bluuebunny/crossref_metadata_2025 huggingface dataset:
An Integral Representation for Ising Models
Authors: Albeverio Sergio , Fei Shao-Ming
Published: October 2003

An integral representation for the partition function for general n-dimensional Ising models with nearest or non-nearest neighbors interactions in a bounded region is given. The representation is used to derive some properties of the partition function. The results are generalized to the case of higher spin Ising models.
Time and Temperature Dependent Correlation Functions of the One-Dimensional Impenetrable Electron Gas
Authors: Göhmann F. , Izergin A. G. , Korepin V. E. , Pronko A. G.
Published: October 2003

We consider the one-dimensional delta-interacting electron gas in the case of infinite repulsion. We use determinant representations to study the long time, large distance asymptotics of correlation functions of local fields in the gas phase. We derive differential equations which drive the correlation functions. Using a related Riemann–Hilbert problem we obtain formulae for the asymptotics of the correlation functions, which are valid at all finite temperatures. At low temperatures these formulae lead to explicit asymptotic expressions for the correlation functions, which describe power law behavior and exponential decay as functions of temperature, magnetic field and chemical potential.
The Bethe Ansatz Solution of a One-Dimensional Fermion Model with Boundary Impurity
Authors: Chen Shu , Wang Yupeng , Yang Fan , Pu Fu-Cho
Published: October 2003

We investigate a 1D fermion model with boundary impurity. When the boundary impurity coupling coefficients fullfil some special relations, this model is integrable. The integrable condition can be determined by the self-consistent condition. Furthermore, the eigenvalue and the Bethe ansatz equation are also obtained by using the coordinate Bethe ansatz method, thus the ground state properties is discussed in some special case.
Extended Van Hove Singularity in Raman Spectra of High-Tc Superconductors
Authors: Misochko O. V. , Ya. Sherman E.
Published: October 2003

We consider theoretically and experimentally how an extended van Hove singularity manifests itself in electronic and phonon Raman scattering in superconducting cuprates. Our analysis and experimental data shows that in the case of singularity carriers redistribution between different parts of the Brillouin zone and/or electronic bands is a common feature of both the electronic and phonon Raman scattering.
Probing the Gauge Structure of High-Temperature Superconductors
Authors: Farakos K. , Koutsoumbas G. , Mavromatos N. E.
Published: October 2003

We suggest that a spin-charge separating ansatz, leading to non-Abelian SU (2)⊗ US(1) gauge symmetries in doped antiferromagnets, proposed earlier as a way of describing Kosterlitz–Thouless superconducting gaps at the nodes of the gap of d-wave (high- Tc ) superconductors, may also lead to a pseudogap phase, characterized by the formation of (nonsuperconducting) pairing and the absence of phase coherence. The crucial assumption is again the presence of electrically charged Dirac fermionic excitations (holons) about the points of the (putative) Fermi surface in the pertinent phase of the superconductor. We present arguments in support of the rôle of nonperturbative effects (instantons) on the onset of the pseudogap phase. As a means of probing such gauge interactions experimentally, we perform a study of the scaling of the thermal conductivity with an externally-applied magnetic field, in certain effective models involving gauge and/or four-fermion (contact) interactions.
Bimerons in Double Layer Quantum Hall Systems
Authors: Ghosh Sankalpa , Rajaraman R.
Published: October 2003

In this paper we discuss bimeron pseudospin textures for double layer quantum Hall systems with filling factor ν = 1. Bimerons are excitations corresponding to bound pairs of merons and anti-merons. Bimeron solutions have already been studied at great length by other groups by minimising the microsopic Hamiltonian between microscopic trial wavefunctions. Here we calculate them by numerically solving coupled nonlinear partial differential equations arising from extremisation of the effective action for pseudospin textures. We also calculate the different contributions to the energy of our bimerons, coming from pseudospin stiffness, capacitance and coulomb interactions between the merons. Apart from augmenting earlier results, this allows us to check how good an approximation it is to think of the bimeron as a pair of rigid objects (merons) with logarithmically growing energy and with electric charge 1/2. Our differential equation approach also allows us to study the dependence of the spin texture as a function of the distance between merons and the inter layer distance. Lastly, the technical problem of solving coupled nonlinear partial differential equations, subject to the special boundary conditions of bimerons is interesting in its own right.
An Investigation of the Electron Transfer and Repulsive Coulomb Interactions in the One-, Two-, Three-Dimensional Isotropic Hubbard Models I: Eigenfunctions and Eigenvalues
Authors: Dixon J. M. , Tuszyński J. A.
Published: October 2003

In this paper we investigate the Hubbard model, with S-orbits on each site, for one-, two- and three-dimensional isotropic lattices with arbitrary values of transfer and on-site repulsion parameters. The method of diagonalization is based on equivalences due to Stevens for number conserving operators and a unitary transformation utilizing permutational symmetries of the problem. We obtained simple analytical formulas for single particle eigenstates and somewhat more involved expression for determinantal multi-electron states. Numerical calculations have been performed which demonstrate the boundaries of the lowest band for various filling factors using examples in one- and two-dimensions. The states within the band have also been identified and degeneracies evaluated numerically. These computations were based on a Monté Carlo technique in conjunction with particle conservation requirements. Agreement with some well-known limiting cases has been found.
An Investigation of the Electron Transfer and Repulsive Coulomb Interactions in the One-, Two-, Three-Dimensional Isotropic Hubbard Models II: Extension to a Wannier and Higher Angular Momentum Basis
Authors: Dixon J. M. , Tuszyński J. A.
Published: October 2003

In the previous paper we have investigated the isotropic Hubbard model, with S-orbits on each site, for one-, two- and three-dimensional isotropic lattices. The method involved the determination of the one-electron energies and eigenfunctions of the transfer interaction, the latter being utilised to form determinantal eigenfunctions for the many-electron system as a whole. In this paper we have examined the consequences of using an atomic basis by introducing Wannier functions and studied physical consequences of this more exact approach. To reduce the transformed two-body interaction to manageable form we have used the rotational properties of individual atomic orbitals incorporating them via irreducible tensorial methods. We find that, although additional components due to Wannier "tails" are small they may modify the nearest neighbor transfer terms significantly and also introduce next-nearest and next-next nearest neighbor transfer interactions.

We have also investigated the effect of using atomic orbitals with a higher angular momentum on each site. Both p-orbitals and d-orbitals have been studied and the broad conclusion is that the corresponding Hubbard model reduces to a series of S-like orbital models, one for each component, m, of the degenerate set on each site. The transfer interaction in each case becomes, in general, m dependent, each m component being effectively decoupled from others.
Magnetic Moment and Saturation of Internal Magnetic Field in Co and Fe Clusters
Authors: Mahendran M. , Iyakutti K.
Published: October 2003

We have investigated the nature of internal magnetic field, exchange interaction and the effective magnetic moment per atom, μ eff , of ferromagnetic transition metal clusters, Fe and Co using the mean field approximation. Interestingly the exchange interaction and the internal magnetic field get saturated and both of them attain a constant value when the number of atoms in the cluster exceeds certain number. We find that, for all the clusters of Fe and Co μ eff decreases with increasing temperature. The surface effects are included in an indirect way through a geometry dependent empirical relation. A new quantity [Formula: see text] emerges out as a constant for a given cluster and this value depends linearly on N, the number of atoms in the cluster.
The Origin and Bifurcation of Disclinations in the Gauge Field Theory of Dislocation and Disclination Continuum
Authors: Yang Guo-Hong , Duan Yishi
Published: October 2003

In the 4-dimensional gauge field theory of dislocation and disclination continuum, the topological current structure and the topological quantization of disclinations are approached. Using the implicit function theorem and Taylor expansion, the origin and bifurcation theories of disclinations are detailed in the neighborhoods of limit points and bifurcation points, respectively. The branch solutions at the limit points and the different directions of all branch curves at 1-order and 2-order degenerated points are calculated. It is pointed out that an original disclination point can split into four disclinations at one time at most. Since the disclination current is identically conserved, the total topological quantum numbers of these branched disclinations will remain constant during their origin and bifurcation processes. Furthermore, one can see the fact that the origin and bifurcation of disclinations are not gradual changes but sudden changes. As some applications of the proposal theory, two examples are presented in the paper.
Ion Probe Diagnostics of Laser Ablated Plumes for Thin Carbon Films Deposition
Authors: Dwivedi R. K. , Singh S. P. , Thareja R. K.
Published: October 2003

The propagation of the carbon plumes in argon atmosphere used for film growth by pulsed laser ablation deposition (PLAD) is studied using ion probe. Spatial and angle resolved analysis of plasma parameters as a function of argon gas pressure is presented. The electron temperature is strongly dependent on the angle with target surface normal as well as the distance from the target surface. The forward directed nature of the PLAD process is strongly influenced by the pressure of the ambient gas. The plasma parameters are used to explain the effect of ambient gas pressure on the characteristics of the deposited film.
Synthesis of Nanophase TiO2 by Ion Beam Sputtering and Cold Condensation Technique
Authors: Saraf L. V. , Patil S. I. , Ogale S. B. , Sainkar S. R. , Kshirsager S. T.
Published: October 2003

Nanoparticles of TiO 2 have been synthesized by an ion beam sputtering-cold condensation (IBS-CC) technique. A sintered TiO 2 was sputtered by an ion beam (Kaufman source, 900 eV, Ar+ ions) and the ejected atoms/radicals were made to condense on a Si(l00) substrate held at -50°C. X-ray diffraction data showed that the average particle size in the as-deposited material is about 3.5 ± 1.5 nm. Upon annealing at 600°C for five hours, the average particle size was seen to increase to about 70 ± 10 nm. Further annealing for one hour at 900°C led to increase of average particle size to 200 ± 20 nm. X-ray spectroscopy, Raman spectroscopy and photoluminescence data have been used to reveal the presence and relative concentrations of rutile and anatase phases of TiO 2 in the as-deposited and annealed samples. The IBS-CC method is found to yield a more compact particle size distribution as compared to the method based on Laser Ablation.
Topological Aspects of Quantum Hall Fluid and Berry Phase
Authors: Basu Banasri , Bandyopadhyay P.
Published: October 2003

We have analyzed here the recent development towards our understanding of the Integral and Fractional Quantum Hall effect. It has been pointed out that the chiral anomaly and Berry phase approach embraces in a unified way the whole spectrum of quantum Hall systems with their various characteristic features. This formalism also helps us to understand the edge states observed in Hall fluids. It is argued that Hall fluids with even denominator filling factor leads to the non-Abelian Berry phase.
The Behavior of the Spin-One-Half Falicov–Kimball Model Close to the Metal–Insulator Transition
Authors: Farkašovský Pavol
Published: October 2003

The spin-one-half Falicov–Kimball model for electronically driven valence and metal–insulator transitions is studied in one and two dimensions using small-cluster exact-diagonalization calculations. Performing an exhaustive study of the model close to the metal–insulator transition we have found that the spin-one-half Falicov–Kimball model can describe much of experimental data of transition-metal and rare-earth compounds. Particularly, except the discontinuous transitions it can provide the qualitative explanation for all typical behaviors of the electrical conductivity observed experimentally in these materials.
Local Moments in Two-Dimensional Quantum Antiferromagnets Doped with Static Vacancies
Authors: Sa Debanand
Published: October 2003

The effect of static impurities in a two-dimensional quantum antiferromagnet has been studied. Creation of a vacancy in an antiferromagnetic lattice yields an impurity Hamiltonian whose x-y part maps to single impurity Anderson model whereas the z-part becomes the X-ray singularity problem. The model is solved where one obtains a bound state giving rise to δ-function density of states. The z-part of the interaction makes this peak asymmetric which follows a power law. It has been argued that the bound state corresponds to a local moment in the system giving rise to Curie-like contribution to the susceptibility. The implications of the present model in the substitutional Zn doped high-T c oxides have been discussed.
Fractional Shapiro Steps and Superstructure in Revivals of Supercurrent in Mesoscopic Josephson Junction
Authors: Zou Jian , Shao Bin , Xing Xiusan
Published: October 2003

Dynamic behavior of supercurrent in mesoscopic Josephson junction interacting with even coherent state (CS) and odd CS of nonclassical microwave with frequency ω1 and classical radiation with frequency ω2 is investigated. It is shown that the supercurrent through the junction has a dc component only for all rational values of ω1/ω2 (fractional Shapiro steps). It is also shown that the dynamic behavior of the supercurrent exhibits collapse and revival phenomenon and the revivals are dominated by the periodical superstructure. Dynamic behavior of supercurrent fluctuation is also studied and it is found that generally when the fluctuation is large, the supercurrent exhibits collapses and when the fluctuation is small, the supercurrent exhibits revivals.
Photon Statistics and Squeezing in the Two-Atom Thermal Jaynes-Cummings Model
Authors: Mir Mubben A.
Published: October 2003

The behavior of the mean photon number, photon number variance and field squeezing is investigated for the two two-level atoms prepared in a multiatom squeezed state coupled to the vacuum or thermal field. The value of superposition angle (θ) of the atomic states is found to affect the photon number variance as well as field squeezing. The bound on the sun-Poissonian field concerning the value of θ and input mean photon number is determined. One of the field quadrature is found to exhibit permanent squeezing for the vacuum input in a small range of the value of θ. It is shown that the input thermal photons degrade the sub-Poissonian character and spontaneous squeezing. However, a considerable amount of squeezing is obtained even in the presence of thermal photons.
Resonant Valence-Band Satellites at the Ni-3p Edge of Ni3Al, Ni3Ga and Ni3In
Authors: Hsu Li-Shing , Tsang K.-L. , Chung S.-C.
Published: October 2003

Valence-band photoemission sspectra of Ni 3 Al , Ni 3 Ga and Ni 3 In were measured with synchrotron radiation at photon energies around the Ni-3p core excitation threshold. The valence-band satellites are resonantly enhanced. The number of holes per Ni atom in the Ni-d band for Ni 3 Al , Ni 3 Ga and Ni 3 In is estimated to be 0.37, 0.30 and 0.34, respectively. The self-energy correction due to the configuration interaction is 0.72, 0.71 and 0.72 eV, respectively, for Ni 3 Al , Ni 3 Ga and Ni 3 In . The Fano parameters and the effective Coulomb interactions between two 3D holes for these intermetallic compounds are determined.
Boltzmann and Zermelo Versus Loschmidt and Poincaré — is there any Recurrence?
Authors: Syros C.
Published: October 2003

It is shown that Poincaré's recurrence theorem incorporates Loschmidt's requirement for velocity reversion in thermodynamic gas systems. It differs essentially from Hamiltonian dynamics from which Boltzmann's H-theorem follows. The inverse automorphism, T-1, on which is based the demonstration of the recurrence theorem does not exist for atoms and molecule systems. Thermodynamic systems need not spontaneously return to states they occupied in the past and a Zermelo paradox has never existed for them. The same conclusion follows a fortiori for quantum systems in chrono-topology. Poincaré's recurrence theorem does not conflict with Boltzmann's H-theorem because they apply to systems described by quite different mathematical structures.
Bounds on Correlation Functions of Quantum Rotators
Authors: Bolina Oscar , Parreira J. Rodrigo
Published: October 2003

We derive a McBryan–Spencer bound to the correlation function of a one-dimensional array of quantum rotators in the Villain approximation of the cosine interaction. We obtain the partition function of the system in the gas representation and establish a lower bound on the external charges correlation function. We also discuss the possible existence of a Kosterlitz–Thouless phase for the quantum rotators in the Villain approximation.
Study of the Large-Dimensional Hubbard Model Via a New Nonperturbative Method
Authors: Hong Jongbae , Song Jinsook
Published: October 2003

A new nonperturbative method calculating the one-particle Green's function is introduced and applied to the Hubbard model. We show the metal–insulator transition in the paramagnetic ground state of the half-filled Hubbard model on a Bethe lattice with infinite connectivity. This method describes the transition regime appropriately. The analytic result using the continued fraction formalism which has a close relationship with present formalism is provided for comparison. As for the Hubbard model with doping, we obtain the optical conductivities and show the interesting features, such as midinfrared band and isobestic point. We also show that the sharp frequency-dependent behavior at large doping is the dispersed Drude type in this model.
Mean-Field Analysis of the Three-Band Hubbard Model
Authors: Yu Yabin , Cao Guanghan , Jiao Zhengkuan
Published: October 2003

A full three-band model for the CuO 2 plane of cuprates, which includes all the essential interaction — Cu-O and O-O hopping and the Coulomb repulsion on the Cu and O sites and between them, is considered. Its antiferromagnetic ground state for the half-filling is studied by using the mean field approximation. The electronic structure and the magnetic properties such as the densities of states, the energy spectra, the composition of holes (Cu or O character), the superexchange interaction and the magnetic moment are calculated and in general, our results are in agreement with the available experimental and other calculation results. Meanwhile, we find that the influence of the O-O hopping and Cu-O intrasite Coulomb repulsion on these properties is considerable. Our estimate of the energy of the spin singlet state above the antiferromagnetic background indicates that the lowest excitation state of the holes is the singlet state and give a charge-transfer energy in agreement with the experiment. We also discuss the hole-doping to the antiferromagnetic background and find the mean field approach invalid. Finally, based on the electronic structure at the half-filling, en effective one-band Hubbard model is presented and the effective parameters are close to the values given by the computation on the clusters.
Fluctuations in the Local Fields Due to Al3+ Ions Substitution in the M-Type Barium Hexaferrites, BaFe12-xAlxO12
Authors: Thongmee S. , Osotchan T. , Winotai P. , Tang I. M.
Published: October 2003

The effects of aluminum substitution into the M-type Ba-hexaferrites on the saturation magnetization and Mossbauer spectrum of these ferrites are studied. The saturation magnetization M s of the BaFe 12-x Al x O 12 is seen to decrease linearly with increasing Al substitution. The Mossbauer spectrums for the x = 0.1, 0.2 and 0.6 specimens were all resolved as superpositions of four sub-spectrums (12k, 4 fiv +2 a , 4 fvi and 4e). The half width of the Mossbauer lines of the 4 fiv+2a sextets is seen to broadening as Al ions are substituted into the ferrite. No broadening is seen in the lines of the other sextets. These behaviors (the line broadening and the linear decrease in M s ) are consistent with the Al 3+ ions substituting preferentially into the 2a sites.
Quantum Friction: A Bell Path Integral Study
Authors: Botelho Luiz C. L. , da Silva Edson P.
Published: October 2003

We write effective propagators associated to: (1) A quantum particle interacting with a classical gas and a quantum bosonic gas, modeling friction environments. (2) A noisy and quantum phonon field modeling friction environment at zero temperature. These studies are implemented by means of the Bell functional integrals representing these external dissipative environments.
On the Dynamical Propeties of a Strongly Anharmonic Face-Centered Cubic Crystal
Authors: Zubov V. I. , Rodrigues C. G. , Pascual M. F.
Published: October 2003

We study the interatomic correlations and mean square relative displacements (MSRD) in anharmonic crystals on the basis of the correlative method of unsymmetrized self-consistent field (CUSF). Here we present general formulae for crystals with the anharmonicity, including the strong one, up to the fourth anharmonic terms and perform calculations of the quadratic correlation moments (QCM) in a crystal with face centered cubic lattice, namely in solid Ar. The second order of CUSF allows one to investigate correlations in this lattice between the nearest, second, third and fourth neighbors. The anharmonicity was demonstrated to have strong effect on the interatomic correlations at temperatures above 0.4 of the melting temperature causing a drastic rise near the spinodal point. The dependence of QCM on the distance between atoms and on the crystallographic direction is discussed.
Patterns Formation in a Primitive Life Form Model
Authors: Andrecut Mircea
Published: October 2003

A computational study of population dynamics in a primitive life form model is presented. The spatio-temporal dependencies of density were calculated for different values of the control parameters (birth and death rates). The calculations have shown that the dynamics of the investigated system lead to the formation of patterns and self-organization only for a certain dependence between the control parameters.
The Importance of Local Disorder in Doped Correlated-Electron Systems; Are "Smoking Guns" Really "Smoking"?
Authors: Bridges Frank
Published: October 2003

The suppression of T c by dopants such as Ni and Zn is discussed. In these systems tiny oxide crystallites can form; when one accounts for the fraction of Zn or Ni in the oxide phase, the suppression of T c is comparable for these two dopants and thus does not provide support for spin-fluctuation models. We also consider the distortions of the axial O in high-T c systems. The unusual broadening of the Hg-O(2) bond in Hg1201 and Cu(1)-O(4) bond in YBCO are suggestive (but not a smoking gun) for polaron formation. We compare with polarons formed in the CMR materials.
The Novel Ladder Superconductors: A Brief Review
Authors: Dagotto Elbio
Published: October 2003

The present status of the study of "ladder" materials is briefly reviewed. One of these materials becomes superconducting upon hole doping and at high pressure. It contains copper-oxide ladders as the dominant component in its structure, with a weak frustrating interladder coupling. Several experimental features of the ladder compounds are in excellent agreement with theoretical calculations. In addition, these intrinsically interesting and challenging systems provide us with useful information for the analysis of the physics of the high-T c cuprates.
Superconductivity in PrBa2Cu3O7: Implications
Authors: Blackstead Howard A. , Dow John D.
Published: October 2003

PrBa 2 Cu 3 O 7 (Pr123) is an intrinsic superconductor with T c ≈ 90 K, as predicted and as recently confirmed by several groups who have synthesized superconducting material under conditions that minimize the number of PrBa defects. Based on evidence that the defect responsible for the failure of ordinary Pr123 to superconduct is PrBa , the cuprate-plane picture of high-temperature superconductivity must be revised.
The Role of the Coulomb Interaction in Superconductivity
Authors: Fan J. D. , Malozovsky Y. M.
Published: October 2003

It is found that particle pairing is nothing else but grouping in the statistical sense and that only in the particle–hole channel does the BCS Hamiltonian have the BCS solution for an attractive interaction, whereas the interaction in the particle–particle channel is still repulsive. A generalized perturbation approach beyond the random phase approximation (RPA), based on Ward's identity, was developed by the authors to deal with both weakly and strongly coupled electronic systems. The full summation of all of the possible Feynman diagrams of two-particle interaction guarantees its validity. A phase transition in this method is determined by instability of the normal state, often referred to as the pairing instability, but better to resonance of interaction, equivalent to the pole condition in the two-particle scattering amplitude. Of more importance and interest is that superconductivity, regardless of low or high temperature, is found to originate from the Coulomb correlations. It was shown that only if interaction in the particle–particle channel is repulsive may the instability occur and the irreducible response function, hence conductivity, tends to infinity as temperature approaches T c . The transition temperature T c is found to be related to the physical, chemical and structural parameters, such as the dielectric constant, concentration of carriers and interlayer spacing. Therefore, low- and high-temperature superconductivity do not have an intrinsic distinction but their observed different properties. An application of the approach to a layered two-dimensional system immediately leads to the metal–superconductor (MS) transition with a possible high transition temperature, while the MS transition in an isotropic three-dimensional system can never exhibit a high transition temperature.
Two-Step Evolution of the Quasiparticle Band with Doping in the Two-Dimensional t-t′-t″-J Model
Authors: Gong Chang-De , Yin Wei-Guo , Leung P. W.
Published: October 2003

We study the doping dependence of photoemission spectra for the t-t′-t″-J model by using the exact diagonalization technique and present a consistent theoretical analysis. Both calculations show that upon doping the enhancement of incoherent motion of holes due to the t′ and t″ terms accounts for the formation of the flat region around (π,0) in the quasiparticle dispersion at underdoped and optimally doped region, despite the absence of the flat band at half filling. Our results are in excellent agreement with resent photoemission experiments on Bi 2 Sr 2 Ca 1 Cu 2 O 8+δ [Marshall et al., Phys. Rev. Lett.76, 4841 (1996)] and Sr 2 CuO 2 Cl 2 [Wells et al., Phys. Rev. Lett.74, 964 (1995)].
What is the Symmetry of the High-Tc Order Parameter?
Authors: Klemm Richard A.
Published: October 2003

In recent years, there has been a raging controversy regarding the orbital symmetry of the superconducting order parameter (OP) in the high temperature superconductors. Many experiments were interpreted in terms of a dx2-y2-wave OP, but many others were interpreted in terms of a more conventional s-wave OP. We review the problems of both intrinsic and extrinsic natures with the phase-sensitive experiments on YBCO. We further show that the photoemission experiments of the purported superconducting gap in Bi2Sr2CaCu2O 8+δ are entirely consistent with charge- and/or spin-density wave formation in that material. The presence of such density waves greatly complicates the analysis of most experiments. Hence, we conclude that the orbital symmetry of the superconducting OP is still unknown in any of the high temperature superconductors.
Electric and Magnetic Field Dependent Dimensionality of Electronic States in Quantum-Scale Semiconducting and Superconducting Heterostructures
Authors: Kopaev Yu. V. , Aleshchenko Yu. A. , Kapaev V. V. , Tokatly I. V.
Published: October 2003

We have generalized our theory of transformation of electronic states dimensionality in semiconductor quantum well (QW) structures with asymmetric barriers to include the HTS heterostructures. It was shown that our theory is capable to explain many features of bulk HTS.
Mechanism of Pseudogap in High-Tc Cuprates
Authors: Liu Fu-Sui
Published: October 2003

This paper shows that the pseudogap (PG) of Y123 and B2212 is caused by two-local- Cu ++-spin-mediated interaction (TLSMI) in the CuO 2 plane and that PG is a gap of quasiparticle excitation which breaks a Cooper pair without long-range phase coherence.
Pseudogap and Unconventional Pairing in the Hubbard Model
Authors: Malozovsky Y. M. , Fan J. D.
Published: October 2003

The attractive (U < 0) and repulsive (U > 0) Hubbard models have been studied using the Fermi liquid perturbation approach. The attractive Hubbard model (U < 0) is an adequate model for 3 He , an incompressible and strongly paramagnetic liquid [Formula: see text], [Formula: see text] for |U|N F = 0.9) with a pseudogap in the charge response. A pairing instability and superfluidity for U < 0 exists in the spin channel only: spin-triplet with l = 0, or spin-singlet with l = 1 (p-wave orbital state j = s + l = 1), where l is the orbital momentum of a pair. The repulsive Hubbard model (U > 0) represents a highly compressible and nearly antiferromagnetic liquid [Formula: see text], [Formula: see text] for UN F = 0.9) with a pseudogap in the spin response. However, for U > 0 a pairing instability and superconductivity exist in the charge channel only: spin-singlet with l = 0 (s-wave), or with l = 2 (d-wave) in the case of an anisotropic Fermi surface.
Acoustic Plasmons and High-Tc Superconductivity of Cuprates with Extended Saddle Point Singularity in Electron Spectrum
Authors: Pashitskii E. A. , Pentegov V. I. , Semenov A. V. , Abraham E.
Published: October 2003

It is shown that the strong anisotropy of the one-particle electron spectrum, due to the existence of extended saddle-point singularities close to the Fermi level in the hole-type cuprates, leads to the appearance of a low-frequency peak in the spectral function of the charge density fluctuations due to the presence of an acoustic plasmon branch in the collective electron spectrum. We argue that the retarded anisotropic electron-plasmon interaction suppresses the static screened Coulomb repulsion for small transferred momenta and provides the effective attraction between electrons in the dx2-y2-wave channel of the Cooper pairing of current carriers.
Pairing Interaction in the Hole-Doped High-Tc Superconductors
Authors: Suck Salk Sung-Ho
Published: October 2003

A scenario for the possibility of hole-pairing is introduced on the basis of the phase diagram involving the spingap phase and the superconducting order for high-T c superconductors. Using our earlier generalized scattering theory, a theory of hole-pairing is proposed by allowing interaction between the negatively charged singlet pair and the positively charged holes as a result of hole doping.
Modelling of Hysteresis of High-Tc Superconductors
Authors: Vajda I. , Szalay A. , Mohácsi L. , Lukács J. , Porjesz T.
Published: October 2003

For the numerical calculation of the magnetic field and forces between HTSC materials and permanent magnets an algorithm and a two-dimensional finite difference computer code has been developed. The superconductor has been treated with its nonlinear and anisotropic dc magnetization curve. The magnetization curves are used as input characteristics for the field calculations. The basic ideas of the physical modelling are discussed.
Non-Phonon Superconductivity in the Flat p-d Electron Systems
Authors: Zaitsev R. O. , Mikhailova Ju. V.
Published: October 2003

The electron structure in the large positive Hubbard energy limit is studied. The condition for Cooper's pairing are defined. The phase diagram for the existence is established.
Superconductivity in R2-zCezCuO4-Type Compounds
Authors: Blackstead Howard A. , Dow John D. , Lehmann Martin
Published: October 2003

The superconductivity of R 2-z Ce z CuO 4 (R214) is discussed and compared with the superconductivity of the 1 × 1 natural superlattice made of it and various oxide layers, R 2-z Ce z Sr 2 Cu 2 MO 10 (R222M10), where R is a tri-valent rare-earth ion and M=Nb, Ta, Ru, or Ti. Using simple models, we are able to understand the dependences on rare-earth size of superconductivity, of the ability to fabricate R214, of the onset of weak ferromagnetism and of the onset of crystal distortions. We can also understand the dependence of R214 superconductivity on angular momentum quantum number L and the superconductivity of Gd222Nb10 when Gd214 does not superconduct. We predict that the R214 superconductors are p-type and that the superconducting layers in R214 are not the same layers of the crystal structures as in R222Nb10.
Theoretical Estimation of Static Dielectric Constants for High Temperature Tℓ-Ba-Ca-Cu-O
Authors: Boudreaux Edward A.
Published: October 2003

Bulk dielectric constants, ∊ c , for the layered BaCaTℓCuO high-T c superconductors have been calculated from polarizability calculations. A correlation between T c and ∊ has been derived. Estimates of the electrons' contribution, ∊ e , have also been reported.
Isothermal Elastic Constants of Flux-Line Lattice in Layered Superconductors
Authors: Cai Zhi-Xiong , Welch David O. , Dubey Girija S.
Published: October 2003

A model of the effective interaction between the magnetic flux-lines in a layered superconductor is derived from the Lawrence–Doniach model. We show analytically that the intralayer interaction energy can be evaluated using the Ewald summation technique. The melting of flux line lattices is studied using Langevin dynamics simulation of the model with various values of interlayer coupling strength and pinning intensities. The thermal fluctuation terms of the isothermal shear modulus are found to increase sharply at the melting transition temperature for systems with or without pinning, while the structural order parameters were close to zero at all temperatures for systems with strong pinning. The melting of the pinned flux-line lattice is discussed in the context of its elastic properties.
A New Theoretical Method and Application to Superconductivity
Authors: Cheng Geng
Published: October 2003

Bifurcation theory is applied to study the spontaneous symmetry breaking and phase transition. The mathematical condition is derived from physical consideration. Generalized parameter imbedding theory is used to solve the bifurcation equation. Using this method, we have solved the Migdal–Elishaberg's equation.
Application of Molecular Dynamics Simulation in Material Structure Research
Authors: Cheng Zhaonian
Published: October 2003

We introduce a bond order parameter (BOP) method to describe and identify the material structure by using the instantaneous configurations generated from molecular dynamics simulation. BOP is a universal method for different simulations and materials. A brief description of some examples of the application of this method in material structure research is given and the theory, method and results can be found in detail in the original references.
Theory of the Leading Edge Gap in Underdoped Cuprates
Authors: Chubukov Andrey V.
Published: October 2003

We present the theory of the leading edge gap in the normal state of underdoped high-T c materials. The consideration is based on a magnetic scenario for cuprates. We show that as doping decreases, the increasing interaction with paramagnons gives rise to a near destruction of the quasiparticle peak near (0, π) and symmetry related points. We then solve the d-wave pairing problem and show that the actual T c decreases with the strength of the coherent quasiparticle peak, but the precursors to d-wave state exist above T c even for fully incoherent quasiparticles.
A Novel Microscopic Two-Stage Fermi (Precursor)-Bose (Superfluid)-Liquid Theory of High-Tc Superconductivity
Authors: Dzhumanov S.
Published: October 2003

We develop a novel microscopic two-stage Fermi (precursor)-Bose (superfluid)-liquid (FBL) model proposed previously beyond the conventional Fermi-liquid (i.e. BCS) and Bose–Einstein condensation (i.e. real-space pairing) theories. The electronic phase diagram of high-T c superconductors is presented. The proposed FBL theory naturally explains many key experimental findings.
Band Structure Features and Concentration Phase Transitions in Oxide High-Tc System
Authors: Golovashkin A. I. , Anshukova N. V. , Ivanova L. I. , Rusakov A. P.
Published: October 2003

It is shown that consideration of harge density wave (CDW) in addition to spin density wave (SDW) allows for the better explanation of the new experimental results.
A New Mechanism for High-Tc: Electron Scattering from Interacting Tunneling Units
Authors: Klein Michael W. , Simanovsky Sergey B.
Published: October 2003

A theoretical model for high-T c superconductivity is presented based on electron pairing due to their interaction with two-level or multi-level tunneling units (TU's) present in high-T c materials. TU's were found experimentally in YBa2Cu3O 7-δ, Bi2CaSr2Cu2O8 , Tl2CaBa2Cu2O8 and in Tl2CaBa2CuO6 . The TU's have specific directions of orientation with respect to the crystal axis and hence cause a strongly anisotropic scattering of the conduction electrons and a strongly anisotropic gap function Δ(k) with wave vector k. For the weak coupling case we obtain, (i) an analytically derived Δ(k) which has a linear combination of s-wave and dx2-y2-wave symmetry with nodes in Δ(k), (ii) a high-T c determined by the elastic interaction potential between the TU's, (iii) an isotope effect consistent with experiment, (iv) a large uniaxial pressure p dependence of T c for YBa2Cu3O 7-δ close to optimal oxygen doping and (dT c /dpa) ≈ -(dT c /dpb) where Pa and Pb are the pressures in the a and b directions, (v) a quantitative agreement with experiment for dT c /dPi (i = a, b) when we use the elastic constant. The recently observed d ± s wave pairing in twinned YBa2Cu3O 7-δ, with a reversal of the sign of s-wave component across the twin boundary, is also in agreement with our model.
Reconsideration of the Doping Effect on High-Tc Superconductivity
Authors: Lin De-Hua , Wu Xingang
Published: October 2003

Studying the change of the cuprate's crystal structure based on doping and then the effect on its high-T c superconductivity, is an effective way to seek the mechanism of superconductivity. We explore the doping effect in the cuprate's crystal structure, such as the property of Cu-O layers, the interlayer spacing, the density of carriers and so on. Then, by considering the effect of the nuclear spin, we construct a three-dimensional superconductivity model, which is a correction of the nonphonon mechanism in a layered Fermi-Liquid.
Phase Diagrams and Mixed States of Unconventional Superconductors
Authors: Shapoval Eugene A.
Published: October 2003

Using the BCS formalism we have calculated the phase diagrams of a tight-binding model on a square lattice as a function of temperature and chemical potential. The results include dx2-y2, dxy and (dx2-y2 ± idxy) superconducting phases.
Pseudogap Phase Formation in the Crossover from Bose-Einstein Condensation to BCS Superconductivity in Low-Dimensional Systems
Authors: Gusynin Valery P. , Loktev Vadim M. , Quick Rachel M. , Sharapov Sergei G.
Published: October 2003

A phase diagram for a two-dimensional metal with variable carrier density has been studied using the modulus-phase representation for the order parameter in fully microscopic treatment. This amounts to splitting the degrees of freedom into neutral fermion and charged boson degrees of freedom. Although true long range order is forbidden in two dimensions, long range order for the neutral fermions is possible since this does not violate any continuous symmetry. The phase fluctuations associated with the charged degrees of freedom destroy long range order in the full system as expected. The presence of the neutral order parameter gives rise to new features in the superconducting condensate formation in low-dimensional systems. The resulting phase diagram contains a new phase which lies above the superconducting (here Berezinskii–Kosterlitz–Thouless) phase and below the normal (Fermi-liquid) phase. We identify this phase with the pseudogap phase observed in underdoped high-T c superconducting compounds above their critical temperature. We also find that the phase diagram persists even in the presence of weak 3-dimensionalisation.
Superconductivity and Magnetic Properties of the t–J Model of Cuprate Perovskites
Authors: Sherman A. , Schreiber M.
Published: October 2003

We apply the spin-wave theory with the additional constraint of zero staggered magnetization to investigate the two-dimensional t–J model in the paramagnetic state in the ranges of hole concentrations 0.02 ≲ x ≲ 0.17 and temperatures T ≲ 100 K. In this region the hole spectrum is nonmetallic and contains a pseudogap with properties similar to those observed in Bi2212 photoemission. The calculated spin correlation length, susceptibility, spin-lattice relaxation times at the Cu and O sites and Cu spin-echo decay time are in qualitative and in some cases in quantitative agreement with experiment in underdoped YBa2Cu3O 6+y. The temperature dependences of these quantities are typical for the quantum disordered regime with a pseudogap in the spectrum of magnetic excitations. In the Eliashberg formalism the hole–magnon interaction was found to be unable alone to give rise to superconductivity. With inclusion of a moderate interaction with apex oxygen vibrations, high-T c 's are obtained for even frequency dx2-y2 pairing.
Superconductivity in the Alkali-Doped Fullerides: Competition of Phonon-Mediated Attractions with Coulomb Repulsions in Polaron Pairing
Authors: Takada Yasutami , Hotta Takashi
Published: October 2003

The mechanism of superconductivity in the alkali-metal-doped fullerides is investigated theoretically based on the Hubbard–Holstein model. Salient observed features such as the anomalous? C isotope effect on the critical temerature T c for 50% molecular substitution, the rapid decrease of T c with the deviation of electron number from half-filling and the difference in the relation of T c with the lattice constant a0 between fcc and sc structures are explained from a unified viewpoint of polaron pairings originating from the competition between phonon-mediated attractions and Coulomb repulsions.
On the Characteristics of the Pseudogapped Metallic State of High-Tc Superconductors
Authors: Zhang Liyuan , Yang Xiaoqiang , Chen Changfeng
Published: October 2003

The pseudogapped metallic state (PS) in high-T c superconductors (HTS) has distinctive characteristics in comparison with the metallic state in tranditional solid state physics. Through careful analyses of experimental data and theoretical calculations, we have correlated the pseudogap in the density of states with the two-component characteristics in the HTS. We propose seven points as a model description for PS.
Effective Properties of Superconducting and Superfluid Composites
Authors: Berlyand Leonid
Published: October 2003

We consider a mathematical model which describes an ideal superfluid with a large number of thin insulating rods and an ideal superconductor reinforced by such rods. We suggest a homogenization procedure for calculating effective properties of both composite media. From the numerical point of view the procedure amounts to solving a linear problem in a periodicity cell of unit size.
Electronic Contribution to the Thermal Diffusivity: DyBa2Cu3O7-y and Y0.9Ca0.1Ba2Cu3O7-z
Authors: Dorbolo S. , Bougrine H. , Ausloos M.
Published: October 2003

The electronic contribution to the specific heat and the thermal conductivity of high-T c superconductors is calculated with a three-dimensional band structure including saddle points and a Lawrence–Doniach coupling between the CuO2 planes. The electronic thermal diffusivity is deduced for s- and d-wave symmetry of the order parameter. Data on DyBa2Cu3O 7-y and Y0.9Ca0.1Ba2Cu3O 7-z and theory are in good agreement on the change of the slope near T c .
A Two-Dimensional Structure Factor Calculation for the Cu-1 Plane in YBa2Cu3O6
Authors: Edis Taner , Fan J. D. , Bagayoko D. , Wang J. T.
Published: October 2003

The anisotropic radial distribution function and two-dimensional (2D) structure factor was calculated for the Cu-1 plane in YBa2Cu3O 6+x when x ≈ 0, based on data generated by a molecular dynamics simulation. The results indicate a stable square lattice and support the validity of layered two-dimensional screened Coulomb potential used in the simulation.
Insulator-Conductor Transition in Doped HTS as a Consequence of -U-Centers Generation
Authors: Ivanenko Olga , Mitsen Kirill
Published: October 2003

It is supposed that the transition from insulator to metal state under doping in high-T c superconductors (HTS) passes through the stage of -U-centers formation, which arise on the neighbor Cu ions when the definite spatial arrangement of the doping atoms is realized. In this doping region the free hole carriers (just as in hole-, so in electron-doped HTS) are generated when electron pairs transfer from the oxygen band into -U-centers.
Transport Properties of Superconductors in the Mixed State
Authors: Koláček Jan , Vašek Petr
Published: October 2003

The Hall voltage sign reversal is consistently explained by the model in which vortices with the superconducting and normal state charge carriers are regarded as three independent subsystems mutually connected by interactions. The equations of motion for these three subsystems are solved simultaneously and a new formula for the Hall resistivity is obtained. It is shown that by this model it is possible to explain qualitatively experimental data. Despite of the fact that the pinning forces have not been taken into account the formula qualitatively explains not only the double Hall voltage sign reversal observed in Bi and Tl based superconductors below the critical temperature but also the magnetic field dependence of the Hall conductivity.
The Pseudogap and Superconductivity Order Parameter Fluctuations
Authors: Kristoffel N. , Örd T.
Published: October 2003

The influence of superconductivity unequilibrium order parameter correlated fluctuations on the normal phase elementary excitations have been investigated. Correspondingly a fluctuating source of pairs appears in the effective Hamiltonian described using the Gaussian free energy functional. A pseudogap (E g ) is found on the Fermi surface decreasing with rising temperature. It is scaled with the parameter α = ∊ F /k B T c . For smaller α-s E g develops more over T c . In non-high-T c systems and overdoped high-T c compounds the pseudogap is suppressed.
Polysomatic Series and Superconductivity in Complex Cuprates with Ladder-Type Structure
Authors: Leonyuk L. , Maltsev V. , Babonas G. J. , Reza A. , Szymczak R.
Published: October 2003

The structure of ladder-type cuprates was considered in polysomatic model. The possibility of the manifestation of superconductivity was estimated considering the structural stability of various cuprates with the ladder-type structure.