arXiv subjects

James Hone

Publications and source records attributed to James Hone.

At least 91 records · Page 5Linked to original sources

arXiv2019

Electron-hole Hybridization in Bilayer Graphene

Band structure determines the motion of electrons in a solid, giving rise to exotic phenomena when properly engineered. Drawing an analogy between electrons and photons, artificially designed optical lattices indicate the possibility of a similar band modulation effect in graphene systems. Yet due to the fermionic nature of electrons, modulated electronic systems promise far richer categories of behaviors than those found in optical lattices. Here, we uncovered a strong modulation of electronic states in bilayer graphene subject to periodic potentials. We observed for the first time the hybridization of electron and hole sub-bands, resulting in local band gaps at both primary and secondary charge neutrality points. Such hybridization leads to the formation of flat bands, enabling the study of correlated effects in graphene systems. This work may also offer a viable platform to form and continuously tune Majorana zero modes, which is important to the realization of topological quantum computation.

cond-mat.mes-hall↗

arXiv2019

Magic continuum in twisted bilayer WSe2

Emergent quantum phases driven by electronic interactions can manifest in materials with narrowly dispersing, i.e. "flat", energy bands. Recently, flat bands have been realized in a variety of graphene-based heterostructures using the tuning parameters of twist angle, layer stacking and pressure, and resulting in correlated insulator and superconducting states. Here we report the experimental observation of similar correlated phenomena in twisted bilayer tungsten diselenide (tWSe2), a semiconducting transition metal dichalcogenide (TMD). Unlike twisted bilayer graphene where the flat band appears only within a narrow range around a "magic angle", we observe correlated states over a continuum of angles, spanning 4 degree to 5.1 degree. A Mott-like insulator appears at half band filling that can be sensitively tuned with displacement field. Hall measurements supported by ab initio calculations suggest that the strength of the insulator is driven by the density of states at half filling, consistent with a 2D Hubbard model in a regime of moderate interactions. At 5.1 degree twist, we observe evidence of superconductivity upon doping away from half filling, reaching zero resistivity around 3 K. Our results establish twisted bilayer TMDs as a model system to study interaction-driven phenomena in flat bands with dynamically tunable interactions.

cond-mat.mes-hall↗

arXiv2019

Blockade of vortex flow by thermal fluctuations in atomically thin clean-limit superconductors

Resistance in superconductors arises from the motion of vortices driven by flowing supercurrents or external electromagnetic fields and may be strongly affected by thermal or quantum fluctuations. The common expectation borne out in previous experiments is that as the temperature is lowered, vortex motion is suppressed, leading to a decreased resistance. A new generation of materials provides access to the previously inaccessible regime of clean-limit superconductivity in atomically thin superconducting layers. We show experimentally that for few-layer 2H-NbSe$_2$ the resistance below the superconducting transition temperature may be non-monotonic, passing through a minimum and then increasing again as temperature is decreased further. The effects exists over a wide range of current and magnetic fields, but is most pronounced in monolayer devices at intermediate currents. Analytical and numerical calculations confirm that the findings can be understood in a two-fluid vortex model, in which a fraction of vortices flow in channels while the rest are pinned but thermally fluctuating in position. We show theoretically that the pinned, fluctuating vortices effectively control the mobility of the free vortices. The findings provide a new perspective on fundamental questions of vortex mobility and dissipation in superconductors.

cond-mat.mes-hall↗

arXiv2019

High-performance monolayer MoS2 field-effect transistor with large-scale nitrogen-doped graphene electrodes for Ohmic contact

A finite Schottky barrier and large contact resistance between monolayer MoS2 and electrodes are the major bottlenecks in developing high-performance field-effect transistors (FETs) that hinder the study of intrinsic quantum behaviors such as valley-spin transport at low temperature. A gate-tunable graphene electrode platform has been developed to improve the performance of MoS2 FETs. However, intrinsic misalignment between the work function of pristine graphene and the conduction band of MoS2 results in a large threshold voltage for the FETs, because of which Ohmic contact behaviors are observed only at very high gate voltages and carrier concentrations (~1013 cm-2). Here, we present high-performance monolayer MoS2 FETs with Ohmic contact at a modest gate voltage by using a chemical-vapor-deposited (CVD) nitrogen-doped graphene with a high intrinsic electron carrier density. The CVD nitrogen-doped graphene and monolayer MoS2 hybrid FETs platform exhibited a large negative shifted threshold voltage of -54.2 V and barrier-free Ohmic contact under zero gate voltage. Transparent contact by nitrogen-doped graphene led to a 214% enhancement in the on-current and a four-fold improvement in the field-effect carrier mobility of monolayer MoS2 FETs compared with those of a pristine graphene electrode platform. The transport measurements, as well as Raman and X-ray photoelectron spectroscopy analyses before and after thermal annealing, reveal that the atomic C-N bonding in the CVD nitrogen-doped graphene is responsible for the dominant effects of electron doping. Large-scale nitrogen-doped graphene electrodes provide a promising device platform for the development of high-performance devices and the study of unique quantum behaviors.

cond-mat.mtrl-sci↗

arXiv2019

Low-loss composite photonic platform based on 2D semiconductor monolayers

Two dimensional materials such as graphene and transition metal dichalcogenides (TMDs) are promising for optical modulation, detection, and light emission since their material properties can be tuned on-demand via electrostatic doping. The optical properties of TMDs have been shown to change drastically with doping in the wavelength range near the excitonic resonances. However, little is known about the effect of doping on the optical properties of TMDs away from these resonances, where the material is transparent and therefore could be leveraged in photonic circuits. Here, we probe the electro-optic response of monolayer TMDs at near infrared (NIR) wavelengths (i.e. deep in the transparency regime), by integrating them on silicon nitride (SiN) photonic structures to induce strong light$-$matter interaction with the monolayer. We dope the monolayer to carrier densities of ($7.2 \pm 0.8$) $\times$ $10^{13} \textrm{cm}^{-2}$, by electrically gating the TMD using an ionic liquid. We show strong electro-refractive response in monolayer tungsten disulphide (WS$_2$) at NIR wavelengths by measuring a large change in the real part of refractive index $Δ$n = $0.53$, with only a minimal change in the imaginary part $Δ$k = $0.004$. The doping induced phase change ($Δ$n), compared to the induced absorption ($Δ$k) measured for WS$_2$ ($Δ$n/$Δ$k $\sim 125$), a key metric for photonics, is an order of magnitude higher than the $Δ$n/$Δ$k for bulk materials like silicon ($Δ$n/$Δ$k $\sim 10$), making it ideal for various photonic applications. We further utilize this strong tunable effect to demonstrate an electrostatically gated SiN-WS$_2$ phase modulator using a WS$_2$-HfO$_2$ (Hafnia)-ITO (Indium Tin Oxide) capacitive configuration, that achieves a phase modulation efficiency (V$_π$L) of 0.8 V $\cdot$ cm with a RC limited bandwidth of 0.3 GHz.

physics.app-ph↗

arXiv2019

Magnetic field mixing and splitting of bright and dark excitons in monolayer MoSe2

Monolayers of semiconducting transition metal dichalcogenides (TMDCs) with unique spin-valley contrasting properties and remarkably strong excitonic effects continue to be a subject of intense research interests. These model 2D semiconductors feature two fundamental intravalley excitons species - optically accessible ' bright' excitons with anti-parallel spins and optically inactive 'dark' excitons with parallel spins. For applications exploiting radiative recombination of bright excitons or long lifetime dark excitons, it is essential to understand the radiative character of the exciton ground state and establish the energy separation between the lowest energy bright and dark excitons. Here, we report a direct spectroscopic measure of dark excitons in monolayer MoSe$_2$ encapsulated in hexagonal boron nitride. By applying strong in-plane magnetic field, we induce mixing and splitting of bright and dark exciton branches, which enables an accurate spectroscopic determination of their energies. We confirm the bright character of the exciton ground state separated by a 1.5~meV gap from the higher energy dark exciton state, much smaller compared to the previous theoretical expectations. These findings provide critical information for further improvement of the accurate theoretical description of TMDCs electronic structure.

cond-mat.mes-hall↗

arXiv2019

Tunable crystal symmetry in graphene-boron nitride heterostructures with coexisting moiré superlattices

In heterostructures consisting of atomically thin crystals layered on top of one another, lattice mismatch or rotation between the layers results in long-wavelength moiré superlattices. These moiré patterns can drive significant band structure reconstruction of the composite material, leading to a wide range of emergent phenomena including superconductivity, magnetism, fractional Chern insulating states, and moiré excitons. Here, we investigate monolayer graphene encapsulated between two crystals of boron nitride (BN), where the rotational alignment between all three components can be varied. We find that band gaps in the graphene arising from perfect rotational alignment with both BN layers can be modified substantially depending on whether the relative orientation of the two BN layers is 0 or 60 degrees, suggesting a tunable transition between the absence or presence of inversion symmetry in the heterostructure. Small deviations ($<1^{\circ}$) from perfect alignment of all three layers leads to coexisting long-wavelength moiré potentials, resulting in a highly reconstructed graphene band structure featuring multiple secondary Dirac points. Our results demonstrate that the interplay between multiple moiré patterns can be utilized to controllably modify the electronic properties of the composite heterostructure.

cond-mat.mes-hall↗

arXiv2019

High quality electrostatically defined hall bars in monolayer graphene

Realizing graphene's promise as an atomically thin and tunable platform for fundamental studies and future applications in quantum transport requires the ability to electrostatically define the geometry of the structure and control the carrier concentration, without compromising the quality of the system. Here, we demonstrate the working principle of a new generation of high quality gate defined graphene samples, where the challenge of doing so in a gapless semiconductor is overcome by using the $ν=0$ insulating state, which emerges at modest applied magnetic fields. In order to verify that the quality of our devices is not compromised by the presence of multiple gates we compare the electronic transport response of different sample geometries, paying close attention to fragile quantum states, such as the fractional quantum Hall (FQH) states, that are highly susceptible to disorder. The ability to define local depletion regions without compromising device quality establishes a new approach towards structuring graphene-based quantum transport devices.

cond-mat.mes-hall↗

arXiv2018

Shedding light on the monolayer-WSe2 exciton's nature by optical effective-mass measurements

Two-dimensional excitons formed in quantum materials such as monolayer transition-metal dichalcogenides and their strong light-matter interaction have attracted unrivalled attention by the research community due to their extraordinarily large oscillator strength as well as binding energy, and the inherent spin-valley locking. Semiconducting few-layer and monolayer materials with their sharp optical resonances such as WSe2 have been extensively studied and envisioned for applications in the weak as well as strong light-matter coupling regimes, for effective nano-laser operation with various different structures, and particularly for valleytronic nanophotonics motivated by the circular dichroism. Many of these applications, which may benefit heavily from the two-dimensional electronic quasiparticle's properties in such films, require controlling, manipulating and first of all understanding the nature of the optical resonances that are attributed to exciton modes. While theory and previous experiments have provided unique methods to the characterization and classification efforts regarding the band structure and optical modes in 2D materials, here, we directly measure the quasiparticle energy-momentum dispersion for the first time. Our results for single-layer WSe2 clearly indicate an emission regime predominantly governed by free excitons, i.e. Coulomb-bound electron-hole pairs with centre-of-mass momentum and corresponding effective mass. Besides uniquely evidencing the existence of free excitons at cryogenic temperatures optically, the fading of the dispersive character for increased temperatures or excitation densities reveals a transition to a regime with profound role of charge-carrier plasma or localized excitons regarding its emission, debunking the myth of free-exciton emission at elevated temperatures.

cond-mat.mes-hall↗

arXiv2018

Magic Angle Spectroscopy

The electronic properties of heterostructures of atomically-thin van der Waals (vdW) crystals can be modified substantially by Moiré superlattice potentials arising from an interlayer twist between crystals. Moiré-tuning of the band structure has led to the recent discovery of superconductivity and correlated insulating phases in twisted bilayer graphene (TBLG) near the so-called "magic angle" of $\sim$1.1°, with a phase diagram reminiscent of high T$_c$ superconductors. However, lack of detailed understanding of the electronic spectrum and the atomic-scale influence of the Moiré pattern has so far precluded a coherent theoretical understanding of the correlated states. Here, we directly map the atomic-scale structural and electronic properties of TBLG near the magic angle using scanning tunneling microscopy and spectroscopy (STM/STS). We observe two distinct van Hove singularities (vHs) in the LDOS which decrease in separation monotonically through 1.1° with the bandwidth (t) of each vHs minimized near the magic angle. When doped near half Moiré band filling, the conduction vHs shifts to the Fermi level and an additional correlation-induced gap splits the vHs with a maximum size of 7.5 meV. We also find that three-fold (C$_3$) rotational symmetry of the LDOS is broken in doped TBLG with a maximum symmetry breaking observed for states near the Fermi level, suggestive of nematic electronic interactions. The main features of our doping and angle dependent spectroscopy are captured by a tight-binding model with on-site (U) and nearest neighbor Coulomb interactions. We find that the ratio U/t is of order unity, indicating that electron correlations are significant in magic angle TBLG. Rather than a simple maximization of the DOS, superconductivity arises in TBLG at angles where the ratio U/t is largest, suggesting a pairing mechanism based on electron-electron interactions.

cond-mat.mes-hall↗

arXiv2018

Spatially controlled electrostatic doping in graphene p-i-n junction for hybrid silicon photodiode

Sufficiently large depletion region for photocarrier generation and separation is a key factor for two-dimensional material optoelectronic devices, but few device configurations has been explored for a deterministic control of a space charge region area in graphene with convincing scalability. Here we investigate a graphene-silicon p-i-n photodiode defined in a foundry processed planar photonic crystal waveguide structure, achieving visible - near-infrared, zero-bias and ultrafast photodetection. Graphene is electrically contacting to the wide intrinsic region of silicon and extended to the p an n doped region, functioning as the primary photocarrier conducting channel for electronic gain. Graphene significantly improves the device speed through ultrafast out-of-plane interfacial carrier transfer and the following in-plane built-in electric field assisted carrier collection. More than 50 dB converted signal-to-noise ratio at 40 GHz has been demonstrated under zero bias voltage, with quantum efficiency could be further amplified by hot carrier gain on graphene-i Si interface and avalanche process on graphene-doped Si interface. With the device architecture fully defined by nanomanufactured substrate, this study is the first demonstration of post-fabrication-free two-dimensional material active silicon photonic devices.

physics.app-ph↗

arXiv2018

Twistable electronics with dynamically rotatable heterostructures

The electronic properties of two-dimensional materials and their heterostructures can be dramatically altered by varying the relative angle between the layers. This makes it theoretically possible to realize a new class of twistable electronics in which device properties can be manipulated on-demand by simply rotating the structure. Here, we demonstrate a new device architecture in which a layered heterostructure can be dynamically twisted, in situ. We study graphene encapsulated by boron nitride where at small rotation angles the device characteristics are dominated by coupling to a large wavelength Moiré superlattice. The ability to investigate arbitrary rotation angle in a single device reveals new features in the optical, mechanical and electronic response in this system. Our results establish the capability to fabricate twistable electronic devices with dynamically tunable properties.

cond-mat.mes-hall↗

arXiv2018

Competing Fractional Quantum Hall and Electron Solid Phases in Graphene

We report experimental observation of the reentrant integer quantum Hall effect in graphene, appearing in the N$=$2 Landau level. Similar to high-mobility GaAs/AlGaAs heterostructures, the effect is due to a competition between incompressible fractional quantum Hall states, and electron solid phases. The tunability of graphene allows us to measure the $B$-$T$ phase diagram of the electron-solid phase. The hierarchy of reentrant states suggest spin and valley degrees of freedom play a role in determining the ground state energy. We find that the melting temperature scales with magnetic field, and construct a phase diagram of the electron liquid-solid transition.

cond-mat.mes-hall↗

arXiv2018

Via Method for Lithography Free Contact and Preservation of 2D Materials

Atomically thin 2D materials span the common components of electronic circuits as metals, semi-conductors, and insulators, and can manifest correlated phases such as superconductivity, charge density waves, and magnetism. An ongoing challenge in the field is to incorporate these 2D materials into multi-layer hetero-structures with robust electrical contacts while preventing disorder and degradation. In particular, preserving and studying air-sensitive 2D materials has presented a significant challenge since they readily oxidize under atmospheric conditions. We report a new technique for contacting 2D materials, in which metal via contacts are integrated into flakes of insulating hexagonal boron nitride, and then placed onto the desired conducting 2D layer, avoiding direct lithographic patterning onto the 2D conductor. The metal contacts are planar with the bottom surface of the boron nitride and form robust contacts to multiple 2D materials. These structures protect air-sensitive 2D materials for months with no degradation in performance. This via contact technique will provide the capability to produce atomic printed circuit boards that can form the basis of more complex multi-layer heterostructures.

cond-mat.mes-hall↗

arXiv2017

Ultrafast Graphene Light Emitter

Ultrafast electrically driven nanoscale light sources are critical components in nanophotonics. Compound semiconductor-based light sources for the nanophotonic platforms have been extensively investigated over the past decades. However, monolithic ultrafast light sources with a small footprint remain a challenge. Here, we demonstrate electrically driven ultrafast graphene light emitters that achieve light pulse generation with up to 10 GHz bandwidth, across a broad spectral range from the visible to the near-infrared. The fast response results from ultrafast charge carrier dynamics in graphene, and weak electron-acoustic phonon-mediated coupling between the electronic and lattice degrees of freedom. We also find that encapsulating graphene with hexagonal boron nitride (hBN) layers strongly modifies the emission spectrum by changing the local optical density of states, thus providing up to 460 % enhancement compared to the grey-body thermal radiation for a broad peak centered at 720 nm. Furthermore, the hBN encapsulation layers permit stable and bright visible thermal radiation with electronic temperatures up to 2,000 K under ambient conditions, as well as efficient ultrafast electronic cooling via near-field coupling to hybrid polaritonic modes. These high-speed graphene light emitters provide a promising path for on-chip light sources for optical communications and other optoelectronic applications.

cond-mat.mes-hall↗

arXiv2017

Ambipolar Landau levels and strong band-selective carrier interactions in monolayer WSe$_2$

Monolayers (MLs) of transition metal dichalcogenides (TMDs) exhibit unusual electrical behavior under magnetic fields due to their intrinsic spin-orbit coupling and lack of inversion symmetry. While recent experiments have also identified the critical role of carrier interactions within these materials, a complete mapping of the ambipolar Landau level (LL) sequence has remained elusive. Here, we use single-electron transistors to perform LL spectroscopy in ML WSe$_2$, for the first time providing a comprehensive picture of the electronic structure of a ML TMD for both electrons and holes. We find that the LLs differ notably between the two bands, and follow a unique sequence in the valence band (VB) that is dominated by strong Zeeman effects. The Zeeman splitting in the VB is several times higher than the cyclotron energy, far exceeding the predictions of a single-particle model, and moreover tunes significantly with doping. This implies exceptionally strong many-body interactions, and suggests that ML WSe$_2$ can serve as a host for new correlated-electron phenomena.

cond-mat.mes-hall↗

arXiv2017

Bright visible light emission from graphene

Graphene and related two-dimensional materials are promising candidates for atomically thin, flexible, and transparent optoelectronics. In particular, the strong light-matter interaction in graphene has allowed for the development of state-of-the-art photodetectors, optical modulators, and plasmonic devices. In addition, electrically biased graphene on SiO2 substrates can be used as a low-efficiency emitter in the mid-infrared range. However, emission in the visible range has remained elusive. Here we report the observation of bright visible-light emission from electrically biased suspended graphenes. In these devices, heat transport is greatly minimised; thus hot electrons (~ 2800 K) become spatially localised at the centre of graphene layer, resulting in a 1000-fold enhancement in the thermal radiation efficiency. Moreover, strong optical interference between the suspended graphene and substrate can be utilized to tune the emission spectrum. We also demonstrate the scalability of this technique by realizing arrays of chemical-vapour-deposited graphene bright visible-light emitters. These results pave the way towards the realisation of commercially viable large-scale, atomically-thin, flexible and transparent light emitters and displays with low-operation voltage, and graphene-based, on-chip ultrafast optical communications.

cond-mat.mes-hall↗

arXiv2017

Electrical 2π phase control of infrared light in a 350nm footprint using graphene plasmons

Modulating the amplitude and phase of light is at the heart of many applications such as wavefront shaping, transformation optics, phased arrays, modulators and sensors. Performing this task with high efficiency and small footprint is a formidable challenge. Metasurfaces and plasmonics are promising , but metals exhibit weak electro-optic effects. Two-dimensional materials, such as graphene, have shown great performance as modulators with small drive voltages. Here we show a graphene plasmonic phase modulator which is capable of tuning the phase between 0 and 2π in situ. With a footprint of 350nm it is more than 30 times smaller than the 10.6$μ$m free space wavelength. The modulation is achieved by spatially controlling the plasmon phase velocity in a device where the spatial carrier density profile is tunable. We provide a scattering theory for plasmons propagating through spatial density profiles. This work constitutes a first step towards two-dimensional transformation optics for ultra-compact modulators and biosensing.

physics.app-ph↗