Query: Interlayer valley excitons in heterobilayers of transition metal dichalcogenides

Candidate papers:

abstract: Stacking different two dimensional crystals into van der Waals heterostructures provides an exciting approach to designing quantum materials that can harness and extend the already fascinating properties of the constituents. Heterobilayers of transition metal dichalcogenides are particularly attractive for low dimensional semiconductor optics because they host interlayer excitons with electrons and holes localized in different layers which inherit valley contrasting physics from the monolayers and thereby possess various novel and appealing properties compared to other solid state nanostructures. This Review presents the contemporary experimental and theoretical understanding of these interlayer excitons. We discuss their unique optical properties arising from the underlying valley physics, the strong many body interactions and electrical control resulting from the electric dipole moment, and the unique effects of a moire superlattice on the interlayer exciton potential landscape and optical properties.This Review discusses the contemporary experimental and theoretical understanding of interlayer excitons in heterobilayers of transition metal dichalcogenides.
author_names: Pasqual Rivera

Hongyi Yu

Kyle L Seyler

N P Wilson

Wang Yao

Xiaodong Xu
title: Interlayer valley excitons in heterobilayers of transition metal dichalcogenides
venue: Nature Nanotechnology
year: 2018

abstract: Van der Waals (vdW) heterobilayers formed by two dimensional (2D) transition metal dichalcogenides (TMDCs) created a promising platform for various electronic and optical properties. ab initio band results indicate that the band offset of type II band alignment in TMDCs vdW heterobilayer could be tuned by introducing Janus WSSe monolayer, instead of an external electric field. On the basis of symmetry analysis, the allowed interlayer hopping channels of TMDCs vdW heterobilayer were determined, and a four level kp model was developed to obtain the interlayer hopping. Results indicate that the interlayer coupling strength could be tuned by interlayer electric polarization featured by various band offsets. Moreover, the difference in the formation mechanism of interlayer valley excitons in different TMDCs vdW heterobilayers with various interlayer hopping strength was also clarified.
author_names: Fengping Li

Wei Wei

Baibiao Huang

Ying Dai
title: Interlayer coupling effect in van der Waals heterostructures of transition metal dichalcogenides
venue: Frontiers of Physics
year: 2020

abstract: We present a theory of optical absorption by interlayer excitons in a heterobilayer formed from transition metal dichalcogenides. The theory accounts for the presence of small relative rotations that produce a momentum shift between electron and hole bands located in different layers, and a moir\'e pattern in real space. Because of the momentum shift, the optically active interlayer excitons are located at the moir\'e Brillouin zone's corners, instead of at its center, and would have elliptical optical selection rules if the individual layers were translationally invariant. We show that the exciton moir\'e potential energy restores circular optical selection rules by coupling excitons with different center of mass momenta. A variety of interlayer excitons with both senses of circular optical activity, and energies that are tunable by twist angle, are present at each valley. The lowest energy exciton states are generally localized near the exciton potential energy minima. We discuss the possibility of using the moir\'e pattern to achieve scalable two dimensional arrays of nearly identical quantum dots.
author_names: Fengcheng Wu

Timothy Lovorn

Allan H MacDonald
title: Theory of optical absorption by interlayer excitons in transition metal dichalcogenide heterobilayers
year: 2017

abstract: We show that, because of the inevitable twist and lattice mismatch in heterobilayers of transition metal dichalcogenides, interlayer excitons have sixfold degenerate light cones anomalously located at finite velocities on the parabolic energy dispersion. The photon emissions at each light cone are elliptically polarized, with the major axis locked to the direction of exciton velocity, and helicity specified by the valley indices of the electron and the hole. These finite velocity light cones allow unprecedented possibilities for optically injecting valley polarization and valley current, and the observation of both direct and inverse valley Hall effects, by exciting interlayer excitons. Our findings suggest potential excitonic circuits with valley functionalities, and unique opportunities to study exciton dynamics and condensation phenomena in semiconducting 2D heterostructures.
author_names: Hongyi Yu

Yong Wang

Qingjun Tong

Xiaodong Xu

Wang Yao
title: Anomalous Light Cones and Valley Optical Selection Rules of Interlayer Excitons in Twisted Heterobilayers.
venue: Physical review letters
year: 2015

abstract: Van der Waals heterostructures offer attractive opportunities to design quantum materials. For instance, transition metal dichalcogenides (TMDs) possess three quantum degrees of freedom: spin, valley index and layer index. Furthermore, twisted TMD heterobilayers can form moire patterns that modulate the electronic band structure according to the atomic registry, leading to spatial confinement of interlayer excitons (IXs) Here we report the observation of spin layer locking of IXs trapped in moire potentials formed in a heterostructure of bilayer 2H MoSe 2 and monolayer WSe 2 The phenomenon of locked electron spin and layer index leads to two quantum confined IX species with distinct spin layer valley configurations. Furthermore, we observe that the atomic registries of the moire trapping sites in the three layers are intrinsically locked together due to the 2H type stacking characteristic of bilayer TMDs. These results identify the layer index as a useful degree of freedom to engineer tunable few level quantum systems in two dimensional heterostructures. The optical properties of two species of localized interlayer excitons in a van der Waals heterostructure are shown to depend on their spin valley layer configuration, enabling the identification of the moire atomic registry and offering insights for engineering quantum states in two dimensional materials.
author_names: Mauro Brotons-Gisbert

Hyeonjun Baek

Alejandro Molina-Sanchez

Aidan Campbell

Eleanor Scerri

Daniel White

Kenji Watanabe

Takashi Taniguchi

Cristian Bonato

Brian D Gerardot
title: Spin layer locking of interlayer excitons trapped in moire potentials
venue: Nature Materials
year: 2020

abstract: Layered two dimensional materials exhibit rich transport and optical phenomena in twisted or lattice incommensurate heterostructures with spatial variations of interlayer hybridization arising from moire interference effects. Here, we report experimental and theoretical studies of excitons in twisted heterobilayers and heterotrilayers of transition metal dichalcogenides. Using MoSe2 WSe2 stacks as representative realizations of twisted van der Waals bilayer and trilayer heterostructures, we observe contrasting optical signatures and interpret them in the theoretical framework of interlayer moire excitons in different spin and valley configurations. We conclude that the photoluminescence of MoSe2 WSe2 heterobilayer is consistent with joint contributions from radiatively decaying valley direct interlayer excitons and phonon assisted emission from momentum indirect reservoirs that reside in spatially distinct regions of moire supercells, whereas the heterotrilayer emission is entirely due to momentum dark interlayer excitons of hybrid layer valleys. Our results highlight the profound role of interlayer hybridization for transition metal dichalcogenide heterostacks and other realizations of multi layered semiconductor van der Waals heterostructures.
author_names: Michael Forg

Anvar S Baimuratov

Stanislav Yu Kruchinin

Ilia A Vovk

Johannes Scherzer

J Forste

Victor Funk

Kenji Watanabe

Takashi Taniguchi

Alexander Hogele
title: Moire excitons in MoSe2 WSe2 heterobilayers and heterotrilayers
venue: Nature communications
year: 2021

abstract: Van der Waals heterobilayers of transition metal dichalcogenides with spin valley coupling of carriers in different layers have emerged as a new platform for exploring spin/valleytronic applications. The interlayer coupling was predicted to exhibit subtle changes with the interlayer atomic registry. Manually stacked heterobilayers, however, are incommensurate with the inevitable interlayer twist and/or lattice mismatch, where the properties associated with atomic registry are difficult to access by optical means. Here, we unveil the distinct polarization properties of valley specific interlayer excitons using epitaxially grown, commensurate WSe2/MoSe2 heterobilayers with well defined (AA and AB) atomic registry. We observe circularly polarized photoluminescence from interlayer excitons, but with a helicity opposite to the optical excitation. The negative circular polarization arises from the quantum interference imposed by interlayer atomic registry, giving rise to distinct polarization selection rules for interlayer excitons. Using selective excitation schemes, we demonstrate the optical addressability for interlayer excitons with different valley configurations and polarization helicities.The interlayer coupling in van der Waals heterostructures is sensitive to the interlayer atomic registry. Here, the authors investigate the polarisation properties of epitaxially grown, commensurate WSe2/MoSe2 heterobilayers with well defined atomic registry, and observe negative, circularly polarized photoluminescence from interlayer excitons.
author_names: Wei-Ting Hsu

Li-Syuan Lu

Po-Hsun Wu

Ming-Hao Lee

Peng-Jen Chen

Pei-Ying Wu

Yi-Chia Chou

Horng-Tay Jeng

Lain-Jong Li

M W Chu

Wen-Hao Chang
title: Negative circular polarization emissions from WSe2/MoSe2 commensurate heterobilayers
venue: Nature Communications
year: 2018

abstract: van der Waals heterostructures formed by stacking two dimensional atomic crystals are a unique platform for exploring new phenomena and functionalities. Interlayer excitons, bound states of spatially separated electron hole pairs in van der Waals heterostructures, have demonstrated potential for rich valley physics and optoelectronics applications and been proposed to facilitate high temperature superfluidity. Here, we demonstrate highly tunable interlayer excitons by an out of plane electric field in homobilayers of transition metal dichalcogenides. Continuous tuning of the exciton dipole from negative to positive orientation has been achieved, which is not possible in heterobilayers due to the presence of large built in interfacial electric fields. A large linear field induced redshift up to ~100 meV has been observed in the exciton resonance energy. The Stark effect is accompanied by an enhancement of the exciton recombination lifetime by more than two orders of magnitude to >20 ns. The long recombination lifetime has allowed the creation of an interlayer exciton gas with density as large as 1.2 x 1011 cm 2 by moderate continuous wave optical pumping. Our results have paved the way for the realization of degenerate exciton gases in atomically thin semiconductors.
author_names: Zefang Wang

Yi-Hsin Chiu

Kevin Honz

Kin Fai Mak

Jie Shan
title: Electrical Tuning of Interlayer Exciton Gases in WSe2 Bilayers.
venue: Nano letters
year: 2018

abstract: Transition metal dichalcogenides (TMDs) constitute a versatile platform for atomically thin optoelectronics devices and spin valley memory applications. In monolayer TMDs the optical absorption is strong, but the transition energy cannot be tuned as the neutral exciton has essentially no out of plane static electric dipole 1 2 In contrast, interlayer exciton transitions in heterobilayers are widely tunable in applied electric fields, but their coupling to light is substantially reduced. In this work, we show tuning over 120 meV of interlayer excitons with a high oscillator strength in bilayer MoS 2 due to the quantum confined Stark effect 3 We optically probed the interaction between intra and interlayer excitons as they were energetically tuned into resonance. Interlayer excitons interact strongly with intralayer B excitons, as demonstrated by a clear avoided crossing, whereas the interaction with intralayer A excitons is substantially weaker. Our observations are supported by density functional theory (DFT) calculations, which include excitonic effects. In MoS 2 trilayers, our experiments uncovered two types of interlayer excitons with and without in built electric dipoles. Highly tunable excitonic transitions with large in built dipoles and oscillator strengths will result in strong exciton exciton interactions and therefore hold great promise for non linear optics with polaritons. Interlayer excitons in bilayer MoS 2 exhibit both a high oscillator strength and highly tunable energies in an applied electric field.
author_names: Nadine Leisgang

Shivangi Shree

Ioannis Paradisanos

Lukas Sponfeldner

Cedric Robert

D Lagarde

Andrea Balocchi

Kenji Watanabe

Takashi Taniguchi

Xavier Marie

Richard John Warburton

Iann C Gerber

Bernhard Urbaszek
title: Giant Stark splitting of an exciton in bilayer MoS2
venue: Nature Nanotechnology
year: 2020

abstract: Geometrical moire patterns, generic for almost aligned bilayers of two dimensional (2D) crystals with similar lattice structure but slightly different lattice constants, lead to zone folding and miniband formation for electronic states. Here, we show that moire superlattice (mSL) effects in MoSe$}_2$/WS$}_2$ and MoTe$}_2$/MoSe$}_2$ heterobilayers that feature alignment of the band edges are enhanced by resonant interlayer hybridization. Such hybridization determines the optical activity of interlayer excitons in transition metal dichalcogenide (TMD) heterostructures, as well as energy shifts in the exciton spectrum. We show that the resonantly hybridized exciton (hX) energy should display a sharp modulation as a function of the interlayer twist angle, accompanied by additional spectral features caused by umklapp electron photon interactions. We analyze the appearance of resonantly enhanced mSL features in absorption and emission of light by the interlayer exciton hybridization with both intralayer A and B excitons in MoSe$}_2$/WS$}_2$ MoTe$}_2$/MoSe$}_2$ MoSe$}_2$/MoS$}_2$ and WSe$}_2$/MoS$}_2$ and anticipate similar features in twisted homobilayers of TMDs.
author_names: David A Ruiz-Tijerina

Vladimir I Fal'ko
title: Interlayer hybridization and moire superlattice minibands for electrons and excitons in heterobilayers of transition metal dichalcogenides
venue: Physical Review B
year: 2019