http://www.hindawi.comThe latest articles from Hindawi Publishing Corporation© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2010/261849.htmlHigh-temperature superconductivity (HTS) emerges in quite different electronic materials: cuprates, diborides, and iron-pnictide superconductors. Looking for unity in the diversity we find in all these materials a common lattice architecture: they are practical realizations of heterostructures at atomic limit made of superlattices of metallic active layers intercalated by spacers as predicted in 1993 by one of us. The multilayer architecture is the key feature for the presence of electronic topological transitions where the Fermi surface of one of the subbands changes dimensionality. The superlattice misfit strain η between the active and spacer layers is shown to be a key variable to drive the system to the highest critical temperature Tc that occurs at a particular point of the 3D phase diagram Tc(δ,η) where δ is the charge transfer or doping. The plots of Tc as a function of misfit strain at constant charge transfer in cuprates show a first-order quantum critical phase transition where an itinerant striped magnetic phase competes with superconductivity in the proximity of a structural phase transition, that is, associated with an electronic topological transition. The shape resonances in these multigap superconductors is associated with the maximum Tc.Nicola Poccia, Alessandro Ricci, and Antonio Bianconi© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2010/484578.htmlCarrier-induced lattice distortion (signature of polaron) in oxypnictide superconductors is found by an instantaneous local probe, extended X-ray absorption fine structure (EXAFS). Polaron formation is detected as two distinct nearest neighbor distances (Fe-As), implying an incoherent local mode that develops coherence at the critical temperature. Comparing the results with the unusual lattice response in cuprate superconductors, intimate correlation between evolution of local lattice mode and superconductivity is revealed. The results suggest that strong electron-lattice interaction is present as a common ingredient in the microscopic mechanism of superconducting transition. The effect of magnetic impurity atoms in cuprates further indicates that magnetic scattering becomes diluted as long as polaron formation is conserved. We argue that polaron coherence dominates electrical conduction and magnetic interaction in oxypnictide and cuprate superconductors.H. Oyanagi and C. J. Zhang© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2010/752943.htmlComplex electronic ground state of molecular and solid state system is analyzed on the ab initio level beyond the adiabatic Born-Oppenheimer approximation (BOA). The attention is focused on the band structure fluctuation (BSF) at Fermi level, which is induced by electron-phonon coupling in superconductors, and which is absent in the non-superconducting analogues. The BSF in superconductors results in breakdown of the adiabatic BOA. At these circumstances, chemical potential is substantially reduced and system is stabilized (effect of nuclear dynamics) in the antiadiabatic state at broken symmetry with a gap(s) in one-particle spectrum. Distorted nuclear structure has fluxional character and geometric degeneracy of the antiadiabatic ground state enables formation of mobile bipolarons in real space. It has been shown that an effective attractive e-e interaction (Cooper-pair formation) is in fact correction to electron correlation energy at transition from adiabatic into antiadiabatic ground electronic state. In this respect, Cooper-pair formation is not the primary reason for transition into superconducting state, but it is a consequence of antiadiabatic state formation. It has been shown that thermodynamic properties of system in antiadiabatic state correspond to thermodynamics of superconducting state. Illustrative application of the theory for different types of superconductors is presented.Pavol Baňacký© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2010/627452.htmlThe critical temperature (TC) of superconductivity in A3C60 compounds is generally lower smaller with alkali atoms (A). Furthermore TC decreases with applied pressure. In the BCS model, these trends are explained by the lower density of states at the Fermi level for a decreased lattice constant (R). There is more than one counterexample, however, suggesting that BCS does not give the whole truth. The most important one is that the compound with the largest lattice constant, Cs3C60, is not superconducting at all at ambient pressure. In this paper we derive a novel model where a negative lattice contribution to Hubbard U, proportional to 1/R, is taken into account. It is possible to explain why A3C60 compounds with A = Li, and Na have a low TC or are not superconducting at all, and why Cs3C60 is superconducting only at applied pressure and then with the highest TC of all C60 alkali fullerides. It is concluded that the density of states mechanism derived in the BCS model is in doubt. Nevertheless superconductivity in A3C60 depends on electron-phonon coupling. The dominating phonon is the bond stretching Ag phonon, a breathing phonon for the whole fullerene molecular ion.Sven Larsson© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2010/681070.htmlExplicit and implicit experimental evidence for charge density wave (CDW) presence in high-Tc superconducting oxides is analyzed. The theory of CDW superconductors is presented. It is shown that the observed pseudogaps and dip-hump structures in tunnel and photoemission spectra are manifestations of the same CDW gapping of the quasiparticle density of states. Huge pseudogaps are transformed into modest dip-hump structures at low temperatures, T, when the electron spectrum superconducting gapping dominates. Heat capacity jumps at the superconducting critical temperature and the paramagnetic limit are calculated for CDW superconductors. For a certain range of parameters, the CDW state in a d-wave superconductor becomes reentrant with T, the main control quantity being a portion of dielectrcally gapped Fermi surface. It is shown that in the weak-coupling approximation, the ratio between the superconducting gap at zero temperature Δ(T=0) and Tc has the Bardeen-Cooper-Schrieffer value for s-wave Cooper pairing and exceeds the corresponding value for d-wave pairing of CDW superconductors. Thus, large experimentally found values 2Δ(T=0)/Tc≈5÷8 are easily reproduced with reasonable input parameter values of the model. The conclusion is made that CDWs play a significant role in cuprate superconductivity.A. M. Gabovich, A. I. Voitenko, T. Ekino, Mai Suan Li, H. Szymczak, and M. Pękała© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2010/167985.htmlThe s-wave three-band Eliashberg theory can simultaneously reproduce the experimental critical temperatures and the gap values of the superconducting materials LaFeAsO0.9F0.1, Ba0.6K0.4Fe2As2 and SmFeAsO0.8F0.2 as exponent of the more important families of iron pnictides. In this model the dominant role is played by interband interactions and the order parameter undergoes a sign reversal between hole and electron bands (s±-wave symmetry). The values of all the gaps (with the exact experimental critical temperature) can be obtained by using high values of the electron-boson coupling constants and small typical boson energies (in agreement with experiments).G. A. Ummarino© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2010/912067.htmlFirst principle band calculations based on local versions of density functional theory (DFT), together with results from nearly free-electron models, can describe many typical but unusual properties of the high-TC copper oxides. The methods and a few of the most important results are reviewed. Some additional calculations are presented, and the problems with the commonly used approximate versions of DFT for oxides are discussed with a few ideas for corrections. It is concluded that rather modest corrections to the approximate DFT, without particular assumptions about strong correlation, can push the ground state towards antiferro magnetic (AFM) order. Spin fluctuations interacting with phonons are crucial for the mechanism of superconductivity in this scenario.T. Jarlborg© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2010/250891.htmlI review the multiple successes of the discrete hard-wired dopant network model ZZIP, and comment on the equally numerous failures of continuum models, in describing and predicting the properties of ceramic superconductors. The prediction of transition temperatures can be regarded in several ways, either as an exacting test of theory, or as a tool for identifying theoretical rules for defining new homology models. Popular “first principle” methods for predicting transition temperatures in conventional crystalline superconductors have failed for cuprate HTSC, as have parameterized models based on CuO2 planes (with or without apical oxygen). Following a path suggested by Bayesian probability, it was found that the glassy, self-organized dopant network percolative model is so successful that it defines a new homology class appropriate to ceramic superconductors. The reasons for this success in an exponentially complex (non-polynomial complete, NPC) problem are discussed, and a critical comparison is made with previous polynomial (PC) theories. The predictions are successful for the superfamily of all ceramics, including new non-cuprates based on FeAs in place of CuO2.J. C. Phillips© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2010/306106.htmlA review of experimental and theoretical results on the spectroscopy of high-temperature superconductors is presented. The models where hole doped into antiferromagnet interacts both with magnetic subsystem and with phonons are considered. Theoretical results of these models for phonon spectra, angle resolved photoemission spectra, and optical conductivity are presented. Comparison with experimental data gives evidence for the strong electron-phonon coupling in the undoped and weakly doped high-temperature superconductors. The strength of electron-phonon coupling decreases with doping though at the optimal doping the compounds are still in the intermediate coupling regime.A. S. Mishchenko© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2010/393526.htmlRecent experimental observations of unconventional isotope effects, multiband superconductivity, and unusual local lattice responses are reviewed and shown to be naturally explained within a two-component scenario where local polaronic effects are important. It is concluded that purely electronic mechanisms of high-temperature superconductivity are incomplete and unable to capture the essential physics of cuprates and other layered superconductors.Hugo Keller and Annette Bussmann-Holder© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2010/815917.htmlThe Holstein Molecular Crystal Model is investigated by a strong coupling perturbative method which, unlike the standard Lang-Firsov approach, accounts for retardation effects due to the spreading of the polaron size. The effective mass is calculated to the second perturbative order in any lattice dimensionality for a broad range of (anti)adiabatic regimes and electron-phonon couplings. The crossover from a large to a small polaron state is found in all dimensionalities for adiabatic and intermediate adiabatic regimes. The phonon dispersion largely smoothes such crossover which is signalled by polaron mass enhancement and on-site localization of the correlation function. The notion of self-trapping together with the conditions for the existence of light polarons, mainly in two- and three-dimensions, is discussed. By the imaginary time path integral formalism I show how nonlocal electron-phonon correlations, due to dispersive phonons, renormalize downwards the e-ph coupling justifying the possibility for light and essentially small 2D Holstein polarons.Marco Zoli© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2010/804343.htmlThe electronic structure and lattice vibrational frequencies of the newly discovered superconductors, LiFeAs and NaFeAs, are calculated within density functional theory. We show that, in the vicinity of the Fermi energy, the density of states is dominated by contributions from Fe 3d states. We also calculate the electron-phonon coupling strength and show that it is too weak to account for the observed values of the superconducting transition temperatures. This seems to indicate that the iron-based superconductors are not of the conventional type.R. A. Jishi and H. M. Alyahyaei© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2009/654190.htmlThe cross-section of light absorption by size-quantized semiconductor quantum dots (QD) is calculated in the case of a resonance with an exciton Γ6×Γ7 in cubical crystals of Td class. The interference of stimulating and induced electric and magnetic fields is taken into account. The cross-section of light absorption is proportional to the exciton nonradiative damping γ.I. G. Lang and S. T. Pavlov© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2009/361080.htmlThe electrical properties of 0.65Pb(Mg1/3Nb2/3)O3-0.35(PbTiO3) ceramics over a wide range of frequencies (102–106 Hz) and temperatures (30–225∘C) were studied using impedance spectroscopy technique. The impedance and electric permittivity were strongly temperature and frequency dependent. The activation energy, calculated from the temperature dependence of AC conductivity of the ceramics was found to be ∼0.5 eV. The relaxation process in the ceramics was found to be of non-Debye type. The nature of Cole-Cole diagram reveals the contribution of grain (bulk) and grain boundary permittivity in the ceramics.Banarji Behera, E. B. Araújo, R. N. Reis, and J. D. S. Guerra© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2009/867638.htmlNonlinear magnetostatic surface wave in a slab waveguide structure has been investigated. The design consisted of dielectric film between two thick nonlinear nonmagnetic negative permittivity material (NPM) layers. A dispersion relation for TE nonlinear Magnetostatic surface waves (NMSSWs) has been derived into the proposed structure and has been numerically investigated. Effective refractive index decreases with thickness and frequency increase have been found. Effective refractive index decrease with optical nonlinearity increase and switching to negative values of effective refractive index at a certain value of optical nonlinearity have been found. This meant that the structure behaved like a left-handed material over certain range. We found that the power flow was changing by changing the operating frequency, the dielectric film thickness, and the optical nonlinearity. Also, the effective refractive index and power flow attained constant values over certain values of dielectric constant values.A. I. Ass'ad and H. S. Ashour© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2009/561201.htmlThe spin interaction energy of different Mn2+ ions with and without an itinerant electron is evaluated for different dot radii. Magnetization is calculated for various concentrations of Mn2+ ions with different dot sizes. Spin polaronic shifts are estimated using a mean field theory. The lowest binding energies of electrons in a Cd1-xMnxTe quantum dot are also calculated. Results are obtained for Cd1−xinMnxinTe/Cd1−xoutMnxoutTe structures as a function of the dot radius variationally. It is found that (i) more number of Mn2+ spins enhance the spin polaronic effect and it varies linearly with the concentration, (ii) spin polarization of Mn2+ ions increases with the concentration for any dot radii, (iii) the magnetization of Mn subsystem increases with the concentration of Mn2+ ions and this feature is predominant for smaller dots, and (iv) variation of increase in ionization energy is sharper for smaller dots with increase in concentration. These results are discussed with the available data in literature.A. John Peter and K. Lily Mary Eucharista© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2008/703479.htmlAluminum doped manganese ferrites MnAlxFe2−xO4 with 0.0≤x≤1.0 have been prepared by the double ceramic route. The formation of mixed spinel phase has been confirmed by X-ray diffraction analysis. The unit cell parameter `aO' is found to decrease linearly with aluminum concentration due to smaller ionic radius of aluminum. The cation distributions were estimated from X-ray diffraction intensities of various planes. The theoretical lattice parameter, X-ray density, oxygen positional parameter, ionic radii, jump length, and bonds and edges lengths of the tetrahedral (A) and octahedral (B) sites were determined. 57Fe Mössbauer spectra recorded at room temperature were fitted with two sextets corresponding to Fe3+ ions at A- and B-sites. In the present ferrite system, the area ratio of Fe3+ ions at the A- and B-sites determined from the spectral analysis of Mössbauer spectra gives evidence that Al3+ ions replace iron ions at B-sites. This change in the site preference reflects an abrupt change in magnetic hyperfine fields at A- and B-sites as aluminum concentration increases, which has been explained on the basis of supertransferred hyperfine field. On the basis of estimated cation distribution, it is concluded that aluminum doped manganese ferrites exhibit a 55% normal spinel structure.R. L. Dhiman, S. P. Taneja, and V. R. Reddy© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2008/817829.htmlAn overview is given of amorphous oxide materials viscosity and glass-liquid transition phenomena. The viscosity is a continuous function of temperature, whereas the glass-liquid transition is accompanied by explicit discontinuities in the derivative parameters such as the specific heat or thermal expansion coefficient. A compendium of viscosity models is given including recent data on viscous flow model based on network defects in which thermodynamic parameters of configurons—elementary excitations resulting from broken bonds—are found from viscosity-temperature relationships. Glass-liquid transition phenomena are described including the configuron model of glass transition which shows a reduction of Hausdorff dimension of bonds at glass-liquid transition.Michael I. Ojovan© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2008/152731.htmlWe investigate theoretically the spin accumulation in a Rashba spin-orbit coupling quantum wire. Using the scattering matrix approach within the effective free-electron approximation, we have demonstrated the three components of spin polarization. It is found that by a few numerical examples, the two peaks for the out-of-plane spin accumulation 〈Sz〉 shift to the edges of quantum wire with the increase of propagation modes. The period of intrinsic oscillations 〈Sx/y〉 inversely proportions to the Rashba SOC strength. This effect may be used to differentiate the intrinsic spin accumulation from the extrinsic one.Xi Fu, Wenhu Liao, and Guanghui Zhou© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2008/839536.htmlNanoparticles of aluminum-substituted manganese zinc ferrites, Mn1.05Zn0.05AlxFe1.9−xO4 with 0.4≤x≤1.0 were synthesized by solid-state reaction route and characterized by XRD, TEM, and Mössbauer spectroscopy measurements. The particle size is found to very from 46 to 28 nm with increase of aluminum ions concentration. The unit cell parameter “aO” is found to decrease linearly with aluminum ions concentration due to smaller ionic radius of aluminum. The cation distributions were estimated from X-ray diffraction intensities of various planes. The theoretical lattice parameter, bulk density, porosity, X-ray density, oxygen positional parameter, ionic radii, jump length, as well as bonds and edges lengths of the tetrahedral (A-) and octahedral (B-) sites were determined. Fe57 Mössbauer spectra recorded at room temperature were fitted with two sextets corresponding to Fe3+ ions at tetrahedral (A-) and octahedral (B-) sites. The magnetic hyperfine fields at A- and B-sites show a gradual decrease with increase of Al3+ ion concentration, which has been explained on the basis of supertransferred hyperfine field. The cation distribution obtained from X-ray intensities and Mössbauer data indicates a decrease in Fe3+(B)/Fe3+(A) ratio with increasing aluminum concentration thereby suggesting a decrease in ferrimagnetic behavior.R. L. Dhiman, S. P. Taneja, and V. R. Reddy© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2008/175862.htmlDolomite mineral samples having white and light green colors of Indian origin have been characterized by EPR, optical, and NIR spectroscopy. The optical spectrum exhibits a number of electronic bands due to presence of Fe(III) ions in the mineral. From EPR studies, the parameters of g for Fe(III) and g,A, and D for Mn(II) are evaluated and the data confirm that the ions are in distorted octahedron. Optical absorption studies reveal that Fe(III) is in distorted octahedron. The bands in NIR spectra are due to the overtones and combinations of water molecules. Thus EPR and optical absorption spectral studies have proven useful for the study of the solid state chemistry of dolomite.S. Lakshmi Reddy, R. L. Frost, G. Sowjanya, N. C. G. Reddy, G. Siva Reddy, and B. J. Reddy© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2008/937054.htmlThe structural and physical properties of xFe2O3-(40-x) B2O3-60V2O5  (0≤x≤20) glass system have been investigated. The samples were prepared by normal melt-quench technique. The structural changes were inferred by means of FTIR by monitoring the infrared (IR) spectra in the spectral range 600–4000 cm-1. The absence of boroxol ring (806 cm-1) in the present glass system suggested that these glasses consist of randomly connected BO3 and BO4 units. The conversion of BO3 to BO4 and VO5 to VO4 tetrahedra along with the formation of non-bridging oxygen's (NBOs) attached to boron and vanadium takes place in the glasses under investigation. The density and molar volume of the present glass system were found to depend on Fe2O3 content. DC conductivity of the glass system has been determined in the temperature range 310–500 K. It was found that the general behavior of electrical conductivity was similar for all glass compositions and found to increase with increasing iron content. The parameters such as activation energy, average separation between transition metal ions (TMIs), polaron radius, and so forth have been calculated in adiabatic region and are found consistent with Mott's model of phonon-assisted polaronic hopping.Virender Kundu, R. L. Dhiman, A. S. Maan, and D. R. Goyal© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2008/941372.htmlIn the present study, the results of investigation of physical properties of heterofullerides A3−xMxC60 (A=K, Rb, Cs, M=Be, Mg, Ca, Al, Fe, Tl, x=1,2); as well as RbCsTlC60, KCsTlC60, and KMg2C60 are described. All of the fullerides were synthesized by the exchange reactions of alkaline fullerides with anhydrous metal halides. Superconductivity was found in RbCsTlC60 and KCsTlC60.V. A. Kulbachinskii, B. M. Bulychev, V. G. Kytin, A. V. Krechetov, E. A. Konstantinova, and R. A. Lunin© 2010, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/acmp/2008/971457.htmlMultiwalled carbon nanotubes (MWCNTs) were prepared by the thermal chemical vapor deposition (CVD) technique. Monometallic and bimetallic Co and Ni combinations were used as a catalyst on MgO support. The mixer of H2/C2H2 was used as a carbon source. The prepared CNTs were found to possess different shapes, morphologies, and sizes. Maximum yield was found for 50% Co (MgO: 50% and Ni: 0%) catalyst at 600C∘. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) techniques were used for structural analysis. Raman spectra were taken to investigate the quality and crystalline perfection of the prepared CNTs. The ratio of D- and G-bands (ID/IG) was measured from these spectra.H. Ryu, B. K. Singh, and K. S. Bartwal© 2010, Hindawi Publishing Corporation. All rights reserved.