Polaronic Charge Carriers Research Articles

Electro-Optical Characterisation of Field Effect Devices With Regioregular Poly-Hexylthiophene Active Layers

Regioregular poly-hexylthiophene (P3HT) is currently one of the most promising active materials for high-mobility polymer field-effect transistors with mobilities reaching 0.05–0.1cm2/Vs. P3HT is a soluble, but highly ordered rigid-rod like polymer with phase separated hexyl side chains. In order to understand the transport physics of this high mobility polymer we present a careful study of the relationship between transistor performance and the electronic properties of the polaronic charge carriers probed by electro-optical modulation spectroscopy.

Oxygen isotope effects in : evidence for polaronic charge carriers and their condensation

Magnetization measurements have been performed on the oxygen-isotope exchanged samples ( and ) of the one-layer cuprate superconductors . From magnetization measurements on the fine-grained and decoupled samples in the Meissner state, we find substantial oxygen-isotope effects on both the penetration depth and . From normal-state susceptibility measurements, we are able to show that there is a negligible oxygen-isotope effect on the carrier density n. The combined results strongly suggest that there is an oxygen-isotope effect on the effective supercarrier mass , which is huge for x = 0.06, and reduced to a smaller value for x = 0.15. We discuss the isotope effects, supercarrier mass anisotropy, normal-state gap, in-plane penetration depth, and mid-infrared spectra for on the basis of the small polaron theory of superconductivity. We find that the agreement between the calculated and experimental results is excellent without any adjustable parameters.

Sr‐ and Ni‐Doped LaCoO3 and LaFeO3 Perovskites: New Cathode Materials for Solid‐Oxide Fuel Cells

An improved cathode material for a solid‐oxide fuel cell would be a mixed electronic and oxide‐ion conductor with a good catalytic activity for oxygen reduction at an operating temperature Top ≥ 700°C and a thermal expansion matched to that of the electrolyte and interconnect. We report on the properties of Sr‐ and Ni‐doped and perovskites that meet these criteria. Single‐phase regions were determined by X‐ray diffraction, and thermogravimetric analysis measurements were used to obtain the temperatures above which oxygen loss, and hence oxide‐ion conductivity, occurs. The conductivity and Seebeck measurements indicate the coexistence of both p‐type and n‐type polaronic charge carriers resulting from an overlap of the redox couple with the low‐spin/intermediate‐spin and high‐spin redox couples. Motional enthalpies ΔHm = 0.03, 0.02, and 0.08 eV, respectively, were estimated for , and polarons. Optimal compositions have percolation pathways between dopants. Comparisons with transport data for the conventional cathode materials and indicate superior cathode performance can be expected.

Evidence of polaronic charge carriers in oxide superconductors

The analysis of photoemission and femtosecond optical absorption spectroscopy in oxide superconductors supports the view that the charge carriers are polaronic in nature. These carriers are found to exist in the (Ba/Sr)-O planes and are coupled toB1g vibrations of the apical oxygen.

Anisotropic Polaron Motion in Polyaniline Studied by Muon Spin Relaxation

Muon spin relaxation has been used to probe the highly anisotropic motion of polaronic charge carriers in the emeraldine base form of polyaniline. The measured diffusion rate along the polymer chain for negative polarons shows a metalliclike temperature dependence, which is saturated at low temperatures but becomes limited by phenylene ring librational scattering above 150 K. The interchain transport at room temperature is more than 4 orders of magnitude slower than the intrachain process and becomes further suppressed on cooling.

Oxygen isotope effects in the manganites and cuprates: Evidence for polaronic charge carriers

We present various oxygen-isotope effects in the manganites from magnetization and thermal-expansion measurements. We found that the ferromagnetic transition temperature T c , the magnitude of the jump in the thermal-expansion coefficient Δβ(T c ) at T c all depend strongly on the oxygen mass. Upon replacing 16O with 18O, T c can be lowered by as much as 20 K, and Δβ(T c ) can be raised by about 20%. The observed novel isotope effects clearly demonstrate that charge carriers in these materials are strongly coupled to the local Jahn-Teller distortions, so that polarons are formed. We also report the experimental evidence for polaronic supercarriers in the cuprate superconductors La 2− x Sr x CuO 4. From magnetization and thermal-expansion measurements, we can quantitatively determine the oxygen-isotope effects on the in-plane penetration depth λ ab (0) and on the carrier density n. We find a negligible isotope effect on n, but a large effect on λ ab (0). Our results show that polaronic charge carriers exist and condense into Cooper pairs in the cuprate superconductors.

Evidence for polaronic supercarriers in the copper oxide superconductors La2–xSrxCuO4

High-temperature superconductivity is known to involve the pairing of charge carriers, but the precise nature of these carriers and the mechanism of their pairing remain unclear. The copper oxides are known to exhibit a strong Jahn–Teller effect — in which spontaneous lattice distortions remove the degeneracy of the electronic ground state — and it has been suggested that the charge carriers are Jahn–Teller polarons (bare charge carriers accompanied by local lattice distortions). In fact, the demonstration1 that a strong Jahn–Teller effect can lead to the formation of such polarons led to the original discovery of high-temperature superconductivity2. Still, direct evidence that Jahn–Teller polarons exist in the superconducting state of the copper oxides has been lacking, although some indirect evidence comes from their recent discovery3 in the structurally similar but non-superconducting manganite La1–xCaxMnO3. Here we report the results of magnetization and thermal expansion measurements on samples of the copper oxide superconductor La2–xSrxCuO4 which characterize the oxygen-isotope effects on the carrier density and on the in-plane penetration depth. We find a negligible isotope effect on the former, but a large effect on the latter. Specific quantitative features of the results show that polaronic charge carriers exist and condense into Cooper pairs in the copper oxide superconductors.

Electrical and magnetic properties of LiPc and LiPcI

We studied the ac electrical (20 Hz ≤ v ≤ 1GHz) and magnetic properties of the molecular semiconductors lithium phthalocyanine (LiPc) and lithium phthalocyanine iodide (LiPcI) at temperatures 1.5 K ≤ T ≤ 300 K. The frequency and temperature dependence of the complex ac conductivity suggest polarons as the dominant species of charge carriers. The higher conductivity of the iodine salt can be attributed to an enhanced mobility of the polaronic charge carriers due to a better overlap of the π-orbitals along the stacking direction of the molecules. Up to 300 K the dc conductivity can be described by σ dc α exp( −( T 0/ T) 1/2), i. e. quasi one-dimensional tunneling of charge carriers is the dominant conduction process in both compounds. Preliminary electron spin resonance (ESR) experiments were performed on LiPc. A single narrow, lorentzian shaped line is observed. Below 30 K a significant decrease in linewidth appears which may indicate the onset of magnetic order or dimerization.

Charge transport in lithium phthalocyanine

The ac electrical properties of monoclinic lithium phthalocyanine (LiPc) and of the iodinated compound LiPcI have been investigated in the frequency and temperature regimes 20 Hz≤ν≤1 GHz and 1.5 K≤T≤300 K, respectively. Both compounds are semiconductors with dielectric constants ε∞≊6 (LiPc) and 20 (LiPcI). Up to room temperature tunneling of charge carriers is the dominant conduction process in both compounds, yielding intrinsic dc conductivities σdc≊5.3×10−4 Ω−1 cm−1 (LiPc) and 0.2 Ω−1 cm−1 (LiPcI). The frequency and temperature dependence of the complex ac conductivity suggests polarons as the dominant species of charge carriers. The higher conductivity of the iodinated compound can be attributed to an enhanced mobility of the polaronic charge carriers which is most probably due to a better overlap of the π orbitals along the stacking direction of the molecules.

Lattice instabilities and c-axis resistivity in high- Tc superconductors

We propose a general model to describe the coupling of polaronic charge carriers with double well potentials due to local lattice instabilities. A path integral formalism accounts for the retardation effects in the attractive polaron-polaron interaction mediated by the double well structure. The model is applied to the normal state of YBa 2Cu 3O 7−δ superconductor and it is shown that the attractive forces are relevant in a strong coupling regime for realistic values of the input parameters. The calculated c-axis electrical resistivity shows a metallic like behaviour with deviations from a T linear dependence due to the role of the attractive interaction. The absolute values of resistivity are consistent with the experimental data in the case of strong electron-lattice coupling.

The polaron scenario for high T c superconductivity

In high T c materials electrons on the whole are only weakly coupled to the phonons except for a few specific local modes. The latter gives rise to the formation of polaronic charge carriers on specific molecular centers in those materials. There is considerable experimental evidence for their existence in form of localized states and we conjecture that localized bi-polaronic states lie inside the valence band of itinerant electrons. With increased doping this valence band is gradually being emptied out, eventually resulting in a situation where the chemical potential coincides with the level of the bi-polaronic state. At this point an insulator→metal transition is triggered off and due to hybridization between the localized bi-polarons and the itinerant electrons the initially localized bi-polaronic states acquire itinerant upon lowering the temperature; eventually undergoing a transition to a superfluid state at a critical temperature T c. A characteristic feature of this scenario of an hybridized Boson-Fermion mixture is the pinning of the chemical potential at the Bosonic (bi-polaronic) level which only weakly depends on doping in the fully developed metallic regime. T c is then essentially controlled by the number of Bosons which does depend on doping and leads to the known universal behaviour of T c as a function of Boson concentration and mass.

Spectral properties of small-polaron systems.

Small polarons, being composite particles in which the charge degrees of freedom and the lattice vibrational modes are locked together, show unique features in their single-particle spectral properties. The occupation number ${\mathit{n}}_{\mathbf{k}}$ of the polaronic charge carriers is finite and roughly constant throughout the entire Brillouin zone; with only a small jump at ${\mathit{k}}_{\mathit{F}}$. Strongly temperature-dependent incoherent contributions to the photoemission spectrum and the occurrence of a polaron-induced vibrational mode in the neutron scattering cross section are indications for small polarons in band states.

Separation of neutral-to-polaron and polaron-to-bipolaron redox events in alkoxy substituted di-2-thienylphenylene polymers

In situ electron paramagnetic resonance (EPR)/electrochemistry methods have been used to elucidate the potential dependence of the presence of polaron and bipolaron charge carriers in poly(di-2-thienyl-2,5-diheptoxyphenylene).

Seebeck coefficients in vanadium spinels

Omission of a negligibly small transport term gives a temperature-independent Seebeck coefficient for polaron charge carriers θ ± = ± 198 log [(N-p)/p], where the polaron concentration p can be determined from chemistry and the concentration N of available polaron sites is model-dependent. The expression contains no adjustable parameters. Application to the Seebeck data for the vanadium spinels shows that the transition from small polarons (identifiable V 4+ ions) to itinerant electrons goes via intermediate polarons trapped at vanadium pairs and four-vanadium clusters.