Dynamic Lattice Disorder and Collective Dipole Coupling Give Rise to Dicke Physics in Perovskite Quantum Dots

Abstract

Halide perovskite quantum dots exhibit cooperative optical phenomena that are absent in conventional semiconductor nanocrystals, including exciton superradiance, superabsorption, and biexciton superradiance within individual dots. Here we develop a microscopic theory that identifies the physical origin of these Dicke effects and establishes how they can be controlled by materials parameters. The central result is that cooperative emission emerges from a competition between collective coupling of optical transition dipoles and lattice-induced disorder, with the balance governed by the Raman-derived phonon spectral density and the excitonic oscillator strength. At elevated temperature, strong Fröhlich coupling and glassy lattice dynamics produce dynamic disorder that suppresses dipole synchronization and yields incoherent emission. Upon cooling, lattice fluctuations freeze and cooperative coherence emerges when the collective coupling exceeds residual static disorder, defining size- and composition-dependent crossover temperatures that we map as phase diagrams. Extending the framework to biexcitons, we show that a confined biexciton constitutes a single correlated charge distribution dressed by a shared lattice configuration, enabling pathway-indistinguishable decay and cooperative radiative enhancement. The theory quantitatively accounts for observed size, composition, and temperature trends in radiative-rate constant ratios and biexciton binding energies, while explaining why full Dicke saturation is not universal. More broadly, the results establish Raman spectral weight and oscillator strength as design parameters for engineering cooperative quantum-optical behavior in quantum materials.

Figures (3)

• Figure 1: Figure 1 | Emergence of Dicke physics in perovskite quantum dots and its lattice origin. (a) Schematic illustration of an ensemble of unit-cell transition dipoles at high temperature (T$>$T$_c$), where strong lattice-induced disorder randomizes dipole phases and suppresses cooperative emission. (b) At low temperature (T$<$T$_c$), suppression of dynamic disorder allows dipole--dipole coupling to synchronize the unit-cell dipoles into a macroscopic polarization, giving rise to Dicke superradiance (and, in reverse, superabsorption). (c) Experimental observation of superradiance from both excitons (X) and biexcitons (XX) in a single 15-nm CsPbBr$_3$ quantum dot, first reported by Kambhampati and coworkers (2023), showing a strong enhancement of the radiative rate constant upon cooling. (d) Single-dot measurements of the size dependence of exciton superradiance in CsPbBr$_3$ quantum dots reported by Kovalenko and collaborators (2024), demonstrating a pronounced low-temperature enhancement that increases with dot diameter, in contrast to the weak and non-systematic behavior at room temperature. (e) Structural motif of the halide perovskite lattice, highlighting its soft, dynamically disordered, and defect-tolerant nature, which enables strong coupling between electronic excitations and lattice degrees of freedom. (f) Raman spectra comparing a perovskite quantum dot and a CdSe quantum dot, illustrating the broad, glassy low-frequency spectral weight and strong exciton--phonon coupling (S$\sim$1) in perovskites, in contrast to the weak, discrete phonon modes (S$\sim$0.1) of CdSe. Together, these panels motivate a microscopic picture in which the balance between collective dipole coupling and lattice-induced disorder governs the emergence of Dicke physics in perovskite quantum dots.
• Figure 2: Figure 2 | Exciton superradiance in perovskite quantum dots: size, composition, and phase behavior. (a) Size dependence of the energy parameter that controls the temperature evolution of the exciton radiative rate constant. This parameter is extracted from the exponential dependence of the radiative-rate constant enhancement on inverse temperature and represents the effective coherence energy arising from the competition between collective dipole coupling and lattice-induced disorder. Its systematic variation with dot diameter and halide composition reflects changes in coherence volume and lattice spectral weight. (b) Corresponding enhancement ratio of the exciton radiative rate constant at 10 K relative to 300 K, showing a strong, superlinear increase with diameter and a clear composition dependence, indicative of the rapid growth of cooperative coherence at low temperature. (c) Temperature--diameter phase diagram separating incoherent and Dicke-superradiant regimes, constructed using a fixed enhancement threshold. Larger dots enter the superradiant regime at higher temperature, reflecting stronger collective coupling. (d) Temperature--composition phase diagram for selected diameters, illustrating how lattice softness and phonon spectral weight tune the crossover temperature between incoherent and superradiant phases. Together, these panels demonstrate that exciton superradiance in perovskite quantum dots is governed by a balance between collective dipole coupling and lattice-induced disorder, and can be systematically tuned by size and composition.
• Figure 3: Figure 3 | Biexciton Dicke physics in perovskite quantum dots: binding, lattice dressing, and cooperative emission. (a) Size-dependent biexciton binding energies extracted from photoluminescence for CdSe and CsPbBr$_3$ quantum dots at 300 K, plotted versus normalized diameter (D/a$_B$). The curves compare Coulomb-only behavior with Coulomb plus lattice stabilization. For perovskites, removing lattice stabilization yields the limiting behavior appropriate to the low-temperature ($\approx$10 K) regime, where the apparent biexciton binding energy is reduced by approximately a factor of two relative to 300 K. (b) Lattice (polaronic) contribution to the biexciton binding energy versus (D/a$_B$) for representative perovskite compositions. The increase and subsequent saturation with size reflects the formation of a shared, spatially coherent lattice polarization field dressing the biexciton, consistent with a single-polaron (not bipolaron) picture and enabling pathway indistinguishability required for cooperative biexciton emission. (c) Predicted temperature dependence of biexciton superradiance quantified by the ratio of biexciton to exciton radiative rate constants, plotted versus inverse temperature for a fixed dot size and multiple compositions. The approximately linear dependence on (1/T) reflects progressive suppression of lattice-induced dephasing and funneling into bright biexciton states upon cooling. (d) Low-temperature (10 K) size dependence of the biexciton-to-exciton radiative-rate constant ratio for multiple compositions, illustrating how closely the system approaches the Dicke limit of 2 and how incomplete saturation depends on size and lattice/compositional disorder.