Van der Waals heterostructures obtained via stacking and twisting have been used to create moire superlattices1, enabling new optical and electronic properties in solid-state systems. Moire lattices in twisted bilayers of transition metal dichalcogenides (TMDs) result in exciton trapping2–5, host Mott insulating and superconducting states6 and act as unique Hubbard systems7–9 whose correlated electronic states can be detected and manipulated optically. Structurally, these twisted heterostructures feature atomic reconstruction and domain formation10–14. However, due to the nanoscale size of moire domains, the effects of atomic reconstruction on the electronic and excitonic properties have not been systematically investigated. Here we use near-0°-twist-angle MoSe2/MoSe2 bilayers with large rhombohedral AB/BA domains15 to directly probe the excitonic properties of individual domains with far-field optics. We show that this system features broken mirror/inversion symmetry, with the AB and BA domains supporting interlayer excitons with out-of-plane electric dipole moments in opposite directions. The dipole orientation of ground-state Γ–K interlayer excitons can be flipped with electric fields, while higher-energy K–K interlayer excitons undergo field-asymmetric hybridization with intralayer K–K excitons. Our study reveals the impact of crystal symmetry on TMD excitons and points to new avenues for realizing topologically non-trivial systems16,17, exotic metasurfaces18, collective excitonic phases19 and quantum emitter arrays20,21 via domain-pattern engineering. Domain-resolved spectroscopy reveals the impact of local atomic registry and crystal symmetry on the exciton properties of individual domains in near-0°-twist-angle MoSe2/MoSe2.

/pdf/broken-mirror-symmetry-in-excitonic-response-of-f3grw5fxho.pdf

Broken mirror symmetry in excitonic response of reconstructed domains in twisted MoSe2/MoSe2 bilayers

In recent time, the optical-analogous skyrmions, topological quasiparticles with sophisticated vectorial structures, have received an increasing amount of interest. Here we propose theortically and experimentally a generalized family of these, the tunable optical skyrmion, unveiling a new mechanism to transform between various skyrmionic topologies, including N\'eel-, Bloch-, and antiskyrmion types, via simple parametric tuning. In addition, Poincar\'e-like geometric representation is proposed to visualize the topological evolution of tunable skyrmions, which we termed Skyrme-Poincar\'e sphere, akin to the spin-orbit representation of complex vector modes. To generate experimentally the tunable optical skyrmions we implemented a digital hologram system based on a spatial light modulator, showing great agreement with our theoretical prediction. 

Generation of Optical Skyrmions with Tunable Topological Textures

Two-dimensional electrons confined to GaAs quantum wells are hallmark platforms for probing electron–electron interactions. Many key observations have been made in these systems as sample quality has improved over the years. Here, we present a breakthrough in sample quality via source-material purification and innovation in GaAs molecular beam epitaxy vacuum chamber design. Our samples display an ultra-high mobility of 44 × 106 cm2 V–1 s–1 at an electron density of 2.0 × 1011 cm–2. These results imply only 1 residual impurity for every 1010 Ga/As atoms. The impact of such low impurity concentration is manifold. Robust stripe and bubble phases are observed, and several new fractional quantum Hall states emerge. Furthermore, the activation gap (Δ) of the fractional quantum Hall state at the Landau-level filling (ν) = 5/2, which is widely believed to be non-Abelian and of potential use for topological quantum computing, reaches Δ ≈ 820 mK. We expect that our results will stimulate further research on interaction-driven physics in a two-dimensional setting and substantially advance the field. Source-material purification and optimized vacuum chamber design lead to a breakthrough in GaAs sample quality.

Ultra-high-quality two-dimensional electron systems

Optical skyrmions and other topological quasiparticles of light

Manipulating polaritons at the extreme scale in van der Waals materials

In modern GaAs/Al$_x$Ga$_{1-x}$As heterostructures with record high mobilities, a two-dimensional electron gas (2DEG) in a quantum well is provided by two remote donor $\delta$-layers placed on both sides of the well. Each $\delta$-layer is located within a narrow GaAs well, flanked by narrow AlAs layers which capture excess electrons from donors. We show that each excess electron is localized in a compact dipole atom with the nearest donor. Nevertheless, excess electrons screen both the remote donors and background impurities. When the fraction of remote donors filled by excess electrons $f$ is small, the remote donor limited quantum mobility grows as $f^{3}$ and becomes larger than the background impurity limited one at a characteristic value $f_c$. We also calculate both the mobility and the quantum mobility limited by the screened background impurities with concentrations $N_1$ in Al$_x$Ga$_{1-x}$As and $N_2$ in GaAs, which allows one to estimate $N_1$ and $N_2$ from the measured mobilities. Taken together, our findings should help to identify avenues for further improvement of modern heterostructures.

https://link.aps.org/accepted/10.1103/PhysRevMaterials.2.064604

Mobility and quantum mobility of modern GaAs/AlGaAs heterostructures

We study a capacitor made of three monolayers of transition-metal dichalcogenide separated by hexagonal boron nitride. We assume that the structure is symmetric with respect to the central layer plane. The symmetry includes the contacts: if the central layer is contacted by the negative electrode, both external layers are contacted by the positive one. As a result a strong enough voltage $V$ induces electron-hole dipoles (indirect excitons) pointing toward one of the external layers. Antiparallel dipoles attract each other at large distances. Thus, the dipoles alternate in the central plane forming a two-dimensional antiferroelectric with negative binding energy per dipole. The charging of a three-layer device is a first-order transition, and we show that if ${V}_{1}$ is the critical voltage required to create a single electron-hole pair and charge this capacitor by $e$, the macroscopic charge ${Q}_{c}=eS{n}_{c}$ ($S$ is the device area) enters the three-layer capacitor at a smaller critical voltage ${V}_{c}l{V}_{1}$. In other words, the differential capacitance $C(V)$ is infinite at $V={V}_{c}$. We also show that in a contactless three-layer device, where the chemically different central layer has lower conduction and valence bands, optical excitation creates indirect excitons that attract each other, and therefore form antiferroelectric exciton droplets. Thus, the indirect exciton luminescence is redshifted compared to a two-layer device.

Attraction of indirect excitons in van der Waals heterostructures with three semiconducting layers

In order to clarify the physics of the gating of solids by ionic liquids (ILs) we have gated lightly doped $p$-Si, which is so well studied that it can be called the ``hydrogen atom of solid state physics'' and can be used as a test bed for ionic liquids. We explore the case where the concentration of induced holes at the Si surface is below ${10}^{12}\phantom{\rule{4.pt}{0ex}}{\text{cm}}^{\ensuremath{-}2}$, hundreds of times smaller than record values. We find that in this case an excess negative ion binds a hole on the interface between the IL and Si becoming a surface acceptor. We study the surface conductance of holes hopping between such nearest neighbor acceptors. Analyzing the acceptor concentration dependence of this conductivity, we find that the localization length of a hole is in reasonable agreement with our direct variational calculation of its binding energy. The observed hopping conductivity resembles that of well studied ${\text{Na}}^{+}$ implanted Si MOSFETs.

Hopping conduction via ionic liquid induced silicon surface states

In modern GaAs/Al$_x$Ga$_{1-x}$As heterostructures with record high mobilities, a two-dimensional electron gas (2DEG) in a quantum well is provided by two remote donor $\delta$-layers placed on both sides of the well. Each $\delta$-layer is located within a narrow GaAs layer, flanked by narrow AlAs layers which capture excess electrons from donors but leave each of them localized in a compact dipole atom with a donor. Still excess electrons can hop between host donors to minimize their Coulomb energy. As a result they screen the random potential of donors dramatically. We numerically model the pseudoground state of excess electrons at a fraction $f$ of filled donors and find both the mobility and the quantum mobility limited by scattering on remote donors as universal functions of $f$. We repeat our simulations for devices with additional disorder such as interface roughness of the doping layers, and find the quantum mobility is consistent with measured values. Thus, in order to increase the quantum mobility this additional disorder should be minimized.

Excess electron screening of remote donors and mobility in modern GaAs/AlGaAs herostructures

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Mobility and quantum mobility of modern GaAs/AlGaAs heterostructures

Attraction of indirect excitons in van der Waals heterostructures with three semiconducting layers

Hopping conduction via ionic liquid induced silicon surface states

Excess electron screening of remote donors and mobility in modern GaAs/AlGaAs herostructures

Mobility and quantum mobility of modern GaAs/AlGaAs heterostructures