arXiv subjects

Amine Ben Mhenni

Publications and source records attributed to Amine Ben Mhenni.

7 recordsLinked to original sources

arXiv2025

Ten-valley excitonic complexes in charge-tunable monolayer WSe$_2$

Excitons dominate the optical response of two-dimensional (2D) semiconductors. Strong interactions produce peculiar excitonic complexes, which provide a testing ground for exciton and quantum many-body theories. Here, we report a hitherto unobserved many-body exciton that emerges upon filling both the K and Q valleys of WSe$_2$. We optically probe the exciton landscape using charge-tunable devices with unusually thin dielectrics that facilitate doping up to several $10^{13}$ cm$^{-2}$. We observe the emergence of the thermodynamically stable complex when 10 valleys are electrostatically filled. We gain insight into its physics using magneto-optical measurements. Our results are well-described by a model where the number of distinguishable Fermi seas interacting with the photoexcited electron-hole pair defines the complex's behavior. In addition to expanding the repertoire of excitons in 2D semiconductors, this complex could probe the limit of exciton models and answer open questions about screened Coulomb interactions in 2D semiconductors.

cond-mat.mes-hall↗

arXiv2025

Engineering strong correlations in a perfectly aligned dual moiré system

Exotic collective phenomena emerge when bosons strongly interact within a lattice. However, creating a robust and tunable solid-state platform to explore such phenomena has been elusive. Dual moiré systems$-$compromising two Coulomb-coupled moiré lattices$-$offer a promising system for investigating strongly correlated dipolar excitons (composite bosons) with electrical control. Thus far, their implementation has been hindered by the relative misalignment and incommensurability of the two moiré patterns. Here we report a dual moiré system with perfect translational and rotational alignment, achieved by utilizing twisted hexagonal boron nitride (hBN) bilayer to both generate an electrostatic moiré potential and separate MoSe$_{2}$ and WSe$_{2}$ monolayers. We observe strongly correlated electron phases driven by intralayer interactions and identify interlayer Rydberg trions, which become trapped in the presence of the Mott insulating state. Importantly, our platform is electrostatically programmable, allowing the realization of different lattice symmetries with either repulsive or attractive interlayer interactions. In particular, we implement the latter scenario by optically injecting charges, which form a dipolar excitonic phase. Our results establish a versatile platform for the exploration and manipulation of exotic and topological bosonic quantum many-body phases.

cond-mat.mes-hall↗

arXiv2025

Quantifying the creation of negatively charged boron vacancies in He-ion irradiated hexagonal boron nitride

Hexagonal boron nitride (hBN) hosts luminescent defects possessing spin qualities compatible with quantum sensing protocols at room temperature. Vacancies, in particular, are readily obtained via exposure to high-energy ion beams. While the defect creation mechanism via such irradiation is well understood, the occurrence rate of optically active negatively charged vacancies ($V_B^-$) is an open question. In this work, we exploit focused helium ions to systematically generate optically active vacancy defects in hBN flakes at varying density. By comparing the density-dependent spin splitting measured by magnetic resonance to calculations based on a microscopic charge model, in which we introduce a correction term due to a constant background charge, we are able to quantify the number of $V_B^-$ defects generated by the ion irradiation. We find a lower bound for the fraction (0.2%) of all vacancies in the optically active, negatively charged state. Our results provide a protocol for measuring the generation efficiency of $V_B^-$, which is necessary for understanding and optimizing luminescent centers in hBN.

cond-mat.mes-hall↗

arXiv2024

Gate-tunable Bose-Fermi mixture in a strongly correlated moiré bilayer electron system

Quantum gases consisting of species with distinct quantum statistics, such as Bose-Fermi mixtures, can behave in a fundamentally different way than their unmixed constituents. This makes them an essential platform for studying emergent quantum many-body phenomena such as mediated interactions and unconventional pairing. Here, we realize an equilibrium Bose-Fermi mixture in a bilayer electron system implemented in a WS$_{2}$/WSe$_{2}$ moiré heterobilayer with strong Coulomb coupling to a nearby moiré-free WSe$_{2}$ monolayer. Absent the fermionic component, the underlying bosonic phase manifests as a dipolar excitonic insulator. By injecting excess charges into it, we show that the bosonic phase forms a stable mixture with added electrons but abruptly collapses upon hole doping. We develop a microscopic model to explain the unusual asymmetric stability with respect to electron and hole doping. By studying the Bose-Fermi mixture via monitoring excitonic resonances from both layers, we demonstrate gate-tunability over a wide range in the boson/fermion density phase space, in excellent agreement with theoretical calculations. Our results further the understanding of phases stabilized in moiré bilayer electron systems and demonstrate their potential for exploring the exotic properties of equilibrium Bose-Fermi mixtures.

cond-mat.str-el↗

arXiv2024

Breakdown of the static dielectric screening approximation of Coulomb interactions in atomically thin semiconductors

Coulomb interactions in atomically thin materials are uniquely sensitive to variations in the dielectric screening of the environment, which can be used to control quasiparticles and exotic quantum many-body phases. A static approximation of the dielectric response, where increased dielectric screening is predicted to cause an energy redshift of the exciton resonance, has been until now sufficient. Here, we use charge-tunable exciton resonances to study screening effects in transition metal dichalcogenide monolayers embedded in materials with dielectric constants ranging from 4 to more than 1000. In contrast to expectations, we observe a blueshift of the exciton resonance exceeding 30 meV for larger dielectric constant environments. By employing a dynamical screening model, we find that while the exciton binding energy remains mostly controlled by the static dielectric response, the exciton self-energy is dominated by the high-frequency response. Dielectrics with markedly different static and high-frequency screening enable the selective addressing of distinct many-body effects in layered materials and their heterostructures, expanding the tunability range and offering new routes to detect and control correlated quantum many-body states and to design optoelectronic and quantum devices.

cond-mat.mes-hall↗

arXiv2023

Nonlinear Dispersion Relation and Out-of-Plane Second Harmonic Generation in MoSSe and WSSe Janus Monolayers

Janus transition metal dichalcogenides are an emerging class of atomically thin materials with engineered broken mirror symmetry that gives rise to long-lived dipolar excitons, Rashba splitting, and topologically protected solitons. They hold great promise as a versatile nonlinear optical platform due to their broadband harmonic generation tunability, ease of integration on photonic structures, and nonlinearities beyond the basal crystal plane. Here, we study second and third harmonic generation in MoSSe and WSSe Janus monolayers. We use polarization-resolved spectroscopy to map the full second-order susceptibility tensor of MoSSe, including its out-of-plane components. In addition, we measure the effective third-order susceptibility, and the second-order nonlinear dispersion close to exciton resonances for both MoSSe and WSSe at room and cryogenic temperatures. Our work sets a bedrock for understanding the nonlinear optical properties of Janus transition metal dichalcogenides and probing their use in the next-generation on-chip multifaceted photonic devices.

cond-mat.mes-hall↗

arXiv2022

Strong light-matter interaction with self-hybridized bound states in the continuum in monolithic van der Waals metasurfaces

Photonic bound states in the continuum (BICs) are a standout nanophotonic platform for strong light-matter coupling with transition metal dichalcogenides (TMDCs), but have so far mostly been employed as all-dielectric metasurfaces with adjacent TMDC layers, incurring limitations related to strain, mode overlap, and material integration. In this work, we experimentally demonstrate for the first time asymmetry-dependent BIC resonances in 2D arrays of monolithic metasurfaces composed solely of the nanostructured bulk TMDC WS$_2$ with BIC modes exhibiting sharp and tailored linewidths, ideal for selectively enhancing light-matter interactions. Geometrical variation enables the tuning of the BIC resonances across the exciton resonance in bulk WS$_2$, revealing the strong-coupling regime with an anti-crossing pattern and a Rabi splitting of 116 meV. The precise control over the radiative loss channel provided by the BIC concept is harnessed to tailor the Rabi splitting via a geometrical asymmetry parameter of the metasurface. Crucially, the coupling strength itself can be controlled and is shown to be independent of material-intrinsic losses. Our BIC-driven monolithic metasurface platform can readily incorporate other TMDCs or excitonic materials to deliver previously unavailable fundamental insights and practical device concepts for polaritonic applications.

physics.optics↗