-mismatch strain. Here, the authors use epitaxial lift-off and transfer to overcome this problem and demonstrate electric field control of a bulk-like magnetization.

When approaching the atomically thin limit, defects and disorder play an increasingly important role in the properties of two-dimensional (2D) materials. While defects are generally thought to negatively affect superconductivity in 2D materials, here we demonstrate the contrary in the case of oxygenation of ultrathin tantalum disulfide (TaS

Publisher's version (útgefin grein), Optical bound states in the continuum (BICs) provide a way to engineer very narrow resonances in photonic crystals. The extended interaction time in these systems is particularly promising for the enhancement of nonlinear optical processes and the development of the next generation of active optical devices. However, the achievable interaction strength is limited by the purely photonic character of optical BICs. Here, we mix the optical BIC in a photonic crystal slab with excitons in the atomically thin semiconductor MoSe2 to form nonlinear exciton-polaritons with a Rabi splitting of 27 meV, exhibiting large interaction-induced spectral blueshifts. The asymptotic BIC-like suppression of polariton radiation into the far field toward the BIC wavevector, in combination with effective reduction of the excitonic disorder through motional narrowing, results in small polariton linewidths below 3 meV. Together with a strongly wavevector-dependent Q-factor, this provides for the enhancement and control of polariton–polariton interactions and the resulting nonlinear optical effects, paving the way toward tuneable BIC-based polaritonic devices for sensing, lasing, and nonlinear optics., The authors acknowledge funding from the Ministry of Education and Science of the Russian Federation through Megagrant No. 14.Y26.31.0015. A.I.T. and D.N.K. acknowledge the UK EPSRC grant EP/P026850/1. I.A.S. acknowledges the project “Hybrid polaritonics” of Icelandic Science Foundation. Numerical calculations of the angle-resolved reflectivity maps were funded by RFBR according to the research project № 18-32-00527. Sample fabrication was funded by RFBR, project No 19-32-90269. Time-resolved measurements were partly funded by the Russian Science Foundation (Grant No. 19-72-30003). V.K. acknowledges support from the Government of the Russian Federation through the ITMO Fellowship and Professorship Program. This work was in part carried out using equipment of the SPbU Resource Centers “Nanophotonics” and “Nanotechnology”. We thank M. Zhukov, A. Bukatin, and A. Chezhegov for their assistance with the sample characterization and A. Bogdanov for the helpful discussion.

Optical bound states in the continuum (BICs) provide a way to engineer resonant response in planar photonic structures with ultimately high-quality factors. Here, we employ optical BIC in a photonic crystal slab (PCS) to realize the regime of strong coupling between light and excitons in a monolayer MoSe2 semiconductor. We experimentally demonstrate BIC-like behavior of the lower polariton branch, with strong suppression of radiation into far-field at Γ-point of the Brillouin zone. With the decrease of the in-plane Bloch wavevector, polariton lifetime rapidly increases, while the group velocity decays. These effects balance each other granting a nearly constant polariton propagation length of less than 10 μm over a broad range of k-vectors. This result suggests the possibilities for the development of ultracompact BIC-based planar polaritonic devices for sensing, lasing, and nonlinear optics.

The way transition metal dichalcogenide (TMD) strains during its transfer from one substrate to another is very interesting and holds a special place in the creation of heterostructures. In our work we observe the spectrum of photoluminescence in TMD during the transfer. For this we use a specially designed transfer system with inverted geometry. During transfer we observe a modification of exciton photoluminescence linewidth and resonance shift in atomically thin layers of TMD. We believe that our results lay grounds for the future work on the assessment of the atomically thin layer inhomogeneity introduced by the typical mechanical transfer.

We have investigated the structure of atomic defects within monolayer NbSe2 encapsulated in graphene by combining atomic resolution transmission electron microscope imaging, density functional theory (DFT) calculations, and strain mapping using geometric phase analysis. We demonstrate the presence of stable Nb and Se monovacancies in monolayer material and reveal that Se monovacancies are the most frequently observed defects, consistent with DFT calculations of their formation energy. We reveal that adventitious impurities of C, N, and O can substitute into the NbSe2 lattice stabilizing Se divacancies. We further observe evidence of Pt substitution into both Se and Nb vacancy sites. This knowledge of the character and relative frequency of different atomic defects provides the potential to better understand and control the unusual electronic and magnetic properties of this exciting two-dimensional material.

The possibility to tailor photoluminescence (PL) of monolayer transition metal dichalcogenides (TMDCs) using external factors such as strain, doping and external environment is of significant interest for optoelectronic applications. Strain in particular can be exploited as a means to continuously vary the bandgap. Micrometer-scale strain gradients were proposed for creating 'artificial atoms' that can utilize the so-called exciton funneling effect and work, for example, as exciton condensers. Here we describe room-temperature PL emitters that naturally occur whenever monolayer TMDC is deposited on an atomically flat substrate. These are hydrocarbon-filled bubbles which provide predictable, localized PL from well-separated submicron areas. Their emission energy is determined by the built-in strain controlled only by the substrate material, such that both the maximum strain and the strain profile are universal for all bubbles on a given substrate, i.e., independent of the bubble size. We show that for bubbles formed by monolayer MoS2, PL can be tuned between 1.72 to 1.81 eV by choosing bulk PtSe2, WS2, MoS2 or graphite as a substrate and its intensity is strongly enhanced by the funneling effect. Strong substrate-dependent quenching of the PL in areas of good contact between MoS2 and the substrate ensures localization of the luminescence to bubbles only; by employing optical reflectivity measurements we identify the mechanisms responsible for the quenching. Given the variety of available monolayer TMDCs and atomically flat substrates and the ease of creating such bubbles, our findings open a venue for making and studying the discussed light-emitting 'artificial atoms' that could be used in applications., 17 pages, 9 figures

Electrons that flow like a fluid Electrons inside a conductor are often described as flowing in response to an electric field. This flow rarely resembles anything like the familiar flow of water through a pipe, but three groups describe counterexamples (see the Perspective by Zaanen). Moll et al. found that the viscosity of the electron fluid in thin wires of PdCoO 2 had a major effect on the flow, much like what happens in regular fluids. Bandurin et al. found evidence in graphene of electron whirlpools similar to those formed by viscous fluid flowing through a small opening. Finally, Crossno et al. observed a huge increase of thermal transport in graphene, a signature of so-called Dirac fluids. Science , this issue p. 1061 , 1055 , 1058 ; see also p. 1026

Graphene on hexagonal boron-nitride (h-BN) is an atomically flat conducting system that is ideally suited for probing the effect of Zeeman splitting on electron transport. We demonstrate by magneto-transport measurements that a parallel magnetic field up to 30 Tesla does not affect the transport properties of graphene on h-BN even at charge neutrality where such an effect is expected to be maximal. The only magnetoresistance detected at low carrier concentrations is shown to be associated with a small perpendicular component of the field which cannot be fully eliminated in the experiment. Despite the high mobility of charge carries at low temperatures, we argue that the effects of Zeeman splitting are fully masked by electrostatic potential fluctuations at charge neutrality., Comment: v2: updated with some modifications/clarifications

It is well known that superconductivity in thin films is generally suppressed with decreasing thickness. This suppression is normally governed by either disorder-induced localization of Cooper pairs, weakening of Coulomb screening, or generation and unbinding of vortex-antivortex pairs as described by the Berezinskii-Kosterlitz-Thouless (BKT) theory. Defying general expectations, few-layer NbSe2 - an archetypal example of ultrathin superconductors - has been found to remain superconducting down to monolayer thickness. Here we report measurements of both the superconducting energy gap and critical temperature in high-quality monocrystals of few-layer NbSe2, using planar-junction tunneling spectroscopy and lateral transport. We observe a fully developed gap that rapidly reduces for devices with the number of layers N < 5, as does their ctitical temperature. We show that the observed reduction cannot be explained by disorder, and the BKT mechanism is also excluded by measuring its transition temperature that for all N remains very close to Tc. We attribute the observed behavior to changes in the electronic band structure predicted for mono- and bi- layer NbSe2 combined with inevitable suppression of the Cooper pair density at the superconductor-vacuum interface. Our experimental results for N > 2 are in good agreement with the dependences of the gap and Tc expected in the latter case while the effect of band-structure reconstruction is evidenced by a stronger suppression of the gap and the disappearance of its anisotropy for N = 2. The spatial scale involved in the surface suppression of the density of states is only a few angstroms but cannot be ignored for atomically thin superconductors., 21 pages, including supporting information

Many layered materials can be cleaved down to individual atomic planes, similar to graphene, but only a small minority of them are stable under ambient conditions. The rest reacts and decomposes in air, which has severely hindered their investigation and possible uses. Here we introduce a remedial approach based on cleavage, transfer, alignment and encapsulation of air-sensitive crystals, all inside a controlled inert atmosphere. To illustrate the technology, we choose two archetypal two-dimensional crystals unstable in air: black phosphorus and niobium diselenide. Our field-effect devices made from their monolayers are conductive and fully stable under ambient conditions, in contrast to the counterparts processed in air. NbSe2 remains superconducting down to the monolayer thickness. Starting with a trilayer, phosphorene devices reach sufficiently high mobilities to exhibit Landau quantization. The approach offers a venue to significantly expand the range of experimentally accessible two-dimensional crystals and their heterostructures., Comment: 4 figures

The constant-voltage electrical stress and 10-keV x-ray irradiation responses of encapsulated graphene-hBN devices are evaluated. Both constant-voltage stress and x-ray exposure induce only modest shifts in the current and the Dirac point of the graphene. Charge trapping at or near the graphene/BN interface is observed after x-ray irradiation. The experimental results suggest that net hole trapping occurs in the BN at low doses and that net electron trapping occurs at higher doses. First-principles calculations also demonstrate that hydrogenated substitutional carbon impurities at B/N sites at or near the graphene/BN interface can play an additional role in the radiation response of these structures.

We have investigated the structure of atomic defects within monolayer NbSe

Heat-loving quantum oscillations The shape of the Fermi surface in a conductor can be gleaned through quantum oscillations—periodic changes in transport properties as an external magnetic field is varied. Like most quantum properties, the phenomenon can usually be observed only at very low temperatures. Krishna Kumar et al. report quantum oscillations in graphene that do not go away even at the temperature of boiling water. Although “ordinary,” low-temperature quantum oscillations die away, another oscillatory behavior sets in that is extremely robust to heating. These resilient oscillations appear only in samples in which graphene is nearly aligned with its hexagonal boron nitride substrate, indicating that they are caused by the potential of the moiré superlattice that forms in such circumstances. Science , this issue p. 181

Khestanova, Ekaterina et al., The polarization of ferroelectric superlattices is determined by both electrical boundary conditions at the ferroelectric/paraelectric interfaces and lattice strain. The combined infl uence of both factors offers new opportunities to tune ferroelectricity. However, the experimental investigation of their individual impact has been elusive because of their complex interplay. Here, a simple growth strategy has permitted to disentangle both contributions by an independent control of strain in symmetric superlattices. It is found that fully strained short-period superlattices display a large polarization whereas a pronounced reduction is observed for longer multilayer periods. This observation indicates that the electrostatic boundary mainly governs the ferroelectric properties of the multilayers whereas the effects of strain are relatively minor., Financial support by the Spanish Government [Projects MAT2014- 56063-C2-1-R and MAT2013-41506 ] and Generalitat de Catalunya ( 2014-SGR-734 and 2014-SGR-672 ) is acknowledged. ICMAB-CSIC authors acknowledge fi nancial support from the Spanish Ministry of Economy and Competitiveness , through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV- 2015-0496 ). I.F. acknowledges Juan de la Cierva – Incorporación postdoctoral fellowship (IJCI-2014- 19102) from the Spanish Ministry of Economy and Competitiveness. The transmission electron microscopy works were conducted in the Laboratorio de Microscopias Avanzadas at Instituto de Nanociencia de Aragón (Universidad de Zaragoza). The authors acknowledge the LMA-INA for offering access to their instruments and expertise. The authors thank Massimiliano Stengel for useful discussions.

We report an experimental study of the influence of pulsed 1064-nm laser light polarisation on the dc photovoltage response of nanographite films. The amplitude of the pulsed potential difference generated in the films across the plane of incidence is an even function of the angle between the plane of polarisation and the plane of incidence of the laser beam, and that along the plane of incidence is an odd function of this angle. We present empirical relations between the incident pulse power, parameters of elliptically polarised laser light and photovoltage amplitude. The results are interpreted in terms of surface electric currents due to the quasi-momentum transfer from the incident light to the electron system via interband quantum transitions and the surface photogalvanic effect.

Superconducting layered transition metal dichalcogenides (TMDs) stand out among other superconductors due to the tunable nature of the superconducting transition, coexistence with other collective electronic excitations (charge density waves) and strong intrinsic spin-orbit coupling. Molybdenum disulphide (MoS2) is the most studied representative of this family of materials, especially since the recent demonstration of the possibility to tune its critical temperature, Tc, by electric-field doping. However, just one of its polymorphs, band-insulator 2H-MoS2, has so far been explored for its potential to host superconductivity. We have investigated the possibility to induce superconductivity in metallic polytypes, 1T- and 1T'-MoS2, by potassium (K) intercalation. We demonstrate that at doping levels significantly higher than that required to induce superconductivity in 2H-MoS2, both 1T and 1T' phases become superconducting, with Tc = 2.8 and 4.6K, respectively. Unusually, K intercalation in this case is responsible both for the structural and superconducting phase transitions. By adding new members to the family of superconducting TMDs our findings open the way to further manipulate and enhance the electronic properties of these technologically important materials., 13 pages, 5 figures plus 7 supplementary figures in Nano Letters, November 27, 2015

When approaching the atomically thin limit, defects and disorder play an increasingly important role in the properties of two-dimensional materials. Superconductivity is generally thought to be vulnerable to these effects, but here we demonstrate the contrary in the case of oxygenation of ultrathin tantalum disulfide (TaS$_2$). Our first-principles calculations show that incorporation of oxygen into the TaS$_2$ crystal lattice is energetically favourable and effectively heals sulfur vacancies typically present in these crystals, thus restoring the carrier density to the intrinsic value of TaS$_2$. Strikingly, this leads to a strong enhancement of the electron-phonon coupling, by up to 80% in the highly-oxygenated limit. Using transport measurements on fresh and aged (oxygenated) few-layer TaS$_2$, we found a marked increase of the superconducting critical temperature ($T_{\mathrm{c}}$) upon aging, in agreement with our theory, while concurrent electron microscopy and electron-energy loss spectroscopy confirmed the presence of sulfur vacancies in freshly prepared TaS$_2$ and incorporation of oxygen into the crystal lattice with time. Our work thus reveals the mechanism by which certain atomic-scale defects can be beneficial to superconductivity and opens a new route to engineer $T_{\mathrm{c}}$ in ultrathin materials.

Van der Waals heterobilayers based on 2D transition metal dichalcogenides have been recently shown to support robust and long-lived valley polarization for potential valleytronic applications. However, the roles of the chemical composition and geometric alignment of the constituent layers in the underlying dynamics remain largely unexplored. Here we study spin–valley relaxation dynamics in heterobilayers with different structures and optical properties engineered via the use of alloyed monolayer semiconductors. Through a combination of time-resolved Kerr rotation spectroscopic measurements and theoretical modeling for Mo1 − x W x Se2/WSe2 samples with different chemical compositions and stacking angles, we uncover the contributions of the interlayer exciton recombination and charge carrier spin depolarization to the overall valley dynamics. We show that the corresponding decay rates can be tuned in a wide range in transitions from a misaligned to an aligned structure, and from a hetero- to a homo-bilayer. Our results provide insights into the microscopic spin–valley polarization mechanisms in van der Waals heterostructures for the development of future 2D valleytronic devices.