James Hone

The ability to separate miniscule solid particles (e.g., nanoparticles) from liquid is important to a wide range of applications. However, directly moving nanoparticles out of liquid is difficult as the capillary force on the nanoparticle at the liquid interface is too large for common body forces to overcome. Here, we demonstrate the ability to eject metallic nanoparticles out of liquid with a laser excitation. The laser applies an optical force on the nanoparticles to drive them toward the liquid surface. In the meantime, it intensely heats the nanoparticle to form a nanobubble encapsulating the nanoparticle (i.e., supercavitation), which achieves the liquid-nanoparticle separation and thus eliminates the trapping force on the nanoparticle at the liquid/air interface. We show that such a mechanism can expel nanoparticles out of liquid using a transient scattering experiment, which is further confirmed by molecular dynamics simulations. We also demonstrate depositing the nanoparticles on a solid surface not in contact with the liquid. Leveraging such a feature, we show an example application where the deposited NPs are used as a template for 2D material nanotent fabrication. This study reveals an interesting fundamental mechanism to separate nanoparticles from liquid and could potentially benefit separation, nanomaterials, and biomedical applications.

One of the most remarkable properties associated with Bose–Einstein condensation (BEC) is superfluidity, in which the system exhibits zero viscosity and flows without dissipation. The superfluid phase has been observed in wide-ranging bosonic systems spanning naturally occurring quantum fluids, such as liquid helium, to engineered platforms such as bilayer excitons and cold atom systems1, 2, 3–4. Theoretical works have proposed that interactions could drive the BEC ground state into another exotic phase that simultaneously exhibits properties of both a crystalline solid and a superfluid—termed a supersolid5, 6, 7–8. Identifying a material system, however, that hosts the predicted BEC solid phase, driven purely by interactions and without imposing an external lattice potential, has remained unknown9, 10–11. Here we report observation of a superfluid-to-insulator transition in the layer-imbalanced regime of bilayer magnetoexcitons. Mapping the transport behaviour of the bilayer condensate as a function of density and temperature suggests that the insulating phase is an ordered state of dilute excitons, stabilized by dipole interactions. The insulator melts into a recovered superfluid on increasing the temperature, which could indicate that the low-temperature solid is also a quantum coherent phase. Transition of superfluid to insulator is observed in the layer-imbalanced regime of bilayer magnetoexcitons.

While scalable graphene synthesis by chemical vapor deposition (CVD) has been in widespread use since 2008, fundamental research requiring consistently high quality continues to rely upon exfoliated graphene. Here we use evaporated Ni to transfer ultrahigh-quality CVD graphene from Cu(111) as continuous films or arrays of predefined shapes. By careful control over the evaporation conditions, we avoid damage to the graphene, preserving intrinsic quality. The dry transfer process minimizes strain and doping. After hBN encapsulation, the CVD-grown graphene shows low-temperature magnetotransport behavior on par with the best exfoliated graphene devices. The CVD-grown graphene can be stacked to create magic-angle twisted bilayer graphene with low twist disorder. These results demonstrate that CVD-grown graphene can replace exfoliated flakes for even the most demanding applications.

The "gold-tape" exfoliation technique can deterministically exfoliate macroscopic transition metal dichalcogenide (TMD) monolayers from bulk single crystals, overcoming limitations of the widely used "Scotch-tape" method, but concerns over the quality of the large-area monolayers remain. Here we introduce a critical step improving the gold-tape method by eliminating a previously unknown polymer residue layer on the TMD surface. The resulting pristine and millimeter-scale TMD monolayers exhibit defect density, charge carrier mobility, and excitonic properties intrinsic to the parent crystal. Imaging, transport, and spectroscopy on length scales spanning 6 orders of magnitude demonstrate the viability of gold-exfoliated TMD monolayers for the deterministic fabrication of high-performance van der Waals devices, including moiré interfaces.

In very clean solid-state systems, where carrier-carrier interactions dominate over any other scattering mechanisms, the flow of electrons can be described within a hydrodynamic framework. In these cases, analogues of viscous fluid phenomena have been experimentally observed. However, experimental studies of electron hydrodynamics have so far been limited to the low velocity, linear response regime. At velocities approaching the speed of sound, the electronic fluid is expected to exhibit compressible behaviour where nonlinear effects and discontinuities such as shocks and choked flow have long been predicted. This compressible regime remains unexplored in electronic systems, despite its promise of strongly nonlinear flow phenomena. Here, we demonstrate compressible electron flow in bilayer graphene through an electronic de Laval nozzle, a structure that accelerates charge carriers past the electronic speed of sound, until they slow down suddenly in a shock. Discontinuities in transport measurements and local flattening of potential in Kelvin probe measurements are consistent with a viscous electron shock front and the presence of supersonic electron flow, and are not consistent with Ohmic or ballistic flow. Breaking the sound barrier in electron liquids opens the door for novel, intrinsically nonlinear electronic devices beyond the paradigm of incompressible flow.

Advancing both the fundamental understanding and technological application of two-dimensional semiconducting transition metal dichalcogenides (TMDs) hinges on precise control and identification of atomic-scale defects. Although self-flux growth yields exceptionally pure TMD crystals, the nature of residual defects has remained an open question. Here, we use scanning tunneling microscopy (STM) to directly image and identify point defects in both monolayer and bulk self-flux grown WSe 2 . We find that the dominant defects reside on chalcogen sites and are unaffected by exfoliation or oxygen exposure. Combining STM observations with first-principles simulations and bulk impurity analysis, we attribute these defects to substitutional oxygen (O Se ). This finding goes against the prevailing wisdom that vacancies are the most common defects in exfoliated TMDs. By establishing substitutional oxygen as the dominant defect, our work provides a crucial reference point for interpreting structure–property relationships and informs ongoing efforts to further improve material quality and device performance.

Using terahertz near-field nanoscopy, we demonstrate that temporal shaping of plasmon polaritons in graphene effectively suppresses their spatial decay. Our experiments and simulations reveal a universal principle for spacetime wave engineering, broadly applicable in photonic, acoustic, plasmonic, and quantum systems.

In addition to a plethora of emergent phenomena, the spatial topology of optical vortices enables an array of applications in optical communications and quantum information science. Multibeam nonlinear optical processes, augmented by optical vortices, are essential in this context, providing robust access to an infinitely large set of quantum states associated with the orbital angular momentum of light. Here, we push the boundaries of vortex nonlinear optics to the ultimate limits of material dimensionality. By exploiting multipulse difference frequency, sum frequency, and four-wave mixing in monolayer quantum materials, we demonstrate their ability to independently control the orbital angular momentum and radial distribution of vortex light-fields in addition to their wavelength. Due to the atomically thin nature of the host crystal, this control spans a broad spectral bandwidth in a highly integrable platform that is unconstrained by the traditional limits of bulk nonlinear optical materials. Our work heralds an innovative path for ultracompact and scalable hybrid nanophotonic technologies empowered by twisted nonlinear light-matter interactions in van der Waals nanomaterials.

High-quality, single-crystal van der Waals (vdW) materials provide a promising platform for constructing Josephson junctions, but systematic studies have been limited by fabrication and measurement challenges. Here researchers characterize 24 vertical vdW superconductor-semiconductor junctions, including microwave spectroscopy of an all-vdW transmon qubit. Transport measurements reveal a crossover from proximity- to tunneling-type behavior with increasing semiconductor thickness. The results demonstrate how band alignment and materials engineering can be used to tailor qubit properties, establishing vdW heterostructures as an emerging platform for next-generation superconducting qubits.

Collective modes in multilayer graphene, such as plasmons and phonons, exhibit sensitivity to displacement fields and interlayer coupling, distinguishing them from their counterparts in single-layer graphene. Here, we engineer collective modes in charge-transfer heterostructures composed of multilayer graphene and <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>α</a:mi> <a:mtext>−</a:mtext> <a:msub> <a:mi>RuCl</a:mi> <a:mn>3</a:mn> </a:msub> </a:math> . In heterostructures with a single <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mrow> <c:mi>α</c:mi> <c:mtext>−</c:mtext> <c:msub> <c:mrow> <c:mi>RuCl</c:mi> </c:mrow> <c:mrow> <c:mn>3</c:mn> </c:mrow> </c:msub> </c:mrow> </c:math> interface, the charge transfer generates displacement fields up to 7 V/nm at the interface between <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mi>α</e:mi> <e:mtext>−</e:mtext> <e:msub> <e:mi>RuCl</e:mi> <e:mn>3</e:mn> </e:msub> </e:math> and the adjacent graphene layer—the highest value achieved through charge-transfer methods. As a result of the broken inversion symmetry, we discover enhanced nonlinear optical response and modified phonon selection rules. Conversely, we find that multilayer graphene sandwiched between two <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mi>α</g:mi> <g:mtext>−</g:mtext> <g:msub> <g:mi>RuCl</g:mi> <g:mn>3</g:mn> </g:msub> </g:math> flakes causes displacement fields to cancel. There, we achieve carrier densities as high as <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mn>8</i:mn> <i:mo>×</i:mo> <i:msup> <i:mn>10</i:mn> <i:mn>13</i:mn> </i:msup> <i:mtext> </i:mtext> <i:mtext> </i:mtext> <i:mrow> <i:msup> <i:mrow> <i:mi>cm</i:mi> </i:mrow> <i:mrow> <i:mo>−</i:mo> <i:mn>2</i:mn> </i:mrow> </i:msup> </i:mrow> </i:math> in multilayer graphene and restore the phonon selection rules to their unperturbed state. Meanwhile, we demonstrate that plasmonic properties derive from the depletion of multiple valence bands. As a result of the quasilinear band dispersion, these “Dirac multiband plasmons” are relatively unaffected by displacement fields. On the other hand, the inverted heterostructure sequence—two multilayer graphene sheets encapsulating <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"> <k:mi>α</k:mi> <k:mtext>−</k:mtext> <k:msub> <k:mi>RuCl</k:mi> <k:mn>3</k:mn> </k:msub> </k:math> —activates significant alteration of the plasmons via interlayer plasmon-plasmon coupling. Hence, multilayer graphene and <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"> <m:mi>α</m:mi> <m:mtext>−</m:mtext> <m:msub> <m:mi>RuCl</m:mi> <m:mn>3</m:mn> </m:msub> </m:math> heterostructures offer a gate-free platform for engineering collective modes derived from inversion symmetry and interlayer coupling.

2D materials offer a unique opportunity to achieve new device functionality and realize novel quantum states. Synthesizing high-purity starting materials is key to achieving these goals. Our synthesis efforts focus on (1) growing ultra-pure crystals of transition metal dichalcogenides (TMDs), and identifying the different types of point defects within these crystals; and (2) new strategies for graphene CVD synthesis that dramatically improve speed, reproducibility, and quality. These improvements in quality lead to dramatically improved performance and open the door to applications in quantum devices. In particular, we are exploring the use of 2D materials for compact qubit architectures in which a single heterostructure can act as a capacitor and Josephson junction.

Surface acoustic waves (SAWs) have recently emerged as a powerful tool for controlling excitonic states in two-dimensional (2D) transition metal dichalcogenides, enabling dynamic energy modulation and transport of carriers over micron-scale distances. Yet, the use of SAWs to realize direct, high-speed manipulation of site-controlled quantum emitters (QEs) in 2D materials remains largely unexplored. Here, we show acoustic manipulation of deterministic, strain-induced QEs in monolayer WSe 2 by interfacing them with SAWs on a lithium niobate substrate. Using gold nanocube stressors to precisely engineer local strain, we overcome the stochastic nature of defect-based QEs. Upon SAW excitation, these deterministic QEs exhibit an energy shift of up to 0.58 meV at tuning bandwidth up to 8 MHz, enabling half-GHz optical modulation speed despite residing 210 nm above the SAW-carrying substrate. Power-dependent experiments reveal a nonlinear response regime of the SAW-driven nanocube stressor for RF excitation near the energetically lowest-lying flexural mode, highlighting the acousto-optical transduction in our device. Our findings provide a robust, scalable approach for fast, dynamic tuning of single photons with strongly suppressed spectral diffusion and may offer opportunities for nanoscale quantum sensors capable of mapping out acoustic fields with subwavelength spatial resolution on a chip.

Single-photon emitters (SPEs) in two-dimensional WSe 2 offer high extraction efficiency and on-chip compatibility, but achieving high purity remains challenging. We present two strategies to suppress classical emission and enhance purity in WSe 2 -based SPEs. In monolayer WSe 2, we exploited the presence and absence of valley–spin locking in free and bound excitons, respectively, to achieve purity of 98.3% via polarization control and 99.0% combined with near-resonant excitation. In bilayer WSe 2, we obtained 97.0% purity without polarization filtering, enabled by the indirect band gap and inversion symmetry. These values represent some of the highest as-measured purities reported for 2D TMD SPEs. Our methods do not require complex fabrication or instrumentation and are supported by first-principles calculations of the vacancy state of Se and spin degeneracy. This work offers practical pathways for realizing high-quality single-photon sources for emerging quantum technologies.

It is undeniable that novel 2D devices and heterostructures will have a lasting impact on the advancement of future technologies. However, the inherent instability of many exfoliated van der Waals (vdW) materials is a well-known hurdle yet to be overcome. Thus, the sustained interest in exfoliated vdW materials underscores the importance of understanding the mechanisms of sample degradation to establish proactive protective measures. Here, the impact of prolonged synchrotron-based X-ray beam exposure on exfoliated flakes of two contemporary vdW materials, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m1"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">N</mml:mi><mml:mi mathvariant="normal">i</mml:mi><mml:mi mathvariant="normal">P</mml:mi><mml:mi mathvariant="normal">S</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m2"><mml:mrow><mml:mi>α</mml:mi></mml:mrow></mml:math> - <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m3"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">R</mml:mi><mml:mi mathvariant="normal">u</mml:mi><mml:mi mathvariant="normal">C</mml:mi><mml:mi mathvariant="normal">l</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math> , is explored using resonant inelastic X-ray scattering (RIXS) and total fluorescence yield X-ray absorption spectroscopy (XAS). In <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m4"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">N</mml:mi><mml:mi mathvariant="normal">i</mml:mi><mml:mi mathvariant="normal">P</mml:mi><mml:mi mathvariant="normal">S</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math> , the resulting RIXS and XAS spectra show a suppression, then vanishing, of NiS 6 multiplet excitations coupled with an upward shift of the peak energy of the XAS as a function of X-ray dose. In <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m5"><mml:mrow><mml:mi>α</mml:mi></mml:mrow></mml:math> - <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m6"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">R</mml:mi><mml:mi mathvariant="normal">u</mml:mi><mml:mi mathvariant="normal">C</mml:mi><mml:mi mathvariant="normal">l</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math> , the signs of beam damage from the RIXS spectra are less evident. However, the post-experiment characterization of both materials using Raman spectroscopy exhibits signals of an amorphous and disordered system compared to pristine flakes; in addition, energy-dispersive X-ray spectroscopy of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="m7"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">N</mml:mi><mml:mi mathvariant="normal">i</mml:mi><mml:mi mathvariant="normal">P</mml:mi><mml:mi mathvariant="normal">S</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math> shows evidence of ligand vacancies. As synchrotron radiation is fast becoming a required probe to study 2D vdW materials, these findings lay the groundwork for the development of future protective measures for synchrotron-based prolonged X-ray beam exposure, as well as for X-ray free electron laser.

Single-layer WSe 2 hosts localized states that are technologically promising solid-state quantum light sources. Their formation requires localized tensile strain and is robust to substrate, material quality, and nanoscale stressors. How these factors affect the quantum emitters is obscured by intrasample heterogeneity that hinders the comparison of individual emitters across different samples. Here, by statistically comparing emitter populations from samples with unique permutations of material quality, substrate composition, and nanoscale stressors, we show that sample architecture affects emitter dynamics at the ensemble level. Systematic differences are revealed by formulating simple numerical descriptors of emitter photoluminescence transients from the different populations. Whereas the substrate does not significantly alter emitter dynamics, both nonlinear relaxation and exciton diffusion to emitter sites are reduced in high-quality WSe 2 on gold nanocones. This study demonstrates how statistical comparisons can reveal important structure–property relationships of the emitters that are important for further developing them into quantum light sources.

Material systems that exhibit tunable spin-selective conductivity are key components of spintronic technologies. Here, we demonstrate a mechanism for spin-selective transport that is based on the unusual Landau-level sequence observed in bilayer WSe2 under large applied magnetic fields. We find that the conductivity depends strongly on the relative ordering between conducting electrons with different spins and valleys in a partially filled Landau level and the localized electrons of lower-energy filled Landau levels. We observe that the conductivity is almost completely suppressed when the spin ratio and field-tuned Coulomb energy exceed a critical threshold. We achieve switching between on and off states through either modulation of the external magnetic or electric fields, with many-body interactions driving a collective switching mechanism. In contrast to magnetoresistive heterostructures, this mechanism achieves electrically tunable spin filtering within a single material, driven by the interaction between free and localized spins residing in energy-separated spin-and-valley-polarized bands. Similar spin-selective conductivity may be realizable in flat-band systems at zero magnetic field. Mechanisms for generating spin-polarized currents may be helpful for applications. Now one such mechanism that uses the unusual Landau-level spectrum of WSe2 under a strong magnetic field is demonstrated.

Atomically layered van der Waals (vdW) materials exhibit remarkable properties, including highly confined infrared waveguide modes and the capacity for infrared emission in the monolayer limit. Here we engineered structures that leverage both of these nano-optical functionalities. Specifically, we encased a photoluminescing atomic sheet of MoTe2 within two bulk crystals of WSe2, forming a vdW waveguide for the embedded light-emitting monolayer. The modified electromagnetic environment offered by the WSe2 waveguide alters MoTe2 spontaneous emission—a phenomenon we directly image with our interferometric nano-photoluminescence technique. We captured spatially oscillating nanoscale patterns prompted by spontaneous emission from MoTe2 into waveguide modes of WSe2 slabs. We quantify the resulting Purcell-enhanced emission rate within the framework of a waveguide quantum electrodynamics model, relating the MoTe2 spontaneous emission rate to the measured waveguide dispersion. Our work marks a substantial advance in the implementation of all-vdW quantum electrodynamics waveguides. A nano-optical probe of the Purcell effect in a van der Waals waveguide is demonstrated, exploiting its highly confined infrared waveguide modes and the capacity for infrared emission in the monolayer limit of atomically layered van der Waals materials.

Materials capable of light upconversion—transforming low-energy photons into higher-energy ones—are pivotal in advancing optoelectronics, energy solutions, and photocatalysis. However, the discovery in various materials pays little attention on few-layer transition metal dichalcogenides, primarily due to their indirect bandgaps and weaker light-matter interactions. Here, we report a pronounced light upconversion in few-layer transition metal dichalcogenides through upconversion photoluminescence spectroscopy. Our joint theory-experiment study attributes the upconversion photoluminescence to a resonant exciton-exciton annihilation involving a pair of dark excitons with opposite momenta, followed by the spontaneous emission of upconverted bright excitons, which can have a high upconversion efficiency. Additionally, the upconversion photoluminescence is generic in MoS2, MoSe2, WS2, and WSe2, showing a high tuneability from green to ultraviolet light (2.34–3.1 eV). The findings pave the way for further exploration of light upconversion regarding fundamental properties and device applications in two-dimensional semiconductors. The authors report upconversion in few-layer transition metal dichalcogenides, and attribute it to a resonant exciton-exciton annihilation involving a pair of dark excitons with opposite momenta, followed by the spontaneous emission of upconverted bright excitons.

Nano-photoluminescence (nano-PL) measurements have proven in the past decade to be an incredibly powerful and direct way of probing the excited state physics beyond the diffraction limit. A prevalent issue with nano-PL measurements is background rejection if the Purcell enhancement from the plasmonic tip does not outright outcompete the far-field PL background. To this end, we have implemented a combination of tapping mode AFM and Fourier analysis to improve the far-field rejection of nano-PL measurements. By conducting tapping mode nano-PL measurements on various Transition Metal Dichalcogenide (TMD) samples, we demonstrate that we can image features with better contrast than with the conventional nano-PL mode of operation.

Spontaneous symmetry breaking, arising from the competition of interactions and quantum fluctuations, is fundamental to understanding ordered electronic phases. Although electrically neutral, optical excitations like excitons can interact through their dipole moment, raising the possibility of optically active ordered phases. The effects of spontaneous ordering on optical properties remain largely unexplored. Recent observations of the excitonic Mott insulating state in semiconducting moiré crystals make them promising for addressing this question. Here, we present evidence for an in-plane ferroelectric phase of dipolar moiré excitons driven by strong exciton-exciton interactions. We discover a surprising speed-up of photon emission at late times and low densities in excitonic decay. This counterintuitive behavior is attributed to collective radiance, linked to the transition between disordered and symmetry-broken ferroelectric phases of moiré excitons. Our findings provide first evidence for strong dipolar inter-site interactions in moiré lattices, demonstrate collective photon emission as a probe for moiré quantum materials, and pave the way for exploring cooperative optical phenomena in strongly correlated systems.

Interlayer excitons (IXs) in layered van der Waals materials are promising for quantum technologies and fundamental studies such as exciton-polariton condensation due to their large permanent dipole moments. However, their indirect bandgap optical transition through the Q-K channel renders them momentum forbidden and thus less relevant for optical applications. Here, we demonstrate a method for brightening momentum indirect Q-K transitions from IX quantum emitters (QEs) in 2H-stacked bilayer WSe 2 by simultaneously employing local strain and plasmonic nanocavity coupling. Initially, long T 1 lifetimes up to 140 ns are indicative of momentum indirect transitions. Magneto-photoluminescence data show a striking bimodal distribution of g -factors between mono- and bilayer QEs, with a well-defined value of g = 9.5 for IX, highlighting their momentum indirect nature and decoupling from local strain variations. In addition, angle-resolved PL measurements reveal that local curvature on the nanostressor induces a dipole orientation tilt of the QEs, affecting cavity coupling. By embedding these strained QEs into plasmonic cavities, we achieve a 10-fold increase in emission intensity and a 24-fold enhancement in the T 1 lifetime in the best case (12-fold average), leading to bright single-photon emission rates up to 1.45 ± 0.1 MHz into the first lens. Moreover, the demonstrated brightening of IX transitions allowed to push the emission wavelength reliably to around 810 nm that enables free-space quantum optical communication.

Forming long wavelength moiré superlattices (MSL) at small-angle twist van der Waals (vdW) bilayers has been a key approach to creating moiré flat bands. The small-angle twist, however, leads to strong lattice reconstruction, causing domain walls and moiré disorders, which pose considerable challenges in engineering such platforms. At large twist angles, the rigid lattices render a more robust, but shorter wavelength MSL, making it difficult to engineer flat bands. Here, we depict a novel approach to tailoring robust super-moiré (SM) structures that combines the advantages of both small-twist and large-twist transition metal dichalcogenides (TMDs) bilayers using only a single twist angle near a commensurate angle. Structurally, we unveil the spontaneous formation of a periodic arrangement of three inequivalent commensurate moiré (CM) stacking, where the angle deviation from the commensurate angle can tune the periodicity. Electronically, we reveal a large set of van Hove singularities (VHSs) that indicate strong band hybridization, leading to flat bands near the valence band maximum. Our study paves the way for a new platform of robust SM bilayers with structural rigidity and controllable wavelength, extending the investigation of the interplay among band topology, quantum geometry, and moiré superconductivity to the large twist angle regime.

We demonstrate a technique for the fabrication of p-type transistors based on monolayer molybdenum telluride (MoTe 2 ). In the device structure, monolayer hexagonal boron nitride ( h BN) protects the channel from oxidation and acts as a tunnel barrier for Pd contacts. P-doping is achieved through charge transfer from oxidized WSe 2 . The contacts show low resistance and Ohmic behavior down to cryogenic temperatures. The resulting FET mobility is the highest reported to date for monolayer MoTe 2 . This architecture can be utilized for electronic and optoelectronic applications of MoTe 2 and will be useful for electrical transport studies of exotic quantum phenomena in MoTe 2 twisted bilayers.

The discovery of superconductivity in twisted bilayer and trilayer graphene1, 2, 3, 4–5 has generated tremendous interest. The key feature of these systems is an interplay between interlayer coupling and a moiré superlattice that gives rise to low-energy flat bands with strong correlations6. Flat bands can also be induced by moiré patterns in lattice-mismatched and/or twisted heterostructures of other two-dimensional materials, such as transition metal dichalcogenides (TMDs)7,8. Although a wide range of correlated phenomena have indeed been observed in moiré TMDs9, 10, 11, 12, 13, 14, 15, 16, 17, 18–19, robust demonstration of superconductivity has remained absent9. Here we report superconductivity in 5.0° twisted bilayer WSe2 with a maximum critical temperature of 426 mK. The superconducting state appears in a limited region of displacement field and density that is adjacent to a metallic state with a Fermi surface reconstruction believed to arise from AFM order20. A sharp boundary is observed between the superconducting and magnetic phases at low temperature, reminiscent of spin fluctuation-mediated superconductivity21. Our results establish that moiré flat-band superconductivity extends beyond graphene structures. Material properties that are absent in graphene but intrinsic among TMDs, such as a native band gap, large spin–orbit coupling, spin-valley locking and magnetism, offer the possibility of accessing a broader superconducting parameter space than graphene-only structures. We report superconductivity, in a limited region of displacement field and density, in 5.0° twisted bilayer WSe2 with a maximum critical temperature of 426 mK, establishing that moiré flat-band superconductivity extends beyond graphene structures.

Van der Waals electrode integration is a promising strategy to create nearly perfect interfaces between metals and 2D materials, with advantages such as eliminating Fermi-level pinning and reducing contact resistance. However, the lack of a simple, generalizable pick-and-place transfer technology has greatly hampered the wide use of this technique. We demonstrate the pick-and-place transfer of prefabricated electrodes from reusable polished hydrogenated diamond substrates without the use of any sacrificial layers due to the inherent low-energy and dangling-bond-free nature of the hydrogenated diamond surface. The technique enables transfer of arbitrary-metal electrodes and an electrode array, as demonstrated by successful transfer of eight different elemental metals with work functions ranging from 4.22 to 5.65 eV. We also demonstrate the electrode array transfer for large-scale device fabrication. The mechanical transfer of metal electrodes from diamond to van der Waals materials creates atomically smooth interfaces with no interstitial impurities or disorder, as observed with cross-section high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy. As a demonstration of its device application, we use the diamond transfer technique to create metal contacts to monolayer transition metal dichalcogenide semiconductors with high-work-function Pd, low-work-function Ti, and semimetal Bi to create n - and p -type field-effect transistors with low Schottky barrier heights. We also extend this technology to air-sensitive materials (trilayer 1T’ WTe 2 ) and other applications such as ambipolar transistors, Schottky diodes, and optoelectronics. This highly reliable and reproducible technology paves the way for new device architectures and high-performance devices.

Among expanding discoveries of quantum phases in moiré superlattices, correlated insulators stand out as both the most stable and most commonly observed. Despite the central importance of these states in moiré physics, little is known about their underlying nature. Here, we use pump-probe spectroscopy to show distinct time-domain signatures of correlated insulators at fillings of one (ν = −1) and two (ν = −2) holes per moiré unit cell in the angle-aligned WSe2/WS2 system. Following photo-doping, we find that the disordering time of the ν = −1 state is independent of excitation density (nex), as expected from the characteristic phonon response time associated with a polaronic state. In contrast, the disordering time of the ν = −2 state scales with $$1/\sqrt{{{{\boldsymbol{n}}}}_{{\mbox{ex}}}}$$ , in agreement with plasmonic screening from free holons and doublons. These states display disparate reordering behavior dominated either by first order (ν = −1) or second order (ν = −2) recombination, suggesting the presence of Hubbard excitons and free carrier-like holons/doublons, respectively. Our work delineates the roles of electron–phonon (e–ph) versus electron–electron (e–e) interactions in correlated insulators on the moiré landscape and establishes non-equilibrium responses as mechanistic signatures for distinguishing and discovering quantum phases. The nature of correlated insulator states commonly observed in moiré superlattices are still under investigation. Here, the authors use pump-probe spectroscopy to study the dynamics of correlated insulator states in a WSe2/WS2 moiré heterostructure at two different fillings, elucidating distinct time-domain signatures associated with these states.

Using Purcell enhanced nanospectrocopy, applied DC bias fields, and nanomechanical strain, we activate emission of dark excitons and trions at room temperature while simultaneously tuning emission to measure competing relaxation processes in WSe 2 .

The discovery of van der Waals superconductors in recent years has generated a lot of excitement for their potentially novel pairing mechanisms. However, their typical atomic-scale thickness and micrometer-scale lateral dimensions impose severe challenges to investigations of pairing symmetry by conventional methods. We demonstrate an improved technique that employs high-quality-factor superconducting resonators to measure the kinetic inductance—up to one part per million—and loss of a van der Waals superconductor. We analyze the equivalent circuit model to extract the kinetic inductance, superfluid stiffness, penetration depth, and ratio of imaginary and real parts of the complex conductivity. We validate the technique by measuring aluminum and finding excellent agreement in both the zero-temperature superconducting gap as well as the complex conductivity data when compared with BCS theory. We then demonstrate the utility of the technique by measuring the kinetic inductance of multilayered niobium diselenide and discuss the limits to the accuracy of our technique when the transition temperature of the sample, <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:msub> <a:mi>NbSe</a:mi> <a:mn>2</a:mn> </a:msub> </a:math> at 7.06 K, approaches our Nb probe resonator at 8.59 K. Our method will be useful for practitioners in the growing fields of superconducting physics, materials science, and quantum sensing, as a means of characterizing superconducting circuit components and studying pairing mechanisms of the novel superconducting states which arise in layered two-dimensional materials and heterostructures. Published by the American Physical Society 2024

The topological properties of Bloch bands are intimately tied to the structure of their electronic wavefunctions within the unit cell of a crystal. Here, we show that scanning tunneling microscopy (STM) measurements on the prototypical transition metal dichalcogenide (TMD) semiconductor WSe$_2$ can be used to unambiguously fix the location of the Wannier center of the valence band. Using site-specific substitutional doping, we first determine the position of the atomic sites within STM images, establishing that the maximum electronic density of states at the $K$-point lies between the atoms. In contrast, the maximum density of states at the $Γ$ point is at the atomic sites. This signifies that WSe$_2$ is a topologically obstructed atomic insulator, which cannot be adiabatically transformed to the trivial atomic insulator limit.

High Resolution Image Download MS PowerPoint Slide Twisted hexagonal boron nitride (thBN) exhibits ferroelectricity due to moiré superlattices with AB/BA domains. These domains possess electric dipoles, leading to a periodic electrostatic potential that can be imprinted onto other materials placed in its proximity. Here we demonstrate the remote imprinting of moiré patterns from thBN onto monolayer MoSe 2 and investigate the changes in the exciton properties. We confirm the imprinted moiré patterns on monolayer MoSe 2 using Kelvin probe force microscopy (KPFM) and hyperspectral photoluminescence (PL) mapping. By creating a large ferroelectric domain (∼8.7 μm), we achieve unprecedented potential modulation (∼387 ± 52 meV). We observe the formation of exciton-polarons by the ferroelectric moiré domains and investigate the optical property changes induced by the moiré pattern in monolayer MoSe 2 by varying the moiré domain size down to ∼110 nm. Our findings highlight the potential of thBN as a platform for controlling the properties of 2D materials for optoelectronic and valleytronic applications.

Nanoribbons (NRs) of atomic layer transition metal dichalcogenides (TMDs) can boost the rapidly emerging field of quantum materials owing to their width-dependent phases and electronic properties. However, the controllable downscaling of width by direct growth and the underlying mechanism remain elusive. Here, we demonstrate the vapor-liquid-solid growth of single crystal of single layer NRs of a series of TMDs (MeX2: Me = Mo, W; X = S, Se) under chalcogen vapor atmosphere, seeded by pre-deposited and respective transition metal-alloyed nanoparticles that also control the NR width. We find linear dependence of growth rate on supersaturation, known as a criterion for continues growth mechanism, which decreases with decreasing of NR width driven by the Gibbs-Thomson effect. The NRs show width-dependent photoluminescence and strain-induced quantum emission signatures with up to ≈ 90% purity of single photons. We propose the path and underlying mechanism for width-controllable growth of TMD NRs for applications in quantum optoelectronics. Size control in quantum materials by direct growth is still difficult to achieve. Here, the authors present the width-dependent growth of single-layer nanoribbons of transition metal dichalcogenides from nanoalloy seeds, achieving strain-induced quantum emission with a purity of up to 90 % for single photons.

Two-dimensional semiconductors exhibit pronounced many-body effects and intense optical responses due to strong Coulombic interactions. Consequently, subtle differences in photoexcitation conditions can strongly influence how the material dissipates energy during thermalization. Here, using multiple excitation spectroscopies, we show that a distinct thermalization pathway emerges at elevated excitation energies, enhancing the formation of trions and charged biexcitons in single-layer WSe 2 by up to 2× and 5×, respectively. Power- and temperature-dependent measurements lend insights into the origin of the enhancement. These observations underscore the complexity of excited state relaxation in monolayer semiconductors, provide insights for the continued development of carrier thermalization models, and highlight the potential to precisely control excitonic yields and probe nonequilibrium dynamics in 2D semiconductors.

Intense electromagnetic fields can result in dramatic changes in the electronic properties of solids. These changes are commonly studied using optical probes of the modified electronic structure. Here we use optical-scanning tunneling microscopy (optical-STM) equipped with near-field continuous wave (CW) laser excitation to directly measure the electronic structure of light-dressed states in a monolayer transition metal dichalcogenide (TMD) semiconductor, WSe2. We find that the effective tunneling gap and tunneling density of states are strongly influenced by the intensity of the electromagnetic field when the applied field is resonant with the bandgap of the semiconductor. Our findings indicate that CW laser excitation can be used to generate light-dressed electronic states of quantum materials when confined strongly to the nanoscale.

Ultraclean graphene at charge neutrality hosts a quantum critical Dirac fluid of interacting electrons and holes. Interactions profoundly affect the charge dynamics of graphene, which is encoded in the properties of its electron-photon collective modes: surface plasmon polaritons (SPPs). Here, we show that polaritonic interference patterns are particularly well suited to unveil the interactions in Dirac fluids by tracking polaritonic interference in time at temporal scales commensurate with the electronic scattering. Spacetime SPP interference patterns recorded in terahertz (THz) frequency range provided unobstructed readouts of the group velocity and lifetime of polariton that can be directly mapped onto the electronic spectral weight and the relaxation rate. Our data uncovered prominent departures of the electron dynamics from the predictions of the conventional Fermi-liquid theory. The deviations are particularly strong when the densities of electrons and holes are approximately equal. The proposed spacetime imaging methodology can be broadly applied to probe the electrodynamics of quantum materials.