Coherent band-edge oscillations and dynamic longitudinal-optical phonon mode splitting as evidence for polarons in perovskites

Liu, Zhaoyu; Vaswani, Chirag; Luo, Liang; Cheng, Di; Yang, Xu; Zhao, Xin; Yao, Yongxin; Song, Zhaoning; Brenes, Roberto; Kim, R. J. H.; Jean, Joel; Bulović, Vladimir; Yan, Yuli; Ho, Kai‐Ming; Wang, Jigang

doi:10.1103/physrevb.101.115125

Cited by 23 publications

(24 citation statements)

References 54 publications

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“…1(a). Rashba-type effects, which control the direct or indirect nature of spin-split bands and fine structure splitting (FSS) [1,2], have been proposed to determine the outstanding, yet mysterious properties in perovskites [3]. These include, e.g., long photocarrier lifetimes [4], spin or charge diffusion lengths [5,6] and nonlinear optics [7].…”

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confidence: 99%

Ultrafast Control of Excitonic Rashba Fine Structure by Phonon Coherence in the Metal Halide Perovskite CH3NH3PbI3<

et al. 2020

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We discover hidden Rashba fine structure in CH3NH3PbI3 and demonstrate its quantum control by vibrational coherence through symmetry-selective vibronic (electron-phonon) coupling. Above a critical threshold of a single-cycle terahertz pump field, a Raman phonon mode distinctly modulates the middle excitonic states with persistent coherence for more than ten times longer than the ones on two sides that predominately couple to infrared phonons. These vibronic quantum beats, together with first-principles modeling of phonon periodically modulated Rashba parameters, identify a threefold excitonic fine structure splitting, i.e., optically forbidden, degenerate dark states in between two bright ones with a narrow, similar to 3 nm splitting. Harnessing of vibronic quantum coherence and symmetry inspires lightperovskite quantum control and sub-THz-cycle "Rashba engineering" of spin-split bands for ultimate multifunction device.

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Ultrafast Control of Excitonic Rashba Fine Structure by Phonon Coherence in the Metal Halide Perovskite CH3NH3PbI3<

et al. 2020

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“…Phase dynamics in SCs can play an important role when spatial variations are considered. We conclude that THz dynamical symmetry breaking during cycles of coherence oscillations is a powerful concept for addressing quantum sensing and coherent control of different quantum materials [10,[64][65][66][67][68][69] and topological phase transitions [70,71] at the ultimate subcycle speed limit necessary for lightwave quantum electronics and magnetoelectronics.…”

Section: Discussionmentioning

confidence: 95%

Lightwave terahertz quantum manipulation of nonequilibrium superconductor phases and their collective modes

2020

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We present a gauge-invariant density matrix description of nonequilibrium superconductor (SC) states with spatial and temporal correlations driven by intense terahertz (THz) lightwaves. We derive superconductor Bloch-Maxwell equations of motion that extend Anderson pseudospin models to include the Cooper pair center-of-mass motion and electromagnetic propagation effects. We thus describe quantum control of dynamical phases, collective modes, quasiparticle coherence, and high nonlinearities during cycles of carrier wave oscillations, which relates to our recent experiments. Coherent photogeneration of a nonlinear supercurrent with a dc component, achieved via condensate acceleration by an effective lightwave field, dynamically breaks the equilibrium inversion symmetry. Experimental signatures include high harmonic light emission at equilibrium-symmetry-forbidden frequencies, Rabi-Higgs collective modes and quasiparticle coherence, and nonequilibrium moving condensate states tuned by few-cycle THz fields. We use such lightwaves as an oscillating accelerating force that drives strong nonlinearities and anisotropic quasiparticle populations to control and amplify different classes of collective modes, e.g., damped oscillations, persistent oscillations, and overdamped dynamics via Rabi flopping. Recent phase-coherent nonlinear spectroscopy experiments can be modeled by solving the full nonlinear quantum dynamics including self-consistent light-matter coupling.

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“…[ 65 ] The assumption of polaron formation is also consistent with published results, since the sub‐ps initial DT spectral dynamics as well as ultrafast coherent oscillations have been described as evidence for polaron formation, in agreement with our interpretation. [ 11,17,66–68 ]…”

Section: Discussionmentioning

confidence: 99%

“…[65] The assumption of polaron formation is also consistent with published results, since the sub-ps initial DT spectral dynamics as well as ultrafast coherent oscillations have been described as evidence for polaron formation, in agreement with our interpretation. [11,17,[66][67][68] The most interesting comparison is perhaps with ultrafast measurements in the THz spectral region, where bound excitons and unbound carriers could be discriminated. THz measurements have provided, together with evidence for free carriers in 3D HPs and ultrafast exciton formation in 2D materials, some interesting signatures of significant presence of free carriers even in 2D perovskites and at low temperatures, [38][39][40][41][42]66,67] where only excitons would have been expected.…”

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confidence: 99%

Polaron Plasma in Equilibrium with Bright Excitons in 2D and 3D Hybrid Perovskites

Simbula¹,

Pau²,

Wang³

et al. 2021

Advanced Optical Materials

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spectrum featuring a marked excitonic resonance, the majority optical excitation in prototypical HP materials for photovoltaics are not bound excitons, but unbound charge carriers. Therefore, electrons and holes excited by solar light can be directed to the electrodes at a negligible energy cost, without the need to split tightly bound excitons as in organics. Taking advantage of the flexibility of the materials class, layered 2D HPs are obtained by inserting bulky organic cations into the formulation, leading to materials inherently more stable than their 3D counterparts against degradation. [8][9][10] However, in 2D HPs the exciton binding energy can be as large as 400 meV, [8] so that it is commonly assumed that their excited states are mostly excitons.A second peculiar characteristic of the excited states in perovskites is the formation of large polarons, that is, charge carriers coupled to lattice deformations and delocalized over many crystal lattice sites. [11][12][13][14][15][16][17][18][19][20] Unlike small polarons in organics, localized in a single molecule, large polarons are compatible with band transport, but are also able to screen the excited states from scattering with defects and reduce non-radiative recombination through trap states, resulting in large mobilities and long lifetimes. Large polarons are also believed to reduce scattering with phonons and have been proposed as an explanation for hot carriers persisting for several nanoseconds at temperatures significantly higher than the lattice one. [12,16,17,[21][22][23][24][25][26] Large polarons may therefore be the enabling microscopic mechanism for efficient solar cells, including innovative architectures that exploit photoconversion with hot carriers. [27,28] Theoretical estimations forecast that the energy associated with polaron formation is comparable with the binding energy gained by forming an exciton, maybe even larger in some materials. [12,14,24,[29][30][31][32] When do polarons form and whether excitons or polarons are the lowest-energy optical excitations is still an open question. The issue is particularly relevant for layered 2D HPs, where polaronic effects have been demonstrated, although it is not clear if small or large polarons are formed. [33][34][35][36][37] In spite of the large exciton binding energy, unbound charge carriers have been reported, so that it is not clear yet how much energy needs to be spent in solar cells to split bound excitons.

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Coherent band-edge oscillations and dynamic longitudinal-optical phonon mode splitting as evidence for polarons in perovskites

Cited by 23 publications

References 54 publications

Ultrafast Control of Excitonic Rashba Fine Structure by Phonon Coherence in the Metal Halide Perovskite CH3NH3PbI3<

Ultrafast Control of Excitonic Rashba Fine Structure by Phonon Coherence in the Metal Halide Perovskite CH3NH3PbI3<

Lightwave terahertz quantum manipulation of nonequilibrium superconductor phases and their collective modes

Polaron Plasma in Equilibrium with Bright Excitons in 2D and 3D Hybrid Perovskites

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