Experimental measurement of the intrinsic excitonic wavefunction
Michael K. L. Man, Julien Madéo, Chakradhar Sahoo, Kaichen Xie, Marshall Campbell, Vivek Pareek, Arka Karmakar, E Laine Wong, Abdullah Al-Mahboob, Nicholas S. C… 展开作者
Michael K. L. Man, Julien Madéo, Chakradhar Sahoo, Kaichen Xie, Marshall Campbell, Vivek Pareek, Arka Karmakar, E Laine Wong, Abdullah Al-Mahboob, Nicholas S. Chan, David R. Bacon, Xing Zhu, Mohamed Abdelrasoul, Xiaoquin Li, Tony F. Heinz, Felipe H. da Jornada, Ting Cao, Keshav M. Dani
An exciton, a two-body composite quasiparticle formed of an electron and hole, is a fundamental optical excitation in condensed-matter systems. Since its discovery nearly a century ago, a measurement of the excitonic wavefunction has remained beyond experimental reach. Here, we directly image the excitonic wavefunction in reciprocal space by measuring the momentum distribution of electrons photoemitted from excitons in monolayer WSe2. By transforming to real space, we obtain a visual of the distribution of the electron around the hole in an exciton. Further, by also resolving the energy coordinate, we confirm the elusive theoretical prediction that the photoemitted electron exhibits an inverted energy-momentum dispersion relationship reflecting the valence band where the partner hole remains, rather than that of conduction-band states of the electron.
We argue that a correlated fluid of electrons and holes can exhibit a fractional quantum Hall effect at zero magnetic field analogous to the Laughlin state at filling $1/m$. We introduce a variant of the Laughlin wavefunction for electrons and holes and show that for $m=1$ it is the exact ground state of a free fermion model that describes $p_x + i p_y$ excitonic pairing. For $m>1$ we develop a simple composite fermion mean field theory, and we present evidence that our wavefunction correctly describes this phase. We derive an interacting Hamiltonian for which our wavefunction is the exact gro…
Perovskites have attracted much attention due to their remarkable optical properties. While it is well established that excitons dominate their optical response, the impact of higher excitonic states and formation of phonon sidebands in optical spectra still need to be better understood. Here, we perform a theoretical study on excitonic properties of monolayered hybrid organic perovskites -- supported by temperature-dependent photoluminescence measurements. Solving the Wannier equation, we obtain microscopic access to the Rydberg-like series of excitonic states including their wavefunctions an…
Topological Frenkel excitons in 2D molecular crystals
Université de Strasbourg（斯特拉斯堡大学）；CNRS（法国国家科学研究中心）；Institute of Physics, Czech Academy of Sciences（捷克科学院物理研究所）；Georg-August-Universität Göttingen（哥廷根大学）；Donostia International Physics Center（多诺斯蒂亚国际物理中心）；Ikerbasque, Basque Foundation for Science（伊克尔巴斯克科学基金会）
The reduced dielectric screening in atomically thin transition metal dichalcogenides allows to study the hydrogen-like series of higher exciton states in optical spectra even at room temperature. The width of excitonic peaks provides information about the radiative decay and phonon-assisted scattering channels limiting the lifetime of these quasi-particles. While linewidth studies so far have been limited to the exciton ground state, encapsulation with hBN has recently enabled quantitative measurements of the broadening of excited exciton resonances. Here, we present a joint experiment-theory …
The quasiparticle band structure and optical properties of single-walled zigzag and armchair SiC nanotubes (SiC-NTs) as well as single SiC sheet are investigated by ab initio many-body calculations using the GW and the GW plus Bethe-Salpeter equation (GW+BSE) approaches, respectively. Significant GW quasiparticle corrections of more than 1.0 eV to the Kohn-Sham band gaps from the local density approximation (LDA) calculations are found. The GW self-energy corrections transform the SiC sheet from a indirect LDA band gap to a direct band gap material. Furthermore, the quasiparticle band gaps of …
We investigate the optical properties of interlayer excitons in heterobilayer transition metal dichalcogenides where moiré pattern is introduced by heterostrain, in comparison with that introduced by twisting (and/or lattice mismatch). Besides being a cause of the moiré texture, strain also effectively introduces a constant gauge potential on either electron or hole, which shifts the dispersion of kinetic energy with respect to the excitonic crystal momenta in the moiré superlattices.This leads to distinct exciton mini-band dispersions and light coupling properties from the twisting induced mo…
We present quantum yield measurements of single layer $\textrm{WSe}_2$ (1L-$\textrm{WSe}_2$) integrated with high-Q ($Q>10^6$) optical microdisk cavities, using an efficient ($η>$90%) near-field coupling scheme based on a tapered optical fiber. Coupling of the excitonic emission is achieved by placing 1L-WSe$_2$ to the evanescent cavity field. This preserves the microresonator high intrinsic quality factor ($Q>10^6$) below the bandgap of 1L-WSe$_2$. The nonlinear excitation power dependence of the cavity quantum yield is in agreement with an exciton-exciton annihilation model. The cavity quant…
It is shown that the strong Coulomb coupling in intrinsic suspended semiconducting transition metal dichalcogenides can exceed the critical value needed for an excitonic ground state. The dipole-allowed optical excitations then correspond to intra-excitonic transitions such that the optically bright excitonic transitions near the Dirac points have a $p$-like symmetry whereas the $s$-like states are dipole forbidden. The large intrinsic coupling strength seems to be a generic property of the semiconducting transition metal dichalcogenides and strong Coulomb-coupling signatures in the form of th…
The monolayer transition metal dichalcogenides are an emergent semiconductor platform exhibiting rich excitonic physics with coupled spin-valley degree of freedom and optical addressability. Here, we report a new series of low energy excitonic emission lines in the photoluminescence spectrum of ultraclean monolayer WSe2. These excitonic satellites are composed of three major peaks with energy separations matching known phonons, and appear only with electron doping. They possess homogenous spatial and spectral distribution, strong power saturation, and anomalously long population (> 6 $μ$s) and…
Tunable Magneto-Excitonic Coupling in Alloyed van der Waals Antiferromagnet
Wroclaw University of Science and Technology（弗罗茨瓦夫科技大学）；University of Warsaw（华沙大学）；Laboratoire National des Champs Magnétiques Intenses（强磁场国家实验室）；Université Grenoble Alpes（格勒诺布尔阿尔卑斯大学）；Université Toulouse（图卢兹大学）；King’s College London（伦敦国王学院）
Hexagonal boron nitride (hBN) has been attracting great attention because of its strong excitonic effects. Taking into account few-layer systems, we investigate theoretically the effects of the number of layers on quasiparticle energies, absorption spectra, and excitonic states, placing particular focus on the Davydov splitting of the lowest bound excitons. We describe how the inter-layer interaction as well as the variation in electronic screening as a function of layer number $N$ affects the electronic and optical properties. Using both \textit{ab initio} simulations and a tight-binding mode…
Electron-hole systems on a Haldane sphere are studied by exact numerical diagonalization. Low lying states contain one or more types of bound charged excitonic complexes Xk-, interacting through appropriate pseudopotentials. Incompressible ground states of such multi-component plasmas are found. A generalized multi-component Laughlin wavefunction and composite Fermion picture are shown to predict the low lying states of an electron-hole gas at any value of the magnetic field.
The light-matter interaction associated with a two-dimensional (2D) excitonic transition coupled to a zero-dimensional (0D) photonic cavity is fundamentally different from coupling localized excitations in quantum dots or color centers, which have negligible spatial extent compared to the cavity-confined mode profile. By calculating the radiation-matter coupling of the exciton transition of a surface deposited 2D material and a 0D photonic crystal nanobeam mode, we found that there is an optimal spatial extent of the monolayer material that maximizes such an interaction strength due to the com…
Preserving a high degree of valley polarization of excitons in photo-excited transition-metal dichalcogenide monolayers (TMD-MLs) is desirable for the valley-based photonic applications, but widely recognized as a hard task hindered by the intrinsic electron-hole exchange interaction. In this study, we present a comprehensive investigation of valley-polarized finite-momentum excitons in WSe$_2$-MLs over the entire Brillouin zone by solving the density-functional-theory(DFT)-based Bethe-Salpeter equation (BSE) under the guidance of symmetry analysis. We reveal that finite-momentum excitons are …
We study the theory of intrinsic photoluminescence of two-dimensional electron systems in the vicinity of the $ν=1$ quantum Hall state. We focus predominantly on the recombination of a band of initial ``excitonic states'' that are the low-lying energy states of our model at $ν=1$. It is shown that the recombination of excitonic states can account for recent observations of the polarization-resolved spectra of a high-mobility GaAs quantum well. The asymmetric broadening of the spectral line in the $σ_-$ polarization is explained to be the result of the ``shake-up'' of spin-waves upon radiative …
Layer-parity-defined surface polarization in Nb$_3$Cl$_8$ for excitonic modulation at van der Waals interfaces
Peking University（北京大学）；Chinese Academy of Sciences（中国科学院）；University of Chinese Academy of Sciences（中国科学院大学）；National Institute for Materials Science（国立材料研究所）；South China Normal University（华南师范大学）；Liaoning Academy of Materials（辽宁省材料研究院）；ShanghaiTech University（上海科技大学）；University of Science and Technology of China（中国科学技术大学）；Songshan Lake Materials Laboratory（松山湖材料实验室）
Because of the reduced dielectric screening and enhanced Coulomb interactions, two-dimensional (2D) materials like phosphorene and transition metal dichalcogenides (TMDs) exhibit strong excitonic effects, resulting in fascinating many-particle phenomena covering both intralayer and interlayer excitons. Their intrinsic bandgaps and strong excitonic emissions allow the possibility to tune the inherent optical, electrical, and optoelectronic properties of 2D materials via a variety of external stimuli, making them potential candidates for novel optoelectronic applications. In this review, we summ…
Layered van der Waals (vdW) magnetic materials have attracted significant research interest to date. In this work, we employ the first-principles many-body perturbation theory to calculate excited-state properties of a prototype vdW magnet, chromium trichloride (CrCl3), covering monolayer, bilayer, and bulk structures. Unlike usual non-magnetic vdW semiconductors, in which many-electron interactions and excited states are sensitive to dimensionality, many-electron interactions are always enhanced and dominate quasiparticle energies and optical responses of both two-dimensional and bulk CrCl3. …
We investigate the quasiparticle excitation of the bilayer quantum Hall (QH) system at total filling factor $ν_{\mathrm{T}} = 1$ in the limit of negligible interlayer tunneling under tilted magnetic field. We show that the intrinsic quasiparticle excitation is of purely pseudospin origin and solely governed by the inter- and intra-layer electron interactions. A model based on exciton formation successfully explains the quantitative behavior of the quasiparticle excitation gap, demonstrating the existence of a link between the excitonic QH state and the composite fermion liquid. Our results pro…
We present a novel, counter-intuitive method, based on dark state protection, for significantly improving exciton transport efficiency through `wires' comprising a chain of molecular sites with an intrinsic energy gradient. Specifically, by introducing `barriers' to the energy landscape at regular intervals along the transport path, we find that undesirable radiative recombination processes are suppressed due to a clear separation of sub-radiant and super-radiant eigenstates in the system. This, in turn, can lead to an improvement in transmitted power by many orders of magnitude, even for very…
Optical properties of transition metal dichalcogenides monolayers are controlled by the Wannier-Mott excitons forming a series of $1s$, $2s$, $2p$,... hydrogen-like states. We develop the theory of the excited excitonic states energy spectrum fine structure. We predict that $p$- and $s$-shell excitons are mixed due to the specific $D_{3h}$ point symmetry of the transition metal dichalcogenide monolayers. Hence, both $s$- and $p$-shell excitons are active in both single- and two-photon processes providing an efficient mechanism of second harmonic generation. The corresponding contribution to th…
The problem of an electron-hole system interacting through a contact potential and moving in a one-dimensional quantum ring threaded by an Aharonov-Bohm flux is considered, both with respect to the system's energetics as well as its optical properties. An exact analytical expression for the energy spectrum is derived using a straightforward method based on boundary conditions for wavefunctions and their derivatives along the ring. The optical properties of this exciton system, namely the linear dynamic polarizability and absorption spectrum are investigated demonstrating certain unusual featur…
Heterobilayers consisting of MoSe$_2$ and WSe$_2$ monolayers can host optically bright interlayer excitons with intriguing properties such as ultralong lifetimes and pronounced circular polarization of their photoluminescence due to valley polarization, which can be induced by circularly polarized excitation or applied magnetic fields. Here, we report on the observation of an intrinsic valley-magnetophonon resonance for localized interlayer excitons promoted by invervalley hole scattering. It leads to a resonant increase of the photoluminescence polarization degree at the same field of 24.2 Te…
Layer-Dependent Phonons, Excitons, and Magneto-Optical Phenomena in CrSBr: A Mini Review
National Center for Nanoscience and Technology（中国科学院国家纳米科学中心）；University of Chinese Academy of Sciences（中国科学院大学）；University of Warsaw（华沙大学）；CNRS–UGA–UPS–INSA–EMFL（法国国家科学研究中心-格勒诺布尔阿尔卑斯大学-图卢兹第三大学-INSA图卢兹-东洛林大学）；University of Sapienza（罗马第一大学）
We investigate an excitonic peak appearing in low-temperature photoluminescence of monolayer transition metal dichalcogenides (TMDCs), which is commonly associated with defects and disorder. First, to uncover the intrinsic origin of defect-related excitons, we study their dependence on gate voltage, excitation power, and temperature in a prototypical TMDC monolayer, $MoS_2$. We show that the entire range of behaviors of defect-related excitons can be understood in terms of a simple model, where neutral excitons are bound to ionized donor levels, likely related to sulphur vacancies, with a dens…
Spectroscopic signatures of persistent exciton condensation in a bulk magnetic topological insulator
Stanford University（斯坦福大学）；Aarhus University（奥胡斯大学）；Università degli Studi di Trieste（的里雅斯特大学）；Elettra - Sincrotrone Trieste S.C.p.A.（Elettra-的里雅斯特同步辐射中心）；Tohoku University（东北大学）；Technical University of Denmark（丹麦技术大学）
