Computational methods for 2D materials modelling

Materials with thickness ranging from a few nanometers to a single atomic layer present unprecedented opportunities to investigate new phases of matter constrained to the two-dimensional plane.Particle-particle Coulomb interaction is dramatically affected and shaped by the dimensionality reduction, driving well-established solid state theoretical approaches to their limit of applicability. Methodological developments in theoretical modelling and computational algorithms, in close interaction with experiments, led to the discovery of the extraordinary properties of two-dimensional materials, such as high carrier mobility, Dirac cone dispersion and bright exciton luminescence, and inspired new device design paradigms. This review aims to describe the computational techniques used to simulate and predict the optical, electronic and mechanical properties of two-dimensional materials, and to interpret experimental observations. In particular, we discuss in detail the particular challenges arising in the simulation of two-dimensional constrained fermions, and we offer our perspective on the future directions in this field.Comment: This submission does not include the third party cited figure

Designer quantum states of matter created atom-by-atom

With the advances in high resolution and spin-resolved scanning tunneling microscopy as well as atomic-scale manipulation, it has become possible to create and characterize quantum states of matter bottom-up, atom-by-atom. This is largely based on controlling the particle- or wave-like nature of electrons, as well as the interactions between spins, electrons, and orbitals and their interplay with structure and dimensionality. We review the recent advances in creating artificial electronic and spin lattices that lead to various exotic quantum phases of matter, ranging from topological Dirac dispersion to complex magnetic order. We also project future perspectives in non-equilibrium dynamics, prototype technologies, engineered quantum phase transitions and topology, as well as the evolution of complexity from simplicity in this newly developing field

Breakdown of the static picture of defect energetics in halide perovskites: the case of the Br vacancy in CsPbBr3

We consider the Br vacancy in CsPbBr3 as a prototype for the impact of structural dynamics on defect energetics in halide perovskites (HaPs). Using first-principles molecular dynamics based on density functional theory, we find that the static picture of defect energetics breaks down; the energy of the Br vacancy level is found to be intrinsically dynamic, oscillating by as much as 1 eV on the ps time scale at room temperature. These significant energy fluctuations are correlated with the distance between the neighboring Pb atoms across the vacancy and with the electrostatic potential at these Pb atomic sites. We expect this unusually strong coupling of structural dynamics and defect energetics to bear important implications for both experimental and theoretical analysis of defect characteristics in HaPs. It may also hold significant ramifications for carrier transport and defect tolerance in this class of photovoltaic materials.Comment: 5 figures, 1 tabl

Theory and modeling of light-matter interactions in chemistry: current and future

Light-matter interaction not only plays an instrumental role in characterizing materials' properties via various spectroscopic techniques but also provides a general strategy to manipulate material properties via the design of novel nanostructures. This perspective summarizes recent theoretical advances in modeling light-matter interactions in chemistry, mainly focusing on plasmon and polariton chemistry. The former utilizes the highly localized photon, plasmonic hot electrons, and local heat to drive chemical reactions. In contrast, polariton chemistry modifies the potential energy curvatures of bare electronic systems, and hence their chemistry, via forming light-matter hybrid states, so-called polaritons. The perspective starts with the basic background of light-matter interactions, molecular quantum electrodynamics theory, and the challenges of modeling light-matter interactions in chemistry. Then, the recent advances in modeling plasmon and polariton chemistry are described, and future directions toward multiscale simulations of light-matter interaction-mediated chemistry are discussed

Coherent exciton-vibrational dynamics and energy transfer in conjugated organics

Coherence, signifying concurrent electron-vibrational dynamics in complex natural and man-made systems, is currently a subject of intense study. Understanding this phenomenon is important when designing carrier transport in optoelectronic materials. Here, excited state dynamics simulations reveal a ubiquitous pattern in the evolution of photoexcitations for a broad range of molecular systems. Symmetries of the wavefunctions define a specific form of the non-adiabatic coupling that drives quantum transitions between excited states, leading to a collective asymmetric vibrational excitation coupled to the electronic system. This promotes periodic oscillatory evolution of the wavefunctions, preserving specific phase and amplitude relations across the ensemble of trajectories. The simple model proposed here explains the appearance of coherent exciton-vibrational dynamics due to non-adiabatic transitions, which is universal across multiple molecular systems. The observed relationships between electronic wavefunctions and the resulting functionalities allows us to understand, and potentially manipulate, excited state dynamics and energy transfer in molecular materials.Fil: Nelson, Tammie R.. Los Alamos National Laboratory; Estados UnidosFil: Ondarse Alvarez, Dianelys. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes; ArgentinaFil: Oldani, Andres Nicolas. Universidad Nacional de Quilmes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Rodríguez Hernández, Beatriz. Universidad Nacional de Quilmes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Alfonso Hernandez, Laura. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes; ArgentinaFil: Galindo, Johan F.. Universidad Nacional de Colombia; ColombiaFil: Kleiman, Valeria D.. University of Florida; Estados UnidosFil: Fernández Alberti, Sebastián. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes; ArgentinaFil: Roitberg, Adrián. University of Florida; Estados UnidosFil: Tretiak, Sergei. Los Alamos National Laboratory; Estados Unido

Correlated electron-hole plasma in organometal perovskites

Organic-inorganic perovskites are a class of solution-processed semiconductors holding promise for the realization of low-cost efficient solar cells and on-chip lasers. Despite the recent attention they have attracted, fundamental aspects of the photophysics underlying device operation still remain elusive. Here we use photoluminescence and transmission spectroscopy to show that photoexcitations give rise to a conducting plasma of unbound but Coulomb-correlated electron-hole pairs at all excitations of interest for light-energy conversion and stimulated optical amplification. The conductive nature of the photoexcited plasma has crucial consequences for perovskite-based devices: in solar cells, it ensures efficient charge separation and ambipolar transport while, concerning lasing, it provides a low threshold for light amplification and justifies a favourable outlook for the demonstration of an electrically driven laser. We find a significant trap density, whose cross-section for carrier capture is however low, yielding a minor impact on device performance

Next-generation single-photon sources using two-dimensional hexagonal boron nitride

With the second quantum revolution unfolding, the realization of optical quantum technologies will transform future information processing, communication, and sensing. One of the crucial building blocks of quantum information architectures is a single-photon source. Promising candidates for such quantum light sources are quantum dots, trapped ions, color centers in solid-state crystals, and sources based on heralded spontaneous parametric down-conversion. The recent discovery of optically active defects hosted by 2D materials has added yet another class to the solid-state quantum emitters. Stable quantum emitters have been reported in semiconducting transition metal dichalcogenides (TMDs) and in hexagonal boron nitride (hBN). Owing to the large band gap, the energy levels of defects in hBN are well isolated from the band edges. In contrast to TMDs, this allows for operation at room temperature and prevents non-radiative decay, resulting in a high quantum yield. Unlike NV centers in diamond and other solid-state quantum emitters in 3D systems, the 2D crystal lattice of hBN allows for an intrinsically ideal extraction efficiency. In this thesis, advances in developing this new type of emitter are described. In the first experiment, quantum emitters hosted by hBN are attached by van der Waals force to the core of multimode fibers. The system features a free space and fiber-coupled single-photon generation mode. The results can be generalized to waveguides and other on-chip photonic quantum information processing devices, thus providing a path toward integration with photonic networks. Next, the fabrication process, based on a microwave plasma etching technique, is substantially improved, achieving a narrow emission linewidth, high single-photon purity, and a significant reduction of the excited state lifetime. The defect formation probability is influenced by the plasma conditions, while the emitter brightness correlates with the annealing temperature. Due to their low size, weight and power requirements, the quantum emitters in hBN are promising candidates as light sources for long-distance satellite-based quantum communication. The next part of this thesis focuses on the feasibility of using these emitters as a light source for quantum key distribution. The necessary improvement in the photon quality is achieved by coupling an emitter with a microcavity in the Purcell regime. The device is characterized by a strong increase in spectral and single-photon purity and can be used for realistic quantum key distribution, thereby outperforming efficient state-of-the-art decoy state protocols. Moreover, the complete source is integrated on a 1U CubeSat, a picoclass satellite platform encapsulated within a cube of length 10cm. This makes the source among the smallest, fully self-contained, ready-to-operate single-photon sources in the world. The emitters are also space-qualified by exposure to ionizing radiation. After irradiation with gamma-rays, protons and electrons, the quantum emitters show negligible change in photophysics. The space certification study is also extended to other 2D materials, suggesting robust suitability for use of these nanomaterials for space instrumentation. Finally, since the nature of the single-photon emission is still debated and highly controversial, efforts are made to locate the defects with atomic precision. The positions at which the defects form correlate with the fabrication method. This allows one to engineer the emitters to be close to the surface, where high-resolution electron microscopy can be utilized to identify the chemical defect. The results so far prove that quantum emitters in hBN are well suited for quantum information applications and can also be integrated on satellite platforms. A device based around this technology would thus provide an excellent building block for a worldwide quantum internet, where metropolitan fiber networks are connected through satellite relay stations