Polaron Formation Time Research Articles

Anomalous Polarons in Two-Dimensional Organometallic Perovskite Ferroelectric.

The concept of ferroelectric polarons is proposed to partially explain the exceptional optoelectronic properties observed in lead halide perovskites (LHPs). It is intriguing but unclear how this proposal, which involves local or transient polarizations, applies in general to 2D LHPs with long-range ferroelectricity. Here, this work presents a pioneering time-domain experimental investigation of polarons in ferroelectric (IA)2(MA)2Pb3Br10 (IMPB; IA is isoamylammonium and MA is methylammonium) using transient absorption spectroscopy. Compared to non-ferroelectric LHPs, IMPB exhibits several distinct polaronic properties closely associated with macroscopic polarizations of ferroelectricity, including a prolonged polaron formation time (≈1.1ps), a Stark splitting of the bleaching (≈63 meV), and a giant polaron Mott density (≈7.6 × 1018 cm-3). These findings broaden the realm of 2D polaron systems and reveal the decisive role of static/unidirectional polarizations on polaron physics in 2D LHPs.

Coherent charge hopping suppresses photoexcited small polarons in ErFeO3 by antiadiabatic formation mechanism.

Polarons are prevalent in condensed matter systems with strong electron-phonon coupling. The adiabaticity of the polaron relates to its transport properties and spatial extent. To date, only adiabatic small polaron formation has been measured following photoexcitation. The lattice reorganization energy is large enough that the first electron-optical phonon scattering event creates a small polaron without requiring substantial carrier thermalization. We measure that frustrating the iron-centered octahedra in the rare-earth orthoferrite ErFeO3 leads to antiadiabatic polaron formation. Coherent charge hopping between neighboring Fe3+─Fe2+ sites is measured with transient extreme ultraviolet spectroscopy and lasts several picoseconds before the polaron forms. The resulting small polaron formation time is an order of magnitude longer than previous measurements and indicates a shallow potential well, even in the excited state. The results emphasize the importance of considering dynamic electron-electron correlations, not just electron-phonon-induced lattice changes, for small polarons for transport, catalysis, and photoexcited applications.

Large Exciton Polaron Formation in 2D Hybrid Perovskites via Time-Resolved Photoluminescence.

We find evidence for the formation and relaxation of large exciton polarons in 2D organic-inorganic hybrid perovskites. Using ps-scale time-resolved photoluminescence within the phenethylammonium lead iodide family of compounds, we identify a red shifting of emission that we associate with exciton polaron formation time scales of 3-10 ps. Atomic substitutions of the phenethylammonium cation allow local control over the structure of the inorganic lattice, and we show that the structural differences among materials strongly influence the exciton polaron relaxation process, revealing a polaron binding energy that grows larger (up to 15 meV) in more strongly distorted compounds.

Critical slowdown of non-equilibrium polaron dynamics

We study the quantum dynamics of a single impurity following its sudden immersion into a Bose–Einstein condensate. The ensuing formation of the Bose polaron in this general setting can be seen as impurity decoherence driven by the condensate, which we describe within a master equation approach. We derive rigorous analytical results for this decoherence dynamics, and thereby reveal distinct stages of its evolution from a universal stretched exponential initial relaxation to the final approach to equilibrium. The associated polaron formation time exhibits a strong dependence on the impurity speed and is found to undergo a critical slowdown around the speed of sound of the condensate. This rich non-equilibrium behavior of quantum impurities is of direct relevance to recent cold atom experiments, in which Bose polarons are created by a sudden quench of the impurity-bath interaction.

Photoexcited Small Polaron Formation in Goethite (α-FeOOH) Nanorods Probed by Transient Extreme Ultraviolet Spectroscopy.

Small polaron formation limits the mobility and lifetimes of photoexcited carriers in metal oxides. As the ligand field strength increases, the carrier mobility decreases, but the effect on the photoexcited small polaron formation is still unknown. Extreme ultraviolet transient absorption spectroscopy is employed to measure small polaron formation rates and probabilities in goethite (α-FeOOH) crystalline nanorods at pump photon energies from 2.2 to 3.1 eV. The measured polaron formation time increases with excitation photon energy from 70 ± 10 fs at 2.2 eV to 350 ± 30 fs at 2.6 eV, whereas the polaron formation probability (85 ± 10%) remains constant. By comparison to hematite (α-Fe2O3), an oxide analogue, the role of ligand composition and metal center density in small polaron formation time is discussed. This work suggests that incorporating small changes in ligands and crystal structure could enable the control of photoexcited small polaron formation in metal oxides.

Large polarons in lead halide perovskites.

Lead halide perovskites show marked defect tolerance responsible for their excellent optoelectronic properties. These properties might be explained by the formation of large polarons, but how they are formed and whether organic cations are essential remain open questions. We provide a direct time domain view of large polaron formation in single-crystal lead bromide perovskites CH3NH3PbBr3 and CsPbBr3. We found that large polaron forms predominantly from the deformation of the PbBr3- frameworks, irrespective of the cation type. The difference lies in the polaron formation time, which, in CH3NH3PbBr3 (0.3 ps), is less than half of that in CsPbBr3 (0.7 ps). First-principles calculations confirm large polaron formation, identify the Pb-Br-Pb deformation modes as responsible, and explain quantitatively the rate difference between CH3NH3PbBr3 and CsPbBr3. The findings reveal the general advantage of the soft [PbX3]- sublattice in charge carrier protection and suggest that there is likely no mechanistic limitations in using all-inorganic or mixed-cation lead halide perovskites to overcome instability problems and to tune the balance between charge carrier protection and mobility.

Dynamics of exciton magnetic polarons in CdMnSe/CdMgSe quantum wells: Effect of self-localization

We study the exciton magnetic polaron (EMP) formation in (Cd,Mn)Se/(Cd,Mg)Se diluted-magnetic-semiconductor quantum wells using time-resolved photoluminescence (PL). The magnetic field and temperature dependencies of this dynamics allow us to separate the non-magnetic and magnetic contributions to the exciton localization. We deduce the EMP energy of 14 meV, which is in agreement with time-integrated measurements based on selective excitation and the magnetic field dependence of the PL circular polarization degree. The polaron formation time of 500 ps is significantly longer than the corresponding values reported earlier. We propose that this behavior is related to strong self-localization of the EMP, accompanied with a squeezing of the heavy-hole envelope wavefunction. This conclusion is also supported by the decrease of the exciton lifetime from 600 ps to 200 - 400 ps with increasing magnetic field and temperature.

Scanning nonlinear absorption in lithium niobate over the time regime of small polaron formation

Nonlinear absorption is studied in presence of small polaron formation in lithium niobate using the z-scan technique and ultrashort laser pulses with pulse durations of 70 – 1,000 fs. A model for the analysis of the transmission loss as a function of pulse duration is introduced that considers (i) the individual contributions of two-photon and small polaron absorption, (ii) the small polaron formation time and (iii) an offset time between the optical excitation of free carriers by two-photon absorption and the appearance of small polarons. It is shown that the model allows for the analysis of the experimentally determined z-scan data with high precision over the entire range of pulse durations using a two-photon absorption coefficient of β = (5.6 ± 0.8) mm/GW. A significant contribution by small polaron absorption to the nonlinear absorption is uncovered for pulse durations exceeding the characteristic small polaron formation time of ≈ 100fs. It can be concluded that the small polaron formation time is as short as (70 – 110) fs and the appearance of small polaron formation is delayed with respect to two-photon absorption by an offset of about 80 fs.

Measurement of the Femtosecond Optical Absorption ofLaAlO3/SrTiO3Heterostructures: Evidence for an Extremely Slow Electron Relaxation at the Interface

The photocarrier relaxation dynamics of an n-type LaAlO3/SrTiO3 heterointerface is investigated using femtosecond transient absorption (TA) spectroscopy at low temperatures. In both LaAlO3/SrTiO3 heterostructures and electron-doped SrTiO3 bulk crystals, the TA spectrum shows a Drude-like free carrier absorption immediately after excitation. In addition, a broad absorption band gradually appears within 40 ps, which corresponds to the energy relaxation of photoexcited free electrons into self-trapped polaron states. We reveal that the polaron formation time is enhanced considerably at the LaAlO3/SrTiO3 heterointerface as compared to bulk crystals. Further, we discuss the interface effects on the electron relaxation dynamics in conjunction with the splitting of the t2g subbands due to the interface potential.

Exciton magnetic polaron in CdMnSe/CdMgSe quantum wells

AbstractWe study exciton magnetic polaron (EMP) formation in (Cd,Mn)Se/(Cd,Mg)Se diluted magnetic semiconductor quantum wells (QWs) using time‐resolved photoluminescence (PL). Magnetic field dependence of the transients allows us to separate the non‐magnetic and magnetic contributions of the exciton localization. We find binding energy of 18 meV and formation time of 500 ps for the EMP. We propose that long polaron formation time is related to auto‐localization process, accompanied with the squeezing of the heavy‐hole envelope wave function. This conclusion is supported by a strong reduction of the exciton radiative lifetime from 600 to 200 ps with increase of magnetic field.

Magnetic-field-induced formation of exciton magnetic polarons in ZnSe/Zn1-xMnxSe quantum-well structures.

cw and time-resolved photoluminescence spectroscopy is used to study ZnSe/${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se quantum wells with semimagnetic barriers in an external magnetic field. The data demonstrate a change of the dominant energy relaxation mechanism from disorder localization of light-hole excitons at zero-field to heavy-hole exciton interface magnetic polaron formation at intermediate fields and, again, disorder localization of heavy-hole excitons at large magnetic fields. The formation of the interface magnetic polaron is promoted by a magnetic-field-induced type-I--type-II transition for heavy-hole excitons. Despite the transition, neither the exciton lifetime nor its optical oscillator strength is dramatically altered. This is, as we confirm by numerical solution of the two-particle Schr\odinger equation, a result of the electron-hole Coulomb interaction. The polaron formation time is initially 110 ps, but decreases with growing magnetic field down to 70 ps (B=5 T). A theoretical investigation of the polaron formation dynamics shows that the associated change of the exciton wave function is smaller, the closer the ${\mathrm{Mn}}^{2+}$ spin system is driven into saturation by the external field. As a consequence, the polaron formation time approaches the characteristic ${\mathrm{Mn}}^{2+}$ spin response time. Our measurement uncovers a fast primary localization prior to the polaron process---but also of magnetic origin---that we believe to be necessary to start the polaron formation. \textcopyright{} 1996 The American Physical Society.

Dynamics of exciton magnetic polarons in [formula omitted] quantum wells

We study the influence of quantum confinement and spin-lattice relaxation on the dynamics of exciton magnetic polaron formation in specially designed semimagnetic quantum wells. Using low temperature time-resolved photoluminescence with variable pulse separation we show that a decrease of the well width does not only enhance the polaron energy but also shortens its formation time. Comparison with numerical calculations reveal that the latter can be associated with smaller changes of the excitonic wave function during polaron formation. In addition, by measuring the polaron formation time as a function of pulse separation we show that spin-lattice coupling influences the polaron dynamics indirectly rather than directly by transferring the polaron induced energy gain of the spin-system to the lattice.

Growth and characterization of ZnTe, ZnMnTe epilayers and single quantum wells

We report the growth of epilayers of ZnTe, Zn1−xMnxTe, and single quantum wells of ZnTe/Zn1−xMnxTe on (001) GaSb by molecular-beam epitaxy. The system has been characterized by photoluminescence spectroscopy and x-ray diffraction. Preliminary calculations indicate that the valence band offset is of the order of 30 meV (x=0.177). Experimental evidence is presented to show that magnetic polaron formation occurs within the ZnTe well region even though absent in the Zn1−xMnxTe epilayers. This has led us to considerations of a dynamic nature and in particular the role of the polaron formation time. By comparing these results with Cd1−xMnxTe, we deduce that the lifetimes of excitons in Zn1−xMnxTe are significantly shorter than in Cd1−xMnxTe.

Magnetic polaron formation of localized excitons in semimagnetic semiconductor alloys of Cd 0.8Mn 0.2Te

Exciton dynamics has been studied in Cd 0.8Mn 0.2Te under selective excitation just below the exciton resonance. In stationary spectra, the exciton luminescence band is found to show a constant Stokes shift of 16.5 meV from the excitation energy at 4.2 K. The Stokes shift decreases considerably with increasing lattice temperature or magnetic field. Within a hundred ps the time- resolved spectra show a rapid red shift of the luminescence peak, which is the main cause of the Stokes shift. The Stokes shift is concluded to be associated with exciton magnetic polaron formation from the directly generated excitons which are localized in alloy potential wells. The polaron formation time is found to be about 40 ps at 2 K and to increase with rising temperature. Under band-to-band excitation the exciton diffusion among the localized states is found to play a competitive role in the exciton dynamics, together with the polaron formation.