Wannier Excitons Confined in Hexagonal Boron Nitride Triangular Quantum Dots

With the ever-growing interest in quantum computing, understanding the behaviour of excitons in monolayer quantum dots has become a topic of great relevance. In this paper, we consider a Wannier exciton confined in a triangular quantum dot of hexagonal Boron Nitride. We begin by outlining the adequate basis functions to describe a particle in a triangular enclosure, analyzing their degeneracy and symmetries. Afterwards, we discuss the excitonic hamiltonian inside the quantum dot and study the influence of the quantum dot dimensions on the excitonic states.Comment: 5 figure

Contribution to excitonic linewidth from free carrier–exciton scattering in layered materials: the example of hBN

Scattering of excitons by free carriers is a phenomenon, which is especially important when considering moderately to heavily doped semiconductors in low-temperature experiments, where the interaction of excitons with acoustic and optical phonons is reduced. In this paper, we consider the scattering of excitons by free carriers in monolayer hexagonal boron nitride encapsulated by a dielectric medium. We describe the excitonic states by variational wave functions, modeling the electrostatic interaction via the Rytova–Keldysh potential. Making the distinction between elastic and inelastic scattering, the relevance of each transition between excitonic states is also considered. Finally, we discuss the contribution of free carrier scattering to the excitonic linewidth, analyzing both its temperature and carrier density dependence.M.F.C.M.Q. acknowledges the International Nanotechnology Laboratory (INL) and the Portuguese Foundation for Science and Technology (FCT) for the Quantum Portugal Initiative (QPI) grant SFRH/BD/151114/2021. N.M.R.P. acknowledges support by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UIDB/04650/2020, COMPETE 2020, PORTUGAL 2020, FEDER, and FCT through projects PTDC/FIS-MAC/2045/2021, EXPL/FIS-MAC/0953/ 2021, and from the European Commission through the project Graphene Driven Revolutions in ICT and Beyond (Ref. No. 881603, CORE 3)

A colloquium on the variational method applied to excitons in 2D materials

In this colloquium, we review the research on excitons in van der Waals heterostructures from the point of view of variational calculations. We first make a presentation of the current and past literature, followed by a discussion on the connections between experimental and theoretical results. In particular, we focus our review of the literature on the absorption spectrum and polarizability, as well as the Stark shift and the dissociation rate. Afterwards, we begin the discussion of the use of variational methods in the study of excitons. We initially model the electron-hole interaction as a soft-Coulomb potential, which can be used to describe interlayer excitons. Using an \emph{ansatz}, based on the solution for the two-dimensional quantum harmonic oscillator, we study the Rytova-Keldysh potential, which is appropriate to describe intralayer excitons in two-dimensional (2D) materials. These variational energies are then recalculated with a different \emph{ansatz}, based on the exact wavefunction of the 2D hydrogen atom, and the obtained energy curves are compared. Afterwards, we discuss the Wannier-Mott exciton model, reviewing it briefly before focusing on an application of this model to obtain both the exciton absorption spectrum and the binding energies for certain values of the physical parameters of the materials. Finally, we briefly discuss an approximation of the electron-hole interaction in interlayer excitons as an harmonic potential and the comparison of the obtained results with the existing values from both first--principles calculations and experimental measurements.The authors thank Eduardo Castro and Joao Lopes dos Santos for comments on a preliminary version of this work, and Bruno Amorim for outlining the derivation in the Appendix A. N.M.R.P. acknowledges support from the European Commission through the project Graphene-Driven Revolutions in ICT and Beyond (Ref. No. 881603 - core 3), and the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Financing UID/FIS/04650/2019. In addition, funding from the projects POCI-01-0145-FEDER-028114, and POCI01-0145-FEDER-029265, and PTDC/NAN-OPT/29265/2017, and POCI-01-0145-FEDER-02888 is acknowledged

Coexistence of Paramagnetic-Charge-Ordered and Ferromagnetic-Metallic Phases in La0.5Ca0.5MnO3 evidenced by ESR

Throughout a complete Electron Spin Resonance (ESR) and magnetization study of La0.5Ca0.5MnO3, we discuss about the nature of the complex phase-segregated state established in this compound below T~210 K. Between TN<T<TC, the ESR spectra shows two lines characteristic of two different magnetic phases. From the resonance field (Hr) derived for each line we argue that the incommensurate-charge-ordering phase (ICO) which coexists with ferromagnetic-metallic (FMM) clusters in this temperature interval, is mainly paramagnetic and not antiferromagnetic. The FMM/ICO ratio can be tuned with a relatively small field, which suggests that the internal energy associated with those phases is very similar. Below TN, there is an appreciable FM contribution to the magnetization and the ESR spectra indicates the presence of FM clusters in an antiferromagnetic matrix (canted). Our results show that ESR could be a very useful tool to investigate the nature of the phase-separated state now believed to play a fundamental role in the physics of mixed valent manganites.Comment: 6 pages, 6 figure

Microscopic theory of the polarizability of transition metal dichalcogenides excitons: Application to WSe2

In this paper we develop a fully microscopic theory of the polarizability of excitons in transition metal dichalcogenides. We apply our method to the description of the excitation $2$p dark states. These states are not observable in absorption experiments but can be excited in a pump-probe experiment. As an example we consider $2$p dark states in WSe\textsubscript{2}. We find a good agreement between recent experimental measurements and our theoretical calculations.Comment: 10 pages, 4 figures; we welcome comments and requests for adding relevant references to the topi

Theory of third-order polarizability of interlayer excitons due to intra-excitonic energy level transitions

n this paper, we employ a fully microscopic approach to the study of interlayer excitons in layered materials. We discuss the utilization of Fowler's and Karplus' method to access the dynamical polarizability of non--interacting interlayer excitons in a $\mathrm{WSe}_{2}/\mathrm{WS}_{2}$--based van der Waals heterostructure. Following from the calculation of the linear polarizability, we consider Svendsen's variational method to the dynamic third--order polarizability. With this variational method, we study both two--photon absorption and third--harmonic generation processes for interlayer excitons in a $\mathrm{WSe}_{2}/\mathrm{WS}_{2}$ hetero--bilayer, discussing the various intra--excitonic energy level transitions observed.Comment: 13 page

Efeito da planta hospedeira na suscetibilidade da mosca-branca, Bemisia tabaci biótipo B a inseticidas sintéticos.

Estudos têm demonstrado que a planta hospedeira pode afetar a susceptibilidade da Bemisia tabaci a inseticidas ou induzir a produção de enzimas específicas no inseto que são altamente efetivas em detoxificar inseticidas sintéticos. Baseado nessas evidências foi conduzido experimento para determinar o efeito da soja, algodão e feijão na suscetibilidade de B. tabaci a thiamethoxam 250 WG a 200 g p.c./ha, acefato 750 BR a 600 g p.c/ha e endosulfan 350 EC a 1500 ml p.c./ha.Resumo

Interação de Beauveria bassiana e óleo de mamona na mortalidade de ninfas de Bemisia tabaci biótipo B.

Fungos entomopatogênicos e inseticidas vegetais são considerados a lternativas promissoras aos inseticidas sintéticos no manejo da mosca-branca. A utilização associada destas estratégias pode potencializar a eficiência de controle da praga. Neste sentido, foi estudada a mistura de Beuveria bassiana com o óleo de mamona Azevedo® na mortalidade de ninfas de Bemisia tabaci biótipo B

Patogenicidade de Metarhizium anisopliae a ovos e ninfas de Tibraca limbativentris (Heteroptera: pentatomidae).

Foi avaliado o efeito do Metarhizium anisopliae isolado CG 168 sobre ovos e ninfas do 1 e 2 instares do percevejo-do-colmo, Tibraca limbativentris, praga importante do arroz irrigado no Brasil

Efeito de adjuvantes químicos na mortalidade de adultos de Bemisia tabaci Biótipo B no feijoeiro comum.

Os adjuvantes, cuja finalidade é de conferir à calda de inseticidas químicos propriedades que mantenham a estabilidade da mesma, têm sido indicados para o controle de mosca-branca Bemisia tabaci biótipo B. Desta forma, este trabalho teve como objetivo avaliar a eficiência de adjuvantes no controle de adultos de mosca-branca no feijoeiro-comum