arXiv subjects

James Hone

Publications and source records attributed to James Hone.

At least 73 records · Page 4Linked to original sources

arXiv2021

Optically Facet-Resolved Reaction Anisotropy in Two-Dimensional Transition Metal Dichalcogenides

Quantifying anisotropy in the chemical reactions of mesoscopic crystals has mostly resorted on the combination of electron microscopy and diffraction. In this work, we established crystal-facet-resolved kinetic measurements of oxidation reactions in 2D transition metal dichalcogenides (TMDs) using optical second-harmonic generation spectroscopy and scanning probe microscopy. We show the in-plane anisotropy of their bond-breaking reactions is governed by their structure and strongly material-dependent among four TMDs. The facet-resolved analysis directly revealed that the reactions proceed fastest (slowest) for chalcogen (metal)-terminated zigzag edges with armchair edges in the middle. The degree of the anisotropy inducing trigonal oxidation patterns was much higher in MoS2 and MoSe2 than WS2 and WSe2. Kinetic Wulff construction based on edge-specific reaction rates verified the material-dependent mesoscopic reaction patterns. We also show that the reactions are initiated at substrate-mediated defects located on the bottom and top surfaces of 2D TMDs.

physics.chem-ph↗

arXiv2021

Moiré heterostructures as a condensed matter quantum simulator

Twisted van der Waals heterostructures have latterly received prominent attention for their many remarkable experimental properties, and the promise that they hold for realising elusive states of matter in the laboratory. We propose that these systems can, in fact, be used as a robust quantum simulation platform that enables the study of strongly correlated physics and topology in quantum materials. Among the features that make these materials a versatile toolbox are the tunability of their properties through readily accessible external parameters such as gating, straining, packing and twist angle; the feasibility to realize and control a large number of fundamental many-body quantum models relevant in the field of condensed-matter physics; and finally, the availability of experimental readout protocols that directly map their rich phase diagrams in and out of equilibrium. This general framework makes it possible to robustly realize and functionalize new phases of matter in a modular fashion, thus broadening the landscape of accessible physics and holding promise for future technological applications.

quant-ph↗

arXiv2021

Intrinsic Donor-Bound Excitons in Ultraclean Monolayer Semiconductors

The monolayer transition metal dichalcogenides are an emergent semiconductor platform exhibiting rich excitonic physics with coupled spin-valley degree of freedom and optical addressability. Here, we report a new series of low energy excitonic emission lines in the photoluminescence spectrum of ultraclean monolayer WSe2. These excitonic satellites are composed of three major peaks with energy separations matching known phonons, and appear only with electron doping. They possess homogenous spatial and spectral distribution, strong power saturation, and anomalously long population (> 6 $μ$s) and polarization lifetimes (> 100 ns). Resonant excitation of the free inter- and intra-valley bright trions leads to opposite optical orientation of the satellites, while excitation of the free dark trion resonance suppresses the satellites photoluminescence. Defect-controlled crystal synthesis and scanning tunneling microscopy measurements provide corroboration that these features are dark excitons bound to dilute donors, along with associated phonon replicas. Our work opens opportunities to engineer homogenous single emitters and explore collective quantum optical phenomena using intrinsic donor-bound excitons in ultraclean 2D semiconductors.

cond-mat.mes-hall↗

arXiv2020

Crossover between Strongly-coupled and Weakly-coupled Exciton Superfluids

In fermionic systems, superconductivity and superfluidity are enabled through the condensation of fermion pairs. The nature of this condensate can be tuned by varying the pairing strength, with weak coupling yielding a BCS-like condensate and strong coupling resulting in a BEC-like process. However, demonstration of this cross-over has remained elusive in electronic systems. Here we study graphene double-layers separated by an atomically thin insulator. Under applied magnetic field, electrons and holes couple across the barrier to form bound magneto-excitons whose pairing strength can be continuously tuned by varying the effective layer separation. Using temperature-dependent Coulomb drag and counter-flow current measurements, we demonstrate the capability to tune the magneto-exciton condensate through the entire weak-coupling to strong-coupling phase diagram. Our results establish magneto-exciton condensates in graphene as a model platform to study the crossover between two Bosonic quantum condensate phases in a solid state system.

cond-mat.mes-hall↗

arXiv2020

Scalable graphene platform for Tbits/s data transmission

To date, no electro-optic platform enables devices with high bandwidth, small footprint, and low power consumption, while also enabling mass production. Here we demonstrate high-yield fabrication of high-speed graphene electro-absorption modulators using CVD-grown graphene. We minimize variation in device performance from graphene inhomogeneity over large area by engineering graphene-mode overlap and device capacitance to ensure high extinction ratio. We fabricate an 8 mm x 1 mm chip with 32 graphene electro-absorption modulators and measure 94% yield with bit error rate below the hard-decision forward error correction limit at 7 Gbits/s, amounting to a total aggregated data rate of 210 Gbits/s. Monte Carlo simulations show that data rates > 0.6 Tbits/s are within reach by further optimizing device cross-section, paving the way for graphene-based ultra-high data rate applications.

physics.app-ph↗

arXiv2020

Diffusivity Reveals Three Distinct Phases of Interlayer Excitons in MoSe2/WSe2 Heterobilayers

Charge separated interlayer excitons in transition metal dichalcogenide (TMDC) heterobilayers are being explored for moiré exciton lattices and exciton condensates. The presence of permanent dipole moments and the poorly screened Coulomb interaction make many body interactions particularly strong for interlayer excitons. Here we reveal two distinct phase transitions for interlayer excitons in the MoSe2/WSe2 heterobilayer using time and spatially resolved photoluminescence imaging: from trapped excitons in the moiré-potential to the modestly mobile exciton gas as exciton density increases to ne/h ~ 1011 cm-2 and from the exciton gas to the highly mobile charge separated electron/hole plasma for ne/h > 1012 cm-2. The latter is the Mott transition and is confirmed in photoconductivity measurements. These findings set fundamental limits for achieving quantum states of interlayer excitons.

cond-mat.mes-hall↗

arXiv2020

Enhanced Superconductivity in Monolayer $T_d$-MoTe$_2$ with Tilted Ising Spin Texture

Crystalline two-dimensional (2D) superconductors with low carrier density are an exciting new class of materials in which superconductivity coexists with strong interactions, the effects of complex topology are not obscured by disorder, and electronic properties can be strongly tuned by electrostatic gating. Very recently, two such materials, 'magic-angle' twisted bilayer graphene and monolayer $T_d$-WTe$_2$, have been reported to show superconductivity at temperatures near 1 K. Here we report superconductivity in semimetallic monolayer $T_d$-MoTe$_2$. The critical temperature $T_\textrm{c}$ reaches 8 K, a sixty-fold enhancement as compared to the bulk. This anomalous increase in $T_\textrm{c}$ is only observed in monolayers, and may be indicative of electronically mediated pairing. Reflecting the low carrier density, the critical temperature, magnetic field, and current density are all tunable by an applied gate voltage, revealing a superconducting dome that extends across both hole and electron pockets. The temperature dependence of the in-plane upper critical field is distinct from that of $2H$ transition metal dichalcogenides (TMDs), consistent with a tilted spin texture as predicted by \textit{ab initio} theory.

cond-mat.supr-con↗

arXiv2020

Nonlinear twistoptics at symmetry-broken interfaces

Broken symmetries induce strong nonlinear optical responses in materials and at interfaces. Twist angle can give complete control over the presence or lack of inversion symmetry at a crystal interface, and is thus an appealing knob for tuning nonlinear optical systems. In contrast to conventional nonlinear crystals with rigid lattices, the weak interlayer coupling in van der Waals (vdW) heterostructures allows for arbitrary selection of twist angle, making nanomechanical manipulation of fundamental interfacial symmetry possible within a single device. Here we report highly tunable second harmonic generation (SHG) from nanomechanically rotatable stacks of bulk hexagonal boron nitride (BN) crystals, and introduce the term twistoptics to describe studies of optical properties in dynamically twistable vdW systems. We observe SHG intensity modulated by a factor of more than 50, polarization patterns determined by moiré interface symmetry, and enhanced conversion efficiency for bulk crystals by stacking multiple pieces of BN joined by symmetry-broken interfaces. Our study provides a foundation for compact twistoptics architectures aimed at efficient, scalable, and tunable frequency-conversion, and demonstrates SHG as a robust probe of buried vdW interfaces.

cond-mat.mes-hall↗

arXiv2020

Deep moiré potentials in twisted transition metal dichalcogenide bilayers

In twisted bilayers of semiconducting transition metal dichalcogenides (TMDs), a combination of structural rippling and electronic coupling gives rise to periodic moiré potentials that can confine charged and neutral excitations. Here, we report experimental measurements of the structure and spectroscopic properties of twisted bilayers of WSe2 and MoSe2 in the H-stacking configuration using scanning tunneling microscopy (STM). Our experiments reveal that the moiré potential in these bilayers at small angles is unexpectedly large, reaching values of above 300 meV for the valence band and 150 meV for the conduction band - an order of magnitude larger than theoretical estimates based on interlayer coupling alone. We further demonstrate that the moiré potential is a non-monotonic function of moiré wavelength, reaching a maximum at around a 13nm moiré period. This non-monotonicity coincides with a drastic change in the structure of the moiré pattern from a continuous variation of stacking order at small moiré wavelengths to a one-dimensional soliton dominated structure at large moiré wavelengths. We show that the in-plane structure of the moiré pattern is captured well by a continuous mechanical relaxation model, and find that the moiré structure and internal strain rather than the interlayer coupling is the dominant factor in determining the moiré potential. Our results demonstrate the potential of using precision moiré structures to create deeply trapped carriers or excitations for quantum electronics and optoelectronics.

cond-mat.mes-hall↗

arXiv2020

Abundance of correlated insulating states at fractional fillings of WSe$_{2}$/WS$_{2}$ moiré superlattices

Quantum particles on a lattice with competing long-range interactions are ubiquitous in physics. Transition metal oxides, layered molecular crystals and trapped ion arrays are a few examples out of many. In the strongly interacting regime, these systems often exhibit a rich variety of quantum many-body ground states that challenge theory. The emergence of transition metal dichalcogenide moiré heterostructures provides a highly controllable platform to study long-range electronic correlations. Here we report an observation of nearly two-dozen correlated insulating states at fractional fillings of a WSe$_{2}$/WS$_{2}$ moiré heterostructure. The discovery is enabled by a new optical sensing technique that is built on the sensitivity to dielectric environment of the exciton excited states in single-layer semiconductor WSe$_{2}$. The cascade of insulating states exhibits an energy ordering which is nearly symmetric about filling factor of half electron (or hole) per superlattice site. We propose a series of charge-ordered states at commensurate filling fractions that range from generalized Wigner crystals to charge density waves. Our study lays the groundwork for utilizing moiré superlattices to simulate a wealth of quantum many-body problems that are described by the two-dimensional t-V model or spin models with long-range charge-charge and exchange interactions.

cond-mat.str-el↗

arXiv2020

Odd- and even-denominator fractional quantum Hall states in monolayer WSe$_2$

Monolayer (ML) semiconducting transition-metal dichalcogenides (TMDs) represent a unique class of two-dimensional (2D) electron systems. Their atomically thin structure -- just like graphene -- facilitates gate-tunability, while the sizable band gap and strong spin-orbit coupling hold promise for properties beyond graphene. Measurements under large magnetic fields have revealed an unusual LL structure, distinct from other 2D electron systems. However, owing to limited sample quality and poor electrical contact, probing the lowest Landau levels (LLs) has been challenging, and observation of electron correlations within the fractionally filled LLs regime has not been possible. Here, through bulk electronic compressibility measurements, we investigate the LL structure of ML WSe$_2$ in the extreme quantum limit, and observe fractional quantum Hall (FQH) states in the lowest three LLs. The odd-denominator FQH sequences demonstrate a systematic evolution with the LL orbital index, which has not been observed in any other system but is consistent with generic theoretical expectations. In addition, we observe an even-denominator state in the second LL that is expected to host non-Abelian statistics. Our results suggest that the 2D semiconductors can provide an experimental platform that closely resembles idealized theoretical models in the quantum Hall regime.

cond-mat.mes-hall↗

arXiv2020

High performance integrated graphene electro-optic modulator at cryogenic temperature

High performance integrated electro-optic modulators operating at low temperature are critical for optical interconnects in cryogenic applications. Existing integrated modulators, however, suffer from reduced modulation efficiency or bandwidth at low temperatures because they rely on tuning mechanisms that degrade with decreasing temperature. Graphene modulators are a promising alternative, since graphene's intrinsic carrier mobility increases at low temperature. Here we demonstrate an integrated graphene-based electro-optic modulator whose 14.7 GHz bandwidth at 4.9 K exceeds the room-temperature bandwidth of 12.6 GHz. The bandwidth of the modulator is limited only by high contact resistance, and its intrinsic RC-limited bandwidth is 200 GHz at 4.9 K.

physics.app-ph↗

arXiv2020

Optical Measurement of Pseudo-Spin Texture of the Exciton Fine-Structure in Monolayer WSe2 within the Light Cone

Several theoretical predictions have claimed that the neutral exciton of TMDCs splits into a transversal and longitudinal exciton branch, with the longitudinal one, which is the upper branch, exhibiting an extraordinary strong dispersion in the meV range within the light cone. Historically, this was linked for semiconductor quantum wells to strong far-field optical dipole coupling, or strong electronic long-range exchange interactions, describing two sides of the same coin. Recently, experiments utilizing Fourier-space spectroscopy have shown that the exciton (exciton-polariton) dispersion can indeed be measured for high-quality hexagonal-BN-encapsulated WSe2 monolayer samples and can confirm the energy scale. Here, the exciton fine-structure's pseudo-spin and the valley polarization are investigated as a function of the centre-of-mass-momentum and excitation-laser detuning. For quasi-resonant excitation, a strong dispersion featuring a pronounced momentum-dependent helicity is observed. By increasing the excitation energy step-wise towards and then above the electronic band gap, the dispersion and the helicity systematically decrease due to contributions of incoherent excitons and emission from plasma. The decline of the helicity with centre-of-mass momentum can be phenomenologically modelled by the Maialle-Silva-Sham mechanism using the exciton splitting as the source of an effective magnetic field.

cond-mat.mes-hall↗

arXiv2020

One-Dimensional Moiré Excitons in Transition-Metal Dichalcogenide Heterobilayers

The formation of interfacial moiré patterns from angular and/or lattice mismatch has become a powerful approach to engineer a range of quantum phenomena in van der Waals heterostructures. For long-lived and valley-polarized interlayer excitons in transition-metal dichalcogenide (TMDC) heterobilayers, signatures of quantum confinement by the moiré landscape have been reported in recent experimental studies. Such moiré confinement has offered the exciting possibility to tailor new excitonic systems, such as ordered arrays of zero-dimensional (0D) quantum emitters and their coupling into topological superlattices. A remarkable nature of the moiré potential is its dramatic response to strain, where a small uniaxial strain can tune the array of quantum-dot-like 0D traps into parallel stripes of one-dimensional (1D) quantum wires. Here, we present direct evidence for the 1D moiré potentials from real space imaging and the corresponding 1D moiré excitons from photoluminescence (PL) emission in MoSe2/WSe2 heterobilayers. Whereas the 0D moiré excitons display quantum emitter-like sharp PL peaks with circular polarization, the PL emission from 1D moiré excitons has linear polarization and two orders of magnitude higher intensity. The results presented here establish strain engineering as a powerful new method to tailor moiré potentials as well as their optical and electronic responses on demand.

cond-mat.mtrl-sci↗

arXiv2020

Spatially mixed moiré excitons in two-dimensional van der Waals superlattices

Moiré superlattices open an unprecedented opportunity for tailoring interactions between quantum particles and their coupling to electromagnetic fields. Strong superlattice potential generates moiré minibands of excitons -- bound pairs of electrons and holes that reside either in a single layer (intralayer excitons) or two separate layers (interlayer excitons). The twist-angle-controlled interlayer hybridization of carriers can also mix the two types of excitons to combine the strengths of both. Here, we report a direct observation of spatially mixed moiré excitons in angle-aligned WSe2/WS2 and MoSe2/WS2 superlattices by optical reflectance spectroscopy. The strongly interacting interlayer and intralayer moiré excitons in WSe2/WS2 manifest energy level anticrossing and oscillator strength redistribution under a vertical electric field. We also observe doping-dependent exciton miniband renormalization and mixing near half filling of the first electron miniband of WS2. Our findings have significant implications for emerging correlated states in two-dimensional semiconductors, such as exciton condensates and Bose-Hubbard models, and optoelectronic applications of these materials.

cond-mat.mtrl-sci↗

arXiv2019

Direct Measurement of the Radiative Pattern of Bright and Dark Excitons and Exciton Complexes in Encapsulated Tungsten Diselenide

The optical properties of particularly the tungsten-based transition-metal dichalcogenides are strongly influenced by the presence of dark excitons. Recently, theoretical predictions as well as indirect experimental insights have shown that two different dark excitons exist within the light cone. While one is completely dark, the other one is only dipole forbidden out-of-plane, hence referred to as grey exciton. Here, we present angle-resolved spectroscopic data of a high-quality hexagonal-BN-encapsulated WSe2 monolayer with which we directly obtain the radiation pattern of this grey exciton that deviates from that of the bright exciton and other exciton complexes obtained at cryogenic temperatures.

cond-mat.mes-hall↗

arXiv2019

Broadband optical parametric amplification by two-dimensional semiconductors

Optical parametric amplification is a second-order nonlinear process whereby an optical signal is amplified by a pump via the generation of an idler field. It is the key ingredient of tunable sources of radiation that play an important role in several photonic applications. This mechanism is inherently related to spontaneous parametric down-conversion that currently constitutes the building block for entangled photon pair generation, which has been exploited in modern quantum technologies ranging from computing to communications and cryptography. Here we demonstrate single-pass optical parametric amplification at the ultimate thickness limit; using semiconducting transition-metal dichalcogenides, we show that amplification can be attained over a propagation through a single atomic layer. Such a second-order nonlinear interaction at the 2D limit bypasses phase-matching requirements and achieves ultrabroad amplification bandwidths. The amplification process is independent on the in-plane polarization of the impinging signal and pump fields. First-principle calculations confirm the observed polarization invariance and linear relationship between idler and pump powers. Our results pave the way for the development of atom-sized tunable sources of radiation with applications in nanophotonics and quantum information technology.

physics.optics↗

arXiv2019

Moiré-less Correlations in ABCA Graphene

Atomically thin van der Waals materials stacked with an interlayer twist have proven to be an excellent platform towards achieving gate-tunable correlated phenomena linked to the formation of flat electronic bands. In this work we demonstrate the formation of emergent correlated phases in multilayer rhombohedral graphene - a simple material that also exhibits a flat electronic band but without the need of having a moiré superlattice induced by twisted van der Waals layers. We show that two layers of bilayer graphene that are twisted by an arbitrary tiny angle host large (micron-scale) regions of uniform rhombohedral four-layer (ABCA) graphene that can be independently studied. Scanning tunneling spectroscopy reveals that ABCA graphene hosts an unprecedentedly sharp flat band of 3-5 meV half-width. We demonstrate that when this flat band straddles the Fermi level, a correlated many-body gap emerges with peak-to-peak value of 9.5 meV at charge neutrality. Mean field theoretical calculations indicate that the two primary candidates for the appearance of this broken symmetry state are a charge transfer excitonic insulator and a ferrimagnet. Finally, we show that ABCA graphene hosts surface topological helical edge states at natural interfaces with ABAB graphene which can be turned on and off with gate voltage, implying that small angle twisted double bilayer graphene is an ideal programmable topological quantum material.

cond-mat.mes-hall↗