bluuebunny/crossref_metadata_2025 huggingface dataset:
MÖSSBAUER EFFECT STUDY OF THE EU-Y-Ba-Cu-O SUPERCONDUCTOR
Authors: Xu Weiming , Liu Xiaoxiang , Jin Mingzhi , He Lanying , Wang Yifeng , Zhou Jianshi , Liu Hongjian , Li Liping , Su Wenhui
Published: November 2004

In this work the hyperfine interactions of (EuxY1−x)0.3Ba0.1CuO3(x=0.5, 0.7, 1.0) superconductors have been studied by 151Eu Mössbauer effect. X-ray diffraction shows that the materials are of single phase. Mössbauer spectra were measured at 300K and 83K. The results show that Eu ions are in the trivalent oxidation state. The isomer shift decreases with the increasing of composition x. The electron density at the 151Eu nuclei is decreased below the zero resistance temperature.
HIGH Tc SUPERCONDUCTING THIN FILMS BY CHEMICAL SPRAY DEPOSITION
Authors: Meng Guang-yao , Wang Xian-dong , Chen Xue-jun , Fu Pei-zhen , Yang Pinghua , Peng Ding-kun , Li Fan-qing , Zhang Wei-de , Zhou Gui-en , Wang Jun , Zhang Yu-heng
Published: November 2004

A Chemical spray deposition process with mixed nitrate solution as source was developed to prepare thin films of Ba-Y-Cu oxide on sapphire substrate. By adjusting the source composition and deposition conditions, we have successfully obtained the films in thickness of 2–15μm with essentially orthorhombic Ba2YCu3O7 phase. The films behave super conducting transition with onset above 90K and zero resistance at 68.4K to date. It is believed that the tails in the resistance transition can be improved by searching proper heat treatment and using suitable substrates such as ZrO2 and SrTiO3.
High temperature superconductivity from magnetism - The Resonating Valence Bond State
Authors: BASKARAN G.
Published: November 2004

In this talk the Resonating Valence Bond (RVB) state of the Mott- Hubbard insulator and the high temperature superconductivity resulting from doping are described as the possible mechanisms for the insulating and high temperature superconducting properties of undoped and doped La2CuO4 and other high Tc oxides. We describe in detail two microscopic approaches to the understanding of the RVB superconductivity- one is a simple mean field theory and the other is a gauge theory.
JAHN-TELLER-EFFECT MEDIATED HOLE-PAIR TUNNELING AND SUPERCONDUCTIVITY IN HIGH Tc COPPER OXIDES
Authors: Kamimura Hiroshi
Published: November 2004

A detailed description is given of a new mechanism of high temperature superconductivity recently proposed by us. In this model a CuO 2 square planar layer is formed by the cooperative Jahn-Teller distortion. As a result the bonding character between Cu and O atoms in a layer becomes partly covalent because of short interatomic distance. In doped (La1−xMx)2CuO4 the extra d holes in | z 2> orbitals in Cu 3+ ions form a hole pair by the superexchange interaction through intervening oxygen p (and/or s) orbitals. Then the coherent tunneling of a hole–pair occurs by a non-adiabatic process and the hole—pairs make a Bose condensation at the zero wavevector state. This causes superconductivity. Based on this mechanism, important key temperatures to give a measure of T c, the Bose condensation temperature and the pair-breaking temperature, are discussed. It is pointed out that T c is determined by a pair-breaking temperature of a hole–pair and thus it does not depend on the electron-lattice interactions. It is also suggested that various means to increase covalency such as the bending of square planar layers and the change of Cu 3+ concentration will lead to the discovery of higher T c materials.
LIGHT-METAL HYDRIDES AS POSSIBLE HIGH-TEMPERATURE SUPERCONDUCTORS
Authors: Overhauser A.W.
Published: November 2004

Light-metal hydrides, such as LiBeH 3, have a valence-electron density similar to that assumed for metallic hydrogen. If metallic hydrides in this class can be found, they may exhibit the high superconducting temperatures that have been predicted for metallic hydrogen.
SOFT-PLASMON MECHANISM FOR HIGH-TEMPERATURE SUPERCONDUCTIVITY
Authors: ASHKENAZI J. , KUPER C.G. , TYK R.
Published: November 2004

Recently-discovered high-temperature superconductors border on a Mott metal insulator transition; pure La2CuO4 undergoes a commensurate charge-density-wave transition and becomes an insulator. At other compositions, the incipient charge-density wave causes a drastic softening of those plasmon modes with wave vectors connecting points on the Fermi surface. These soft plasmons (typically tenths of an electron-volt) serve as the attraction which binds Cooper pairs, instead of the usual phonons. Consequently, transition temperatures are an order of magnitude larger than for phonon-induced superconductivity. One of the factors responsible for the strong electron - plasmon coupling is the influence of the empty f-orbitals of La or Ba.
PHENOMENOLOGICAL THEORY OF HIGH Tc SUPERCONDUCTORS
Authors: Das M.P. , Saif A.G.
Published: November 2004

We present a phenomenological approach in terms of two coexisting macroscopic order parameters correponding to the superconducting and the insulating states relevant to the high. temperature superconducting ceramics. On using the mean field approach we discuss the electrodynamical responses and from the fluctuations of the order parameters we obtain results for the electrical conductivity, which is shown to possess anomalies as in the granular materials.
EVIDENCE FOR A POSSIBLE ORDER-DISORDER TRANSITION IN THE JOSEPHSON JUNCTION LATTICE OF A GRANULAR La1.85Sr0.15CuO4 SUPERCONDUCTING OXIDE
Authors: Giovannella C. , Chappert C. , Beauvillain P. , Collin G.
Published: November 2004

We present a detailed low field susceptibility study of a sintered La1.85Sr0.15CuO4 superconductor. We find that two critical temperatures should be considered in order to explain our observations. The lower one is characteristic of an order-disorder transition in the Josephson Junction lattice and scales as H .55. Moreover, it is inferred that field cooled data do not give the true bulk Meissner effect.
ENERGY GAP OF HIGH-Tc SUPERCONDUCTOR YBa2Cu3O9−x
Authors: Ye H.J. , Lu W. , Yu Z.Y. , Shen X.C. , Miao B.C. , Cai Y.M. , Qian Y.J. , Hua Z.Y.
Published: November 2004

The far infra-red (IR) reflectances of the high-Tc superconductor YBa2Cu3O9−x(Tc=90 K) are measured in the frequency range 25–150 cm −1 and the temperature range 4.2–200 K. The reflectance curve at 4.2 K rapidly drops near 125 cm −1, whereas the curves at T>90 K go smoothly. Such a superconductor might have an energy gap 2Δ=15.5 meV , and the ratio 2Δ / kTc=2.0 is close to that of La–Ba–Cu–O and La–Sr–Cu–O superconductors, but well smaller than the BCS value 3.5.
THE OPTICAL GAP IN TERNARY PHOSPHIDES
Authors: BURDETT JEREMY K. , CODDENS BARRY A.
Published: November 2004

Some factors are described which influence the band gap in ternary phosphides with superstructures of the rocksalt, zincblende and perovskite arrangements. The gap is strongly controlled by cation size, since as these atoms become smaller the phosphorus sheets are pulled closer together with an increase in energy of the top of the valence band via P-P overlap. The electronegativity of the cations are also important since the bottom of the conduction band is largely composed of metal d orbitals in the case of a transition metal. It is shown how good estimates of observed band gaps in ZnSiP 2 and ZnGeP 2 may be obtained by the inclusion of d orbitals on the anions in the tight-binding calculations.
PHASE STABILITY OF RANDOM BRASSES: PSEUDOPOTENTIAL THEORY REVISITED
Authors: MUJIBUR RAHMAN S. M.
Published: November 2004

We review the theoretical development concerning the phase stability of random brasses. The introductory discussion of the subject embraces the rules of metallurgy in general, but we emphasize the roles of electron-per-atom ratio in the major bulk of our discussion. Starting from the so-called rigid-band model the discussion goes up to the recent higher-order pseudopotential theory. The theoretical refinements within the pseudopotential framework are discussed briefly. The stability criteria of the random phases are analysed both in the static lattice and dynamic lattice approximations.
HIGH-DENSITY EXPANSION FOR THE SPINLESS FERMION MODEL II: SECOND AND THIRD ORDER IN 1/z
Authors: BARTKOWIAK M. , MÜNGER P. , CHAO K. A.
Published: November 2004

The selfconsistent high-density expansion for thermodynamic quantities of the quantum spin-polarized fermion lattice gas, derived in the preceding paper, is extended to higher orders. The (1/z)2 terms in the free energy are evaluated by summing two-loop diagrams and the corresponding equation for critical temperature in the classical limit is discussed. Ground state phase diagram of the system for arbitrary electron density is derived in the first and second order of the expansion. Properties of the half-filled, one-dimensional spinless fermion model at T = 0, obtained here in the first three orders in 1/z, are compared with the results of exact calculations and of other approximate methods. This provides a test for the unrenormalized high-density expansion method.
U(1) Gauge Theory as a Collective Field Theory for Hubbard Model
Authors: Matsui Tetsuo
Published: November 2004

I construct a collective field theory for Hubbard model of high Tc superconductivity, using a path-integral method in the third quantized (slave boson) form. It is a U(1) gauge invariant theory consisting of a U(1) gauge field and a Higgs scalar. The gauge field stands for resonating valence bonds and describes a (short range) antiferro-paramagnet phase transition by a condensation machanism. The Higgs scalar represents spinless holes carrying electric charges. Through the confining gauge force, there formed bounded hole pairs on each link, which correspond to the vector mesons in lattice QCD. A superconducting phase is to be described by a condensation of a gauge invariant order parameter for these hole pairs, and to be compared with the color confining chirally broken phase in QCD. A Ginzburg-Landau theory for the vector hole-pair field is proposed.
THE ELECTRONS KINEMATIC INTERACTION AND HIGH-Tc SUPERCONDUCTIVITY IN TRANSITION METALS COMPOUNDS
Authors: Ivanov V A , Zaitsev R O
Published: November 2004

In the strong short-ranged repulsion conditions the scattering picture of excitations is equivalent to the scattering of identical Fermi particles. In the wave lengths, large in comparison with the elementary cell dimensions (in Γ-point), the physical situation corresponds to the scattering of slow particles. For the latest case the scattering amplitude is positive always. With the energy increasing the scattering amplitude is decreases and changes his sign for momenta of order of inverse scattering centre dimension. In the crystal lattice the elementary cell dimension is the representative dimension. Hence we conclude that with energy increasing from Γ-point to the Brillouin zone boundary the scattering amplitude can to change the sign at the momentum of order of reciprical lattice vector . The negative sign of the scattering amplitude on the Fermi energy surface results to the normal state instability and the superconductivity condencate prescipitation. Therefore in the filling of lower Hubbard band the superconductivity begins from the some electrons finite concentration, corresponding to the amplitude sign change of the Fermi level.
EXCITONIC SUPERCONDUCTIVITY IN COPPER OXIDES
Authors: Tešanović Zlatko , Bishop A. R. , Martin R. L. , Harris C.
Published: November 2004

We discuss the possibility of excitonic superconductivity in high T c copper oxides. The Hamiltonian describing CuO 2 planes supports both antiferromagnetism and low-lying Cu ↔ 0 intra and interband charge fluctuations. One crosses from one regime to another as the number of holes per unit cell increases. The high T c superconductivity takes place at hole concentrations most favorable for intraband charge transfer excitations. The dynamic polarizability of the environment surrounding CuO 2 planes plays an important role in enhancing T c.
GENERALIZED GUTZWILLER ANSATZ FOR THE HALF-FILLED HUBBARD CHAIN
Authors: Fazekas Patrik , Penc Karlo
Published: November 2004

The well-known Gutzwiller wave function is generalized by including new variational parameters to control nearest-neighbour charge-charge, charge-spin, and spin-spin correlations. The non-magnetic state of the one-dimensional, half-filled Hubbard model is studied. Within the Gutzwiller approximation, the expression for the ground state energy can be worked out analytically. The correlation between empty and doubly occupied sites is found to play the most essential role. Minimization in the large-U limit shows that the Brinkman-Rice transition has been pushed to U → ∞, and the leading term of the ground state energy density is of order −t2/ U . In contrast to results obtained with the Gutzwiller wave function, we find that the band occupation number nk is monotonically decreasing both above and below kF. The dominant k–dependence is given by ~(t/U) cos k, in agreement with t/U–expansion results. nk has also a weak step-function component, with the discontinuity at kF vanishing as (t/U)2 in the limit U/t ≫ 1.
HIGH Tc STUDIES IN SVERDLOVSK
Authors: GOSHCHITSKII B. N. , KOZHEVNIKOV V. L. , SADOVSKII M. V.
Published: November 2004

This review reports the main experimental results on superconducting lattice and electron properties of high-temperature superconductors of the type La 2−x Sr x CuO 4 and YBa 2 Cu 3 O 7 obtained at three institutes of the Ural Branch of the USSR Academy of Sciences in Sverdlovsk. Special attention is paid to investigations of structural phase transitions, heat capacity, optic and magnetic properties, NMR relaxation and the role of radiation disordering under the influence of fast neutron irradiation. In conclusion, a brief review of possible theories to explain high-temperature superconductivity in metal oxides is given.
SOME THERMAL AND ELECTRICAL PROPERTIES OF THE HIGH-Tc SUPERCONDUCTORS Y-Ba-Cu AND Bi-Sr-Ca-Cu OXIDES
Authors: COLLOCOTT S. J. , ANDRIKIDIS C. , DRIVER R. , WHITE G. K.
Published: November 2004

The heat capacity, Cp, has been measured in the range 0.4 to 20 K and the linear thermal expansivity, α, in the range 2 to 283 K of YBa 2 Cu 3 O 7. Below 4 K Cp exhibits a T-term (≈ 7.5 T mJ/mol.K ), but in general both Cp(T) and α(T) have magnitudes and T-dependences that are not very different from other ceramics. The electrical resistance of YBa 2 Cu 3 O 7 and Bi 2 Sr 1.7 CaCu 2 O 8 samples have been measured in the range 20 to 120 K and in magnetic fields of up to 7 T. In all cases the value of T c is highly sensitive to the applied magnetic field. The upper critical field, B c2 (0), is estimated to be 68.5 T for YBa 2 Cu 3 O 7 and 55 T for Bi 2 Sr 1.7 CaCu 2 O 8.
MAGNETIC SUSCEPTIBILITY OF YBa2Cu3O7−Δy
Authors: NAKAHIGASHI K. , NAKANISHI S. , KOGACHI M. , KAWANO R. , INOUE J. , NOGUCHI S. , OKUDA K.
Published: November 2004

Magnetic properties of YBa 2 Cu 3 O 7−Δy was investigated by dc- and ac-magnetic susceptibility as a function of oxygen deficiency Δy. The susceptibility in the normal state at low temperatures is dominated by the Curie-Weiss law, while the susceptibility at high temperatures deviates from the Curie-Weiss law. The deviations were successfully explained by assuming the triplet spin-pair excitation from the antiferromagnetically coupled Cu 2+ ions in the ground state.
BEING AND BECOMING AT THE MICROSCOPIC LEVEL
Authors: SCHOMMERS W.
Published: November 2004

The laws of classical mechanics and quantum theory do not distinguish between the past and the future. But a lot of phenomena tell us that the "arrow of time" points only in one direction. For example, in biology we discover evolutionary processes leading to diversification and increasing complexity; it is a typical feature of such evolutionary processes that "forward" in time is possible but "backward" in time is impossible. At what level can phenomena like these be described?

There is another problem in connection with time: within usual quantum theory, space-coordinates and time are not symmetrical to each other, which is in contrast to the basic results of special theory of relativity. In usual quantum theory coordinates are operators and play the role of statistical quantities; time however does not behave statistically and remains a simple parameter when we go from classical to quantum mechanics. For this reason there is no uncertainty relation for the time and energy within usual quantum theory which would agree in its physical content with that for the coordinates and momenta.

The purpose of this paper is to show, on the basis of new aspects, that both one-sidedness of time and its uncertainty are charateristics at the fundamental level of quantum theory. We have extended usual quantum theory on the basis of space-time arguments. In this connection a time-operator appeared and there is in general temporal asymmetry at the microscopic level. The picture might be important in the description of phenomena with an "arrow of time", i.e., processes which belong to the physics of becoming. Furthermore, we have derived an uncertainty relation for the energy and time which agrees in its physical content with the uncertainty relation for the momentum and the coordinate; this relation is different from that of usual quantum theory. Within the frame of this physical picture all the concepts (diversity, natural selection, mutation), which are presently used in connection with the evolution of biological systems, are already established at the quantum mechanical level and, therefore, our theoretical picture might also be the foundation for the explanation of the phenomena at the biological (macro-molecular) level.
DERIVATION OF EQUATIONS FOR HIGH Tc BY MEANS OF SLAVE BOSON TECHNIQUE
Authors: VAN HIEU NGUYEN , VINH TAN HA , TOAN THANG NGUYEN
Published: November 2004

The "slave boson" technique is applied for studying the superconductivity of the system of strongly correlated electrons with the Hubbard Hamiltonian. On the basis of the Green functions equations for the new boson and fermion operators we derive the dynamical equations determining the order parameters of the given RVB model.
Tc IN MULTI-LAYER OXIDE SUPERCONDUCTORS
Authors: TAO RUIBAO , HU XIAO , SUZUKI MASUO
Published: November 2004

A possible relation of the supeconducting transition temperature T c to the number of Cu-O planes coupled tightly in a Tl-Ca-Ba-Cu-O compound is discussed by means of Anderson's RVB theory. A model Hamiltonian has been suggested to include the tunneling of singlet pairs between the nearest neighbour Cu-O planes and our calculation shows that the experimental T c can be fitted well and raised obviously by increasing the number of Cu-O planes at first, then it becomes more and more insensitive as the number is increased beyond 4 or 5. The maximum T c may approach 144 K.
FLAT CONNECTIONS AND INTEGRABLE SYSTEMS
Authors: SATTINGER D. H.
Published: November 2004

The theory of completely integrable systems based on n × n first order isospectral problems is reviewed. The topics discussed are the inverse scattering theory for n × n systems; hierarchies of completely integrable Hamiltonian systems based on semi-simple Lie algebras; flat connections and gauge transformations; and the gauge theory of Bäcklund transformations.
ELECTROSTATIC MECHANISM AND THEORY OF HIGH-Tc SUPERCONDUCTIVITY IN LAYERED MATERIALS
Authors: MATTIS DANIEL C.
Published: November 2004

Electrostatic forces among stacked metal planes provide a viable mechanism for high-T c superconductivity. Dimensionless parameters which involve interplanar separations, density-of-states, and carrier density, govern the outcome. Our mechanism favors holes over electrons, and works best in layered materials. We identify regions in parameter space where superconductivity is favored, and others where charge density waves are a possible outcome. We find a distribution of superconducting energy gaps, and obtain the resulting quasiparticle density-of-states.
THE ROLE OF QUASI-ONE-DIMENSIONAL STRUCTURES IN HIGH-Tc SUPERCONDUCTIVITY
Authors: BOGOLIUBOV N. M. , KOREPIN V. E.
Published: November 2004

The critical exponents describing the decrease of correlation functions on long distances for the one-dimensional Hubbard model is obtained. The behaviour of correlators shows that Cooper pairs of electrons are formed. The electron tunneling between the chains leads to the existence of the anomalous mean values and to the superconductive current. The anisotropy of the quasi-one-dimensional system leads to the rise of critical temperature T c .
ELECTRON HOLOGRAPHY AND AHARONOV-BOHM EFFECT
Authors: TONOMURA AKIRA
Published: November 2004

The Aharonov-Bohm (AB) effect was tested under conditions where an electron wave and a magnetic field did not overlap: the Meissner effect of the superconductor shielding a toroidal ferromagnet eliminated the leakage field. Using the newly-developed technique of electron holography, conclusive evidence for the AB effect was obtained by detecting a relative phase shift of π between the two electron waves passing through the hole and outside the toroid. The detected phase shift value of exactly π comes from the quantization of the magnetic flux within the superconductor. This quantization assures the complete shielding of the magnetic field.
EFFECTS OF CHEMICAL DOPING AND STOICHIOMETRIC VARIATION IN HIGH-Tc SUPERCONDUCTORS
Authors: KAO Y. H. , YAO Y. D. , SONG L. W.
Published: November 2004

Superconducting properties of various chemically-doped high-T c materials are investigated. By using the results obtained with Cu- and Bi-deficient Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O systems as basis, variations of the transition temperature T c , critical current density J c , room temperature resistivity ρ, and oxygen content are compared for different dopant contents. In several cases, J c and ρ are found to be correlated as both quantities show an exponential dependence on the dopant concentration. Most dopants result in a monotonic decrease of T c and J c , except for Ag, F, and Cl. Substitution of Ag and Pb for Cu and Bi in Bi-Sr-Ca-Cu-O indicates a condition for compatibility between the impurity atom and the local structure in the host matrix. Results suggest that the predominant mechanism for high-T c superconductivity is most likely associated with short-range-order structure in the material.
SINTERING CONDITIONS AND SUPERCONDUCTIVITY IN THE Y-Ba-Ca-Cu-O SYSTEM
Authors: VASSILEV P. G. , MARINOVA M. , DIMITROV I. , NENKOV K. , KOVACHEV V.
Published: November 2004

Superconducting samples from starting compositions Y ( Ba 1−x Ca x)2 Cu 3 O 7−δ at 0 ≤ x ≤ 1 are prepared at various T sint . It was found that the sintering temperature needs to be increased with x. Superconducting chains of Y-Ca-Cu-O with a volume fraction below 5% were formed in samples with a very high overall normal state resistivity, when sintered above 1070°C. The zero resistance point for such samples was at about 70–80 K.
PERCOLATION IN SUPERCONDUCTING CUPRATES: RESISTIVITY AND CRITICAL CURRENTS
Authors: HALBRITTER J.
Published: November 2004

Weak links in metallic cuprates yield complicated percolative conduction networks enhancing the intrinsic resistivity ρi(T) = αiT + ρi0L to: [Formula: see text] with ρb describing the weak links and a geometrical factor p ≥ 1. In most cases "single crystals" and "epitaxial" films show insulating intragrain weak links yielding p ≈ 3 and Σρb ≪ ρi(100 K ), like good sintered YBa 2 Cu 3 O x. Rarely are the intrinsic values αi ≈ 0.5 μΩ cm/K and ρi(100 K ) ≈ 50 μΩ cm observed. In the superconducting state the intrinsic resistivity pi(T) vanishes and then the critical transport current density jc (B, T) is given by the superconducting aspects of conducting weak links: [Formula: see text] Here, jCJ (jCG) stands for Josephson junctions at intergrain (intragrain) weak links and jCP for pinning. ps(B, T) stands for current path systems changing with B-field and temperature. jCJ and jCG depend crucially on the care in preparation and alignment of the [Formula: see text]-planes, where the current flows, but values jCJ (0.77 K ) ≈ 103 A/cm 2 and jCG (0.77 K ) ≈ 105 A/cm 2 seem typical. Weak links are related to planar defects where large and small angle grain boundaries are dominant. jCP is shadowed by jCJ and jCG but jCP ≈ 106 A/cm 2 are inferred from experiment.

The time-, temperature-, B-field- and voltage-dependencies of the current in — and in the transition to — the superconducting state are analyzed. Evidence is presented for arrays of microbridges being "electrically" the planar defects. These microbridges are typical superconductor-normal-conductor-superconductor (S-N-S) junctions, where pair tunneling is reduced by factors 10–103 as compared to single particle, normal tunneling. The reduced pair tunneling is due to normal conductors coating cuprates being related to O-disorder and the tetragonal phase, which also occur by radiation damage. The time and voltage dependence implies rapid flux motion or flux creep along the planar defects causing noise and rf residual losses.
DEEP LEVELS DUE TO VACANCY PAIRS IN SILICON
Authors: XU HONGQI , LINDEFELT U.
Published: November 2004

The recursion method is used to investigate the electronic structure of undistorted vacancy pairs in silicon up to the seventh nearest-neighbour divacancy. The many energy levels associated with these vacancy pairs in and around the band gap region are calculated. The results of the calculation show that the strength of the interaction between a pair of vacancies depends as much on their relative positions as on the inter-vacancy distance, but the mean value of the gap-state energy levels remains essentially constant at the monovacancy level.
NUMERICAL SIMULATION OF THE ISOTOPE EFFECT IN THE HIGH-Tc SUPERCONDUCTORS
Authors: GALBAATAR T. , RAKAUSKAS R-J. , SULSKUS J.
Published: November 2004

The results of a numerical simulation are presented for the oxygen isotope effect in the high-T c oxides by solving the Schrödinger equation for a local vibrational mode of oxygen ions in a double well anharmonic potential of the form −Ax2/2+Bx4/4. The features of the energy spectrum, eigenfunctions and the matrix elements of the dipole and quadrupole moments are discussed. The applicability region of a two-level approximation to describe the electron-phonon interaction is given. The constant of the isotope effect α is determined to be comparable to that of La 2−x Sr x CuO 4 but in disagreement with the up-to-date experimental observations for the YBaCuO system.
ISING MODEL ON A LAYERED SQUARE LATTICE
Authors: LIN K.Y. , HSU K.J.
Published: November 2004

We have considered the Ising model on a layered square lattice where each layer has a different set of horizontal and vertical interactions. The free energy is determined exactly by the method of Pfaffian at two values of the magnetic field, H=0 and H=iπkT/2. The free energy at H=0 was first derived by Wolff et al. using the method of transfer matrix.
THE MICROSCOPIC APPROACH IN THE THEORY OF SUPERCONDUCTORS WITH A MANY-COMPONENT ORDER PARAMETER
Authors: IZYUMOV Y.A. , LAPTEV V.M. , SYROMYATNIKOV V.N.
Published: November 2004

In terms of the generalized Bardeen-Cooper-Schrieffer model for a superconductor with degenerate orbital electron states, we derive a Ginzburg-Landau energy functional with a microscopic deciphering of the coefficients involved. The superconducting order parameter is separated out by expanding the wave function of a Cooper pair in the basis functions of crystal point group irreducible representations. It is shown that a many-component superconducting order parameter may arise from the degeneracy of the electron states on the Fermi surface—either owing to the degeneracy of the orbital states or owing to the degeneracy over the arms of the star, or as a result of both degeneracies simultaneously. Allowance for the orbital degeneracy extends the physical basis on which superconducting modes are realized and perturbs the classification of singlet and triplet states according to even and odd point group representations. It is also shown that allowance for the orbital degeneracy may alter the conditions for the superconducting gap to go to zero in individual Fermi surface points or lines as compared with the conditions that follow from arm degeneracy alone.
ON THE LATTICE INSTABILITY BETWEEN 100 AND 300 K OF YBa2Cu3O7−y HIGH Tc SUPERCONDUCTING CERAMIC
Authors: HE Y. , XIANG J. , LIN S. , LIU Z. , CAI L. , SUN X. , CHANG S. , LI F. , CUI W. , LIN C. , DU J. , LI J. , LI C. , YIN D.
Published: November 2004

The lattice instability of YBa 2 Cu 3 O 7−y superconducting ceramic between 100 and 300 K has been studied by means of ultrasonic measurements, TEM study, X-ray diffraction experiments, thermal analysis and specific heat measurements for samples from different laboratories. It was shown that there are anomalous changes in lattice constants near 250 K, 170 K and 130 K, together with specific heat anomalies near this temperature region, which is consistent with the longitudinal and transverse ultrasonic results. Such anomalies are caused by an intrinsic lattice instability related to the ordering readjustments of the oxygen vacancies, which was confirmed by the observed tendency of reversible orthohombic to tetragonal phase transition near 170 K.
CONSTRUCTION OF MEDIAN ITINERARIES WITHOUT USING ANTI-HARMONICS
Authors: ZHENG WEI-MOU
Published: November 2004

A theorem for the co-existence of three adjacent maximal periodic itineraries is proved. Based on this period window theorem a modified version of the MSS-algorithm for median itineraries is presented which avoids the use of the artificial anti-harmonics. The construction rule is then generalized to the antisymmetric cubic map, and the U-sequence for the map may be obtained in a natural way.
DYNAMICS IN DICKE MODEL
Authors: KADANTSEVA E.P. , CHMIELOWSKI W. , SHUMOVSKY A.S.
Published: November 2004

We derive an exact differential operator equation for the operator level populations. Upon averaging over the initial state of the system and applying a mean-field approximation, we reduce the operator equation to an equation for c-numbers. Particular cases of one-and two-photon processes are considered for which the equations are integrated in quadratures and the solutions are the Weierstrasse elliptic functions. The approach allows us to describe a periodic emission of an electromagnetic field in the perfect resonator that alternates with absorption of the field for arbitrary photon multiplication of the transition. Superradiative intensity have a specific asymmetry with respect to the time delay.

It is shown that the system of two-level atoms is in the upper state for a longer time than in the lower state. Switching-on of detuning increases the time of collectivization. The results are in agreement with experiment.
SINGLE SPIN FLIP IN THE INFINITE U HUBBARD MODEL: HUBBARD OPERATORS, THREE-BODY FADEEV EQUATIONS AND GUTZWILLER WAVE FUNCTIONS
Authors: Ruckenstein Andrei E. , Schmitt-Rink Stefan
Published: November 2004

We investigate a recently proposed many-body theory for composite (Hubbard) operators (A.E. Ruckenstein and S. Schmitt-Rink, Phys. Rev. B38, 7188 (1988)) in the context of the problem of a single spin flip in the saturated ferromagnetic state of the infinite U Hubbard model. We prove that the suitably defined strong coupling Hartree-Fock mean field theory leads to results identical to those obtained from the Gutzwiller wave function through exact evaluation of the kinetic energy. Most interestingly, we also show how exactly the same results can be obtained starting from the weak coupling limit by solving analytically the three-body t-matrix (Fadeev) equations in the infinite U limit. This work also sheds light on the physical content of slave boson approximations to which our approach was previously shown to be equivalent in the limit of large spin or orbital degeneracy. For the single spin flip problem we compare our results with those obtained by Bethe-Goldstone perturbation theory, Bethe Ansatz in one dimension, and exact diagonalization studies.
PRELIMINARY GROUP ANALYSIS OF A 4 X 4 TWO DIMENSIONAL HUBBARD MODEL
Authors: Fano G. , Ortolani F. , Semeria F.
Published: November 2004

A finite size two-dimensional Hubbard model is considered; it is shown that the consideration of the spatial symmetry group of the Hamiltonian reduces the dimension of the problem of two orders of magnitude in the case of a 4 X 4 lattice. Exact diagonalization of the Hamiltonian can then be performed with the aid of modern high-speed computers.
Incommensurate Antiferromagnetism and Magnetic Domain Walls in the Two-Dimensional Hubbard Model
Authors: SCHULZ H.J.
Published: November 2004

The antiferromagnetism of the two-dimensional Hubbard model is investigated analytically and numerically in the vicinity of half-filling. It is found that for a small but finite concentration of holes a regularly spaced array of domain walls is formed, with the holes localized at the domain walls, i.e. one has an insulating incommensurate antifer romagnet, with a linearly polarized magnetisation pattern and a modulation wavevector proportional to the hole concentration. Above a critical concentration of holes, pockets of free carriers appear, coexisting with incommensurate antiferromagnetic order. Only at even higher hole concentration does the antiferromagnetism vanish completely. Based on a Landau expansion, the different possible antiferromagnetic states are discussed.
Self-Consistent Hole Motion and Spin Excitations in A Quantum Antiferromagnet
Authors: Su Z.B. , Li Y.M. , Lai W.Y. , Yu L.
Published: November 2004

A new quantum Bogoliubov-de Gennes (BdeG) formalism is developed to study the self-consistent motion of holes and spin excitations in a quantum antiferromagnet within the generalized t-J model. On the one hand, the effects of local distortion of spin configurations and the renormalization of the hole motion due to virtual excitations of the distorted spin background are treated on an equal footing to obtain the hole wave function and its spectrum, as well as the effective mass for a propagating hole. On the other hand, the change of the spin excitation spectrum and the spin correlations due to the presence of dynamical holes are studied within the same adiabatic approximation. The stability of the hole states with respect to such changes justifies the self-consistency of the proposed formalism.
RAMAN MODES AND STRONG INTRAPLANAR OXYGEN-OXYGEN CHARGE FLUCTUATIONS IN YBa2Cu3O7
Authors: Barišić S. , Kupčić I. , Batistić I.
Published: November 2004

It is shown that symmetric and antisymmetric (340 cm −1) Raman modes are strongly coupled respectively to the intracell, intraplanar oxygen-copper and oxygen-oxygen charge fluctuations in YBa 2 Cu 3 O 7. The dominant coupling arises from the variations of the crystal field with deformations. The anomalous behavior of the 340 cm −1 mode is thus attributed to the corresponding behavior of the in-plane oxygen-oxygen charge correlation function. The symmetric modes are continuous at Tc, and this contrast is examined within the existing theories. It appears that the Emery model is a suitable starting point for the explanation of the Raman mode behavior. The increase of the ep coupling from one high Tc family to another turns out to correlate with increase of Tc.
SUPERCONDUCTING GLASSY STATE IN THE EXTENDED HUBBARD MODEL WITH OFF-DIAGONAL DISORDER
Authors: Kopeć T.K. , Wróbel P.
Published: November 2004

The phase diagram of the extended Hubbard model with on-site attraction and random inter-site Coulomb energies is studied for the half-filled band case. In the strong coupling limit the problem is mapped onto the system of hard-core bosons (bipolarons) on a lattice, described by the anisotropic pseudo-spin model with infinite-range random exchange interactions—in close analogy with the Sherrington-Kirkpatrick spin glass approach. It is found that the disorder and frustration strongly affects properties of the system leading to the appearance of the bipolaronic superconducting (BSC) phase, the bipolaronic superconducting glass (BSCG) state as well as the bipolaronic charge glass (BCG) phase, depending on the temperature range and the degree of the disorder.
CBED AND SAED STUDY OF HIGH Tc Ba-Y-Cu-O SUPERCONDUCTORS
Authors: YANG C.Y. , ZOU X.D. , ZHOU Y.Q. , FUNG K.K.
Published: November 2004

We have studied two kinds of superconducting samples of Tc about 90K with nominal compositions Ba3Y2Cu5O12+x and Ba0.5Y4.5Cu5O12+x, respectively, by means of convergent beam electron diffraction (CBED) and selected area electron diffration (SAED) in more detail. The common phase has been found to be a pseudotetragonal superstructure based on a cubic perovskite subcell of a=0.39 nm. Its lattice parameters are a=ap, c=3ap and its space group is Pmmm. This phase is considered to be responsible for the superconductivity at about 90K. This is the major phase in Ba3Y2Cu5O12+x, but the minor phase in Ba0.5Y4.5Cu5O12+x. The lattice parameters and the symmetry of the other phases in Ba0.5Y4.5Cu5O12+x have also been determined.
THE YBa2Cu3O7−δ SYSTEM: STRUCTURE, STOICHIOMETRY AND ORBITAL ORDERING AND THEIR ROLE IN SUPERCONDUCTIVITY
Authors: SUBBANNA G.N. , GANGULY P. , RAO C.N.R.
Published: November 2004

In this communication, alternative structures of YBa2Cu3O7−δ (δ=0.0−0.5) are discussed making use of the results from thermogravimetric analysis and electron microscopy. Possible orbital orderings in YBa2Cu3O7 corresponding to the different structures are also discussed. Magnetic susceptibility data of the non-superconducting δ=0.5 phase are briefly examined.
A PHYSICAL MODEL FOR HIGH TcSUPERCONDUCTIVITY OF THE OXYGEN DEFICIENT PEROVSKITE Y-Ba-Cu-O
Authors: GAN ZI-ZHAO , DAI YUAN-DONG , HAN RU-SHAN
Published: November 2004

The high Tc superconductivity of the oxygen deficient perovskite Y1Ba2Cu3O7 has a closed relation to the ordering distribution of the cations with various valencies and the oxygen vacancies in this structure. The special electronic structure related to such ordering distribution leads to the phonon softening and the excitonic mechanism for superconducting pairing. These effects provide a physical model for understanding the high Tc superconductivity in this material.
TERBIUM MOLYBDATE: GROUP THEORY AND FLUCTUATIONS
Authors: GALAM SERGE
Published: November 2004

A new mechanism to explain the first order ferroelastic—ferroelectric transition in Terbium Molybdate (TMO) is presented. From group theory analysis it is shown that in the two-dimensional parameter space ordering along either an axis or a diagonal is forbidden. These symmetry-imposed singularities are found to make the unique stable fixed point not accessible for TMO. A continuous transition even if allowed within Landau theory is thus impossible once fluctuations are included. The TMO transition is therefore always first order. This explanation is supported by experimental results.
HIGH-Tc SUPERCONDUCTIVITY IN THE Y-Ba-Al-Cu OXIDES SYSTEM
Authors: XU YUHUAN , LI ZHONGRONG , LI CHAORUI , LIU BIN , LIN XIANDUAN , WANG HONG , LI WU
Published: November 2004

The ceramics with the composition of YBa2AlxCu3−xO9−n(0.5≤x≤2) have been synthesized directly from a solid-state reaction of Y2O3, BaCO3, A12O3, and CuO by general ceramic manufacture. The stable high-Tc superconductivity with a resistive mid-point of 101K and zero resistance temperature of 99K has been observed in ceramic samples with the composition of YBa2Al0.5Cu2.5O9−n.
ELECTRON MICROSCOPY STUDY OF THE SUPERCONDUCTOR “Bi2Sr2CaCu2O8+δ”
Authors: HERVIEU M. , MICHEL C. , DOMENGES B. , LALIGANT Y. , LEBAIL A. , FEREY G. , RAVEAU B.
Published: November 2004

The structural study of the superconductor Bi 2 Sr 2 CaCu 2 O 8+δ was performed by X-ray diffraction on a single crystal, electron diffraction and electron microscopy. The structure of this oxide which crystallizes in the space group Amaa, with [Formula: see text] and c≃30.7 Å can be described as an intergrowth of an oxygen deficient double perovskite layer [Sr2CaCu2O6]∞ with ( BiO y)2 layers. The E.D. patterns show the existence of satellites in incommensurate positions. The HREM images and their simulation suggest that the [BiOy]∞ layers are not [ Bi 2 O 2] Aurivillius type layers. Modulations along the a axis and much more rarely along b are observed. Stacking defects are rarely observed along c: they correspond to the intergrowth of a phase Bi 2 Sr 2 CuO 6+δ which is assumed to be the superconductor at 22K previously detected for the nominal composition “ Bi 2 Sr 2 Cu 2 O 7”.
SPIN PEIERLS INSTABILITY AGAINST S-LIKE ANISOTROPIC SUPERCONDUCTIVITY IN FRAMEWORK OF THE MEAN FIELD RVB-HUBBARD MODEL
Authors: WRÓBEL P. , JACAK L.
Published: November 2004

It is shown theoretically that the superconducting transition in the framework of RVB mean field treatment in nearly half-filled band Hubbard model is substantially influenced by spin density wave instability. The reasonable SDW and SC ordering phase diagram for doped La 2 CuO 4 compounds is found.
THE 120K-SUPERCONDUCTING PHASE IN Bi-Ca-Sr-Cu-O
Authors: MENG R.L. , HOR P.H. , SUN Y.Y. , HUANG Z.J. , GAO L. , XUE Y.Y. , WANG Y.Q. , BECHTOLD J. , CHU C.W. , HAZEN R.M. , PREWITT C.T. , ANGEL R.J. , ROSS N.L. , FINGER L.W. , HADIDIACOS C.G.
Published: November 2004

The formation of the high temperature superconducting phases in the quinary Bi-Ca-Sr-Cu-O compound system has been investigated. The 120K-superconducting transition appears in samples prepared between 850 and 900°C. A certain homogeneity range is found to exist in this superconducting system, suggesting that higher Tc may be achievable in the same system.
HIGH Tc SUPERCONDUCTIVITY AND SUBSTRUCTURE OF Bi-Sr-Ca-Cu-O SYSTEM
Authors: ZHU XING , FENG SUNQI , LU GUO , WEI CHONGDE , ZHANG HANG , SHEN ZHAOHUI , ZHANG WENBIN , GU HUI , ZHANG JIAN , WEN QINGZHE , CHEN KAIXUAN , XUE LIXIN , YIN DAOLE , GAN ZIZHAO , ZHU JING , LIN XIWEI
Published: November 2004

Bi x Sr 1 Ca 1 Cu y O z(x, y=1, 2 or 3) high Tc superconductors were synthesized by solid state reaction method. There are two superconducting phases in Bi-Sr-Ca-Cu-O system: 110 K phase and 80 K phase. The relative portion of the two phases and the state of superconducting connection are closely related to chemical compositions and the conditions of processing. X-ray powder diffraction shows that there exists a main phase with orthorhombic symmetry in the samples of different compositions and conditions of processing. According to X-ray diffraction data, the lattice constants of the main phase in the sample of nominal composition Bi 1 Sr 1 Ca 1 Cu 2 O z have been determined as aο=5.44Å, bο=5.35Å and co=30.66Å. Electron diffraction shows complicated modulation structure along b− and c− axes. High resolution image along [001] zone axis indicates the modulation structure along b− axis.
Tl2Ba2CaCu2O8: STRUCTURE AND SUPERCONDUCTIVITY
Authors: MAIGNAN A. , MICHEL C. , HERVIEU M. , MARTIN C. , GROULT D. , RAVEAU B.
Published: November 2004

Tl 2 Ba 2 CaCu 2 O 8 has been synthesized as a pure phase by reaction in sealed quartz tube. The structural model of this oxide has been established from X-ray diffractograms and high resolution electron microscopy. This tetragonal structure (a=3.860 Å, c=29.388 Å, space group I4/mmm) can be described as formed of [ Ba 2Tl2Cu2 O 8]∞ layers interleaved with calcium ions. These layers are themselves described as the intergrowth of sodium chloride type [TIO] ∞ layers of corner-sharing pyramids [ BaCuO 3]∞ leading to the formulation [( BaCuO 3)2 (TIO) 2]∞. Such a perfect structure Tl2 Ba 2 CaCu 2O8, which contains only Cu(II) cannot be at the origin of the superconductivity at 110K. The superconducting properties may in fact be explained by the observation by HREM of extended defects of local composition “ TlBa 2 CaCu 2O7” for which a structural model is proposed. In a more general way the creation of thallium deficiency, according to the formulation Tl2−x Ba 2 CaCu 2 O 8 could be at the origin of superconductivity.
ANOMALOUS BEHAVIOR ABOVE Tc AND 90K SUPERCONDUCTIVITY IN YBa2Cu3Ox
Authors: XIAOHUA CHENG , LINHAI SUN , YENING WANG , HUIMIN SHEN , ZHENG YU
Published: November 2004

Internal friction and lattice parameter were measured by Marx’s three components oscillator and X-ray polycrystalline diffraction respectively. As a function of temperature, there exist a few anomalies above Tc, of which the 110K-130K anomaly is found to be closely connected with 90K superconducting transition, but no obvious relation was observed for the 210K anomaly. In addition, a relaxation type internal friction peak was measured at about 100K.
ANALYTICAL SPECTRA AND INTENSITY-INTENSITY CORRELATIONS IN COOPERATIVE OPTICAL DOUBLE RESONANCE
Authors: JAGATAP B.N. , LAWANDE S.V.
Published: November 2004

We study the cooperative effects in the fluorescent radiation emitted from a system of N three-level atoms driven by two intense coherent fields. It is shown that the analytical expressions for fluorescent spectra and intensity-intensity correlations can be obtained for arbitrary N in a special case. These expressions manifest the essential cooperative effects.
Co-EVAPORATED HIGH TcY-Ba-Cu-O FILMS
Authors: IVANOV Z.
Published: November 2004

A co-evaporated Y-Ba-Cu-O film on a sapphire substrate, which was post-annealed in oxygen at 930°C could show a sharp superconducting transition, 89–93K, when the composition agreed with the stoichiometric one. Semiconducting and hysteretic behaviors were registered in other cases.
HIGH RESOLUTION ELECTRON MICROSCOPY OF THE HIGH Tc SUPERCONDUCTOR Bi2Sr2CaCu2O8+δ
Authors: HERVIEU M. , DOMENGES B. , MICHEL C. , RAVEAU B.
Published: November 2004

The new superconductor Bi 2 Sr 2 CaCu 2 O 8+δ with T c ranging from 80 to 105K, was studied by electron microscopy. The electron diffraction study shows a pseudo-tetragonal symmetry with a≈b≈5.4 Å and c≈30.7 Å and satellites along a, which settle in an incommensurate way. The high resolution images agree with the proposed basic structure. The stacking of the ( BiO y)2 and [ Sr 2 CaCu 2 O 6] layers is quite regular, with only some defects corresponding to c≈24 Å. The lamellar character of the oxide results in splitting and bending of the crystals.
DISSIPATIVE QUANTUM TUNNELLING AND THERMAL ACTIVATION: AN EXACTLY SOLVABLE MODEL
Authors: DEKKER H.
Published: November 2004

By balancing energy diffusion (inside a local potential hole) with ohmic quantum tunnelling (through a parabolic barrier), an exact quasi-equilibrium metastable state is found with a decay rate which includes Bell’s result (for moderate damping), Melnikov’s formula (for weak damping), the Büttiker, Harris and Landauer expression (in the classical limit), both Kramers’ weak and strong damping results, and the correct quantum limit as special cases.
SUPERCONDUCTING FLUCTUATION EFFECT IN YBa2Cu3O7−δ FILMS UP TO 170~180K
Authors: AKINAGA MASAHIRO , ABUKAY DOGAN , RINDERER LÉO
Published: November 2004

We have prepared high-T c YBa 2 Cu 3 O 7−δ films onto the cooled substrates by RF magnetron sputtering. The temperature dependences of the electrical resistivity and I-V characteristics of these films have been measured in detail. The very large superconducting fluctuation effect up to 170~180K is strongly suggested from the unusual experimental results about both the resistance behavior and I-V characteristics due to superconducting weak link.
A BRIEF REPORT ON THE CURRENT STATUS OF RESONATING-VALENCE-BOND THEORY
Authors: ZOU Z. , HSU T.C.
Published: November 2004

We report on the current status of the Resonating-Valence-Bond theory, reviewing recent progress made by the Princeton group. We outline new developments concerning the topology of spinons, the geometric “meron” description, the SU(2) gauge theory for the insulating state and the possible connection between fractional quantum Hall state and an RVB state on a square lattice. The current understanding of the normal metallic and superconducting states is also summarized.