Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session C53: Excitonic and Photonic Behavior in 2D - IFocus Live
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Sponsoring Units: DMP DCMP Chair: Tony Heinz, Stanford Univ |
Monday, March 15, 2021 3:00PM - 3:36PM Live |
C53.00001: Equilibrium exciton insulator and exciton quasi-condensation in semiconductor atomic double layers Invited Speaker: Kin Fai Mak Exciton insulator, a conductor for excitons but an insulator for charges, is expected to emerge in a material when its energy gap for charge excitations becomes smaller than the exciton binding energy, beyond which an exciton gas is spontaneously formed. Although the concept is known to the physics community for nearly seventy years, the thermodynamic evidence of an exciton insulator remains elusive. Atomic double layers of transition metal dichalcogenide (TMD) semiconductor, which support large interlayer exciton binding energy, provide an ideal platform to realize an exciton insulator. In this talk, I will present direct thermodynamic evidence for an equilibrium exciton insulator in TMD double layers. Using a new exciton-contact device design, we have created a charge-free equilibrium exciton fluid by continuously reducing the charge gap of the system below its exciton binding energy. Compressibility measurements show that the fluid is exciton-compressible but charge-imcompressible, directly demonstrating its exciton insulating character. I will further discuss the phase diagram for the strongly interacting exciton fluid and provide direct thermodynamic evidence of 2D quasi-condensation of excitons. |
Monday, March 15, 2021 3:36PM - 3:48PM Live |
C53.00002: Interlayer exciton polarons in a MoSe2/WSe2 heterostructure Mauro Brotons-Gisbert, Aidan Campbell, Hyeonjun Baek, Kenji Watanabe, Takashi Taniguchi, Brian D Gerardot Monolayer transition metal dichalcogenide semiconductors (TMDs), with a large variety of robust exciton species, represent a versatile platform for investigating the interactions between excitons and a Fermi sea of charges. Recent experiments in charge-tunable monolayer TMDs show robust evidence that the interaction of intralayer excitons with a Fermi sea is best described by exciton–polarons, rather than trions [1-3]. In this work, we report the observation of interlayer (rather than intralayer) exciton (IX) polarons in the low-temperature photoluminescence of a gate-tunable MoSe2/WSe2 heterostructure. Increasing electron density leads to a splitting of the neutral IXs peaks into attractive and repulsive IX polaron branches. Interestingly, we observe two species of repulsive polarons in which the exciton carriers located in different layers present parallel and antiparallel alignment. Finally, we investigate the magneto-optical properties of the IX polarons in the Faraday geometry. Our results reveal that, although the attractive polarons show a conventional positive diamagnetic shift, the repulsive polaron branches present anomalous negative diamagnetic shifts. |
Monday, March 15, 2021 3:48PM - 4:00PM Live |
C53.00003: Equilibrium exciton insulator in semiconductor atomic double layers Liguo Ma, Phuong Nguyen, Zefang Wang, Kenji Watanabe, Takashi Taniguchi, Jie Shan, Kin Fai Mak Exciton insulator, a conductor for excitons but an insulator for charges, is expected to emerge in a material when its energy gap for charge excitations becomes smaller than the exciton binding energy, beyond which an exciton gas is spontaneously formed. Atomic double layers of transition metal dichalcogenide (TMD) semiconductor, which support large interlayer exciton binding energy, provide an ideal platform to realize exciton insulators with high ionization temperatures. We provide direct thermodynamic evidence for an equilibrium exciton insulator in TMD double layers. Using a new exciton-contact device design, we have created a charge-free equilibrium exciton fluid by continuously reducing the charge gap of the system below the exciton binding energy. Compressibility measurements show that the fluid is exciton-compressible but charge-imcompressible, directly demonstrating its exciton insulating character. We have further obtained a phase diagram for the strongly interacting exciton fluid. |
Monday, March 15, 2021 4:00PM - 4:12PM Live |
C53.00004: Electrical generation of equilibrium exciton fluid in semiconductor atomic double layers Phuong Nguyen, Liguo Ma, Zefang Wang, Kenji Watanabe, Takashi Taniguchi, Jie Shan, Kin Fai Mak Bose-Einstein condensation of spatially indirect excitons is a fascinating macroscopic quantum phenomenon. Atomic double layers of transition metal dichalcogenide (TMD) semiconductor, which support large interlayer exciton binding energy, is an attractive platform to explore exciton condensation at high-temperatures and provides a route to realize high-temperature superconductivity. Although evidence of non-equilibrium exciton condensation has been observed in TMD double layers, the realization of an equilibrium exciton fluid, which is important for the realization of exciton superfluidity, remains elusive. Here we report the electrical generation of an equilibrium interlayer exciton fluid in WSe2-hBN-MoSe2 double layers. This is enabled by an improved dual-gate device design, in which excitons can be injected into the double layer system while interlayer tunneling is suppressed. Capacitance measurements further allow the quantification of the band alignment and exciton binding energy. |
Monday, March 15, 2021 4:12PM - 4:24PM Live |
C53.00005: Interlayer excitons in the heterostrain induced moire of heterobilayer transition metal dichalcogenides Huiyuan Zheng, Dawei Zhai, Wang Yao Here we investigate the optical properties of interlayer excitons in heterobilayer transition metal dichalcogenides where moire pattern is introduced by the uniaxial heterostrain, in comparison with that introduced by twisting. Besides being a cause of the moire texture, the uniaxial strain also effectively introduces a constant gauge potential on carriers, which results in distinct exciton mini-band dispersions from the twisting induced moire, even though the excitonic superlattice potential can have the same real space profile for the two cases. As the uniaxial strain breaks the three-fold rotational symmetry, the exciton wavepackets trapped at the superlattice potential minima now have elliptically polarized valley optical selection rule, in contrast to the circularly polarized ones in the twisting moire. We investigate the evolution of the excitonic mini-bands and the optical dipoles of the states inside the light cones with the change of the moire periodicity. |
Monday, March 15, 2021 4:24PM - 4:36PM Live |
C53.00006: Enhancing exciton DOLP by scattering valley excitons in mono- and few-layer MoS2 Garima Gupta, Kausik Majumdar The generated exciton valley coherence on Linearly Polarized light excitation is affected by strong exchange interaction and (impurity, phonon) scattering in Transition Metal Dichalcogenides (TMDs). The final state is a mixed ensemble, and the detected Degree of Linear Polarization (DOLP) quantifies the residual coherence at the time of emission. We decouple the role of multiple factors degrading valley coherence in Monolayer (ML) and Bilayer (BL) MoS2. We observe that impurity scattering degrades DOLP in clean MoS2 with low defect emission but significantly enhances it when MoS2 has huge defect emission. This is because of the Motional narrowing effect due to the dense charge fluctuating environment in excitons' vicinity in defected MoS2. We artificially improve the DOLP by stacking MoS2 on Graphene as (a) due to fast exciton transfer to Gr, the collection of long-lived decoherent excitons is reduced (Gr filtering effect) and (b) the exciton wavefunction overlap and hence the exchange interaction is reduced due to dielectric screening. The DOLP degrades with temperature (T) in a ML due to phonon scattering inside the light cone but shows a slight improvement in Bi- and Tri-layer due to transfer (, filtering) to the low energy indirect peak. |
Monday, March 15, 2021 4:36PM - 4:48PM Live |
C53.00007: Localization of interlayer excitons in nanoscale wrinkles investigated by nano-PL and nano-Raman spectroscopies Thomas Darlington Interlayer excitons (ILE) in TMD-heterostructures have the potential to form complex multiparticle excitonic phases and are of intense recent scientific interest. However, the low photoluminescence quantum yield of ILE’s makes optical investigation of localized ILE's especially challenging, forcing many optical measurements of ILE emission to be done at cryogenic temperatures and with excitation spots >1 um in diameter. Such large probe areas average over local perturbations, complicating analysis. Here, I will show advanced near-field optical characterization of localized ILE’s in a MoSe2-WSe2-Au heterostructure. Hyperspectral imaging with few-nm resolution shows a rich variety of ILE states with distinct emission peaks between 850 nm to past 1000 nm that are localized to nanobubble edge regions 1 - 5 nm wide, and have emission intensity that is enhanced by >100x. Using nano-Raman scattering we find that these edge regions have similar vibrational spectra to lattice wrinkles observed to 1L-WSe2 nanobubbles, suggesting a “double-wrinkle” of the heterolayer as the source. Our results illustrate the potential for strain engineering ILE’s in TMD heterostructures for use in tailoring exciton states for future 2D optoelectronic devices. |
Monday, March 15, 2021 4:48PM - 5:00PM Live |
C53.00008: Multiple vibrational resonances and Franck-Condon palisades in the low temperature photocurrent spectrum of interlayer excitons Fatemeh Barati, Trevor Arp, Shanshan Su, Roger Lake, Justin Song, Mark Rudner, Vivek M Aji, Nathaniel Monroe Gabor Engineering the interactions between atomic motion and excitons is challenging since the interaction strength is fixed by atomic-scale configuration and electronic structure. Yet, current advances in stack-engineering of atomically thin vdw crystals allow careful tuning of both atomic and electronic structure. Here we report the emergence of strong exciton-phonon coupling in an atomically thin heterostructure composed of WSe2 and MoSe2. Strong coupling manifests as numerous photocurrent sidebands that form a palisade of multiple vibrational excited state resonances above the lowest excited state absorption feature. This can be understood through strong coupling between vibrations of the atomic lattice and interlayer excitons, giving rise to Frank-Condon transitions to multiple phonon states. Discretely spaced peaks with a period of 30 meV in the photocurrent spectra match the energy of the vibrational modes and are observed only when the device is finely tuned to charge neutrality, the condition at which electrons and holes across the interface are fully compensated. Strong vibrational coupling and multiple sidebands in the e-h pair generation process are hallmarks of localized excitons and may indicate self-trapping of excitons through strong interactions with the phonon field. |
Monday, March 15, 2021 5:00PM - 5:12PM Live |
C53.00009: Separating electron-hole pairs: photocurrent imaging of interlayer exciton dissociation in van der Waals heterojunctions Jedediah Kistner-Morris, Trevor Arp, Nathaniel Monroe Gabor In van der Waals heterostructures, the separation of interlayer excitons requires that the oppositely charged electron and hole overcome an electrostatic binding energy before undergoing free charge carrier motion. We report optoelectronic transport imaging studies of WS2-MoSe2 heterojunctions over a range of device layer thickness. Using Multi-Parameter Dynamic Photoresponse Microscopy, we generate ~10x photocurrent images of the heterostructure response as a function of source-drain and gate voltages. In bulk heterojunctions these images reveal rectifying behavior within the overlap region, modulated by a reduction in the interlayer photoconduction that spatially evolves with increasing source drain voltage. Monolayer junctions of the same materials reveal negative differential photoresistance that evolves with the same gate and source drain voltage dependence as the photosuppression in bulk junctions. We attribute these effects to electric field-assisted dissociation of interlayer excitons, which we analyze as a function of device thickness. Our measurements indicate that strong two-body correlations arise precisely at the onset to a reduction in interlayer conductance, which unambiguously establishes the crossover from bound interlayer exciton to free electron and hole. |
Monday, March 15, 2021 5:12PM - 5:24PM Live |
C53.00010: Davydov Splitting and Evolution of Molecular Excitons in Two-Dimensional PTCDA Crystals Dogyeong Kim, Sunmin Ryu Two-dimensional (2D) van der Waals materials have recently gained much research interest because of their potential in various applications. Unlike 2D atomic crystals represented by graphene, MoS2, and hexagonal BN, however, 2D molecular crystals have rarely been studied. In this work, single and few-layer PTCDA (perylene-3,4,9,10-tetracarboxylic dianhydride) crystals were grown on hexagonal BN by self-limited vapor assembly. Polarized photoluminescence (PL) spectra revealed high and low-energy excitonic emissions with a strong dependence on thickness. The former Frenkel exciton exhibited significant vibronic progressions with pronounced Davydov splitting for 0-0 peak, indicating the presence of multiple basis molecules in unit cells, which was also confirmed in absorption spectra. The temperature-dependence of PL signals suggested thermal equilibration between the two Davydov states. By determining transition dipole moments using polarized PL analysis, we directly show the charge-transfer character in the latter is enhanced for thicker layers with increased π-π interactions. |
Monday, March 15, 2021 5:24PM - 5:36PM Live |
C53.00011: Intralayer moiré exciton states in transition metal dichalcogenide heterostructures: a GW-BSE study Mit Naik, Yang-hao Chan, Zhenglu Li, Chin Shen Ong, Felipe Da Jornada, Steven G Louie Recent experimental measurements have shown signatures of novel exciton states in the moiré superlattices of transition metal dichalcogenide bilayer heterostructures. In the WS2/WSe2 heterostructure, the WSe2 “A” exciton resonance is found to split into multiple peaks as the twist angle between the two layers approaches zero [1]. This splitting is directly correlated to the formation of a large-scale (~8.5 nm) moiré pattern at zero twist angle. Computing the optical properties of this system by brute force using first principles is intractable owing to the large number of atoms (~4000) in the moiré superlattice and the requirement of a fine k-point sampling of the Brillouin zone. We demonstrate an efficient approximation to solve the GW plus Bethe-Salpeter equation of the reconstructed moiré which yields excitations with meV accuracy relative to the traditional approach. Using this method, we study the twist-angle dependence of the absorption spectrum of intralayer excitons in the WS2/WSe2 moiré superlattice. |
Monday, March 15, 2021 5:36PM - 5:48PM Live |
C53.00012: Probing interlayer excitons in transition-metal dichalcogenide heterostructures using time- and angle-resolved photoemission spectroscopy Ouri Karni, Elyse Barre, Chakradhar Sahoo, Michael Man, Julien Madéo, Johnathan Dimitrios Georgaras, Henrique Ribeiro, Aidan Luke O'beirne, Abdullah Al-Mahboob, Mohamed Abdelrasoul, Nicholas S Chan, Arka Karmakar, Vivek Pareek, Andrew Winchester, Bumho Kim, Kenji Watanabe, Takashi Taniguchi, Katayun Barmak, Felipe Da Jornada, Tony Heinz, Keshav M. Dani Van der Waals heterostructures of monolayer transition metal dichalcogenides (TMDCs) host interlayer excitons (ILX), electron-hole pairs bound across the interlayer gap. Their charge separation leads to long lifetimes and the possibility of reaching high densities[1], with pronounced exciton-exciton many-body effects like Mott transitions[1] and exciton condensates[2]. However, so far ILXs in TMDC heterostructures have been probed mostly by their luminescence[3,4], accessing only a narrow range of momentum space, and leaving many open questions about the ILXs' density and size, which are key for studying their many-body interactions. In this talk, we present our direct observation of the full momentum-space image of ILXs in a WSe2/MoS2 heterostructure using time-resolved, angle-resolved photoemission spectroscopy. These data enable us to determine for the first time the location, distribution size, and density of ILXs in momentum space, without prior assumptions about their lifetime or formation dynamics. |
Monday, March 15, 2021 5:48PM - 6:00PM On Demand |
C53.00013: Luminescence anomaly of dipolar valley excitons in homobilayer semiconductor moiré superlattices Hongyi Yu, Wang Yao In twisted homobilayer transition metal dichalcogenides, intra- and inter-layer valley excitons hybridize with the layer configurations spatially varying in the moiré. The ground state valley excitons are trapped at two high-symmetry points with opposite electric dipoles in a moiré supercell, forming a honeycomb superlattice of nearest-neighbor dipolar attraction. We find that the spatial texture of layer configuration results in a luminescence anomaly of the moiré trapped excitons, where a tiny displacement by interactions dramatically increases the brightness and changes polarization from circular to linear. At full filling, radiative recombination predominantly occurs at edges and vacancies of the exciton superlattice. The anomaly also manifests in the cascaded emission of small clusters, producing chains of polarization entangled photons. An interlayer bias can switch the superlattice into a single-orbital triangular lattice with repulsive interactions only, where the luminescence anomaly can be exploited to distinguish ordered states and domain boundaries at fractional filling. |
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