Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session J62: Excitons and Exciton Transport in NanostructuresFocus
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Sponsoring Units: DMP Chair: Prashant Padmanabhan Room: Mile High Ballroom 4C |
Tuesday, March 3, 2020 2:30PM - 3:06PM |
J62.00001: Novel exciton transport in double-layer graphene structures Invited Speaker: Jia Li A spatially indirect exciton is created when an electron and a hole, confined to separate layers of a double quantum well system, bind to form a composite boson. A system of such excitons is demonstrated to host a Bose–Einstein condensate phase at low temperature, featuring dissipationless exciton flow and perfect Coulomb drag. In this talk, I will discuss novel exciton phases in various double-layer structures characterized by exotic transport properties. In double monolayer graphene, an insulating exciton phase is stabilized in the presence of large density imbalance. With increasing temperature, the insulating phase “melts” into an exciton superfluid exhibiting perfect Coulomb drag. In double bilayer graphene, Coulomb drag measurement reveals strong anisotropy in exciton flow when n = 0 and n = 1 orbitals states are degenerate in one of the layers. Along the easy direction, drag measurement is consistent with an exciton condensate, whereas an anomalous drag response is observed in the orthogonal direction. These exotic exciton phases are studied in a multidimensional phase space defined by a variety of experimental parameters available to double-layer structures, which provides insight into the nature of electron correlation and the resulting physical structures. |
Tuesday, March 3, 2020 3:06PM - 3:18PM |
J62.00002: Tunable polarons in a carbon nanotube electromechanical resonator Sergio De Bonis, Chandan Samanta, Wei Yang, Carles Urgell, Biljana Stamenic, Brian Thibeault, Fabio Pistolesi, Adrian Bachtold We demonstrate the formation of polarons in a nanotube electromechanical resonator by increasing the effect of the electron-phonon |
Tuesday, March 3, 2020 3:18PM - 3:30PM |
J62.00003: Optical transition dipoles in quasi-two dimensional semiconductor nanomaterials Xuedan Ma Optical transition dipole moment is the key parameter in determining the interactions between an optical emitter and external electromagnetic fields. Understanding and having control over the transition dipole moments could lead to optoelectronic and photonic devices with unprecedented performance.[1,2] Transition dipole moments of materials can vary significantly depending on the materials’ band structures and compositions. In this work, we investigate the intrinsic optical transition dipole moments in quasi-two dimensional semiconductor nanomaterials, namely semiconductor nanoplatelets and quantum rings, to understand the influence of geometry and electronic structures on the transition dipole moments. Combining advanced quantum optical microscopy techniques with empirical tight-binding theory, we reveal strikingly different optical transition dipole properties in the two types of structures,[3,4] indicating the importance of material geometry and electronic structures in determining the transition dipole moments. |
Tuesday, March 3, 2020 3:30PM - 3:42PM |
J62.00004: True Bilayer Exciton Condensate of One-Dimensional Electrons Adrian Kantian We theoretically predict that a true bilayer exciton condensate, characterized by off-diagonal long-range order and global phase coherence, can be created in one-dimensional solid state electron systems. The mechanism by which this happens is to introduce a single particle hybridization of electron and hole populations, which locks the phase of the relevant mode and hence invalidates the Mermin-Wagner theorem. Electron-hole interactions then amplify this tendency towards off-diagonal long-range order, enhancing the condensate properties by more than an order of magnitude over the noninteracting limit. We show that the temperatures below which a substantial condensate fraction would form could reach hundreds of Kelvin, a benefit of the weak screening in one-dimensional systems. |
Tuesday, March 3, 2020 3:42PM - 3:54PM |
J62.00005: Can trions crystallize in TMDC monolayers? Roman Kezerashvili Positively and negatively charged trions in monolayers of transition metal dichalcogenides (TMD) are bound with binding energies of 20-30 meV [1]. Based on the existence of the trions in TMD monolayers, one can propose that a dilute gas of trions, as composite fermions, forms a 2D Wigner crystal. This is similar to the 2D Wigner crystal formed by a dilute system of electrons. The formation of 2D Wigner crystal of the dilute system of trions in TMDC monolayers is studied. We show how the reduction of dimensionality affects the binding energy of trions and their crystallization in TMD monolayers. The 2D trion gas in TMD is described using a Charge–Charge, Charge-Exciton, Exciton-Exciton interactions based on the screened Coulomb interaction - Keldysh potential [2]. The formation of the 2D Wigner crystal in the dilute system of trions in TMD monolayers, when the average distance between the trions is much larger than the radius of each trion is predicted. It is shown that the critical density of the formation of the trion Wigner crystal is greater than the critical density of the electron Wigner crystal in the same material. |
Tuesday, March 3, 2020 3:54PM - 4:06PM |
J62.00006: Direct Visualization of Exciton Transport in Semiconductor Quantum Dot Nanostructures Using Time-Resolved Superresolution Microscopy Alan Van Orden, Megan Dunlap, Duncan P. Ryan, Peter Goodwin, James Werner, Jennifer A Hollingsworth, Martin Paul Gelfand A time-resolved superresolution microscope to localize single emitters with nanometer precision and image their lifetimes with sub-nanosecond time resolution is described. This technique has been used to image semiconductor quantum dot (QD) nanostructures composed of single QD emitters that interact via resonance energy transfer. Photoemission from the nanostructures is imaged onto a 2x2 optical fiber array, and the fiber outputs are monitored using time-correlated single photon counting. The relative intensities reveal changes in the emission center as the QDs blink on and off. Emission centers separated by 10-nm or less can be distinguished based on changes in intensity and lifetime that occur when energy is transferred from donor to acceptor QDs. The distribution of centroid positions provide a direct visualization of the energy transport pathway through the nanostrucure. |
Tuesday, March 3, 2020 4:06PM - 4:18PM |
J62.00007: Electron-hole superfluidity controlled by a periodic potential in double layers of two-dimensional material Oleg Berman, Roman Kezerashvili, Yurii Lozovik, Klaus Ziegler We propose to control of electron-hole superfluidity in semiconductor coupled quantum wells and double layers of 2D material by an external periodic potential [1]. The latter can either be created by periodic gates attached to quantum wells or the double layers of 2D material or by the Moiré pattern of two twisted layers. Treating the electron-hole pairing within the mean-field approach, we apply the tight-binding approximation of the single electron spectrum and study the effect of the additional periodic potential on the electron-hole plasma-superfluid transition. The electron-hole pairing order parameter as a function of the temperature, the charge carrier density, and the gate parameters are obtained by minimization of the mean-field free energy. The second order phase transition between superfluid and electron-hole plasma, controlled by the external periodic potential, is studied for various parameters. |
Tuesday, March 3, 2020 4:18PM - 4:30PM |
J62.00008: Exciton- and phonon-mediated electron ultrafast dynamics in semiconducting carbon nanotubes Stefano Dal Forno, Marco Battiato Semiconducting single-wall carbon nanotubes (SWCNTs) are ideal one-dimensional materials that have attracted much attention in the construction of novel optoelectronic devices. Their exeptional properties arise from the tunability of quantities such as chirality, twist angle and diameter. The understanding of scattering processes in CNTs is complex due to the non trivial interplay of different quasiparticles such as electrons, holes, phonons and excitons. In particular, due to the low dimensionality and the relatively high dielectric constant, excitons in SWCNTs possess very high binding energy and large spatial extent. It is therefore critical to model this theoretically hard problem to allow for the exploitation of CNTs properties in applications like low dimensional electronics or THz emission. |
Tuesday, March 3, 2020 4:30PM - 4:42PM |
J62.00009: Electroluminescence from multi-particle exciton complexes in transition metal dichalcogenide semiconductors Matthias Paur, Aday J. Molina-Mendoza, Rudolf Bratschitsch, Kenji Watanabe, Takashi Taniguchi, Thomas Mueller Light emission from higher-order correlated excitonic states has been recently reported in hBN-encapsulated monolayer WSe2 and WS2 upon optical excitation. These exciton complexes are found to be bound states of excitons residing in opposite valleys in momentum space, a promising feature that could be employed in valleytronics or other novel optoe- lectronic devices. However, electrically-driven light emission from such exciton species is still lacking. Here we report electroluminescence from bright and dark excitons, negatively charged trions and neutral and negatively charged biexcitons, generated by a pulsed gate voltage, in hexagonal boron nitride encapsulated monolayer WSe2 and WS2 with graphene as electrode. By tailoring the pulse parameters we are able to tune the emission intensity of the different exciton species in both materials. We find the electroluminescence from charged biexcitons and dark excitons to be as narrow as 2.8 meV. |
Tuesday, March 3, 2020 4:42PM - 4:54PM |
J62.00010: Strong plasmon coupling to high-energy band-nested excitons in 2D TMDCs Aaron Rose, Jeremy R. Dunklin, Hanyu Zhang, Sanjini U Nanayakkara, Elisa M Miller, Jao van de Lagemaat We show Rabi splitting of 244 meV at the C-exciton in the two-dimensional (2D) transition metal dichalcogenide (TMDC) MoS2. The splitting is 9.5 % of the transition energy (2.57 eV) in k-space (607 meV or 23.6 % in θ-space), which is ~2.5× the ratio observed in similar experiments at the A- and B-excitons and approaches the ultrastrong coupling limit. The large enhancement in the coupling strength is likely due to the nested conduction and valence bands found in the 2D TMDCs which leads to strong absorption. We measured the k-space dispersion of few-layer MoS2 in a tunable plasmonic cavity at room temperature to observe the Rabi splitting. The band-nesting region of 2D TMDCs is under study for spontaneous charge-separation and slowed hot carrier cooling. Our observation suggests that strong coupling of the C-exciton in the 2D TMDCs may improve carrier lifetimes as well as enhance other quantum coherent phenomena such as polariton condensation or molecular hybridization for chemical reactions. |
Tuesday, March 3, 2020 4:54PM - 5:06PM |
J62.00011: Study of exciton diffusion in carbon-nanotube thin-films Yichen Li, Samuel W Belling, Amirhossein Davoody, Irena Knezevic Carbon nanotubes have attracted considerable attention for their potential use in organic thin-film photovoltaic devices. Given the fact that the exciton-transfer properties are directly related to improving the efficiency of these devices, it is crucial to develop a model of exciton diffusion in various carbon nanotube composites and study the diffusion mechanism. Here, we focus on the role of different properties of carbon nanotubes, including chirality and spacing, on exciton diffusion. We generate realistic nanotube meshes with different properties through which the motion of excitons is tracked using a Monte Carlo simulation. Finally, we compare the calculated exciton diffusion rate and diffusion length to experiment and develop an understanding of how different conditions of carbon nanotube aggregation affect diffusion properties. |
Tuesday, March 3, 2020 5:06PM - 5:18PM |
J62.00012: Electric Field Effect of TC in Thin Flakes of the Excitonic Insulator Ta2NiSe5 Michael Mastalish, Arash Fereidouni, Krishna Pandey, Rabindra Basnet, Jin Hu, Hugh Churchill The layered, small-gap semiconductor Ta2NiSe5 is a prominent excitonic insulator candidate with a TC of 326 K. We investigated the electric field effect of the excitonic insulator transition temperature in a device fabricated with thin (15 nm) Ta2NiSe5. Flakes of Ta2NiSe5 were obtained by mechanical exfoliation and transferred onto narrow bottom gates insulated with Al2O3, and the Ta2NiSe5 was then contacted with Cr/Au. Measurements of source-drain current as a function of temperature revealed a small yet sharp drop in current near TC. A modest, reproducible shift of TC by approximately 1 K was observed under a transverse electric field of +/- 0.5 V/nm. Future experiments with this and similar devices will probe the origin of this field effect on TC, as well as the unexpected sign of the change in current at TC. |
Tuesday, March 3, 2020 5:18PM - 5:30PM |
J62.00013: Potential gradient control of room-temperature triangular-whispering gallery polariton condensate Hyun Gyu Song, Sunghan Choi, Chung Hyun Park, Su-Hyun Gong, Chulwon Lee, Min Sik Kwon, Dae Gwang Choi, Kie Young Woo, Yong-Hoon Cho Exciton polaritons offer hybrid nature of excitons and photons, which are interactive quasi-particles providing the potential energy manipulation. Although wide bandgap semiconductors form room-temperature polaritons, the fabrication of homogeneous planar microcavities is challenging due to low refractive index contrast and large lattice misfit in III-nitride systems. Consequently, lateral localization of planar cavities by disorder (potential fluctuation and dephasing) hinder the establishment of ballistic extensions of polariton condensates, an essential factor for any quantum controls. We introduce a room-temperature polariton system with low disorder capable of one-dimensional (1D) ballistic extension.1 Selectively grown GaN wires reduce disorder in both excitons by dislocation bending and photons by crystallographically defined hexagonal facet. This high-quality wire allows us to form room-temperature triangular-whispering gallery polariton condensates with ballistic extension controlled by the optical potential gradient. The correlation between transport characteristics from momentum and real space offers a strong indication of ballistic transport in this 1D system. |
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