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1. Moiré pattern of interference dislocations in condensate of indirect excitons

   https://doi.org/10.1038/s41467-021-21353-7  

   Leonard, J. R. ; Hu, Lunhui ; High, A. A. ; Hammack, A. T. ; Wu, Congjun ; Butov, L. V. ; Campman, K. L. ; Gossard, A. C. ( December 2021 , Nature Communications)

   null (Ed.)

   Abstract Interference patterns provide direct measurement of coherent propagation of matter waves in quantum systems. Superfluidity in Bose–Einstein condensates of excitons can enable long-range ballistic exciton propagation and can lead to emerging long-scale interference patterns. Indirect excitons (IXs) are formed by electrons and holes in separated layers. The theory predicts that the reduced IX recombination enables IX superfluid propagation over macroscopic distances. Here, we present dislocation-like phase singularities in interference patterns produced by condensate of IXs. We analyze how exciton vortices and skyrmions should appear in the interference experiments and show that the observed interference dislocations are not associated with these phase defects. We show that the observed interference dislocations originate from the moiré effect in combined interference patterns of propagating condensate matter waves. The interference dislocations are formed by the IX matter waves ballistically propagating over macroscopic distances. The long-range ballistic IX propagation is the evidence for IX condensate superfluidity. 

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2. Pancharatnam–Berry phase in condensate of indirect excitons

   https://doi.org/10.1038/s41467-018-04667-x  

   Leonard, J. R. ; High, A. A. ; Hammack, A. T. ; Fogler, M. M. ; Butov, L. V. ; Campman, K. L. ; Gossard, A. C. ( June 2018 , Nature Communications)

   The Pancharatnam–Berry phase is a geometric phase acquired over a cycle of parameters in the Hamiltonian governing the evolution of the system. Here, we report on the observation of the Pancharatnam–Berry phase in a condensate of indirect excitons (IXs) in a GaAs-coupled quantum well structure. The Pancharatnam–Berry phase is directly measured by detecting phase shifts of interference fringes in IX interference patterns. Correlations are found between the phase shifts, polarization pattern of IX emission, and onset of IX spontaneous coherence. The evolving Pancharatnam–Berry phase is acquired due to coherent spin precession in IX condensate and is observed with no decay over lengths exceeding 10 μm indicating long-range coherent spin transport.

    

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3. Anisotropic Excitons Reveal Local Spin Chain Directions in a van der Waals Antiferromagnet

   https://doi.org/10.1002/adma.202206585  

   Kim, Dong Seob ; Huang, Di ; Guo, Chunhao ; Li, Kejun ; Rocca, Dario ; Gao, Frank Y. ; Choe, Jeongheon ; Lujan, David ; Wu, Ting‐Hsuan ; Lin, Kung‐Hsuan ; et al ( March 2023 , Advanced Materials)

   A long‐standing pursuit in materials science is to identify suitable magnetic semiconductors for integrated information storage, processing, and transfer. Van der Waals magnets have brought forth new material candidates for this purpose. Recently, sharp exciton resonances in antiferromagnet NiPS₃have been reported to correlate with magnetic order, that is, the exciton photoluminescence intensity diminishes above the Néel temperature. Here, it is found that the polarization of maximal exciton emission rotates locally, revealing three possible spin chain directions. This discovery establishes a new understanding of the antiferromagnet order hidden in previous neutron scattering and optical experiments. Furthermore, defect‐bound states are suggested as an alternative exciton formation mechanism that has yet to be explored in NiPS₃. The supporting evidence includes chemical analysis, excitation power, and thickness dependent photoluminescence and first‐principles calculations. This mechanism for exciton formation is also consistent with the presence of strong phonon side bands. This study shows that anisotropic exciton photoluminescence can be used to read out local spin chain directions in antiferromagnets and realize multi‐functional devices via spin‐photon transduction.

    

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4. Exciton dispersion and exciton–phonon interaction in solids by time-dependent density functional theory

   https://doi.org/10.1063/5.0137326  

   Liu, Junyi ; Lu, Gang ; Zhang, Xu ( January 2023 , The Journal of Chemical Physics)

   Understanding, predicting, and ultimately controlling exciton band structure and exciton dynamics are central to diverse chemical and materials problems. Here, we have developed a first-principles method to determine exciton dispersion and exciton–phonon interaction in semiconducting and insulating solids based on time-dependent density functional theory. The first-principles method is formulated in planewave bases and pseudopotentials and can be used to compute exciton band structures, exciton charge density, ionic forces, the non-adiabatic coupling matrix between excitonic states, and the exciton–phonon coupling matrix. Based on the spinor formulation, the method enables self-consistent noncollinear calculations to capture spin-orbital coupling. Hybrid exchange-correlation functionals are incorporated to deal with long-range electron–hole interactions in solids. A sub-Hilbert space approximation is introduced to reduce the computational cost without loss of accuracy. For validations, we have applied the method to compute the exciton band structure and exciton–phonon coupling strength in transition metal dichalcogenide monolayers; both agree very well with the previous GW-Bethe–Salpeter equation and experimental results. This development paves the way for accurate determinations of exciton dynamics in a wide range of solid-state materials.

    

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5. Magnetic-gateable valley exciton emission

   https://doi.org/10.1038/s41524-020-00356-w  

   Bragança, Helena ; Zeng, Hao ; Dias, Alexandre Cavalheiro ; Correa, Jorge Huamani ; Qu, Fanyao ( July 2020 , npj Computational Materials)

   The use of valley excitonic states of transition metal dichalcogenides to store and manipulate information is hampered by fast carrier recombination and short valley lifetime. We propose theoretically a scheme to overcome such an obstacle, by applying a tilted exchange field through the magnetic proximity effect on monolayer MoS₂. While the in-plane component of the exchange field brightens the dark exciton by spin mixing, the out-of-plane field can effectively gate the emission with an ON/OFF ratio of 2700. Importantly, the brightening is valley selective, leading to nearly 100% valley and spin polarization at room temperature. The resulting strongly gateable dark-exciton emission with long lifetime and near unity valley polarization makes it convenient to manipulate the valley degree of freedom, which may offer new paradigm for information processing and transmission.

    

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