Polaron Contribution Research Articles

Confinement effect on the intradonor 1s–2p+ transition energies in GaN quantum wells

In this work, we report a calculation of the 1s–2p+ transition energies of the donor impurity levels in a GaN–AlGaN quantum well (QW) in the presence of an external magnetic field. The impurity level energies are calculated through the variational method and the polaronic contribution is calculated via second order perturbation theory. The band non-parabolicity effects on the binding energies are included in the calculations. We have compared our results with that for GaN-bulk and verified the enhancement of the transition energy due to the electron confinement.

Breakdown of the Wiedemann-Franz law in strongly-coupled electron-phonon system, application to the cuprates

With the superconducting cuprates in mind, a set of unitary transformations was used to decouple electrons and phonons in the strong-coupling limit. While phonons remain almost unrenormalised, electrons are transformed into itinerent singlet and triplet bipolarons and thermally excited polarons. The triplet/singlet exchange energy and the binding energy of the bipolarons are thought to account for the spin and charge pseudogaps in the cuprates, respectively. We calculated the Hall Lorenz number of the system to show that the Wiedemann-Franz law breaks down due to the interference of the polaron and bipolaron contributions to heat flow. The model provides a quantitative fit to magnetotransport data in the cuprates. Furthermore we are able to extract the phonon component of the thermal conductivity with the use of experimental data and the model. Our results further validate the use of a charged Bose gas model to describe normal and superconducting properties of unconventional superconductors.

Influence of electron–phonon scattering on optical absorption in ZnSe

We investigate the influence of quantum kinetic effects connected with the scattering of carriers with plasmons and LO-phonons on the excitonic absorption in ZnSe within a linear response theory. Within our approach the quantum-kinetic effects are reflected in a dependence of the many-body effects on the energy. In contrast to earlier treatments we use a unified description of the longitudinal dielectric function, including coupled plasmon and LO-phonon modes. We present a way to handle the well known polaron contributions in order to provide the correct excitonic features at low excitation. We find, that after elimination of the polaron contributions the phonons only weakly influence the interband self-energy for small densities at about 1014 − 1015 cm−3 in comparison to the plasmons. With increasing density structures around the LO-phonon-energy ± ωLO appear. These are more pronounced inside the band, but increasingly affect the region of excitonic absorption too. Surprisingly there is a stronger coupling of plasmon and phonon modes as expected from the pure dispersions.

Lorenz number in high T(c) superconductors: evidence for bipolarons.

The strong electron-phonon interaction in cuprates has gathered support over the last decade in a number of experiments. While phonons remain almost unrenormalized, electrons are transformed into itinerant bipolarons and thermally excited polarons when the electron-phonon interaction is strong. We calculate the Lorenz number of the system to show that the Wiedemann-Franz law breaks down because of the interference of polaron and bipolaron contributions in the heat flow. The model fits numerically the experimental Hall Lorenz number, which provides direct evidence for bipolarons in the cuprates.

Synthesis, Characterization, and Conductivity Studies of Poly- o-Methoxyaniline Intercalated into V 2O 5 Xerogel

In this work we study the conductivity properties of poly- o-methoxyaniline/V 2O 5 intercalation compounds obtained through intercalative polymerization of o-methoxyaniline with V 2O 5· nH 2O in hydrogel form and by reacting directly with V 2O 5 film in de-hydrated form (xerogel). These new compounds were characterized using Fourier-transform infrared and ultra-violet/visible spectroscopies, electron paramagnetic resonance, elemental analysis (C, N, H), thermogravimetric analysis, scanning electron microscopy, dc-conductivity and powder X-ray diffraction. For samples formed from V 2O 5 xerogel in film form, an increase in dc-conductivity and a decrease in the thermal activation energy in comparison with pure matrix was observed. The increase in conductivity is attributed to an increase of carrier density in the vanadium oxide lattice and the contribution of polarons from the polymer. On the other hand, for the intercalation compound obtained with V 2O 5· nH 2O in hydrogel form presents a very low room temperature conductivity value. The decrease in conductivity is due to the lack of connectivity of the various parts that compose the material.

High-pressure effects on the resistivity and ferromagnetic transition of ceramic manganite Ca 1− xY xMnO 3

The effect of hydrostatic pressure up to 1 GPa on the electrical resistivity and on the ferromagnetic transition temperature T c is systematically investigated for the electron-doped manganite Ca 1− x Y x MnO 3 for low electron density ( x⩽0.2). We find that T c is nearly constant for pressures roughly below 0.5 GPa and shows a linear increase for higher pressures, with a slope varying with the x content. Our results are discussed considering the enhancement of the electronic bandwidth with pressure related to double exchange and polaronic contributions. The unusual pressure dependence of T c obtained can also be related to a competition scenario between double exchange mechanism and antiferromagnetic superexchange interactions proposed for electron-doped manganites.

The polaron and bipolaron contributions to the electronic structure of polypyrrole films

UPS and NEXAFS spectroscopy have been used to study the changes in the electronic structure of polypyrrole films by varying the dopant anion. In particular, we find the density of states within the electronic gap to be influenced by the chemical structure of the dopant anion. From the position of the Fermi level in the bandgap, the charge carriers in the polymer can be identified either as polarons or as bipolarons. For the first time, we used copper phthalocyanine trisulfonate as dopant, resulting in a high concentration of bipolarons which have a high contribution from the wavefunctions of the pyrrole–nitrogen atoms. Interaction of the π-systems of the polymer and the dopant causes a change in the chemical nature of the nitrogen from amino-like to imino-like.

Polaron contribution to the infrared reflectivity of the (Ca, Sr, Nd)CuO 2 infinite layer structure

We studied the polaronic contribution to the optical conductivity of three (Ca, Sr, Nd)CuO 2 “infinite layer” compounds. In the most conducting film the binding energy and the product of the mass and the radius of the polaron are lower than in the more insulating samples, consistently with the metallization process.

Effect of Electron(Hole)-Confined Longitudinal Optical Phonon Interactions on the Exciton Binding Energy in Quantum-Well Wires

We have calculated the effect of the electron(hole)–confined LO-phonon interactions on the exciton binding energy in rectangular quantum-well wires (QWW) as a function of the length of one side of the wire for several values of the length of the other side. Our results indicate that the polaronic contribution to the binding increases as the confinement length increases.

Dielectric properties of iron-bearing trioctahedral phyllosilicates

Measurements of the real part of conductivity (σ′) and dielectric function (ɛ′) were performed on large crystal flakes of biotite (Moen) and vermiculite (Benahavis), at variable frequencies (0.1 to 1000 kHz) and as a function of temperature (300 K<T<900 K). By heating, cooling and repeated heating experiments under inert atmospheric conditions effects involving water (H2O) diffusion and electrical transport were separated. The effect of dehydration dominates the functional dependence of ɛ′ on T at frequencies below about 100 kHz, within the first heating run. Six dehydration steps for vermiculite Benahavis and two broad features for biotite Moen are observed, the water being transported effectively only in the interlayers. Electrical transport occurs along Fe paths within the octahedral layers and can be described by σ′=σdc+A·ws. Values of s are between 0.45 and 0.8. σdc shows a temperature dependence according to exp(−Ea(T)/kT) with Ea(T) between 0.3 and 0.6 eV. Ea is suggested to be composed of a polaron plus disorder contribution.

Effects of confined longitudinal optical phonons on the binding energy of the impurity in a quantum well

AbstractThe effects of the confined LO phonon‐electron interaction on the binding energy of shallow donors confined in a GaAsGa1−xAlxAs quantum well are investigated. The barrier potential height is taken into account. Using the variational method of Lee‐Low‐Pines, we find that the effect of this interaction is negligible for very narrow quantum wells. For thick quantum wells, the polaronic contribution is quite large.

Polarons in the infrared spectra of high-T c materials

Polaronic absorption features, induced by excess charges in both electron and hole doped cuprates, have been measured. The features here reported include additional phonon-like peaks in the far infrared and a broad overtone band centered at ∼1000 cm−1 (d band). The peaks in the far infrared are attributed to local modes. Thed band is produced by overtone and combination bands of a few of these local modes and is well fitted in a polaron model. Most of these polaronic contributions survive in the metallic phase, where they are superimposed on the Drude term due to free carriers.

Hole transport in triarylamine doped polymers

Hole mobilities have been measured in N,N′,N″,N‴-tetrakis(4-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine(TTB) and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) doped poly(styrene) (PS). The results are described within the framework of a formalism based on disorder, due to Bässler and coworkers. The formalism is based on the assumption that charge transport occurs by hopping through a manifold of localized states with superimposed energetic and positional disorder. The key parameters are σ, the energy width of the hopping site manifold, and Σ, the degree of positional disorder. For 20% TTB and TPD doped PS, σ = 0.102 eV. The positional disorder parameter is Σ = 3.9 for TTB and Σ = 3.8 for TPD doped PS. The width of the hopping site manifold is described by a model based on dipolar disorder. The model is based on the assumption that the width is comprised of a dipolar component due to the dopant molecule and a van der Waals component. The results show that transport in both TTB and TPD can be described by disorder-controlled hopping without invoking polaronic contributions. Deviations from ideal disordercontrolled behavior have been observed for both compounds at low fields.

The spectral assessment of the origin of the polaronic contribution to charge transport in a highly conjugated triarylamine donor molecule

Abstract Site-selective fluorescence spectra have been measured to investigate the origin of the polaronic contribution to charge transport in 4-[bis(4-ethylphenyl)amino]phenyl-N,N,N′,N′-tetrakis(4-ethylphenyl)-[1,1′:3′1″-terphenyl]-4,4″-diamine doped poly(styrene). The spectra feature an unusually large Stokes shift of σ = 750 cm−1 and remain broad over the entire range of excitation energies. This is a clear indication of a torsional mode of relaxation that can account for the polaronic contribution observed in charge transport experiments.

A comparative study of hole transport in vapor-deposited molecular glasses of N,N′,N″,N‴-tetrakis(4-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine

Abstract Hole mobilities have been measured in vapor-deposited layers of N,N′,N″,N‴-tetrakis(4-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TTB) and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD). The results are described within the framework of a formalism based on disorder, due to Bassler and co-workers. The formalism is based on the assumption that charge transport occurs by hopping through a manifold of localized states with superimposed energetic and positional disorder. The key parameters of the formalism are σ, the energy width of the distribution of hopping site energies, and Σ, the degree of positional disorder. For TTB, σ = 0.078 eV and Σ = 1.7. The values for TPD are 0.077 eV and 1.6. The width of the hopping site energies can be described by a model based on dipolar disorder. The degree of positional disorder is in agreement with results observed for a wide range of vapor-deposited molecular glasses and is attributed to packing constraints. The results show that transport in both compounds can be described by simple disorder-controlled hopping without invoking polaronic contributions.

Hopping conduction and possible Coulomb gap effects in manganese and zinc ferrous ferrite

Abstract Electrical conductivity and thermopower measurements have been made on zinc and manganese ferrous ferrite, ZnxFe3−xO4 and MnxFe3 −xO4 with 0 ≤ X < 1, in the temperature range 100–300 K. The results for X < 0.5 are tentatively interpreted in terms of the formation of a Coulomb gap at low temperatures in an energy band of localized states. For X ≥ 0.5 the electrical conductivity is symptomatic of nearest-neighbour hopping, and the thermopower varies as In c where c=(1 − X)/2. The energy bandwidth of conduction states has been extracted from the thermopower and electrical conductivity data and is shown to be roughly proportional to the standard deviation of the Zn and Mn distributions [X(1 − X)1/2. A polaron contribution to the conductivity activation energy of approximately 0.03 eV at room temperature has been deduced for Mn0.8Fe2.2O4 and Mn0.8Fe2.1O4.

Magnetization of bound magnetic polarons: Direct determination via photomemory effect.

Light-induced persistent change in the concentration of occupied shallow donors (photomemory effect) in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$${\mathrm{Te}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Se}}_{\mathit{y}}$:In has been used for determining the contribution of donor-bound magnetic polarons to the total magnetization of the crystal. This contribution must be taken into account in any precise description of low temperature properties of diluted magnetic semiconductors containing shallow donors. The observed agreement between experimental data and the theory of bound magnetic polarons provides additional support for the negative-U model of DX-like centers in II-VI semiconductors.

Photoconduction in poly(3-alkylthiophene) II. Charge transport

Hole motion in poly(dodecylthiophene) has been investigated by means of time of flight (TOF) studies and space charge limited conduction involving hole injection from a gold anode. The decrease of the hole mobility with temperature above 290 K parallels the increase of disorder manifest in the absorption spectra. At lower temperatures TOF signals become dispersive. The results are interpreted in terms of hopping motion within an energetically random array of hopping sites formed by ordered chain segments. The polaron contribution towards the activation energy for charge carrier motion is found negligible relative to the disorder contribution.

Diffusion of hydrogen and deuterium on stepped tungsten surfaces II. W(023)

The diffusion of 1H and 2H on a (023) surface of a tungsten field emitter has been studied by the fluctuation method. This surface consists of (110) oriented terraces separated by (100)-oriented steps. Activated and tunneling diffusion are observed both parallel and perpendicular to steps with very little anisotropy. The activation energies and prefactors for diffusion differ considerably from those found on W(110) and it is hypothesized that the observed values are dominated by diffusion across steps. The activation energies for 1H are higher than those for 2H by ~ 2 kcal and prefactors for 1H are also much higher than for 2H. This is interpreted as activated polaronic diffusion largely controlled by substrate deformation, with the energy differences arising from zero point energy differences for the two isotopes during lattice distortion. At half coverage a switch from an initial activated to a slower one is seen at 151 ⩽ T ⩽ 166 K and interpreted as a hydrogen-assisted reconstructive phase transition involving the non-close-packed (100) oriented steps. Tunneling diffusion shows only very small isotope effects probably because of polaronic contributions which counteract the isotope effect for pure hydrogen tunneling.

Infrared magnetospectroscopy of n-type InP at magnetic fields up to 150 T.

Infrared transmission experiments of n-type InP reveal the cyclotron resonance and donor-shifted cyclotron resonance at magnetic fields up to 150 T. Pronounced band nonparabolicity is observed at megagauss fields, as well as a large polaron correction at fields above the longitudinal-optical-phonon energy. The experimental results are compared to a five-level P\ensuremath{\cdot}p model, which includes resonant and nonresonant polaron contributions. The five-level model agrees very well with the data for energies up to approximately 50 meV above the band edge. At higher energies, the theory predicts a stronger band nonparabolicity than experimentally observed, amounting to a 5% discrepancy at megagauss field strengths.