arXiv subjects

Mahdi Ghafariasl

Publications and source records attributed to Mahdi Ghafariasl.

8 recordsLinked to original sources

arXiv2026

Probing Quantum Anomalous Hall States in Twisted Bilayer WSe2 via Attractive Polaron Spectroscopy

Moiré superlattices in semiconductors exhibit a rich variety of interaction-induced topological states, including quantum anomalous Hall (QAH) effects. A recent study hinted that twisted WSe2 homobilayer (tWSe2) could host a QAH state but lacked direct evidence of ferromagnetism, a key hallmark of this phase. Here, we report the first direct evidence of QAH states in tWSe2 with spontaneous ferromagnetism. Specifically, we employ polarization-resolved attractive polaron spectroscopy on a dual-gated, 2 degree tWSe2 and observe direct signatures of spontaneous time-reversal symmetry breaking at hole filling ν= 1. Together with a Chern number measurement via Streda formula analysis, we identify this magnetized state as a topological state, characterized by C = 1. Furthermore, we demonstrate that these topological and magnetic properties are tunable via a finite displacement field, between a QAH ferromagnetic state and an antiferromagnetic state. Our findings position tWSe2 as a highly versatile, stable, and optically addressable platform for investigating topological order and strong correlations in two-dimensional landscapes.

cond-mat.str-el↗

arXiv2026

Nanoscopy of Excitons in Atomically Thin In-Plane Heterostructures with Nanointerfaces

Atomically sharp 2D in-plane heterostructures with nanoscale interfaces provide a powerful platform for tailoring optical and electrical properties at the nanoscale, enabling novel device engineering and the exploration of new physical phenomena. However, direct experimental correlation between local dielectric response and excitonic properties across such interfaces has remained elusive. Here, we probed the nanoscale complex dielectric function and the corresponding localized photoluminescence (PL) modulations in heterostructure domains of lateral monolayer MoxW1-xS2 - WxMo1-xS2, synthesized using a liquid-phase precursor-assisted approach. Near-field nano imaging across the visible-near-infrared range enables real space mapping of sharp amplitude and phase changes at the heterointerface, resolving the local complex dielectric function with nano-meter scale spatial resolution. Excitation energy-dependent nano spectroscopy reveals a reversal of dielectric contrast between Mo-rich and W-rich domains at their respective excitonic resonances, consistent with Lorentz-oscillator fits. Complementary hyperspectral nano-PL mapping resolves the evolution of excitonic emission across the lateral heterointerface, with neutral-exciton intensities varying continuously from W-rich to Mo-rich regions. Effective-medium theory modeling of the imaginary part of the effective dielectric function of the heterostructure as a function of photon energy and Mo filling fraction reproduces the observed excitonic trends, linking the PL evolution to a composition-dependent dielectric response. Together, these results provide direct nanoscale correlation between dielectric and excitonic boundaries in laterally stitched monolayer heterostructures and establish a multimodal near-field spectroscopy framework for probing excitonic phenomena at the nanoscale.

cond-mat.mtrl-sci↗

arXiv2025

Excitonic Landscapes in Monolayer Lateral Heterostructures Revealed by Unsupervised Machine Learning

Two-dimensional (2D) in-plane heterostructures including compositionally graded alloys and lateral heterostructures with defined interfaces display rich optoelectronic properties and offer versatile platforms to explore one-dimensional interface physics and many-body interaction effects. Graded $\mathrm{Mo}_x\mathrm{W}_{1-x}\mathrm{S}_2$ alloys show smooth spatial variations in composition and strain that continuously tune excitonic emission, while $\mathrm{MoS}_2$--$\mathrm{WS}_2$ lateral heterostructures contain atomically sharp interfaces supporting one-dimensional excitonic phenomena. These single-layer systems combine tunable optical and electronic properties with potential for stable, high-performance optoelectronic devices. Hyperspectral and nano-resolved photoluminescence (PL) imaging enable spatial mapping of optical features along with local variations in composition, strain, and defects, but manual interpretation of such large datasets is slow and subjective. Here, we introduce a fast and scalable unsupervised machine-learning (ML) framework to extract quantitative and interpretable information from hyperspectral PL datasets of graded $\mathrm{Mo}_x\mathrm{W}_{1-x}\mathrm{S}_2$ alloys and $\mathrm{MoS}_2$--$\mathrm{WS}_2$ heterostructures. Combining principal-component analysis (PCA), t-distributed stochastic neighbor embedding (t-SNE), and density-based spatial clustering (DBSCAN), we uncover spectrally distinct domains associated with composition, strain, and defect variations. Decomposition of representative spectra reveals multiple emission species, including band-edge excitons and defect-related transitions, demonstrating that ML-driven analysis provides a robust and automated route to interpret rich optical properties of 2D materials.

cond-mat.mtrl-sci↗

arXiv2025

Multi-timescale frequency-phase matching for high-yield nonlinear photonics

Integrated nonlinear photonic technologies, even with state-of-the-art fabrication with only a few nanometer geometry variations, face significant challenges in achieving wafer-scale yield of functional devices. A core limitation lies in the fundamental constraints of energy and momentum conservation laws. Imposed by these laws, nonlinear processes are subject to stringent frequency and phase matching (FPM) conditions that cannot be satisfied across a full wafer without requiring a combination of precise device design and active tuning. Motivated by recent theoretical and experimental advances in integrated multi-timescale nonlinear systems, we revisit this long-standing limitation and introduce a fundamentally relaxed and passive framework: nested frequency-phase matching. As a prototypical implementation, we investigate on-chip multi-harmonic generation in a two-timescale lattice of commercially available silicon nitride (SiN) coupled ring resonators, which we directly compare with conventional single-timescale counterparts. We observe distinct and striking spatial and spectral signatures of nesting-enabled relaxation of FPM. Specifically, for the first time, we observe simultaneous fundamental, second, third, and fourth harmonic generation, remarkable 100 percent multi-functional device yield across the wafer, and ultra-broad harmonic bandwidths. Crucially, these advances are achieved without constrained geometries or active tuning, establishing a scalable foundation for nonlinear optics with broad implications for integrated frequency conversion and synchronization, self-referencing, metrology, squeezed light, and nonlinear optical computing.

physics.optics↗

arXiv2023

Photodegradation and Thermal Effects in Violet Phosphorus

Violet phosphorus (VP) has garnered attention for its appealing physical properties and potential applications in optoelectronics. We present a comprehensive investigation of the photo-degradation and thermal effects of exfoliated VP on SiO2 substrate. The degradation rate of VP was found to be strongly influenced by the excitation wavelength and light exposure duration. Light exposure to the above bandgap (λ > 532 nm) leads to faster degradation, attributed to interactions with reactive oxygen species. Power-dependent photoluminescence (PL) measurements at low temperature (T=4 K) showed neutral exciton (X0) and trion (T) intensities linearly increased with excitation power, while the energy difference between their peak energies decreased, indicating changes in the exciton energy gap due to degradation. At room temperature X0 and T peaks were observed with higher X0 spectral weight, indicating reduced thermal stability of T. As the temperature decreased to 4 K, both X0 and T emissions intensified with blue-shifted peak positions. The T/X0 spectral weight ratio increased from 0.28 at 300 K to 0.69 at 4 K, suggesting enhanced T formation due to reduced phonon scattering. Temperature-dependent Raman spectroscopy revealed the presence of VP up to 673K. By tracking the peak position of 9 Raman modes with temperature the linear first-order temperature coefficient were obtained and found to be linear for all modes up to 673 K. Our results provide a deeper understanding of VP's degradation behavior and implications for optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2023

Sulfur Vacancy Related Optical Transitions in Graded Alloys of MoxW1-xS2 Monolayers

Engineering the electronic bandgap is of utmost importance in diverse domains ranging from information processing and communication technology to sensing and renewable energy applications. Transition metal dichalcogenides (TMDCs) provide an ideal platform for achieving this goal through techniques including alloying, doping, and creating in-plane or out-of-plane heterostructures. Here, we report on the synthesis and characterization of atomically controlled two-dimensional graded alloy of MoxW1-xS2, wherein the center region is Mo rich and gradually transitions towards a higher concentration of W atoms at the edges. This unique alloy structure leads to a continuously tunable bandgap, ranging from 1.85 eV in the center to 1.95 eV at the edges consistent with the larger band gap of WS2 relative to MoS2. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy showed the presence of sulfur monovacancy, VS, whose concentration varied across the graded MoxW1-xS2 layer as a function of Mo content with the highest value in the Mo rich center region. Optical spectroscopy measurements supported by ab initio calculations reveal a doublet electronic state of VS, which was split due to the spin-orbit interaction, with energy levels close to the conduction band or deep in the band gap depending on whether the vacancy is surrounded by W atoms or Mo atoms. This unique electronic configuration of VS in the alloy gave rise to four spin-allowed optical transitions between the VS levels and the valence bands. Our work highlights the potential of simultaneous defect and optical engineering of novel devices based on these 2D monolayers.

cond-mat.mtrl-sci↗

arXiv2023

Infrared Nanoimaging of Hydrogenated Perovskite Nickelate Synaptic Devices

Solid-state devices made from correlated oxides such as perovskite nickelates are promising for neuromorphic computing by mimicking biological synaptic function. However, comprehending dopant action at the nanoscale poses a formidable challenge to understanding the elementary mechanisms involved. Here, we perform operando infrared nanoimaging of hydrogen-doped correlated perovskite, neodymium nickel oxide (H-NdNiO3) devices and reveal how an applied field perturbs dopant distribution at the nanoscale. This perturbation leads to stripe phases of varying conductivity perpendicular to the applied field, which define the macroscale electrical characteristics of the devices. Hyperspectral nano-FTIR imaging in conjunction with density functional theory calculations unveil a real-space map of multiple vibrational states of H-NNO associated with OH stretching modes and their dependence on the dopant concentration. Moreover, the localization of excess charges induces an out-of-plane lattice expansion in NNO which was confirmed by in-situ - x-ray diffraction and creates a strain that acts as a barrier against further diffusion. Our results and the techniques presented here hold great potential to the rapidly growing field of memristors and neuromorphic devices wherein nanoscale ion motion is fundamentally responsible for function.

physics.app-ph↗

arXiv2023

Substrate Induced van der Waals Force Effect on the Stability of Violet Phosphorous

The van der Waals (vdWs) forces between monolayers has been a unique distinguishing feature of exfoliable materials since the first isolation of graphene. However, the vdWs interaction of exfoliable materials with their substrates and how this interface force influences their interaction with the environment is yet to be well understood.Here, we experimentally and theoretically unravel the role of vdWs forces between the recently rediscovered wide band gap p-type vdW semiconductor violet phosphorus (VP), with various substrates (including, SiO$_2$, mica, Si, Au) and quantify how VP stability in air and its interaction with its surroundings is influenced by the interface force.Using a combination of infrared nanoimaging and theoretical modeling we find the vdWs force at the interface to be a main factor that influences how VP interacts with its surroundings.In addition, the hydrophobicity of the substrate and the substrate surface roughness modify the vdWs force there by influencing VP stability. Our results could guide in the selection of substrates when vdW materials are prepared and more generally highlight the key role of interface force effects that could significantly alter physical properties of vdWs materials.

cond-mat.mtrl-sci↗