Spin Polarons Research Articles

DFT and EPR Study of Spin Excitations in Oxidized Thiophene-Based Conjugated Oligomers

Comparison of data obtained using the density functional theory (DFT) and electron paramagnetic resonance (EPR) method made it possible to establish the composition and state of spin charge carries in the poly(3, 4-ethylenedioxythiophene) (PEDOT) and poly(3-alkylthiophene) (P3AT) oligomers. The coexistence of several polarons on the P3AT chain, the properties of which are governed by the structure of the substituents, is identified. All terms of the spin Hamiltonians of different oligomers were obtained, and their high-resolution EPR spectra were calculated. The structure of the P3AT substituents is shown to influence the polaron spin state and hyperfine interaction with its own microenvironment. The prospects of the approaches used in the work to develop electronic and spintronic elements with spin-controlled parameters are shown.

Vertex corrections on the conductivity of high-Tc superconductors in the spin polaron formulation

Vertex corrections on the conductivity of high-Tc superconductors in the spin polaron formulation

Polaron spin states in chiral systems

Polaron spin states in chiral systems

The chiral dependence of effective masses of polaron spin states in chiral structures

The renormalization of effective masses of polaron spin states in chiral structures is studied based on the direct spin–phonon coupling, in which the simple formulas for the renormalization of effective masses are derived for polarons with spin-up and spin-down states using a linear-combination operator method. The theoretical calculations show that the effective masses are renormalized significantly with increasing the cut-off wave vector of phonon modes and the renormalized magnitudes are different for right-handed and left-handed structures. Moreover, the renormalized magnitude for the spin-down state is larger than the spin-up one. This asymmetrical behavior naturally gives rise to very different conductivities for spin-up and spin-down states as electrons transmitted through the systems. These results not only enrich the microscopic origin of CISS mechanism, but also enlighten more experimental ways to explore CISS.

Directly imaging spin polarons in a kinetically frustrated Hubbard system.

The emergence of quasiparticles in quantum many-body systems underlies the rich phenomenology in many strongly interacting materials. In the context of doped Mott insulators, magnetic polarons are quasiparticles that usually arise from an interplay between the kinetic energy of doped charge carriers and superexchange spin interactions1-8. However, in kinetically frustrated lattices, itinerant spin polarons-bound states of a dopant and a spin flip-have been theoretically predicted even in the absence of superexchange coupling9-14. Despite their important role in the theory of kinetic magnetism, a microscopic observation of these polarons is lacking. Here we directly image itinerant spin polarons in a triangular-lattice Hubbard system realized with ultracold atoms, revealing enhanced antiferromagnetic correlations in the local environment of a hole dopant. In contrast, around a charge dopant, we find ferromagnetic correlations, a manifestation of the elusive Nagaoka effect15,16. We study the evolution of these correlations with interactions and doping, and use higher-order correlation functions to further elucidate the relative contributions of superexchange and kinetic mechanisms. The robustness of itinerant spin polarons at high temperature paves the way for exploring potential mechanisms for hole pairing and superconductivity in frustrated systems10,11. Furthermore, our work provides microscopic insights into related phenomena in triangular-lattice moiré materials17-20.

Effects of Ni2+ modified La0.67Ca0.33MnO3 ceramics on their temperature coefficient of resistivity and magnetoresistance properties

Element-modified La0.67Ca0.33MnO3 (LCMO) ceramics have been a major research subject in the past decades due to their various electromagnetic properties and potential applications in infrared detection and magnetic storage devices. However, its electromagnetic transport mechanism has not been fully elucidated and its properties need to be further optimized for practical applications. Here, we report the effect of Ni2+ doping on the structural, magnetic, the temperature coefficient of resistivity (TCR) and magnetoresistance (MR) properties of La0.67Ca0.33Mn1-xNixO3 (LCMNO) ceramics. All samples were synthesized by the sol-gel method and were in pure phase. XRD refinement results show a decrease in lattice constants and cell volume of LCMNO ceramics with increasing Ni2+ content. The XPS study confirms the reduction of the Mn3+/Mn4+ ratio and indicates that oxygen vacancies occur in LCMNO ceramics due to the Ni2+ doping. Curie-Weiss fitting results suggest ferromagnetic superexchange coupling between Ni2+ and Mn3+(4+) ions. The electrical transport properties of LCMNO ceramics were further investigated using the spin polaron hopping model in the high temperature range and the various competing scattering mechanisms in the low temperature range. In addition, double resistivity peak behavior is observed at x = 0.04 and 0.05. Furthermore, the TCR of the LCMNO ceramics increased to 45.4 %·K−1 (x = 0.01), while the MR increased to 67.9 % (x = 0.03), respectively. Our studies contribute to a better understanding of the electromagnetic properties of LCMNO ceramics.

Observation of spin polarons in a frustrated moiré Hubbard system

Observation of spin polarons in a frustrated moiré Hubbard system

Experimental evidence for ultrashort-lived spin polarons in EuSe

We investigated the time evolution of the density of giant spin polarons in a magnetic semiconductor. Spin polarons (SPs) were photoexcited and observed using time-resolved Faraday rotation. We find the existence of two types of SPs, a short-lived spin polaron with a lifetime of around τsl = 0.5 ns and a long-lived spin polaron with a lifetime of τll = 0.45 ± 0.03 µs, at T = 5 K. The stark difference of three order of magnitude between these lifetimes suggests that in the long lived SP the electron-hole pair is relaxed and its recombination is forbidden. The short-lived SP is probably associated with SP recombination before such relaxation has occured. An extraordinary finding is that the magnitude of τsl, as well as its decrease with increasing temperature, reproduces exactly the characteristic time for SP growth. This suggests that the thermal fluctuations, responsible for SP magnetic moment growth, are also responsible for increasing the recombination probability of SPs.

Spin Polarons and Ferromagnetism in Doped Dilute Moiré-Mott Insulators

Spin Polarons and Ferromagnetism in Doped Dilute Moiré-Mott Insulators

Helicity-dependent optical modulation of polaron spin current in organic semiconductors

The optical response of the transmitting polaron in π-conjugated organic semiconductor to the circularly polarized pulse is theoretically studied. Depending on the helicity of the circularly polarized light, polaron spin flip is achieved within femtoseconds without applying magnetic field, leading to the spin polarization of the polaron current. The magnitude of spin polarization can be modulated by tuning the frequency and amplitude of the circularly polarized light. Furthermore, we demonstrate that the conversions between spin-depolarization and spin-polarization of the polaron current can be repeatedly triggered by applying a series of circularly polarized pulses until the polaron dissociates. Our work suggests a new prospect for all-optically controlling spin current in the organic spintronic devices.

Itinerant spin polaron and metallic ferromagnetism in semiconductor moiré superlattices

Itinerant spin polaron and metallic ferromagnetism are theoretically predicted in the Mott insulator in semiconductor moir\'e superlattices doped below and above half filling of the narrow moir\'e band, respectively. The existence of a spin polaron can be directly identified from the kink in the dependence of the charge gap on the magnetic field.

Pseudogap metal and magnetization plateau from doping moiré Mott insulator

The problem of doping Mott insulators is of fundamental importance and long-standing interest in the study of strongly correlated electron systems. The advent of semiconductor based moiré materials opens a new ground for simulating the Hubbard model on the triangular lattice and exploring its rich phase diagram as a function of doping and external magnetic field. Based on our recent identification of spin polaron quasiparticle in Mott insulator, in this work we predict the emergence of a pseudogap metal phase at small doping below half filling and an intermediate range of fields, which exhibits a single-particle gap and a doping-dependent magnetization plateau.

Baby skyrmions in Chern ferromagnets and topological mechanism for spin-polaron formation in twisted bilayer graphene

The advent of moiré materials has galvanized interest in the nature of charge carriers in topological bands. In contrast to conventional materials with electron-like charge carriers, topological bands allow for more exotic possibilities where charge is carried by nontrivial topological textures, such as skyrmions. However, the real-space description of skyrmions is ill-suited to address the limit of small skyrmions and to account for momentum-space band features. Here, we develop a momentum-space approach to study the formation of the smallest skyrmions – spin polarons, formed as bound states of an electron and a spin flip – in topological ferromagnets. We show that, quite generally, there is an attraction between an electron and a spin flip that is purely topological in origin, promoting the formation of spin polarons. Applying our results to twisted bilayer graphene, we identify a range of parameters where spin polarons are formed and discuss their possible experimental signatures.

(Invited) Effect of Polaron Formation on Optical and Carrier Transport Properties of Transition Metal Oxides As Photoelectrodes from First-Principles Calculations

Transition metal oxides are promising photoelectrode materials for solar-to-fuel conversion applications. However, their performance is limited by the low carrier mobility (especially electron mobility) due to the formation of small polarons. Recent experimental studies have shown improved carrier mobility and conductivity by atomic doping; however the underlying mechanism is not understood. A fundamental atomistic-level understanding of the effects on small polaron transport is critical to future material design with high conductivity. In this talk, we will discuss the effect of small polaron formation on optical and carrier transport properties of transition metal oxides from first-principles calculations.First, we resolve the conflicting findings that have been reported on the optical gap of a well-known catalysis Co3O4 as an example[1]. We confirm that the formation of small hole polarons significantly influences the optical absorption spectra and introduces extra spectroscopic signature below the intrinsic band gap, leading to a 0.8 eV transition that is often misinterpreted as the band edge that defines the fundamental gap.Then we discuss the formation of small polarons' effect on carrier concentration, by resolving the controversy of nature of "shallow" or "deep" impurities of intrinsic oxygen vacancies in BiVO4 as an example[2], i.e. how to unify different experiments with the correct definition of ionization energy in polaronic oxides. We further discuss why certain dopants can have very low optimal concentrations (or very early doping bottleneck) in polaronic oxides such as Fe2O3, through a novel "electric-multipole" clustering between dopants and polarons[3]. These multipoles can be very stable at room temperature and are difficult to fully ionize compared to separate dopants, and thus they are detrimental to carrier concentration improvement. This allows us to uncover mysteries of the doping bottleneck in hematite and provide guidance for optimizing doping and carrier conductivity in polaronic oxides toward highly efficient energy conversion applications. In addition, we show the importance of synthesis condition such as synthesis temperature and oxygen partial pressure on dopant and polaron concentrations, and how to optimize the synthesis condition based on theoretical predictions[4].At the end, we show different theoretical models for polaron mobility calculations from a macroscopic dielectric continuum picture with an example of spin polarons in CuO[5] and a microscopic polaron hopping picture by combining generalized Landau-Zener theory and kinetic Monte-Carlo samplings for doped oxides[6].Our first-principles calculations provide important insights and suggest design principles for optimal optical and transport properties of polaronic oxides.

Acceleration of polaron induced by site effective mass increment in organic ferromagnets

The polaron dynamics in organic ferromagnets (OFs) is investigated theoretically by considering the increment of site effective mass originating from the attachment of spin radicals. In spite of the increase of polaron effective mass, the results reveal two opposite effects on the polaron velocity caused by the increment of site effective mass. One is the normal decrease of the polaron velocity with the increment of site effective mass. The other is the abrupt acceleration of the polaron from below the sound velocity to the supersonic velocity upon a critical value of the site effective mass increment. Mechanism analysis shows that the acceleration of the polaron is attributed to the reduction of critical decoupling field for optical and acoustic deformations caused by the site effective mass increment. The second effect further depends on the polaron spin due to the existence of spin-dependent intrinsic dipole of the polarons. This work indicates the potential of high polaron mobility and efficient spin filtering effect under low electric fields in OFs.

Closing the hybridization charge gap in the Kondo semiconductor SmB6 with an ultrahigh magnetic field

We have investigated the high-frequency magnetoresistance of Kondo semiconductor ${\mathrm{SmB}}_{6}$ under ultrahigh magnetic fields generated by the electromagnetic flux-compression technique. The semiconductor-metal transition due to closing of the hybridization charge gap was observed at approximately 180 T. The critical magnetic field observed is substantially higher than that reported previously. At temperatures below around 10 K, another transition was observed at a lower field (approximately 80 T). We suggest that the observed 80-T transition relates with the energy scale of quasiparticles such as the spin polaron in the in-gap state.

Spin precession of polarons in organic ferromagnets

Based on an extended Su-Schrieffer-Heeger model including spin correlation with radicals and spin-orbit interaction, the static and dynamic properties of a polaron in organic ferromagnets are investigated theoretically. The static property shows that the above two interactions are competitive in determining the spatial symmetry of polaron spin density. The nonadiabatic dynamic simulation demonstrates that the spin-orbit interaction induces spin precession of the polaron accompanied with periodic spin flipping, but such spin precession is suppressed when the spin correlation is considered in the presence of spin radicals. Periodic spin decline appears instead of the spin flipping process with the increase of spin correlation strength. The mechanism analysis indicates that the spin splitting energy caused by spin correlation weakens the exchange of electron occupation probability between polaron levels with different spin components. This work reveals the role of spin radicals in spin precession of polarons in organic ferromagnets.

Bound photoinduced giant spin polaron in EuTe

Previously, we showed that at low temperatures and very low excitation powers, EuTe displays band-edge photoluminescence with a well-resolved phonon structure, the so-called MX0 band, which is associated with the recombination of giant spin polarons (SPs) of magnetic moments of several hundred Bohr magnetons. Here, we investigate the MX0 band both experimentally and by Monte Carlo simulations. Results indicate that the SPs are bound to defects, with a localization energy of 0.29 eV. The density of defects harboring the SPs is estimated to be 4.3×1015 cm−3.

Anomalous Change in the Size of a Spin Polaron in the Paramagnetic Temperature Range in La1.2Sr1.8Mn2O7

Anomalous Change in the Size of a Spin Polaron in the Paramagnetic Temperature Range in La1.2Sr1.8Mn2O7

Density-matrix renormalization group study of optical conductivity of the Mott insulator for two-dimensional clusters

The real part of optical conductivity $\text{Re}\sigma(\omega)$ of the Mott insulators has a large amount of information on how spin and charge degrees of freedom interact with each other. By using the time-dependent density-matrix renormalization group, we study $\text{Re}\sigma(\omega)$ of the two-dimensional Hubbard model on a square lattice at half filling. We find an excitonic peak at the Mott-gap edge of $\text{Re}\sigma(\omega)$ not only for the two-dimensional square lattice but also for two- and four-leg ladders. For the square lattice, however, we do not clearly find a gap between an excitonic peak and continuum band, which indicates that a bound state is not well defined. The emergence of an excitonic peak in $\text{Re}\sigma(\omega)$ implies the formation of a spin polaron. Examining the dependence of $\text{Re}\sigma(\omega)$ on the on-site Coulomb interaction and next-nearest neighbor hoppings, we confirm that an excitonic peak is generated from a magnetic effect. Electron scattering due to an electron-phonon interaction is expected to easily suppress an excitonic peak since spectral width of an excitonic peak is very narrow. Introducing a large broadening in $\text{Re}\sigma(\omega)$ by modeling the electron-phonon coupling present in La$_{2}$CuO$_{4}$ and Nd$_{2}$CuO$_{4}$, we obtain $\text{Re}\sigma(\omega)$ comparable with experiments.