Session M51: Optical Studies of 2D Materials and Their Twisted Heterostructures

Fabrication of Single-Photon Emitters in 2D Hexagonal Boron Nitride via Carbon Annealing

As the most recent addition to the single-photon emitter library, defects in hexagonal boron nitride (hBN) have attracted much attention due to their high brightness at room temperature and stability. These emitters also have the potential to integrate with cavities and photonic waveguides thanks to their 2D hosting system. Here, we developed a new method to activate single-photon emitters in 2D hBN, which is highly desired for their further application. We annealed mechanically exfoliated hBN under mixed atmosphere with methane as carbon source, and our simple one-step annealing process yields a tremendous increase in the concentration of emitters in hBN. With several related experiments, the single defects fabricated by our method are confirmed to be quantum emitters. Some other photophysical properties indicate that the emitters show higher brightness and become more stable after carbon annealing. Our approach to the design of single photon emitters in 2D hBN opens up new avenues for the generation of single photon emitters and their integration in quantum nanophotonic devices.   

Density functional theory of strain engineering on single photon emitter in hexagonal boron nitride

Hexagonal boron nitride has been found as excellent host for color centers as single photon emitters. The complex defect which is a nitrogen vacancy next to a nitrogen antisite (V_NN_B) is regarded as promising candidate due to the nontrivial electronic structure.^1,2 Here, using density functional theory, we provide detailed investigation of the geometric and electronic evolution of V_NN_B under external strain. The V_NN_B prefers the C_1h symmetry making the defect as a dynamic Jahn-Teller system.³ By lowing the symmetry, the first transition which considered to be a dark state can be activated, inducing the compete between two possible excitation pathways.⁴ The zero phonon line shows non-linear evolution as a function of in plane strain indicating the important role of the out of plane strain between defect atom and its ambient structure. It is predicted that a significantly stronger redshift than blueshift of emission can be realized by applying armchair strain. Our analysis provides insightful understanding of the color centers as single photon emitters.

(1) Tran, T. T.; et al. Nature Nanotech 2016, 11 (1), 37–41.
(2) Abdi, M.; et al. ACS Photonics 2018, 5 (5), 1967–1976.
(3) Noh, G.; et al. Nano Lett. 2018, 18 (8), 4710–4715.
(4) Wu, F.; et al. Phys. Rev. B 2019, 100, 081407(R).
  

Optical detection of correlated insulating states in WSe2/WS2 moiré superlattices

Moiré superlattices can be used to engineer strongly correlated phenomena in van der Waals heterostructures, as recently demonstrated in graphene-based systems. Aligned transition metal dichalcogenide (TMDC) heterostructures may also support correlated electronic states, with additional opportunities arising from the strong-light matter interaction and large spin-orbital coupling. Using a novel optical detection technique, we observe correlated electronic phases in semiconducting WSe₂/WS₂ moiré superlattices. Further, we demonstrate that the spin-valley optical selection rules in TMDCs can be used to optically create and study low-energy spin excitations in the correlated states.
  

Light Emission of 2D Interlayer Moiré Excitons

Recently, 2D twisted bilayers with interlayer lattice mismatch and/or rotational misalignment have emerged as a powerful platform for studying unconventional properties such as the interlayer exciton (IX), where electrons and holes located in different monolayers couple to form a bound state. The IXs can further be spatially concentrated into periodic potential minima modulated via the moiré landscape, forming uniform periodic quantum-dot-like emitter array. In this talk, I will introduce optical characterization of 2D heterobilayer transition metal dichalcogenides (TMDCs) interlayer excitons under different configurations of interlayer twisting angles and material selections. 2D heterobilayers with deterministic interlayer twist angle were prepared using a custom-built layer stacking system. Here, we studied the light emission properties with time-resolved photoluminescence (PL), temperature-dependent PL, and magneto-PL. Through the PL studies, we were able to extract key parameters of interlayer excitons such as lifetime, binding energy, valley polarization, etc. These results represent an advance in the understanding of the physics of 2D interlayer excitations and the future implementation of these TMDC materials within tunable light-emitting devices.   

Anomalous bright narrow interlayer exciton photoluminescence in MoSe2-WSe2 heterostructures

Van der Waals heterostructures provide a new platform for studying spatially indirect interlayer excitons, where the constituent electrons and holes are located in different layers. These heterostructures are made by stacking two or more two-dimensional materials via a polymer stamping technique. Stacking layers with a lattice mismatch and/or twist angle between layers will form a moiré pattern in real space. This moiré lattice can modulate the exciton energy, resulting in so-called moiré excitons where excitons are trapped at the moiré potential minima. Recent studies on - heterostructures have shown narrow lines in low power photoluminescence(PL), which were attributed to moiré excitons. Optical measurements of our high-quality - heterostructures encapsulated by hBN shows an anomalous bright narrow interlayer exciton PL localized to a small area of the sample. The PL of this structure shows interlayer PL emission whose linewidth is comparable to delocalized interalayer excitons in high quality samples. We will discuss the density and temperature dependence of this bright interlayer exciton PL and how it relates to other recent reports of interlayer moiré excitons in transition metal dichalcogenide heterostructures.   

Emergent magnetic field from the moiré of homobilayer 2D semiconductors

The spatial texture of internal degree of freedom of electrons has profound effects on material properties. Such texture in real space can manifest as an emergent magnetic field, which is expected to induce interesting transport phenomena. Moiré pattern as a spatial variation at the interface of 2D atomic crystals provides a natural platform for investigating such real space Berry phase effect [1]. Here we study Moiré structures formed in homobilayer TMD due to twisting, uniform strains, and their combinations [2], where electrons can reside in either layer with the layer index serving as an internal degree of freedom. The layer pseudospin exhibits vortex/antivortex textures in the Moiré supercell, which leads to a giant magnetic field. We will show that strain and interlayer bias can be used to engineer the in-plane and out-of-plane layer pseudospin texture, respectively. Therefore, the profile, intensity, and flux of the magnetic field are highly tunable, rendering TMD Moiré structures promising for transport and topological material applications.

[1] H. Yu, M. Chen, and W. Yao, Natl. Sci. Rev. (2019).
[2] D. Zhai et al., in preperation.   

Induced spin-orbit coupling in twisted graphene-TMDC heterobilayers: Twistronics meets spintronics

Transport experiments in graphene deposited on monolayer transition metal dichalcogenide (TMDC) have demonstrated weak antilocalization, Shubnikov-de Haas oscillations, spin lifetime anisotropy, spin Hall effect and Rashba-Edelstein effect [1]. This suggests that spin-orbit coupling (SOC) in graphene is enhanced by the strong intrinsic SOC of the nearby TMDC layer. Setting up a theoretical model of the graphene-TMDC heterobilayer we are able to explain the origin and predict the interlayer twist angle dependence of the induced SOC [2]. We derive the induced valley Zeeman and Rashba type SOC in terms of the TMDC band structure parameters and interlayer tunneling matrix elements. We find that the strength of the induced SOC can be strongly tuned by the twist angle. Provided that the energy of the Dirac point is close to the TMDC conduction band, up to a tenfold increase of the valley Zeeman (at 18 degrees twist angle) and a fourfold increase of the Rashba type induced SOC can be achieved. Moreover, we have extended our model to the case of graphene encapsulated in two TMDC layers, where the induced SOC is also modulated by the additional twist angle.

[1] Avsar, et al., arXiv:1909.09188 (2019).
[2] David, Rakyta, Kormányos, and Burkard, Phys. Rev. B 100, 085412 (2019).   

The optical evidence of atomic reconstruction in twisted bilayer MoS2

In two-dimensional materials, Moiré superlattices formed by stacking two monolayers with lattice constant mismatch or rotational misalignment have been widely used to manipulate their electronic and optical properties. Here, we employ low-frequency Raman measurements and DFT calculations to demonstrate atomic reconstruction in the MoS₂ homobilayer with a small twist angle (θ). For 0°＜θ≤3.5°, only interlayer Raman mode of 3R stacking with stable peak position is presented, indicating the overwhelming expansion of stable stacking in small twist angle due to structural relaxation. For 3.5°＜θ≤6°, the small Moiré supercell with weak relaxation results in mixed in-plane and out-of-plane vibrations. The shear mode quickly disappears with an increasing twist angle, while mixed breathing modes are observed. Our work provides a novel strategy to understand and monitor the evolution of structural relaxation in moiré superlattices in two-dimensional van der Waals materials.   

Interlayer Excitons in van der Waals Heterostructures in a Magnetic Field

We investigate particle-hole excitations in bilayer 2D van der Waals heterostructures in a magnetic field. Such systems are interesting because the two constituents of the excitons can reside in band environments of very different Berry’s curvatures. This allows the neutral excitations to carry signatures of the fermionic electronic structures of the individual materials. Moreover, the heterostructures can give rise to a large moiré unit cell. The effective periodicity relaxes momentum conservation so that the exciton dispersion at finite momentum could be detected by light absorption. As an example, we study an MoS₂-graphene heterostructure, for which both valleys host Berry’s curvature in only one layer. We consider variational single particle-hole pair wavefunctions, both in terms of the Landau levels and eigenstates of magnetic translation operators. The formalism allows one to identify how wavefunction overlaps determine the exciton state and energy. We show for this heterostructure that the energy disperses remarkably slowly. The impacts of many-body effects and the quasi-periodic tunneling between the layers associated with the moiré pattern on exciton dispersions and light absorption are also considered.   

Near field study of twisted transition metal dichalcogenide bilayers

Twistronics has recently emerged as a novel tuning knob to engineer the properties of electrons and excitons in two dimensional van der Waals heterostructures. However, directly probing the properties of vdW heterostructures within a single moiré unit cell remains elusive. In this talk, we utilize near-field optical microscopy to explore twisted transition metal dichalcogenides (TMD) bilayers. The atomically thin vertical p-n junction of WSe₂/MoSe₂ bilayer is fabricated by mechanical exfoliation and stacking. The type-II band alignment of the WSe₂/MoSe₂ facilitates electron-hole separation permitting photocurrent measurements at the nanoscale. Features with ~100 nm periodicity are observed and show systematic evolution under electro-static gating. More experiments are carrying out to conclusively assign these features to the moiré. Our study demonstrates a viable pathway towards probing and manipulating moiré patterned TMD heterostructures at the nanoscale.   

Floquet-Engineered Topological Flat Bands in Irradiated Twisted Bilayer Graphene

We propose a tunable optical setup to engineer topologically nontrivial flat bands in twisted bilayer graphene under circularly polarized light. Using both analytical and numerical calculations, we demonstrate that nearly flat bands can be engineered at small twist angles near the magic angles of the static system. The flatness and the gaps between these bands can be tuned optically by varying laser frequency and amplitude. We study the effects of interlayer hopping variations on Floquet flat bands and find that lattice relaxation favors their formation. Furthermore, we find that, once formed, the flat bands carry nonzero Chern numbers. We show that at currently known values of parameters, such topological flat bands can be realized using circularly polarized UV laser light. Thus, our work opens the way to creating optically tunable, strongly correlated topological phases of electrons in moire superlattices.   

Doping-dependence of exciton energies and MCD in TMD moiré superlattices

The long-period pattern formed by two-dimensional crystals that are slightly misaligned or have slightly different lattice constants is known
as a moire superlattice. It was recently discovered that the moiré superlattices have a pronounced effect on the electronic properties of many two-dimensional materials. I will discuss the influence of a moiré superlattice on excitons in transition metal dichalcogenide heterostructures.
Existing effective boson theories of the moiré excitons do not allow the effect of electron or hole doping to be predicted. I will present a microscopic calculation of the optical response at finite doping, as well as a simple phenomenological model, which captures the main results.
When the carriers are valley polarized the optical response exhibits circular dichroism. I will discuss the use of the circular dichroism of the exciton spectrum as a probe of ground state valley polarization.   

Excitonic insulator and spin-valley superfluid in twisted bilayer transition metal dichalcogenide

Recently striking correlated phenomena emerge in twisted bilayer WSe$_2$ -- including a correlated moir\'e insulating dome at half-filling -- over a range of small twist angles. In this work, we show the half-filling state is an intervalley excitonic insulator, where the electron-hole pairing is strongly enhanced for a certain range of displacement field due to the proximity of a van Hove singularity to the Fermi level. The mean field phase diagram as a function of displacement field, temperature and magnetic field shows a good agreement with the experiment. Furthermore, the excitonic insulator hosts a spin-valley density wave order which can be viewed as a spin superfluid. We propose an all-optical setup to study the superflow of spins in the twisted bilayer transition metal dichalcogenide systems.   

Dielectric engineeringin monolayer WSe2/graphene heterostructures

The strong Coulomb interaction between charge carriers in atomically thin transition metal dichalcogenides (e.g. MoS₂, WSe₂) is sensitive to the nearby dielectric environment. Here, we investigate the optical response of monolayer WSe₂, which is in contact with gate-tunable graphene in van der Waals heterostructures, by optical reflection spectroscopy. We find that both the exciton binding energy and quasiparticle bandgap of WSe₂can be renormalized by the dielectric screening effect. The effect can also be modulated upon the formation of Landau levels in graphene under perpendicular magnetic fields. Furthermore, we directly observe inter-band Landau level transitions of WSe₂, which helps to accurately determine the quasiparticle bandgap and effective cyclotron mass in WSe₂ as a function of dielectric screening.   

Flat bands and optical activity in twisted boron nitride

We present a minimal continuum model describing the electronic reconstruction in twisted bilayers of hexagonal boron nitride. The relative twist creates regions of unnatural stacking within the moiré cell, where ions of the same polarity lie on top of each other. The gap is strongly modulated, giving rise to very narrow quasi-bands at relative large twist angles. The absence of a magic angle condition emphasizes the different nature of these flat bands as compared to the case of twisted graphene systems. In particular, changing the parameters of our model we can describe both situations, which are separated by a topological phase transition controlled by inter-band coherences of the envelope wave functions around the moiré zone center. Transitions between these flat bands are manifested as sharp resonances in the optical absorption spectrum. We also evaluate the Hall counter-flow ascribed to the chiral symmetry of the system, which gives rise to different absorption cross sections for right-handed and left-handed circularly-polarized light (circular dichroism).