Self-trapped Polarons Research Articles

Spontaneous formation of spin lattices in semimagnetic exciton-polariton condensates

An exciton-polariton microcavity that incorporates magnetic ions can exhibit a spontaneous self-trapping phenomenon which is an analog of the classical polaron effect. We investigate in detail the full model of a polariton condensate that includes pumping and losses, the spin degree of freedom, external magnetic field and energy relaxation. In the quasi-one-dimensional case, we show that the polaron effect can give rise to a spontaneous lattice of perfectly arranged polarization domains in an antiferromagnetic configuration. We find that partial polarization of the condensate at moderate magnetic field strengths facilitates the formation of such "polaron lattices", which are qualitatively different from self-trapped polarons that appear in a fully polarized condensate. Within the Bogoliubov-de Gennes approximation, we calculate the instability condition which marks the appearance of the patterns. Surprisingly, we find that the stability condition displays a discontinuity at the point of partial-full polarization threshold.

5p-F-18 EuSeに於けるself-trapped magnetic polaronの存在の傍証?

5p-F-18 EuSeに於けるself-trapped magnetic polaronの存在の傍証?

Magnetic polarons in a nonequilibrium polariton condensate

We consider a condensate of exciton-polaritons in a diluted magnetic semiconductor microcavity. Such system may exhibit magnetic self-trapping in the case of sufficiently strong coupling between polaritons and magnetic ions embedded in the semiconductor. We investigate the effect of the nonequilibrium nature of exciton-polaritons on the physics of the resulting self-trapped magnetic polarons. We find that multiple polarons can exist at the same time, and derive a critical condition for self-trapping which is different to the one predicted previously in the equilibrium case. Using the Bogoliubov-de Gennes approximation, we calculate the excitation spectrum and provide a physical explanation in terms of the effective magnetic attraction between polaritons, mediated by the ion subsystem.

Femtosecond time-resolved X-ray absorption spectroscopy of anatase TiO2 nanoparticles using XFEL.

The charge-carrier dynamics of anatase TiO2 nanoparticles in an aqueous solution were studied by femtosecond time-resolved X-ray absorption spectroscopy using an X-ray free electron laser in combination with a synchronized ultraviolet femtosecond laser (268 nm). Using an arrival time monitor for the X-ray pulses, we obtained a temporal resolution of 170 fs. The transient X-ray absorption spectra revealed an ultrafast Ti K-edge shift and a subsequent growth of a pre-edge structure. The edge shift occurred in ca. 100 fs and is ascribed to reduction of Ti by localization of generated conduction band electrons into shallow traps of self-trapped polarons or deep traps at penta-coordinate Ti sites. Growth of the pre-edge feature and reduction of the above-edge peak intensity occur with similar time constants of 300–400 fs, which we assign to the structural distortion dynamics near the surface.

Ab initio prediction of fast non-equilibrium transport of nascent polarons in SrI2: a key to high-performance scintillation

AbstractThe excellent light yield proportionality of europium-doped strontium iodide (SrI2:Eu) has resulted in state-of-the-art γ-ray detectors with remarkably high-energy resolution, far exceeding that of most halide compounds. In this class of materials, the formation of self-trapped hole polarons is very common. However, polaron formation is usually expected to limit carrier mobilities and has been associated with poor scintillator light-yield proportionality and resolution. Here using a recently developed first-principles method, we perform an unprecedented study of polaron transport in SrI2, both for equilibrium polarons, as well as nascent polarons immediately following a self-trapping event. We propose a rationale for the unexpected high-energy resolution of SrI2. We identify nine stable hole polaron configurations, which consist of dimerised iodine pairs with polaron-binding energies of up to 0.5 eV. They are connected by a complex potential energy landscape that comprises 66 unique nearest-neighbour migration paths. Ab initio molecular dynamics simulations reveal that a large fraction of polarons is born into configurations that migrate practically barrier free at room temperature. Consequently, carriers created during γ-irradiation can quickly diffuse away reducing the chance for non-linear recombination, the primary culprit for non-proportionality and resolution reduction. We conclude that the flat, albeit complex, landscape for polaron migration in SrI2 is a key for understanding its outstanding performance. This insight provides important guidance not only for the future development of high-performance scintillators but also of other materials, for which large polaron mobilities are crucial such as batteries and solid-state ionic conductors.

Muon-Spin-Rotation study of yttria-stabilized zirconia (ZrO2:Y): Evidence for muon and electron separate traps

This paper is part of an extended study of oxide materials with the μSR technique. As an example, we present here experimental data on yttria-stabilized zirconia (ZrO2 doped with 8% Y2O3). Three different muon states can be distinguished: i) Deep muonium (less than 17(1)% fraction), seen as a fast-relaxing signal or indirectly via decoupling measurements in high longitudinal fields, ii) μ+ in a paramagnetic environment 62(6)% fraction), characterized by a very weak but clearly-visible hyperfine interaction, and iii) diamagnetic muon 21(1)% fraction); the diamagnetic signal is broadened only by the interaction with nuclear moments. The state corresponding to μ+ in a paramagnetic environment and the diamagnetic state are attributed to the same (oxygen-bound) muon configuration, but we assume that they have different electron surroundings (with or without an unpaired electron in the vicinity). The paramagnetic electron is not captured in the Coulomb potential of the positive muon but is self-trapped (polaron formation) at a nearby Zr ion. The distant electron interacts with the muon only via dipolar magnetic fields. This explains the very weak hyperfine interaction felt by the μ+ state in a paramagnetic environment. A further result of the experiment is that the disappearance of this signal with increasing temperature is not due to ionization of an electron shallowly bound to the muon but is caused by rapid spin fluctuations of the electron, averaging the hyperfine interaction to zero.

X-ray study of femtosecond structural dynamics in the 2D charge density wave compound 1T-TaS2

1T-TaS2 is a 2D metallic compound which undergoes a series of electronically driven phase transitions toward charge density wave and Mott phases. Its intricate electron–phonon interactions and electron–electron correlations have been promising peculiar out-of-equilibrium dynamics. In this paper, we provide the first direct information on the atomic structure response to an ultra-fast infrared laser pulse in the commensurate phase of 1T-TaS2, by using femtosecond time-resolved X-ray diffraction. We show that ultra-fast excitation with near-infrared photons drives a displacive excitation of the amplitude mode of the commensurate charge density wave. About 3ps after laser excitation, the system reaches a new, photo-induced state that is maintained for at least 10ps. We give evidence that this long-lived state exhibits the same structural modulation as in the thermodynamically stable commensurate phase, with a large correlation length. Only the average amplitude of the modulation is found to decrease. We propose that the long-lived state is formed from the commensurate phase by reducing the modulation amplitude on few superlattice nodes. The underlying mechanism proposed is the annihilation of self-trapped polarons.

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.

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.

Hole Localization in Molecular Crystals from Hybrid Density Functional Theory

We use first-principles computational methods to examine hole trapping in organic molecular crystals. We present a computational scheme based on the tuning of the fraction of exact exchange in hybrid density functional theory to eliminate the many-electron self-interaction error. With small organic molecules, we show that this scheme gives accurate descriptions of ionization and dimer dissociation. We demonstrate that the excess hole in perfect molecular crystals forms self-trapped molecular polarons. The predicted absolute ionization potentials of both localized and delocalized holes are consistent with experimental values.

Feynman path-integral treatment of the BEC-impurity polaron

The description of an impurity atom in a Bose-Einstein condensate can be cast in the form of Frohlich's polaron Hamiltonian, where the Bogoliubov excitations play the role of the phonons. An expression for the corresponding polaronic coupling strength is derived, relating the coupling strength to the scattering lengths, the trap size and the number of Bose condensed atoms. This allows to identify several approaches to reach the strong-coupling limit for the quantum gas polarons, whereas this limit was hitherto experimentally inaccessible in solids. We apply Feynman's path-integral method to calculate for all coupling strengths the polaronic shift in the free energy and the increase in the effective mass. The effect of temperature on these quantities is included in the description. We find similarities to the acoustic polaron results and indications of a transition between free polarons and self-trapped polarons. The prospects, based on the current theory, of investigating the polaron physics with ultracold gases are discussed for lithium atoms in a sodium condensate.

Localization-delocalization transition of a polaron near an impurity

We solve the problem of polaron localization on an attractive impurity by means of direct-space diagrammatic Monte Carlo implemented for the system in the thermodynamic limit. In particular we determine the ground-state phase diagram in dependence on the electron-phonon coupling and impurity potential strength for the whole phonon frequency range. Including the quantum phonon effects we find and characterize a phase where self-trapped polarons are not localized at shallow impurities, which is missing in the adiabatic approximation. We show that near the localization transition a region with a mixture of weak- and strong-coupling spectral responses is realized.

Spin-Wave Excitations in Effectively Half-Filled Mott Insulators

We study spin-wave excitations in an effectively half-filled Hubbard model, where the effectively half-filled situation is realized when all holes doped in the exactly half-filled parent compound become self-trapped lattice polarons, and their hopping rate is much slower than the neutron scattering time of magnetic excitation measurements. It is further assumed that the doped holes become stabilizing centers of spin vortices; thus, the spin texture is not necessarily that of an antiferromagnet. We derive a new spin Hamiltonian that is suitable for this situation by extending the usual antiferromagnetic Heisenberg model. It is shown that two modes of spin-wave excitations exist in this new spin Hamiltonian: the first is the one exhibits antiferromagetic dispersion in high energy excitations; the other shows a sharp commensurate peak at the excitation energy maximum, and a broadened dispersion at energies below. It is shown that the sum of these two modes form an hourglass-shaped magnetic excitation spectru...

Phase diagram of the Holstein polaron in one dimension

The behavior of the 1D Holstein polaron is described, with emphasis on lattice coarsening effects, by distinguishing between adiabatic and nonadiabatic contributions to the local correlations and dispersion properties. The original and unifying systematization of the crossovers between the different polaron behaviors, usually considered in the literature, is obtained in terms of quantum to classical, weak coupling to strong coupling, adiabatic to nonadiabatic, itinerant to self-trapped polarons and large to small polarons. It is argued that the relationship between various aspects of polaron states can be specified by five regimes: the weak-coupling regime, the regime of large adiabatic polarons, the regime of small adiabatic polarons, the regime of small nonadiabatic (Lang-Firsov) polarons, and the transitory regime of small pinned polarons for which the adiabatic and nonadiabatic contributions are inextricably mixed in the polaron dispersion properties. The crossovers between these five regimes are positioned in the parameter space of the Holstein Hamiltonian.

Loop Current Excitations in Effectively Half-Filled Mott Insulators

We show that spin vortex excitations in a two-dimensional half-filled antiferromagnetic insulator are accompanied by loop currents; they are the collective flow of electrons generated by a fictitious magnetic field, where the field arises from a phase factor that ensures the single-valuedness of wave functions for electron hopping motion. We study effectively half-filled states of a U ≫ t Hubbard model in which all holes added to the exactly half-filled antiferromagnetic insulator become self-trapped polarons. Using the perturbation theory, we demonstrate that loop currents arise when spin vortices are created and partially retrieve the itineracy of electrons in the upper Hubbard band. We also perform mean-field calculations to gain qualitative insights on the loop current state using parameters appropriate for cuprate superconductors. It is shown that a collection of loop currents creates a large current flow region or a “river” of current. A calculated ARPES profile exhibits a Fermi arc feature seen in ...

What is the role of strong hole-lattice interaction in the hole-doped cuprate superconductivity?

We first present our molecular orbital cluster calculation results that indicate the presence of strong interaction between doped holes and the underlying lattice in the cuprate. It is also indicated that the naive Zhang-Rice singlet picture for the local electronic state needs to be revised: the doped hole is mainly concentrated in a single bridging oxygen, and the bridging oxygen p orbital and two nearby copper dx2-y2 orbitals form a valence bond type bonding; this state is stabilized by local lattice distortions and the doped hole becomes a small polaron.Next, we consider the current generation mechanism compatible with the above small polaron formation; actually, we present a new current generation mechanism that is effective even if all the doped holes become self-trapped small polarons. The current generation here reverses a long-standing belief that at half-filing the effective Hamiltonian for the U >> t Hubbard model is the Heisenberg model, thus, possible low energy excitations are those include spin degrees of freedom only. The new current generation mechanism yields the Fermi arc seen in the ARPES simulation; it also suggests that the phase of the superconductivity order parameter in underdoped cuprates arises without Cooper pair formation; i.e., the London current formula with effective charge 2e is realized without the Cooper pair formation.The electric current here is generated by a fictitious magnetic field induced by spin vortices: because the presence of spin vortices causes a sign ambiguity of wave functions for electron hoping motion, a phase factor that ensures the single-valuedness of the wave function appears; and this phase factor gives rise to a vector potential for the fictitious magnetic field. The strong hole-lattice interaction stabilizes spin vortices by pinning each center at a polaron occupied site; thus, it will also stabilize the spin vortex induced current. Based on the new current generation mechanism, possibility of an artificial superconducting nano wire is presented. It is made in the Cu-O plane, and consists of pining centers of nonmagnetic atoms arranged in two parallel lines; here the nonmagnetic atoms play the role of lattice small polarons; and the region between the two arrays of vortex centers is a superconducting wire where a thermodynamically stable current flows.

Dynamic process of self-trapped polaron in photoexcited SrTiO 3

Relaxation process of self-trapped polaron is investigated by a nonadiabatic molecular dynamic method. We show localized disorder due to lattice fluctuations can give rise to a tightly-bound electronic state in ultraviolet illuminated SrTiO 3 crystal. This bound state is actually a self-trapped polaron in accordance with the experimentally observed large Stokes-shift. The formation of the self-trapped polaron is shown to be an ultrafast process.

Semiclassical and Quantum Polarons in Crystalline Acetanilide

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Semiclassical and quantum polarons in crystalline acetanilide

Crystalline acetanilide is a an organic solid with peptide bond structure similar to that of proteins. Two states appear in the amide I spectral region having drastically different properties: one is strongly temperature dependent and disappears at high temperatures while the other is stable at all temperatures. Experimental and theoretical work over the past twenty five years has assigned the former to a selftrapped state while the latter to an extended free exciton state. In this article we review the experimental and theoretical developments on acetanilide paying particular attention to issues that are still pending. Although the interpretation of the states is experimentally sound, we find that specific theoretical comprehension is still lacking. Among the issues that that appear not well understood is the effective dimensionality of the selftrapped polaron and free exciton states.

Impurity conduction and magnetic polarons in antiferromagnetic oxides

Low-temperature transport and magnetization measurements for the antiferromagnets SrMnO(3) and CaMnO(3) identify an impurity band of mobile states separated by energy E from electrons bound in Coulombic potentials. Very weak electric fields are sufficient to excite bound electrons to the impurity band, increasing the mobile carrier concentration by more than three orders of magnitude. The data argue against the formation of self-trapped magnetic polarons (MPs) predicted by theory, and rather imply that bound MPs become stable only for kT<<E.