bluuebunny/crossref_metadata_2025 huggingface dataset:
Superconductivity Deduced from Deterministic Theory of Atomic Structure
Authors: Lu K. U.
Published: October 2003

The standard Quantum Mechanics is statistical in nature and cannot provide a detailed picture of an atom. The Deterministic Theory of Atomic Structure, derived from a system of equations that govern the electrodynamics in atoms, can account for the stability of the atoms. We describe the chirality of an atom, the rules for forming molecules, and the assemblies of the superconducting material from Aluminum and Silicon.
On the Ground State of Hubbard Model with Strong Repulsion
Authors: Mikhailova Ju. V.
Published: October 2003

For square (cubic) Hubbard lattice with infinite repulsion energy U exact result has been obtained: the ferromagnetic state with maximum total spin is not the ground state of system, if the hole number is equal to two.
The Electronic Liquid Model of Superconductivity
Authors: Peng Kuangding
Published: October 2003

We propose an electronic liquid model of superconductivity. We assume that in a crystal most of the movable electrons are in the liquid phase. Applying the quasi-crystal lattice model of a liquid and Toda's potential, we derive the existence of solitary waves of electron pairs described by a GL equation. From this, superconductivity, Josephson effect and Meissner effect can be proved. We then derive an approximate formula for T c .
Polaron Crystallization and the Metal–Insulator Transition
Authors: Quémerais P. , Fratini S.
Published: October 2003

The Crystallization of polarons at finite density, due to the long-range Coulomb forces — when no bipolarons can be formed — is discussed close to the metal–insulator transition (MIT). As a function of the density, the melting is examined at zero temperature. By calculating the quantum fluctuations of both the electron and the polarization, we show that at strong electron–phonon coupling the dissociation of the polarons at the MIT is favored, rather than the melting towards a polaron liquid. In this regime, we demonstrate, that an instability of the transverse vibrational modes of the polaron crystal occurs as the density increases. This provides a new physical mechanism for the MIT in polar materials, for which an experimental signature is predicted.
Interface Phonon Modes in Strained Semiconductor Superlattices
Authors: Ren Shang-Fen , Stanfield Jason
Published: October 2003

Phonon modes in strained ZnTe/CdSe superlattices are studied. The macroscopic interface modes and two different types of microscopic interface modes are identified. Interface phonon modes in (ZnTe)8(CdSe)8 superlattice with interchange of atomic layers across interfaces are calculated and compared with the results of superlattice with ideal interfaces.
The Simple Generalized BCS-Type Model for the Acoustic Plasmon Induced d-Wave Superconductivity in High-Tc Layered Cuprate Compounds
Authors: Semenov A. V.
Published: October 2003

A simple analytically treated model is presented for high-T c superconductivity in layered cuprate compounds. This model assumes a cylindrical Fermi-surface with circular cross-section, but anisotropic Fermi-velocity (density of states), which is approximated by a step-like function on azimuth angle. The system under these assumptions is shown to support in the long wavelength limit collective charge-density excitations with acoustic spectrum (acoustic plasmons), which suppress the Coulomb interelectron repulsion for small transferred momenta. This result leads to the second essential feature of the proposed model: besides the standard assumptions of the BCS scheme the effective potential of interelectron interaction is characterized by additional step-like dependence on the absolute value of two-dimensional transferred quasimomenta. The effective constants of the Cooper pairing in anisotropic s- and d-wave channels are evaluated. It is shown, that the plasmon mechanism gives rise to the pairing in the d-wave channel despite the fully isotropic (in two dimensions) model potential. The Umklapp processes are taken into account and are shown to increase the pairing constants.
Transport Properties of Normal and Quasinormal States of Poor Superconductors
Authors: Gantmakher Vsevolod
Published: October 2003

There are some doubts whether materials near the superconductor–insulator transition (SIT), either on the insulating side, or with superconductivity suppressed by the magnetic field, behave like an ordinary metal. Two experiments in which this issue is addressed will be reviewed here, namely, transport measurements of two different materials: single crystals YBa2Cu3O 6+x (x ≈ 0.37) — a representative of the HTS family and amorphous InO x where x is the oxygen content which could be changed through moderate-temperature annealing. The low-temperature normal resistivity ρ(T) of YBaCuO crystals on both sides of SIT behaves as usual "bad" metal with conductivity below the Mott's minimum value. The amorphous In-O films behave differently. Those films with superconductivity do not display any characteristic magnetic field which can be interpreted as Bc2. Instead, they give out an example of a quantum phase transition and follow Fisher's model1 for "the field-tuned SIT in disordered two-dimensional superconductors" though they are not precisely two-dimensional and their high-magnetic-field state may not be insulating at all. Apparently, the normal state of InO x films comprises localized Cooper pairs.
Internal Pressure Effect on Some Manganite Oxides
Authors: Huang C. Y. , Gundakaram R. , Lin J. G.
Published: October 2003

In this paper, we investigate the effects of internal pressure on various colossal magnetoresistance materials. By substituting a smaller ion for a larger ion, one can exert the lattice pressure to the crystal structures and, hence, changes the physical properties of these materials. Since the characteristic of the perovskite oxides is a layered structure, the created strains owing to the substitutions are mostly anisortropy and the resulting changes are presumably complicated. We have found that the substitutions can fine-tune the electronic structures of various materials and can result in many interesting phenomena.
Photo-Assisted MOCVD Fabrication of YBCO Thick Films and Buffer Layers on Flexible Metal Substrates for Wire Applications
Authors: Ignatiev A. , Chou P. C. , Zhong Q. , Zhang X. , Chen Y. M.
Published: October 2003

Photo-assisted MOCVD has been developed for application to high-rate, high quality YBa2Cu3O 7-δ thin film production. This technique is now being applied to the fabrication of thick film YBa2Cu3O 7-δ on flexible metal substrates for wires and tapes. The unique application of photo-irradiation as the sole source of energy for the MOCVD reaction has resulted in extremely rapid growth rates for YBCO films with accompanying high superconducting performance. Typical YBCO films of thickness >1 μ m exhibit J c ≥ 1 × 106 A/cm2 and T c > 90 K on single crystal oxide substrates. Such films have now been integrated to metal substrates such as single crystal Ni(100) and roll-textured nickel foil. YBCO films grown on textured Ni foils with CeO2 and YSZ buffer layers show J c ≈ 3 to 5 × 105 A/cm2 for films of 1 μ m thickness grown in 2 to 4 minutes time. Such performance for thick films made in such short growth time is promising for YBCO wire fabrication. The development of effective YBCO wire through thick film growth on flexible substrates also requires buffer layer application. Photo-assisted MOCVD has also been applied to buffer layer formation with promising results.
Electronic Properties of Layered Oxides: Pulsed Laser Deposition of YBCO Films for In-Situ Studies by Photoemission Spectroscopy
Authors: Pavuna D. , Ariosa D. , Berger H. , Christensen S. , Frazer B. , Gatt R. , Grioni M. , Margaritondo G. , Misra S. , Onellion M. , Schmauder T. , Vobornik I. , Xi X. , Zacchigna M. , Zwick F.
Published: October 2003

Due to imperfect surfaces of most cuprate samples, almost all Photoemission studies in the past decade were performed on Bi2Sr2CaCu2O 8+x, even though a large fraction of other studies and electronic applications was reported for YBa2Cu3O 7-δ (YBCO) family of superconducting compounds. In order to systematically study the gap parameter and the Fermi surface variation in high symmetry directions of YBCO and related oxide films we have constructed a new facility at the Wisconsin Synchrotron Radiation Center. We use the pulsed laser ablation (PLD) system that is directly linked to the photoemission chamber. In our unique approach, the samples never leave the controlled ambient and we oxidize our films, either by molecular oxygen or by ozone. In this paper, we, summarize some of the most relevant recent results on electronic properties of layered oxides and describe our new facility for the study of YBCO and related oxide films.
Positron Annihilation in Superconducting 123 Compounds
Authors: Peter M. , Manuel A. A. , Erb A.
Published: October 2003

After a brief review of the theory of angular correlation of positron annihilation radiation (ACAR), we illustrate experimental principles and give examples of successful determination of electron momentum density (EMD) and of positron lifetime in solids. The central question which we try to answer concerns the contribution of positron spectroscopy to the knowledge and understanding of the new high temperature superconducting oxides. We find that in these oxides also, partially filled bands exist and we can observe parts of their Fermi surface and measure lifetimes in accordance with band theoretical calculations. There are characteristic differences, however. The intensity of the anisotropy of the ACAR signal is below theoretical expectation and signals depend on sample preparation. Recent studies by the Geneva group have concerned dependence of the signals on impurities, on oxygen content and on the thermal history of preparation. Of particular interest are correlations between the variations of these signals and between the variations of structural and transport properties in these substances. Besides deliberate additions of impurities, the Geneva group also reports progress in the preparations of samples of highest purity (barium zirconate crucibles). The alloy series Pr x Y 1-x Ba2Cu3O 7-δ is of special interest because of exceptional transport properties. The recent positron results on these materials will be presented and commented in the light of theoretical models and in the light of the reported superconductivity of the Pr-compound.
NMR of High-Tc Superconductors
Authors: Asayama K. , Kitaoka Y. , Zheng G.-Q. , Ishida K. , Magishi K. , Tokunaga Y. , Yoshida K.
Published: October 2003

Recent NMR measurements on high-T c materials by our group are reviewed. Study of YBa2Cu3O7+Ni has revealed that the attractive force is spin in origin. Analysis of T1, the Knight shift K and T2G in Tl2Ba2Ca2CuO10 (Tl2223), HgBa2Ca2Cu3O8 (Hg1223), HgBa2Ca3Cu4O10 (Hg1234) and YBa2Cu4O8 (Y1248) supports the antiferromagnetic spin fluctuation (AFSF)-mediated-superconductivity-model. The temperatures at which a pseudogap starts to open obtained by 1/T1T, dK/dT, photoemission, resistivity and STM all agree with each other in Bi2Sr2CaCu2O8 (Bi2212). The origin of the pseudogap is discussed.
Cooper Pair-Like Systems at High Temperature and their Role on Fluctuations Near the Critical Temperature
Authors: Ausloos M. , Dorbolo S.
Published: October 2003

A logarithmic behavior is hidden in the linear temperature regime of the electrical resistivity R(T) of some YBCO sample below 2T c where "pairs" break apart, fluctuations occur and "a gap is opening". An anomalous effect also occurs near 200 K in the normal state Hall coefficient. In a simulation of oxygen diffusion in planar 123 YBCO, an anomalous behavior is found in the oxygen-vacancy motion near such a temperature. We claim that the behavior of the specific heat above and near the critical temperature should be reexamined in order to show the influence and implications of fluctuations and dimensionality on the nature of the phase transition and on the true onset temperature.
Superconductivity in (110)-PrBa2Cu3O7-δ Thin Films Pseudomorphically Grown on (110)-YBa2Cu3O7-δ Single Crystal Substrates
Authors: Usagawa Toshiyuki , Ishimaru Yoshihiro , Wen Jianguo , Utagawa Tadashi , Koyama Satoshi , Enomoto Youichi
Published: October 2003

The vertical four-probe measurements for (110)- PrBa2Cu3O 7-δ/ YBa2Cu3O 7-δ bilayers pseudomorphically grown on (110)- YBa2Cu3O 7-δ (YBCO) single crystal substrates show clearly two superconducting transition steps associated with two different T c 's of 92.0 K of YBa2Cu3O 7-δ and 75.5 K for 220 nm thick PrBa2Cu3O 7-δ. The simultaneous PrBa2Cu3O 7-δ/ YBa2Cu3O 7-δ bilayers on (001)- YBa2Cu3O 7-δ (YBCO) single crystal substrates show a non-superconducting nonlinear I-V curve.
Pressure Effects on Oxygen Deficient Superconducting PrBa2Cu3Ox
Authors: Ye Jinhua , Zou Zhigang
Published: October 2003

Resistive measurements in a cubic anvil apparatus showed that the T c of an oxygen deficient PrBa2Cu3O x can be increased by 50 K by applying a pressure of 9.3 GPa. The enhancement rate dT c /dP is as large as 7.8 K/GPa at initial stage, decreases with increase of applied pressure, but still keeps positive value in the experimental range of 10 GPa. The unusually large T c enhancement far exceeds that observed for the isostructural YBa2Cu3O x compounds, where the T c was reported to saturate at around 92 K. X-ray diffraction under high pressure indicated that the compressibility of PrBa2Cu3O x is about 7.4 × 10-3 GPa -1, a value comparable to that of YBa2Cu3O x.
Characterization of Superconducting PrBa2Cu3Ox
Authors: Zou Zhigang , Ye Jinhua , Oka Kunihiko , Nishihara Yoshikazu
Published: October 2003

The crystal structures of superconducting and nonsuperconducting PrBa2Cu3O x crystals were investigated using a single crystal X-ray precession camera. A large amount of stacking faults were observed in h0l reflections of as-grown supconducting PrBa2Cu3O x single crystals. These diffusing could be relaxed after oxygen annealing. Meanwhile, intensities of 003n (n = 1, 2, 3…) diffraction reflections of annealed crystal were increased significantly, indicating that the structural ordering of 1/3n unit cell was improved. Magnetic measurement exhibited that the superconducting state of PrBa2Cu3O x , is sensitive to magnetic fields and the magnetic fields dependence of the flux melting temperature showed that the sample has the smaller critical magnetic field than YBa2Cu3O x.
Anomalous Thermal Expansion in HTS
Authors: Anshukova N. V. , Golovashkin A. I. , Ivanova L. I. , Krinetskiy I. B. , Rusakov A. P.
Published: October 2003

The data about anomalous thermal expansion α(T) at low temperatures are presented for number of oxide HTS system. The anomaly of thermal expansion in such systems is explained in the terms of charge density waves instability.
PrBa2Cu3Ox Superconducting Powders
Authors: Cooley J. C. , Hults W, L. , Peterson E. J. , Smith J. L.
Published: October 2003

We have been preparing polycrystalline PrBa2Cu3O x (PBCO) to look for superconductivity at 89 K as has been reported in single crystals and thin films. We have found two transitions one at ≈ 90 K and another one at ≈ 12 K in samples, which we are still unable to make single phase. Additional phases include PrBaO 3 and BaCuO 2. The PBCO samples all appear to have the usual YBCO crystal structure and this is the phase that we believe contains the ≈ 90 K transition. The ≈ 12 K transition is most likely antiferromagnetic ordering of the PrBaO 3 phase. We are continuing to investigate sample preparation. Here we present and discuss the superconducting and magnetic properties of our samples.
Variability of Superconducting Transition Temperatures of Ag-Clad Bi(2223) Tapes
Authors: De Mayo Ben
Published: October 2003

The zero-field intersample variability of the superconducting transition temperature was measured for two commercially supplied Ag-clad Bi2Sr2Ca2Cu3O x, (Bi(2223)) 37 filament tapes, one 103 m and the second 193 m long. The resistivity of six 30 cm long pieces of each tape was measured with increasing and decreasing temperature. Hysteresis and time-dependent effects were observed. For the 103 m tape, upon cooling, the resistivity deviates from the conventional behavior at (114.3.3.2) K and complete superconductivity is achieved at (105.1.3.9) K. With increasing temperature deviation from superconductivity starts at (112.5.4.6) K and superconductivity is completely gone at (121.8.3.1) K. The corresponding temperatures for the 193 m tape are (113.4.4.9), (108.6.3.6), (108.9.7.4) and (118.1.7.4) K. For one sample of the 193 m tape, the corresponding temperatures are (107.9.3.1), (102.5.5.9), (100.8.4.3) and (112.0.3.3) K. The anomalously broad foot structure in the zero field resistive transition previously seen by Zhao et al. (Ref. 1) in single crystals of Bi2Sr2CaCu2O x, (Bi(2212)) was observed here, too.
Anisotropic Upper Critical Field of Single Crystal RNi2B2C (R = Y, Lu)
Authors: Du Mar A. C. , Rathnayaka K. D. D. , Naugle D. G. , Canfield P. C.
Published: October 2003

Measurements of the anisotropic upper critical field curves have been extended for YNi 2B2C and LuNi2B2C (T c = 15.6 K and 16.1 K, respectively). Measurements of R(T, H) were taken along the crystallographic directions of <100> and <110>. The resulting critical fields show an upward curvature near T c and a linear behavior at low temperature, which continues to below 2 K with little, if any, sign of saturation.
Flux Pinning and Defects in As-Grown and Neutron Irradiated Y-123 Crystals
Authors: Farmer J. W. , Kornecki M. , Cowan D. L.
Published: October 2003

A model of cooperative pinning where the pinning potential U(j) is inversely proportional to j accurately describes flux creep in high quality single crystals of Y-123 in the single fluxoid creep regime. In contrast, for strong pinning the pinning potential varies as (1-j/j max )3/2. Plots of the function S(j)=d ln (U)/d ln (j) provide a sensitive test for distinguishing between these two pinning mechanisms. Several crystals grown and annealed under different conditions have been analyzed. We find that we can prepare single crystals in either pinning class, depending on the growth and annealing process. It is suggested that strong pinning occurs when the growth and annealing processing lead to the formation of precipitates with nearly optimum size distribution. Strong pinning effects are also observed in fast neutron irradiated samples.
Observation of Pseudogap in Underdoped Bi2Sr2CaCu2O8+δ by Tunneling Spectroscopy
Authors: Gupta Anjan K. , Ng K.-W.
Published: October 2003

We have performed tunneling measurements on underdoped and optimally doped single crystals of Bi2Sr2CaCu2O 8+δ(Bi2212). The tunneling is in the ab-plane, in the maximum gap direction for all of our experiments. Measurement temperature ranges from 4.2 K to well above 100 K. The superconducting gap has been measured for T < T c . 2Δ/kT c for underdoped Bi2212 (~ 13.4) is significantly larger than that of optimally doped Bi2212 (~ 8.8). As the temperature is raised above T c , the gap vanishes for the optimally doped sample. In contrary, the gap like feature for the underdoped Bi2212 persists with larger gap value at higher temperatures. There are interesting differences in the nature of superconducting gap and pseudogap.
PrBa2Cu3Ox Polycrystalline Superconductor Preparation
Authors: Hults W. L. , Cooley J. C. , Peterson E. J. , Smith J. L. , Blackstead Howard A. , Dow John D.
Published: October 2003

We prepared many samples of nominal PrBa2Cu3O x as ceramic, polycrystalline pellets with an enormous variety of preparation conditions and can confirm its superconductivity at T c = 89 K. We found that the superconductivity is in material with the YBa2Cu3O7 (YBCO) crystal structure. We observed weak ferromagnetism (canted antiferromagnetism) in samples prepared at high temperatures with a low partial pressure of oxygen; these samples also contained the phase PrBaO3 . The electromagnetic properties of the material depend sensitively on preparation conditions, especially the sintering temperature and the oxygen level.
Direct Determination of the Velocity of Vortices in High-Tc Superconductors by a Non-Equilibrium Method
Authors: Kirschner I. , Laiho R. , Bódi A. C. , Lähderanta E. , Vajda I.
Published: October 2003

As is shown, thermally assisted vortex motion can come into being in high-T c superconductors due to the applied temperature gradient. Its behavior strongly depends on the local and global microstructure of the samples, moreover on the temperature and magnetic field. Investigation of the density, size and intensity of the pinning centers of specimens leads to the conclusion that the higher homogeneity immediately weakens and the lower one strenghtens the pinning, thus the former promotes and the latter impedes the vortex motion. The non-equilibrium experimental technique together with a.c. susceptibility measurements render possible the direct determination of the velocity of vortices. Depending on the actual microstructural state of samples it has the values between 6 × 10-2 mm/s and 18 × 10-2 mm/s in the case of Y-Ba-Cu-O specimens investigated.
Hysteresis of the Resonance Absorption in Y–Ba–Cu–O
Authors: Kovács Gy. , Khatiasvili N. , Porjesz T. , Vajda I.
Published: October 2003

The effect of weak d.c. magnetic field on the interaction between high–T c superconductors and high frequency field has been studied. The magnetic field (Bres) at which the most intensive change in the absorption could be obtained depended on the previously applied field and displayed a hysteretic behaviour but it did not depend on the frequency.
Cu K-Edge Study of (Tl0.5Pb0.5)Sr2Ca1-xYxCu2O7-δ
Authors: Liang G. , Liu R. S. , Wang L. V.
Published: October 2003

Near-edge x-ray-absorption spectra at the Cu K-edge has been measured for (Tl0.5Pb0.5)Sr2Ca 1-x Y x Cu2O 7-δ compound series. It is found that both the intensity of the quadruple 1s → 3d transition feature and the edge-energy decrease with increasing x. Our results show that the Cu valence decreases slightly with the substitution of Ca +2 by Y +3. This result, together with recent O K-edge result the O -2p holes are reduced by the substitution, indicates that electronic charge is transferred into the entire CuO 2 layers by the Y +3 for Ca +2 substitution.
Correlations Between Physical and Crystallographic Parameters in the (Bi, Pb)2Sr2 (Ca, Y)Cu2O8+δ Superconductors
Authors: Manifacier Laurent , Collin Gaston , Blanchard Nicole
Published: October 2003

Systematic studies of the lead and yttrium substituted Bi-2212 phase were performed on various compositions ranging between 0 and 0.8 for Pb and between 0 and 0.9 for Y. Crystallographic measurements reveal that the two modulations observed in unleaded and highly Pb-substituted samples are closely linked, the main differences being that the commensurate component disappears in type II (lead type) modulation, along with the modulation of the Bi 3+ ion. We argue that this is due to a covalent bonding between Bi-O planes. The study of internal parameters, including modulation amplitudes, of the Bi2Sr2 ( Ca 1-x, Y x) Cu2O 8+x/2+δ system reveals three distinct regimes with very different behaviors, corresponding to the overdoped, underdoped and insulating regimes. By comparing these results to the variation of the diamagnetic shielding with Y and Pb contents, we conclude that yttrium and lead affect more the mobility of the holes than the hole concentration itself. Y seems to induce, rather than a metal/insulator transition, a switch from a three-dimensional to a two-dimensional regime with a hole localization in the CuO 2 planes.
Spectroscopic Properties of the High-Tc Materials by the Ab Initio Quantum Chemistry Paradigm
Authors: Panas Itai , Johnson Tobias , Snis Anders
Published: October 2003

The σ-π splitting for an embedded O - ion in a CuO2 plane is estimated and effects of crystal fields are evaluated. High fields favor σ-hole while the π-hole scenario is favored in low lattice fields. The results are discussed in the context of the tow-bands Raman continuum understanding put forward by Monien and Zawadowski2 and argued to support a recently proposed two-bands two-physics mechanism for superconductivity in the cuprates.
Extended-Drude Model to Fit Infrared Conductivity Cuprate Laser-Ablated Films
Authors: Pessaud Sandrine , De Sousa Domingos , Lobo Ricardo , Gervais François
Published: October 2003

An "extended-Drude" model, implying a simple form for the self-energy function of the mobile charge-carrier response, has been applied to fitting the infrared and visible reflectivity spectra of simple cuprates. Excellent fits are obtained in a wide spectral range, from 4 meV to 4 eV, with a very restricted number of adjustable parameters. The optical conductivity obtained with this procedure is highly different from the Kramers–Kronig transformation of reflectivity spectra. The same procedure has been applied to characterise the infrared conductivity of multi-target laser-ablated films built via intergrowth of YBa2Cu3O7 and MCuO2 (M = Ca, Sr).
New Features of the Freezing Vortex Matter in Single Crystals of YBa2Cu3O7-δ
Authors: Rykov A. I. , Tajima S. , Kusmartsev F. V.
Published: October 2003

We report on the new features observed in the magnetization of a high-T c superconductor at cooling through the first order transition. The single crystal of YB2Cu3O 7-δ cooled in the magnetic field H‖c at cooling (FCC) exhibits a multipolar magnetic moment which results from the summing up over the sample of both the negative (magnetization of the uncompressed vortex lattice) and the positive local magnetic moments. The positive local moments correspond to the enhanced density of the trapped flux lines exceeding locally that for the external magnetic field. At large fields (> 3T) below freezing temperature Tf, the dipolar contribution into the magnetic moment is fully reversible, in agreement with zero density of critical current. The magnetization of the field-cooled sample measured at warming (FCW) exhibit jumps at clearly larger temperature Tm(>Tf). The jump from zero to a finite value in the octupolar magnetizations [Formula: see text] occurs at the same temperatures Tf, Tm as the conventional jump in the field-cooled dipolar magnetization [Formula: see text] related to the vortex ordering entropy (ΔS < k B ). The high-pole magnetic moments are metastable and exhibit long sample-specific relaxation, quickest in a narrow temperature range at T<Tm close to Tm.
Spin Dynamics in the Metallic State of YBa2(Cu0.98 Zn0.02) 3O6+x
Authors: Sidis Y. , Bourges P. , Hennion B. , Villeneuve R. , Collin G. , Marruco J. F.
Published: October 2003

Inelastic neutron scattering measurements have been carried out on a YBa2(Cu0.98-Zn0.02)3O 6+x single crystal in both underdoped (x = 0.7) and overdoped (x = 0.97) regimes. In the zinc substituted system, spin dynamics is drastically changed in respect to the pure compound: (i) the "resonance peak" almost vanishes, (ii) the spin gap is filled, (iii) new antiferromagnetic excitations are found at low energy. These new magnetic fluctuations, which persist in the normal state, account for a local enhancement of AF correlations around nonmagnetic impurities. Besides, it is worth emphasizing that features, not directly related to superconductivity, i.e., the contribution to the spin dynamics apart from the resonance peak and the "spin pseudo-gap" observed in the underdoped regime above T c , coexist with the new low energy magnetic fluctuations.
Nb and Ta Substituted Bi(Pb)-Sr-Ca-Cu-O Superconductors
Authors: Smrčková O. , Sýkorová D. , Vaśek P.
Published: October 2003

The influence of stoichiometric replacement of Bi by Nb and Ta in the Bi(Pb)-based superconducting system has been investigated. The properties were detected by the measurement of resistivity, magnetic susceptibility and critical current density. X-ray diffraction technique has been used for crystallographic and phase analysis. The study reveals that for both substituents an addition of small amount of them enhances the content of the high temperature phase and improves superconducting properties. On the other hand greater amount of substituents deteriorates weak links in the sample.
Bose-Glass Behavior in Bi2Sr2Ca1-xYxCu2O8 Crystals with Columnar Defects: Experimental Evidence for Variable-Range Hopping
Authors: Phuoc V. Ta , De Sousa R. , Ammor L. , Soret J. C. , Ruyter A. , Wahl A.
Published: October 2003

We report on vortex transport in Bi2Sr2Ca 1-x Y x Cu2O8 crystals irradiated at different doses of heavy ions. We show evidence of a flux-creep resistivity typical of a variable-range vortex hopping mechanism as predicted by Nelson and Vinokur.
TEM Study of Coherently Tilted YBa2Cu3O7-δ Thin Films on (305) SrTiO3 Substrates
Authors: Yang Y. , Gao J.
Published: October 2003

Microstructural study has been carried out on the YBa2Cu3O 7-δ (YBCO) thin films grown on (305) SrTiO3 (STO) substrates. The results show that the YBCO films are coherently tilted with the c-axis parallel to the [001] direction of STO. Small antidomains are nucleated at the initial growth stage. When the film thickness increases, antidomains disappear leading to the formation of single domain thin films. Some particles are observed above the defective substrate surface, but the film with the favorable orientation suppresses their further growth.
Band and Percolation Approaches to Low Temperature Properties of Manganites
Authors: Gor'kov Lev P. , Kresin Vladimir Z.
Published: October 2003

Properties of the parent compound, LaMnO3 are greatly affected by strong Hund's coupling. Assuming the antiferromagnetic ordering (with a low Neel temperture) along one of the cubic axes, this coupling would result in formation of disconnected ferromagnetic layers. Furthermore, in the presence of the cooperative Jahn–Teller effect the compound becomes a band insulator. At small doping ( La 1-x Ca x MnO 3, x ≪ 1) the band insulator phase coexists with the presence of localized holes. An insulator-ferromagnetic metal transition occurs at the percolation point x c =0.16.
Optical Properties of the Half-Metallic La3/4Ca1/4MnO3
Authors: Bagayoko D. , Zhao G. L. , Fan J. D. , Wang J. T.
Published: October 2003

We studied the electronic structure and optical properties of the low temperature (T = 0 K for the calculation) ferromagnetic phase of La3/4Ca1/4MnO3 using a self-consistent ab-initio LCAO method. We employed a local density potential. The calculated electronic structure suggests that the material is in a half-metallic state. The calculated average magnetic moment for the half-metallic phase is 3.75 μ B per Mn. This value is very close to the experimental values of 3.7 μ B per Mn. The real part of the optical conductivity, σ1(ω), from the direct band transitions, is predicted. The calculated optical conductivity shows that for photon energies below 2.4 eV, the electronic states of minority spin do not contribute to the optical transitions.
Dependence of Infrared Photoresponse of Pyroelectric Thin Film Detector with Respect to Chopper Frequency
Authors: Chen Yansong , Wu N. J. , Ignatiev A.
Published: October 2003

In this paper, the dependence of infrared photoresponse of a pyroelectric thin film detector with respect to chopper frequency is studied theoretically and experimentally. The expressions of photocurrent, photovoltage and a special chopper frequency f m at which the maximum photovoltage is generated, are derived. A PMSZT/YBCO heterostructure is taken as the pyroelectric infrared detector and its photocurrent and photovoltage dependence on chopper frequency was measured. The experimental results are satisfactory and in agreement with theoretical analysis.
Phase Separation in Models for Manganites
Authors: Moreo Adriana
Published: October 2003

Using computational techniques the phase diagram of models for manganites is studied. In the absence of Jahn–Teller phonons three regimes are found to dominate the low temperature properties of these models: (i) a ferromagnetic region induced by the double-exchange mechanism; (ii) a region with incommensurate spin correlations at small Hund coupling; and (iii) a novel regime of phase separation (PS) between hole-rich ferromagnetic and hole-poor antiferromagnetic domains. PS occurs both at high- and low density of eg electrons. Experiments for manganites compatible with the presence of PS are discussed. In the presence of dynamical Jahn–Teller phonons, PS is also observed and, in addition, staggered orbital order exists in some regions of parameter space.
Spin-Dependence of the Electron Scattering Cross Section by a Magnetic Layer System and the Magneto-Resistance
Authors: Wang J. T. , Tang F. , Brown W. D. , Bagayoko D.
Published: October 2003

We present a theoretical model for calculating the spin-dependent cross section of the scattering of electrons by a magnetic layer system. Our model demonstrates that the cross sections of the scattering are different for spin up and spin down electrons. The model assumes that the electrical resistivity in a conductor is proportional to the scattering cross section of the electron in it. It is believed to support the two channel mechanism in interpreting magneto-resistance (MR). Based on the model without considering the scattering due to the interfacial roughness and the spin flipping scattering, we have established a relationship between MR and the square of the magnetic moment in the bulk sample without considering the scattering due to the interfacial roughness and the spin flipping scattering. It can also qualitatively explain the MR difference between the current in plane (CIP) and current perpendicular to the plane (CPP) configurations. The predictions by the model agree well with the experimental findings.
Perovskite Oxide Thin Film Heterostructures for IR Detector
Authors: Wu N. J. , Ignatiev A. , Chen Y. S.
Published: October 2003

Mn and Sb-doped PbZrTiO 3 (PMSZT) ferroelectric thin films were integrated to high-T c superconducting YB2Cu3O 7-x (YBCO) thin films on LaAlO 3 (100) and YSZ/Si (100) substrates by the pulsed laser deposition technique. The PZT/YBCO and PM-SZT/YBCO infrared detectors were fabricated and examined as to their photovoltage and photocurrent in response to IR source temperature, and IR wavelength. The detector has relatively high IR sensitivity with a detectivity of ~ 108 cm · Hz 1/2/ W in the wavelength range of 3–16 μm.
Properties and Applications of High-Temperature Superconductor/Ferroelectric Heterostructures
Authors: Xi X. X.
Published: October 2003

The properties and applications of high temperature superconductor/ferroelectric heterostructures are discussed, in particular for the case of electric-field effects and tunable microwave devices. Such epitaxially grown heterostructures provide unique platforms to study various physical phenomena in both material systems.
Dominant Role of a Pseudogap in the Physics of the Colossal Magnetoresistive Oxides
Authors: Dessau D. S. , Saitoh T. , Park C.-H. , Shen Z.-X. , Moritomo Y. , Tokura Y.
Published: October 2003

We have performed direct electronic structure measurements of the colosssal magnetoresistive oxides using high energy resolution photoemission spectroscopy. A key result of these measurements is the discovery of a pseudogap which drastically supresses or completely removes the spectral weight (density of states) at the Fermi energy E F . It is shown that the pseudogap plays a dominant role in the "colossal" conductivity changes which occur across the magnetic ordering temperature. We briefly discuss possible origins for the pseudogap.
Non-Conventional Infrared Conductivity ofLa2CuO4.06andPr0.7Sr0.2Ca0.1MnO3
Authors: Gervais François , Daulan Christine , Maignan Antoine , Lobo Ricardo P. S. M.
Published: October 2003

Pr0.7Sr0.2Ca0.1MnO3displays a ferromagnetic metallic-like to non-ferromagnetic insulator-like phase transition near 200 K and shows colossal magneto-resistance. The temperature evolution of its optical conductivity was measured and the results were compared to those of La2CuO4.06which is a superconductor below 34 K. The growth of a peak in the conductivity around 10 meV upon cooling Pr0.7Sr0.2Ca0.1MnO3strongly suggests a trapped-polaron contribution to the conductivity.
Conductivity and Isotope Effect in the Colossal Magnetoresistance
Authors: Malozovsky Y. M. , Fan J. D. , Bagayoko D.
Published: October 2003

The diffusion of nearly localized electrons is considered in the presence of the spin-dependent and multiple scattering on the randomly oriented and/or distributed magnetic moments of scatterers. The spin diffusion coefficient in the case of the exchange interaction between the diffusive but nearly localized electrons is evaluated. It is shown that the electron localization leads to the vanishing of the spin diffusion and hence the ferromagnetic phase transition. The isotope effect in the Curie temperature in the case of the electron–phonon inelastic relaxation is obtained, as experimentally observed in the manganese oxides by Zhao and co-workers.11 The strong spin-polaron effect caused by the interaction of the diffusive electrons with the diffusive paramagnons significantly reduces conductivity near the Curie temperature. The temperature dependence of conductivity in the absence and presence of an external magnetic field is discussed as well.
The Isotope Effect in Hydrogen-Bonded Systems
Authors: Bussmann-Holder A. , Michel K.-H.
Published: October 2003

The giant isotope effect on the ferro- and antiferroelectric transition temperature upon deuteration of hydrogen-bonded systems is well known experimentally since various decades. Yet, theoretically only recently a microscopic understanding of this effect has been achieved which, specifically, took into account the geometry of the O ⋯ H ⋯ O bond. The implications of this modeling are multiple as numerous hydrogen-bonded organic systems show the same effects as ferro- and antiferroelectrics, i.e., cooperative proton tunneling at a well-defined temperature.
The Dielectric Properties of the Piezoelectric PZT Thick Films
Authors: Tang F. , Chinkhota M. , Brown W. D. , Wang J. T.
Published: October 2003

The piezoelectric PZT thick films were fabricated by spinning and painting techniques in our laboratory. The purpose of this paper is to study the characteristics of the piezoelectric films that could be used as the sensors in devices, smart materials and structures. The effects of processing methods and processing conditions on dielectric properties of thick films are investigated. The temperature dependencies of the dielectric constants and the loss of the sample are measured before and after poling. Our experimental results might provide a better understanding of the behavior of these types of materials regarding their applications in monitoring, active controlling and diagnosing.
Crystal Symmetry Dependence of the Polarization of Brillouin Scattered Light
Authors: Wang J. T. , Tang F. , Tashakkori R. , Schmidt V. H. , Chen T. P.
Published: October 2003

The Brillouin scattering is the light scattering by the phonons in the solid. It is very useful for studying the structural phase transition in the crystal and measuring the elastic and photoelastic constants of the materials. To accomplish these goals, one has to be able to determine the linkage between the polarization of the scattered light and the phonon modes which can also provide insight into the origin of the phase transition. In our paper, we calculated the scattering factors using elastic equations, photoelastic relation and radiation equations for an orthorhombic crystal. The results are summerized in the Chart. From the Chart provided below one can easily find the correspondence of the phonon mode to the polarization of the scattered light.
Role of the Berry Phase in the Formation of Stripes in Manganese Oxides
Authors: Hotta Takashi , Takada Yasutami , Koizumi Hiroyasu
Published: October 2003

In order to gain an insight into the formation of paired Jahn–Teller (JT) stripes, we have investigated the electronic and vibronic structures in the manganese oxides by paying due attention to the effect of the Berry phase associated with the JT distortions on the band motion of an eg electron. In terms of a band-insulator picture, we have succeeded in obtaining the JT stripes and the stabilization mechanism of the CE-type antiferromagnetism in agreement with experiment.
Impurity Entropy for the Two-Channel Kondo Model
Authors: Rozhkov A. V.
Published: October 2003

In this paper the entropy of the two-channel Kondo impurity is calculated. We demonstrate that the exactly solvable system of finite size can be described in terms of free fermions. None of these fermions is Majorana fermion. This allows us to find the impurity entropy S(T, L) for arbitrary values of T and L. As found before, S(T = 0, L = ∞) equals to (1/2) ln 2. For finite L the entropy shows a cross-over to the infinite-size behavior at T > TL= v F /L, in agreement with results of other researchers.
Fermi-Liquid Features of the One-Dimensional Luttinger Liquid
Authors: Wang Yupeng
Published: October 2003

We show that the one-dimensional (1D) electron systems can also be described by Landau's phenomenological Ferimi-liquid theory. Most of the known results derived from the Luttinger-liquid theory can be retrieved from the one-dimensional Fermi-liquid theory. Exact correspondence between the Landau parameters and Haldane parameters is established. The exponents of the dynamical correlation functions and the impurity problem are also discussed based on the finite size corrections of elementary excitations with the predictions of the conformal field theory, which provides a bridge between the one-dimensional Fermi-liquid and the Luttinger liquid.
Coherence Length in a Two-Dimensional Anisotropic Model of Superconductivity
Authors: Bujkiewicz L. , Jacak L.
Published: October 2003

The coherence length as a mean-square radius of a Cooper pair is calculated for various forms of the anisotropic order parameter. A wide range of values of this quantity is obtained and the smallest values are of the order of magnitude of a few lattice spacings.
Micropatterning of BiSrCaCuO Thin Films Using Excimer Laser
Authors: Higo S. , Hakuraku Y. , Shimada Y. , Minamizono E. , Tadokoro M. , Ogushi T.
Published: October 2003

Micropatterning of BiSrCaCuO(BSCCO) superconducting thin films has been successfully accomplished by using a pulsed KrF excimer laser with a wavelength of λ=248 nm and a duration of 25 ns. The optimum energy density of the laser was 0.36 Jcm -2 for etching of these films. For example, a superconducting microstructure of BSCCO film with nominally 3 μm wide and 2 μm long, showed no degradation in T c and J c . Excimer laser patterning technique yields reproducible patterning without any degradation of superconducting properties. The etching bordering and surface morphology was examined using an atomic force microscope.
Applying the 1/N-Energy Solution of the Harper Equation to the Derivation of Thermodynamic Properties of Bloch Electrons
Authors: Micu C. , Papp E.
Published: October 2003

The novel 1/N-energy solution to the Harper equation presented recently is applied to the derivation of thermodynamic properties of Bloch electrons on a two-dimensional lattice penetrated by a perpendicular uniform magnetic field. One procceeds by using an almost typical density of states such as proposed previously for a two-dimensional electron gas. Comparisons with recent data concerning rare earth componds as well as with results obtained before with the help of Dingle- and quantum-gas densities are done. One assumes that the Fermi level is fixed.
Effects of Molecular Orientational Disorder on the Electronic Structure of K3C60: With the Inclusion of the Tangentially Directed C σ-Orbital
Authors: Lu Jing , Zhao Xiangeng , Zhang Xinwei , Zhang Liyuan
Published: October 2003

Effects of molecular orientational disorder on the electronic structure of K3C60 are studied using a tight-binding model in which both the 2s and 2p orbitals of C atoms are taken into account. Nearly free of the cut-off distance of the interculster interaction and the form of the distance dependence of the hopping parameters, the orientational disorder always smears out the structure in the conduction-band density of states (DOS), while the value of the DOS at the Fermi level (N(E F )) changes slightly upon disordering. These results are in excellent agreement with the earlier ones based on the single molecular orbital approximation.
Learning Rule for Pattern Recognition in Hopfield Neuronal Network
Authors: Andrecut Mircea
Published: October 2003

The Hopfield model of associative memories is reconsidered in terms of pattern recognition in presence of noise. A learning rule for the investigated model is deduced. With the obtained learning rule the storing capacity of Hopfield neuronal network is increased from α≃0.14 to the optimal value α=1. The pattern recognition capabilities of the model increase exponentially with the dimensionality of the patterns.
Phase Transition in a Two-State Chain Interacting with a Phonon Bath
Authors: Xu Jing , Yuan Qingshan , Chen Hong
Published: October 2003

The properties of a two-state chain interacting with a phonon bath is studied in this paper. We discuss the renormalization of the single pseudospin tunneling parameter by phonons' interaction, as well as other pseudospins' correlation. We find that for Ohmic dissipation, the correlation between pseudospins vanishes and the phonons' interaction induces the localization–delocalization transition. For our discussed super-Ohmic dissipation, the phonons' interaction only reduces the tunneling parameter and does not induce phase transition. In this case the pseudospins' correlation has its effect and reduces the tunneling parameter at weak coupling. In addition, some application of our results is also discussed.
Generalized Superposition of Two Coherent States and Interference Effects
Authors: Ragi R. , Baseia B. , Bagnato V. S.
Published: October 2003

We introduce a generalized superposition of two coherent states to investigate interference effects between them and their influences upon nonclassical effects. The role played by the involved parameters on each nonclassical effect is shown, including a proposal for generating this state and its application. The present scheme constitutes an unified approach, including various important states studied in the literature.
Discrete Solid-On-Solid Model of Interface with Bending Rigidity: Restricted and Unrestricted Step Sizes
Authors: Maiti Prabal K. , Chowdhury Debashish
Published: October 2003

We report the results of imposing a physically motivated restriction on the step sizes in a (d + 1)-dimensional discrete solid-on-solid (SOS) model, which we have proposed recently (Phys. Rev.E54, 2670 (1996)) for a d-dimensional interface with bending stiffness and interfacial tension. We clarify some features of this model in the absence of such restrictions. We also draw analogies between the relaxation to thermally roughened equilibrium states of these SOS models and nonequilibrium growth of some kinetically roughened model surfaces.
The Paradox of Globalization
Authors: Thelen Kathleen
Published: October 2003

What is the impact of globalization on labor in the advanced industrial countries? One body of work holds that globalization pushes all countries toward neoliberalism; another argues that the impact of globalization will vary according to prevailing institutional arrangements and predicts continued divergence between the so-called liberal and coordinated market economies. In an analysis of the German case that combines case-study and formal-modeling methods, we refute the first, neoliberal convergence, theory, showing that globalization in some ways empowers unions by rendering employers extremely vulnerable to industrial unrest. Furthermore, we suggest some revisions to the second, varieties of capitalism, perspective. We show that, in Germany, employers' increased vulnerability to conflict has shored up centralized bargaining arrangements in the short run, but at the same time, has also set in motion feedback effects that are deeply destabilizing to the system in the longer term.