305 research outputs found
Second derivative of the log-likelihood in the model given by a Levy driven stochastic differential equations
By means of the Malliavin calculus, integral representation for the second
derivative of the loglikelihood function are given for a model based on
discrete time observations of the solution to SDE driven by a Levy process.Comment: arXiv admin note: substantial text overlap with arXiv:1301.514
Malliavin calculus approach to statistical inference for Levy driven SDE's
By means of the Malliavin calculus, integral representations for the
likelihood function and for the derivative of the log-likelihood function are
given for a model based on discrete time observations of the solution to
equation dX_t=a_\theta(X_t)dt + dZ_t with a tempered \alpha-stable process Z.
Using these representations, regularity of the statistical experiment and the
Cramer-Rao inequality are proved
Model for the electronic structure, Fermi arcs, and pseudogap in cuprate superconductors
We suggest a model for electronic structure of cuprate superconductors that
makes it possible to describe evolution of this structure with the doping and
provides a new explanation for a number of typical features of cuprates,
including the pseudogap and the Fermi arcs. According to this model, the unique
electronic structure of cuprates is favorable for the formation of two-atomic
negative-U centers (NUCs) and realization of a peculiar mechanism of the
electron-electron interaction.Comment: 9 pages, 4 figure
On the possible common nature of the ground state in Cu- and Fe-based HTSCs
A qualitative model describing the ground state and the mechanism of
superconducting pairing in Cu- and Fe-based high-temperature superconductors
(HTSCs) is suggested. In this model, doping by localized charges (as well as
physical or chemical pressure) is supposed to be responsible for transition of
Cu- and Fe-based HTSC to new ground state common for both HTSC classes where
specific mechanism of superconductivity takes place. The resulting HTSC ground
state is strongly correlated insulator with not fully filled exciton-electronic
band, where the incoherent electron transport is impossible but coherent
superconducting transport is possible because the band is not fully occupied.
It is shown also that such electronic system is inherently predisposed to
superconductive pairing because each pair of nearest cations acts as a two-atom
negative-U center. The nature of Fermi arcs and mechanism of pseudogap are
considered. It is shown that both of these features result from d-wave pairing
and therefore have to be observed only in cuprates. We believe that the
considered ground state is common for various families of HTSCs including
cuprates, pnictides, selenides, bismutates and probably some other.Comment: 16 pages, 7 figures; abstract, text and figures modified, typos
corrected. arXiv admin note: substantial text overlap with arXiv:1011.3623
and arXiv:1007.032
Insulator-metal transition in high-T superconductors as result of percolation over -U centers
The mechanism of -U center formation in high- superconductors (HTS)
with doping is considered. It is shown that the transition of HTS from
insulator to metal passes through the particular dopant concentration range
where the local transfer of singlet electron pairs from oxygen ions to pairs of
neighboring cations (-U centers) are allowed while the single-electron
transitions are still forbidden. We believe it is this concentration range that
corresponds to the region of high-Tc superconductivity and the interelectron
attraction results from the interaction of electron pairs with -U centers.
Additional hole carriers are generated as the result of singlet electron pair
transitions from oxygen ions to -U-centers. The orbitals of the arising singlet
hole pairs are localized in the nearest vicinity of -U center. In such a system
the hole conductivity starts up at the dopant concentration exceeding the
classical 2D-percolation threshold for singlet hole pair orbitals. In the
framework of the proposed model the phase diagram Ln-214 HTS compounds is
constructed. The remarkable accord between calculated and experimental phase
diagrams may be considered as the confirmation of the supposed model. The main
features of hole carriers in HTS normal state are found to be the nondegenerate
distribution and the dominant contribution of electron-electron scattering to
the hole carrier relaxation processes. Various experimentally observed
anomalies of HTS properties are shown to be the consequences of the
above-mentioned features. The conclusion is made that HTS compounds are the
special class of solids where the unusual mechanism of superconductivity
different from BCS is realized.Comment: 8 pages, 6 figure
Phase diagrams of La2-xSrxCuO4 and YBa2Cu3O{6+/delta} as the keys to understanding the nature of high-Tc superconductors
The model that explains many of the details of superconducting and magnetic
phase diagrams of YBa2Cu3O{6+/delta} and La2-xSrxCuO4 is presented. The model
is based on the assumption of rigid localization of doped charges in the close
vicinity of doped ion. This localization results in local variation of
electronic structure of the parent charge-transfer insulator that depends on
the local mutual arrangement of the doped charges. It is shown that in such
system the negative-U centers (NUCs) may form under certain conditions on the
pairs of neighboring Cu cations in CuO2 plane. We consider the mechanism of
hole generation. The calculated dependences of hole concentration in
YBa2Cu3O{6+/delta} on doping and temperature are found to be in a perfect
quantitative agreement with experimental data. In the framework of the model
the phase diagrams of YBa2Cu3O{6+/delta} is considered and the interpretation
of pseudogap and 60 K-phases in YBa2Cu3O{6+/delta} is offered. The pseudogap
has superconducting nature and arises at temperature T*>Tc/inf>Tc in small
clusters uniting a number of NUCs due to large fluctuations of NUC occupation.
(Tc/inf and Tc are the temperatures of superconducting transition for infinite
and finite clusters, accordingly). The calculated T*(/delta) and Tc(/delta)
dependences are in accordance with experiment. The range between T*(/delta) and
Tc(/delta) corresponds to the range of fluctuations where small clusters
fluctuate between superconducting and normal states owing to fluctuations of
NUC occupation. The phase diagram of La2-xSrxCuO4 is calculated. It is shown
the features of the superconducting phase diagram and the characteristics of
stripe textures of La2-xSrxCuO4 only reflect the geometrical relations existing
in a square lattice and the competition of different types of dopant ordering.Comment: 26 pages, 17 figures. New references are include
Towards the issue of the origin of Fermi surface, pseudogaps and Fermi arcs in cuprate HTSCs
Earlier we have proposed a new approach to the analysis of superconducting
phase diagrams for cuprates and pnictides and have shown that the positions of
superconducting domes on the diagrams can be predicted with high accuracy
proceeding from only the crystal structure of a particular compound. The
proposed approach uses the concept of the self-localization of doped carriers
due to their formation of trion complexes that represent a bound state of the
doped carrier and charge transfer excitons emerging under its influence. Here,
as exemplified by cuprates, we show that the use of the proposed approach to
the analysis of the transformation of an electronic structure with doping
enables an explanation to a range of their anomalies: Fermi arcs, large and
small pseudogaps etc. The basic conclusion is that the role of the Fermi
surface in cuprates is played by an isoenergetic contour that emerges at the
sectioning of the surface of a band dispersion by a dispersionless "biexciton"
pair level. This level additionally plays the role of an acceptor to lead to
the emergence of hole carriers on the isoenergetic contour and to a jump of the
chemical potential. Based on the conducted consideration, we propose a possible
mechanism of superconducting pairing genetically inherent in such a system.Comment: 13 pages, 5 figures, typos correctio
Superconducting phase diagrams of cuprates and pnictides as a key to the HTSC mechanism
This paper reviews experimental phase diagrams of cuprates and pnictides to
demonstrate that specific features of the superconducting phase diagrams in
bothHTSC families can be understood within the framework of the proposed
approach,which assumes the formation, under heterovalent doping, of localized
trion complexes consisting of a doped carrier and charge transfer (CT)
excitons. The geometry of such cells containing CT excitons (CT plaquettes) in
the basal plane of the crystal is determined by its crystal structure and the
type of dopant, so that the dopant concentration range corresponding to the
existence of a percolation cluster of CT plaquettes can be readily determined
for each particular compound. These dopant concentration ranges coincide with
good accuracy with the experimental ranges of superconducting domes in the
phase diagrams of the HTSC compounds considered. The generation of free
carriers and the mechanism of superconducting pairing in this pattern is
related to biexciton complexes (Heitler-London centers) emerging in neighboring
CT plaquettes.Comment: 15 pages, 13 figure
Parameter estimation in models generated by SDE's with symmetric alpha stable noise
The article considers vector parameter estimators in statistical models
generated by Levy processes. An improved one step estimator is presented that
can be used for improving any other estimator. Combined numerical methods for
optimization problems are proposed. A software has been developed and a
correspondent testing and comparison have been presented
On the possible mechanism of superconductivity in cuprates and pnictides
The nature of the normal state and the mechanism of superconductivity in two
families of high-temperature superconductors, cuprates and pnictides, remain a
matter of intense discussions. According to band-structure calculations,
confirmed by ARPES data, the electronic structure of pnictides differs
considerably from that of cuprates. However, in spite of these differences, it
looks like in both cases there exists some general and fairly "coarse"
mechanism independent of fine details of the band structure and responsible for
superconductive pairing in these materials. Here, we suggest a qualitative
model describing the ground state and the mechanism of superconductive pairing
in cuprates and pnictides that, in our opinion, can explain many of their
unusual properties.Comment: 6 pages, 1 figure, submitted to FPS`11, 3-7 October 2011, Zvenigoro
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