arXiv subjects

Sihem Jaziri

Publications and source records attributed to Sihem Jaziri.

15 recordsLinked to original sources

arXiv2024

Berry curvature contribution towards $ 1s-2p_{\pm} $ interlayer exciton ultrafast transition within a $R-WSe_{2}/MoSe_{2} $ heterobilayer

We calculate the spectrum of interlayer neutral excitons in transition-metal-dichalcogenide WeSe$_2$/MoSe$_2$ heterobilayers in the $R$-stacking configuration. Most saliently, we show that, similarly to neutral excitons and trions in monolayer transition-metal dichalcogenides, the spectrum is sensitive to the Berry curvature and thus quantum-geometric effects underlying the electron and hole wave functions. Due to the spatial separation between the electron and hole constituting the exciton in different layers, the Berry-curvature-induced splitting of the between the $2p_+$ and the $2p_-$ exciton states is smaller than for monolayer excitons. Furthermore, we investigate the dependence of the exciton spectra on the dielectric environment and the twist angle between the two layers. Finally, the long-lived moiré interlayer exciton ground state ($1s$) enhances the possibility of creating brightened $2p_{\pm}$ states using a circularly polarized medium-infrared probe from the $1s$ ground state. As a result, we determine the polarizability of the $1s-2p_{\pm}$ transition, following by two-level dressed model for the optical Stark effect.

cond-mat.mes-hall↗

arXiv2024

The optical absorption in indirect semiconductor to semimetal PtSe2 arises from direct transitions

$\rm{PtSe_2}$ is a van der Waals material transitioning from an indirect bandgap semiconductor to a semimetal with increasing thickness. Its absorption threshold has been conjectured to originate from interband indirect transitions. By quantitative comparison between broadband ($0.8 - 3.0\,\rm{eV}$) optical absorption of high-quality exfoliated crystals and DFT ab-initio simulations, we prove instead that the optical absorption arises only from direct transitions. This understanding allows us to shed light on the semiconductor to semimetal transition and to explore the effect of stacking and excitons on the optical absorption.

cond-mat.mtrl-sci↗

arXiv2024

Layer-number and strain effects on the structural and electronic properties of PtSe2 material

Bandgap engineering of low-dimensional materials forms a robust basis for advancements in optoelectronic technologies. Platinum diselenide (PtSe2) material exhibits a transition from semi-metal to semiconductor (SM-SC) when going from bulk to monolayer (ML). In this work, density functional theory (DFT) with various van der Waals (vdW) corrections has been tested to study the effect of the layer-number on the structural and electronic properties of the PtSe2 material. The considered vdW corrections gave different results regarding the number of layers at which the SM-SC transition occurs. This variation is due to the different interlayer distances found for each correction, revealing the sensitivity of the bandgap to this distance in addition to the layer number. In fact, the bandgap increases with the increasing of the interlayer distance, due to the energy shift of conduction and valence bands dominated by Se-pz orbitals. According to the comparison with the available experimental data, the vdW corrections vdW-DF and rVV10 gave the most accurate results. Moreover, the control of the interlayer distance via vertical compressive strain led to the bandgap tuning of semiconductor PtSe2 BL. Indeed, a semi-metal character of PtSe2 BL can be obtained under 17% vertical strain. Our work shows a deep understanding of the correlation between the structural and electronic properties, and thus a possibility to tune the bandgap by strain means.

cond-mat.mtrl-sci↗

arXiv2023

Optical properties of orthorhombic germanium sulfide: Unveiling the Anisotropic Nature of Wannier Exciton

To fully explore exciton-based applications and improve their performance, it is essential to understand the exciton behavior in anisotropic materials. Here, we investigate the optical properties of anisotropic excitons in GeS encapsulated by h-BN, using different approaches that combine polarization- and temperature-dependent photoluminescence (PL) measurements, \textit{ab initio} calculations, and effective mass approximation (EMA). Using the Bethe-Salpeter Equation (BSE) method, we found that the optical absorption spectra in GeS are significantly affected by the Coulomb interaction included in the BSE method, which shows the importance of excitonic effects besides it exhibits a significant dependence on the direction of polarization, revealing the anisotropic nature of bulk GeS. Combining \textit{ab initio} calculations and EMA methods, we investigate the quasi-hydrogenic exciton states and oscillator strength (OS) of GeS along the zigzag and armchair axes. We found that the anisotropy induces lifting of the degeneracy and mixing of the excitonic states in GeS, which results in highly nonhydrogenic features. Very good agreement with the experiment is observed.

cond-mat.mes-hall↗

arXiv2023

Atomic Layer-controlled Nonlinear Terahertz Valleytronics in Dirac Semi-metal and Semiconductor PtSe2

Platinum diselenide (PtSe2) is a promising two-dimensional (2D) material for the terahertz (THz) range as, unlike other transition metal dichalcogenides (TMDs), its bandgap can be uniquely tuned from a semiconductor in the near-infrared to a semimetal with the number of atomic layers. This gives the material unique THz photonic properties that can be layer-engineered. Here, we demonstrate that a controlled THz nonlinearity - tuned from monolayer to bulk PtSe2 - can be realised in wafer size polycrystalline PtSe2 through the generation of ultrafast photocurrents and the engineering of the bandstructure valleys. This is combined with the PtSe2 layer interaction with the substrate for a broken material centro-symmetry permitting a second order nonlinearity. Further, we show layer-dependent circular dichroism, where the sign of the ultrafast currents and hence the phase of the emitted THz pulse can be controlled through the excitation of different bandstructure valleys. In particular, we show that a semimetal has a strong dichroism that is absent in the monolayer and few layer semiconducting limit. The microscopic origins of this TMD bandstructure engineering is highlighted through detailed DFT simulations and show that circular dichroism can be controlled when PtSe2 becomes a semimetal and when the K-valleys can be excited. As well as showing that PtSe2 is a promising material for THz generation through layer controlled optical nonlinearities, this work opens up new class of circular dichroism materials beyond the monolayer limit that has been the case of traditional TMDs, and impacting a range of domains from THz valleytronics, THz spintronics to harmonic generation.

cond-mat.mtrl-sci↗

arXiv2021

Hyperfine Interaction in a MoS$_2$ Quantum Dot: Decoherence of a Spin-Valley Qubit

A successful and promising device for the physical implementation of electron spin-valley based qubits is the Transition Metal Dichalcogenide monolayer (TMD-ML) semiconductor quantum dot. The electron spin in TMD-ML semiconductor quantum dots can be isolated and controlled with high accuracy, but it still suffers from decoherence due to the unavoidable coupling with the surrounding environment, such as nuclear spin environments. A common tool to investigate systems like the one considered in this work is the density matrix formalism by presenting an exact master equation for a central spin (spin-qubit) system in a time-dependent and coupled to a nuclear spin bath in terms of hyperfine interaction. The master equation provides a unified description of the dynamics of the central spin. Analyzing this in more detail, we calculate fidelity loss due to the Overhauser field from hyperfine interaction in a wide range number of nuclear spins $\mathcal{N}$.

cond-mat.mes-hall↗

arXiv2021

Quantum beats of coherent 1s 2s excitons in two dimensional transition metal

Motivated by recent experimental measurement of the intrinsic excitonic wave-function in 2D Transition-metal dichalcogenides (TMDs) by angle-resolved photoemission spectroscopy (ARPES), we developed a theoretical study to resolve some characteristics of these excitons and some of the many open issues in these systems. The system is assumed to be embedded in an environment with average dielectric constant, below which electrostatic interactions in the corresponding TMD layer are screened. We adopt the long range approximation, which gives the electron-hole interaction in the Rytova - Keldysh form. Latter allows understanding the role of screening in TMDs structures. The bound state 1s 2s energy eigenvalues for the twodimensional are reformulated in momentum space leads to an integral form of the Wannier equation. The eigenfunctions are then expanded in terms of spherical harmonics. To evaluate the dynamic of the angle resolved photoemission spectrum arising from the dissociation of excitons given their steady states 1s 2s expressions, we follow the semi perturbative theoretical description developed by previous calculations. We discuss the dielectric environment effect on the dispersive features of the spectrum for different 1s and 2s exciton distributions. Quantum beat signatures in photoemission intensity demonstrate coherent coupling between 1s and 2s excitons. The beating contribution due to excitonic coherence is also discussed. The periodic oscillations arising from coherent superposition states, quantum beats, enable exploration of novel coherent phenomena.

cond-mat.mes-hall↗

arXiv2021

DFT+U Investigation of magnetocrystalline anisotropy of Mn-doped transition-metal dichalcogenides monolayers

Doped transition-metal dichalcogenides monolayers exhibit exciting magnetic properties for the benefit of two-dimensional spintronic devices. Using density functional theory (DFT) incorporating Hubbard-type of correction (DFT$+U$) to account for the electronic correlation, we study the magnetocrystalline anisotropy energy (MAE) characterizing Mn-doped MS$_2$ (M=Mo, W) monolayers. A single isolated Mn dopant exhibits a large perpendicular magnetic anisotropy of 35 meV (8 meV) in the case of Mn-doped WS$_2$ (MoS$_2$) monolayer. This value originates from the Mn in-plane orbitals degeneracy lifting due to the spin-orbit coupling. In pairwise doping, the magnetization easy axis changes to the in-plane direction with a weak MAE compared to single Mn doping. Our results suggest that diluted Mn-doped MS$_2$ monolayers, where the Mn dopants are well separated, could potentially be a candidate for the realization of ultimate nanomagnet units.

cond-mat.mtrl-sci↗

arXiv2020

Phonon-assisted exciton and trion conversion efficiency in transition metal Dichalcogenides

Photoluminescence spectra, shows that monolayer Transition-metal dichalcogenides (MLTMDCs), possess charged exciton binding energies, conspicuously similar to the energy of optical phonons. This enigmatic coincidence has offered opportunities to investigate many-body interactions between trion, exciton and phonon and led to efficient excitonic anti-Stokes processes with the potential for laser refrigeration and energy harvesting. In this study, we show that in WSe2 materials, the trion binding energy matches two phonon modes, the outofplane HP and the in-plane LO mode. In this respect, using the Fermi golden rule together with the effective mass approximation, we investigate the rate of the population transfers between exciton and trion, mediated by a single phonon. We demonstrate that, while the absolute importance of the two phonon modes on the upconversion process strongly depend on the experimental conditions such as the temperature and the dielectric environment (substrate), both modes lead to an up-conversion process on time scales in the range of few picoseconds to sub-nanosecond, consistent with recents experimental findings. The conjugate process is also investigated in our study, as a function of temperature and electron density . We prove that exciton to trion down-conversion process is very unlikely at low electron density and high temperature while it increases dramatically to reach few picoseconds time scale at low temperature and for electron density . Finally, our results show that conversion process occurs more rapidly in exemplary monolayer molybdenum-based dichalcogenides (MoSe2 and MoTe2) than tungsten dichalcogenides .

cond-mat.mes-hall↗

arXiv2020

Optical properties of exciton in two-dimensional transition metal dichalcogenide nanobubbles

Strain in two-dimensional (2D) transition metal dichalcogenide (TMD) has led to localized states with exciting optical properties, in particular in view of designing one photon sources. The naturally formed of the MoS2 monolayer deposed on hBN substrate leads to a reduction of the bandgap in the strained region creating a nanobubble. The photogenerated particles are thus confined in the strain-induced potential. Using numerical diagonalization, we simulate the spectra of the confined exciton states, their oscillator strengths and radiative lifetimes. We show that a single state of the confined exciton is optically active, which suggests that the MoS2/hBN nanobubble is a good candidate for the realisation of single-photon sources. Furthermore, the exciton binding energy, oscillator strength and radiative lifetime are enhanced due to the confinement effect.

cond-mat.mtrl-sci↗

arXiv2019

Dependence of the magnetic interactions in MoS$_2$ monolayer on Mn-doping configurations

Understanding the magnetic properties of the various Mn doping configurations that can be encountered in $2H$-MoS$_2$ monolayer could be beneficial for its use in spintronics. Using density functional theory plus Hubbard U (DFT$+$U) approach, we study how a single isolated, double- and triple-substitution configurations of Mn atoms within a MoS$_2$ monolayer could contribute to its total magnetization. We find that the doping-configuration plays a critical role in stabilizing a ferromagnetic state in a Mn-doped MoS$_2$ monolayer. Indeed, the Mn-Mn magnetic interaction is found to be ferromagntic and strong for Mn in equidistant substitution positions where the separation average range of 6-11 Å. The strongest ferromagnetic interaction is found when substitutions are in second nearest neighbors Mo-sites of the armchair chain. Clustering is energetically favorable but it strongly reduces the ferromagnetic exchange energies. Our results suggest that ordering the Mn dopants on MoS$_2$ monolayer is needed to increase its potential ferromagnetism.

cond-mat.mtrl-sci↗

arXiv2018

Exciton-Polaritons Dynamics of a Monolayer Tungsten Disulphide (WS2) Coupled to a Semiconductor Microcavity

We present a theoretical model that allows us to describe the dynamics of exciton polaritons in the strong-coupling regime in a monolayer WS2 based semiconductor microcavity. Numerical simulations using Boltzmann equations give an overall description of polariton kinetics over a temperature range of 130-230K, under nonresonant excitation. Here, only the scattering rate via optical phonons (LO) from the upper (UP) towards lower (LP) polariton branches is considered. According this model we show the importance of the radiative lifetime of the polaritonic states relative to polariton relaxation rate. Our results show as the cavity detuning changes from negative (130 K) to positive (230 K) values, the UP branch can be tuned from a more excitonlike to a more photonlike state at small wave vector k. We deduce that the UP states have a faster lifetime and the polariton relaxation time into the LP energy states becomes very efficient. Thus, the UP occupation starts to decrease, and we observe a simultaneous increase in the occupation of the LP branch. Furthermore, the polariton states are less occupied for small excitation pump rate, the calculated behavior is linear. In the high pumping regime, we observe the nonlinear behavior of cavity polaritons and occupation factors much larger than unity are reached.

cond-mat.mes-hall↗

arXiv2018

Radiative lifetime of localized excitons in transition metal dichalcogenides

Disorder derived from defects or strain in monolayer TMDs can lead to a dramatic change in the physical behavior of the interband excitations, producing inhomogeneous spectral broadening and localization; leading to radiative lifetime increase. In this study, we have modeled the disorder in the surface of the sample through a randomized potential in monolayer WSe2. We show that this model allows us to simulate the spectra of localized exciton states as well as their radiative lifetime. In this context, we give an in depth study of the influence of the disorder potential parameters on the optical properties of these defects through energies, density of states, oscillator strengths, photoluminescence (PL) spectroscopy and radiative lifetime at low temperature (4K). We demonstrate that localized excitons have a longer emission time than free excitons, in the range of tens of picoseconds or more, and we show that it depends strongly on the disorder parameter and dielectric environment. Finally, in order to prove the validity of our model we compare it to available experimental results of the literature.

cond-mat.mes-hall↗

arXiv2017

Comparative study of the binding energy in a thin and ultra-thin organic-inorganic perovskite within dielectric mismatches effects

The multi-quantum well (MQW) organic-inorganic perovskite offer an approach of tuning the exciton binding energy based on the well-barrier dielectric mismatch effect, which called the image charge effect. The exfoliation from MQW organic-inorganic perovskite forms a twodimensional (2D) nano-sheet. As with other 2D materials, like graphene or transition metal dichalcogenides (TMDs), the ultra-thin perovskites layers are highly sensitive to the dielectric environment. We investigate the ultrathin crystalline 2D van-der-Waals (vdW) layers of organic-inorganic perovskite crystals close to a surface of the substrate. We show that binding exciton energy is strongly influenced by the surrounding dielectric environment. We find that the Keldysh model somehow estimates the strong dependence of the exciton binding energies on environmental screening. We compare our binding energies results with experimental results in the (C6H13NH3)2PbI4 perovskite, and we estimate the binding energy values of (C4H9NH3)2PbBr4.

cond-mat.mes-hall↗

arXiv2016

Exciton, trion and localized exciton in monolayer Tungsten Disulfide

The ultrathin transition metal dichalcogenides (TMDs) have emerged as promising materials for various applications using two dimensional (2D) semiconductors. They have attracted increasing attention due to their unique optical properties originate from neutral and charged excitons. Here, we report negatively charged exciton formation in monolayer TMDs, notably tungsten disulfide WS2. Our theory is based on an effective mass model of neutral and charged excitons, parameterized by ab-initio calculations. Taking into the account the strong correlation between the monolayer WS2 and the surrounding dielectric environment, our theoretical results are in good agreement with one-photon photoluminescence (PL) and reflectivity measurements. We also show that the exciton state with p-symmetry, experimentally observed by two-photon PL emission, is energetically below the 2s-state. We use the equilibrium mass action law, to quantify the relative weight of exciton and trion PL. We show that exciton and trion emission can be tuned and controlled by external parameters like temperature, pumping and injection electrons. Finally, in comparison with experimental measurements, we show that exciton emission in monolayer tungsten dichalcogenides is substantially reduced. This feature suggests that free exciton can be trapped in disordered potential wells to form a localized exciton and therefore offers a route toward novel optical properties.

cond-mat.mes-hall↗