Properties Of Polarons Research Articles

Spin polaronics: Static and dynamic properties of spin polarons in La-doped CaMnO3

The static and dynamic properties of spin polarons in La-doped CaMnO3 are explored theoretically, by means of an effective low-energy Hamiltonian. All parameters of the effective Hamiltonian are ev ...

Influence of magnetic field on the properties of polaron in an asymmetric quantum dot

Influence of the magnetic field on the properties of the polaron in an asymmetric quantum dot is studied by using the Pekar variation method. The expression of the magnetopolaron ground-state energy is obtained by theoretical derivation. The relationship between the ground-state energy of the magnetopolaron with the transverse confinement strength, the longitudinal confinement strength and the magnetic field cyclotron resonance frequency are further discussed by us. Due to the crystal structure inversion asymmetry and the time inversion asymmetry, the polaron energy causes Rashba spin–orbit splitting and Zeeman splitting. Under the strong and weak magnetic fields, we discuss the dominant position of Rashba effect and Zeeman effect, respectively. Due to the presence of phonons, the polaron is more stable than the bare electron state, and the energy splitting is more stable.

Spectral properties of spin-orbital polarons as a fingerprint of orbital order

Transition metal oxides are a rich group of materials with very interesting physical properties that arise from the interplay of the charge, spin, orbital, and lattice degrees of freedom. One interesting consequence of this, encountered in systems with orbital degeneracy, is the coexistence of long range magnetic and orbital order, and the coupling between them. In this paper we develop and study an effective spin-orbital superexchange model for $e_g^3$ systems and use it to investigate the spectral properties of a charge (hole) injected into the system, which is relevant for photoemission spectroscopy. Using an accurate, semi-analytical, magnon expansion method, we gain insight into various physical aspects of these systems and demonstrate a number of subtle effects, such as orbital to magnetic polaron crossover, the coupling between orbital and magnetic order, as well as the orbital order driving the system towards one-dimensional quantum spin liquid behavior. Our calculations also suggest a potentially simple experimental verification of the character of the orbital order in the system, something that is not easily accessible through most experimental techniques.

Two-dimensional repulsive Fermi polarons with short- and long-range interactions

We study the repulsive polaron problem in a two-component two-dimensional system of fermionic atoms. We use two different interaction models: a short-range (hard-disk) potential and a dipolar potential. In our approach, all the atoms have the same mass and we consider the system to be composed of a uniform bath of a single species and a single atomic impurity. We use the diffusion Monte Carlo method to evaluate polaron properties such as its chemical potential and pair distribution functions, together with a discussion on the deficit of volume induced by the impurity. We also evaluate observables that allow us to determine the validity of the quasi-particle picture: the quasi-particle residue and the effective mass of the polaron. Employing two different potentials allows us to identify the universality regime, where the properties depend only on the gas parameter $n a_s^2$ fixed by the bath density and the two-dimensional scattering length.

Analyzing a Bose polaron across resonant interactions

Recently, two independent experiments reported the observation of long-lived polarons in a Bose-Einstein condensate, providing an excellent setting to study the generic scenario of a mobile impurity interacting with a quantum reservoir. Here, we expand the experimental analysis by disentangling the effects of trap inhomogeneities and the many-body continuum in one of these experiments. This makes it possible to extract the energy of the polaron at a well-defined density as a function of the interaction strength. Comparisons with quantum Monte-Carlo as well as diagrammatic calculations show good agreement, and provide a more detailed picture of the polaron properties at stronger interactions than previously possible. Moreover, we develop a semi-classical theory for the motional dynamics and three-body loss of the polarons, which partly explains a previously unresolved discrepancy between theory and experimental observations for repulsive interactions. Finally, we utilize quantum Monte-Carlo calculations to demonstrate that the findings reported in the two experiments are consistent with each other.

Cation Alloying Delocalizes Polarons in Lead Halide Perovskites.

Recently, mixed-cation perovskites have promised enhanced performances concerning stability and efficiency in optoelectronic devices. Here, we report a systematic study on the effects of cation alloying on polaronic properties in cation-alloyed perovskites using first principle calculations. We find that cation alloying significantly reduces the polaron binding energies for both electrons and holes compared to pure methylammonium lead iodide (MAPbI3). This is rationalized in terms of crystal symmetry reduction that causes polarons to be more delocalized. Electron polarons undergo large Jahn-Teller distortions (∼15-30%), whereas hole polarons tend to shrink the lattice by ∼5%. Such different lattice distortion footprints could be utilized to distinguish the type of polarons. Finally, our simulations show that Cs, formamidinium (FA), and MA mixtures can effectively minimize polaron binding energy while weakly affecting band gap, in a good agreement with experimental findings. These modeling results can guide future development of halide perovskite materials compositions for optoelectronic applications.

The Properties of the Polarons’ Ground State in Coupling Spherical Quantum Dots

Investigations on the properties of polarons in coupling quantum dots (QDs) are useful for the designs of quantum devices and applications of QDs. Based on accurately solving the time-independent Schrodinger equation, the famous Lee-Low-Pines unitary transformation (LLPUT) and variation methods, we have obtained the ground state energy of polarons in two spherical quantum dots (TSQD) with Coulomb interaction. The numerical results show that Coulomb interaction energy, electronic kinetic energy, induced potential and the ground state energy all decrease with increasing the interval of spherical centers or radius of quantum dot (QD).

Dynamical Simulations of Polaron Spin-Filtering and Rectification in an Organic Magnetic–Nonmagnetic Co-oligomer: The Interfacial Effect

The interfacial effect on the dynamical properties of spin-dependent polarons in an organic magnetic/nonmagnetic co-oligomer is investigated by using a tight-binding model coupled with a nonadiabatic dynamic method. It is found that the dynamical behaviors of spin-up and spin-down polarons are quite different at the interface. In the presence of an external electric field, polarons with a specific spin (up or down) can get trapped near the magnetic/nonmagnetic interface while the motion of polarons with an opposite spin is not affected, which leads to the phenomenon of polaron spin-filtering. Interestingly, when the electric field is reversely applied, only polarons with opposite spin can pass through the co-oligomer and spin rectification can be observed. The spin-filtering and rectification can be improved by increasing the strength of the spin correlation in the magnetic part of the co-oligomer and they can also be affected by the strength of the interfacial electron–lattice coupling and the electronic...

Stationary and Dynamical Properties of Polarons in Anisotropic C60-Crystals

Two-dimensional C60 crystals are treated with a Holstein–Peierls model that takes into account both intra- and intermolecular vibrational modes to describe charge transport. By virtue of this procedure, we obtained the set of values for intra- and intermolecular electron–phonon coupling that makes it possible to study stationary properties of polarons as well as to investigate the transport regime of the charge carriers. This is carried out by considering the presence of anisotropy of electronic terms and effects because of the application of external electric fields. We have mapped the regimes in which polarons of different nature—each endowing the system with different properties—are observed.

Polaron properties in 2D organic molecular crystals: directional dependence of non-local electron-phonon coupling.

In organic molecular crystals, the polaronic hopping model for the charge transport assumes that the carrier lies at one or a small number of molecules. Such a kind of localization suffers the influence of the non-local electron-phonon (e-ph) interactions associated with intermolecular lattice vibrations. Here, we developed a model Hamiltonian for numerically describing the role played by the intermolecular e-ph interactions on the stationary and dynamical properties of polarons in a two-dimensional array of molecules. We allow three types of electron hopping mechanisms and, consequently, for the nonlocal e-ph interactions: horizontal, vertical, and diagonal. Remarkably, our findings show that the stable polarons are not formed for isotropic arrangements of the intermolecular transfer integrals, regardless of the strengths of the e-ph interactions. Interestingly, the diagonal channel for the e-ph interactions changes the transport mechanism by sharing the polaronic charge between parallel molecular lines in a breather-like mode.

Polaron properties in pentathienoacene crystals

A two-dimensional tight-binding model is used to study static and dynamical properties of polarons in pentathienoacene crystals. Our computational approach determines the spacial parameter to obtain stable polarons in these materials by using a version of the Holstein–Peierls Hamiltonian. The model takes intra and intermolecular electron-lattice interactions into account. Remarkably, the results show that the charge transport mechanism is essentially one-dimensional in pentathienoacene crystals. This property is due to the nature of the polaron localization. Moreover, we show that the polaron dynamics can occur for an electric field threshold that overcomes the standard value of organic crystalline semiconductors.

From Large to Small Polarons in Lead, Tin, and Mixed Lead-Tin Halide Perovskites.

The origin of the long carrier lifetime in lead halide perovskites is still under debate, and, among different hypotheses, the formation of large polarons preventing the recombination of charge couples is one of the most fascinating. Using state-of-the art ab initio calculations, we report a systematic study of the polaron formation process in metal halide perovskites, focusing on the influence of the chemical composition of the perovskite on the polaron properties. We examine variations in A-site cations (FA, MA, Cs, and Cs-MA), B-site cations (Pb, Sn, and Pb-Sn), and X-site anions (Br, I). Our study confirms that stronger structural distortions occur for Cs than for MA and FA, with the effect of different A-site cations being almost additive. For the same A cation, bromide features stronger distortions than iodide perovskites. The pure Sn phase has an almost double polaron stabilization energy compared with the pure Pb phase. Surprisingly, the trend of polaron stabilization energy is nonmonotonic in mixed Sn-Pb perovskites, with a maximum for small Sn percentages. Polaron formation is found to be promoted by bond asymmetry, ranging from small to large polarons in mixed Sn-Pb perovskites depending on the relative Sn percentage.

Transport properties of electron small polarons in a V2O5 cathode of Li-ion batteries: a computational study.

Employing the first-principles plane-wave approach, we explored the behavior of electron transport in the V2O5 cathode. Polaron migrations along different crystallographic directions in the presence and absence of Li+ ions were systematically examined using linear interpolation (LE) and nudged elastic band (NEB) methods. We find that the NEB calculations, based on structural optimizations of TS structures, generally exhibit lower hopping barriers than those obtained from the LE calculations. Both methods consistently predict that the [010] hopping, in the presence and absence of a nearby Li+ ion, is kinetically least favorable since the migration involves displacements of rigid 3-coordinated O atoms. Computations based on the LE method reveal anisotropic polaron mobilities where the estimated hopping frequencies within the layer are approximately one order of magnitude higher than the normal. The prediction based on the LE calculations is consistent with the experimental results. Lithiation dramatically affects the behavior of polaron movement. It significantly increases the reaction energies and hopping barriers due to the strong polaron-ion interaction. In addition, it is predicted that polaron hopping in the V2O5 cathode is non-adiabatic where lithiation has negligible effects on the adiabaticity.

Ground-state properties of interacting Bose polarons

We theoretically investigate the role of multiple impurity atoms on the ground state properties of Bose polarons. The Bogoliubov approximation is applied for the description of the condensate resulting in a Hamiltonian containing terms beyond the Fr\"ohlich approximation. The many-body nature of the impurity atoms is taken into account by extending the many-body description for multiple Fr\"ohlich polarons, revealing the static structure factor of the impurities as the key quantity. Within this formalism various experimentally accessible polaronic properties are calculated such as the energy and the effective mass. These results are examined for system parameters corresponding to two recent experimental realizations of the Bose polaron, one with fermionic impurities and one with bosonic impurities.

Properties of One-Dimensional Highly Polarized Fermi Gases**Supported by the National Natural Science Foundation of China under Grant Nos 11374331, 11304364 and 11534014.

Using both the exact Bethe ansatz method and the variational method, we study properties of the one-dimensional Fermi polaron. We focus on the binding energy, effective mass, momentum distributions, Tan contact and correlation functions. As the attraction increases, the impurity is more tightly bound and correlated with the surrounding particles, and the size of formed polaron decreases. In addition, compared with the Bethe ansatz method, the variational method is totally qualified to study the one-dimensional Fermi polaron. The intrinsic reason is that the number of particle-hole excitations in a Fermi sea, caused by a single impurity, is always rather small. The variational method can be well extended to other impurity systems.

Ambient-Stable Cubic-Phase Hybrid Perovskite Reaching the Shockley–Queisser Fill Factor Limit via Inorganic Additive-Assisted Process

Additive-assisted organic–inorganic perovskite materials have attracted substantial attention as photovoltaic light absorbers which lead to outstanding power conversion efficiency. Here we report an easy and effective fabrication of cubic-phase perovskite with an inorganic molecule additive like hydrazinium chloride (N2H5Cl, to be denoted as HZCl). We predict that this inorganic cation of N2H5+, which can substitute for the organic A-site in the perovskite structure, can tune Frohlich polaron properties by controlling the interaction strength and the number of proton coordinations to halide. This prediction is experimentally demonstrated with an optimized perovskite device with 2% N2H5Cl additive, which exhibits an unprecedented 85% fill factor (FF) with the highest value close to the Shockley–Queisser limit. An extra power conversion efficiency (PCE) of 2.3% and a fill factor (FF) efficiency of 14% are boosted. These optimized performances by additive effects lead to a new approach based on the theoretic...

Polaron in a non-Abelian Aubry-André-Harper model with p -wave superfluidity

We theoretically investigate the behavior of a mobile impurity immersed in a one-dimensional quasi-periodic Fermi system with topological $p$-wave superfluidity. This polaron problem is solved by using a standard variational approach, the so-called Chevy ansatz. The polaron states are found to be strongly affected by the strength of the quasi-disorder and the amplitude of the $p$-wave pairing. We analyze the phase diagram of the polaron ground state and find four phases: two extended phases, a weakly-localized phase and a strongly-localized phase. It is remarkable that these polaron phases are directly corresponding to the four distinct phases experienced by the underlying background Fermi system. In particular, the weakly-localized polaron phase corresponds to an intriguing critical phase of the Fermi system. Therefore, the different phases of the background system can be unambiguously probed by measuring the polaron properties via radio-frequency spectroscopy. We also investigate the high-lying excited polaron states at an infinite temperature and address the possibility of studying many-body localization (MBL) of these states. We find that the introduction of $p$-wave pairing may delocalize the many-body localized states and make the system easier to thermalize. Our results could be observed in current state-of-the-art cold-atom experiments.

Contribution of bulk and surface phonons to the properties of polaron in a Zn1−XCdXSe∕ZnSe heterojunction confined in a triangular potential

The contributions of bulk and surface phonons to the properties of a polaron in a Zn1 −XCdXSe∕ZnSe heterojunction confined in a triangular potential have been studied using the Lee-Low-Pines variational method and the Tokuda linear-combination operator method. We obtain expressions for the wave function of the system, the ground state energy, effective mass, the average number of phonons, the mobility and the lifetime of the polaron. Our numerical results show that the two Interface-optical (IO)-phonon modes and the longitudinal-optical (LO)-phonon mode contribute to the ground state energy, the average number of phonons, the mobility, the effective mass and the lifetime of the polaron. It is seen that both the ground state energy and the average number of phonons increase with an increase in the Cd composition X while the effective mass, mobility and lifetime of polaron decrease as the Cd composition X increases. We observe that there are more phonons in the bulk than at the surface. It is also noticed that the polaron disintegrates when the temperature is increased to certain values. Thus, at high temperatures, the electron-phonon interaction is destroyed. It is seen that even though the contribution of the LO phonon is more important; the contribution of the IO phonon to the polaron's properties is also quite significant.

Attractive Fermi polarons at nonzero temperatures with a finite impurity concentration

We theoretically investigate how quasi-particle properties of an attractive Fermi polaron are affected by nonzero temperature and finite impurity concentration. By applying both non-self-consistent and self-consistent many-body $T$-matrix theories, we calculate the polaron energy (including decay rate), effective mass, and residue, as functions of temperature and impurity concentration. The temperature and concentration dependences are weak on the BCS side with a negative impurity-medium scattering length. Toward the strong attraction regime across the unitary limit, we find sizable dependences. In particular, with increasing temperature the effective mass quickly approaches the bare mass and the residue is significantly enhanced. At the temperature $T\sim0.1T_{F}$, where $T_{F}$ is the Fermi temperature of the background Fermi sea, the residual polaron-polaron interaction seems to become attractive. This leads to a notable down-shift in the polaron energy. We show that, by taking into account the temperature and impurity concentration effects, the measured polaron energy in the first Fermi polaron experiment {[}A. Schirotzek \textit{et al.}, Phys. Rev. Lett. \textbf{102}, 230402 (2009){]} can be better theoretically explained.

Polaron in dilute 2D Bose gas at low temperatures

The properties of a Bose polaron immersed in a dilute two-dimensional medium at finite temperatures are discussed. Assuming that the impurity is weakly coupled to bath particles, we have perturbatively calculated the polaron energy, effective mass, quasiparticle residue and damping rate. The parameters of the impurity spectrum are found to be well-defined in the whole temperature region whereas the pole structure of the impurity Green’s function is visible only at absolute zero. At any finite temperatures the quasiparticle residue is logarithmically-divergent signalling of the branch-cut behavior of the polaron propagator.