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 C55: Excitons in Nanoscale Structures and SystemsFocus Live
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Sponsoring Units: DMP Chair: William Rice, University of Wyoming |
Monday, March 15, 2021 3:00PM - 3:36PM Live |
C55.00001: Exciton localization and multiexciton enhancement in single-layer WSe2 Invited Speaker: Nicholas Borys The merger of intense coulombic interactions with a unique 2D band structure in atomically thin semiconductors gives rise to rich excitonic behavior that opens new scientific and technological opportunities but occurs on experimentally challenging ultrafast timescales and ultrasmall length scales. By combining powerful optical techniques such as nano-optical and low-temperature photoluminescence spectroscopies, deep insight can be gained at these scales on how the excitonic properties of 2D materials produce, for instance, new quantum functionalities. Recent nano-optical studies by our team have revealed a new mechanism by which strain localizes excitons on nanometer scales at room-temperature, providing key insight into single-photon emitters in WSe2. And low-temperature studies have demonstrated that optical excitation of an electron-hole plasma and its ultrafast relaxation enhances the formation of multiexcitons, which are known emitters of entangled photon pairs. These studies elucidate structure-property relationships and many-body dynamics that are unique to 2D semiconductors and important for harnessing these materials for next-generation quantum optoelectronics. |
Monday, March 15, 2021 3:36PM - 3:48PM Live |
C55.00002: Moiré pattern of interference dislocations and superfluidity in condensate of indirect excitons Jason Leonard, Lunhui Hu, Alexander A High, Aeron T Hammack, Congjun Wu, Leonid V Butov, Ken L Campman, Arthur C Gossard Interference patterns provide direct measurement of coherent propagation of matter waves in quantum systems. Dislocations in interference patterns is a basic phenomenon, which is observed in a variety of quantum systems including atoms, polaritons, magnons, and optical systems. Interference dislocations have been associated with vortices. In this work, we present a new mechanism – the moiré effect, which leads to the appearance of dislocations in interference patterns. We show that this mechanism is the origin of interference dislocations in condensate of indirect excitons (IXs). These interference dislocations are formed by IX condensate matter waves ballistically propagating over macroscopic distances. The long-range ballistic IX propagation is the evidence for IX condensate superfluidity. Superfluidity was predicted for IXs decades ago. The finding of IX condensate superfluidity in interference experiments is consistent with this theoretical prediction. |
Monday, March 15, 2021 3:48PM - 4:00PM Live |
C55.00003: Optical excitations in poly(p-phenylene vinylene) polymer crystals in the GW plus Bethe-Salpeter equation approach Fangzhou Zhao, Yang-hao Chan, Zhenglu Li, Steven G Louie Conjugated polymers have been promising candidates for applications such as light-emitting diodes due to their fascinating electronic and optical properties. The optical absorption spectra of different kinds of conjugated polymers have been widely studied both experimentally and computationally. However, first-principles calculations in the literature commonly underestimate the optical gaps of conjugated polymers [1], and do not reproduce the shape of the optical absorption spectra qualitatively [2]. We investigate the optical absorption spectra and excitons in the poly(p-phenylene vinylene) (PPV) from first-principles by the GW plus Bethe-Salpeter equation (GW-BSE) approach to include relevant many-body effects. The dependence of the calculated absorption spectrum on various approximations employed in the first-principles calculations has been studied, and a calculated absorption spectrum with better match to the experimental results has been achieved. |
Monday, March 15, 2021 4:00PM - 4:12PM Live |
C55.00004: Modeling Interlayer Exciton Trapping in Two-dimensional Heterostructures with Discrete, Random-walks Darien Morrow, Xuedan Ma Excitons in transition metal dichalcogenides (TMDCs) host a wealth of many-body physics and exciting applications in photonic and optoelectronic devices. Local strain modulation in TMDCs creates potential traps which localize excitons. These strain-induced traps can localize interlayer excitons (IXs) in TMDC vertical heterostructures.[1,2] Trapped IXs in TMDCs are ideal candidates for condensate formation and long-wavelength single photon emitters.[3] Using a discrete, random-walk model, we simulate trapping of IXs in traps of varying depth, density, and shape to explore the experimentally relevant parameter space for efficient trapping of IXs. Our results show that dipole-dipole interactions between IXs play an important role in regulating IX trapping. The effects of these dipole interactions are mitigated with many small, deep traps which are synthetically possible using defects and moiré potentials. |
Monday, March 15, 2021 4:12PM - 4:24PM Live |
C55.00005: Dipolar Attraction and Repulsion Between Indirect Excitons Darius Choksy, Chao Xu, Michael Fogler, Leonid V Butov, Justin Norman, Arthur C Gossard We examine the attractive and repulsive dipolar interaction between indirect excitons (IXs) formed by electrons and holes in separated layers. Repulsive dipolar IX interactions have been studied thoroughly in the past but attractive interaction have yet to be similarly explored. Each layer of IXs forms in a single pair of couple quantum wells (CQW) where the out-of-plane IX electric dipoles lead to the repulsive interaction between IXs. The attractive interaction appears between IXs in different IX layers created in 2-CQW heterostructures with 2 layers of IX dipoles. In both experimental measurements and theoretical simulations, we found that increasing the density of IXs in one layer causes a monotonic energy reduction for IXs in the other layer. Additionally, we found an in-plane shift of a cloud of IXs in one layer toward a cloud of IXs in the other layer. This behavior is qualitatively consistent with attractive dipolar interaction. The measured IX energy reduction and IX cloud shifts are higher than the values given by the correlated liquid theory. |
Monday, March 15, 2021 4:24PM - 4:36PM Live |
C55.00006: Strain induced quantum Hall phenomena of excitons in graphene Oleg L. Berman, Roman Kezerashvili, Yurii E. Lozovik, Klaus G. Ziegler We predict quantum Hall phenomena for excitons such as the Integer and Fractional Quantum Hall effects, and the state of composite fermions at ν = 1/2 in a mono and double layer of gapped graphene under strain induced gauge pseudomagnetic field. When electrons and holes are excited only in one valley of the honeycomb lattice of gapped graphene, the strain induced pseudomagnetic field acts on electrons and holes the same way. The latter leads to the formation of single pseudomagnetoexcitons (PMEs) whose properties are cardinally different from those of magnetoexcitons. Wave functions and the energy spectrum of direct and indirect PMEs in a mono and double layer of gapped graphene are obtained. The valley Hall flows of direct and indirect PME’s, similar to Hall currents of charged particles, can be excited in a mono or double layer of gapped graphene, respectively. The predicted quantum Hall phenomena for the PMEs are important, since they imply that the quantum Hall physics and valleytronics phenomena can be observed in the novel system of neutral bosons without magnetic field. |
Monday, March 15, 2021 4:36PM - 4:48PM Live |
C55.00007: Direct Visualization of Exciton Transport in Semiconductor Quantum Dot Nanostructures Using Time-Resolved Super-Resolution Microscopy Megan Dunlap, Duncan P. Ryan, Peter Goodwin, James Werner, Jennifer A Hollingsworth, Martin Paul Gelfand, Alan Van Orden A time-resolved superresolution microscope to localize single emitters with nanometer precision and image their lifetimes with subnanosecond 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 the aperture of a 2x2 optical fiber array, and the output of each is monitored using time-correlated single photon counting. The relative intensities of the four detection channels 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, plotted as a function of lifetime and intensity, provide a direct visualization of the energy transport pathway through the nanostrucure. |
Monday, March 15, 2021 4:48PM - 5:24PM Live |
C55.00008: Dipole Interactions Between Localized Interlayer Excitons Invited Speaker: Ajit Srivastava Selected by Focus Topic Organizer (Tom Harris) |
Monday, March 15, 2021 5:24PM - 5:36PM Live |
C55.00009: Momentum-dependent Resonant Inelastic X-ray Spectroscopy of Anatase TiO2 Thin Films on LaAlO3 (001) Kidae Shin, Sangjae Lee, Guillaume Marcaud, Jonathan Pelliciari, Jiemin Li, Valentina Bisogni, Ignace Jarrige, Charles Ahn, Frederick Walker Materials that host two-dimensional excitons exhibit a strong bonding of the electron-hole pair. Anatase TiO2 is an interesting host candidate, as it has recently been reported to support strongly bound 2D excitons, even in the bulk. In addition, exciton energies in single crystal anatase have been modulated by as much as 30 to 50 meV at room temperature using coherent acoustic phonons. We employ resonant inelastic X-ray scattering (RIXS) to characterize excitons and phonons in anatase TiO2 thin films grown on LaAlO3 (001) using molecular beam epitaxy. RIXS is an ideal tool to characterize excitations at multiple energy scales due to the momentum dependence of the scattering from both excitons and phonons simultaneously, which can be challenging to achieve with optical methods. Initial results show phonon features in the 50 ~ 300 meV range that exhibit polarization, momentum, and film thickness dependence, revealing the influence of confinement. |
Monday, March 15, 2021 5:36PM - 5:48PM Live |
C55.00010: Exciting transport properties of Ta2NiSe5 Dung Vu, Michael Scudder, Joshua Goldberger, Joseph P C Heremans Ta2NiSe5 is a candidate excitonic insulator that crystalizes as stacked van de Waals layers. Its direct narrow-gap (~ 0.1 eV) promotes spontaneous exciton formation with electrons in Ta chains and holes in Ni chains. We report resistivity r, thermopower S, Hall effect rxy, Nernst effect N and thermal conductivity k in Ta2NiSe5 from 10K to 400K, specific heat Cp from 2K to 395K. Cp shows an anomaly at 324 K like [1], but deviates from the Debye behavior at other temperatures. S has a maximum of 938 mV/K at 80 K and is zero for 10K<T<50K. At 90K<T<300K both S and r (similar to [2]) show activated behavior with activation energy ~0.1eV. Carrier concentration from rxy freeze-out from 1021cm-3 at 300K to 6.3x1015 at 40K. Both N and k are reported here for the first time and we describe the magnetic field dependence of these quantities. |
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