255 research outputs found
Metal-insulator transition and the Pr/Pr valence shift in (PrY)CaCoO
The magnetic, electric and thermal properties of the
(Y)CaCoO perovskites (~=~Pr, Nd) were
investigated down to very low temperatures. The main attention was given to a
peculiar metal-insulator transition, which is observed in the praseodymium
based samples with and 0.15 at and 132~K, respectively.
The study suggests that the transition, reported originally in
PrCaCoO, is not due to a mere change of cobalt ions from
the intermediate- to the low-spin states, but is associated also with a
significant electron transfer between Pr and Co/Co sites,
so that the praseodymium ions occur below in a mixed
Pr/Pr valence. The presence of Pr ions in the insulating
phase of the yttrium doped samples (PrY)CaCoO
is evidenced by Schottky peak originating in Zeeman splitting of the ground
state Kramers doublet. The peak is absent in pure PrCaCoO
in which metallic phase, based solely on non-Kramers Pr ions, is
retained down to the lowest temperature.Comment: 10 figure
Interpreting of fuzzy dispersion model by contour map method
Akarsularda boyuna dispersiyon katsayısının belirlenmesi için geliştirilen denklemler genellikle doğrusal olmayan regresyon analizine, istatistik metotlara veya benzeri kara kutu yaklaşımlarına dayanmaktadır. Geliştirilmeleri sırasında yapılan kabuller nedeniyle bu denklemler dikkatli kullanılmadıkları taktirde gerçekçi olmayan sonuçlarla karşılaşılabilir. Bu çalışmada bulanık mantık yaklaşımı ile geliştirilen yeni bir modelin, ölçümler sonucu elde edilen verilere uygulanmasının sonuçları sunulmuştur. Veri kümeleri,bağımsız değişkenler olarak akımın derinliği, genişliği, kesit ortalama ve kayma hızlarını, bağımlı değişken olarak da boyuna dispersiyon katsayısını içermektedir. Bulanık model sonuçları, eldeki veriler ve farklı yedi çalışmanın sonuçları kontur harita yöntemi kullanılarak karşılaştırılmış ve bulanık modelin, mevcut çalışmalardan daha gerçekçi sonuçlar verdiği ve kontur harita yönteminin model sonuçlarının karşılaştırılmasında güvenle kullanılabileceği görülmüştür.Anahtar Kelimeler: Boyuna dispersiyon katsayısı, Bulanık mantık, difüzyon, dispersiyon, kontur harita, modelleme.In order to investigate the dispersion process, researchers have used differential equations written under certain assumptions with a dispersion coefficient. These differential equations cannot be used in practice unless the dispersion coefficient is either directly measured or calculated from measurements. Many empirical equations, statistical methods or several black box approaches are used to determine the longitudinal dispersion coefficients in natural channels. The empirical equations most depend procedurally on the least squares techniques through regression equations of non-linear types. Unfortunately, many critical assumptions in the derivation of these regression equations are not considered and consequently the resulting equations can be used with precautious and reservations. Otherwise, the results will not be reliable. The main goal of this study is to apply a new fuzzy model to the 96 data sets concerning more than 30 rivers at different times. The data sets include four independent and one dependent variables, which are, the depth, the width and the mean cross-sectional velocity of the flow, shear velocity and longitudinal dispersion coefficient. Results of the model have been compared with existing data and the results of seven equations using contour map method. It is observed that the fuzzy model yields more reliable results than other approaches and it can be used more easily and efficiently.Keywords: Contour map, dispersion, fuzzy-logic, longitudinal dispersion coefficient, modelin
Inelastic Quantum Transport
We solve a Schrodinger equation for inelastic quantum transport that retains
full quantum coherence, in contrast to previous rate or Boltzmann equation
approaches. The model Hamiltonian is the zero temperature 1d Holstein model for
an electron coupled to optical phonons (polaron), in a strong electric field.
The Hilbert space grows exponentially with electron position, forming a
non-standard Bethe lattice. We calculate nonperturbatively the transport
current, electron-phonon correlations, and quantum diffusion. This system is a
toy model for the constantly branching ``wavefunction of the universe''.Comment: revtex, 13 pages, 4 figure
Approaching disorder-free transport in high-mobility conjugated polymers.
Conjugated polymers enable the production of flexible semiconductor devices that can be processed from solution at low temperatures. Over the past 25 years, device performance has improved greatly as a wide variety of molecular structures have been studied. However, one major limitation has not been overcome; transport properties in polymer films are still limited by pervasive conformational and energetic disorder. This not only limits the rational design of materials with higher performance, but also prevents the study of physical phenomena associated with an extended π-electron delocalization along the polymer backbone. Here we report a comparative transport study of several high-mobility conjugated polymers by field-effect-modulated Seebeck, transistor and sub-bandgap optical absorption measurements. We show that in several of these polymers, most notably in a recently reported, indacenodithiophene-based donor-acceptor copolymer with a near-amorphous microstructure, the charge transport properties approach intrinsic disorder-free limits at which all molecular sites are thermally accessible. Molecular dynamics simulations identify the origin of this long sought-after regime as a planar, torsion-free backbone conformation that is surprisingly resilient to side-chain disorder. Our results provide molecular-design guidelines for 'disorder-free' conjugated polymers.We gratefully acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) through a programme grant (EP/G060738/1) and the Technology Strategy Board (TSB) (PORSCHED project). D. Venkateshvaran acknowledges financial support from the Cambridge Commonwealth Trust through a Cambridge International Scholarship. K. Broch acknowledges post-doctoral fellowship support from the German Research Foundation (DFG). Mateusz Zelazny acknowledges funding from the NanoDTC in Cambridge. The work in Mons was supported by the European Commission / Région Wallonne (FEDER – Smartfilm RF project), the Interuniversity Attraction Pole program of the Belgian Federal Science Policy Office (PAI 7/05), Programme d’Excellence de la Région Wallonne (OPTI2MAT project) and FNRS-FRFC. D.B. and J.C. are FNRS Research Fellows.This is the accepted manuscript. The final version's available from Nature at http://www.nature.com/nature/journal/vaop/ncurrent/full/nature13854.html
Particle Path Correlations in a Phonon Bath
The path integral formalism is applied to derive the full partition function
of a generalized Su-Schrieffer-Heeger Hamiltonian describing a particle motion
in a bath of oscillators. The electronic correlations are computed versus
temperature for some choices of oscillators energies. We study the perturbing
effect of a time averaged particle path on the phonon subsystem deriving the
relevant temperature dependent cumulant corrections to the harmonic partition
function and free energy. The method has been applied to compute the total heat
capacity up to room temeperature: a low temperature upturn in the heat capacity
over temperature ratio points to a glassy like behavior ascribable to a time
dependent electronic hopping with variable range in the linear chain.Comment: To be published in J.Phys.:Condensed Matte
Optical conductivity of the nonsuperconducting cuprate La(8-x)Sr(x)Cu(8)O(20)
La(8-x)Sr(x)Cu(8)O(20) is a non-superconducting cuprate, which exhibits a
doubling of the elementary cell along the c axis. Its optical conductivity
sigma (omega) has been first measured here, down to 20 K, in two single
crystals with x = 1.56 and x = 2.24. Along c, sigma (omega) shows, in both
samples, bands due to strongly bound charges, thus confirming that the cell
doubling is due to charge ordering. In the ab plane, in addition to the Drude
term one observes an infrared peak at 0.1 eV and a midinfrared band at 0.7 eV.
The 0.1 eV peak hardens considerably below 200 K, in correspondence of an
anomalous increase in the sample dc resistivity, in agreement with its
polaronic origin. This study allows one to establish relevant similarities and
differences with respect to the spectrum of the ab plane of the superconducting
cuprates.Comment: Revised version submitted to Phys. Rev. B, including the elimination
of Fig. 1 and changes to Figs. 4 and
Optical Conductivity of Manganites: Crossover from Jahn-Teller Small Polaron to Coherent Transport in the Ferromagnetic State
We report on the optical properties of the hole-doped manganites Nd_{0.7}Sr
_{0.3}MnO_{3}, La_{0.7}Ca_{0.3}MnO_{3}, and La_{0.7}Sr_{0.3}MnO_{3}. The
low-energy optical conductivity in the paramagnetic-insulating state of these
materials is characterized by a broad maximum near 1 eV. This feature shifts to
lower energy and grows in optical oscillator strength as the temperature is
lowered into the ferromagnetic state. It remains identifiable well below Tc and
transforms eventually into a Drude-like response. This optical behavior and the
activated transport in the paramagnetic state of these materials are consistent
with a Jahn-Teller small polaron. The optical spectra and oscillator strength
changes compare well with models that include both double exchange and the
dynamic Jahn-Teller effect in the description of the electronic structure.Comment: 27 pages (Latex), 6 figures (PostScript
Bose-Einstein condensation of strongly correlated electrons and phonons in cuprate superconductors
The long-range Froehlich electron-phonon interaction has been identified as
the most essential for pairing in high-temperature superconductors owing to
poor screening, as is now confirmed by optical, isotope substitution, recent
photoemission and some other measurements. I argue that low energy physics in
cuprate superconductors is that of superlight small bipolarons, which are
real-space hole pairs dressed by phonons in doped charge-transfer Mott
insulators. They are itinerant quasiparticles existing in the Bloch states at
low temperatures as also confirmed by continuous-time quantum Monte-Carlo
algorithm (CTQMC) fully taking into account realistic Coulomb and long-range
Froehlich interactions. Here I suggest that a parameter-free evaluation of Tc,
unusual upper critical fields, the normal state Nernst effect, diamagnetism,
the Hall-Lorenz numbers and giant proximity effects strongly support the
three-dimensional (3D) Bose-Einstein condensation of mobile small bipolarons
with zero off-diagonal order parameter above the resistive critical temperature
Tc at variance with phase fluctuation scenarios of cuprates.Comment: 35 pages, 10 figures, to appear in the special volume of Journal of
Physics: Condensed Matte
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