725 research outputs found
Anisotropic linear and non-linear excitonic optical properties of buckled monolayer semiconductors
The optical properties of two-dimensional materials are exceptional in
several respects. They are highly anisotropic and frequently dominated by
excitonic effects. Dipole-allowed second order non-linear optical properties
require broken inversion symmetry. Hence, several two-dimensional materials
show strong in-plane (IP) non-linearity but negligible out-of-plane (OOP)
response due to vertical symmetry. By considering buckled hexagonal monolayers,
we analyze the critical role of broken vertical symmetry on their excitonic
optical response. Both linear as well as second order shift current and second
harmonic response are studied. We demonstrate that substantial OOP non-linear
response can be obtained, in particular, through off-diagonal tensor elements
coupling IP excitation to OOP response. Our findings are explained by excitonic
selection rules for OOP response and the impact of dielectric screening on
excitons is elucidated.Comment: 20 pages, 8 figure
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
Optimización y revalidación del análisis de nitrógeno por el método
En el presente artÃculo, se ha optimizado y se ha revalidado el procedimiento de análisis de nitrógeno en muestras de taurina mediante el método Kjeldahl. La Asociación Española de Farmacéuticos de la Industria (AEFI) propone un modelo de cálculo que permite determinar el número de replicados a realizar, en función del coeficiente de variación de la repetibilidad del procedimiento (C.V.%) y el valor lÃmite aceptado, que en el caso de la determinación de la pureza, corresponde al valor máximo o mÃnimo de las especificaciones de la muestra. Al aplicar dicho modelo, considerando la incertidumbre de calibración del equipo, como el mÃnimo valor posible para el C.V.%, y teniendo en cuenta las especificaciones establecidas para la taurina en la U.S. Pharmacopeia 30, se obtiene que el número de replicados a realizar es de cinco o seis, según las condiciones de trabajo
A Polynomial Approach to the Spectrum of Dirac-Weyl Polygonal Billiards
The Schr\"odinger equation in a square or rectangle with hard walls is solved
in every introductory quantum mechanics course. Solutions for other polygonal
enclosures only exist in a very restricted class of polygons, and are all based
on a result obtained by Lam\'e in 1852. Any enclosure can, of course, be
addressed by finite element methods for partial differential equations. In this
paper, we present a variational method to approximate the low-energy spectrum
and wave-functions for arbitrary convex polygonal enclosures, developed
initially for the study of vibrational modes of plates. In view of the recent
interest in the spectrum of quantum dots of two dimensional materials,
described by effective models with massless electrons, we extend the method to
the Dirac-Weyl equation for a spin-1/2 fermion confined in a quantum billiard
of polygonal shape, with different types of boundary conditions. We illustrate
the method's convergence in cases where the spectrum in known exactly and apply
it to cases where no exact solution exists.Comment: 17 pages, 10 figure
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
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