Polaron Formation Research Articles

Thermodynamic evidence for polaron stabilization inside the antiferromagnetic order of Eu5In2Sb6

Materials exhibiting electronic inhomogeneities at the nanometer scale have enormous potential for applications. Magnetic polarons are one such type of inhomogeneity which link the electronic, magnetic and lattice degrees of freedom in correlated matter and often give rise to colossal magnetoresistance. Here, we investigate single crystals of Eu5In2Sb6 by thermal expansion and magnetostriction along different crystallographic directions. These data provide compelling evidence for the formation of magnetic polarons in Eu5In2Sb6 well above the magnetic ordering temperature. More specifically, our results are consistent with anisotropic polarons with varying extent along the different crystallographic directions. A crossover revealed within the magnetically ordered phase can be associated with a surprising stabilization of ferromagnetic polarons within the global antiferromagnetic order upon decreasing temperature. These findings make Eu5In2Sb6 a rare example of such coexisting and competing magnetic orders and, importantly, shed new light on colossal magnetoresistive behavior beyond manganites.

Cation Vacancy-Mediated Ultrafast Hole Transport in CuBi2O4 Photocathodes.

In this study, the ultrafast transport dynamics of the valence band hole states in CuBi2O4 (CBO) photocathodes were investigated by varying the atomic composition and manipulating their p-type character. As a comprehensive ultrafast optical transient absorption spectroscopy (TAS) investigation of compositionally manipulated CBO that combines both ex situ and in situ TAS experiments with photoelectrochemical (PEC) performance tests, the study reveals the polaron formation tendencies of the valence band (VB) holes at cationic vacancy sites. Therefore, it draws a complete picture of the ultrafast hole transport dynamics and provides valuable insights into the hindrance of the photocurrent generated in CBO.

Direct and Inverse Photoelectron Spectroscopy Evidence for a Revised Picture of Electronic States of Negative Polarons in n‐Doped C60

AbstractDetermining the electronic levels associated with polarons, the fundamental charge carriers in organic semiconductors, is key to understanding the charge transport properties of these materials. Recent findings challenge the traditional view of these electronic levels by highlighting the importance of intra‐molecular Coulomb interactions in polarons. Experimental evidence was previously presented for a revised model of the negative polaron in the case of the polymer semiconductor poly(NDI2OD‐T2); there, the addition of an excess electron was seen to lead to the emergence of a singly occupied state within the energy gap of the undoped material and an unoccupied state above the edge of the conduction states. Here, focus is on a small‐molecule semiconductor, C60, and spectral evidence is provided of a similar picture for the new states appearing upon polaron formation. Specifically, direct and inverse photoemission spectroscopy is used to investigate the density of states in C60 films n‐doped with two dimeric dopants. The Coulomb interaction energy (Hubbard U) of the C60 anion is experimentally determined to be ≈1.1 eV, a value that aligns closely with theoretical predictions.

Polaron formation via doping process in an organic semiconductor polymer based on thiophene-phenylene

In this research, we synthesized and analyzed the polymer 5-(5-(10-chlorodecyloxy)-2-methoxy-4-(5-methylthiophen-2-yl)phenyl)-5′-(2-(2-ethylhexyloxy)-5-methoxy-4-(5-methylthiophen-2-yl)phenyl)-2,2′-bithiophene (PTBT46). The polymer was dissolved in chloroform and subjected to gamma radiation doses ranging from 0 to 100 Gy (gray). We observed that PTBT46 can be doped with H+ions, as indicated by visible color changes, optical measurements, and a tenfold increase in conductivity. When analyzing PTBT46 in solution using THF (tetrahydrofuran) and chloroform, we found a notable solvent dependence. In chloroform, new absorption bands appeared at approximately 680 nm and 2630 nm after radiation doses of 25 to 100 Gy, which were not present in THF. These bands are attributed to polarons and bipolarons resulting from the doping process. Additionally, we observed a spectral “blue shift” with increasing H+concentration due to changes in molecular conjugation, without degradation of the polymer chain. This shift, confirmed by Fourier-transform infrared (FTIR) spectroscopy, is reversible and suggests potential applications in optoelectronic devices and advanced radiation sensors.

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.

Substitutional Cu doping at the cation sites in Ba2YNbO6 toward improved visible-light photoactivity—A first-principles HSE06 study

Artificial photosynthesis holds immense promise for sustainable clean energy harvesting, with recent strides in material engineering with the earth abundant elements enabling efficient utilization of the visible solar spectrum for photoelectrochemical catalytic water splitting. Here, we have investigated the impact of substitutional Cu doping at all three cation sites in Ba2YNbO6 (BYN) using density functional theory calculations at the Heyd-Scuseria-Ernzerhof-06 level. One of the key findings is that the defect formation energy follows the hierarchy Nb > Ba > Y. The presence of an oxygen vacancy (OV) enhances the co-solubility of Cu substitution of Nb, particularly when placed outside the CuO6 unit, while it has a contrary effect for Y substitution. Cu replacement reduces the bandgap as Nb > Y ≫ Ba vis-à-vis pure BYN, while extending it into the visible part of the solar spectrum for Nb and Y replacement cases, albeit with OV causing a slight blue shift to them, without reducing the oxidation state of Cu due to strong charge-delocalization. Cu doping at Y and Nb sites retains the direct band transition character of BYN, a feature removed by OV. While all the bare Cu doped systems exhibit formation of a weak electron polaron, the placement of OV tends to annihilate this except for the system comprising first nearest neighbor placement of an OV relative to the Cu substitution at an Nb site. Notably, Cu doping at the Nb site significantly enhances optical activity, particularly ∼2.0–2.5 eV, resulting in promising candidates for photoelectrochemical catalysts.

Unconventional insulator-to-metal phase transition in Mn3Si2Te6

The nodal-line semiconductor Mn3Si2Te6 is generating enormous excitment due to the recent discovery of a field-driven insulator-to-metal transition and associated colossal magnetoresistance as well as evidence for a new type of quantum state involving chiral orbital currents. Strikingly, these qualities persist even in the absence of traditional Jahn-Teller distortions and double-exchange mechanisms, raising questions about exactly how and why magnetoresistance occurs along with conjecture as to the likely signatures of loop currents. Here, we measured the infrared response of Mn3Si2Te6 across the magnetic ordering and field-induced insulator-to-metal transitions in order to explore colossal magnetoresistance in the absence of Jahn-Teller and double-exchange interactions. Rather than a traditional metal with screened phonons, the field-driven insulator-to-metal transition leads to a weakly metallic state with localized carriers. Our spectral data are fit by a percolation model, providing evidence for electronic inhomogeneity and phase separation. Modeling also reveals a frequency-dependent threshold field for carriers contributing to colossal magnetoresistance which we discuss in terms of polaron formation, chiral orbital currents, and short-range spin fluctuations. These findings enhance the understanding of insulator-to-metal transitions in new settings and open the door to the design of unconventional colossal magnetoresistant materials.

Coexistence of multiple magnetic interactions in oxygen-deficient V2O5 nanoparticles

This paper reports on the spin glass-like coexistence of competing magnetic orders in oxygen-deficient V2O5 nanoparticles with a broad size distribution. X-ray photoelectron spectroscopy yields the surface chemical stoichiometry of nearly V2O4.65 due to significant defect density. Temperature-dependent electrical conductivity and thermopower measurements demonstrate a polaronic conduction mechanism with a hopping energy of about 0.112 eV. The V2O5-δ sample exhibits strong field as well as temperature-dependent magnetic behaviour when measured with a SQUID magnetometer, showing positive magnetic susceptibility across the temperature range of 2-350 K. Field-cooled and zero-field-cooled data indicate hysteresis, suggesting glassy behaviour. The formation of small polarons due to oxygen vacancy defects, compensated by V4+ charge defects, results in Magneto-Electronic Phase Separation (MEPS) and various magnetic exchanges, as predicted by first-principle calculations. This is evidenced by the strong hybridisation of V orbitals in the vicinity of vacant oxygen site. An increase in V4+ defects shows an antiferromagnetic (AFM) component. The magnetic diversity in undoped V2O4.9 originates from defect density and their random distribution, leading to MEPS. This involves localised spins in polarons and ferromagnetic (FM) clusters on a paramagnetic (PM) background, while V4+ dimers induce AFM interactions. Electron paramagnetic resonance spectra measured at different temperatures indicate a dominant paramagnetic signal at a g-value of 1.97 due to oxygen defects, with a broad FM resonance-like hump. Both signals diminish with increasing temperature. Neutron diffraction data rules out long-range magnetic ordering, reflecting the composition as V2O4.886. Despite the FM hysteresis, no long-range order is observed in neutron diffraction data, consistent with the polaron cluster-like FM with MEPS nature. This detailed study shall advance the understanding of the diverse magnetic behaviour observed in undoped non-magnetic systems.

Optical Conductivity and Photo-Induced Polaronic Formation in Co2MnGa Topological Semimetal.

Topological materials occupy an important place in the quantum materials family due to their peculiar low-energy electrodynamics, hosting emergent magneto-electrical, and nonlinear optical responses. This manuscript reports on the optical responses for the magnetic topological nodal semimetal Co2MnGa, studied in a thin film geometry at various thicknesses. The thickness-dependent optical conductivity is investigated, observing a substantial dependence of the electronic band structure on thickness. Additionally, details on the ultrafast response of the low energy excitations in the terahertz frequency are reported by employing optical pump-terahertz probe (OPTP) spectroscopy. In particular, the photocarrier dynamics of Co2MnGa thin films is studied at varying pump fluence, pump wavelength, and film thickness, observing a negative THz photoconductivity which is assigned to a dynamical formation of large polarons in thematerial.

Role of polaron dressing in superradiant emission dynamics

Cooperative effects of multiple quantum emitters are characterized by transitions via delocalized collective states with altered emission properties due to the existence of interemitter coherences. When realized with excitonic condensed-matter nanostructures, these effects are significantly affected by the presence of strong emitter-phonon coupling, which leads to the formation of polarons. We show that, while for single-emitter emission into free space this formation has no impact on its radiative lifetime, the same is not true for superradiant emission. Considering the case of two indistinguishable quantum emitters, we analyze how polaron dressing affects collective photon emission by mixing bright and dark Dicke states. Our numerical simulations show that this mixing crucially depends on the circumstances of the excitation of the system: Depending on the pulse length of an exciting laser, one can choose to either prepare polaronic Dicke states, or bare electronic Dicke states, changing the superradiant decay characteristics of the system. Additionally, we derive analytic expressions for these limiting cases, which match the results of numerically exact calculations. Published by the American Physical Society 2024

Hole Polaron-Mediated Suppression of Electron-Hole Recombination Triggers Efficient Photocatalytic Nitrogen Fixation.

In the pursuit of successful photocatalytic transformations, challenges persist due to limitations in charge carrier utilization and transfer efficiency, which stemming from rapid recombination. Overcoming these limitations necessitates the exploration of novel mechanisms that enhance the effective separation of photogenerated electron-hole pairs. Herein, deviating from the conventional approach of enhancing carrier migration to separate photogenerated charges and extend their lifetime, the proposal is to directly prevent the recombination of photogenerated electrons and holes by forming hole polarons. Specifically, disordered pores are introduced on the surface of KTaO3 ultrathin sheets, and the clear-cut evidences in electron paramagnetic resonance, photoluminescence, and ultrafast spectroscopy unambiguously confirm the enhanced carrier-phonon coupling, which results in the formation of hole polarons to impede the recombination of photogenerated electron-hole pairs. Taking the challenging nitrogen oxidation reaction as an example, it is found that the hole polarons in atomic-disordered pore KTaO3 ultrathin nanosheets trigger outstanding photo-oxidation performance of nitrogen (N2)to nitrate, with a nitrate-producing rate of 2.1 mg g-1 h-1. This scenario is undoubtedly applicable to a wide variety of photocatalytic reactions due to the common challenge of charge carrier recombination in all photocatalytic processes, manifesting broad implications for promoting photocatalysis performance.

Small polaron hopping and tunneling mechanisms in Ba0.9La0.1Fe12O19 hexaferrite ceramic at low temperatures

The correlation between dielectric relaxation and conduction mechanisms in the Ba0.9La0.1Fe12O19 ceramic hexaferrite, at temperatures below 100 K was investigated. The sample was prepared using the conventional solid-state reaction method. X-ray diffraction analysis indicated that the sample is composed of a hexagonal BaFe12O19 phase (lattice parameters a = 5.8778(6) Å and c = 23.170(3) Å) and a small proportion of hematite (α-Fe2O3). The average grain size is 1.48 ± 0.4 μm, with irregular hexagonal and agglomerated grains. The magnetic hysteresis curve shows characteristics typical of ferro and ferrimagnetic materials, with an effective magnetic anisotropy coefficient of 0.960×106 emu/cm3 and an anisotropy field of 1.587×104 Oe. A dielectric relaxation process is observed between 50 and 100 K, with an activation energy of 60.4 ± 0.4 meV and a characteristic relaxation time of 2.09 ×10−10 s, mainly attributed to the electrical behavior of the grains. Two conduction mechanisms were identified: small polaron tunneling (high temperatures and low frequencies) and correlated barrier hopping of electrons (low temperatures and high frequencies). The activation energy for free polaron formation, according to the Komine-Iguchi hopping polarization model, was 7.12 ± 0.02 meV. In the dielectric modulus relaxation process between 30 and 54 K, the activation energy was 40.8 ± 0.9 meV, with a characteristic relaxation time of 2.41×10−10 s. The small polaron conductivity in the grain region was 67.52 meV and 54.0 meV in the grain boundary region. The results highlight that the polarization and conduction mechanisms in this ceramic at low temperatures are dominated by small polaron hopping and tunneling, and emphasize the influence of La3+ doping on the structural, magnetic, and electrical properties of barium hexaferrite.

A momentum-resolved view of polaron formation in materials

An ab-initio computational methodology for interrogating the phonon contribution to polaron formation in real materials is developed that can be directly compared to experiment. Using LiF as an example, we show that the recent ab-initio theory of Sio et al.1 makes predictions of the momentum- and branch dependent phonon amplitudes in polaron quasiparticles that are testable using ultrafast electron diffuse scattering (UEDS) and related techniques. The large electron polaron in LiF has UEDS signatures that are qualitatively similar to those expected from a simple isotropic strain field model, but the small hole polaron exhibits a profoundly anisotropic UEDS pattern that is in poor agreement with an isotropic strain field. We also show that these polaron diffuse scattering signatures are directly emblematic of the underlying polaron wavefunction. The combination of new time and momentum resolved experimental probes of nonequilibrium phonons with novel computational methods promises to complement the qualitative results obtained via model Hamiltonians with a first principles, material-specific quantitative understanding of polarons and their properties.

Coherent Charge Hopping Suppresses Photoexcited Polarons in ErFeO3

We measure how coherent charge hopping competes with and slows small polaron formation. The coupling of photoexcited charge carriers with phonons in the crystal lattice that results in polaron formation causes carrier localization and limits transport to site-to-site hopping. ErFeO3, a rare-earth orthoferrite, is designed where the large rare earth atom frustrates the FeO6 octahedra, reducing the energetic favorability of polaron formation. We compare transient extreme ultraviolet (XUV) measurements with ab initio theory and transport measurements to quantify the polaron formation rate and mobility. Polaron formation occurs within several picoseconds following multiple coherent charge hopping events between neighboring Fe3+–Fe2+ sites. This timescale is more than an order of magnitude longer compared to previously studied materials. The delay in polaron formation is notable because the photoexcited carriers shed excess thermal energy before forming the polaron. To date, polarons have been measured to form instantaneously with the first electron-phonon scattering event. The measured interplay between optical phonons, electron coupling, and on-site lattice deformation emphasizes the importance of dynamic electron correlations for tuning polaron formation in photocatalysts.

(Invited) How to Mitigate Excited State Polaron Formation in Transition Metal Oxide Photocatalysts

We measure how lattice versus electronic coupling in transition metal oxide photocatalysts can mitigate photoexcited polaron formation. Despite well-aligned energetics for photocatalysis, transition metal oxide materials have limited carrier mobilities due to the formation of polarons. The coupling of photoexcited charge carriers with phonons in the crystal lattice causes carrier localization and results in transport that relies on site-to-site hopping. Models that predict polaron formation are limited to ground-state predictions, which limits the exploration of new photocatalysts. Small polaron formation rates have been measured to be independent of defects, dopants, surface treatments, and other material tuning parameters. Our transient extreme ultraviolet (XUV) measurements show the first hints about how electronic correlations can be used to counter polaron formation for increased transport. For example, in CuFeO2, we add Cu layers between FeO6 octahedra, which increases photoexcited transport compared to α-Fe2O3. In ErFeO3, where Er frustrates FeO6 octahedra, coherent Fe-Fe charge hopping due to correlated electron interactions slows polaron formation. We frame our discussion in terms of the Hubbard-Holstein Hamiltonian, which we are slowly proving can be used to guide materials design rules for non-polaronic photocatalysts using only ground-state DFT parameters.

(Invited) Charge Separation and Stabilisation in Photocatalyst Materials for Solar Driven Water Splitting

In photocatalytic systems for solar energy conversion, a key challenge for efficient operation is the efficient separation and stabilisation of photogenerated charges, and thereby the minimisation of undesired recombination losses. This challenge is particularly great for photocatalytic systems because of the long charge lifetimes required to drive catalysis A further consideration is to minimise the energy losses required to drive this charge separation. My talk will cover examples of recent work from my group addressing aspects of this challenge. I will start by considering the role of polaron localisation / relaxation in driving charge separation in metal oxides, and quantification of the energetic loss associated with this polaron formation. Kinetic competion between polaron formation and ligand field state mediated charge recombination will be highlighted as the key challenge limiting the performance of many visible light absorbing metal oxides. The roles of metal oxide dielectric constant, surface facet energies, traps, ‘photocharging’ and heterojunctions in aiding charge separation in metal oxides will be discussed, drawing on examples from BiVO4 and SrTiO3. I will then go on to discuss charge separation and stabilisation in alternative photocatalyst materials, including organic semiconductor nanoparticles and metal-organic frameworks, highlighting the observation of remarkably long lived charge photogeneration in both systems and their relevance to the efficiency photocatalytic performance.

(Invited) Probing Spin Selective Surface Chemistry with Circularly Polarized XUV Light

Controlling spin states of surface reaction intermediates is an important but often overlooked parameter to designing semiconductor photocatalysts to mimic nature’s best photosynthetic complexes. Extreme Ultraviolet Reflection-Absorption (XUV-RA) spectroscopy enables direct observation of surface electron dynamics by combining element, oxidation, and spin state resolution, with surface sensitivity and ultrafast time resolution. While absorption of linearly polarized XUV light can probe the local oxidation and spin states of individual elements, it is insensitive to absolute spin alignment. To enable the study of spin-selective electron dynamics at interfaces, we have recently implemented ultrafast XUV Magnetic Circular Dichroism (XUV-MCD). Employing XUV-MCD in a reflection geometry, we are now studying the ultrafast surface electron dynamics that give rise to spin polarized photocurrents in magnetic and chiral semiconductors. As an example, yttrium iron garnet (Y3Fe5O12, YIG) is a ferrimagnetic oxide with a visible band gap, consisting of two sub-lattices based on octahedrally and tetrahedrally coordinated Fe(III) centers. Ultrafast XUV measurements show that electron-phonon coupling and polaron formation rates differ significantly between octahedral and tetrahedral centers leading to very different charge carrier recombination kinetics. Circular measurements show that this difference in recombination rate gives rise to long-lived, spin-polarized charge accumulation at the YIG surface that drives spin selective water oxidation. This new ability to observe real-time spin polarization at surfaces promises to provide key design parameters for semiconductor photocatalysts capable of spin selective surface chemistry.

Effect of Surface Electron Trapping and Small Polaron Formation on the Photocatalytic Efficiency of Copper(I) and Copper(II) Oxides.

Cu2O, CuO, and mixed phase Cu2O/CuO represent promising candidates for photoelectrochemical H2 evolution due to their strong visible light absorption, earth-abundance, and chemical stability. However, the photoelectrochemical efficiency in these materials remains far below the theoretical limit, largely due to poorly understood surface electron dynamics. These dynamics depend on defect states, such as Cu atom vacancies and phase boundaries, which control electron trapping, charge carrier separation, and recombination. In this work, we study the photoinduced electron and hole dynamics at the surface of various Cu oxides using ultrafast extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy. In Cu2O we find that photoexcitation occurs as electron promotion from primarily Cu 3d valence band to Cu 4s conduction band states compared to O 2p valence band to Cu 4s conduction band states in CuO. In catalysts with a significant concentration of Cu vacancies, we observe fast electron trapping to the Cu 3d defect band occurring in less than 100 fs. In contrast, photoexcited electrons in phase pure CuO do not trap to midgap states; rather these electrons form small polarons within approximately 500 fs. Photoelectrochemical measurements of these catalysts show that Cu vacancy-mediated electron trapping correlates with a significant loss of photocurrent. Together, these results provide a detailed picture of the defect states and associated ultrafast carrier dynamics that govern the photocatalytic efficiency in widely studied Cu2O and CuO photocatalysts.

Hole Localization in Bulk and 2D Lead-Halide Perovskites Studied by Time-Resolved Infrared Spectroscopy.

Scattering and localization dynamics of charge carriers in the soft lattice of lead-halide perovskites impact polaron formation and recombination, which are key mechanisms of material function in optoelectronic devices. In this study, we probe the photoinduced lattice and carrier dynamics in perovskite thin films (CsFAPbX3, X = I, Br) using time-resolved infrared spectroscopy. We examine the CN stretching mode of formamidinium (FA) cations located within the lead-halide octahedra of the perovskite structure. Our investigation reveals the formation of an infrared mode due to spatial symmetry breaking within a hundred picoseconds in 3D perovskites. Experiments at cryogenic temperatures show much-reduced carrier localization, in agreement with a localization mechanism that is driven by the dynamic disorder. We extend our analysis to 2D perovskites, where the precise nature of charge carriers is uncertain. Remarkably, the signatures of charge localization we found in bulk perovskites are not observed for 2D Ruddlesden-Popper perovskites ((HexA)2FAPb2I7). This observation implies that the previously reported stabilization of free charge carriers in these materials follows different mechanisms than polaron formation in bulk perovskites. Through the exploration of heterostructures with electron/hole excess, we provide evidence that holes drive the formation of the emerging infrared mode.

Collective photon emission in solid state environments: Concatenating non-Markovian and Markovian dynamics

Collective light emission and multiqubit dynamics of solid-state quantum emitters are affected both by their coupling to the light field and to lattice vibrations. The effect of phonons on quantum emitters is twofold: polaron formation is described by ultrafast non-Markovian dynamics, while slower dephasing is well described by exponential decay. Of the two temperature-dependent processes, the effect of the former on the collective emission and the entanglement decay of emitters is usually not modeled, and also the latter is sometimes neglected. Here we propose and compare two methods that are efficient also for several emitters: the first method concatenates the fast and slow phonon dynamics, and the second is the polaron method. For a single quantum emitter, we show that the dynamical equations are identical in both methods, while predictions for two or more emitters also agree very well. Both of our methods incorporate non-Markovian dynamics due to phonons demonstrating the temperature sensitivity of the collective photon emission. Utilizing a simplified Markovian model instead may not be accurate enough especially for quantum information applications: for example, we show how the Markovian model may considerably overestimate the two-emitter concurrence, except at very low temperatures. Our concatenation and polaron methods can be applied to an arbitrary number and type of quantum emitters, and beyond the bulk GaAs environment that we consider here. Especially the concatenation method can take phonon effects into account at the same computational cost as modeling the emitter-photon interaction alone. Finally, we present approximate analytical expressions for the collective emission spectrum for N emitters on a one-dimensional chain. Published by the American Physical Society 2024