Search query: strongly bound excitons in gaples two dimensional

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abstract: Monolayer and few layer phosphorene are anisotropic quasi two dimensional (quasi 2D) van der Waals (vdW) semiconductors with a linear dichroic light matter interaction and a widely tunable direct band gap in the infrared frequency range. Despite recent theoretical predictions of strongly bound excitons with unique properties, it remains experimentally challenging to probe the excitonic quasiparticles due to the severe oxidation during device fabrication. In this study, we report observation of strongly bound excitons and trions with highly anisotropic optical properties in intrinsic bilayer phosphorene, which are protected from oxidation by encapsulation with hexagonal boron nitride (hBN) in a field effect transistor (FET) geometry. Reflection contrast and photoluminescence spectroscopy clearly reveal the linear dichroic optical spectra from anisotropic excitons and trions in the hBN encapsulated bilayer phosphorene. The optical resonances from the exciton Rydberg series indicate that the neutral exciton binding energy is over 100 meV even with the dielectric screening from hBN. The electrostatic injection of free holes enables an additional optical resonance from a positive trion (charged exciton) 30 meV below the optical bandgap of the charge neutral system. Our work shows exciting possibilities for monolayer and few layer phosphorene as a platform to explore many body physics and novel photonics and optoelectronics based on strongly bound excitons with two fold anisotropy.
author_names: Sangho Yoon

Taeho Kim

Seung-Young Seo

Seung-Hyun Shin

Su-Beom Song

B J Kim

Kenji Watanabe

Takashi Taniguchi

Gil-Ho Lee

Moon-Ho Jo

Diana Yuan Qiu

Jonghwan Kim
title: Strongly bound excitons and trions in anisotropic 2D semiconductors
year: 2020

abstract: Common wisdom asserts that bound excitons cannot form in high dimensional (d>1) metallic structures because of their overwhelming screening and unavoidable resonance with nearby continuous bands. Strikingly, here we illustrate that this prevalent assumption is not quite true. A key ingredient that has been overlooked is that of viable decoherence that thwarts the formation of resonances. As an example of this general mechanism, we focus on an experimentally relevant material and predict bound excitons in twisted bilayer graphene, which is a two dimensional gapless structure exhibiting metallic screening. The binding energies calculated by first principles simulations are surprisingly large. The low energy effective model reveals that these bound states are produced by a unique destructive coherence between two alike subband resonant excitons. In particular, this destructive coherent effect is not sensitive to the screening and dimensionality, and hence may persist as a general mechanism for creating bound excitons in various metallic structures, opening the door for excitonic applications based on metallic structures.
author_names: Yufeng Liang

Ryan Soklaski

Shouting Huang

Matt W Graham

Robin W Havener

Jiwoong Park

Li Yang
title: Strongly bound excitons in gapless two dimensional structures
year: 2014

abstract: Moire superlattices open an unprecedented opportunity for tailoring interactions between quantum particles and their coupling to electromagnetic fields. Strong superlattice potential generates moire minibands of excitons bound pairs of electrons and holes that reside either in a single layer (intralayer excitons) or two separate layers (interlayer excitons) The twist angle controlled interlayer hybridization of carriers can also mix the two types of excitons to combine the strengths of both. Here, we report a direct observation of spatially mixed moire excitons in angle aligned WSe2/WS2 and MoSe2/WS2 superlattices by optical reflectance spectroscopy. The strongly interacting interlayer and intralayer moire excitons in WSe2/WS2 manifest energy level anticrossing and oscillator strength redistribution under a vertical electric field. We also observe doping dependent exciton miniband renormalization and mixing near half filling of the first electron miniband of WS2. Our findings have significant implications for emerging correlated states in two dimensional semiconductors, such as exciton condensates and Bose Hubbard models, and optoelectronic applications of these materials.
author_names: Yanhao Tang

Jie Gu

Song Liu

Kenji Watanabe

Takashi Taniguchi

James C Hone

Kin Fai Mak

Jie Shan
title: Spatially mixed moir\'e excitons in two dimensional van der Waals superlattices
year: 2020

abstract: Generation of photocurrents in semiconducting materials requires dissociation of excitons into free charge carriers. While thermal agitation is sufficient to induce dissociation in most bulk materials, an additional push is required to induce efficient dissociation of the strongly bound excitons in monolayer transition metal dichalcogenides (TMDs) Recently, static in plane electric fields have proven to be a promising candidate. In the present paper, we introduce a numerical procedure, based on exterior complex scaling, capable of computing field induced exciton dissociation rates for a wider range of field strengths than previously reported in literature. We present both Stark shifts and dissociation rates for excitons in various TMDs calculated within the Mott Wannier model. Here, we find that the field induced dissociation rate is strongly dependent on the dielectric screening environment. Furthermore, applying weak field asymptotic theory (WFAT) to the Keldysh potential, we are able to derive an analytical expression for exciton dissociation rates in the weak field region.
author_names: Hogni C Kamban

Thomas Garm Pedersen
title: Field induced dissociation of two dimensional excitons in transition metal dichalcogenides
venue: Physical Review B
year: 2019

abstract: Anatase TiO2 is among the most studied materials for light energy conversion applications, but the nature of its fundamental charge excitations is still unknown. Yet it is crucial to establish whether light absorption creates uncorrelated electron hole pairs or bound excitons and, in the latter case, to determine their character. Here, by combining steady state angle resolved photoemission spectroscopy and spectroscopic ellipsometry with state of the art ab initio calculations, we demonstrate that the direct optical gap of single crystals is dominated by a strongly bound exciton rising over the continuum of indirect interband transitions. This exciton possesses an intermediate character between the Wannier Mott and Frenkel regimes and displays a peculiar two dimensional wavefunction in the three dimensional lattice. The nature of the higher energy excitations is also identified. The universal validity of our results is confirmed up to room temperature by observing the same elementary excitations in defect rich samples (doped single crystals and nanoparticles) via ultrafast two dimensional deep ultraviolet spectroscopy.Here the authors combine steady state angle resolved photoemission spectroscopy, ellipsometry and ultrafast two dimensional ultraviolet spectroscopy to examine the role of many body correlations in anatase TiO2, revealing the existence of strongly bound excitons in single crystals and nanoparticles.
author_names: Edoardo Baldini

Letizia Chiodo

A Dominguez

Maurizia Palummo

Simon Moser

Meghdad Yazdi-Rizi

Gerald Aubock

B P P Mallett

Helmuth Berger

Arnaud Magrez

Christian Bernhard

M Grioni

Angel Rubio

Majed Chergui
title: Strongly bound excitons in anatase TiO2 single crystals and nanoparticles
venue: Nature Communications
year: 2017

abstract: Two dimensional (2D) semiconducting materials are promising building blocks for optoelectronic applications, many of which require efficient dissociation of excitons into free electrons and holes. However, the strongly bound excitons arising from the enhanced Coulomb interaction in these monolayers suppresses the creation of free carriers. Here, we identify the main exciton dissociation mechanism through time and spectrally resolved photocurrent measurements in a monolayer WSe2p n junction. We find that under static in plane electric field, excitons dissociate at a rate corresponding to the one predicted for tunnel ionization of 2D Wannier Mott excitons. This study is essential for understanding the photoresponse of 2D semiconductors and offers design rules for the realization of efficient photodetectors, valley dependent optoelectronics, and novel quantum coherent phases.In two dimensional semiconductors excitons are strongly bound, suppressing the creation of free carriers. Here, the authors investigate the main exciton dissociation pathway in p n junctions of monolayer WSe2 by means of time and spectrally resolved photocurrent measurements.
author_names: Mathieu Massicotte

Fabien Vialla

P Schmidt

Mark B Lundeberg

Simone Latini

Sten Haastrup

Mark Danovich

Diana Davydovskaya

Kenji Watanabe

Takashi Taniguchi

Vladimir I Fal'ko

Kristian Sommer Thygesen

Thomas Garm Pedersen

Frank H L Koppens
title: Dissociation of two dimensional excitons in monolayer WSe2
venue: Nature Communications
year: 2018

abstract: Using the quasiclassical concept of Berry curvature we demonstrate that a Dirac exciton a pair of Dirac quasiparticles bound by Coulomb interactions inevitably possesses an intrinsic angular momentum making the exciton effectively self rotating. The model is applied to excitons in two dimensional transition metal dichalcogenides, in which the charge carriers are known to be described by a Dirac like Hamiltonian. We show that the topological self rotation strongly modifies the exciton spectrum and, as a consequence, resolves the puzzle of the overestimated two dimensional polarizability employed to fit earlier spectroscopic measurements.
author_names: Maxim Trushin

Mark Oliver Goerbig

Wolfgang Belzig
title: Model Prediction of Self Rotating Excitons in Two Dimensional Transition Metal Dichalcogenides.
venue: Physical review letters
year: 2018

abstract: Two dimensional hybrid organic inorganic perovskites with strongly bound excitons and tunable structures are desirable for optoelectronic applications. Exciton transport and annihilation are two key processes in determining device efficiencies; however, a thorough understanding of these processes is hindered by that annihilation rates are often convoluted with exciton diffusion constants. Here we employ transient absorption microscopy to disentangle quantum well thickness dependent exciton diffusion and annihilation in two dimensional perovskites, unraveling the key role of electron hole interactions and dielectric screening. The exciton diffusion constant is found to increase with quantum well thickness, ranging from 0.06 0.03 to 0.34 0.03 cm 2 s 1 which leads to long range exciton diffusion over hundreds of nanometers. The exciton annihilation rates are more than one order of magnitude lower than those found in the monolayers of transition metal dichalcogenides. The combination of long range exciton transport and slow annihilation highlights the unique attributes of two dimensional perovskites as an exciting class of optoelectronic materials. Two dimensional hybrid perovskites are promising excitonic materials; however, there currently lacks understanding on exciton diffusion and annihilation. Here Deng et al. employ transient absorption microscopy to disentangle quantum well thickness dependent exciton transport and annihilation in these materials.
author_names: Shibin Deng

Enzheng Shi

Long Yuan

Linrui Jin

Letian Dou

Libai Huang
title: Long range exciton transport and slow annihilation in two dimensional hybrid perovskites
venue: Nature Communications
year: 2020

abstract: Abstract We studied theoretically the screening of the Coulomb interaction in the 2D layered halide organic inorganic perovskite PEPI. We show that the screening reduces considerably the exciton binding energy at high temperature, acting on the displacement of the equilibrium exciton plasma densities for the studied system. The presence of strongly bound excitons and trions in the perovskite layers is predicted by an analytical calculation using a mass action law and we discuss the trion formation with excess background carriers. Our study gives some tracks to find the right samples and the right experimental conditions to observe the trions in such materials.
author_names: Haitham Zahra

Sihem Jaziri
title: Screened excitons and trions by free carriers in the two dimensional perovskite structure
venue: Physica E: Low dimensional Systems and Nanostructures
year: 2019

abstract: While polarons charges bound to a lattice deformation induced by electron phonon coupling are primary photoexcitations in bulk metal halide hybrid organic inorganic perovskites (HOIP) excitons Coulomb bound electron hole pairs are the stable quasi particles in their two dimensional (2D) analogues. However, are polaronic effects consequential for excitons in 2D HOIPs? We argue that these are manifested intrinsically in the exciton spectral structure, which is comprised of multiple non degenerate resonances with constant inter peak energy spacing. We highlight population and dephasing dynamics that point to the apparently deterministic role of polaronic effects. We contend that an interplay of long range and short range exciton lattice couplings give rise to exciton polarons, which fundamentally establishes their effective mass and radius, and consequently, their quantum dynamics. Finally, we highlight opportunities for the community to develop the rigorous description of exciton polarons in 2D HOIPs to advance their fundamental understanding as model systems for condensed phase materials with strong lattice mediated correlations.
author_names: Ajay Ram Srimath Kandada

Carlos Silva
title: Exciton Polarons in Two Dimensional Hybrid Metal Halide Perovskites.
venue: The journal of physical chemistry letters
year: 2020