Polaron Binding Energy Research Articles

Spin-orbit interaction enhanced polaron effect in two-dimensional semiconductors

It is shown that in two-dimensional semiconductors, the electron-phonon interaction and polaron mass correction are both significantly enhanced by the Rashba spin-orbit coupling. The mass correction is positive for the upper Rashba branch and negative for the lower Rashba branch. Both Rashba branches have the same polaron binding energy, which is higher than that for systems in the absence of spin-orbit interaction. To the leading order, the correction to the binding energy is proportional to the square of the spin-orbit coupling strength.

A-site deficiency effects on the structure, magnetic and transport properties of Pr 0.6− x□ xSr 0.4MnO 3 (0 ≤ x ≤ 0.2) perovskite manganite

Praseodymium-deficient Pr 0.6− x □ x Sr 0.4MnO 3 ( x = 0, 0.05, 0.1, 0.15 and 0.2) polycrystalline samples have been studied by X-ray diffraction, magnetic and transport measurements. Our samples have been synthesized by the solid state reaction method at high temperature. Structural characterization showed that all our samples crystallize in the orthorhombic system with Pbnm space group. Praseodymium deficiency leads to a decrease of the unit cell volume. Magnetization measurements versus temperature in a magnetic applied field of 50 mT showed that all our samples are ferromagnetic at low temperature. With increasing praseodymium-deficient amount, the Curie temperature T C shifts from 310 K for x = 0 to 254 K for x = 0.2. All our samples exhibit a semiconducting-metallic transition with decreasing temperature. The maximum of the resistivity increases and the semiconductor–metallic transition temperature T MI decreases as the deficiency content increases. In the ferromagnetic metallic region ( T < T MI), the resistivity of the samples are well fitted using the expression ρ = ρ 0 + ρ 2 T 2 + ρ 9 / 2 T 9 / 2 implying the importance of scattering process. In the high temperature regime, the resistivity data can be well described by the small polaron model. The polaron binding energy varies from 192 to 202 meV with increasing deficient amount. Under a magnetic applied field, a significant reduction in the resistivity and a shift of T MI to higher values are observed. With increasing temperature, the magnetoresistance ratio for Pr 0.4□ 0.2Sr 0.4MnO 3 sample remains constant up to 200 K and then decreases, it is found to be about 15% at 300 K.

Polarons in quantum chains with XY exchange and Dzyaloshinsky-Moriya interaction

Excitations of the polaron types are investigated in the spin-1/2 quantum chain with XY exchange and Dzyaloshinsky-Moriya interaction, both coupled to acoustic vibrations of the substrate lattice. The study is carried out via Jordan-Wigner transformation with the help of which the spin chain is mapped onto a chain of spinless fermions. From the resulting effective fermion-lattice Hamiltonian, the discrete equations of motion are derived. These equations are solved in the continuum limit for self-trapped states near the bottom of the fermion spectrum interacting with long-wavelength acoustic lattice modes. The associate polaron solution, which has a pulse shape, is shown to propagate bound to the induced lattice kink distortion by translation along the chain at a constant velocity v. The pair can also experience an additional acceleration ϑ0 when the free fermion charge is excited above its groundstate. The polaron binding energy is strongly reduced, depending quadratically on the ratio D/J of the Dzyaloshinsky-Moriya interaction strength D to the isotropic XY exchange interaction J. It is also found that polaron parameters depend only on the XY spin-lattice coupling but not on the Dzyaloshinsky-Moriya contribution.

A comprehensive study of the effect of reactive end groups on the charge carrier transport within polymerized and nonpolymerized liquid crystals

Polymerizable liquid crystalline semiconductors, referred to as reactive mesogens (RMs), consist of π-conjugated cores with reactive end groups decoupled by an aliphatic spacer. These can be polymerized within the mesophase, maintaining the self-assembled morphology and charge transport characteristics. The polymerized films can then be used in organic electronic applications such as charge transport layers in organic light emitting diodes and field effect transistors. We present a systematic study of the effect of reactive end groups on charge transport in calamitic liquid crystals (RMs) using the time-of-flight technique. Several different compounds were synthesized with a variation in both the liquid crystal (LC) mesogenic core group and the functional end groups. The reactive end groups in most cases affect the mesophase charge transport compared to the nonreactive LC mesophase transport. This manifests itself as a reduction in mobility, varying from a factor of 4 in the best case to as large as two orders of magnitude. In the best systems studied, however, the reactive end group effect on the transport, compared to the nonreactive mesophase transport, is negligible. Polymerized reactive mesogens do maintain long-range transport, with comparable mobilities to those of the phase in which they were polymerized over a broad temperature range, including room temperature. The hole and electron mobilities found in polymerized systems are explored using the Holstein small polaron model in the nonadiabatic limit, yielding the relevant polaron binding energies and bandwidths, and using the Bässler Gaussian disorder model, yielding the relevant energetic disorder parameters.

Carrier heating in disordered organic semiconductors

We propose a semi-implicit model for hopping transport in disordered media with application to organic semiconductors. The results show excellent agreement with both Monte Carlo and standard master-equation calculations. In organic LEDs the applied field would result in heating of the charge carrier population by up to 100 °C above the lattice temperature and is more effective at lower temperatures. We show that the voltage dependence of the mobility in space charge limited LEDs is largely due to carrier heating and not to the previously considered charge density or barrier lowering effects. At the end we look into the effect of accounting for the soft nature of organic materials via the inclusion of polaronic rate binding energy and we find that carrier heating is suppressed at polaron binding energies above 0.1 eV.

Homogeneous spectroscopic properties in manganite films

The correlation between topographic features and spectroscopic properties was studied by means of room-temperature scanning tunnelling microscopy on single-crystalline fully strained La 0.77 Ca 0.23 MnO 3 films. Growth terraces are atomically flat and present similar semiconducting-like tunnel current–voltage characteristics. The STM-estimated gap is in agreement with the polaron binding energy obtained from the insulating-phase resistivity. Conductance maps at different voltages do not present recognizable spatial patterns. In conclusion, ‘extrinsic’ phase separation is not likely in the manganite films studied.

Optical spectra of strong-coupling polarons

The photodissociation spectra of Landau-Pekar polarons are calculated using the theory of quantum coherent states. It is shown that the number of phonons emitted in one dissociation event can differ and that their energy is equal to the doubled polaron binding energy Ep only on the average. It is established that the absorption spectrum is a superposition of bands corresponding to different numbers of phonons emitted during the dissociation of one polaron and that the half-width of each of the bands is much greater than the distance between the bands (which is equal to the phonon energy ℏθ). Therefore, the absorption spectrum looks like a very wide unstructured band with the low-frequency edge lying at E p + ℏθ, a maximum at an energy of about 5E p (for the band carrier mass equal to (1–3)m e ), and the half-width being of the order of the energy corresponding to the maximum.

Temperature and field dependence of hole mobility in poly(9,9-dioctylfluorene)

Detailed studies of hole mobility in the electroluminescent conjugated polymer poly($9,{9}^{\ensuremath{'}}$-dioctylfluorene) by the time-of-flight technique are reported. The hole mobility is studied in a number of samples over a wide range of temperatures and electric field strengths. Significant sample-to-sample variations are observed in both hole mobility and the character of hole transport in as-spun films of different thicknesses. Hole mobility is found to improve irreversibly as a result of thermal annealing at elevated temperatures. Analysis of the experimental results is carried out within the Gaussian disorder model (GDM), the polaronic correlated disorder model (PCDM), and the geometry fluctuation model based on torsional interactions (known here as the Los Alamos model). The PCDM is found to be more successful than the GDM in explaining the experimental data. Moreover, the Los Alamos model succeeds in explaining the data when modified to include polaron hopping. Analysis with both PCDM and Los Alamos models leads to the conclusion that the improvement in hole mobility upon annealing is consistent with reduced energetic disorder and reduced polaron binding energy, both of which may result from crystallization of the material. Spectroscopic ellipsometry is used to confirm that crystallization occurs upon annealing.

Effect of strain and tetragonal lattice distortions in doped perovskite manganites

A series of high-quality, coherently strained $[{\mathrm{La}}_{2∕3}(\mathrm{Ca}\phantom{\rule{0.3em}{0ex}}\mathrm{or}\phantom{\rule{0.3em}{0ex}}\mathrm{Ba}{)}_{1∕3}{\mathrm{MnO}}_{3}∕{\mathrm{SrTiO}}_{3}]\ifmmode\times\else\texttimes\fi{}N$ superlattices has been prepared on $(100)\phantom{\rule{0.3em}{0ex}}{\mathrm{SrTiO}}_{3}$ and ${\mathrm{NdGaO}}_{3}$ substrates by laser molecular beam epitaxy. The manganite layers are biaxially strained due to lattice mismatch. A quadratic decrease of the metal-to-insulator transition temperature ${T}_{p}$ with increasing biaxial distortion ${\ensuremath{\epsilon}}_{\mathrm{bi}}^{2}$ both for tensile and compressive in-plane strain is found. For $Tg{T}_{p}$, the resistivity versus temperature curves could be well described by the small polaron hopping model with the polaron binding energy increasing with increasing ${\ensuremath{\epsilon}}_{\mathrm{bi}}^{2}$. Furthermore, the magnetoresistance of the manganite films was found to strongly increase with increasing ${\ensuremath{\epsilon}}_{\mathrm{bi}}^{2}$ or decreasing ${T}_{p}$, respectively, following a universal behavior. An anomalous upturn of resistivity in the low-temperature regime $(Tl25\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ was detected, which may be attributed to enhanced Coulomb interaction of the charge carriers resulting from disorder due to the lattice distortion. Our analysis clearly demonstrates the importance of biaxial strain and Jahn-Teller-type lattice distortions for the physics of the doped manganites. It is shown that epitaxial coherency strain can be used to deliberately modify the materials properties.

Effect of Ga doping on magneto-transport properties in colossal magnetoresistive La 0.7Ca 0.3Mn 1−xGa xO 3 (0< x

Samples of La 0.7Ca 0.3Mn 1− x Ga x O 3 with x=0, 0.025, 0.05 and 0.10 were prepared by standard solid-state reaction. They were first characterized chemically, including the microstructure. The magnetic properties and various transport properties, i.e. the electrical resistivity, magnetoresistivity (for a field below 8 T), thermoelectric power and thermal conductivity measured each time on the same sample, are reported. The markedly different behaviour of the x=0.1 sample from those with a smaller Ga content, is discussed. The dilution of the Mn 3+/Mn 4+ interactions with Ga doping considerably reduces the ferromagnetic double exchange interaction within the manganese lattice leading to a decrease of the Curie temperature. The polaron binding energy varies from 224 to 243 meV with increased Ga doping.

Nanocharacterization of electrocoated polymers on carbon fibers

Electropolymerization of carbazole and its copolymers onto carbon fibers were performed by potentiodynamic and potentiostatic methods. Electrocoated polymer thin films on carbon fiber microelectrodes (CFME) were characterized by combination of a variety of techniques, i.e., cyclo-voltammetry, electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy (XPS), reflectance FTIR spectroscopy (FTIR-ATR). From XPS results the peak at 285.7 eV is assigned to C S bonding, for the poly[methylthiophene-co-carbazole]. Two distinct S2p peaks in a ratio of 3/2 was observed, which can be assigned to S in polaronic (binding energy: ∼164.7 eV) and S in bipolaronic forms. Broad band in FTIR-ATR centred in ∼3100 cm −1 supports these findings. S and N contents of copolymeric films (XPS) formed (at different initial feed ratio of monomers [Cz] 0/[MeTh] 0 = 0.35–1.85) on the CFME surface copolymeric composition was found to be ranging from 1 to 16 carbazole unit against 1 MeTh unit in the resulting chain.

Electron-molecular-vibration coupling for small polarons in DNAs

Within a tight-binding electron-phonon interacting model, we calculated the spectrally resolved polaron binding energy between electrons and phonons and between holes and phonons on poly(dA).poly(dT) and poly(dG).poly(dC).Poly(dA).Poly(dT) is a DNA where one single strand consists only of adenine(A) and the other single strand consists only of thymine(T), while Poly(dG).Poly(dC) is a DNA where one single strand consist only of guanine(G) and the other of cytosine(C). We found that the polaron binding energies of poly(dA).poly(dT) were larger than those of poly(dG).poly(dC), and that the polaron binding energy and the electrical conductance were strongly temperature dependent. These findings agree well with the current experimental data. We concluded that small polaron hopping occurs by a conduction mechanism on the DNA molecules examined.

Voltage-tunable ferromagnetism in semimagnetic quantum dots with few particles: Magnetic polarons and electrical capacitance

Magnetic semiconductor quantum dots with a few carriers represent an interesting model system where ferromagnetic interactions can be tuned by voltage. By designing the geometry of a doped quantum dot, one can tailor the anisotropic quantum states of magnetic polarons. The strong anisotropy of magnetic polaron states in disk-like quantum dots with holes comes from the spin splitting in the valence band. The binding energy and spontaneous magnetization of quantum dots oscillate with the number of particles and reflect the shell structure. Due to the Coulomb interaction, the maximum binding energy and spin polarization of magnetic polarons occur in the regime of Hund's rule when the total spin of holes in a quantum dot is maximum. With increasing number of particles in a quantum dot and for certain orbital configurations, the ferromagnetic state becomes especially stable or may have broken symmetry. In quantum dots with a strong ferromagnetic interaction, the ground state can undergo a transition from a magnetic to a nonmagnetic state with increasing temperature or decreasing exchange interaction. The characteristic temperature and fluctuations of magnetic polarons depend on the binding energy and degeneracy of the shell. The capacitance spectra of magnetic quantum dots with few particles reveal the formation of polaron states.

Thickness-dependent optical properties of La0.7Sr0.3MnO3 thin films

We report on a systematic study of the thickness dependence of the optical properties of La0.7Sr0.3MnO3 thin films epitaxially grown on a LaAlO3 substrate. The x-ray powder-diffraction data indicate that the c-axis lattice constant is enhanced with decreasing the film thickness due to the compressive strain in the film plane produced by lattice mismatch. Magnetization curves show a decrease of the Curie temperature (TC) for decreasing thickness of films. Optical reflectance and transmittance measurements provide evidence that the position of Mn-O stretching mode shifts toward low frequency and the energy of the charge-transfer transition between O 2p and Mn 3d states increases with the decrease of film thickness. Most importantly, an analysis of the small-polaron absorption in the mid-infrared region shows that the polaron binding energy increases with decreasing the film thickness, suggesting that the strain dependence of TC mainly results from the strain-induced electron–phonon coupling.

Polarons in semiconducting polymers: Study within an extended Holstein model

We present a study of electron- (hole-) phonon interaction and polaron formation in semiconducting polymers within an extended Holstein model. A minimization of the lowest electronic state of this Hamiltonian with respect to lattice degrees of freedom yields the polaronic ground state. Input parameters of this Hamiltonian are obtained from ab initio calculations based on the density-functional theory. We calculate optical phonon modes and the coupling constants of these modes to the highest occupied and lowest unoccupied molecular orbital bands, respectively. For the studied polymers [polythiophene, poly(phenylenevinylene), poly(para-phenylene)] the polaron binding energy, its size, and the lattice deformation as a function of conjugation length have been determined. Self-trapped polarons are found for long conjugation lengths. Energies of prominent PPV modes involved in polaron formation agree with infrared spectra. The polaron binding energies we find are much smaller than the width of the energy disorder in polymeric systems of practical importance, thus self-trapping effects can be ignored in practice.

Pairing interactions and pairing mechanism in high-temperature copper oxide superconductors

The polaron binding energy ${E}_{p}$ in undoped parent cuprates has been determined to be about $1.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ from the unconventional oxygen-isotope effect on the antiferromagnetic ordering temperature. The deduced value of ${E}_{p}$ is in quantitative agreement with that estimated from independent optical data and that estimated theoretically from the measured dielectric constants. The substantial oxygen-isotope effect on the in-plane supercarrier mass observed in optimally doped cuprates suggests that polarons are bound into the Cooper pairs. We also identify the phonon modes that are strongly coupled to conduction electrons from the angle-resolved photoemission spectroscopy, tunneling spectra, and optical data. We consistently show that there is a very strong electron-phonon coupling feature at a phonon energy of about $20\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ along the antinodal direction and that this coupling becomes weaker towards the diagonal direction. We further show that high-temperature superconductivity in cuprates is caused by strong electron-phonon coupling, polaronic effect, and significant coupling with $2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ $\mathrm{Cu}\ensuremath{-}\mathrm{O}$ charge transfer fluctuation.

Vibron-polaron critical localization in a finite size molecular nanowire

The small polaron theory is applied to describe the vibron dynamics in an adsorbed nanowire with a special emphasis onto finite size effects. It is shown that the finite size of the nanowire discriminates between side molecules and core molecules which experience a different dressing mechanism. Moreover, the inhomogeneous behavior of the polaron hopping constant is established and it is shown that the core hopping constant depends on the lattice size. However, the property of a lattice with translational invariance is recovered when the size of the nanowire is greater than a critical value. Finally, it is pointed out that these features yield the occurrence of high energy localized states in which both the nature and the number are summarized in a phase diagram in terms of the relevant parameters of the problem (small polaron binding energy, temperature, lattice size).

Small polaron formation in porous WO3−x nanoparticle films

Porous tungsten oxide nanoparticle films were prepared by reactive gas evaporation. The structure was studied by x-ray diffraction and scanning electron microscopy, and the oxygen nonstoichiometry was inferred by x-ray photoelectron spectroscopy, elastic recoil detection analysis, and neutron scattering. Specifically, the films consisted of WO3−x with 0.25&amp;lt;x&amp;lt;0.4. The optical and electrical data were consistent with the formation of small polarons having a radius of 5–6Å. The infrared optical data, used to extract information on phonon energies, were instrumental to reach this conclusion. The polaron hopping energy was about half the polaron binding energy, as expected from the theory.

BINDING ENERGY OF THE SELF-TRAPPED MAGNETIC POLARON: EFFECT OF THE INTRA-SUBLATTICE ELECTRON HOPPING

We study the energetic stability of the self-trapped magnetic polaron in an antiferromagnetic lattice of localized spins, including the effects of the intra-sublattice hopping which allows electron transfer within the same magnetic sublattice. The model consists of an itinerant electron moving in a one-dimensional antiferromagnetic lattice of localized spins. In addition to the first and the second neighbor electron hopping, terms describing the superexchange between the localized spins as well as the Hund's-rule coupling between the electron and the localized spins are included in the model Hamiltonian. The ground-state energy of the self-trapped polaron with the resulting ferromagnetic core region is compared with the energy of the propagating state in the antiferromagnetic lattice. We find the magnetic polaron to be self-trapped for all values of the Hamiltonian parameters, although the intra-sublattice hopping substantially reduces the polaron binding energy.

Density-functional-based tight-binding approach to polarons in conjugated polymers

Whether or not charge injection leads to polaron formation in conjugated polymers remains a matter of controversy with profound qualitative discrepancies between different methods. In this paper we show that the Density-functional-based tight-binding (DFTB) approach of Porezag et al. predicts delocalization of an excess charge and thus no polaron formation in infinite conjugated polymer chains. Since real polymers have finite conjugation lengths, we also present the first comparative study of electron and hole polaron binding energies in oligomers of the conjugated polymers trans-polyacetylene (tPA), poly( para-phenylene) (PPP) and poly( para-phenylene vinylene) (PPV).