Polaron Parameters Research Articles

Physical and electrical properties of Sm2O3 doped Boro-Zinc-Vanadate glasses

Sm2O3 doped boro-zinc-vanadate glass systems were synthesised by following the melt quenching method. XRD patterns indicated largely non-crystalline nature with few nano-crystallites. Room temperature density was measured. Molar volume and various polaron parameters were estimated. Density and molar volume are found to vary non-linearly with samarium concentration. Conductivity has been measured by two probe technique for temperature range 303K - 573K. High temperature conductivity obeyed the small polaron hopping (SPH) theory. Activation energy for conduction in the temperature regime of small polaron theory is found to vary from 0.249 eV to 0.368 eV non-linearly with Sm2O3 concentration. The conductivity data at low temperature deviated has been looked into using Mott’s VRH model and the density of states at Fermi level were determined. Shimakawa’s multiphonon tunnelling model has also been applied to the low temperature conductivity and found linearity between logarithmic conductivity, ln(σ) and logarithmic temperature ln(T) as predicted by the model. The temperature exponent values obtained from Shimakawa’s model fit are found to be in good agreement with literature. Therefore, it is concluded that at low temperature, carrier multiphonon tunnelling is the charge transport mechanism in the present glasses.

Chalcogenide Perovskites (ABS3; A = Ba, Ca, Sr; B = Hf, Sn): An Emerging Class of Semiconductors for Optoelectronics.

Chalcogenide perovskites have received considerable interest in the photovoltaic research community because of their stability, nontoxicity, and lead-free composition. However, because of the huge computational cost, theoretical study focusing on excitonic and polaronic properties is not explored rigorously. Herein, we capture the excitonic and polaronic effects in a series of chalcogenide perovskites ABS3, where A = Ba, Ca, Sr and B = Hf, Sn, by employing state-of-the-art hybrid density functional theory and many-body perturbative approaches, viz., GW and BSE. We find that they possess an exciton binding energy larger than that of 3D inorganic-organic hybrid perovskites. We examine the interplay of electronic and ionic contributions to the dielectric screening and conclude that the electronic contribution is dominant over the ionic contribution. Using the Feynman polaron model, polaron parameters are computed, and charge-separated polaronic states are less stable than bound excitons. Finally, the theoretically calculated spectroscopic limited maximum efficiency suggests that among all chalcogenide perovskites, CaSnS3 could serve as the best choice for photovoltaic applications.

Thermal and electrical properties of (B2O3-TeO2-Li2O-CoO) glasses

Borotellurite glasses in the composition, (B2O3)0.2–(TeO2)0.3–(CoO)x–(Li2O)0.5-x; x = 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45 and 0.50 have been synthesized and their non-crystalline nature confirmed by XRD. Density of the glasses increased and molar volume decreased up to x = 0.3 and reversed their trends for x > 0.3. The glass transition temperature is increased up to x = 0.30 and decreased for x > 0.3. Changes in density, molar volume and glass transition temperature around x = 0.3 may be ascribed to structural/topological modifications occurring in the glasses. Polaron parameters have been determined. Thermal stability is found to increase with increasing x. Activation energy for dc conduction at high temperature has been determined by applying Mott's small polaron hopping model. Conductivity and activation energy were found to pass through minimum and maximum respectively for x around 0.30, which may be due to a changeover of conduction mechanism from predominantly ionic regime to electronic regime. This transition can be attributed to the interaction between electrons and alkali ions. This result is of significance both scientifically and technologically. Mott's variable range hopping model has been employed to understand conductivity at low temperature for the present glasses.

High and low temperature DC electrical properties of vanadium borate glasses

Semiconducting binary borate glass samples doped with vanadium of composition (H3BO3)1 + (V2O5)1-x where x ranging from 0.1, to 0.5 has been prepared by melt quench technique. The samples were annealed and their non-crystalline nature was confirmed by X-ray diffraction studies. Archimedes principle was adopted to measure room temperature density of the glass samples. High and low temperature DC electrical conductivity of the samples were analyzed and various polaron related parameters were investigated in the light of Mott’s SPH when T > and Mott’s VRH and Greaves VRH when T < (where Debye’s temperature). The investigations in these glass systems showed to be non-adiabatic electrical conduction. As per as above theories, the calculated activation energies, conductivities and various polaron parameters at different temperatures were compared. The detailed results of all the above parameters were discussed and which reveals that at high temperature regime Mott’s SPH model is adequate and at low temperature regime Greaves VRH model is valid to explain the variation of electrical conductivity of the glasses.Within the studied composition range it is for the first time the pure borate glasses doped with vanadium ions subjected to dc electrical conduction mechanism.

Temperature- and doping-dependent optical absorption in the small-polaron systemPr1−xCaxMnO3

Small polaron optical properties are studied comprehensively in thin film samples of the narrow bandwidth manganite $\mathrm{P}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{a}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ by optical absorption spectroscopy as a function of doping and temperature. A broad near infrared double-peak absorption band in the optical conductivity spectras is observed and interpreted in the framework of photon-assisted small polaron intersite hopping and on-site Jahn-Teller excitation. Application of quasiclassical small polaron theory to both transitions allows an approximate determination of polaron specific parameters like the polaron binding energy, the characteristic phonon energy, as well as the Jahn-Teller splitting energy as a function of temperature and doping. Based on electronic structure calculations, we consider the impact of the hybridization of $\mathrm{O}\phantom{\rule{0.28em}{0ex}}2p$ and Mn $3d$ electronic states on the Jahn-Teller splitting and the polaron properties. The interplay between hopping and Jahn-Teller excitations is discussed in the alternative pictures of mixed valence $\mathrm{M}{\mathrm{n}}^{3+}/\mathrm{M}{\mathrm{n}}^{4+}$ sites (Jahn-Teller polaron) and equivalent $\mathrm{M}{\mathrm{n}}^{(3+x)+}$ sites (Zener polaron). We give a careful evaluation of the estimated polaron parameters and discuss the limitations of small polaron quasiclassical theory for application to narrow bandwidth manganites.

Current-voltage characteristics of manganite-titanite perovskite junctions.

After a general introduction into the Shockley theory of current voltage (J–V) characteristics of inorganic and organic semiconductor junctions of different bandwidth, we apply the Shockley theory-based, one diode model to a new type of perovskite junctions with polaronic charge carriers. In particular, we studied manganite–titanate p–n heterojunctions made of n-doped SrTi1−yNbyO3, y = 0.002 and p-doped Pr1−xCaxMnO3, x = 0.34 having a strongly correlated electron system. The diffusion length of the polaron carriers was analyzed by electron beam-induced current (EBIC) in a thin cross plane lamella of the junction. In the J–V characteristics, the polaronic nature of the charge carriers is exhibited mainly by the temperature dependence of the microscopic parameters, such as the hopping mobility of the series resistance and a colossal electro-resistance (CER) effect in the parallel resistance. We conclude that a modification of the Shockley equation incorporating voltage-dependent microscopic polaron parameters is required. Specifically, the voltage dependence of the reverse saturation current density is analyzed and interpreted as a voltage-dependent electron–polaron hole–polaron pair generation and separation at the interface.

Elastic modulus, optical phonon modes and polaron properties in Al1−xBxN alloys

The results of first-principles plane-wave-based pseudopotential method calculations of the elastic modulus, optical phonon modes, and polaron properties of zinc-blende Al1−xBxN are presented. Good agreement is obtained with the exciting experimental data for AlN and BN compounds. Our results provide predictions for the remaining mixed crystals Al1−xBxN (0 < x < 1) for which no direct experimental or theoretical data are presently available. The compositional dependence of features such as elastic constants and their related mechanical parameters, phonon frequencies, dielectric constants, and polaron parameters was investigated. The study may be useful for the characteristic analysis of AlBN-based quantum well devices.

Phonon–polaron effect in diluted magnetic semiconductor nanotubes

We report the calculation results of the polaron binding energy and a polaron correction to the mass in a diluted magnetic semiconductor (DMS) nanotube. Analytical expressions for the polaron binding energy and the polaron correction to the mass are obtained in dependence on magnetic field and on the radius of the nanotube. Numerical calculations performed for the lowest spin subband of the conduction band show that the binding energy of the polaron and the polaron correction to the mass decreases with increasing magnetic field in the presence of the exchange interaction. In the absence of the exchange interaction, a slight increase in the polaron binding energy and in the polaron correction to the mass takes place with increasing magnetic field due to orbital magnetism. Both polaron parameters decrease with increasing radius of the nanotube.

Fermi polaron in two dimensions: Importance of the two-body bound state

We investigate a single impurity interacting with a free two-dimensional atomic Fermi gas. The interaction between the impurity and the gas is characterized by an arbitrary attractive short-range potential, which, in two dimensions, always admits a two-particle bound state. We provide analytical expressions for the energy and the effective mass of the dressed impurity by including the two-body bound state, which is crucial for strong interactions, in the integral equation for the effective interaction. Using the same method we give also the results for the polaron parameters in one and three dimensions finding a pretty good agreement with previous known results. Thus our relations can be used as a simple way to estimate the polaron parameters once the two-body bound state of the interaction potential is known.

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.

Electronic structure and polarons inCaMnO3−δsingle crystals: Optical data

The ellipsometry measurements (in the energy range $E=0.5--5.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$) on cleavage surfaces of single crystals, the measurements of the absorption and reflectance spectra $(E=0.04--0.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$, electrical resistivity, and magnetic susceptibility were carried out for determination of peculiarities of the fundamental absorption spectrum and the conductivity mechanism in $\mathrm{Ca}\mathrm{Mn}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ single crystals. The energy of fundamental absorption edge associated with the direct transitions was determined as ${E}_{\mathrm{g}}=1.55\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The absorption band with two maxima at 2.2 and $3.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ corresponding to the charge transfer transitions was revealed. The polaron band at $0.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ was discovered and polaron parameters were determined. Peculiarities of IR absorption and dc conductivity of $\mathrm{Ca}\mathrm{Mn}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ in dependence on oxygen stoichiometry are determined by the superposition of hopping and band conductivity of spin-lattice polarons, and by additional localization of charge carriers under oxygen vacancies ordering.

Coherent state polarons in quantum wells

We investigate the polaronic effects of an electron confined in a quantum well, which we describe through its algebraic properties using su(1,1), taking into account the electron-bulk longitudinal-optical phonon interaction. We construct the variational wave function as the direct product of an electronic part and a part describing coherent phonons generated by the Low–Lee–Pines transformation from the vacuum state. We use two explicit forms of coherent states, Perelomov and Barut-Girardello states, to represent the electronic part in the quantum well spectrum. Our results show that in a coherent state basis for electrons the basic polaron parameters such as the energy gap shift and effective mass are further enhanced compared to those obtained with the conventional sinusoidal form of the basis. The difference between the two types of quantum well coherent states appears in polaronic interactions in quantum wells. We extend the calculations in order to estimate polaron lifetimes for a variety of different material systems.

Anderson localization in strongly coupled disordered electron–phonon systems

Based on the statistical dynamic mean-field theory, we investigate, in a generic model for a strongly coupled disordered electron–phonon system, the competition between polaron formation and Anderson localization. The statistical dynamic mean-field approximation maps the lattice problem to an ensemble of self-consistently embedded impurity problems. It is a probabilistic approach, focusing on the distribution instead of the average values for observables of interest. We solve the self-consistent equations of the theory with a Monte Carlo sampling technique, representing distributions for random variables by random samples, and discuss various ways to determine mobility edges from the random sample for the local Green function. Specifically, we give, as a function of the ‘polaron parameters’, such as adiabaticity and electron–phonon coupling constants, a detailed discussion of the localization properties of a single polaron, using a bare electron as a reference system.

Influence of small polarons on the optical properties ofMg:LiNbO3crystals

We have measured refractive indices and absorption coefficients for ${\mathrm{M}\mathrm{g}:\mathrm{L}\mathrm{i}\mathrm{N}\mathrm{b}\mathrm{O}}_{3}$ single crystals of different Mg contents (0--7 mol %) in the visible and IR ranges up to the phonon absorption edge. The obtained dependencies of optical characteristics on Mg concentration in an IR region of 4--5 \ensuremath{\mu}m indicate the four-step character of structural changes under subsequent Mg doping. Spectral dependencies of optical characteristics reveal the existence of two polaron resonance bands, centered at 1.3 and 3.2 \ensuremath{\mu}m, which are not observed in the pure ${\mathrm{LiNbO}}_{3}.$ The theory approximation of the band at 1.3 \ensuremath{\mu}m, enhanced after chemical reduction, enabled us to determine the main polaron parameters for the 1.3-\ensuremath{\mu}m band, and to make a possible assignment of the 3.2 \ensuremath{\mu}m band. We assign this to direct transitions between energy levels of polarons trapped at ${\mathrm{Nb}}_{\mathrm{Nb}}$ sites and those of polarons trapped at ${\mathrm{Nb}}_{\mathrm{Li}}$ antisites in a lattice. A number of peculiarities are revealed in the absorption behavior at the low-energy edge from 1000 to 3000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The values of the uninduced part of absorption are sufficiently less than those predicted by a phonon oscillator model. Absorption spectral dependencies are modulated at a rather stable period, being of the order of frequencies of the lowest-energy longitudinal optical phonons, excited at a room temperature.

Polarons in an ellipsoidal potential well

Applying the Feynman variational principle, we analyze the basic polaron parameters – the ground-state energy, the polaron effective mass, the number of phonons in the polaron cloud, and the polaron radius – for a three-axis ellipsoidal potential well. Numerical calculations are performed for the specific cases when the potential well determines (i) a cylindrical quantum wire, (ii) a planar quantum wire, and (iii) a spherical quantum dot. The polaron parameters are derived analytically for the limiting cases of large and small cross-section sizes of a quantum wire, and of large and small radii of a quantum dot. A boundary between the weak and strong coupling regions is studied as a function of sizes of the structures under consideration. It is shown that with confinement strengthening, regions of the weak and intermediate coupling shorten, while the strong-coupling region widens. For a “squeezed” polaron state in a quantum dot, when the radius of confinement R is smaller than the polaron radius R p =(ℏ/ m ̄ ω 0) 1/2 (with the electron band mass m ̄ and the LO phonon frequency ω 0), the electron–phonon coupling constant α is scaled as αR p/ R.

Parameters of the magnetic polaron state in diluted magnetic semiconductors Cd-Mn-Te with low manganese concentration.

A method for the determination of magnetic polaron parameters is suggested for diluted magnetic semiconductors with low manganese concentration x, when the magnetic polaron shift of the exciton energy cannot be observed in practice. We show that the average exchange field, created by the localized exciton, and the magnetic polaron energy can be determined from the experimentally measured magnetic-field dependences of the circular polarization of the luminescence and Zeeman splitting of the free exciton. Theoretical dependences of the circular polarization on the magnetic field have been calculated in the two-dimensional case for different temperatures. Results of the calculations are in good agreement with our experimental data for CdTe/Cd12xMnxTe quantum wells with x50.117 and 0.13. @S0163-1829~96!06731-8# In semimagnetic semiconductors the strong exchange interaction between the charge carriers and electrons in the deep magnetic shell of magnetic ions manifests itself in the giant Zeeman splitting of the valence- and conduction-band states, the large Faraday rotation, and the formation of magnetic polarons ~MP’s !. 1,2 The magnetic polaron represents a small region in the crystal with strongly correlated spins of magnetic ions and carriers localized in this region. This spin ordering results in a decrease of the carrier energy, which is usually detected in optical experiments as a Stokes shift between the maximum of the photoluminescence line and the maximum of the photoluminescence excitation spectrum. 3‐5 Magnetic polaron states of localized excitons have been studied in the semimagnetic semiconductors Cd12xMnxTe with considerable magnetic-ion concentrations ~x>0.2!. 6,7 Three-dimensional polaron dynamics have been studied

Doping dependence of the a-b-plane optical conductivity of single-crystal La2-xSrxNiO4+ delta.

Results of room-temperature a-b-plane optical-reflection studies of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{NiO}}_{4+\mathrm{\ensuremath{\delta}}}$ (\ensuremath{\delta}=0.0; x=0, 0.05, 0.10, 0.20, 0.21, and \ensuremath{\delta}=0.085; x=0) are presented and cover the photon energy range 0.0056 eV. A Kramers-Kronig analysis is used to compute the optical conductivity \ensuremath{\sigma}(\ensuremath{\omega}) and energy-loss function Im(-1/\ensuremath{\varepsilon}(\ensuremath{\omega})). The doping dependence of the TO and LO a-b-plane phonon modes, anomalous midinfrared absorption (mid-IR band), and interband transitions are discussed. The mid-IR band (band maximum ${\mathrm{\ensuremath{\omega}}}_{\mathit{m}}$\ensuremath{\sim}0.5 eV) is found to be similar to that observed in the isostructural materials ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$, and is identified with photon-assisted hopping of small polarons. A microscopic theory developed by Reik is used to obtain polaron parameters whose values are compared to those obtained similarly for other small-polaron materials.

Polaronic hopping conduction in vanadium and iron phosphate glasses

A.c. mechanism of polaronic hopping in vanadium and iron phosphate glasses is discussed in the light of polaron tunnelling model. The polaron parameters calculated from this model appear reasonable and are in agreement with values obtained from analyses of d.c. conductivity data.

Optical phonon interaction effects in layered semiconductor structures

Various aspects of the electron-LO-phonon interaction effects on the electronic properties of a single two-dimensional electron layer (as occurring, for example, in artificially structured single quantum wells or heterojunctions made of III–V or II–VI semiconducting materials) are discussed theoretically. In particular, perturbation theory is carried to the second order in the coupling constant to obtain the two-dimensional polaron energy in the weak-coupling limit. Intermediate coupling (the so-called Lee-Low-Pines theory) and strong coupling theories for the two-dimensional polaron problem are developed and interpolation (Padé approximations) formulae valid for arbitrary coupling are derived. Effects of the band non-parabolicity and of the free-carrier screening on the weak-coupling theory are discussed. The real and the imaginary parts of the two-dimensional polaron self-energy are obtained in a many-body perturbation calculation. Comparison with the known three-dimensional results is made wherever possible, showing that the electron-LO-phonon interaction effects are substantially enhanced in confined structures. Explicit formulas valid for two-dimensional systems are given for various polaron parameters like the binding energy, the effective mass, the scattering rate, the average phonon density in the polaron cloud, etc.

A.c. conductivity in binary V2O5-P2O5 glasses

Abstract The a.c. conductivity of binary V2O5-P2O5 glasses containing 40, 50, 60 and 70mol% V2O5 has been measured at temperatures between 100 and 423 K, and for frequencies up to 100 MHz. The results are interpreted in terms of Long's polaron hopping model. The polaron parameters calculated from the above model appear reasonable and are in agreement with values obtained by other means.