Polaron mobility modulation by bandgap engineering in black phaseα-FAPbI_(3)

Lead halide hybrid perovskites(LHP)have emerged as one of the most promising photovoltaic materials for their remarkable solar energy conversion ability.The transportation of the photoinduced carriers in LHP could screen the defect recombination with the help of the large polaron formation.However,the physical insight of the relationship between the superior optical-electronic performance of perovskite and its polaron dynamics related to the electron-lattice strong coupling induced by the substitution engineering is still lack of investigation.Here,the bandgap modulated thin films ofα-FAPbI_(3)with different element substitution is investigated by the time resolved Terahertz spectroscopy.We find the polaron recombination dynamics could be prolonged in LHP with a relatively smaller bandgap,even though the formation of polaron will not be affected apparently.Intuitively,the large polaron mobility in(FAPb I_(3))0.95(MAPbI_(3))0.05thin film is~30%larger than that in(FAPb I_(3))0.85(MAPbBr_(3))0.15.The larger mobility in(FAPb I_(3))0.95(MAPb I_(3))0.05could be assigned to the slowing down of the carrier scattering time.Therefore,the physical origin of the higher carrier mobility in the(FAPb I_(3))0.95(MAPbI_(3))0.05should be related with the lattice distortion and enhanced electron–phonon coupling induced by the substitution.In addition,(FAPbI_(3))0.95(MAPbI_(3))0.05will lose fewer active carriers during the polaron cooling process than that in(FAPb I_(3))0.85(MAPbBr_(3)),indicating lower thermal dissipation in(FAPbI_(3))0.95(MAPbI_(3))0.05.Our results suggest that besides the smaller bandgap,the higher polaron mobility improved by the substitution engineering inα-FAPbI_(3)can also be an important factor for the high PCE of the black phaseα-FAPbI_(3)based solar cell devices.

Charge self-trapping in two strand biomolecules:Adiabatic polaron approach

We investigate the properties of the excess charge(electron, hole) introduced into a two-strand biomolecule. We consider the possibility that the stable soliton excitation can be formed due to interaction of excess charge with the phonon subsystem. The influence of overlap of the molecular orbitals between adjacent structure elements of the macromolecular chain on the soliton properties is discussed. Special attention is paid to the influence of the overlapping of the molecular orbitals between structure elements placed on the different chains. Using the literature values of the basic energy parameters of the two-chain biomolecular structures, possible types of soliton solutions are discussed.

Infrared optical absorption of Fr?hlich polarons in metal halide perovskites

The formation of Frohlich polarons in metal halide perovskites,arising from the charge carrier-longitudinal optical(LO)phonon coupling,has been proposed to explain their exceptional properties,but the effective identification of polarons in these materials is still a challenging task.Herein,we theoretically present the infrared optical absorption of Frohlich polarons based on the Huang-Rhys model.We find that multiphonon overtones appear as the energy of the incident photons matches the multiple LO phonons,wherein the average phonon number of a polaron can be directly evaluated by the order of the strongest overtone.These multiphonon structures sensitively depend on the scale of electronic distribution in the ground state and the dimensionality of the perovskite materials,revealing the effective modulation of competing processes between polaron formation and carrier cooling.Moreover,the order of the strongest overtone shifts to higher ones with temperature,providing a potential proof that the carrier mobility is affected by LO phonon scattering.The present model not only suggests a direct way to verify Frohlich polarons but also enriches our understanding of the properties of polarons in metal halide perovskites.

Photothermal‐boosted polaron transport in Fe_(2)O_(3)photoanodes for efficient photoelectrochemical water splitting

Introduction of the photothermal effect into transition-metal oxide photoanodes has been proven to be an effective method to improve the photoelectrochemical(PEC)water-splitting performance.However,the precise role of the photothermal effect on the PEC performance of photoanodes is still not well understood.Herein,spinel-structured ZnFe_(2)O_(4)nanoparticles are deposited on the surface of hematite(Fe_(2)O_(3)),and the ZnFe_(2)O_(4)/Fe_(2)O_(3)photoanode achieves a high photocurrent density of 3.17 mA cm^(-2)at 1.23 V versus a reversible hydrogen electrode(VRHE)due to the photothermal effect of ZnFe_(2)O_(4).Considering that the hopping of electron small polarons induced by oxygen vacancies is thermally activated,we clarify that the main reason for the enhanced PEC performance via the photothermal effect is the promoted mobility of electron small polarons that are bound to positively charged oxygen vacancies.Under the synergistic effect of oxygen vacancies and the photothermal effect,the electron conductivity and PEC performance are significantly improved,which provide fundamental insights into the impact of the photothermal effect on the PEC performance of small polaron-type semiconductor photoanodes.

Magnetic polaron-related optical properties in Ni(Ⅱ)-doped Cd S nanobelts: Implication for spin nanophotonic devices

Emissions by magnetic polarons and spin-coupled d-d transitions in diluted magnetic semiconductors(DMSs)have become a popular research field due to their unusual optical behaviors.In this work,high-quality NiI_(2)(Ⅱ)-doped CdS nanobelts are synthesized via chemical vapor deposition(CVD),and then characterized by scanning electron microscopy(SEM),x-ray diffraction,x-ray photoelectron spectroscopy(XPS),and Raman scattering.At low temperatures,the photoluminescence(PL)spectra of the Ni-doped nanobelts demonstrate three peaks near the band edge:the free exciton(FX)peak,the exciton magnetic polaron(EMP)peak out of ferromagnetically coupled spins coupled with FXs,and a small higher-energy peak from the interaction of antiferromagnetic coupled Ni pairs and FXs,called antiferromagnetic magnetic polarons(AMPs).With a higher Ni doping concentration,in addition to the d-d transitions of single Ni ions at 620 nm and 760 nm,two other PL peaks appear at 530 nm and 685 nm,attributed to another EMP emission and the d-d transitions of the antiferromagnetic coupled Ni^(2+)-Ni^(2+)pair,respectively.Furthermore,single-mode lasing at the first EMP is excited by a femtosecond laser pulse,proving a coherent bosonic lasing of the EMP condensate out of complicated states.These results show that the coupled spins play an important role in forming magnetic polaron and implementing related optical responses.

Crossover of large to small radius polaron in ionic crystals

The crossover of large to small radius polaron is studied in terms of the inverse-relaxation time and temperature.It is found that the small radius polaron exists at higher temperature than the large radius polaron.A formula which relates the inverse-relaxation time to the ratio of arbitrary temperature and Debye temperature of the crystal is derived.The polaron crossover temperatures in NaCl and KBr are found from plotted graphs.The straight line emerging at the Debye temperature T_d of a graph reflects the increase of the inverse relaxation time for increasing temperature up to the collapse of the small radius polaron.The relationship between the small and large radius polarons is found and known ratios of the effective and the bare masses of the electrons for the two substances are used to validate our results.The small radius polaron's mass is later compared with the mass obtained from the hopping formula and is found to be approximately equal.Finally,we point out that the crossover temperature is material-specific since it depends on the Debye and the effective dielectric function.

Polaron effect on the optical rectification in spherical quantum dots with electric field

The polaron effect on the optical rectification in spherical quantum dots with a shallow hydrogenic impurity in the presence of electric field is theoretically investigated by taking into account the interactions of the electrons with both confined and surface optical phonons. Besides, the interaction between impurity and phonons is also considered. Numerical calculations are presented for typical Zn_(1-x)Cd_xSe/ZnSe material. It is found that the polaronic effect or electric field leads to the redshifted resonant peaks of the optical rectification coefficients. It is also found that the peak values of the optical rectification coefficients with the polaronic effect are larger than without the polaronic effect, especially for smaller Cd concentrations or stronger electric field.

Variational Path-Integral Study on Bound Polarons in Parabolic Quantum Dots and Wires

The expression of the ground-state energy of the electron coupled simultaneously with a Coulomb potential and a longitudinal-optical phonon field in parabolic quantum dots and wires is derived within the framework of Feynman variational path integral theory.We obtain a general result with arbitrary electron-phonon coupling constant,Coulomb binding parameters,and confining potential strength,which could be used for further numerical calculation of polaron properties.Moreover,it is shown that all the previous path-integral formulae for free polarons,bound polarons,and polarons confined in parabolic quantum dots and wires can be recovered in the present formalism.

Mechanism of Insulator-Metal Transition in La_(2/3)Ca_(1/3)MnO_(3):Collapse of Small Polaron

The insulator-metal(I-M)transition in colossal magnetoresistance material La_(2/3)Ca_(1/3)MnO_(3) has been studied by electrical transport,magnetization and electron spin resonance(ESR)measurements.Around the I-M transition,the ESR signal shows a crossover from paramagnetic resonance line to ferromagnetic resonance line,which implies a coexistence of localized small polarons and itinerant carriers during the I-M transition.Based on the concept of‘small polaron collapse’,a two-phase model is proposed to quantitatively describe the electrical transport behaviour across the insulator-metal transition.The calculations of resistivity in different applied magnetic fields have close trends with the experimental data,which strongly proves that the I-M transition results from the collapse of small polarons and two phases coexist during the transition.

The energy-level and vibrational frequency properties of a polaron weak-coupled in a quantum well with asymmetrical Gaussian confinement potential

The vibrational frequency(VF), the ground state(GS) energy and the GS binding energy of the weak electron-phonon coupling polaron in a quantum well(QW) with asymmetrical Gaussian confinement potential are calculated. First we introduce the linear combination operator to express the momentum and coordinates in the Hamilton and then operate the system Hamilton using unitary transformation. The results indicate the relations of the quantities(the VF, the absolute value of GS energy and the GS binding energy) and the parameters(the QW barrier height and the range of Gaussian confinement potential in the growth direction of the QW).

On Optical Polarons in One Dimensional Molecular-Crystal Chains

An improved solution for the optical polarons in the molecular-crystal model with the dispersion term in one dimensional chains is found.Under approximation neglecting the dispersion term,the improved solution tends to usual optical polarons solution.The self-trapped well of the electron is calculated.

POLARON IN POLAR CRYSTAL INTERFACE IN ELECTRIC FIELD

ＰＯＬＡＲＯＮＩＮＰＯＬＡＲＣＲＹＳＴＡＬＩＮＴＥＲＦＡＣＥＩＮＥＬＥＣＴＲＩＣＦＩＥＬＤ①ＹａｎｇＢｉｎｇｃｈｕ，ＣｈｅｎｇＺｈｅｎｇｈｕａＩｎｓｔｉｔｕｔｅｏｆＮｏｎｅｑｕｉｌｉｂｒｉｕｍＭａｔｅｒｉａｌｓＳｃｉｅｎｃｅａｎｄＥｎｇｉｎｅｒｉｎｇ...

The effect of interface hopping on inelastic scattering of oppositely charged polarons in polymers

The inelastic scattering of oppositely charge polarons in polymer heterojunctions is believed to be of fundamental importance for the light-emitting and transport properties of conjugated polymers.Based on the tight-binding SSH model,and by using a nonadiabatic molecular dynamic method,we investigate the effects of interface hopping on inelastic scattering of oppositely charged polarons in a polymer heterojunction.It is found that the scattering processes of the charge and lattice defect depend sensitively on the hopping integrals at the polymer/polymer interface when the interface potential barrier and applied electric field strength are constant.In particular,at an intermediate electric field,when the interface hopping integral of the polymer/polymer heterojunction material is increased beyond a critical value,two polarons can combine to become a lattice deformation in one of the two polymer chains,with the electron and the hole bound together,i.e.,a self-trapped polaron-exciton.The yield of excitons then increases to a peak value.These results show that interface hopping is of fundamental importance and facilitates the formation of polaron-excitons.

The Electronic and Lattice Structures of the Ground and the Polaron States of Polymer Poly (Phenylene Vinylene) (PPV)

The electronic and lattice structures of poly (phenylene vinylene) (PPV) are studied theoretically. Both the electron-electron and electron-phonon interactions are taken into account in the Pariser-Parr-Pople model. The electronic band and the lattice structure of the ground state and the polaronic state are calculated by means of the unrestricted Hartree-Fock method. In the ground state, there exist eight bands in PPV including four valence bands and four conduction bands, and the benzenes can be considered to be rigid. The polaron induces the split of energy bands. There are four localized electronic states within the energy gap. The defect of the polaron appears to extend over about 5 units. The benzenes are strongly affected by the electron-phonon interaction. Our calculation for the energy band structure of the ground and polaron states are consistent with experimental absorption spectra. The results of our calculation show that the electron-phonon and inter-site electron-electron interactions play an important role in determining the electronic and lattice structures.

Probability density and oscillating period of magnetopolaron in parabolic quantum dot in the presence of Rashba effect and temperature

We study the property of magnetopolaron in a parabolic quantum dot under the Rashba spin–orbit interaction(RSOI)by adopting an unitary transformation of Lee–Low–Pines type and the variational method of Pekar type with and without considering the temperature.The temporal spatial distribution of the probability density and the relationships of the oscillating period with the RSOI constant,confinement constant,electron–phonon coupling strength,phonon wave vector and temperature are discussed.The results show that the probability density of the magnetopolaron in the superposition of the ground and first excited state takes periodic oscillation(T0/period)in the presence or absence of temperature.Because of the RSOI,the oscillating period is divided into different branches.Also,the results indicate that the oscillating period increases(decreases)when the RSOI constant,electron-phonon coupling strength and phonon wave vector(the confinement constant)increase in a proper temperature,and the temperature plays a significant role in determining the properties of the polaron.

Quasi-One-Dimensional Organic Polymer Ferromagnet Based on Polaron

A new kind of organic polymer ferromagnet based on polaron is proposed.In this kind of polymer,there are no organic free-radicals,which are rather reactive and unstable,instead spins are provided by polarons.Allowing for full lattice relaxation,the system is studied self-consistently by Peierls-Hubbard model.The formation energies of polarons are calculated.The possible ferromagnetism in polymers of this type is demonstrated.

Strong Coupling Polaron in a Slab of Polar Crystal

By using the extensive Pekar's variational method,the strong coupling polaron energy and the effective mass in a slab of polar crystal including both LO-and SO-phonons are derived as a function of the thickness of slab and the coupling constant.When the slab is thin,the SO-phonons not only influence the ground state energy,but also make the polaronic effective mass become non-isotropic.

Polaron and molecular states of a spin-orbit coupled impurity in a spinless Fermi sea

We investigate the polaron and molecular states of a fermionic atom with one-dimensional spin-orbit coupling(SOC)coupled to a three-dimensional spinless Fermi sea.Because of the interplay among the SOC,Raman coupling and spinselected interatomic interactions,the polaron state induced by the spin-orbit coupled impurity exhibits quite unique features.We find that the energy dispersion of the polaron generally has a double-minimum structure,which results in a finite center-of-mass(c.m.)momentum in the ground state,different from the zero-momentum polarons where SOC are introduced into the majority atoms.By further tuning the parameters such as the atomic interaction strength,a discontinuous transition between the polarons with different c.m.momenta may occur,signaled by the singular behavior of the quasiparticle residue and effective mass of the polaron.Meanwhile,the molecular state as well as the polaron-to-molecule transition is also strongly affected by the Raman coupling and the effective Zeeman field,which are introduced by the lasers generating SOC on the impurity atom.We also discuss the effects of a more general spin-dependent interaction and mass ratio.These results would be beneficial for the study of impurity physics brought by SOC.

Non-universal Fermi polaron in quasi two-dimensional quantum gases

We consider an impurity problem in a quasi-two-dimensional Fermi gas, where a spin-down impurity is immersed in a Fermi sea of N spin-up atoms. Using a variational approach and an effective two-channel model, we obtain the energy for a wide range of interaction strength and for various different mass ratios between the impurity and the background fermion in the context of heteronuclear mixture. We demonstrate that in a quasi-two-dimensional Fermi gas there exists a transition of the ground state from polaron in the weakly interacting region to molecule in the strongly interacting region.The critical interaction strength of the polaron–molecule transition is non-universal and depends on the particle density of the background Fermi sea. We also investigate the excited repulsive polaron state, and find similar non-universal behavior.

Translational Motion of a Free Large Polaron and Broadening of Absorption Spectra

The translational motion of a large polaron as whole is analyzed in the context of its effect on the broadening of an absorption optical spectrum. It was open question how important the role of translational degrees of freedom and the corresponding velocities are on the broadening. The Bogolyubov method of canonical transformation of coordinates is formulated for a system of an electron and field, taking into account rigorous fulfillment of the conservation laws. Separation of variables is carried out for the coordinates describing the translational degrees of freedom and the electron oscillations in a polarization well. The equations obtained for the electronic states explicitly depend on the velocity of the free polaron as a whole. An estimate is made for free polaron in ammonia.